Method and apparatus for Fourier transform spectroscopy

Abstract

A Fourier transform spectrometer is disclosed comprising a beam splitting arrangement operable to define a first radiation source and a second radiation source from a common radiation source, and at least one detector operable to detect the first radiation source and the second radiation source, the detector is operable to detect interferogram data as a function of a detection position at least in a first detection plane direction of the detection plane, the interferogram data resulting from interference of a first divergent beam emitted from the first radiation source and a second divergent beam emitted from the second radiation source. The processor is operable to: perform a linearization correction on the interferogram data to obtain linearized interferogram data; and performing Fourier transform on the linearized interferogram data to obtain spectral characteristic data related to the common radiation source.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22187745.9, filed on July 29, 2022, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a lithographic apparatus and / or a metrology apparatus, and to a spectrometry method related to such an apparatus. Background Art

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

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

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

[0007] Metrology tools are used in many aspects of the IC manufacturing process, such as alignment tools for properly positioning the substrate before exposure, leveling tools for measuring the surface topology of the substrate, and tools based on, for example, focus control and scatterometry, for inspecting / measuring exposed and / or etched products in process control.

[0008] During photolithography, it is desirable to frequently measure the structures being formed, for example, for process control and verification. Various tools are known for performing such measurements, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers. Examples of known scatterometers typically rely on providing a dedicated metrology target, such as an unfilled target (a target in the form of a simple grating or overlapping gratings in different layers that is large enough so that the measurement beam produces a spot smaller than the grating) or an overfilled target (where the illumination spot partially or completely encompasses the target). Furthermore, the use of a metrology tool (e.g., an angle-resolved scatterometer that illuminates an unfilled target, such as a grating) allows for the use of so-called reconstruction methods, in which the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0009] A scatterometer is a versatile instrument that allows for measurement of parameters of a lithography process by placing a sensor in the pupil of the scatterometer's objective lens or in a plane conjugate to the pupil (measurements are often referred to as pupil-based measurements), or by placing a sensor in the image plane or in a plane conjugate to the image plane (in which case measurements are often referred to as image-based or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entirety. Such scatterometers can measure multiple targets from multiple gratings in a single image using light from soft x-rays and visible to near-IR wavelengths.

[0010] In lithography tools (such as scanners) and / or various metrology tools (e.g., alignment sensors, level sensors, scatterometers, holographic microscopes), it is often necessary to measure the spectrum of the source radiation used by the tool (i.e., perform spectral analysis on the radiation). This can be achieved, for example, by using a dispersive optical element (such as a grating) and measuring the intensity of the radiation diffracted from the grating. This known spectroscopic arrangement requires additional optical components to direct the measured radiation at a well-defined angle toward the dispersive grating, as well as collection optics to collect the diffracted light and project it onto a detector array. Achieving this with the required level of accuracy over a wide wavelength range places relatively high demands on the spectrometer design, which often increases its cost.

[0011] It would be desirable to improve this spectrometric method. Summary of the Invention

[0012] According to a first aspect of the present invention, there is provided a Fourier transform spectrometer comprising: a beam splitting arrangement operable to define a first radiation source and a second radiation source from a common radiation source, at least one detector operable to detect interference pattern data as a function of a detection position in at least a first detection plane direction of a detection plane, the interference pattern data being generated by interference of a first divergent beam emitted from the first radiation source and a second divergent beam emitted from the second radiation source; and a processor operable to: perform a linearization correction on the interference pattern data to obtain linearized interference pattern data; and perform a Fourier transform on the linearized interference pattern data to obtain spectral characteristic data associated with the common radiation source.

[0013] According to a second aspect of the present invention, there is provided a Fourier transform spectrometer comprising: a beam splitting arrangement operable to define a first radiation source and a second radiation source from a common radiation source, at least one detector operable to detect interference pattern data, the interference pattern data describing an interference pattern as a function of a detection position in at least a first detection plane direction of a detection plane, the interference pattern data being generated by interference of a first divergent beam emitted from the first radiation source and a second divergent beam emitted from the second radiation source; wherein the Fourier transform spectrometer does not include an optical device interposed between the beam splitting arrangement and the at least one detector, so that the detector is arranged to directly detect the interference pattern in an overlapping region of the first divergent beam and the second divergent beam.

[0014] According to a third aspect of the present invention, a method is provided, comprising: dividing the common radiation source into a first radiation source and a second radiation source; emitting the first divergent beam from the first radiation source; emitting the second divergent beam from the second radiation source; detecting interference pattern data as a function of the detection position in at least a first detection plane direction of a detection plane, the interference pattern data being generated by interference between the first divergent beam and the second divergent beam; performing linearization correction on the interference pattern data to obtain linearized interference pattern data; and performing Fourier transform on the linearized interference pattern data to obtain spectral characteristic data associated with the common radiation source. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

[0019] Figure 4 depicts a schematic overview of a scatterometry apparatus for use as a metrology device, which may include a radiation source according to an embodiment of the present invention;

[0020] Figure 5 depicts a schematic overview of a level sensor device that may include a radiation source according to an embodiment of the present invention;

[0021] Figure 6 depicts a schematic overview of an alignment sensor apparatus that may include a radiation source according to an embodiment of the present invention;

