WO2025131523A1

WO2025131523A1 - Metrology method for a digital holographic microscope and associated computer program

Info

Publication number: WO2025131523A1
Application number: PCT/EP2024/083057
Authority: WO
Inventors: Marvin Gerold FEIL; Theodorus Thomas Marinus VAN SCHAIJK; Robin Daniel BUIJS
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2023-12-21
Filing date: 2024-11-21
Publication date: 2025-06-26
Anticipated expiration: 2026-06-21

Abstract

A method of monitoring a path-length difference (PLD) between a scattered beam and a reference beam in a digital holographic microscope comprising obtaining at least one hologram image associated with at least a wavelength of the illumination radiation and identifying a point of coincidence for the at least one hologram image, the point of coincidence being a point in the at least one hologram image for which the PLD between the reference beam and the scattered beam of each of said beam pairs is zero.

Description

METROLOGY METHOD FOR A DIGITAL HOLOGRAPHIC MICROSCOPE AND ASSOCIATED COMPUTER PROGRAM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23219184.1 which was filed on 21 December 2023 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to digital holographic microscopy and in particular high speed dark field digital holographic microscopy and in relation to metrology applications in the manufacture of integrated circuits.

BACKGROUND

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

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

[0005] Low-ki lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula may be expressed as CD = kix /NA, where /. is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case half -pitch) and ki is an empirical resoluti

[0006] on factor. In general, the smaller ki the more difficult it becomes to reproduce the pattern on the substrate that resembles the shape and dimensions planned by a circuit designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine- tuning steps may be applied to the lithographic projection apparatus and/or design layout. These include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase shifting patterning devices, various optimization of the design layout such as optical proximity correction (OPC, sometimes also referred to as “optical and process correction”) in the design layout, or other methods generally defined as “resolution enhancement techniques” (RET). Alternatively, tight control loops for controlling a stability of the lithographic apparatus may be used to improve reproduction of the pattern at low kl .

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

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

[0009] Digital holographic microscopy is an imaging technology that combines holography with microscopy. Different from other microscopy methods that record projected images of an object, digital holographic microscopy records holograms formed by interference between object radiation obtained by irradiation of a three-dimensional (3D) object with object radiation and reference radiation that is coherent with the object radiation. Thus, this imaging technique is inherently sensitive to changes in optical path-length difference (due to optical path-length drifts of one or both of the object radiation and the reference radiation) between the object radiation and the reference radiation.

[00010] It is the object of the present disclosure to propose methods for monitoring optical path-length difference between the object radiation and the reference radiation and correcting drifting-induced measurement errors.

SUMMARY

[00011] In a first aspect of the invention, there is provided a method of monitoring a path-length difference (PLD) between a scattered beam and a reference beam in a digital holographic microscope, comprising: obtaining at least one hologram image comprising at least one interference pattern relating to a structure having been illuminated by at least one illumination beam, the at least one hologram image associated with a plurality of different center wavelengths, wherein said at least one interference pattern directly or indirectly results from interference between at least one beam pair, each said at least one beam pair comprising a reference beam and a scattered beam, said scattered beam having been scattered from the structure after being illuminated by the illumination beam, wherein the reference beam and scattered beam of each beam pair comprises substantially the same center wavelength; identifying a point of coincidence for the at least one hologram image, the point of coincidence being a point in the at least one hologram image for which the PLD between the reference beam and the scattered beam of each of said beam pairs is zero; determining a distance between the identified point of coincidence and a calibration position in the at least one hologram image; and determining the PLD between the scattered beam and the reference beam based on the determined distance. [00012] In a second aspect of the invention, there is provided a method of overlay metrology with a df- DHM, comprising: determining a first PLD using the method according to the first aspect while performing a first measurement over a target structure; determining a second PLD using the method according to while performing a second measurement over a calibration structure; and determining a spurious overlay error based on a difference between the first PLD and the second PLD.

[00013] In a third aspect of the invention, there is provided a computer program comprising program instructions operable to perform the method according to the first aspect, when run on a suitable apparatus.

[00014] In a fourth aspect of the invention, there is provided a lithographic tool (e.g., df-DHM) for measuring and/or inferring properties of a patterned structure on a substrate, comprising a non-transient computer program carrier comprising a computer program according to the third aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

[00015] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a schematic overview of a lithographic apparatus;

Figure 2 depicts a schematic overview of a lithographic cell;

Figure 3 depicts a schematic representation of holistic lithography, representing a cooperation between three key technologies to optimize semiconductor manufacturing;

Figure 4 depicts a schematic overview of a scatterometry apparatus used as a metrology device, which may comprise a dark field digital holographic microscope according to embodiments of the invention;

Figure 5 depicts a schematic overview of a level sensor apparatus which may comprise a dark field digital holographic microscope according to embodiments of the invention;

Figure 6 depicts a schematic overview of an alignment sensor apparatus which may comprise a dark field digital holographic microscope according to embodiments of the invention;

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

Figure 8 depicts schematically a different example of a diffraction-based dark field metrology device operated in a sequential acquisition scheme;

Figure 9 depicts schematically an example dark field digital holographic microscope operated in a sequential acquisition scheme;

Figure 10 depicts schematically an example dark field digital holographic microscope operated in a parallel acquisition scheme;

Figure 11 A depicts schematically the simulated relationship between path-length drifts 8L between the illumination beam and the reference beam in between target structure and calibration measurements for both the +l^st diffraction order (as indicated by the thick line) and the -1^st diffraction order (as indicated by the thin line) and the resultant spurious overlay error 80V,'

Figure 11B depicts schematically a reflectance image of a typical DBO target, wherein the thick solid box shows the region of interest for the plus biased pad and the thin solid box shows the region of interest for the minus biased pad;

Figure 12 shows a flowchart of a method of monitoring a method of monitoring a PLD between a scattered beam and a reference beam in a dark field digital holographic microscope (df-DHM) (e.g., as shown in Figure 10);

Figures 13A and 13B show an example implementation of the method of Figure 12; wherein Figure 13A depicts schematically a plurality of hologram images acquired by illuminating the structure respectively with a plurality of different center wavelengths in a df-DHM (e.g., as shown in Figure 10) and Figure 13B depicts schematically a hyperspectral hologram image comprising a composite interference pattern obtained by digitally combining the plurality of hologram images shown in Figure 13 A; and

Figure 14 depicts a block diagram of a computer system for controlling a dark field digital holographic microscope.

DETAILED DESCRIPTION

[00016] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5- 100 nm).

[00017] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[00018] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[00019] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[00020] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[00021] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253B2, which is incorporated herein by reference.

[00022] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and/or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[00023] In addition to the substrate support WT, the lithographic apparatus LA may comprise 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 multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[00024] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[00025] As shown in Figure 2 the lithographic apparatus LA may form part of a lithographic cell LC, also sometimes referred to as a lithocell or (litho)cluster, which often also includes apparatus to perform pre- and post-exposure processes on a substrate W. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH and bake plates BK, e.g. for conditioning the temperature of substrates W e.g. for conditioning solvents in the resist layers. A substrate handler, or robot, RO picks up substrates W from input/output ports I/O I , I/O2, moves them between the different process apparatus and delivers the substrates W to the loading bay LB of the lithographic apparatus LA. The devices in the lithocell, which are often also collectively referred to as the track, are typically under the control of a track control unit TCU that in itself may be controlled by a supervisory control system SCS, which may also control the lithographic apparatus LA, e.g. via lithography control unit LACU.

[00026] In order for the substrates W exposed by the lithographic apparatus LA to be exposed correctly and consistently, it is desirable to inspect substrates to measure properties of patterned structures, such as overlay errors between subsequent layers, line thicknesses, critical dimensions (CD), etc. For this purpose, inspection tools (not shown) may be included in the lithocell LC. If errors are detected, adjustments, for example, may be made to exposures of subsequent substrates or to other processing steps that are to be performed on the substrates W, especially if the inspection is done before other substrates W of the same batch or lot are still to be exposed or processed.

[00027] An inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrates W, and in particular, how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on the substrate W and may, for example, be part of the lithocell LC, or may be integrated into the lithographic apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure the properties on a latent image (image in a resist layer after the exposure), or on a semi-latent image (image in a resist layer after a post-exposure bake step PEB), or on a developed resist image (in which the exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[00028] Typically the patterning process in a lithographic apparatus LA is one of the most critical steps in the processing which requires high accuracy of dimensioning and placement of structures on the substrate W. To ensure this high accuracy, three systems may be combined in a so called “holistic” control environment as schematically depicted in Fig. 3. One of these systems is the lithographic apparatus LA which is (virtually) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key of such “holistic” environment is to optimize the cooperation between these three systems to enhance the overall process window and provide tight control loops to ensure that the patterning performed by the lithographic apparatus LA stays within a process window. The process window defines a range of process parameters (e.g. dose, focus, overlay) within which a specific manufacturing process yields a defined result (e.g. a functional semiconductor device) - typically within which the process parameters in the lithographic process or patterning process are allowed to vary.

