Dark-field digital holographic microscope and related metrology methods

Abstract

A dark-field digital holographic microscope configured to determine properties of interest of a structure is disclosed. The dark-field digital holographic microscope includes 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 one or more optical elements operable to capture first and second scattered radiation scattered by the structure, resulting from the first and second illumination beams, respectively. The beams of the first beam pair are mutually coherent, and the beams of the second beam pair are mutually coherent. The illumination device is configured to provide incoherence between the first and second beam pairs.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to European Application No. 19216970.4, filed December 17, 2019, and European Application No. 20167524.6, filed April 1, 2020, which are incorporated herein by reference in their entireties.

[0002] [Technical field] The present invention relates to dark-field digital holographic microscopes, and in particular to high-speed dark-field digital holographic microscopes, for metrology applications in integrated circuit manufacturing. [Background technology]

[0003] A lithographic apparatus is a machine built to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern (also called a "design layout" or "design") in a patterning device (e.g., mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., wafer).

[0004] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Lithographic apparatus using extreme ultraviolet (EUV) radiation with wavelengths in the range of 4-20 nm, e.g. 6.7 nm or 13.5 nm, can be used to form smaller features on a substrate than lithographic apparatus using radiation with a wavelength of, e.g., 193 nm.

[0005] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of the lithographic tool. In such processes, the equation for resolution can be expressed as CD=k1×λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics of the lithographic tool, CD is the "critical dimension" (usually the smallest feature size to be printed, but in this case the half-pitch) and k1 is an empirical resolution factor. In general, the smaller k1 is, the more difficult it is to reproduce on the substrate a pattern similar to the shape and dimensions planned by the circuit designer to achieve a particular electrical function and performance. To overcome these difficulties, advanced fine-tuning steps can be applied to the lithographic projection tool and / or the design layout. These include, for example, but are not limited to, optimization of the NA, customized illumination schemes, the use of phase-shifting patterning devices, various optimizations of the design layout such as optical proximity correction (OPC), also known as "optical and process correction", in the design layout, or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, tight control loops for controlling the stability of the lithographic apparatus can be used to improve reproduction of patterns at low k1.

[0006] During the manufacturing process, it is necessary to inspect the manufactured structures and / or measure properties of the manufactured structures. Suitable inspection and metrology tools are known in the art. One known metrology tool is a scatterometer, for example a dark field scatterometer.

[0007] Patent application publication US2016 / 0161864A1, patent application publication US2010 / 0328655A1, and patent application publication US2006 / 0066855A1 discuss embodiments of photolithographic apparatus and embodiments of scatterometers. The cited documents are incorporated herein by reference. Summary of the Invention [Problem to be solved by the invention]

[0008] Darkfield microscopes, such as the instrument described above, more generally suffer from a limited angular range for illumination and detection due to the need to share the full range of angles (corresponding to the area within the angle-resolved pupil) between the illumination and detection paths. This limits the effective numerical aperture (NA) of illumination and / or detection. The problem of increasing the effective NA of both illumination and detection has been addressed by implementing sequential acquisition schemes. This results in an unnecessarily slower measurement speed.

[0009] It would be desirable to provide a dark field microscope in which the effective NA of at least the detection optics is increased in order to improve resolution by capturing diffracted light over a wider range of diffraction angles. [Means for solving the problem]

[0010] In a first aspect of the invention, a dark field digital holographic microscope configured to determine a characteristic of interest of a structure is provided. It comprises an illumination device configured to provide at least a first beam pair comprising a first illumination radiation beam and a first reference radiation beam, and a second beam pair comprising 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 resulting from the structure being illuminated by the first illumination radiation beam and to detect a second scattered radiation scattered by the structure resulting from the structure being illuminated by the second illumination radiation beam, the imaging branch having a detection NA greater than 0.1. The illumination device is configured such that the first illumination radiation beam and the first reference radiation beam are at least partially coherent in time and space, and the second illumination radiation beam and the second reference radiation beam are at least partially coherent in time and space. The illumination device is configured to provide a temporal and / or spatial incoherence between the first beam pair and the second beam pair.

[0011] In a second aspect of the invention, there is provided a method for determining a property of interest of a target formed on a substrate by a lithographic process. The method comprises illuminating the target with a first illumination radiation beam and capturing a resultant first scattered radiation scattered from the target; illuminating the target with a second illumination radiation beam and capturing a resultant second scattered radiation scattered from the target; imparting spatial and / or temporal incoherence between a first beam pair comprising the first illumination beam and a first reference beam and a second beam pair comprising the second illumination beam and a second reference beam, wherein the beams of the first beam pair are at least partially spatially and temporally coherent and the beams of the second beam pair are at least partially spatially and temporally coherent, and both beams of the first beam pair are spatially and / or temporally incoherent with respect to both beams of the second beam pair; and simultaneously generating a first interference pattern resulting from interference of the first scattered radiation with the first reference radiation beam and a second interference pattern resulting from interference of the second scattered radiation with the second reference beam. [Brief description of the drawings]

[0012] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings. [Figure 1] 1 depicts a schematic diagram of a lithographic apparatus; [Diagram 2] FIG. 1 is a schematic diagram of a lithography cell. [Diagram 3] FIG. 1 is a schematic diagram of overall lithography illustrating the collaboration between three main technologies to optimize semiconductor manufacturing. [Figure 4] FIG. 1 is a schematic diagram of a scatterometry apparatus used as a metrology apparatus, which may include a dark field digital holographic microscope according to an embodiment of the present invention; [Diagram 5] 1 is a schematic diagram of a level sensor apparatus that may include a dark-field digital holographic microscope according to an embodiment of the present invention. [Figure 6]1 is a schematic diagram of an alignment sensor apparatus that may include a dark-field digital holographic microscope according to an embodiment of the present invention. [Figure 7] FIG. 1 shows a schematic diagram of an example of a diffraction-based dark field metrology setup operating in a parallel acquisition mode. [Figure 8] FIG. 13 shows a schematic diagram of another example of a diffraction-based dark field metrology apparatus operating in a sequential acquisition mode. [Figure 9] FIG. 1 shows a schematic diagram of an example of a dark-field digital holographic microscope operating in a sequential acquisition mode. [Figure 10] FIG. 1 illustrates a schematic diagram of a dark-field digital holographic microscope (df-DHM) operating in a parallel acquisition mode, according to one embodiment. [Figure 11] 1 illustrates a schematic diagram of an illumination device capable of providing multiple beams of radiation according to an embodiment; [Figure 12] 1A-1D illustrate schematic diagrams of illumination devices capable of providing multiple radiation beams according to different embodiments; [Figure 13] FIG. 2 illustrates a Fourier transform image in the spatial frequency domain. [Figure 14] FIG. 4 shows a flowchart of a method for determining the amplitude and phase of a complex field according to another different embodiment. [Figure 15] FIG. 1 is a block diagram of a computer system for controlling a dark-field digital holographic microscope. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] As used herein, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet (e.g., having wavelengths of 365, 248, 193, 157, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having wavelengths in the range of about 5 to 100 nm).

[0014] The terms "reticle," "mask," or "patterning device," as used herein, may be broadly interpreted to refer to a general patterning device that can be used to impart an incident radiation beam with a patterned cross-section that corresponds to a pattern to be created on a target portion of a substrate. The term "light valve" may also be used in this context. In addition to standard masks (transmissive or reflective; binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0015] 1 illustrates diagrammatically a lithographic apparatus LA, comprising: an illumination system (also called illuminator) IL configured to condition a radiation beam B (e.g. UV, DUV or EUV radiation), a mask support (e.g. mask table) MT constructed to support a patterning device (e.g. mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. wafer table) WT constructed to hold a substrate (e.g. resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0016] In operation, the illumination system IL receives a beam from the radiation source SO, for example via the beam delivery system BD. The illumination system IL may include various optical elements, or any combination of these, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical elements for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B such that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0017] The term "projection system" PS as used herein should be interpreted broadly as covering any type of projection system including, for example, refractive, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. When the term "projection lens" is used herein, it can be considered as synonymous with the more general term "projection system" PS.

[0018] The lithographic apparatus LA may be of a type in which at least a part of the substrate is covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a gap between the projection system PS and the substrate W. This is also referred to as immersion lithography. Further details about immersion techniques are found in US Pat. No. 6,952,253, which is incorporated herein by reference.

[0019] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also called "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel and / or steps in preparation for subsequent exposure of the substrate W may be performed on a substrate W located on one of the substrate supports WT. The substrate W on the other substrate support WT is being used to expose a pattern on the other substrate W.

[0020] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold a number of sensors. The cleaning device may be configured to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of a system for providing immersion liquid. When the substrate support WT is away from the projection system PS, the measurement stage may be moved below the projection system PS.

[0021] In operation, the radiation beam B is incident on a patterning device, for example a mask MA, which is held on the mask support MT, and is patterned according to a pattern (design layout) present on the patterning device MA. After passing the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be precisely moved to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the first positioner PM and possibly further position sensors (not explicitly shown in FIG. 1 ) can be used to precisely position the patterning device MA with respect to the path of the radiation beam B. The patterning device MA and substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may also be located in spaces between the target portions. The substrate alignment marks P1, P2, when they are located between target portions C, are known as scribe-lane alignment marks.

[0022] As shown in FIG. 2, the lithographic apparatus LA may form part of a lithographic cell LC, sometimes also referred to as a lithocell or (litho)cluster, which often also includes apparatus for performing pre-exposure 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 cooling plate CH, and a bake plate BK, for example to adjust the temperature of the substrate W, for example to adjust the solvent of the resist layer. A substrate handler or robot RO picks up the substrates W from input / output ports I / O1, I / O2, moves them between the different process apparatus, and delivers the substrates W to a loading bay LB of the lithographic apparatus LA. The apparatus of the lithocell (often also collectively referred to as the track) are typically under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which may control the lithographic apparatus LA, for example via a lithography control unit LACU.

[0023] In order to accurately and consistently expose an exposed substrate W by the lithographic apparatus LA, it is desirable to inspect the substrate to measure characteristics of the pattern structures such as overlay error between subsequent layers, line width, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithocell LC. If an error is detected, for example, the exposure of a subsequent substrate or other processing steps performed on the substrate W may be adjusted, especially if inspection is done before other substrates W of the same batch or lot are yet to be exposed or processed.

[0024] The inspection apparatus, sometimes called metrology apparatus, is used to determine the properties of the substrate W, in particular how the properties of different substrates W vary or how the properties associated with different layers of the same substrate W vary from layer to layer. Alternatively, the inspection apparatus may be configured to identify defects on the substrate W and may for example be part of a lithocell LC or integrated in the lithography apparatus LA or may be a stand-alone apparatus. The inspection apparatus may measure properties of the latent image (image of the resist layer after exposure), or the semi-latent image (image of the resist layer after a post-exposure bake step PEB), or the developed resist image (image in which exposed or unexposed parts of the resist have been removed), or the etched image (after a pattern transfer step such as etching).

[0025] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires high accuracy of the dimensioning and placement of the structures on the substrate W. To guarantee this high accuracy, three systems can be combined in a so-called "holistic" controlled environment, as shown diagrammatically in FIG. 3. One of these systems is the lithography apparatus LA, which is (virtually) connected to a metrology tool MT (second system) and a computer system CL (third system). The key to such a "holistic" environment is to optimize the cooperation between these three systems to enforce the overall process window and to provide a strict control loop that ensures that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the set of process parameters (e.g. dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g. a functional semiconductor device), typically within which the process parameters of the lithography process or patterning process are allowed to vary.

[0026] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement technique to use and perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings will achieve the largest overall process window of the patterning process (indicated by the double arrow at the first scale SC1 in FIG. 3). Typically, the resolution enhancement technique is configured to match the patterning capabilities of the lithography apparatus LA. The computer system CL may also be used to detect where in the process window the lithography apparatus LA is currently operating (e.g., using input from the metrology tool MT) in order to predict whether defects are present due to suboptimal processing (indicated by the arrow pointing to "0" at the second scale SC2 in FIG. 3).