[0022] Figure 7 A schematic overview diagram depicting an alternative lithographic apparatus configuration for EUV lithography is provided;

[0023] Figure 8 schematically depicts a dark-field digital holographic microscope (df-DHM) operating with a parallel acquisition scheme according to an embodiment;

[0024] Figure 9 schematically depicts a Fourier transform spectrometry arrangement according to an embodiment;

[0025] Figure 10 is a method for processing a device according to an embodiment of the present invention. Figure 9A flowchart of a method for processing an interference pattern obtained by an arrangement; and

[0026] Figure 11 (a) shows a graph of Fourier transformed, non-linearized detected signal level versus spatial frequency; Figure 11 (b) shows a linearization method for a detected interference pattern according to an embodiment; Figure 11 (c) shows a graph of Fourier transformed, linearized detected signal level versus spatial frequency; and

[0027] Figure 12 A block diagram of a computer system for controlling a method according to an embodiment is depicted. DETAILED DESCRIPTION

[0028] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365, 248, 193, 157, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5-100 nm).

[0029] As used herein, the terms "reticle," "mask," or "patterning device" should be broadly interpreted to refer to a general-purpose patterning device that can be used to impart an incident radiation beam with a patterned cross-section corresponding to the pattern to be produced in a target portion of the substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective, binary, phase-shift, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0030] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also called illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., mask table) MT configured to support a patterning device (e.g., mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to specific parameters, a substrate support (e.g., wafer stage) WT configured 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 according to specific parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0031] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0032] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, which projection system is appropriate to the exposure radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0033] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g., water) to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US6952253 (which is incorporated herein by reference).

[0034] The lithographic apparatus LA may also be of a type having two (also referred to as "dual stage") or more substrate supports WT. In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps for subsequent exposure preparation of a substrate W may be performed on a substrate W on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0035] In addition to the substrate support WT, the lithographic apparatus LA can include a measurement stage. The measurement stage is arranged to hold sensors and / or cleaning devices. The sensors can be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement stage can hold multiple sensors. The cleaning devices can be arranged to clean part of the lithographic apparatus, such as part of the projection system PS or part of the system for providing immersion liquid. The measurement stage can be moved beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0036] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. After passing through the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example, in order to position different target portions C in a focused and aligned position in the path of the radiation beam B. Similarly, a first positioner PM and possibly another position sensor (which is not in the Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 are shown occupying dedicated target portions, they may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe lane alignment marks.

[0037] like Figure 2 As shown, the lithography apparatus LA may form part of a lithocell LC, sometimes also referred to as a litho cell or (lithography) cluster. The lithocell LC typically also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Typically, this equipment includes a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH, and a bake plate BK, for example, to adjust the temperature of the substrate W, such as for adjusting the solvent in the resist layer. A substrate handling device (or robot) RO picks up substrates W from input / output ports I / O1 and I / O2, moves them between various processing devices, and delivers them to a loading station LB of the lithography apparatus LA. The equipment within the lithocell (often collectively referred to as a track) is typically controlled by a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which in turn controls the lithography apparatus LA, for example, via a litho control unit LACU.

[0038] In order to correctly and consistently expose substrates W exposed by the lithographic apparatus LA, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CDs), etc. To this end, an inspection tool (not shown) may be included in the lithography cell LC. If an error is detected, particularly if the inspection is performed before exposing or processing other substrates W from the same batch or lot, for example, adjustments may be made to the exposure of subsequent substrates and / or other processing steps to be performed on the substrates W.

[0039] Inspection equipment (which may also be referred to as metrology equipment) is used to determine properties of substrate W, in particular, to determine how properties vary between different substrates W or how properties associated with different layers of the same substrate W vary from layer to layer. Alternatively, the inspection equipment may be configured to identify defects on substrate W and may, for example, be part of a lithography cell LC, integrated into a lithography apparatus LA, or even a standalone device. The inspection equipment may measure properties on a latent image (the image in the resist layer after exposure), a semi-latent image (the image in the resist layer after a post-exposure bake step (PEB), a developed resist image (in which exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0040] Typically, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process requiring high-precision sizing and placement of structures on the substrate W. To ensure this high precision, the three systems can be combined into a so-called "holistic" control environment, such as Figure 3 One of these systems is a lithography apparatus LA, which is (physically) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to this "holistic" environment is optimizing the collaboration between these three systems to enhance the overall process window and providing a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined outcome (e.g., a functional semiconductor device). Typically, process parameters in the lithography or patterning process are allowed to vary within this range.

[0041] The computer system CL can use (a portion of) the design layout to be patterned to predict which resolution enhancement technology to use and to perform computational lithography simulations and calculations to determine which mask layouts and lithographic equipment settings achieve the largest overall process window for the patterning process (in Figure 3 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. by using input from a metrology tool MT) to predict whether defects may be present due to, for example, a suboptimal process (e.g., in the example of FIG. Figure 3 , which is represented by an arrow pointing to “0” in the second scale SC2).

[0042] 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, for example, possible drift in the calibration state of the lithographic apparatus LA (e.g., Figure 3 represented by multiple arrows in the third scale SC3).