[00029] The computer system CL may use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and to perform computational lithography simulations and calculations to determine which mask layout and lithographic apparatus settings achieve the largest overall process window of the patterning process (depicted in Fig. 3 by the double arrow in the first scale SCI). Typically, the resolution enhancement techniques are arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from the metrology tool MT) to predict whether defects may be present due to e.g. sub-optimal processing (depicted in Fig. 3 by the arrow pointing “0” in the second scale SC2).

[00030] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drifts, e.g. in a calibration status of the lithographic apparatus LA (depicted in Fig. 3 by the multiple arrows in the third scale SC3).

[00031] In lithographic processes, it is desirable to make frequently measurements of the structures created, e.g., for process control and verification. Tools to make such measurement are typically called metrology tools MT. Different types of metrology tools MT for making such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments which allow measurements of the parameters of a lithographic process by having a sensor in the pupil or a conjugate plane with the pupil of the objective of the scatterometer, measurements usually referred as pupil based measurements, or by having the sensor in the image plane or a plane conjugate with the image plane, in which case the measurements are usually referred as image or field based measurements. Such scatterometers and the associated measurement techniques are further described in patent applications US2010/0328655A1, US2011/102753A1, US2012/0044470A1, US2011/0249244A1, US2011/0026032A1 or EP1628164A2, incorporated herein by reference in their entirety. Aforementioned scatterometers may measure gratings using light from soft x-ray and visible to near-IR wavelength range. [00032] In a first embodiment, the scatterometer MT is an angular resolved scatterometer. In such a scatterometer reconstruction methods may be applied to the measured signal to reconstruct or calculate properties of the grating. Such reconstruction may, for example, result from simulating interaction of scattered radiation with a mathematical model of the target structure and comparing the simulation results with those of a measurement. Parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[00033] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such spectroscopic scatterometer MT, the radiation emitted by a radiation source is directed onto the target and the reflected or scattered radiation from the target is directed to a spectrometer detector, which measures a spectrum (i.e. a measurement of intensity as a function of wavelength) of the specular reflected radiation. From this data, the structure or profile of the target giving rise to the detected spectrum may be reconstructed, e.g. by Rigorous Coupled Wave Analysis and non-linear regression or by comparison with a library of simulated spectra.

[00034] In a third embodiment, the scatterometer MT is a ellipsometric scatterometer. The ellipsometric scatterometer allows for determining parameters of a lithographic process by measuring scattered radiation for each polarization states. Such metrology apparatus emits polarized light (such as linear, circular, or elliptic) by using, for example, appropriate polarization filters in the illumination section of the metrology apparatus. A source suitable for the metrology apparatus may provide polarized radiation as well. Various embodiments of existing ellipsometric scatterometers are described in US patents US7791724B2, US7701577B2, US8115926B2, US8553227B2, US8681312B2, US8792096B2, US8823922B2, US8692994B2, US13/533110 and US8797554B2 incorporated herein by reference in their entirety.

[00035] A metrology apparatus, such as a scatterometer, is depicted 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 a spectrum 6 (i.e. a measurement of intensity as a function of wavelength) of the specular reflected radiation. From this data, the structure or profile 8 giving rise to the detected spectrum may be reconstructed by processing unit PU, e.g. by Rigorous Coupled Wave Analysis and non-linear regression or by comparison with a library of simulated spectra as shown at the bottom of Figure 3. In general, for the reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process by which the structure was made, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer may be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[00036] Overall measurement quality of a lithographic parameter via measurement of a metrology target is at least partially determined by the measurement recipe used to measure this lithographic parameter. The term “substrate measurement recipe” may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in a substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the incident angle of radiation relative to the substrate, the orientation of radiation relative to a pattern on the substrate, etc. One of the criteria to select a measurement recipe may, for example, be a sensitivity of one of the measurement parameters to processing variations. More examples are described in US patent application US2016/0161863A1 and published US patent application US 2016/0370717Alincorporated herein by reference in its entirety.

[00037] Another type of metrology tool used in IC manufacture is a topography measurement system, level sensor or height sensor. Such a tool may be integrated in the lithographic apparatus, for measuring a topography of a top surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as height map, may be generated from these measurements indicating a height of the substrate as a function of the position on the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in a properly focus position on the substrate. It will be understood that “height” in this context refers to a dimension broadly out of the plane to the substrate (also referred to as Z-axis). Typically, the level or height sensor performs measurements at a fixed location (relative to its own optical system) and a relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.

[00038] An example of a level or height sensor LS as known in the art is schematically shown in Figure 5, which illustrates only the principles of operation. In this example, the level sensor comprises an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a beam of radiation LSB which is imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband light source, such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO may include a plurality of radiation sources having different colors, or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not restricted to visible radiation, but may additionally or alternatively encompass UV and/or IR radiation and any range of wavelengths suitable to reflect from a surface of a substrate.

[00039] The projection grating PGR is a periodic grating comprising a periodic structure resulting in a beam of radiation BE1 having a periodically varying intensity. The beam of radiation BE1 with the periodically varying intensity is directed towards a measurement location MLO on a substrate W having an angle of incidence ANG with respect to an axis perpendicular (Z-axis) to the incident substrate surface between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. At the measurement location MLO, the patterned beam of radiation BE1 is reflected by the substrate W (indicated by arrows BE2) and directed towards the detection unit LSD. [00040] In order to determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET produces a detector output signal indicative of the light received, for example indicative of the intensity of the light received, such as a photodetector, or representative of a spatial distribution of the intensity received, such as a camera. The detector DET may comprise any combination of one or more detector types.

[00041] 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 strength as measured by the detector DET, the signal strength having a periodicity that depends, amongst others, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[00042] 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 beam of radiation between the projection grating PGR and the detection grating DGR (not shown).

[00043] In an embodiment, the detection grating DGR may be omitted, and the detector DET may be placed at the position where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[00044] In order to cover the surface of the substrate W effectively, a 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.

[00045] Various height sensors of a general type are disclosed for example in US7265364B2 and US7646471B2, both incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in US2010/233600A1, incorporated by reference. In W02016102127A1, incorporated by reference, a compact height sensor is described which uses a multi-element detector to detect and recognize the position of a grating image, without needing a detection grating.

[00046] Another type of metrology tool used in IC manufacture is an alignment sensor. A critical aspect of performance of the lithographic apparatus is therefore the ability to place the applied pattern correctly and accurately in relation to features laid down in previous layers (by the same apparatus or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be measured at a later time using a position sensor, typically an optical position sensor. The position sensor may be referred to as “alignment sensor” and marks may be referred to as “alignment marks”.

[00047] A lithographic apparatus may include one or more (e.g. a plurality of) alignment sensors by which positions of alignment marks provided on a substrate can be measured accurately. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatus is based on a self-referencing interferometer as described in US6961116B2. Various enhancements and modifications of the position sensor have been developed, for example as disclosed in US2015/261097A1. The contents of all of these publications are incorporated herein by reference.

[00048] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as is described, for example, in US6961116B2, and which is incorporated by reference. Radiation source RSO provides a beam RB of radiation of one or more wavelengths, which is diverted by diverting optics onto a mark, such as mark AM located on substrate W, as an illumination spot SP. In this example the diverting optics comprises a spot mirror SM and an objective lens OL. The illumination spot SP, by which the mark AM is illuminated, may be slightly smaller in diameter than the width of the mark itself. [00049] Radiation diffracted by the alignment mark AM is collimated (in this example via the objective lens OL) into an information-carrying beam IB. The term “diffracted” is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI, e.g. of the type disclosed in US6961116B2 mentioned above, interferes the beam IB with itself after which the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide separate beams in case more than one wavelength is created by the radiation source RSO. The photodetector may be a single element, or it may comprise a number of pixels, if desired. The photodetector may comprise a sensor array.

[00050] The diverting optics, which in this example comprises the spot mirror SM, may also serve to block zero order radiation reflected from the mark, so that the information-carrying beam IB comprises only higher order diffracted radiation from the mark AM (this is not essential to the measurement, but improves signal to noise ratios).

[00051] Intensity signals SI are supplied to a processing unit PU. By a combination of optical processing in the block SRI and computational processing in the unit PU, values for X- and Y-position on the substrate relative to a reference frame are output.

[00052] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. Coarser measurement techniques are used in conjunction with this to identify which period of a sine wave is the one containing the marked position. The same process at coarser and/or finer levels may be repeated at different wavelengths for increased accuracy and/or for robust detection of the mark irrespective of the materials from which the mark is made, and materials on and/or below which the mark is provided. The wavelengths may be multiplexed and demultiplexed optically so as to be processed simultaneously, and/or they may be multiplexed by time division or frequency division.

[00053] In this example, the alignment sensor and spot SP remain stationary, while it is the substrate W that moves. The alignment sensor can thus be mounted rigidly and accurately to a reference frame, while effectively scanning the mark AM in a direction opposite to the direction of movement of substrate W. The substrate W is controlled in this movement by its mounting 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. A measurement of the position of the marks provided on the substrate support allows the position of the substrate support as determined by the position sensor to be calibrated (e.g. relative to a frame to which the alignment system is connected). A measurement of the position of the alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[00054] In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured.