[0027] The metrology tool MT can provide input to the computer system CL to enable accurate simulation and prediction, and can provide feedback to the lithographic apparatus LA to identify possible drift, for example in the calibration state of the lithographic apparatus LA (indicated by multiple arrows at a third scale SC3 in Figure 3).

[0028] In lithographic processes, it is frequently desirable to perform measurements of the created structures, e.g. for process control and verification. Tools for performing such measurements are typically called metrology tools MT. Various types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow 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 scatterometer objective lens (usually referred to as pupil-based measurements) or in the image plane or in a plane conjugate to the image plane (in this case the measurements are typically referred to as image or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164A, which are incorporated herein by reference in their entireties. The scatterometers described above can measure gratings using light from soft x-rays and light in the visible to near infrared wavelength range.

[0029] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signal in order to reconstruct or calculate the properties of the diffraction grating. Such a reconstruction can result, for example, from simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the one observed from the real target.

[0030] 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 towards a target and reflected or scattered radiation from the target is directed towards a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. a measurement of the intensity as a function of wavelength). From this data, the structure or profile of the target giving rise to the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

[0031] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. The ellipsometric scatterometer allows to determine parameters of the lithographic process by measuring the scattered radiation of each polarization state. Such a metrology tool emits polarized light (such as linear, circular or elliptical), for example by using a suitable polarizing filter in the illumination section of the metrology tool. A radiation source suitable for the metrology tool may also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. patent application Ser. Nos. 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 by reference herein in their entireties.

[0032] A metrology device such as a scatterometer is shown in Figure 4. It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate W. The reflected or scattered radiation is passed to a spectrometer detector 4, which measures the spectrum 6 (i.e. a measurement of the intensity as a function of wavelength) of the radiation due to specular reflection. From this data, the structure or profile 8 giving rise to the detected spectrum can be reconstructed by a processing unit PU, for example by rigorous coupled wave analysis and non-linear regression, or by comparison with a library of simulated spectra, as shown in the lower part of Figure 3. Typically, in the reconstruction, the general form of the structure is known, some parameters are assumed from knowledge of the process by which the structure was created, and only a few parameters of the structure are determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0033] The overall measurement quality of a lithography parameter from a metrology target measurement is at least partially determined by the measurement recipe used to measure this lithography parameter. The term "substrate measurement recipe" may include one or more parameters of the measurement itself, one or more parameters of the measured pattern, or both. For example, if the measurement used in a substrate measurement recipe is a diffraction-based optical measurement, the one or more parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, the orientation of the radiation to the pattern on the substrate, etc. One of the criteria for selecting a measurement recipe may be, for example, the sensitivity of one measurement parameter to process variations. More examples are described in U.S. Patent Application US2016-0161863 and published U.S. Patent Application US2016 / 0370717, which are incorporated by reference herein in their entirety.

[0034] Another type of metrology tool used in IC manufacturing is a topography measurement system, a level sensor or height sensor. Such a tool can be integrated into a lithography apparatus to measure the topography of the top surface of a substrate (or wafer). A map of the topography of the substrate, also called a height map, can be generated from these measurements, showing the height of the substrate as a function of position on the substrate. This height map can then be used to correct the position of the substrate during transfer of a pattern to the substrate, to provide an aerial image of the patterning device at the correct focus position on the substrate. In this context, it will be understood that "height" refers generally to the out-of-plane dimension (also called the Z-axis) relative to the substrate. Typically, a level or height sensor performs measurements at a fixed location (relative to its optics), and relative movement between the substrate and the optics of the level or height sensor results in height measurements at each location across the substrate.

[0035] An example of a level or height sensor LS known in the art is shown diagrammatically in FIG. 5. The figure illustrates only the principle of operation. In this example, the level sensor comprises an optical system including a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a radiation beam LSB provided by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrow or broadband light source, such as a supercontinuum light source, polarized or unpolarized, such as a polarized or unpolarized laser beam, pulsed or continuous. The radiation source LSO may include multiple radiation sources with different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but may additionally or alternatively encompass UV and / or IR radiation and any range of wavelengths suitable for reflection from the surface of the substrate.

[0036] The projection grating PGR is a periodic grating with a periodic structure resulting in a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed towards a measurement location MLO on the substrate W with an angle of incidence ANG of 0 to 90 degrees, typically 70 to 80 degrees, relative to an axis perpendicular to the incident substrate surface (Z-axis). At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0037] To determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grid DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grid DGR may be identical to the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, e.g. indicative of the intensity of the received light as in a photodetector or representative of the spatial distribution of the received intensity as in a camera. The detector DET may comprise any combination of one or more detector types.

[0038] By means of triangulation techniques the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends inter alia on the design of the projection grating PGR and on the (oblique) angle of incidence ANG.

[0039] The projection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0040] In one embodiment, the detection grid DGR may be omitted and the detector DET may be placed at the location of the detection grid DGR, such a configuration providing a more direct detection of the image of the projection grid PGR.

[0041] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0042] Various height sensors of the general type are disclosed, for example, in U.S. Patent Nos. 7,265,364 and 7,646,471, which are incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in U.S. Patent Publication No. 2010233600A1, which is incorporated by reference. WO 2016102127A1, which is incorporated by reference, describes a compact height sensor that uses a multi-element detector to detect and recognize the position of a grid image without the need for a detection grid.

[0043] Another type of metrology tool used in IC manufacturing is the alignment sensor. Therefore, an important aspect of the performance of a lithographic apparatus is the ability to correctly and accurately position an applied pattern with respect to features defined in a previous layer (by the same or a different lithographic apparatus). For this purpose, one or more sets of marks or targets are provided on the substrate. Each mark is a structure whose position can be subsequently measured by a position sensor, typically an optical position sensor. The position sensor is sometimes called an "alignment sensor" and the marks are sometimes called "alignment marks".

[0044] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors, by which the position of an alignment mark provided on a substrate can be accurately measured. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information from an alignment mark formed on the substrate. An example of an alignment sensor used in current lithographic apparatus is based on a self-referencing interferometer, as described in US Patent 6,961,115. Various extensions and modifications of the position sensor have been developed, as disclosed, for example, in US Patent Publication 2015261097A1. The contents of all these publications are incorporated herein by reference.

[0045] 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, for example as described in US Pat. No. 6,961,116, which is incorporated herein by reference. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is diverted by diverting optics onto a mark, such as a mark AM, positioned on a substrate W, as an illumination spot SP. In this example, the diverting optics includes a spot mirror SM and an objective lens OL. The illumination spot SP illuminating the mark AM may be of a diameter slightly smaller than the width of the mark itself.

[0046] Radiation diffracted by the alignment mark AM is collimated (in this example via an objective lens OL) into an information-bearing beam IB. The term "diffracted" is intended to include zero order diffraction (which may be referred to as reflection) from the mark. A self-referencing interferometer SRI, for example of the type disclosed in the above-mentioned US Pat. No. 6,961,116, interferes with the beam IB with itself, which is then received by a photodetector PD. If more than one wavelength is produced by the radiation source RSO, additional optics (not shown) may be included to provide separate beams. The photodetector may be a single element, or may include many pixels if desired. The photodetector may include a sensor array.

[0047] Guiding optics, which in this example include a spot mirror SM, may serve to block the zero order radiation reflected from the mark, so that the information-bearing beam IB contains only higher order diffracted radiation from the mark AM (this is not essential for the measurement, but improves the signal-to-noise ratio).

[0048] The intensity signal SI is fed to a processing unit PU. The combination of optical processing in block SRI and computational processing in unit PU results in values ​​of the X and Y position on the substrate relative to a reference frame.

[0049] A single measurement of the type shown only fixes the position of the mark within a certain range corresponding to one pitch of the mark. to identify the period of the sine wave that contains this mark location. The same process can be repeated at different wavelengths and at coarse and / or fine levels for improved accuracy and / or robust detection of the mark, independent of the material the mark is made of and the material provided below and / or above the mark. The wavelengths can be optically multiplexed and demultiplexed and / or multiplexed by time or frequency division to be processed simultaneously.

[0050] In this example, the alignment sensor and the spot SP remain fixed, and it is the substrate W that moves. The alignment sensor can therefore scan the mark AM in a direction substantially opposite to the direction of movement of the substrate W, while being rigidly and precisely mounted to a reference frame. This movement of the substrate W is controlled by the substrate W being mounted on a substrate support and a substrate positioning system controlling the movement of the substrate support. A substrate support position sensor (e.g. an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. By measuring the positions of the marks provided on the substrate support, the position of the substrate support as determined by the position sensor can be calibrated (e.g. relative to a frame to which the alignment system is connected). By measuring the positions of the alignment marks provided on the substrate, the position of the substrate can be determined relative to the substrate support.

[0051] To monitor the lithographic process, parameters of the patterned substrate are measured. Parameters include, for example, the overlay error between successive layers formed in or on the patterned substrate. The measurements can be performed on the product substrate and / or on dedicated metrology targets. There are various techniques for measuring the microstructures formed in the lithographic process, including the use of scanning electron microscopes and various dedicated tools. A fast, non-invasive form of specialized inspection tool is a scatterometer, in which a beam of radiation is directed at a target on the surface of the substrate and properties of the scattered or reflected beam are measured.

[0052] Examples of known scatterometers include angle resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The targets used in such scatterometers are relatively large, e.g. a 40 μm×40 μm grating, and the measurement beam produces a spot smaller than the grating (i.e. the grating is underfilled). In addition to measuring feature shapes by reconstruction, such devices can be used to measure diffraction-based overlay, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders allows overlay measurements on smaller targets. Examples of dark-field imaging metrology can be found in International Patent Applications WO2009 / 078708 and WO2009 / 106279, which are incorporated herein by reference in their entireties. Further developments of this technology are described in published patent applications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets can be smaller than the illumination spot and can be surrounded by product structures on the wafer. Multiple grating targets can be used to measure multiple gratings in one image. The contents of all of these applications are also incorporated herein by reference.

[0053] Dark field microscopes, such as the metrology devices mentioned above, more generally suffer from the problem that the range of angles for illuminating the target and / or detecting light diffracted by the target may be required to be shared between the illumination and detection paths, thus limiting the effective NA of the illumination and detection.

[0054] In a diffraction-based dark field metrology apparatus, a beam of radiation is directed at a measurement target and one or more properties of the scattered radiation are measured to determine a property of interest of the target. The properties of the scattered radiation may include, for example, the intensity at a single scattering angle (e.g., as a function of wavelength), or the intensity at one or more wavelengths as a function of scattering angle.

[0055] The measurement of a target in dark field metrology is, for example, the first diffraction order I +1 The first intensity and the -1st diffraction order (I -1 ) and the intensity asymmetry (A=I +1 -I -1 ) from which the parameter of interest may be inferred from such intensity asymmetry measurements. For example, the target is designed such that the asymmetry of the target varies with the parameter of interest. For example, in overlay metrology, the target may include at least one composite grating formed by at least a pair of overlapping sub-gratings patterned in different layers of a semiconductor device. The asymmetry of the target thus depends on the alignment, i.e., overlay, of the two layers. Other targets may be formed with structures that are exposed with different degrees of variation based on the focus setting used during exposure, the measurement of which allows that focus setting to be inferred (again via the asymmetry of the intensity).

[0056] Figures 7 and 8 show two schematic examples of diffraction-based dark-field metrology devices. Note that for simplicity, both figures show only a small set of components sufficient for the purpose of explaining the working principles of the two devices.

[0057] As shown in FIG. 7, a first illumination beam of radiation IB1 may be obliquely incident on an overlay target on a substrate WA from one side of the device. The grating-based overlay target may diffract the first illumination beam into several diffraction orders. Because the device is configured for dark-field imaging, the 0th diffraction order is configured to be blocked by optical components or completely outside the numerical aperture of the objective lens OB. At least one non-0th diffraction order, e.g., the positive 1st diffraction order +1 st DF can be collected. 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 diffracted orders, e.g. the positive 1st diffraction order +1 st The DF can be focused onto the image sensor IS such that a first image IM1 is formed at a first location.