[0043] During a lithographic process, it is often desirable to measure the resulting structures, for example, for process control and verification. The tool that performs such measurements is typically referred to as a metrology tool MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow parameters of the lithographic process to be measured by placing a sensor in the pupil of the scatterometer objective, or in a plane conjugate to the pupil, typically referred to as pupil-based measurements, or by placing the sensor in the image plane, or in a plane conjugate to the image plane, typically referred to as image-based or field-based measurements. Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entireties. The scatterometer described above can measure gratings using light from soft X-rays and visible to near IR wavelength ranges.

[0044] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measurement signal to reconstruct or calculate the properties of the grating. For example, such a reconstruction can be obtained by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0045] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed toward a target, and radiation reflected or scattered from the target is directed toward a spectroscopic detector, which measures the spectrum of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

[0046] In a third embodiment, the scatterometer MT is an ellipsometer. Ellipsometers allow parameters of a lithography process to be determined by measuring scattered radiation for each polarization state. Such metrology equipment emits polarized light (such as linear, circular, or elliptical) by, for example, using appropriate polarization filters in the illumination portion of the metrology equipment. A source suitable for the metrology equipment can also provide polarized radiation. Various embodiments of existing ellipsometers are described in U.S. patent applications Ser. Nos. 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, the entire contents of which are incorporated herein by reference.

[0047] In one embodiment of a scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or an asymmetry in the detection configuration, the asymmetry being related to the degree of overlap. The two (typically overlapping) grating structures can be applied to two different layers (not necessarily consecutive layers) and can be formed at substantially the same location on the wafer. The scatterometer can have a symmetrical detection configuration, such as that described in commonly owned patent application EP1,628,164A, so that any asymmetry can be clearly distinguished. This provides a straightforward method for measuring grating misalignment. Other examples of measuring overlay error between two layers comprising periodic structures when measuring targets through asymmetry of the periodic structures can be found in PCT Patent Application Publication No. WO 2011 / 012624 or US Patent Application US 20160161863, the entire contents of which are incorporated herein by reference.

[0048] Other parameters of interest may be focus and dose. Focus and dose can be determined simultaneously using a scatterometer (or alternatively, a scanning electron microscope), as described in U.S. Patent Application No. US2011-0249244, the entire contents of which are incorporated herein by reference. A single structure can be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the focus energy matrix (FEM—also known as the focus exposure matrix). If these unique combinations of critical dimensions and sidewall angles are available, focus and dose values ​​can be uniquely determined based on these measurements.

[0049] The metrology target can be a composite grating formed by the photolithography process, primarily in the resist, but also, for example, after an etching process. Typically, the pitch and linewidth of the structures in the grating are largely determined by the measurement optics (particularly the optical aperture) to capture the diffraction orders from the metrology target. As previously described, the diffraction signal can be used to determine the drift between two layers (also known as "overlay") or to reconstruct at least a portion of the original grating produced by the photolithography process. This reconstruction can provide guidance on the quality of the photolithography process and can be used to control at least part of the photolithography process. The target can have smaller subsegments configured to mimic the dimensions of functional portions of the design layout within the target. Due to this subsegmentation, the target's behavior will more closely resemble that of the functional portions of the design layout, resulting in overall process parameter measurements that better resemble those of the functional portions of the design layout. The target can be measured in either an underfill mode or an overfill mode. In underfill mode, the spot size generated by the measurement beam is smaller than the overall target. In overfill mode, the spot size generated by the measurement beam is larger than the overall target. In this overfill mode, different targets can also be measured simultaneously, thereby simultaneously determining different process parameters.

[0050] The overall measurement quality of a lithography parameter using a particular target is determined at least in part by the measurement profile used to measure the lithography parameter. The term "substrate measurement profile" may include one or more parameters of the measurement itself, one or more parameters of one or more measured patterns, or both. For example, if the measurement used in the substrate measurement profile is a diffraction-based optical measurement, the one or more parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and the like. For example, one of the criteria for selecting a measurement profile may be the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US 2016 / 0370717A1, the entire contents of which are incorporated herein by reference.

[0051] Figure 4 A measurement device, such as a scatterometer, is depicted. The measurement device comprises a broadband (white light) radiation projector 2, which projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectral detector 4, which measures the spectrum 10 of the specularly reflected radiation (i.e., a measurement of the intensity as a function of wavelength λ). From this data, for example, a scatterometer is used as in Figure 3The processing unit PU can reconstruct the structure or profile that produced the detected spectrum using rigorous coupled wave analysis and nonlinear regression, as shown at the bottom of the figure, or by comparison with a library of simulated spectra. Typically, for reconstruction, the general form of the structure is known, and some parameters are set based on knowledge of the process that formed the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. This scatterometer can be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0052] The overall measurement quality of a lithography parameter via measurement of a measurement target is determined at least in part by the measurement profile used to measure the lithography parameter. The term "substrate measurement profile" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement profile is a diffraction-based optical measurement, the one or more parameters measured may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and the like. One of the criteria for selecting a measurement profile may, for example, be the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. patent application US2016 / 0161863 and published U.S. patent application US2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0053] Another type of metrology tool used in IC manufacturing is a topography measurement system, level sensor, or height sensor. Such a tool can be integrated into a lithographic apparatus to measure the topography of a substrate (or wafer). A topography map (also called a height map) of the substrate can be generated from these measurements indicating the height of the substrate as a function of position on the substrate. This height map can then be used to correct the position of the substrate during transfer of the pattern onto the substrate so as to provide an aerial image of the pattern forming device at an appropriate focus position on the substrate. It will be understood that "height" in this context refers to a dimension that is significantly out of plane (also called the Z-axis) relative to the substrate. Typically, the level sensor or height sensor performs measurements at a fixed location (relative to its own optical system), and the relative motion between the substrate and the optical system of the level sensor or height sensor produces height measurements at multiple locations across the substrate.