[00055] Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The targets used by such scatterometers are relatively large, e.g., 40pm by 40pm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark field imaging of the diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications W02009/078708A1 and W02009/106279 Al which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US2011/0027704A1, US2011/0043791 Al, US2011/102753A1, US2012/0044470A1,

US2012/0123581A1, US2013/0258310A1, US2013/0271740A1 and WO2013/178422A1. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. The contents of all these applications are also incorporated herein by reference.

[00056] Dark field microscopes, such as the metrology device mentioned above and more generally, have the problem of having a limited range of angles for illumination of the target and/or detection of the light that is diffracted by the target, as it may be required that the total range of angles (corresponding to regions within the angle resolved pupil) are shared between the illumination path and detection path. This limits the effective NA in illumination and detection.

[00057] In a diffraction-based dark field metrology device, a beam of radiation is directed onto a metrology target and one or more properties of the scattered radiation are measured so as to determine a property of interest of the target. The properties of the scattered radiation may comprise, for example, intensity at a single scattering angle (e.g., as a function of wavelength) or intensity at one or more wavelengths as a function of scattering angle.

[00058] Measurement of targets in dark field metrology may comprise, for example, measuring the a first intensity of the 1^st diffraction order I+i and a second intensity of the -1^st diffraction order (I-i) and calculating an intensity asymmetry (A = I_+i - Li), which is indicative of asymmetry in the target. The metrology targets may comprise one or more grating structures from which a parameter of interest may be inferred from such intensity asymmetry measurements, e.g., the targets are designed such that the asymmetry in the target varies with the parameter of interest. For example, in overlay metrology a target may comprise at least one composite grating formed by at least a pair of overlapping sub-gratings that are patterned in different layers of the semiconductor device. Asymmetry of the target will therefore be dependent on alignment of the two layers and therefore overlay. Other targets may be formed with structures which are exposed with different degrees of variation based on the focus setting used during the exposure; the measurement of which enabling that focus setting to be inferred back (again through intensity asymmetry).

[00059] Figure 7 and Figure 8 schematically illustrate two examples of diffraction-based dark field metrology devices. Note that for the sake of simplicity, both figures only show some of the components that are sufficient for the purpose of describing working principle of the two devices.

[00060] As illustrated in Figure 7, a first illumination beam of radiation IB1 may be obliquely incident onto an overlay target of a substrate WA from one side of the device. The grating based overlay target may diffract the first illumination beam into a number of diffraction orders. Since the device is configured for dark field imaging, the zeroth diffraction order may be either blocked by an optical component or configure to fall completely outside the numerical aperture of the objective lens OB. At least one non-zeroth diffraction order, e.g., positive first diffraction order +l^st DF, may be collected by the objective lens OB. At the pupil plane of the objective lens OB, a first wedge WG1 may be used to re-direct the diffracted radiation to follow a desired beam path. Finally, an imaging lens may be used to focus the diffraction order, e.g., positive first diffraction order +l^st DF, onto an image sensor IS such that a first image IM1 is formed at a first location.

[00061] Similarly, a second illumination beam of radiation IB2 may be obliquely incident onto the same overlay target OT of the substrate WA from the opposite side of the system. The incident angle of the second illumination beam IB2 may be same as that of the first illumination beam IB1. At least one non- zeroth diffraction order, e.g., negative first diffraction order -1^st DF, may be collected by the objective lens OB and subsequently redirected by a second wedge WG2. The negative first diffraction order -1^st DF may then be focused by the imaging lens IL onto the image sensor IS such that a second image IM2 is formed at a second location.

[00062] The example of Figure 7 is operated in a parallel acquisition scheme. The overlay target is illuminated simultaneously by both illumination beams IB1, IB2. Correspondingly, the two spatially separated images IM1, IM2 of the overlay target are acquired at the same time. Such a parallel acquisition scheme allows for a fast measurement speed and hence high throughput. However, the pupil plane of the objective lens OB has to be shared by the two diffraction orders, e.g., +l^st DF and -1^st DF. A consequence of dividing the pupil into mutually exclusive illumination and detection pupils is that there is a consequent reduction in the illumination NA and in the detection NA. While there is some flexibility in trade-off between the illumination NA and detection NA, ultimately having both the illumination NA the detection NA as large as is often desirable is not possible within a single pupil. This results in a limited range of angles for each corresponding illumination beam and for the +l^st DF and -1^st DF beams, which in turn limits the range of allowable grating pitch sizes and/or illumination wavelengths and hence imposes a tight requirement for designing such a metrology system.

[00063] Figure 8 schematically illustrates another exemplary dark field metrology device (or different operation mode of the device of Figure 7). The main difference is that the metrology device of Figure 8 is operated in a sequential acquisition scheme. In the sequential acquisition scheme, a metrology target OT is only illuminated by one illumination beam from one direction at any time instance and thereby only one image of the target is formed and acquired at any point in time. Referring to Figure 8, at a first time instance t=Tl, a first illumination beam IB1 may be switched on and directed obliquely onto an overlay target OT of a substrate WA from one side of the metrology device. After interaction with the gratings of the overlay target, a number of diffraction orders may be generated. At least one of the non- zeroth diffraction order, e.g., positive first diffraction order +l^st DF, may be collected by an objective lens OB and subsequently focused by an imaging lens IL onto an image sensor IS.

[00064] Subsequent to the first image IM1 of the overlay gratings being acquired, at a second time instance t=T2, the first illumination beam IB 1 is switched off and a second illumination beam IB2 is switched on. The second illumination beam IB2 may be directly obliquely onto the same overlay target from an opposite side of the metrology device. At least one of the generated diffraction orders, e.g., negative first diffraction order -1^st 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 position on the image sensor.

[00065] With this time multiplexed acquisition scheme, the full NA of the objective lens OB is made available for detecting the diffracted beams +l^st DF and -1^st DF. No limitation in objective NA means a wider range of relevant design parameters, such as grating pitch sizes, illumination wavelengths and illumination angles, is allowed and a greater flexibility in system design can be obtained. However, the fact that multiple image acquisitions are needed means measurement speed is reduced and hence system throughput is impacted.

[00066] In addition, accurate determination of e.g., overlay error, relies on accurate measurement of a minute relative intensity difference (or intensity asymmetry) between the two acquired images IM1, IM2. The typical relative intensity difference is on the order of 10^ of the intensity of one of the acquired images, e.g., IM1 or IM2. Such a small intensity difference could easily be dwarfed by any intensity and/or wavelength fluctuations of illumination radiation. Therefore, the illumination beams are required to stay stable during consecutive image acquisitions. This can be achieved by using a stable light source providing desired intensity and wavelength stabilities. Alternatively, additional hardware and software, such as for example intensity/wavelength monitoring device and corresponding feedback control loop, should be incorporated into the metrology device such that intensity and/or wavelength fluctuations of the illumination beams are actively monitored and well compensated. In some cases, an intensity monitoring device may be used to actively track the intensity of the illumination beams. The signal generated from the intensity monitoring device may be used to (e.g., electronically) correct the intensity fluctuations of the illumination beams. All these solutions add complexity and cost to the overall system.

[00067] Some or all of aforementioned problems could be addressed by using digital holographic microscopy, in particular dark field digital holographic microscopy. Digital holographic microscopy is an imaging technology that combines holography with microscopy. Different from other microscopy methods that record projected images of an object, digital holographic microscopy records holograms formed by interference between object radiation obtained by irradiation of a three-dimensional (3D) object with object radiation and reference radiation that is coherent with the object radiation. Images may be captured using, for example a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). Since the object radiation is radiation scattered from the object, wave-front of the object radiation is therefore modulated or shaped by the object. Said scattered radiation may comprise reflected radiation, diffracted radiation, or transmitted radiation. Therefore, the wavefront of the object radiation carries information of the irradiated object, e.g., 3D shape information. Based on the captured images of holograms, images of the object can be numerically reconstructed by using a computer reconstruction algorithm. An important advantage of hologram based metrology over intensity based metrology, as described in examples of Figures 7 and 8, is that hologram based metrology allows both intensity and phase information of an object to be obtained. With additional phase information, characteristics of the object can be determined with better accuracy.

[00068] The international patent application WO2019/197117A1, incorporated herein by reference, discloses a method and metrology apparatus based on a dark field digital holographic microscope (df- DHM) to determine a characteristic, e.g., overlay, of a structure manufactured on a substrate. For the purpose of description, Figure 3 of the international patent application WO2019/197117A1 is replicated in Figure 9. Figure 9 schematically illustrates the disclosed df-DHM specifically adapted for use in lithographic process metrology.

[00069] In comparison to the former examples shown in Figures 7 and 8, the df-DHM in Figure 9 further comprises a reference optical unit 16, 18 which is used to provide additional two reference radiation beams 51, 52 (the reference radiation). Such two reference radiation beams 51, 52 are respectively paired with two corresponding portions 41, 42 of the scattered radiation beams 31, 32 (the object radiation). The two scattered-reference beam pairs are used sequentially to form two interference patterns. Coherence control is provided by way of adjusting the relative optical path-length difference (OPD) between the two scattered-reference beams within each beam pair. However, no coherence control is available between the two beam pairs.