[0058] Similarly, a second illumination beam IB2 of radiation may be obliquely incident on the same overlay target OT on the substrate WA from the opposite side of the system. The angle of incidence of the second illumination beam IB2 may be the same as the angle of incidence of the first illumination beam IB1. At least one non-zero diffraction order, e.g. the negative first diffraction order −1 st DF can be collected by the objective lens OB and subsequently redirected by the second wedge WG2. Then, the negative first diffraction order −1 st The DF can be focused by an imaging lens IL onto the image sensor IS such that a second image IM2 is formed at a second location.

[0059] The example of FIG. 7 is operated in a parallel acquisition scheme. The overlay target is illuminated simultaneously by both illumination beams IB1, IB2. Correspondingly, two spatially separated images IM1, IM2 of the overlay target are acquired simultaneously. Such a parallel acquisition scheme allows for a high measurement speed and a high throughput. However, the pupil plane of the objective lens OB is illuminated simultaneously by two diffraction orders, e.g. +1 stDF and -1 st The result of splitting the pupil into mutually exclusive illumination and detection pupils is a reduction in the illumination and detection NA. There is some flexibility in the tradeoff between illumination and detection NA, but ultimately it is not possible within a single pupil to make both the illumination and detection NA as large as necessary. This allows each corresponding illumination beam and a +1 st DF and -1 st The angular range of the DF beam is limited, restricting the range of allowable grating pitch sizes and / or illumination wavelengths, thus placing stringent requirements on the design of such metrology systems.

[0060] FIG. 8 shows a schematic diagram of another exemplary dark-field metrology device (or a different operation mode of the device of FIG. 7). The main difference is that the metrology device of FIG. 8 operates in a sequential acquisition mode. In the sequential acquisition mode, the metrology target OT is always illuminated with only one illumination beam from one direction, so that only one image of the target is formed and acquired at any one time. With reference to FIG. 8, at a first time t=T1, a first illumination beam IB1 is switched on and directed obliquely from one side of the metrology setup onto the overlay target OT of the substrate WA. After interaction with the grating of the overlay target, multiple diffraction orders can be generated. At least one non-zero diffraction order, e.g., a positive first diffraction order +1 st The DF may be collected by an objective lens OB and subsequently focused by an imaging lens IL onto an image sensor IS.

[0061] After the first image IM1 of the overlay grating is acquired, at a second time t=T2, the first illumination beam IB1 is switched off and the second illumination beam IB2 is switched on. The second illumination beam IB2 is directed obliquely directly onto the same overlay target from the opposite side of the metrology setup. At least one generated diffraction order, e.g. the negative first diffraction order −1 stThe DF may be collected by the objective lens OB and subsequently focused onto the image sensor IS to form a second image IM2 of the overlay target. Note that both images IM1 and IM2 may be formed at a common location on the image sensor.

[0062] In this time-multiplexed acquisition scheme, the total NA of the objective lens OB is 1 st DF and -1 st The lack of restriction on the objective NA means that the range of relevant design parameters, such as grating pitch size, illumination wavelength, and illumination angle, is wider, allowing for greater flexibility in system design. However, the fact that multiple image acquisitions are required means that the measurement speed is slower, impacting the system throughput.

[0063] In addition, accurate determination of, for example, overlay errors depends on accurate measurement of small relative intensity differences (or intensity asymmetries) between the two acquired images IM1, IM2. A typical relative intensity difference is 10 times the intensity of one of the acquired images, e.g., IM1 or IM2. -4 Such small intensity differences are easily dwarfed by variations in the intensity and / or wavelength of the illumination radiation. Therefore, the illumination beam needs to remain stable during successive 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, e.g., an intensity / wavelength monitoring device and a corresponding feedback control loop, must be incorporated into the measurement device so that variations in the intensity and / or wavelength of the illumination beam are actively monitored and adequately compensated for. In some cases, an intensity monitoring device can be used to actively track the intensity of the illumination beam. The signal generated from the intensity monitoring device can be used to correct (e.g., electronically) the intensity variations of the illumination beam. All these solutions increase the complexity and cost of the overall system.

[0064] Some or all of the aforementioned problems can be addressed by using digital holographic microscopy, in particular dark-field digital holographic microscopy. Digital holographic microscopy is an imaging technique that combines holography and microscopy. Unlike other microscopic observation methods that record a projected image of an object, digital holographic microscopy records the object radiation obtained by illumination of a three-dimensional (3D) object and a hologram formed by the interference between the object radiation and a coherent reference radiation. The image can be acquired, for example, using a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS). Since the object radiation is radiation scattered from the object, the wavefront of the object radiation is modulated or shaped by the object. The scattered radiation can include reflected radiation, diffracted radiation, or transmitted radiation. Thus, the wavefront of the object radiation carries information of the illuminated object, e.g., 3D shape information. Based on the image of the captured hologram, the image of the object can be numerically reconstructed by using computer reconstruction algorithms. As explained in the examples of Figures 7 and 8, an important advantage of hologram-based metrology over intensity-based metrology is that hologram-based metrology can obtain both intensity and phase information of the object. Using the additional topological information, the properties of the object can be determined more accurately.

[0065] International Patent Application WO2019197117A1, which is incorporated herein by reference, discloses a method and metrology apparatus based on dark-field digital holographic microscopy (df-DHM) for determining properties, such as overlay, of structures fabricated on a substrate. For illustrative purposes, Figure 3 of International Patent Application WO2019197117A1 is reproduced in Figure 9. Figure 9 shows a schematic diagram of the disclosed df-DHM, which is particularly adapted for use in lithography process metrology.

[0066] Compared to the previous examples shown in Figures 7 and 8, the df-DHM in Figure 9 further comprises a reference optical unit 16, 18 used to provide two additional reference radiation beams 51, 52 (reference radiation). Such two reference radiation beams 51, 52 are paired with two corresponding portions 41, 42 of the scattered radiation beams 31, 32 (object radiation), respectively. The two scattered reference beam pairs are used in turn to form two interference patterns. Coherence control is provided by adjusting the relative optical path-length difference (OPD) between the two scattered reference beams in each beam pair. However, coherence control is not available between the two beam pairs.

[0067] Due to the use of a single light source and poor coherence control, all four radiation beams, i.e. the first part 41 of the scattered radiation 31, the first reference radiation 51, the second part 42 of the scattered radiation 32 and the second reference radiation 52, are mutually coherent. If these four mutually coherent radiation beams could reach the same position of the sensor 6 at the same time, i.e. when operating in a parallel acquisition scheme, multiple interference patterns would be superimposed on each other, including the desired information containing pattern and undesired artefact-contributing patterns. Undesired interference patterns may be formed, for example, by interference between the part 41 of the first scattered radiation 31 and the part 42 of the second scattered radiation 32. Parallel acquisition in this arrangement is impractical, since completely separating the overlapping interference patterns is technically difficult and time consuming.

[0068] Similar to the example in Figure 8, the sequential acquisition scheme in the example in Figure 9 allows the full NA of the objective to be utilized for both illumination and detection. However, this system suffers from the same problem of slower measurement speed due to sequential acquisition. Therefore, it is desirable to have a df-DHM that can perform parallel acquisition so that high measurement speed and high design freedom can be obtained at the same time.

[0069] 10 shows a schematic of an imaging branch of a dark-field digital holographic microscope (df-DHM) 1000 according to one embodiment. The dark-field digital holographic microscope (df-DHM) comprises an imaging branch and an illumination branch. In this embodiment, a metrology target 1060 including a structure on a substrate 1050 is illuminated by two illumination beams of radiation, i.e. a first illumination beam of radiation 1010 and a second illumination beam of radiation 1020. In one embodiment, such two illumination beams 1010, 1020 can illuminate the metrology target 1060 simultaneously.

[0070] In one embodiment, the first illumination beam 1010 may be incident on the metrology target 1060 at a first angle of incidence in a first direction relative to the optical axis OA. The second illumination beam 1020 may be incident on the metrology target 1060 at a second angle of incidence in a second direction relative to the optical axis OA. The first angle of incidence of the first illumination beam 1010 and the second angle of incidence of the second illumination beam 1020 may be substantially the same. The angle of incidence of each illumination beam may be, for example, in the range of 70 degrees to 90 degrees, in the range of 50 degrees to 90 degrees, in the range of 30 degrees to 90 degrees, or in the range of 10 degrees to 90 degrees. Illumination of the metrology target 1060 may result in radiation being scattered from the target. In one embodiment, the first illumination beam 1010 may be incident on the metrology target 1060 at a first azimuthal angle corresponding to a first direction. The second illumination beam 1020 may be incident on the metrology target 1060 at a second azimuthal angle corresponding to a second direction. The first azimuth angle of the first illumination beam 1010 and the second azimuth angle of the second illumination beam 1020 may be different (eg, opposite angles 180 degrees apart).

[0071] Depending on the structure of the metrology target 1060, the scattered radiation may include reflected radiation, diffracted radiation, or transmitted radiation. In this embodiment, the metrology target may be a diffraction-based overlay target. Each illumination beam may correspond to a scattered beam that includes at least one non-zero diffraction order. Each scattered beam carries information of the illuminated metrology target. For example, the first illumination beam 1010 may include a positive first diffraction order +1 stThe second illumination beam 1020 corresponds to the first scattered beam 1011 having the negative first diffraction order −1 st The DF-DHM may correspond to the second scattered beam 1021 including the DF. The zeroth diffraction order and other undesired diffraction orders may be blocked by a beam blocking element (not shown) or configured to be completely outside the NA of the objective lens 1070. As a result, the df-DHM may be operated in dark field mode. It should be noted that in some embodiments, a combination of one or more optical elements, e.g., lenses, may be used to achieve the same optical effect of the objective lens 1070.

[0072] Due to the small size of the metrology target 1060, the imaging branch can have a net positive magnification (eg, greater than 10x, greater than 20x, or 30x or more).

[0073] Both scattered beams 1011, 1021 may be collected by the objective lens 1070 and then refocused onto the image sensor 1080. Note that the objective lens 1070 in the imaging branch may be an imaging objective lens that is used only in the detection path (as shown) and not for illumination (e.g., focusing the illumination onto the metrology target 1060). As such, the illumination does not necessarily have to pass through the same objective lens as the scattered light. In other embodiments, the objective lens may be shared between the imaging branch and the illumination branch to collect the scattered radiation and focus the illumination onto the metrology target 1060.

[0074] It is desirable to collect as much scattered / diffracted light as possible, and therefore a high NA detection path or imaging branch is desirable. In this regard, a high NA may be, for example, greater than 0.1, greater than 0.2, greater than 0.3, or greater than 0.4. In other embodiments, a high NA may refer to an NA of 0.8 or greater.

[0075] The objective lens 1070 may include multiple lenses, and / or the df-DHM 1000 may include a lens system having two or more lenses, e.g., an objective lens and an imaging lens similar to the exemplary df-DHG of FIG. 9, thereby defining a pupil plane of the objective lens between the two lenses and an image plane at the focus of the imaging lens. In this embodiment, a portion 1012 of the first scattered beam 1011 and a portion 1022 of the second scattered beam 1021 are incident simultaneously on a common location of the image sensor 1080. At the same time, two reference beams of radiation, i.e., a first reference beam 1030 and a second reference beam 1040, are incident at the same location of the image sensor 1080. Such four beams may be grouped into two pairs of scattered radiation and reference radiation. For example, the first scattered reference beam pair may include the portion 1012 of the first scattered beam 1011 and the first reference beam 1030. Similarly, the second scattered reference beam pair may include portion 1022 of the second scattered beam 1021 and the second reference beam 1040. These two scattered reference beam pairs may then form two interference patterns (holographic images) that at least partially overlap in the spatial domain.

[0076] In one embodiment, to separate two at least partially spatially overlapping interference patterns (e.g., in the spatial frequency domain), the first reference beam 1030 may have a first angle of incidence with respect to the optical axis OA and the second reference beam 1040 may have a second angle of incidence with respect to the optical axis OA. The first and second angles of incidence are different. Alternatively or additionally, the first reference beam 1030 may have a first azimuthal angle with respect to the optical axis OA and the second reference beam 1040 may have a second azimuthal angle with respect to the optical axis OA. The first and second azimuthal angles are different.