[0054] Figure 5An example of a level sensor or height sensor LS known in the art is shown schematically, which merely illustrates the operating principle. In this example, the level sensor comprises an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO, which provides a radiation beam LSB imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO can, for example, be a narrowband light source or a broadband light source, such as a supercontinuum light source, a polarized or unpolarized radiation source, a pulsed or continuous radiation source, such as a polarized or unpolarized laser beam. The radiation source LSO can comprise a plurality of radiation sources of different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but can additionally or alternatively encompass UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0055] Projection grating PGR is a periodic grating comprising a periodic structure that generates a radiation beam BE1 having a periodically varying intensity. This periodically varying intensity radiation beam BE1 is directed toward a measurement position MLO on a substrate W at an angle of incidence ANG between 0 and 90 degrees, typically between 70 and 80 degrees, relative to an axis perpendicular to the substrate surface (the Z axis). At measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed toward a detection unit LSD.

[0056] To determine the height level at the measurement location MLO, the level sensor further includes 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 can be equivalent to the projection grating PGR. The detector DET generates a detector output signal indicative of the received light, for example, a detector output signal indicative of the intensity of the received light (e.g., a photodetector) or a detector output signal representative of the spatial distribution of the received intensity (e.g., a camera). The detector DET can include any combination of one or more detector types.

[0057] By means of triangulation techniques the height level at the measurement position MLO can be determined. The detected height level is generally related to the signal intensity measured by the detector DET, which has a periodicity that depends inter alia on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0058] The projection unit LSP and / or the detection unit LSD may comprise further optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0059] In an embodiment, the detection grating DGR can be omitted and the detector DET can be placed where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0060] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0061] Various general types of height sensors are disclosed, for example, in US7265364 and US7646471, both of which are incorporated by reference. A height sensor that uses UV radiation rather than visible or infrared radiation is disclosed in US2010233600A1, which is incorporated by reference. In WO2016102127A1, incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without requiring a detection grating.

[0062] Another type of metrology tool used in IC manufacturing is the alignment sensor. A key aspect of the performance of a lithographic apparatus (either the same apparatus or a different one) is its ability to correctly and accurately place an applied pattern relative to features laid down in previous layers. To this end, 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 an "alignment sensor," and the mark may be referred to as an "alignment mark."

[0063] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors that accurately measure the positions of alignment marks provided on a substrate or wafer. Alignment sensors (or position sensors) may utilize optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. Examples of alignment sensors currently used in lithographic apparatus are based on self-referencing interferometers, as described in US Pat. No. 6,961,116. Various enhancements and modifications to position sensors have been developed, for example, as disclosed in US Pat. No. 2015,261,097, A1. All of these disclosures are incorporated herein by reference.

[0064] Figure 6FIG2 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as that described in, for example, US Pat. No. 6,961,116, incorporated by reference. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is steered by steering optics onto a mark (such as mark AM on a substrate W) as an illumination spot SP. In this example, the steering optics include a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP used to illuminate the mark AM can be slightly smaller than the width of the mark itself.

[0065] Radiation diffracted by alignment mark AM is collimated (in this example via objective lens OL) into an information-bearing beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI (e.g., of the type disclosed in the aforementioned US Pat. No. 6,961,116) causes beam IB to interfere with itself, after which it is received by photodetector PD. In the event that more than one wavelength is generated by radiation source RSO, additional optics (not shown) may be included to provide separate beams. If desired, the photodetector may be a single element, or it may comprise multiple pixels. The photodetector may comprise an array of sensors.

[0066] The steering optics, which in this example comprise a spot mirror SM, can also be used to block zero-order radiation reflected from the mark so that the information-carrying beam IB comprises only higher-order diffracted radiation from the mark AM (which is not necessary for the measurement but improves the signal-to-noise ratio).

[0067] The intensity signal SI is provided to the processing unit PU. Through a combination of optical processing in block SRI and computational processing in unit PU, the values ​​of the X position and Y position on the substrate relative to the reference frame are output.

[0068] A single measurement of the type shown only fixes the position of the mark within a specific range corresponding to one pitch of the mark. In conjunction with this, a coarse measurement technique is used to identify which period of the sine wave includes the position of the mark. The same process can be repeated at coarser or finer levels with different wavelengths to improve accuracy and / or robustly detect the mark, regardless of the material the mark is made of and what materials are disposed above and / or below it. The wavelengths can be optically multiplexed and demultiplexed for simultaneous processing, and / or they can be multiplexed using time or frequency division.