[00070] Due to the use of a single light source and insufficient coherence control, all four radiation beams, i.e. the first portion 41 of the scattered radiation 31, the first reference radiation 51, the second portion 42 of the scattered radiation 32 and the second reference radiation 52, are mutually coherent. If these four mutually coherent radiation beams were allowed to reach the same position of the sensor 6 at the same time, namely operating in a parallel acquisition scheme, multiple interference patterns comprising desired information containing patterns and undesired artefact-contributing patterns would overlap each other. The undesired interference patterns may be formed by interference between e.g., the portion 41 of the first scattered radiation 31 and the portion 42 of the second scattered radiation 32. Since it would be technically challenging and time consuming to completely separate the superimposed interference patterns, parallel acquisition is impractical this arrangement.

[00071] Similar to the example of Figure 8, the use of a sequential acquisition scheme in the example of Figure 9 allows the full NA of the objective lens to be available for both illumination and detection. However, the system suffers the same problem of low measurement speed due to sequential acquisition. Therefore, it is desirable to have a df-DHM capable of performing parallel acquisition such that a high measurement speed and a high design flexibility can be simultaneously obtained.

[00072] Figure 10 schematically illustrates the imaging branch of a dark field digital holographic microscope (df-DHM) 1000 in accordance with an embodiment. A dark field digital holographic microscope (df-DHM) comprises an imaging branch and an illumination branch. In this embodiment, a metrology target 1060 comprising a structure on a substrate 1050 is illuminated by two illumination beams of radiation, i.e., a first illumination beam of radiation 1010 and a second illumination beam of radiation 1020. In an embodiment, such two illumination beams 1010, 1020 may simultaneously illuminate the metrology target 1060.

[00073] In an embodiment, the first illumination beam 1010 may be incident on the metrology target 1060 at a first angle of incidence in a first direction with respect 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 with respect to the optical axis OA. The first angle of incidence of the first illumination beam 1010 and the second angle of incidence of the second illumination beam 1020 may be substantially the same. The angle of incidence of each illumination beam may be, for example in the range of 70 degrees to 90 degrees, in the range of 50 degrees to 90 degrees, in the range of 30 degrees to 90 degrees, in the range of 10 degrees to 90 degrees. The illumination of the metrology target 1060 may result in radiation being scattered from the target. In an embodiment, the first illumination beam 1010 may be incident on the metrology target 1060 at a first azimuthal angle, corresponding to the first direction. The second illumination beam 1020 may be incident on the metrology target 1060 at a second azimuthal angle, corresponding to the second direction. The first azimuthal angle of the first illumination beam 1010 and the second azimuthal angle of the second illumination beam 1020 may be different; e.g., opposing angles 180 degrees apart.

[00074] Depending on the structure of the metrology target 1060, the scattered radiation may comprise reflected radiation, diffracted radiation or transmitted radiation. In this embodiment, the metrology target may be a diffraction-based overlay target; and each illumination beam may correspond to a scattered beam comprising at least one non-zeroth diffraction order. Each scattered beam carries information of the illuminated metrology target. For example, the first illumination beam 1010 may correspond to the first scattered beam 1011 comprising the positive first diffraction order +l^st DF; the second illumination beam 1020 may correspond to the second scattered beam 1021 comprising the negative first diffraction order -1^st DF. The zeroth diffraction order and other undesired diffraction orders may either be blocked by a beam blocking element (not shown) or configured to completely fall outside the NA of the objective lens 1070. As a result, the df-DHM may be operated in a dark field mode. Note that, in some embodiments, one or more optical elements, e.g., a lens combination, may be used to achieve same optical effect of the objective lens 1070.

[00075] Both scattered beams 1011, 1021 may be collected by objective lens 1070 and subsequently re-focused onto an image sensor 1080. Objective lens 1070 may comprise multiple lenses, and/or df- DHM 1000 may comprise a lens system having two or more lenses, e.g., an objective lens and an imaging lens similar to the exemplary df-DHG of Figure 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 simultaneously incident at a common position 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 on the same position 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 comprise the portion 1012 of the first scattered beam 1011 and the first reference beam 1030. Eikewise, the second scattered-reference beam pair may comprise the portion 1022 of the second scattered beam 1021 and the second reference beam 1040. These two scattered-reference beam pairs may subsequently form two interference patterns (holographic images) which at least partially overlap in spatial domain.

[00076] In an embodiment, in order to separate the 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 angle of incidence and the second angle of incidence being different. Alternatively or in addition, 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 being different. [00077] In order to generate an interference pattern, the two beams of each scattered-reference beam pair should be at least partially coherent to each other, to a degree which is sufficient to form an interference pattern. Note that each scattered radiation beam may have a phase offset with respect to its corresponding illumination radiation. For example, at the image plane of the image sensor 1080, such a phase offset may comprise contributions due to the optical path-length (OPD) from the metrology target 1060 to the image sensor 1080, and by the interaction with the metrology target. As described above, it is necessary to control the coherence between the first scattered-reference beam pair and the second scattered-reference beam pair such that each beam of one pair is incoherent to any beam of the other pair. In other words, interference should only occur between the beams within the same beam pair and suppressed between different beam pairs. In such a manner, only desired interference patterns, e.g., the two interference patterns formed by respect scattered-reference beam pairs, are formed in a superimposed manner on the image sensor 1080, thus obviating the problem of separating or removing undesired interference patterns.

[00078] The characteristic of the structure of the metrology target 1060 is determined by a processing unit 1090 of the metrology apparatus. The processing unit 1090 uses the first interference pattern and the second interference pattern recorded by the image sensor 1080 to determine the characteristic of the structure of the metrology target 1060. In an embodiment, the processing unit 1090 is coupled to the image sensor 1080 to receive a signal comprising information about the first interference pattern and the second interference pattern recorded by the sensor 1090. In an embodiment, the processing unit 1090 corrects for aberrations of the objective lens 1070 of the df-DHM 1000. In an embodiment, the measurements of the first interference pattern and the second interference pattern are performed with radiation simultaneously in time (in parallel) and the processing unit 1090 is configured to use the measurements simultaneously in time (in parallel) to determine the characteristic of the structure of the metrology target 1060 on the substrate 1050.

[00079] In an embodiment, the processing unit 1090 uses the first interference pattern to calculate a complex field of radiation at the sensor 1080 (“complex” here meaning that both amplitude and phase information is present) 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. Such calculation of a complex field of radiation from an interference pattern formed by interfering reference radiation with radiation scattered from an object is known in general terms from holography. Further details about how to perform such calculations in the context of metrology for lithography may be found for example in US2016/0061750A1, which is hereby incorporated by reference.

[00080] If the optical characteristics 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 metrology target 1060. [00081] Having knowledge of the complex field provides additional information for determining the characteristic of the metrology target 1060 on the substrate 1050, relative to alternative modes in which phase and amplitude information are not both available. For example, in European patent application EP18158745.2, filed on February 27, 2018, it has been disclosed how phase information of the scattered radiation can be used to determine overlay errors between structures of different layers on the substrate (an example of a characteristic of the structure to be determined). European patent application EP3531191A1 is hereby incorporated by reference.

[00082] In an embodiment, the processing unit 1090 may be a computer system. The computer system may be equipped with an image reconstruction algorithm which is used to perform all the aforementioned tasks, comprising performing Fourier transform, extracting each individual high order spatial spectrum, performing inverse Fourier transform, calculating complex fields and determining a characteristic of the structure based on the results.

[00083] As described above, in a conventional diffraction based dark field metrology device (e.g., as shown in Figure 7 and Figure 8), intensity images of +l^st and -1^st diffraction orders scattered from a structure are captured and used to determine overlay of the structure. By comparison, in a df-DHM, overlay of a structure is determined using hologram images, wherein each hologram image comprises multiple (e.g., two) interference patterns respectively formed by multiple (e.g., two) scattered-reference beam pairs (e.g., the first scattered-reference beam pair comprising the portion 1012 of the first scattered beam 1011 (corresponding to +l^st diffraction order) and the first reference beam 1030, and the second scattered-reference beam pair comprising the portion 1022 of the second scattered beam 1021 (corresponding to -1^st diffraction order) and the second reference beam 1040 shown in Figure 10).