[0077] To generate an interference pattern, the two beams of each scattered reference beam pair must be at least partially coherent with each other to a sufficient degree to form an interference pattern. It should be noted that each scattered radiation beam may have a phase offset with respect to the corresponding illumination radiation. For example, at the image plane of the image sensor 1080, such phase offset may include contributions due to the optical path length (OPD) from the metrology target 1060 to the image sensor 1080 and due to interactions with the metrology target. As mentioned above, the coherence between the first scattered reference beam pair and the second scattered reference beam pair needs to be controlled so that each beam of one pair is incoherent with respect to any beam of the other pair. That is, interference only occurs between beams in the same beam pair and needs to be suppressed between different beam pairs. In this way, only the desired interference pattern, e.g., the two interference patterns formed by the respective scattered reference beam pairs, are formed superimposed on the image sensor 1080, thus avoiding the problem of separating or removing the undesired interference patterns.

[0078] More specifically, the coherence between beams in the same beam pair must be temporal and spatial coherence. The mutual coherence function between these beams depends on space and time. A typical approximation is to factor this function into spatial and temporal components. However, when the beams move at an angle (such as in off-axis systems), this approximation is no longer ideal. Sufficient coherence is required so that the object and reference beams interfere at the camera. To simplify temporal and spatial coherence, this means that all points on the camera have the same optical path length from the source (e.g., within the "temporal coherence" length). For limited spatial coherence, all points of the reference beam must also be mapped to corresponding points in the object beam (this may mean that both the object and reference arms must create images of the beam splitter on the camera). If single-mode optical fibers are used, the spatial coherence can be very large.

[0079] In addition to being spatially and temporally coherent with the reference beam (of the same beam pair), each object beam may be smooth (e.g., uniformly illuminated) across the entire area of ​​the overlay target.

[0080] In an embodiment, both illumination beams 1010, 1020 and both reference beams 1030, 1040 used in the df-DHM of FIG. 10 may be provided by an illumination branch comprising an illumination device. FIG. 11 shows a schematic of an illumination arrangement according to an embodiment. As shown in FIG. 11, a light source 1110 may emit an at least partially coherent primary radiation beam 1111. The primary radiation beam 1111 may include wavelengths ranging from soft x-ray and visible to near IR. The primary radiation beam 1111 may be split into two beams by a first beam splitter 1120, namely a first radiation beam 1112 and a second radiation beam 1114. In this embodiment, the first beam splitter 1120 may include a 50 / 50 splitting ratio, whereby the first radiation beam 1112 and the second radiation beam 1114 may have substantially the same power level. The two beams 1112, 1114 can then follow two different beam paths, respectively.

[0081] In one of the beam paths 1112, 1114, the second beam path in the example shown here, the second radiation beam 1114 (or the first beam 1112) may undergo a delay, although this is largely arbitrary. In the example shown here, the delay is performed via an incoherence delay arrangement, such as an adjustable optical delay line AD1 including a prism 1132. The adjustable optical delay line AD1, or more generally a delay, can be used to control the OPD (or coherence) between the beams in the first beam path and the beams in the second beam path. This can be done to ensure that these beams are not coherent, so that the first beam pair does not interfere with the second beam pair. Instead of this delay line, a "hard" path difference may be intentionally introduced between the beam paths 1112, 1114.

[0082] In one embodiment, the tunable optical delay line AD1 may be operable to impose incoherence while keeping the time delay between the two paths as short as possible to cause both beams to have similar intensity variations.

[0083] In the first beam path, the first radiation beam 1112 may be incident on a second beam splitter 1122. The second beam splitter 1122 may split the first radiation beam 1112 into another two beams, namely the first illumination beam 1010 and the first reference beam 1030. Depending on the splitting ratio of the second beam splitter 1122, the first illumination beam 1010 and the first reference beam 1030 may have different powers. The splitting ratio of the second beam splitter 1122 may be 90 / 10, 80 / 20, 70 / 30, 60 / 40, or 50 / 50. In this embodiment, the power of the first illumination beam 1010 may be higher than the power of the first reference beam 1030. Then, each of the two beams 1010, 1030 may be reflected by a reflecting element 1140 into an optical delay line. Each optical delay line may be fixed or adjustable and may comprise a reflective optical element that retroreflects the incident radiation. The reflective optical element may be a right angle prism 1130 or 1131. In some embodiments, the reflective optical element may be a pair of reflective mirrors. In this embodiment, the first illumination beam 1010 may pass through a fixed optical delay line comprising prism 1131, while the first reference beam 1030 may pass through an adjustable optical delay line AD2 comprising prism 1130. In a different embodiment, the first illumination beam 1010 may pass through an adjustable optical delay line comprising prism 1130, while the first reference beam 1030 may pass through a fixed optical delay line comprising prism 1131. In either of these two scenarios, the relative OPD between the two beams 1010, 1030 is adjustable. The two beams 1010, 1030 form a first pair of output beams. It should be noted that this is just one example of an adjustable path length configuration that allows for adjustment of the OPD between beams 1010, 1030. Other suitable configurations for achieving this may be used instead.

[0084] The second beam of radiation 1114 may be split by a third beam splitter 1124 into two beams, for example a second illumination beam 1020 and a second reference beam 1040. Depending on the splitting ratio of the third beam splitter 1124, the two beams may have different powers. In this embodiment, the splitting ratio of the third beam splitter 1124 may be the same as the splitting ratio of the second beam splitter 1122, such that the first illumination beam 1010 and the second illumination beam 1020 have substantially the same power level, the first reference beam 1020 has substantially the same power level, and the first reference beam 1030 and the second reference beam 1040 have substantially the same power level. In this embodiment, the power of the second illumination beam 1020 may be higher than the power of the second reference beam 1040. The two beams 1020, 1040 may then be reflected by a reflecting element 1142 respectively into two optical delay lines (one fixed and the other adjustable). In this embodiment, the second illumination beam 1020 may pass through a fixed optical delay line comprising prism 1133, while the second reference beam 1040 may pass through an adjustable optical delay line AD3 comprising prism 1134. In a different embodiment, the second illumination beam 1020 may pass through an adjustable optical delay line comprising prism 1134, while the second reference beam 1040 may pass through a fixed optical delay line comprising prism 1133. In either of these two scenarios, the relative OPD between the two beams 1020, 1040 may be adjustable. The two beams 1020, 1040 form a second pair of output beams. It should be noted that this is just one example of an adjustable path length configuration that allows for adjustment of the optical path length between the beams 1020, 1040. Other suitable configurations to achieve this may be used instead.

[0085] After exiting the respective optical delay lines, the four radiation beams, i.e., first illumination beam 1010, first reference beam 1030, second illumination beam 1020, and second reference beam 1040, may exit the illumination device 1100 and may be used as illumination and reference beams in the df-DHM, e.g., the corresponding beams of the df-DHM of FIG. 10. In an embodiment, before exiting the illumination device 1100, some or all of these four beams may pass through additional optical elements, such as optical beam shaping elements, optical steering mirrors, optical polarizing elements, and optical power control elements, respectively, so that the beam parameters, propagation direction, polarization state, and / or optical power of each beam can be individually controlled. The beam parameters may include beam shape, beam diameter, and beam divergence. In an embodiment, any two beams of the first pair of output beams and the second pair of output beams may have different power levels. One beam of the first pair of output beams may have substantially the same power level as one beam of the second pair of output beams.

[0086] The propagation direction of the radiation beam emerging from the illuminator in Figure 11 determines the incidence angle and azimuth angle of the beam with respect to the optical axis OA of the df-DHM in Figure 10. The orientation of the Cartesian coordinate system of reference is shown at the top of Figure 10. The incidence angle of the beam refers to the angle in the xz plane between the optical axis (dashed line) or z-axis of the microscope and the incident beam or its projection onto the xz plane. The azimuth angle of the beam refers to the angle between the x-axis and the incident beam or its projection onto the xy plane.

[0087] Correspondingly, the relative OPD between the first illumination beam 1010 and the first reference beam 1030 can be adjusted using the optical delay line AD2, while the relative OPD between the second illumination beam 1020 and the second reference beam 1040 can be adjusted using the optical delay line AD2. The coherence between each scattered beam radiation (e.g. the first scattered beam 1011 or the second scattered beam 1021) and its paired reference beam radiation (e.g. the first reference beam 1030 or the second reference beam 1040) can be independently controlled or optimized, as long as the relative OPD induced phase delay is sufficient to cover the phase offset between each illumination beam of radiation and its associated scattered radiation beam. In addition, an adjustable optical delay line AD1 can be used to intentionally add sufficient phase delay between the two illumination reference beam pairs or the two scattered reference beam pairs (e.g., a first scattered reference beam pair including portion 1012 of the first scattered beam 1011 and the first reference beam 1030; a second scattered reference beam pair including portion 1022 of the second scattered beam 1021 and the second reference beam 1040) so that any beam of one beam pair does not interfere with any beam of the other beam pair. In this way, only two desired interference patterns are formed by the two scattered reference beam pairs on the image sensor 1080, respectively.

[0088] FIG. 12 shows a schematic of an illumination device 1200 according to a different embodiment. The illumination device of FIG. 12 is similar to the illumination device of FIG. 11. The main difference is that in the embodiment of FIG. 12, the three beam splitters 1220, 1222, 1224 may have different splitting ratios compared to their respective counterparts in the embodiment of FIG. 11. As a result of using different splitting ratios, some or all of the four radiation beams 1010, 1020, 1030, 1040 output from the device may have different powers compared to their respective counterparts in the embodiment of FIG. 11. For example, in this embodiment, the two beams 1010, 1020 output from the upper branch (from the beam splitter 1222) have similar powers to each other, and similarly for the two beams 1030, 1040 output from the lower branch (from the beam splitter 1224), the power of the pair of beams 1010, 1020 being different from the power of the pair of beams 1030, 1040. Thus, in this embodiment, the first and second illumination beams 1010, 1020 may be output as a pair via a beam splitter 1222, and the first and second reference beams 1030, 1040 may be output as a pair via a beam splitter 1224.

[0089] Thus, Figure 11 shows a configuration in which the first output branch (via beam splitter 1122) includes a first beam pair branch (e.g., +1 diffraction order illumination and +1 reference beam) and the second output branch (via beam splitter 1124) includes a second beam pair branch (e.g., -1 diffraction order illumination and -1 reference beam). In contrast, in Figure 12, the first output branch is an illumination branch (e.g., +1 and 1 diffraction order illumination beams) and the second output branch is a reference branch (e.g., +1 and 1 diffraction order reference beams).

[0090] 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 to ensure that only the desired interference pattern is formed on the image sensor 1080 in Figure 10. In one embodiment, the delay line AD3′ in the reference arm is much longer (tens of mm) than the delay line AD2′ in the illumination arm.

[0091] Thus, in the configuration of FIG. 11, the adjustable optical delay line AD1 can implement an incoherence delay configuration operable to impose a delay on one of the first or second branches relative to the other of the first or second branches. The adjustable optical delay lines AD2, AD3 implement a coherence matching configuration for coherence matching of the beams in each beam pair. In contrast, in the configuration of FIG. 12, the coherence matching configuration and the incoherence delay configuration can be implemented together by joint 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, the optimization would be different if the prism 1131 instead of the prism 1130 were adjustable, or if the prism 1133 instead of the prism 1134 were adjustable.

[0092] The characteristics of the structure of the metrology target 1060 are determined by a processing unit 1090 of the metrology device. The processing unit 1090 uses the first and second interference patterns recorded by the image sensor 1080 to determine the characteristics of the structure of the metrology target 1060. In one embodiment, the processing unit 1090 is coupled to the image sensor 1080 to receive a signal including information about the first and second interference patterns recorded by the sensor 1080. In one embodiment, the processing unit 1090 corrects aberrations of the objective lens 1070 of the df-DHM 1000. In one embodiment, measurements of the first and second interference patterns are performed with radiation simultaneously in time (in parallel), and the processing unit 1090 is configured to use the measurements simultaneously (in parallel) to determine the characteristics of the structure of the metrology target 1060 on the substrate 1050.