[0069] In this example, the alignment sensor and light spot SP remain stationary, while the substrate W moves. Thus, the alignment sensor can be fixedly and accurately mounted to a reference frame while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is mounted on a substrate support and controlled during this movement by a substrate positioning system that controls the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In one embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the position of the marks provided on the substrate support allows calibration of the position of the substrate support determined by the position sensor (e.g., relative to a frame to which the alignment system is attached). Measuring the position of the alignment marks provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0070] Figure 7 shows a lithographic apparatus comprising a radiation source SO and a photolithographic apparatus LA. Figure 1 An alternative lithography system to the lithography system of FIG. A radiation source SO is configured to generate an EUV radiation beam B and provide the EUV radiation beam B to a lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0071] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Thereby, the illumination system IL may comprise a faceted field mirror arrangement 10 and a faceted pupil mirror arrangement 11. The faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11 together provide a desired cross-sectional shape and a desired intensity distribution for the EUV radiation beam B. The illumination system IL may comprise other mirrors or arrangements in addition to or instead of the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11.

[0072] After this conditioning, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To this end, the projection system PS may comprise a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thereby forming an image having features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although in Figure 1The projection system PS is shown in FIG. 2 with only two mirrors 13 , 14 , but the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0073] The substrate W may include a previously formed pattern. In this case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0074] A relative vacuum, ie, a small amount of gas (eg, hydrogen) at a pressure well below atmospheric pressure, may be provided in radiation source SO, illumination system IL, and / or projection system PS.

[0075] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL), or any other radiation source capable of generating EUV radiation.

[0076] Figure 8 The imaging branch of a dark-field digital holographic microscope (df-DHM) 800 according to an embodiment is schematically illustrated. The dark-field digital holographic microscope (df-DHM) comprises an imaging branch and an illumination branch. In this embodiment, a metrology target 860 comprising structures on a substrate 850 is illuminated by two radiation illumination beams: a first radiation illumination beam 810 and a second radiation illumination beam 820. In embodiments, these two illumination beams 810 and 820 can illuminate the metrology target 860 simultaneously.

[0077] In one embodiment, a first illumination beam 810 may be incident on a measurement target 860 at a first angle of incidence along a first direction relative to the optical axis OA. A second illumination beam 820 may be incident on the measurement target 860 at a second angle of incidence along a second direction relative to the optical axis OA. The first angle of incidence of the first illumination beam 810 and the second angle of incidence of the second illumination beam 820 may be substantially the same. For example, the angle of incidence of each illumination beam may be within a range of 70 to 90 degrees, within a range of 50 to 90 degrees, within a range of 30 to 90 degrees, or within a range of 10 to 90 degrees. Irradiating the measurement target 860 may cause radiation to scatter from the target. In one embodiment, the first illumination beam 810 may be incident on the measurement target 860 at a first azimuth angle corresponding to the first direction. The second illumination beam 820 may be incident on the measurement target 860 at a second azimuth angle corresponding to the second direction. The first azimuth angle of the first illumination beam 810 and the second azimuth angle of the second illumination beam 820 can be different; for example, opposite angles that are 180 degrees apart.

[0078] Depending on the structure of the measurement target 860, the scattered radiation may include reflected radiation, diffracted radiation, or transmitted radiation. In this embodiment, the measurement target may be an overlapping target based on diffraction; and each illumination beam may correspond to a scattered beam including at least one non-zero diffraction order. Each scattered beam carries information about the measurement target being illuminated. For example, the first illumination beam 810 may correspond to a scattered beam including a positive first diffraction order +1 st The first scattered beam 811 of DF; the second illumination beam 820 may correspond to the first diffraction order including the negative -1 st Second scattered beam 821 of DF. The zeroth diffraction order and other undesirable diffraction orders can be blocked by a beam stop element (not shown) or configured to fall completely outside the NA of objective lens 870. Therefore, df-DHM can operate in dark field mode. Note that in some embodiments, one or more optical elements (e.g., lens combinations) can be used to achieve the same optical effect of objective lens 870.

[0079] Both scattered beams 811, 821 may be collected by an objective lens 870 and subsequently refocused onto an image sensor 880. The objective lens 870 may include multiple lenses, and / or the df-DHM 800 may include a lens having two or more lenses (e.g., an objective lens and an imaging lens, similar to Figure 9 An exemplary df-DHG lens system (e.g., an 812-μm df-DHG lens system) is employed, thereby defining the pupil plane of the objective lens between the two lenses and the image plane at the focus of the imaging lens. In this embodiment, a portion 812 of the first scattered beam 811 and a portion 822 of the second scattered beam 821 are simultaneously incident on the image sensor 880 at a common location. Simultaneously, two reference radiation beams (i.e., a first reference beam 830 and a second reference beam 840) are incident on the same location on the image sensor 880. These four beams can be grouped into two pairs of scattered radiation and reference radiation. For example, a first scattered beam-reference beam pair can include portion 812 of the first scattered beam 811 and the first reference beam 830. Similarly, portion 822 of the second scattered beam-reference beam pair can include the second scattered beam 821 and the second reference beam 840. These two scattered beam-reference beam pairs can then form two interference patterns (holographic images) that at least partially overlap in the spatial domain.