[00084] From a hologram image captured with a DHM (e.g., the df-DHM shown in Figure 10) a holographic intensity may be obtained for each interference pattern it compromises e.g., corresponding to +l^st and -1^st diffraction order. This holographic intensity is the product of the actual intensity of the scattered beam , the actual intensity of the reference beam, and the inhomogeneous contrast function. For + 1^st or -1^st diffraction order it can be expressed as:

where I_H + (u) , abbreviated for I_{H +} (u) or I_H _(u) , denotes the holographic intensity for the +l^st diffraction order and -1^st diffraction order respectively; I+(u), abbreviated for I₊(u) or I_(u), denotes the actual (dark field) intensity of the scattered beam in the +l^st diffraction order and the scattered beam in the -1^st diffraction order respectively; 1^, abbreviated for

denotes the actual intensity of the reference beam for interfering with the scattered beam in the +l^st diffraction order and the scattered beam in the -1^st diffraction order respectively; C+(u, L+) , abbreviated for C₊ (u, L₊) or C_(u, L_), denotes the inhomogeneous contrast function for the +l^st diffraction order and -1^st diffraction order respectively. As shown in equation [1], the inhomogeneous contrast function C_± (u, L+) is the absolute square of the temporal coherence function y of the light source concatenated with the scalar product of image location u and sensitivity vector S±, abbreviated for S₊ or S_ plus the path-length difference (PLD) L± , abbreviated for L₊ or L_ , between the scattered beam and the reference beam for the + 1 ^st and - 1 ^st diffraction order respectively . The sensitivity vector S_± can be solely determined by the df-DHM’ s configuration, such as for example the incidence angle of the illumination beam 1010/1020, the incidence angle of the reference beam 1030/1040, the NA and focal length of the objective lens 1070, the magnification of the lens system. It is known that decreasing the spectral bandwidth of the illumination (and reference) beam increases coherence length which leads to a flatter temporal coherence function and thus a flatter contrast function. Here, a flatter contrast function means the intensity of the interference fringes reduces more slowly from the zeroth-order fringe (where the contrast is maximum) to fringes of higher orders.

[00085] The holographic intensity I_H + (u) can be calibrated by dividing the holographic intensity of a target by the holographic intensity of a homogeneous calibration sample (e.g., a fiducial mark). The calibration samples holographic intensities are expressed as: i_H ^cai = c_± ^ai\^r _± ^ef , p]

or 1^1, denotes the calibration samples holographic intensity for the + l^st diffraction order and -1^st diffraction order respectively; C+, abbreviated for C₊ or C_, denotes the inhomogeneous contrast function for the +l^st diffraction order and -1^st diffraction order respectively; l^^al, abbreviated

or /Z^H/, denotes the actual intensity of the +l^st diffraction order and -1^st diffraction order for the calibration sample respectively;

abbreviated for X™ or IT^, denotes the actual intensity of the reference beam for interfering with the scattered beam in the +l^st diffraction order and the scattered beam in the -1^st diffraction order respectively,. With this calibration method, one can cancel the contrast up to a constant and subsequently calculate overlay. However, complete cancellation of contrast requires perfect stability of the optical PLD between the scattered beam and the reference beam for both the 1^st and -1^st diffraction orders. If there is a path-length drift between the calibration measurement and the actual target measurement which results in a change in the PLD for either or both of the +l^st and -1^st diffraction orders, the respective contrast function moves on the wafer between the two measurements (see below with reference to Figure 1 IB); that is to say, the line of the maximum contrast (corresponding to the zeroth order fringe) of the measurement hologram no longer overlaps with the line of the maximum contrast of the calibration hologram. Consequently, the calibrated intensity (obtained after a path-length drift) of the + 1^st or -1^st diffraction order is not equal to the actual dark field intensity of + 1^st or -1^st diffraction order. In such a case, the calibrated intensity I±, 1^st or -1^st diffraction order is expressed as:

[3]

L'_± = L_± + 8L_±, [4] where 8L±, abbreviated for 8L₊ or 8L_, denotes a path-length drift for the +l^st diffraction order and - 1^st diffraction order; L’₊, abbreviated for L’₊ or L'_, denotes the new PLD between the scattered beam and the reference beam for, respectively, the +l^st diffraction order and -1^st diffraction order as a result of the path- length drift 8L±.

[00086] The incomplete contrast cancellation results in an overlay error 80V which is dependent on the path-length drifts 8L±. The path-length drifts 8L± induce a shift of the respective line of maximum contrast defined by S₊u + L₊ = 0 by Au₊ = — in direction of S₊. In the case of df-DHMs where - - - |s_±| optical fibers are used e.g., for outputting and/or transporting illumination beams (illumination beams 1010, 1020 shown in Figure 10), path-length drifts 8L, which may result from thermal drifts and/or other effects, are expected to be on the order of 100 nm in several seconds.

[00087] Figure 11A shows the simulated relationship between path-length drifts 8L± between the illumination beam and the reference beam in between target structure and calibration measurements for both the +l^st diffraction order (as indicated by the thick line) and the -1^st diffraction order (as indicated by the thin line) and resultant spurious overlay error 80V wherein simulations are based on a typical DBO target (e.g., a pDBO (micro-DBO) target with a size of 16 pm x 16 pm, a pitch of 800 nm, and a proportionality factor of 0.01 nm ¹) in thin-element approximation, a typical DHM configuration, and homogeneous illumination (e.g., an illumination spectrum with a 5 nm wide Gaussian line-shape and centered at 532 nm). As shown in the simulation plot, a path-length drift of around 100-200 nm can result in an overlay error 80V of around 50-100 picometer (pm) which may fill a significant part of the precision error budget for overlay measurement. Therefore, it is desirable to reduce or minimize path- length drifting induced overlay errors so as to improve the performance (e.g., measurement accuracy) of a DHM.

[00088] Figure 1 IB shows a reflectance image of a typical DBO target (e.g., the example pDBO target mentioned above), wherein the thick solid box shows the region of interest ROI+ for the plus biased pad and the thin solid box ROI. shows the region of interest for the minus biased pad. The temporal coherence function for both the +l^st diffraction order and -1^st diffraction order is centered on the target, that is to say the PLD between the reference beam and the scattered beam for both the +l^st diffraction order and the -1^st diffraction order is zero (i.e. L₊ = L_ = 0) at the image center (i.e. Px=0, Py=0). The thick dashed line LMC+ and thin dashed line LMC. show the line of maximum contrast defined by S±u + L_± = 0 for the +l^st diffraction order and -1^st diffraction order, respectively. For either the +l^st or - 1^st diffraction orders, pixel positions (Px, Py) closer to the line of maximum contrast will have higher intensities than those farther away from the same line. In this particular example, both lines of maximum contrast are centered at the image center (i.e. Px=0, Py=0) or the zero PLD position. The line of maximum contrast C will displace from the position shown in Figure 11B if the PLD between the reference beam and the scattered beam deviates from zero e.g., as a result of a thermal drift. Therefore, the positional information of the line of maximum contrast may be utilized to monitor or determine path-length drifts in a df-DHM.

[00089] It is found that for single -pad overlay targets, path-length drifting induced overlay errors are approximately inversely proportional to the square of the coherence length of the illumination radiation. Thus, a narrower illumination bandwidth which enables a larger coherence length is expected to result in a smaller overlay error than a broader illumination bandwidth. However, this goes against the current trend of using broad illumination bandwidths to increase optical power and therefore measurement throughput of existing df-DHMs.

[00090] Therefore, it is the object of the present disclosure to mitigate the above-described issue, i.e. to reduce or minimize path-length drifting induced overlay errors in df-DHMs. In an aspect of the present disclosure, there is provided a method 1200 of monitoring a PLD between a scattered beam and a reference beam in a digital holographic microscope (e.g., df-DHM 1000 shown in Figure 10). The method 1200 may be implemented in a computer system (e.g., a processing unit in the digital holographic microscope or df-DHM). Figure 12 shows a flowchart of method 1200 according to an embodiment. As shown in Figure 12, method 1200 may comprise the following four main steps 1210- 1240.

[00091] At step 1210, at least one hologram image may be obtained. The at least one hologram image may comprise at least one interference pattern relating to a structure (e.g., a measurement target or a calibration target) having been illuminated by at least one illumination beam, the at least one hologram image associated with a plurality of different center wavelengths. The at least one interference pattern of each of the at least one hologram image directly or indirectly may result from interference between at least one beam pair, each said at least one beam pair comprising a reference beam and a scattered beam, said scattered beam having been scattered from the structure after being illuminated by the illumination beam. The reference beam and scattered beam of each beam pair may comprise substantially the same center wavelength.

[00092] Step 1220 may comprise identifying at least one point of coincidence for the at least one hologram image. The point of coincidence may be a point in the at least one hologram image for which the PLD between the reference beam and the scattered beam of all the plurality of different center wavelengths of each of said beam pairs is zero. [00093] Step 1230 may comprise determining a distance between the identified point of coincidence and a calibration position in the at least one hologram image. In an embodiment, the calibration position may be a pre-drift position in the at least one hologram image where the PLD between the scattered beam and the reference beam was initially zero. In an embodiment, the calibration position is a center position of the at least one hologram image.

[00094] Step 1240 may comprise determining the PLD based on the determined distance.

[00095] In an embodiment, the at least one hologram image may comprise a plurality of individually acquired hologram images each comprising an interference pattern and associated with a respective one of the plurality of different center wavelengths. The at least one beam pair may comprise a plurality of beam pairs, each beam pair associated with a respective one of the plurality of different center wavelengths and forming one interference pattern in a respective one of the plurality of individually acquired hologram images. The gap in time between any two successive image acquisitions may be negligible compared to the optical path-length drifting time.

[00096] In an embodiment, the plurality of individually acquired hologram images may be obtained by illuminating the structure with the illumination beam having one of the plurality of different center wavelengths; acquiring one of the plurality of individually acquired hologram images comprising an interference pattern formed by interference between the reference beam and the scattered beam of one of said beam pairs; and repeating the steps of illuminating and acquiring with different said beam pairs of the plurality of beam pairs.