[0093] In one embodiment, the processing unit 1090 uses the first interference pattern to calculate a complex field of radiation (here "complex" means that both amplitude and phase information is present) at the sensor 1080 associated with the portion 1012 of the first scattered radiation 1011. Similarly, the processing unit 1090 uses the second interference pattern to calculate a complex field of radiation at the sensor 1080 associated with the portion 1022 of the second scattered radiation 1021. Calculating a complex field of radiation from an interference pattern formed by interfering a reference radiation with radiation scattered from an object is generally known from holography. Details of how to perform such calculations in the context of metrology in lithography are described, for example, in US 2016 / 0061750 A1, which is incorporated herein by reference.

[0094] If the optical properties of the df-DHM 1000 are known, it is possible to mathematically and computationally back-propagate each of the calculated complex fields to obtain the corresponding complex fields of the first scattered radiation 1011 and the second scattered radiation 1021 at the measurement target 1060.

[0095] Having knowledge of the complex field provides additional information for determining properties of a metrology target 1060 on a substrate 1050 over alternative modes where both phase and amplitude information is not available. For example, European Patent Application EP18158745.2, filed on 27 February 2018, discloses a method for determining overlay error between structures of different layers on a substrate (one example of a property of the structures to be determined) using phase information of the scattered radiation. European Patent Application EP18158745.2 is incorporated herein by reference.

[0096] In one embodiment, a characteristic of the structure of the metrology target 1060 is determined by comparing the first and second interference patterns. In one embodiment, the characteristic of the structure is determined based on a difference between the first and second interference patterns. The difference between the first and second interference patterns may, for example, include information about an asymmetry in the structure of the metrology target 1060. Obtaining information about the asymmetry in the structure of the metrology target 1060 may provide information about overlay. In one embodiment, the overlay information is obtained using phase information obtained from the calculated complex field, as described in EP18158745.2, filed on 27 February 2018. Overlay represents an undesired misalignment between different patterns in the metrology target 1060, such as patterns formed at different times, patterns formed using different processes, and / or patterns formed at different layers. In other embodiments, the characteristic of the structure of the metrology target 1060 that is determined may include an error indicative of an error in the focus of radiation used in a lithography process to fabricate the structure of the metrology target 1060. In yet other embodiments, the determined characteristics of the metrology target 1060 features may include errors indicative of errors in the dose of radiation used in a lithography process to manufacture the metrology target 1060 features.

[0097] It is important to minimize contributions to the difference between the first and second interference patterns that are not due to structures in the metrology target 1060, such as the aforementioned unwanted interference patterns. By using three adjustable optical delay lines AD1, AD2, AD3, these contributions are effectively suppressed by applying sufficient coherence control to the four beams of radiation, namely the first scattered beam 1011, the first reference beam 1030, the second scattered beam 1021 and the second reference beam 1040.

[0098] In order to accurately calculate the two complex fields, the two interference patterns should be completely separated from background stray light and / or residual zeroth diffraction order. Furthermore, to extract target information from each interference pattern, the two overlapping interference patterns also need to be separated. By using spatial frequency multiplexing, the overlapping interference patterns can be completely separated. Such a method is described in detail in U.S. Patent Application US20180011022A1, which is incorporated herein by reference.

[0099] By spatial frequency multiplexing, the processing unit 1090 performs a two-dimensional (2D) Fourier transform on the recorded image including the multiple overlapping interference patterns to obtain a Fourier transform image. The horizontal and vertical axes of the obtained Fourier transform image correspond to the two axes fx and fy in the spatial frequency coordinate system (fx, fy), respectively. The obtained Fourier transform image has multiple spatial spectra, each of which corresponds to a part of the recorded image.

[0100] Figure 13 shows an exemplary 2D Fourier image in the spatial frequency domain obtained by subjecting the recorded image containing the two overlapping interference patterns to a 2D Fourier transform. As shown in Figure 13, the 2D Fourier image includes five spatial spectra: a base spatial spectrum 1301 including a zeroth order Fourier component, a first higher order spatial spectrum 1311 corresponding to a first interference pattern formed with a portion 1012 of the first scattered beam of radiation 1011, a second higher order spatial spectrum 1312 corresponding to a second interference pattern formed with a portion 1022 of the second scattered beam of radiation 1021, a first conjugate spatial spectrum 1321 that is conjugated to the first higher order spatial spectrum 1311, and a second conjugate spatial spectrum 1322 that is conjugated to the second higher order spatial spectrum 1312.

[0101] The center of the base spectrum is the origin O of the spatial frequency coordinates. The position of the base spectrum is fixed. However, the positions of the higher order spatial spectra and their conjugate spectra can be adjusted with respect to the base spectrum, for example, by changing the angle of incidence and / or the azimuthal angle of each reference beam. The radial distance between the center of each higher order spatial spectrum and the center of the beam spatial spectrum is related to the angle between the optical axis of the part 1012 or 1022 of the scattered beam 1011 or 1021 and the optical axis of the reference beam 1030 or 1040. The larger the angle, the further away (relative to the base spatial spectrum) the higher order spatial spectrum is. Thus, by providing a sufficiently large angle between the axis of the part of the scattered beam and the axis of the reference beam, the higher order spatial spectrum 1311 or 1312 can be completely separated from the base spatial spectrum 1301. However, the angle of the reference beam cannot be made arbitrarily high, since an increase in the angle between the optical axis of the portion 1012 or 1022 of the scattered beam 1011 or 1021 and the optical axis of the reference beam 1030 or 1040 results in a decrease in the fringe spacing of the hologram fringes. Ultimately, the angle is limited by the pixel pitch of the image sensor 1080. The fringes (or interference pattern) of the hologram must be adequately sampled by the sensor pixels. The maximum frequency of the hologram must satisfy the Nyquist criterion for sampling.

[0102] Furthermore, the azimuthal angle of each reference beam affects the circumferential position of the spatial spectrum relative to the origin O. The circumferential position of the high-order spatial spectrum is represented by the angle between the high-order spatial spectrum and the spatial frequency axis fx. For example, the circumferential position of the first high-order spatial spectrum is represented by the angle 1331. Thus, the high-order spatial spectra 1311, 1312 can be completely separated from each other by ensuring that the difference in azimuthal angles of the two reference beams is large enough.

[0103] Once separated, the processing unit 1090 extracts each high-order spatial spectrum from the Fourier image and then performs an inverse Fourier transform on the extracted high-order spatial spectrum. It should be noted that since both reference beams of radiation are directly provided by the illuminator 1100, the information of the reference radiation, such as its 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 of the reference radiation, a complex field of paired scattered radiation can be obtained, as will be described in more detail below. Finally, the complex fields of the two scattered beams of radiation are used to determine the characteristics of the structure of the metrology target 1060 and / or to correct the optical aberrations of the objective lens 1070 of the df-DHM 1000.

[0104] Continuing to refer to FIG. 13, existing methods use only the higher spatial spectrum or sidebands (SB) 1311 or 1312 of the 2D Fourier image 1300 to determine the amplitude and phase of the complex field of the portion 1012 or 1022 of the scattered beam 1011 or 1021. The information contained in the base spatial spectrum or center band (CB) 1301 is completely discarded in the determination process. Thus, existing methods are prone to noise limitations, e.g., low signal-to-noise ratios, thereby leading to loss of throughput. According to different aspects of the present embodiment, a method is provided that improves on existing methods by providing a better, more accurate method for determining the amplitude and phase of the complex field of the scattered radiation at the image sensor. This is achieved by considering the information contained in both the CB 1301 and the SB 1311 or 1312.

[0105] Note that another term for the Fourier images in Figure 13 is often called the Fourier representation of the hologram. The four images in Figure 13 are obtained by converting the hologram into its Fourier representation or Fourier spectrum in the spatial frequency domain via the (2d) Fourier transform.

[0106] FIG. 14 shows a flow chart of a method for determining the amplitude and phase of a complex field according to an embodiment (which may be performed, for example, by the processing unit 1090 or otherwise). With reference to FIG. 14, in step 1401, a hologram (or interference pattern) is generated after illumination of an object or target, which may then be converted into its Fourier representation or Fourier spectrum in the spatial frequency domain by Fourier transformation. This Fourier representation has the advantageous property that the CB and the respective SB are spatially separated if the tilt angle of the reference wave used in the hologram is large enough to take into account the spatial frequency components of the SB. It is further noted that in FIG. 13, the SBs are displayed in pairs, one pair consisting of SBs 111 and 1321, and the other pair consisting of SBs 1312 and 1322. For each SB pair, the two SBs carry identical information. Since they are complex conjugates of each other, it is sufficient to select one SB for each SB pair. In step 1402, a CB in the Fourier representation is selected, which may then be used to calculate the corresponding component in the image plane via the inverse Fourier transformation of the selected CB. In step 1403, one or more distinct SBs may be selected in the Fourier representation, and each of the selected SBs may be used to calculate a corresponding component in the image plane via an inverse Fourier transform of the selected SB. In step 1404, the amplitude and / or phase of the complex field may be determined based on the calculated CB and SB components in the image plane. Details of the implementation of the method are described below.

[0107] It should be noted that the embodiment of Figure 14 is only a non-limiting example, and other embodiments may include more or less steps as determined by particular requirements. For example, some embodiments may further include a step of illumination of the object or target, which may 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 CB and the inverse Fourier transform of SB may be performed in parallel rather than sequentially in time.

[0108] If a single interference pattern is sufficient, one of the illumination and reference beam pairs provided by the illumination device 1100 or 1200 can be used to illuminate the target 1060. Subsequently, in step 1401, the scattered radiation from the object or target forms the desired single interference pattern together with the reference radiation provided from the illumination-reference beam pair. Such a single interference pattern can be transformed into a 2D Fourier representation in the spatial frequency domain via Fourier transformation. In this case, the 2D Fourier representation (not shown) can include one CB and one pair of mutually conjugated SBs. The mutual tilt angles of the respective beams of the scattered reference beam pair can be adjusted so that the resulting CBs and SBs do not overlap in the spatial frequency domain. Then, in step 1402, a CB in the Fourier representation is selected and its corresponding component in the image plane (i.e., the CBs described below) is transformed into a 2D Fourier representation by an inverse Fourier transformation. exp Subsequently, in step 1403, one of the mutually conjugate SBs in the Fourier representation is selected, and the corresponding component in the image plane (i.e., SB exp Finally, in step 1404, the calculated information (i.e., CB exp (R) and S.B. exp Based on (R), the amplitude and phase of the complex field of scattered radiation from the object or target may be determined. Step 1404 is further explained by the following mathematical description.

[0109] In the following mathematical description, we apply the inverse Fourier transform to CB and SB separately, and denote the corresponding components of the image plane by the real-valued function CB(R) and the complex-valued function SB(R), respectively, where R is the 2D coordinate of 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

number

[0110] The complex-valued field in the image plane is Φ p (R), which is equal to the convolution of the sample field Φ(R) (the field scattered from the sample / target) with the point spread function of the imaging optics (e.g., objective lens 1070), denoted p(R), i.e.:

number

[0111] The complex-valued field Φ in the image plane p (R) can be expressed in terms of amplitude A(R) and phase φ(R) as follows:

number

[0112] The hologram H(R) or interference pattern is modeled as follows:

number

number

[0113] The least squares function for estimating the amplitude and phase of a complex-valued field can be defined as follows:

number

[0114] 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 their Fourier representation. After incorporating the respective contributions from CB and the two conjugates SB that are separable from CB, the above equation can be expressed more explicitly as

number

number

number

[0115] The parameter fitting of the amplitude A(R) and phase φ(R) is 2 , that is, obtained by the respective derivatives of equation [6].

number

number

[0116] S 2 The latter derivative of, i.e., equation

[10] , gives (for a particular value of R):

number

number

[0117] Since the phase φ(R) can only be measured from the sidebands, its estimate is given by the coefficient s or the amplitude |Φ of the reference beam.ref (R)|. However, for optimal signal-to-noise ratio in the sidebands, the value of s may be selected as s=0.5.