[0080] In one embodiment, to separate two at least partially spatially overlapping interference patterns (e.g., in the spatial frequency domain), first reference beam 830 may have a first angle of incidence relative to optical axis OA, and second reference beam 840 may have a second angle of incidence relative to optical axis OA; the first angle of incidence and the second angle of incidence are different. Alternatively or additionally, first reference beam 830 may have a first azimuth angle relative to optical axis OA, and second reference beam 840 may have a second azimuth angle relative to optical axis OA; the first azimuth angle and the second azimuth angle are different.

[0081] To generate an interference pattern, the two beams of each scattered-reference beam pair should be at least partially coherent with each other to a sufficient degree to form an interference pattern. Note that each scattered radiation beam may have a phase offset relative to its corresponding illuminating radiation. For example, at the image plane of image sensor 880, this phase offset may include contributions due to the optical path length (OPD) between measurement target 860 and image sensor 880, as well as interactions with the measurement target. As described above, it is necessary to control the coherence between the first scattered-reference beam pair and the second scattered-reference beam pair so that each beam in one pair is incoherent with any beam in the other pair. In other words, interference should only occur between beams within the same beam, and interference should be suppressed between different beam pairs. In this way, only the desired interference pattern (e.g., the two interference patterns formed by the respective scattered-reference beam pairs) is formed in a superimposed manner on image sensor 880, thereby avoiding the problem of separating or removing undesired interference patterns.

[0082] A measurement tool MT (such as a scatterometer, a topography measurement system, a position measurement system or a DHM as described above) can use radiation originating from a radiation source to perform measurements. The nature of the radiation used by the measurement tool may affect the type and quality of the measurements that may be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example broadband radiation may be used. Multiple different frequencies may be able to propagate, illuminate and scatter the measurement target with no or minimal interference from other frequencies. Thus, for example, more measurement data may be obtained simultaneously using different frequencies. Different radiation frequencies may also be able to query and discover different properties of the measurement target. Broadband radiation may be used for a measurement system MT, such as, for example, a level sensor, an alignment mark measurement system, a scatterometry tool, or an inspection tool. The broadband radiation source may be a supercontinuum source.

[0083] Many of the optical metrology techniques, lithography techniques, and associated metrology and lithography tools described above can use their desired or expected radiation to measure the spectral characteristics of the radiation used in the tool, for example, to perform appropriate measurements or exposures. By way of specific example, some of these tools can use spatially coherent, quasi-monochromatic light; for example, a specific example of this is digital holographic microscopy (DHM). The remainder of this disclosure will be described in terms of DHM, but it will be clear that the concepts disclosed herein can be used to determine the spectral characteristics of any radiation source or any radiation beam used in any of the tools and devices described herein.

[0084] In DHM, the central wavelength is tunable and can be varied over a wide range (e.g., from visible to infrared (IR) wavelengths). The bandwidth of the radiation can be on the order of a few nanometers and correspond to a temporal coherence length of tens of micrometers. The limited bandwidth of the radiation used in metrology sensors such as DHM can adversely affect signal formation. In some cases, this can be corrected computationally, but this requires accurate knowledge of the central wavelength and spectral shape of the quasi-monochromatic light used in the measurement.

[0085] Existing methods for measuring spectral characteristic data, such as central wavelength and spectral shape, involve the use of spectrometers with dispersive elements. To achieve sufficient spectral resolution, gratings are often used as dispersive elements. Spectrometers are capable of achieving sufficient accuracy levels for metrological purposes. However, in addition to the grating, such spectrometers require additional optical elements to direct the beam at a well-defined angle toward the dispersive grating, as well as collection optics to collect the diffracted light and project it onto a detector array. Achieving this with the required accuracy over a wide wavelength range places relatively high demands on the spectrometer design, often resulting in high complexity and cost.

[0086] Another known method for measuring the desired spectral characteristic data includes Fourier-transform spectroscopy. In Fourier-transform spectroscopy, a Michelson interferometer is used to measure the interference pattern as a function of the optical path difference (OPD) between two interfering beams. Fourier transforming this interference pattern yields the spectral shape of the light source. Fourier-transform spectroscopy is a proven high-resolution spectroscopy technique. However, it requires highly accurate motorized translation of at least one mirror (or other optical element) to change the OPD, which is difficult to achieve.

[0087] Methods and apparatus for achieving highly accurate Fourier transform spectrometry without the use of moving parts requiring high-precision control will be described. Specifically, the Fourier transform spectrometry method described herein can also include no optics between the light source (other than the light source for splitting the light into two point sources, as will be described) and the detector for detecting the resulting interferogram data from which desired spectral characteristic data can be inferred. Such optics can be used in prior art arrangements to linearize the relationship between the OPD and the position on the detector. However, such optics can themselves be sources of error due to aberrations and imperfections, and can also increase the cost and complexity of the overall arrangement.

[0088] The basic concept is essentially based on Young's double-slit experiment. The radiation being measured originates from a common source and is split into two point sources: a first source emitting a first diverging beam and a second source emitting a second diverging beam. The first and second diverging beams substantially overlap, with the overlapping beams directly illuminating a detector (which can, for example, comprise a pixel array). Thus, light arrives at each detector pixel (at least in the detector's overlapping region) by arriving from each of the two point sources, but the two beams follow different optical path lengths for each detector pixel.