[00097] In an embodiment, the DHM may comprise a wavelength selection arrangement (WSA) configured to output radiation at one or more of the plurality of different center wavelengths, said radiation being used to provide the illumination beam and the reference beam. In such a case, method 1200 may further comprise outputting from the WSA individually and sequentially a plurality of different beam pairs, each comprising a respective different center wavelength.

[00098] In some implementations, the WSA may comprise a plurality of individual radiation sources for providing the plurality of different center wavelengths. Each of the plurality of individual radiation sources may be operable to output a spectrum having a different center wavelength. The output beam of each of the plurality of radiation sources may be used to provide one or both of the two scattered- reference beam pairs (e.g., the first scattered-reference beam pair 1012 and 1030, and the second scattered-reference beam pair 1022 andl044 shown in Figure 10). As mentioned above, the two beam pairs may be configured such that interference should only occur between the beams within the same beam pair and suppressed between different beam pairs.

[00099] Therefore, to obtain the plurality of individually acquired hologram images, the DHM or WSA may be configured to (1) select at least some of the plurality of radiation sources to provide the plurality of different center wavelengths; (2) sequentially enable the selected at least some of the plurality of radiation sources to provide the illumination beam and the reference beam for obtaining the plurality of individually acquired hologram images. The plurality of individual radiation sources may be sequentially enabled in a predefined or arbitrary order. In some implementations, the radiation sources may be enabled one after the other in an order according to their respective output center wavelengths, e.g., the output center wavelength increasing from the shortest wavelength to the longest wavelength (of a selected wavelength range), or decreasing from the longest wavelength to the shortest wavelength (of a selected wavelength range). The selected wavelength range within which the center wavelength is switched may be between 300 nm and 1600 nm, between 300 nm and 1200 nm, between 400 nm and 1000 nm, between 400nm and 800 nm, or between 400 nm and 700 nm.

[000100] In some implementations, the WSA may comprise a tunable radiation source operable to output a spectrum having a center wavelength which is tuneable across a wavelength range (e.g., between 400 nm and 1000 nm) comprising all the plurality of different center wavelengths. The output radiation of the tuneable radiation source may be used to provide the illumination beam and the reference beam for obtaining the plurality of individually acquired hologram images. As such, the DHM or WSA may be configured to tune or vary the center wavelength of the tuneable radiation source so as to individually and sequentially output the plurality of different center wavelengths.

[000101] In some implementations, the WSA may comprise a broadband radiation source operable to output a broadband spectrum comprising the plurality of different center wavelengths. The WSA may comprise a spectral filter configured to selectively spectrally filter the broadband output of the broadband radiation source so as to selectively output one or more different portions of the broadband spectrum to provide each said beam pair. During the image acquisition, the DHM or WSA may be configured to tune or vary the spectral filter to vary (in either a smooth or non-smooth (stepwise) manner) the center wavelength of the output of the broadband radiation source across a predefined wavelength range.

[000102] The spectrally filtered output of the broadband radiation source may be used to provide one or both of the first scattered-reference beam pair and the second scattered-reference beam pair. During the image acquisition, the DHM or WSA may be configured to tune or vary the spectral filter so as to vary the center wavelength of the output of the broadband radiation source from the first center wavelength to the second center wavelength, thereby across the predefined wavelength range (e.g., 400 nm - 1000 nm).

[000103] In an example implementation, the spectral filter may comprise a physical aperture having an opening of a fixed width. Said aperture may filter out a portion of the broadband spectrum having a certain center wavelength by allowing high transmission of said portion while substantially blocking the remaining portion of the broadband spectrum. During an image acquisition, said aperture may be operable to move across a sub-range or the entire range of the broadband spectrum so as to vary the center wavelength of the output portion across the predefined wavelength range.

[000104] In another example implementation, the spectral filter may comprise a plurality of narrowband filters each having a different center wavelength. In operation, only one of the plurality of narrowband filters is used to filter out a portion of the broadband radiation source. During an image acquisition, the DHM or WSA may be configured to rapidly change or vary the selected filter so as to vary the center wavelength of the output portion across the predefined wavelength range.

[000105] In an embodiment, the point of coincidence may be a point where zeroth-order fringes of interference patterns of all the plurality of different center wavelengths coincide. To identify such a point, method 1200 may further comprise combining the plurality of individually acquired hologram images (e.g., by exploiting their mutual incoherence) so as to combine their respective interference patterns into a composite interference pattern, identifying a region of the composite interference pattern having a highest average intensity and determining the point of coincidence as a point within the identified region. For example, the point of incidence may be a substantially central point within said identified region.

[000106] As described above in relation to Figure 10, each acquired hologram image may comprise two interference patterns formed respectively by the first scattered-reference beam pair (corresponding to the +l^st diffraction order) and the second scattered-reference beam pair (corresponding to the -1^st diffraction order). Thus, it may be necessary to first process each of the plurality of individually acquired hologram images to separate interference patterns formed with light associated with a +l^st diffraction order and light associated with a -1^st diffraction order, respectively. Such image processing may result in a first subset of hologram images each of which comprises an interference pattern associated with the +l^st diffraction order and a second subset of hologram images each of which comprises one interference pattern associated with the -1^st diffraction order. Then, the first subset of hologram images may be combined such that their respective interference patterns are combined into a first composite interference pattern and the second subset of hologram images may be combined such that their respective interference patterns are combined into a second composite interference pattern. The first composite interference and the second composite interference may be individually used to determine the PLD between the two beams of the first scattered-reference beam pair and the second scattered-reference beam pair, respectively.

[000107] The following paragraphs will explain the working principle of determining the PLD between the scattered beam and the reference beam for either +l^st or -1^st diffraction order in a df-DHM (e.g., as shown in Figure 10). Figures 13A-13B schematically depict an example implementation of method 1200. Figure 13 A shows a plurality of hologram images HIM i , HIM2, .. . HIM_n acquired by illuminating the structure respectively with a plurality of different center wavelengths i, 2...L1 in the df-DHM. Each hologram image HIMi, HIM2,...HIM_n is on a gray scale where the minimum value is white, and the maximum value is black. For overlay measurement, hologram images are obtained respectively with illumination spectra centered around several different center wavelengths

... A_n in rapid succession (corresponding to step 1210 of Figure 12). The time between any two immediately adjacent image acquisitions is in the order of several milliseconds, which is significantly slower (e.g., at least one order of magnitude slower) than the time required for typical optical path-length drifts to occur. [000108] Once obtained, the plurality of the hologram images HIMi, HIM2,...HIM_n are combined in a digital manner to form a hyperspectral hologram image HYM comprising a composite interference pattern, as shown Figure 13B (corresponding to step 1210 of Figure 12). The plurality of hologram images HIMi, HIM2,...HIM_n may correspond to either the first subset of hologram images associated with the +l^st diffraction order or the second subset of hologram images associated with the -1^st diffraction order.

[000109] Assuming that no significant optical path length drifts 8L occur on the short time scale over which multiple hologram image acquisitions can happen, for instance a few milliseconds, one can recover the optical path length difference L± from the color (wavelength) series of interference patterns (or interferograms). In first order approximation, the fringe maxima of the interferograms are described by

A₊u + = n G TL , [5]

- Ti

where A_± , abbreviated for A₊ or A_ , denotes the fringe frequency vector, 2₀ denotes the center wavelength of the illumination spectrum, Uj denotes the unit vector on the image plane pointing orthogonal to the grating lines of the target, n denotes the order of the interference fringe, the P denotes the grating pitch of the target, M denotes magnification of the lens system of the df-DHM. According to equations [5] and [6], the interference fringe spacing (determined by | A + 1 ) and the interference fringe angle (determined by A±/ 1 A_± 1 ) both change with the center wavelength 2₀.

[000110] With reference to Figure 13B, the composite interference pattern is on a gray scale where the minimum value is white, and the maximum value is black. For each interference fringe order (e.g., n = — 1, 0, or + 1), the +l^st or -1^st interference fringes, as indicated by lines LMC1 (2₁ ), LMC2(2₂) in the figure, of all different center wavelengths 2₁,2₂ respectively coincide at a single point, i.e. the negative first-order (n=-l) fringes of all different center wavelengths coincide at point CP(-l), the zeroth-order (n=0) fringes of all different center wavelengths coincide at point CP(0), the positive first-order (n=+l) fringes of all different center wavelengths coincide at point CP(+l).In the case of the scattered beam and/or the reference beam having a finite linewidth, the zeroth-order point of coincidence CP(0) may have the highest intensity or be the brightest among all these points of coincidence. Such a point of coincidence can thus be used for the subsequent PLD determination. With continued reference to Figure 13B, the zeroth-order point of coincidence CP(0), located at a position u* = vu₂, is displaced from the center position of the hologram image (i.e. Px=0, Py=0) in direction of the grating lines by a distance v = (corresponding to step 1230 of Figure 12). Accordingly, by determining how much the S±,2 zeroth-order point of coincidence CP(0) is displaced from the image center (e.g., the horizontal displacement Vx and the vertical displacement Vy), one can determine the PLD L± between the scattered beam and the reference beam for either +l^st or -1^st diffraction order (corresponding to step 1240 in Figure 12 of Figure 12).