[0118] S 2 The former derivative of, i.e., equation [9], gives (for any value of position in the image plane R):

number

[0119] The above relationship can be simplified to (omitting the obvious R) as follows:

number

[0120] In this relation, the first term is related to CB and the second term is related to SB. We can take the amplitudes of each of these two terms separately for the following reasons: This gives us an estimate of their amplitudes at CB and the following:

number

number

[0121] Using these two equations, i.e., equation

[15] and equation

[16] , we can obtain S with respect to A. 2 The derivative of equation

[14] can be simplified as follows:

number

[0122] The true solution (positive and real-valued) of the amplitude A at any image position R can be easily solved from the above cubic equation using standard mathematical approaches

[17] . Some interesting properties of the true solution can be further derived. From the facts that A > 0 and 0 < s < 1, two regimes can be considered, either of the following:

Number

Number

[0123] In other words, this means either of the following.

Number

Number

[0124] In situations where two (overlapping) interference patterns are desired, such as the operation described with respect to the embodiment of FIG. 10, both of the illumination reference beam pairs provided by the illumination device 1100 or 1200 can be used to illuminate the target 1060. As described above, two (overlapping) interference patterns can be formed by two pairs of scattered reference beams, respectively. Thus, the processing unit needs to determine the amplitude and phase of two complex fields, each corresponding to one scattered beam. In some embodiments, the processing unit 1070 can also perform the above four steps (i.e., 1401 to 1404 in FIG. 14) to complete the determination process. However, this time, the 2D Fourier representation can include a superposition of two respective CDs and two pairs of mutually conjugated and well-separated SBs (or four SBs in total), as in the example Fourier representation of FIG. 13. Each pair of mutually conjugated SBs contains information of one of the two interference patterns. For example, the first pair of mutually conjugated SBs contains information of the first interference pattern formed by the first pair of scattered reference beams. On the other hand, the second pair of mutually conjugate SBs contains information of the second interference pattern formed by the second pair of scattered reference beams. Both scattered reference beam pairs can be positioned such that the resulting CBs and SBs do not overlap in the spatial frequency domain.

[0125] After step 1401, where the captured image of the interference pattern is Fourier transformed into the spatial frequency domain, in step 1402, the central region of the Fourier representation containing each of the two overlapping CBs is selected and its corresponding component, i.e., the CBs at the image plane by inverse Fourier transformation, is calculated. exp (R). For simplicity of notation, we use CB expNote that (R) represents the superposition CB of two respective scattered reference beam pairs. The same notation is used for the modeled version of this component. Subsequently, in step 1403, one of each pair of SB in the Fourier representation is selected and used to calculate the corresponding component in the image plane by inverse Fourier transformation. The result is two image plane components SB with indices "1" and "2", which refer to two different interference patterns (or complex fields). 1,exp (R) and S.B. 2,exp (R) can be obtained. Finally, in step 1404, the calculated information, e.g., CB exp (R), S.B. 1,exp (R) and S.B. 2,exp Based on (R), the amplitude and phase of both complex fields can be determined. Step 1404 is further explained by the following mathematical description, which is an extension of the previous mathematical description (Equations [1] through

[21] ) for the case of a single hologram or interference pattern.

[0126] In the following mathematical description, the inverse Fourier transform is applied separately to the CB in the Fourier image, e.g., 1301 in FIG. 13, and each of the two selected SBs, e.g., 1311 and 1312 in FIG. 13, to obtain the corresponding components in the image plane. Such image plane components are represented by real-valued functions CB(R) and complex-valued functions SB1(R) and SB2(R), respectively. Here, R is the 2D coordinate of the image plane, and the indices "1" and "2" refer to two distinct interference patterns (or complex fields). The multiplexed hologram H(R) is modeled as an incoherent superposition of two separate holograms or interference patterns (the indices "1" and "2" refer to the two separate holograms), and without loss of generality we assume the same splitting ratio s for both individual holograms.

number

[0127] The Fourier transform of the above multiplexed hologram contains a single CB in the Fourier representation (originating from the two individual CBs of the two respective scattered reference beam pairs) and two separate SBs, which are modeled as follows:

number

number

number

[0128] The least squares function that is minimized for the multiplexed hologram is:

number

[0129] The optimization of phases φ1(R) and φ2(R) is carried out by S 2 and is therefore the same as for non-multiplexed (single) holograms, since the phase is only detectable in the respective SB (and not in the CB, where overlap occurs for the two respective scattered reference beam pairs). This means that the same solution can be derived for the phase as for the individual holograms (since the sidebands are separated in Fourier space for the multiplexed hologram).

number

number

[0130] Similarly, the optimization of the amplitudes A1(R) and A2(R) is carried out by the S 2This involves taking the derivative of A1(R) and A2(R), resulting in the following two equations:

number

number

[0131]

number

[29] and

[30] can be rewritten as follows:

number

number

[0132] The above set of equations can be solved in a variety of ways. Without loss of generality, one particular approach is described here as an example. Dividing the two equations

[31] and

[32] gives the ratio of the two amplitudes.

number

[0133] Applying equation

[33] to equation

[31] to eliminate A2(R) gives us an equation with only one unknown parameter, A1(R) (omitting the obvious R).

number

[34] . The value of the amplitude A2 of the second hologram in the multiplexed hologram can then be derived from the ratio relationship, i.e., equation

[33] .

[0134] It should be noted that the above-described embodiment can be further generalized to df-DHM with multiple pairs of illumination and reference radiation beams. In some embodiments, each of the multiple illumination radiation beams can include a different azimuth angle and / or a different angle of incidence. Similarly, each of the multiple reference beams can include a different azimuth angle and / or a different angle of incidence. For example, in one embodiment, in addition to the two illumination radiation beams 1010, 1020 that are primarily in the xz plane, another two illumination radiation beams (not shown) that are primarily in the yz plane can be used to illuminate the target 1060. Two additional reference radiation beams can also be used to pair with two additional illumination radiation beams, respectively. This results in four at least partially spatially overlapping interference patterns, each of which can correspond to a pair of illumination and reference radiation beams. By appropriately configuring the azimuth angle and / or angle of incidence of each additional reference radiation beam, the four at least partially spatially overlapping interference patterns can be separated in the spatial frequency domain. In this way, more information about the structure of the target can be obtained, for example, y-axis asymmetry in the structure of the target.

[0135] In some embodiments, the illuminator may provide multiple pairs of illumination and reference radiation beams. The illuminator may also provide sufficient coherence control between the radiation beams such that only the desired interference pattern is formed on the image sensor. Multiple pairs of illumination and reference radiation beams may result in the formation of multiple mutually incoherent, spatially overlapping interference patterns. Thus, the multiplexed hologram H(R) may be modeled as an incoherent superposition of multiple separate holograms or interference patterns. Equation

[22] for the case of two overlay holograms may then be further extended to include the amplitude and phase functions of all the individual holograms. Thus, the hologram index needs to be extended from “1,2” to “1,2,3,··· and n” when n holograms are formed. It should be noted that the determination process illustrated in FIG. 14 is equally applicable to any number of overlay holograms, for example, three or more overlay holograms.

[0136] It should also be noted that different embodiments of the method for determining the amplitude and phase of one or more complex fields, e.g., the embodiment of Figure 14, can be used in combination with or independently of the embodiments of Figures 10 to 12. When used alone, other types of df-DHM can be used to generate holograms or interference patterns.

[0137] In one embodiment, the processing unit 1090 may be a computer system that may include image reconstruction algorithms that are used to perform all the aforementioned tasks, including performing a Fourier transform, extracting each of the individual higher order spatial spectra, performing an inverse Fourier transform, calculating complex fields, and determining the properties of the structures based on the results.

[0138] 15 is a block diagram illustrating a computer system 1500 that can assist in implementing the methods and flows disclosed herein. The computer system 1500 includes a bus 1502 or other communication mechanism for communicating information and a processor 1504 (or multiple processors 1504 and 1505) coupled to the bus 1502 for processing information. The computer system 1500 also includes a main memory 1506, such as a random access memory (RAM) or other dynamic storage device, coupled to the bus 1502 for storing information and instructions 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.

[0139] The computer system 1500 may be coupled via a bus 1502 to a display 1512, such as a cathode ray tube (CRT) or flat panel or touch panel display, for displaying information to a computer user. An input device 1514, including alphanumeric and other keys, is coupled to the bus 1502 for communicating information and command selections to the processor 1504. Another type of user input device is a cursor control 1516, such as a mouse, trackball, or cursor direction keys, for communicating directional information and command selections to the processor 1504 and for controlling cursor movement on the display 1512. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), allowing the device to specify a position in a plane. A touch panel (screen) display may also be used as an input device.

[0140] One or more of the methods described herein may be performed by the computer system 1500 in response to the processor 1504 executing one or more sequences of one or more instructions contained in the main memory 1506. Such instructions may be read into the main memory 1506 from another computer-readable medium, such as the storage device 1510. Execution of the sequence of instructions contained in the main memory 1506 causes the processor 1504 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be used to execute the sequence of instructions contained in the main memory 1506. In alternative embodiments, hard-wired circuitry may 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.

[0141] The term "computer-readable medium" as used herein refers to any medium that participates in providing instructions to the processor 1504 for execution. Such media can take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1510. Volatile media include dynamic memory, such as main memory 1506. Transmission media include coaxial cables, copper wire, and fiber optics, including the wires that comprise bus 1502. Transmission media can also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, floppy disks, flexible disks, hard disks, magnetic tape, other magnetic media, CD-ROMs, DVDs, other optical media, punch cards, paper tape, other physical media with patterns of holes, RAM, PROMs, and EPROMs, FLASH-EPROMs, other memory chips or cartridges, carrier waves as described below, or other media that can be read by a computer.

[0142] Various forms of computer readable media may be used to carry one or more sequences of one or more instructions to the processor 1504 for execution. For example, the instructions may initially reside on a magnetic disk of a remote computer. The remote computer may load the instructions into a dynamic memory and send the instructions over a telephone line using a modem. A modem local to the computer system 1500 can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to the bus 1502 can receive the data carried in the infrared signal and place the data on the bus 1502. The bus 1502 carries the data to main memory 1506, from which the processor 1504 retrieves and executes the instructions. The instructions received by main memory 1506 may optionally be stored on storage device 1510 either before or after execution by the processor 1504.

[0143] 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 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 such an implementation, the communication interface 1518 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0144] Network link 1520 typically provides data communication through one or more networks to other data devices. For example, network link 1520 may provide a connection through a local network 1522 to a host computer 1524 or to data equipment operated by an Internet Service Provider (ISP) 1526. The ISP 1526 in turn provides data communication services through a worldwide packet data communication network now commonly referred to as the "Internet" 1528. Local network 1522 and Internet 1528 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 1520 and through communication interface 1518, which carry the digital data to and from computer system 1500, are exemplary forms of carrier waves transporting the information.

[0145] The computer system 1500 can send messages and receive data, including program code, through the network, the network link 1520, and the communication interface 1518. In the Internet example, a server 1530 might transmit a requested code for an application program through the Internet 1528, the ISP 1526, the local network 1522 and the communication interface 1518. One such downloaded application may, for example, provide one or more of the techniques described herein. The received code may be executed by the processor 1504 as received, and / or stored in the storage device 1510 or other non-volatile storage for later execution. In this manner, the computer system 1500 may obtain the application code in the form of a carrier wave.