[0089] While the optical path difference (OPD) between the beams differs for each pixel, the relationship between OPD and pixel / detection position is not linear. Therefore, a processing step is proposed to perform a nonlinear correction to linearize the captured interferogram data. The interferogram data can include intensity values ​​at the detector caused by the interference between the two beams. The processed or linearized interferogram data can then be Fourier transformed to determine the expected spectral characteristics of the measured radiation.

[0090] Figure 9 is a schematic diagram of an arrangement according to the concepts / principles disclosed herein. Radiation to be measured can be coupled from a radiation source into a (e.g., single-mode) waveguide beam splitter WS (or other suitable beam splitting arrangement) to define a first radiation source or point source and a second radiation source or point source from a common radiation source, the two sources being separated by a distance D. Both radiation sources can be spatially coherent. For example, the radiation sources can both lie in a plane parallel to the detection plane (e.g., the plane defined by detector DET). Each radiation source can emit a respective first diverging beam B1 and a second diverging beam B2.

[0091] The splitting ratio applied by the waveguide beam splitter WS is not critical but may be approximately 50:50.

[0092] Optionally, the waveguide beam splitter WS can be configured such that, in the case where the detector is a rectangular (i.e. oblong) or linear detector whose longer dimension extends substantially in the direction of the first detection plane, the first divergent beam B1 and the second divergent beam B2 have a greater divergence in the direction of the first detection plane than in the direction of the second detection plane.

[0093] Thus, the divergence of each beam can be in the direction of the first detection plane (eg, marked as x in the figures). c The detector DET may be large in the direction of x in the detection plane and small or minimized in the direction of the second detection plane (eg, in the direction perpendicular to the plane of the depicted figure). c direction (e.g., a high-resolution array of pixels) and perpendicular to the x-axis in the detection plane. cThe beams may comprise only a very small number of pixels (e.g., one or more) in the direction of the first detection plane. For example, the detector DET may be a linear detector (e.g., a 1D pixel array). The beams may be arranged to maximize the overlap area on the detector DET, or to ensure a larger overlap area on the detector DET, at least in the direction of the first detection plane.

[0094] The distance D between the two waveguides directly determines the OPD between the interfering beams on the detector array. In one embodiment, sufficient spectral resolution can be achieved with an OPD of at least 400 μm, which can be achieved when D is on the order of millimeters (e.g., between 1 mm and 10 mm, between 2 mm and 8 mm, between 2 mm and 6 mm, or between 3 mm and 5 mm; e.g., approximately 4 mm).

[0095] The diverging beams (e.g., spherical beams) B1, B2 emitted from the waveguide aperture travel through free space over a distance H. The distance H can be on the order of hundreds of millimeters, such as between 100 mm and 400 mm, between 100 mm and 300 mm, between 150 mm and 300 mm, or between 150 mm and 250 mm; for example, about 200 mm.

[0096] The optical path difference OPD between the interfering beams on the image sensor array can be given by:

[0097]

[0098] where x c Describes a detection position in at least a first detection plane direction of a detection plane.

[0099] Near the center of the detector DET (x c ≪H), OPD will change with the position x on the detector c However, in order to obtain a sufficiently large OPD, it is necessary to c Measurements are made where higher order nonlinear terms in the above equations become relevant.

[0100] To address this, a processing step is proposed to perform a linearization correction or distortion correction to remove the nonlinear terms. This can be implemented computationally (e.g. using a processor or processing unit PU) to determine a linear representation of the detected interferogram (linearized interferogram data) such that it represents the OPD versus the detected position variable x in the first detection direction. c This linearization correction can be achieved according to the equation for OPD provided above, noting that the relevant parameters (the distance H between the source and the detector, the detector position x c, and the point source separation D) are well defined and accurately known. Based on this, a linearization correction can be determined, which effectively replaces the detection position variable x with the transformed position c , the transformed position is the linearization of the OPD. This correction can then be applied to the detected interferogram data to obtain corrected or linearized interferogram data.

[0101] Figure 10 1 is a flow chart describing the processing steps for processing detected interferogram data IGM according to the method disclosed herein. A linearization step LIN or a distortion correction step is performed on the detected interferogram data IGM to remove nonlinear terms and determine linearized interferogram data. This linearized interferogram data is then Fourier transformed (e.g., via a Fast Fourier Transformation (FFT)) into the Fourier domain, which directly yields spectral characteristic data SP.

[0102] An example of the linearization step will now be described. The reason for the linearization will be explained in the context of the following idealized example: for a wave with wavelength λ c For a monochromatic beam, the intensity detected on the detector array is given by:

[0103]

[0104] According to the following formula, the optical path difference OPD (x c ) depends (at least approximately) on the position x in the detector array c :

[0105]

[0106] Due to the third-order and fifth-order terms in this expression, the detected interference pattern will not be a pure sinusoidal shape. Compared to the sinusoidal intensity variation detected near OPD=0, the sinusoidal intensity variation will be more exaggerated for larger OPD values. Therefore, in the case of the detected intensity I d (x c ) The measured spectrum obtained after Fourier transformation will not include the single spatial frequency 1⁄λ c , but as Figure 11 (a) is widened, Figure 11 (a) is the Fourier transformed signal level (arbitrary units) of the nonlinear interference pattern and 1⁄λ c Graph of the curve.