[000111] In an embodiment, the at least one hologram image may comprise a hyperspectral hologram image comprising a composite interference pattern associated with all the plurality of different center wavelengths.

[000112] In an embodiment, the hyperspectral hologram image may be obtained by: illuminating the structure with the illumination beam having a first center wavelength (e.g., 400 nm); acquiring a hyperspectral hologram image comprising a composite interference pattern formed by interference between the reference beam and the scattered beam; and during the image acquisition, varying (in either a smooth or non-smooth manner) simultaneously the center wavelength of the illumination beam and the center wavelength of the reference beam across a predefined wavelength range (e.g., 400 nm - 1000 nm) starting at the first wavelength and ending at a second wavelength (e.g., 1000 nm) to obtain the plurality of beam pairs. In an embodiment, the center wavelength may be incrementally increased or decreased from one end of the predefined range to the other end. For example, the center wavelength may be increased at an increment of 100 nm from 400 nm to 1000 nm.

[000113] In this embodiment, the hyperspectral hologram image may comprise a first composite interference pattern associated with the +l^st diffraction order and a second composite interference pattern associated with the -1^st diffraction order. Therefore, method 1200 may further comprise a step of processing the hyperspectral hologram image to separate the two composite interference patterns. Once separated, the first and second composite interference patterns can be used to determine the PLD for the beam pairs associated respectively with the +l^st diffraction order and the -1^st diffraction order.

[000114] Similar to the foregoing embodiments, the DHM may comprise a WSA configured to output radiation the center wavelength of which can be tuned across a desired wavelength range comprising the plurality of different center wavelengths. The WSA may comprise either a tunable radiation source operable to output a spectrum having a center wavelength which is tuneable across a desired wavelength range or alternatively the WSA may comprise a broadband radiation source configured to output a broadband radiation having a broadband spectrum and a spectral filter configured to spectrally filter the broadband spectrum to selectively output a portion of the spectrum. During the image acquisition, the DHM or WSA may be configured to tune the tunable radiation source or the spectral filter so as to rapidly vary the center wavelength of the output from the first center wavelength to the second center wavelength, thereby across the predefined wavelength range (e.g., 400 nm - 1000 nm).

[000115] By varying the center wavelength of the illumination beam and the reference beam over a large wavelength range in the course of an acquisition, all fringes except for the zeroth-order point of coincidence will smear out. The acquired hyperspectral hologram will comprise a composite interference pattern similar to the one shown in Figure 13B wherein the brightest spot corresponds to the zeroth-order point of coincidence (as described above). This is the analog equivalent of the digital procedure described in the foregoing embodiment.

[000116] In an embodiment, the WSA may comprise a broadband radiation source operable to output a broadband spectrum comprising the predefined wavelength range. The DHM or WSA may be configured to enable the output of the broadband radiation source to provide the illumination beam and reference beam, both having the broadband spectrum, illuminate the structure with the broadband illumination beam; and acquire a hyperspectral hologram image comprising a composite interference pattern formed by interference between the broadband reference beam and the broadband scattered beam.

[000117] In another aspect of the present disclosure, there is provided a method of overlay metrology with a DHM, comprising: determining a first path-length difference (PLD) using the above-described PLD monitoring method while performing a first measurement over a target structure (e.g., pDBO target); determining a second path-length difference (PLD) using the above-described PLD monitoring method while performing a second measurement over a calibration structure; and determining a spurious overlay error based on a difference between the first PLD and the second PLD. It is known that the coherence function shifts proportionally to the PLD (e.g., the first PLD or the second PLD) between the illumination beam and the reference beam with a geometric factor dependent on wavelength, target design and reference beam orientation. Hence, the determined first PLD and the determined second PLD may be used to infer a first position of the coherence function for the first measurement and a second position of the coherence function for the second measurement, respectively. The coherence function modulates the true diffraction efficiency to produce a lower perceived intensity. The shape of the coherence function may be known based on earlier calibration or calculated from the spectrum of the light source. If the perceived intensity is divided by the coherence function which is centered on the position inferred by the measured PLD (e.g., first PLD or the second PLD), true intensity is retrievable. From this retrieved intensity, overlay of the target structure and the calibration structure is calculated as typical for pDBO targets. After obtaining the overlay information of both structures, the phase length drift induced spurious overlay error can be determined.

[000118] In an embodiment, the method of overlay metrology with a DHM may further comprise processing computationally overlay data obtained from the first measurement to correct for the determined overlay error. The computational processing of the overlay data may comprise determining a first overlay using the data obtained from the first measurement over the target structure and subtracting, computationally, the spurious overlay error from the first overlay to obtain the corrected target structure overlay.

[000119] In an embodiment, the method may further comprise adjusting, continuously or intermittently, the PLD between the scattered beam and the reference beam to correct for the determined overlay error. The PLD may be actively adjusted to compensate any path-length drift and thus correct for the path- length drift induced spurious overlay error. This may be achieved by active optical path length stabilization which may be implemented with for example a fiber heater or cooler or a piezo controlled delay line. The use of the active optical path length stabilization obviates the need of continuous computational correction of spurious overlay error.

[000120] The above-described methods may be implemented by a computer system.

[000121] Figure 14 is a block diagram that illustrates a computer system 1400 that may assist in implementing the methods and flows disclosed herein. Computer system 1400 includes a bus 1402 or other communication mechanism for communicating information, and a processor 1404 (or multiple processors 1404 and 1405) coupled with bus 1402 for processing information. Computer system 1400 also includes a main memory 1406, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 1402 for storing information and instructions to be executed by processor 1404. Main memory 1406 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 1404. Computer system 1400 further includes a read only memory (ROM) 1408 or other static storage device coupled to bus 1402 for storing static information and instructions for processor 1404. A storage device 1410, such as a magnetic disk or optical disk, is provided and coupled to bus 1402 for storing information and instructions.

[000122] Computer system 1400 may be coupled via bus 1402 to a display 1412, such as a cathode ray tube (CRT) or flat panel or touch panel display for displaying information to a computer user. An input device 1414, including alphanumeric and other keys, is coupled to bus 1402 for communicating information and command selections to processor 1404. Another type of user input device is cursor control 1416, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 1404 and for controlling cursor movement on display 1412. 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), that allows the device to specify positions in a plane. A touch panel (screen) display may also be used as an input device.

[000123] One or more of the methods as described herein may be performed by computer system 1400 in response to processor 1404 executing one or more sequences of one or more instructions contained in main memory 1406. Such instructions may be read into main memory 1406 from another computer- readable medium, such as storage device 1410. Execution of the sequences of instructions contained in main memory 1406 causes processor 1404 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory 1406. In an alternative embodiment, 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.

[000124] The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor 1404 for execution. Such a medium may 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 1410. Volatile media include dynamic memory, such as main memory 1406. Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise bus 1402. 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, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read.

[000125] Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor 1404 for execution. For example, the instructions may initially be borne on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 1400 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 bus 1402 can receive the data carried in the infrared signal and place the data on bus 1402. Bus 1402 carries the data to main memory 1406, from which processor 1404 retrieves and executes the instructions. The instructions received by main memory 1406 may optionally be stored on storage device 1410 either before or after execution by processor 1404.

[000126] Computer system 1400 also preferably includes a communication interface 1418 coupled to bus 1402. Communication interface 1418 provides a two-way data communication coupling to a network link 1420 that is connected to a local network 1422. For example, communication interface 1618 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, communication interface 1418 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 1418 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[000127] Network link 1420 typically provides data communication through one or more networks to other data devices. For example, network link 1420 may provide a connection through local network 1422 to a host computer 1424 or to data equipment operated by an Internet Service Provider (ISP) 1426. ISP 1426 in turn provides data communication services through the worldwide packet data communication network, now commonly referred to as the “Internet” 1428. Local network 1422 and Internet 1428 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 1420 and through communication interface 1418, which carry the digital data to and from computer system 1400, are exemplary forms of carrier waves transporting the information.

[000128] Computer system 1400 may send messages and receive data, including program code, through the network(s), network link 1420, and communication interface 1418. In the Internet example, a server 1430 might transmit a requested code for an application program through Internet 1428, ISP 1426, local network 1422 and communication interface 1418. One such downloaded application may provide for one or more of the techniques described herein, for example. The received code may be executed by processor 1404 as it is received, and/or stored in storage device 1410, or other non-volatile storage for later execution. In this manner, computer system 1400 may obtain application code in the form of a carrier wave.

[000129] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[000130] Although specific reference may be made in this text to embodiments of the invention in the context of a 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 mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[000131] Further embodiments according to the present invention are presented in below numbered clauses:

1. A method of monitoring a path-length difference (PLD) between a scattered beam and a reference beam in a digital holographic microscope (DHM), comprising: obtaining at least one hologram image comprising at least one interference pattern relating to a structure having been illuminated by at least one illumination beam, the at least one hologram image associated with a plurality of different center wavelengths, wherein said at least one interference pattern directly or indirectly results from interference between at least one beam pair, each said at least one beam pair comprising a reference beam and a scattered beam, said scattered beam having been scattered from the structure after being illuminated by the illumination beam, wherein the reference beam and scattered beam of each beam pair comprises substantially the same center wavelength; identifying a point of coincidence for the at least one hologram image, the point of coincidence being a point in the at least one hologram image for which the PLD between the reference beam and the scattered beam of each of said beam pairs is zero; determining a distance between the identified point of coincidence and a calibration position in the at least one hologram image; and determining the PLD between the scattered beam and the reference beam based on the determined distance. 2. A method according to any preceding clause, wherein the calibration position comprises a predrift position in the at least one hologram image at which the PLD between the scattered beam and the reference beam was initially zero.