[0146] Further embodiments are discussed in subsequent numbered paragraphs. 1. A dark-field digital holographic microscope configured to determine a property of interest of a structure, comprising: an illumination device configured to provide at least a first beam pair comprising a first illumination radiation beam and a first reference radiation beam, and a second beam pair comprising a second illumination radiation beam and a second reference radiation beam; an imaging branch operable to detect at least a first scattered radiation scattered by the structure resulting from the structure being illuminated by the first illumination radiation beam and to detect a second scattered radiation scattered by the structure resulting from the structure being illuminated by the second illumination radiation beam, the imaging branch having a detection NA greater than 0.1, optionally greater than 0.8; Equipped with The lighting device includes: the first illumination radiation beam and the first reference radiation beam are at least partially temporally and spatially coherent; the second illumination radiation beam and the second reference radiation beam are configured to be at least partially temporally and spatially coherent; A dark-field digital holographic microscope, wherein the illumination device is configured to provide temporal and / or spatial incoherence between the first beam pair and the second beam pair. 2. The dark field digital holographic microscope described in item 1, wherein the illumination device is operable to direct the first illumination radiation beam to illuminate the structure from a first direction and to direct the second illumination radiation beam to illuminate the structure from a second direction, the second direction being different from the first direction. 3. A dark-field digital holographic microscope as described in item 1 or 2, wherein the imaging branch comprises a sensor, and the dark-field digital holographic microscope is operable to simultaneously capture an interference image on the sensor comprising a first interference pattern resulting from interference of the first scattered radiation with a first reference beam and a second interference pattern resulting from interference of the second scattered radiation with a second reference beam. 4. A dark-field digital holographic microscope as described in item 3, operable such that the first interference pattern and the second interference pattern at least partially spatially overlap on the sensor. 5. A dark-field digital holographic microscope as described in item 3 or 4, configured such that the first reference radiation beam and the second reference radiation beam are arranged to be incident at different azimuthal angles relative to the optical axis of the dark-field digital holographic microscope. 6. A dark-field digital holographic microscope as described in paragraph 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 difference that is sufficiently large so that the two interference patterns are separable in the spatial frequency domain. 7. A dark-field digital holographic microscope according to any one of items 3 to 6, configured so that the first reference radiation beam and the second reference radiation beam are arranged to be incident at different angles of incidence relative to the optical axis of the dark-field digital holographic microscope. 8. Transforming the interference image of the first interference pattern and the second interference pattern into a Fourier representation, the Fourier representation including a central band and at least a pair of sidebands; determining at least an amplitude of a complex field of at least one of the first scattered radiation and the second scattered radiation from the central band and at least one sideband of the at least pair of sidebands; 8. A dark-field digital holographic microscope according to any one of claims 3 to 7, comprising a processor capable of executing the above. 9. The at least one pair of sidebands comprises: a first pair of conjugate sidebands containing first information regarding the first interference pattern of a scattered reference beam pair; a second pair of conjugate sidebands containing second information regarding the second interference pattern of the scattered reference beam pair; Item 9. The dark-field digital holographic microscope according to item 8, comprising: 10. The processor: calculating a first component in the interference image of the first interference pattern and the second interference pattern by an inverse Fourier transform using the central band; calculating a second component in the interference image of the first interference pattern and the second interference pattern by an inverse Fourier transform using the first sideband; calculating a third component in the interference image of the first interference pattern and the second interference pattern by inverse Fourier transformation using the second sideband; determining a first amplitude and a first phase of a first complex field of the first scattered radiation and a second amplitude and a second phase of a second complex field of the second scattered radiation from the first, second and third components of the interference image; Item 10. The dark-field digital holographic microscope according to item 9, which is capable of performing the above. 11. Determining a first amplitude and a first phase of the complex field of the first scattered radiation and determining a second amplitude and a second phase of the complex field of the second scattered radiation comprises: obtaining 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; defining a performance function that describes the match between the interference image and the model image; optimizing (e.g., minimizing) the performance function to obtain values ​​of one or more of the first phase, the second phase, the first amplitude, and the second amplitude; Item 11. The dark-field digital holographic microscope of item 10, wherein the processor is configured to further include: 12. The processor, wherein the optimizing of the performance function further comprises: fitting a value of the first phase by taking a derivative of the performance function with respect to the first phase; fitting values ​​of the second phase by taking a derivative of the performance function with respect to the second phase; fitting a value of the first amplitude by taking a derivative of the performance function with respect to the first amplitude; fitting values ​​of the second amplitude by taking a derivative of the performance function with respect to the second amplitude; Item 12. The dark-field digital holographic microscope according to item 11, configured to include: 13. A dark-field digital holographic microscope as described in any of the preceding paragraphs, wherein the illumination device further comprises a coherence matching arrangement operable to adjustably 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 to maintain the beams of the first beam pair at least partially coherent, and to adjustably 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 to maintain the beams of the second beam pair at least partially coherent. 14. The dark-field digital holographic microscope described in paragraph 13, wherein the illumination device includes a time delay arrangement configured to provide 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 described in paragraph 14, wherein the time delay arrangement includes an adjustable time delay arrangement operable to adjustably 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 to provide the incoherence. 16. The illumination device includes a first branch operable to provide the first beam pair and a second branch operable to provide the second beam pair; the time delay arrangement comprises at least a delay line operable to impart a delay to one of the first branch or the second branch relative to the other of the first branch or the second branch; Item 16. The dark-field digital holographic microscope of item 14 or 15, wherein the coherence matching configuration includes a first coherence matching configuration in the first branch operable to adjustably delay at least one of the first reference beam and the first illumination beam, and a second coherence matching configuration in the second branch operable to adjustably delay at least one of the second reference beam and the second illumination beam. 17. The illumination device comprises a first branch operable to provide the first illumination beam and the second illumination beam, and a second branch operable to provide the first reference beam and the second reference beam; 16. The dark-field digital holographic microscope of claim 14 or 15, wherein the coherence matching configuration and the time delay configuration are implemented through joint optimization of at least a first adjustable delay line operable to impart an adjustable delay to one of the first branch or the second branch with respect to the other of the first branch or the second branch, a second adjustable delay line in the first branch operable to impart an adjustable delay to at least one of the first illumination beam and the second illumination beam, and a third adjustable delay line in the second branch operable to impart an adjustable delay to at least one of the first reference beam and the second reference beam. 18. A dark-field digital holographic microscope according to any of the preceding paragraphs, wherein the first illumination radiation beam is configured to illuminate the structure at a first angle of incidence and 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. A dark-field digital holographic microscope as described in any of the preceding paragraphs, wherein the illumination device includes a single radiation source and the illumination device is configured to generate the first beam pair and the second beam pair from the single radiation source. 20. The dark-field digital holographic microscope of claim 19, wherein the single light source is configured to emit at least partially coherent radiation. 21. A dark-field digital holographic microscope as described in any of the preceding paragraphs, wherein the illumination device is configured such that the first reference beam and the second reference beam are each generated at a first power level, and the first illumination beam and the second illumination beam are each generated at a second power level, the second power level being greater than the first power level. 22. A dark-field digital holographic microscope as described in any of the preceding paragraphs, further comprising one or more optical elements operable to capture first scattered radiation scattered by the structure resulting from the structure being illuminated by the first illumination radiation beam, and to capture second scattered radiation scattered by the structure resulting from the structure being illuminated by the second illumination radiation beam. 23. A dark-field digital holographic microscope according to any of the preceding paragraphs, wherein the imaging branch further comprises an objective lens operable to capture at least the first scattered radiation and the second scattered radiation. 24. A dark-field digital holographic microscope according to any preceding claim, wherein the imaging branch includes a net positive magnification. 25. A dark-field digital holographic microscope as described in any of the preceding paragraphs, wherein the illumination device is configured such that the first illumination beam and the second illumination beam each include a smooth profile to substantially uniformly illuminate the structure. 26. A method for determining a property of interest of a target formed on a substrate by a lithographic process, comprising: illuminating the target with a first illumination radiation beam and capturing resulting first scattered radiation scattered from the target; illuminating the target with a second illumination radiation beam and capturing resulting second scattered radiation scattered from the target; providing spatial and / or temporal incoherence between a first beam pair comprising the first illumination beam and the first reference beam and a second beam pair comprising the second illumination beam and the second reference beam, the beams of the first beam pair being at least partially spatially and temporally coherent; the beams of the second beam pair being at least partially spatially and temporally coherent; Any beam in the first beam pair is spatially and / or temporally incoherent with respect to any beam in the second beam pair; and simultaneously generating a first interference pattern resulting from interference of the first scattered radiation with a first reference radiation beam, and a second interference pattern resulting from interference of the second scattered radiation with a second reference beam. 27. The method of claim 26, further comprising directing the first illumination radiation beam to illuminate the target at a first angle of incidence and directing the second illumination radiation beam to illuminate the target at a second angle of incidence, the first angle of incidence being different from the second angle of incidence. 28. The method of claim 26 or 27, further comprising directing the first illumination radiation beam to illuminate the target at a first azimuthal angle and directing the second illumination radiation beam to illuminate the target at a second azimuthal angle, the first azimuthal angle being different from the second azimuthal angle. 29. A method according to any of clauses 26 to 28, further comprising orienting the first and second reference radiation beams so that they are each incident at a different azimuthal angle relative to an optical axis of the dark-field digital holographic microscope. 30. A method according to claim 29, wherein the azimuthal angle of the first reference radiation beam and the azimuthal angle of the second reference radiation beam comprise a difference which is sufficiently large such that the two interference patterns are separable in the spatial frequency domain. 31. The method of any of clauses 29 or 30, further comprising directing the first reference radiation beam and the second reference radiation beam so that they are each incident at a different angle of incidence relative to an optical axis of the dark-field digital holographic microscope. 32. The method of any of clauses 26 to 31, further comprising adjustably 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 to maintain the beams of the first beam pair at least partially coherent, and adjustably 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 to maintain the beams of the second beam pair at least partially coherent. 33. The method of claim 32, further comprising adjustably delaying 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 to provide incoherence between the first beam pair and the second beam pair. 34. The method of any of clauses 26 to 33, further comprising generating the first beam pair comprising the first illumination radiation beam and the first reference radiation beam, and the second beam pair comprising the second illumination radiation beam and the second reference radiation beam, from a common radiation source. 35. The method of any of clauses 26 to 34, further comprising 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. 36. A method according to any of clauses 26 to 35, comprising maintaining a time delay between the first beam pair and the second beam pair as short as possible when providing the incoherence between the beam pairs. 37. A method according to any of clauses 26 to 36, 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. Transforming the interference image of the first interference pattern and the second interference pattern into a Fourier representation, the Fourier representation including a central band and at least a pair of sidebands; determining at least an amplitude of a complex field of at least one of the first scattered radiation and the second scattered radiation from the central band and at least one sideband of the at least pair of sidebands; 38. The method of claim 37, further comprising: 39. The at least one pair of sidebands comprises: a first pair of conjugate sidebands containing first information about the first interference pattern; a second pair of conjugate sidebands containing second information about the second interference pattern; and 39. The method of claim 38, comprising: 40. The determining step comprises: calculating a first component in the interference image of the first interference pattern and the second interference pattern by an inverse Fourier transform using the central band; calculating a second component in the interference image of the first interference pattern and the second interference pattern by an inverse Fourier transform using the first sideband; calculating a third component in the interference image of the first interference pattern and the second interference pattern by inverse Fourier transformation using the second sideband; determining a first amplitude and a first phase of a first complex field of the first scattered radiation and a second amplitude and a second phase of a second complex field of the second scattered radiation from the first, second and third components of the interference image; 40. The method of claim 39, further comprising: 41. The method of 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. Determining a first amplitude and a first phase of the complex field of the first scattered radiation and determining a second amplitude and a second phase of the complex field of the second scattered radiation comprises: obtaining 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; defining a performance function that describes the match between the interference image and the model image; optimizing (e.g., minimizing) the performance function to obtain values ​​of one or more of the first phase, the second phase, the first amplitude, and the second amplitude; 42. The method of claim 40 or 41, further comprising: 43. The optimization of the performance function further comprises: fitting a value of the first phase by taking a derivative of the performance function with respect to the first phase; fitting values ​​of the second phase by taking a derivative of the performance function with respect to the second phase; fitting a value of the first amplitude by taking a derivative of the performance function with respect to the first amplitude; fitting values ​​of the second amplitude by taking a derivative of the performance function with respect to the second amplitude; 43. The method of claim 42, comprising: 44. A metrology apparatus for measuring a property of interest of a structure on a substrate, comprising a dark-field digital holographic microscope according to any of items 1 to 22 or 52 to 56. 45. An inspection apparatus for inspecting a structure on a substrate, comprising a dark-field digital holographic microscope according to any one of items 1 to 25 or items 52 to 56. 46. ​​A method for determining at least the amplitude of a complex field representative of a structure, comprising: illuminating the structure with a first illumination radiation beam and capturing a resulting first scattered radiation scattered from the structure; illuminating the structure with a second illumination radiation beam and capturing resulting second scattered radiation scattered from the structure; imaging a first interference pattern resulting from interference of the first scattered radiation with a first reference radiation beam and a second interference pattern resulting from interference of the second scattered radiation with a second reference radiation beam, such that the first interference pattern and the second interference pattern at least partially spatially overlap to obtain an interference image; converting the interference image of the first interference pattern and the second interference pattern into a Fourier representation, the Fourier representation including a central band and at least a pair of sidebands; determining at least an amplitude of a complex field of at least one of the first scattered radiation and the second scattered radiation from the central band and the at least one pair of sidebands; A method for providing the above. 47. The at least one pair of sidebands comprises: a first pair of conjugate sidebands containing first information about the first interference pattern; a second pair of conjugate sidebands containing second information about the second interference pattern; and Item 47. The method of item 46, comprising: 48. The determining step comprises: calculating a first component in the interference image of the first interference pattern and the second interference pattern by an inverse Fourier transform using the central band; calculating a second component in the interference image of the first interference pattern and the second interference pattern by an inverse Fourier transform using the first sideband; calculating a third component in the interference image of the first interference pattern and the second interference pattern by inverse Fourier transformation using the second sideband; determining a first amplitude and a first phase of a first complex field of the first scattered radiation and a second amplitude and a second phase of a second complex field of the second scattered radiation from the first, second and third components of the interference image; 48. The method of claim 47, further comprising: 49. The method of claim 48, 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. 50. Determining a first amplitude and a first phase of the complex field of the first scattered radiation and determining a second amplitude and a second phase of the complex field of the second scattered radiation comprises: obtaining 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; defining a performance function that describes the match between the interference image and the model image; optimizing (e.g., minimizing) the performance function to obtain values ​​of one or more of the first phase, the second phase, the first amplitude, and the second amplitude; 50. The method of claim 48 or 49, further comprising: 51. The optimization of the performance function further comprises: fitting a value of the first phase by taking a derivative of the performance function with respect to the first phase; fitting values ​​of the second phase by taking a derivative of the performance function with respect to the second phase; fitting a value of the first amplitude by taking a derivative of the performance function with respect to the first amplitude; fitting values ​​of the second amplitude by taking a derivative of the performance function with respect to the second amplitude; 51. The method of claim 50, comprising: 52. A dark-field digital holographic microscope configured to determine a characteristic of interest of a structure, comprising: an illumination device configured to provide (e.g. simultaneously) at least a first beam pair comprising a first illumination radiation beam and a first reference radiation beam, and a second beam pair comprising a second illumination radiation beam and a second reference radiation beam, such that the dark field digital holographic microscope is operable to (e.g. simultaneously) capture first scattered radiation scattered by the structure resulting from the structure being illuminated by the first illumination radiation beam, and to capture second scattered radiation scattered by the structure resulting from the structure being illuminated by the second illumination radiation beam; a sensor operable to simultaneously capture an interference image comprising a first interference pattern resulting from interference of the first scattered radiation with the first reference beam and a second interference pattern resulting from interference of the second scattered radiation with the second reference beam; transforming the interference image of the first interference pattern and the second interference pattern into a Fourier representation, the Fourier representation including a central band and at least a pair of sidebands; determining at least an amplitude of a complex field of at least one of the first scattered radiation and the second scattered radiation from the central band and the at least one pair of sidebands; A dark-field digital holography microscope comprising: 53. The at least one pair of sidebands comprises: a first pair of conjugate sidebands containing first information about the first interference pattern; a second pair of conjugate sidebands containing second information about the second interference pattern; and Item 53. The dark-field digital holography microscope according to item 52, comprising: 54. The processor: calculating a first component in the interference image of the first interference pattern and the second interference pattern by an inverse Fourier transform using the central band; calculating a second component in the interference image of the first interference pattern and the second interference pattern by an inverse Fourier transform using the first sideband; calculating a third component in the interference image of the first interference pattern and the second interference pattern by inverse Fourier transformation using the second sideband; determining a first amplitude and a first phase of a first complex field of the first scattered radiation and a second amplitude and a second phase of a second complex field of the second scattered radiation from the first, second and third components of the interference image; Item 55. A dark-field digital holography microscope as described in Item 54, which is capable of operating. 55. Determining a first amplitude and a first phase of the complex field of the first scattered radiation and determining a second amplitude and a second phase of the complex field of the second scattered radiation comprises: obtaining 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; defining a performance function that describes the match between the interference image and the model image; optimizing (e.g., minimizing) the performance function to obtain values ​​of one or more of the first phase, the second phase, the first amplitude, and the second amplitude; 55. The dark-field digital holographic microscope of claim 54, wherein the processor is configured to further include: 56. The processor further comprises: fitting a value of the first phase by taking a derivative of the performance function with respect to the first phase; fitting values ​​of the second phase by taking a derivative of the performance function with respect to the second phase; fitting a value of the first amplitude by taking a derivative of the performance function with respect to the first amplitude; fitting values ​​of the second amplitude by taking a derivative of the performance function with respect to the second amplitude; Item 12. The dark-field digital holographic microscope according to item 11, configured to include:

[0147] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, including the manufacture of guidance and detection patterns for integrated optical systems, magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0148] Although specific reference may be made herein to embodiments of the invention in relation to lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object, such as a wafer (or other substrate) or a mask (or other patterning device). These apparatus may be generally referred to as lithography tools. Such lithography tools may use vacuum or ambient (non-vacuum) conditions.

[0149] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, the invention is not limited to optical lithography and may also be used in other applications, for example imprint lithography, where the context permits.

[0150] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative and not limiting. Thus, it will be apparent to one skilled in the art that modifications can be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. 1. A dark field digital holographic microscope configured to determine a property of interest of a structure, comprising: an illumination device configured to simultaneously provide at least a first beam pair comprising a first illumination radiation beam and a first reference radiation beam, and a second beam pair comprising a second illumination radiation beam and a second reference radiation beam; an imaging branch operable to at least capture a first scattered radiation scattered by the structure resulting from the structure being illuminated by the first illumination radiation beam and to capture a second scattered radiation scattered by the structure resulting from the structure being illuminated by the second illumination radiation beam, the imaging branch having a detection NA greater than 0.1; Equipped with The lighting device includes: the first illumination radiation beam and the first reference radiation beam are at least partially temporally and spatially coherent; the second illumination radiation beam and the second reference radiation beam are configured to be at least partially temporally and spatially coherent; the illumination system is configured to provide temporal and / or spatial incoherence between the first beam pair and the second beam pair; the imaging branch further comprises a sensor, the dark-field digital holographic microscope being operable to simultaneously capture on the sensor an interference image comprising a first interference pattern resulting from interference of the first scattered radiation with a first reference beam and a second interference pattern resulting from interference of the second scattered radiation with a second reference beam; A dark field digital holographic microscope, wherein the illumination device is operable to direct the first illumination radiation beam to illuminate the structure from a first direction and to direct the second illumination radiation beam to illuminate the structure from a second direction, the second direction being different from the first direction.

2. The dark-field digital holographic microscope of claim 1 , wherein the dark-field digital holographic microscope is operable such that the first interference pattern and the second interference pattern at least partially spatially overlap on the sensor.

3. the first and second reference radiation beams are arranged to be incident at different azimuthal angles relative to an optical axis of the dark-field digital holographic microscope; 3. A dark-field digital holographic microscope as described in claim 1 or 2, optionally configured such that the azimuthal angle of the first reference radiation beam and the azimuthal angle of the second reference radiation beam include a difference that is sufficiently large so that the two interference patterns are separable in the spatial frequency domain.

4. A dark-field digital holographic microscope as described in any one of claims 1 to 3, configured so that the first reference radiation beam and the second reference radiation beam are arranged to be incident at different angles of incidence relative to an optical axis of the dark-field digital holographic microscope.

5. 5. A dark-field digital holographic microscope according to claim 1 , wherein the illumination device further comprises a coherence matching arrangement operable to adjustably delay one of the first illumination radiation beam and the first reference radiation beam relative to the other of the first illumination radiation beam and the first reference radiation beam to maintain the beams of the first beam pair at least partially coherent, and to adjustably delay one of the second illumination radiation beam and the second reference radiation beam relative to the other of the second illumination radiation beam and the second reference radiation beam to maintain the beams of the second beam pair at least partially coherent.

6. the illumination device comprising a time delay arrangement configured 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, thereby providing incoherence between the first beam pair and the second beam pair; 6. The dark-field digital holographic microscope of claim 5, optionally wherein the time delay arrangement comprises an adjustable time delay arrangement operable to adjustably 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 to provide the incoherence.

7. the illumination device includes a first branch operable to provide the first beam pair and a second branch operable to provide the second beam pair; the time delay arrangement comprises at least a delay line operable to impart a delay to one of the first branch or the second branch relative to the other of the first branch or the second branch; 7. The dark-field digital holographic microscope of claim 6, wherein the coherence matching arrangement includes a first coherence matching arrangement in the first branch operable to adjustably delay at least one of the first reference radiation beam and the first illumination radiation beam, and a second coherence matching arrangement in the second branch operable to adjustably delay at least one of the second reference radiation beam and the second illumination radiation beam.

8. the illumination device comprises a first branch operable to provide the first and second illumination radiation beams and a second branch operable to provide the first and second reference radiation beams; 7. The dark-field digital holographic microscope of claim 6, wherein the coherence matching arrangement and the time delay arrangement are implemented through joint optimization of at least a first adjustable delay line operable to impart an adjustable delay to one of the first branch or the second branch with respect to the other of the first branch or the second branch, a second adjustable delay line in the first branch operable to impart an adjustable delay to at least one of the first illumination radiation beam and the second illumination radiation beam, and a third adjustable delay line in the second branch operable to impart an adjustable delay to at least one of the first reference radiation beam and the second reference radiation beam.

9. 9. A dark-field digital holographic microscope as described in any one of claims 1 to 8, wherein the first illumination radiation beam is configured to illuminate the structure at a first angle of incidence and the second illumination radiation beam is configured to illuminate the structure at a second angle of incidence different from the first angle of incidence.

10. 10. A dark-field digital holographic microscope as described in any one of claims 1 to 9, wherein the illumination device includes a single radiation source, and the illumination device is configured to generate the first beam pair and the second beam pair from the single radiation source.

11. The dark-field digital holographic microscope of claim 10 , wherein the single light source is configured to emit at least partially coherent radiation.

12. 12. A dark-field digital holographic microscope as described in any one of claims 1 to 11, wherein the illumination device is configured such that the first reference radiation beam and the second reference beam are each generated at a first power level and the first illumination radiation beam and the second illumination beam are each generated at a second power level, the second power level being greater than the first power level.

13. 1. A method for determining a property of interest of a target formed on a substrate by a lithographic process, comprising: illuminating the target from a first direction with a first illumination radiation beam and capturing a resulting first scattered radiation scattered from the target; illuminating the target with a second illumination radiation beam from a second direction different from the first direction and capturing resulting second scattered radiation scattered from the target; providing spatial and / or temporal incoherence between a first beam pair comprising the first illumination radiation beam and a first reference radiation beam, and a second beam pair comprising the second illumination radiation beam and a second reference radiation beam, the beams of the first beam pair being at least partially spatially and temporally coherent; the beams of the second beam pair being at least partially spatially and temporally coherent; any beam of the first beam pair being spatially and / or temporally incoherent with respect to any beam of the second beam pair; the first beam pair and the second beam pair are provided simultaneously; and simultaneously generating a first interference pattern resulting from interference of the first scattered radiation with a first reference radiation beam, and a second interference pattern resulting from interference of the second scattered radiation with a second reference beam.

14. A metrology apparatus for measuring a property of interest of a structure on a substrate, comprising a dark field digital holographic microscope according to any one of claims 1 to 12.

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