3. A method according to any preceding clause, where the calibration position is a center position of the at least one hologram image.

4. A method according to clause 1, 2 or 3, wherein the at least one hologram image comprises a plurality of individual hologram images, each comprising a respective interference pattern and associated with a respective one of the plurality of different center wavelengths.

5. A method according to clause 4, further comprising processing each hologram image of the at least one hologram image to separate interference patterns associated with different diffraction orders.

6. A method according to clause 5, wherein the different diffraction orders comprise a +l^st diffraction order and a -1^st diffraction order and the step of processing results in generation of at least a first hologram image for the +l^st diffraction order and at least a second hologram image for the -1^st diffraction order.

7. A method according to any of clauses 4 to 6, wherein the step of identifying a point of coincidence comprises identifying a point where zeroth-order fringes of interference patterns of all the plurality of different center wavelengths coincide.

8. A method according to any of clauses 4 to 7, further comprising: combining the plurality of individually acquired hologram images so as to combine their respective interference patterns into a composite interference pattern; identifying a region of the composite interference pattern having a highest average intensity; and determining said point of coincidence as a point within said identified region.

9. A method according to clause 8, wherein said determining said point of coincidence as a point within said identified region comprises determining said point of coincidence as a substantially central point within said identified region.

10. A method according to any of clauses 4 to 9, wherein the plurality of individual hologram images are obtained by: illuminating the structure with the illumination beam having one of the plurality of different center wavelengths; acquiring one of the plurality of individual hologram images comprising an interference pattern formed by interference between the reference beam and the scattered beam of one of said beam pairs; repeating the steps of illuminating and acquiring with different said beam pairs of the plurality of beam pairs.

11. A method according to clause 1, 2 or 3, wherein the at least one hologram image comprises a hyperspectral hologram image comprising a composite interference pattern associated with all the plurality of different center wavelengths.

12. A method according to clause 11, wherein the hyperspectral hologram image is obtained by: illuminating the structure with the illumination beam; acquiring a hyperspectral hologram image comprising a composite interference pattern formed by interference between the reference beam and the scattered beam; and during the image acquisition, varying simultaneously the center wavelength of the illumination beam and the center wavelength of the reference beam across a predefined wavelength range starting from the first wavelength and ending at a second wavelength to obtain said plurality of beam pairs.

13. A method according to clause 11, wherein said at least one beam pair comprises a broadband illumination beam and a broadband reference beam, each having a broadband spectrum, and the hyperspectral hologram image is obtained by: illuminating the structure with the broadband illumination beam; and acquiring a hyperspectral hologram image comprising a composite interference pattern formed by interference between said beam pair.

14. A method according to any preceding clause, wherein the DHM comprises a wavelength selection arrangement configured to output radiation at one or more of the plurality of different center wavelengths, said radiation being used to provide each said at least one beam pair.

15. A method according to clause 14, further comprising outputting from the wavelength selection arrangement individually and sequentially a plurality of different beam pairs, each comprising a respective different center wavelength.

16. A method according to clause 14 or 15, wherein the wavelength selection arrangement comprises a plurality of individual radiation sources each operable to output a spectrum having a different center wavelength.

17. A method according to clause 14 or 15, wherein the wavelength selection arrangement comprises a tunable radiation source operable to output a spectrum having a center wavelength which is tuneable in a wavelength range comprising said plurality of different center wavelengths.

18. A method according to clause 14 or 15 , wherein the wavelength selection arrangement comprises a broadband radiation source operable to output a broadband spectrum comprising the plurality of different center wavelengths.

19. A method according to clause 18, wherein the method further comprises selectively spectrally filtering the output of the broadband radiation source to selectively output one or more different portions of the broadband spectrum to provide each said beam pair.

20. A method of overlay metrology, comprising: determining a first path-length difference (PLD) using the method of any preceding clause while performing a first measurement over a target structure; determining a second path-length difference (PLD) using the method of any preceding clause while performing a second measurement over a calibration structure; and determining a spurious overlay error based on a difference between the first PLD and the second

PLD. 21. A computer program comprising program instructions operable to perform the method of any of clauses 1 to 9, when run on a suitable apparatus.

22. A lithographic tool for measuring and/or inferring properties of a patterned structure on a substrate, comprising a non-transient computer program carrier comprising a computer program according to clause 21.

23. A lithographic tool according to clause 22 comprises a digital holographic microscope (DHM).

24. A lithographic tool according to clause 23, wherein the digital holographic microscope is a dark-field digital holographic microscope (df-DHM).

[000132] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[000133] 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 descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. A method of monitoring a path-length difference (PLD) between a scattered beam and a reference beam in a digital holographic microscope (DHM), comprising: obtaining at least one hologram image comprising at least one interference pattern relating to a structure having been illuminated by at least one illumination beam, the at least one hologram image associated with a plurality of different center wavelengths, wherein said at least one interference pattern directly or indirectly results from interference between at least one beam pair, each said at least one beam pair comprising a reference beam and a scattered beam, said scattered beam having been scattered from the structure after being illuminated by the illumination beam, wherein the reference beam and scattered beam of each beam pair comprises substantially the same center wavelength; identifying a point of coincidence for the at least one hologram image, the point of coincidence being a point in the at least one hologram image for which the PLD between the reference beam and the scattered beam of each of said beam pairs is zero; determining a distance between the identified point of coincidence and a calibration position in the at least one hologram image; and determining the PLD between the scattered beam and the reference beam based on the determined distance.

2. A method as claimed in any preceding claim, wherein the calibration position comprises a predrift position in the at least one hologram image at which the PLD between the scattered beam and the reference beam was initially zero.

3. A method as claimed in any preceding claim, where the calibration position is a center position of the at least one hologram image.

4. A method as claimed in claim 1, 2 or 3, wherein the at least one hologram image comprises a plurality of individual hologram images, each comprising a respective interference pattern and associated with a respective one of the plurality of different center wavelengths.

5. A method as claimed in claim 4, further comprising processing each hologram image of the at least one hologram image to separate interference patterns associated with different diffraction orders.

6. A method as claimed in claim 5, wherein the different diffraction orders comprise a +l^st diffraction order and a -1^st diffraction order and the step of processing results in generation of at least a first hologram image for the +l^st diffraction order and at least a second hologram image for the -1^st diffraction order.

7. A method as claimed in any of claims 4 to 6, wherein the step of identifying a point of coincidence comprises identifying a point where zeroth-order fringes of interference patterns of all the plurality of different center wavelengths coincide.

8. A method as claimed in any of claims 4 to 7, further comprising: combining the plurality of individually acquired hologram images so as to combine their respective interference patterns into a composite interference pattern; identifying a region of the composite interference pattern having a highest average intensity; and determining said point of coincidence as a point within said identified region.

9. A method as claimed in claim 8, wherein said determining said point of coincidence as a point within said identified region comprises determining said point of coincidence as a substantially central point within said identified region.

10. A method as claimed in any of claims 4 to 9, wherein the plurality of individual hologram images are obtained by: illuminating the structure with the illumination beam having one of the plurality of different center wavelengths; acquiring one of the plurality of individual hologram images comprising an interference pattern formed by interference between the reference beam and the scattered beam of one of said beam pairs; repeating the steps of illuminating and acquiring with different said beam pairs of the plurality of beam pairs.

11. A method as claimed in claim 1, 2 or 3, wherein the at least one hologram image comprises a hyperspectral hologram image comprising a composite interference pattern associated with all the plurality of different center wavelengths.

12. A method as claimed in claim 11, wherein the hyperspectral hologram image is obtained by: illuminating the structure with the illumination beam; acquiring a hyperspectral hologram image comprising a composite interference pattern formed by interference between the reference beam and the scattered beam; and during the image acquisition, varying simultaneously the center wavelength of the illumination beam and the center wavelength of the reference beam across a predefined wavelength range starting from the first wavelength and ending at a second wavelength to obtain said plurality of beam pairs.

13. A method as claimed in claim 11, wherein said at least one beam pair comprises a broadband illumination beam and a broadband reference beam, each having a broadband spectrum, and the hyperspectral hologram image is obtained by: illuminating the structure with the broadband illumination beam; and acquiring a hyperspectral hologram image comprising a composite interference pattern formed by interference between said beam pair.

14. A method as claimed in any preceding claim, wherein the DHM comprises a wavelength selection arrangement configured to output radiation at one or more of the plurality of different center wavelengths, said radiation being used to provide each said at least one beam pair.

15. A computer program comprising program instructions operable to perform the method of any of claims 1 to 9, when run on a suitable apparatus.