[0107] Now combine Figure 11 (b) Explanation of the purely exemplary linearization algorithm. The black horizontal arrows indicate the linearly increasing OPD scale. The non-equidistant dashed lines above the OPD scale indicate the position x in the detector array. cThe OPD values ​​corresponding to the pixels at locations 'A' and 'B' are given in bold. The dashed lines below the OPD scale represent equidistant samples of OPD. The linearization step aims to find the intensities (or other measured parameters) at these OPD locations. This can be achieved using established interpolation techniques. For example, the intensity at OPD location 'C' can be determined by linearly interpolating between the measured intensities at samples 'A' and 'B' according to the following equation:

[0108]

[0109] where the weight coefficient w is given by the relative OPD distance between samples B and C:

[0110]

[0111] This linear interpolation can be performed for all intensity samples, resulting in a corrected interference pattern which, in the case of monochromatic light, will vary perfectly sinusoidally according to:

[0112]

[0113] Figure 11 (c) is related to Figure 11 The equivalent graph of (a) but for the Fourier-transformed interpolated signal. The result is spectral lines that are no longer broadened due to the nonlinear sampling of the OPD.

[0114] The reconstruction step can also include optimizing the sampling grid (i.e., resolution and / or number of samples) to further sharpen the peaks of the Fourier-transformed interpolated signal. This can be combined with iterative refinement to improve the accuracy of the reconstructed spectrum. Alternatively or additionally, the reconstruction step can also include adjusting the number of samples to a power of 2 (or other low prime factorization). This improves the computational speed of the FFT.

[0115] It will be appreciated that other (eg more complex) interpolation methods are also possible, such as quadratic interpolation or spline interpolation. Linear interpolation as described herein was chosen for its simplicity and effectiveness.

[0116] Thus, the concept disclosed herein provides a simple and low-cost method that can be implemented due to the absence of intervening optics (e.g. lenses) between the point source and the detector and due to the relative sizes (particularly the detection position x c Since both the source separation (D) and the point source separation (D) are precisely known, high accuracy can be inherently achieved. This is particularly true when the waveguide beam splitter and the waveguides of the image sensor are photolithographically defined. The distance H between the waveguide and the detector array can also be determined with very good accuracy.

[0117] While the described embodiments may not be very light efficient, this is not expected to be a problem since speed of application is not critical for the intended application and averaging can be performed over many frames to improve the signal to noise ratio.

[0118] While the above-described embodiments use waveguides to perform beam splitting (eg, a waveguide beam splitter), beam splitting may be achieved in any suitable manner using any suitable beam splitting arrangement that produces two (eg, spherical) beams (eg, a prism-based beam splitting arrangement).

[0119] To maximize the detected spectral range, the detector can actually include two or more detectors, each for a different wavelength range. For example, two linear image sensor arrays can be arranged overlapping (one above the other) or directly adjacent to each other: one for a first wavelength range (e.g., the visible range) and the other for a second wavelength range (e.g., the IR range).

[0120] Figure 12 1100 is a block diagram illustrating a computer system 1100 that can facilitate implementation of the methods and processes disclosed and / or claimed herein. Computer system 1100 includes a bus 1102 or other communication mechanism for communicating information, and a processor 1104 (or multiple processors 1104 and 1105) coupled to bus 1102 for processing information. Computer system 1100 also includes a main memory 1106, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 1102 for storing instructions and information to be executed by processor 1104. Main memory 1106 can also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by processor 1104. Computer system 1100 also includes a read-only memory (ROM) 1108 or other static storage device coupled to bus 1102 for storing static information and instructions for processor 1104. A storage device 1110, such as a magnetic disk or optical disk, is provided and coupled to bus 1102 for storing information and instructions.

[0121] The computer system 1100 can be coupled via bus 1102 to a display 1112 for displaying information to a computer user, such as a cathode ray tube (CRT) or a flat-panel display or a touch panel display. An input device 1114, including alphanumeric keys and other keys, is coupled to bus 1102 for communicating information and command selections to processor 1104. Another type of user input device is a cursor control 1106, such as a mouse, trackball, or cursor direction keys, for communicating directional information and command selections to processor 1104 and for controlling cursor movement on display 1112. Such input devices typically have two degrees of freedom along two axes, namely a first axis (e.g., x) and a second axis (e.g., y), which allow the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0122] One or more of the methods described herein can be performed by the computer system 1100 in response to the processor 1104 executing one or more sequences of one or more instructions contained in the main memory 1106. These instructions can be read into the main memory 1106 from another computer-readable medium, such as the storage device 1110. Execution of the sequences of instructions contained in the main memory 1106 causes the processor 1104 to perform the process steps described herein. One or more processors in a multi-processing arrangement can also be employed to execute the sequences of instructions contained in the main memory 1106. In alternative embodiments, hard-wired circuitry can be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0123] As used herein, the term "computer-readable medium" refers to any medium that participates in providing instructions to processor 1104 for execution. Such media can take many forms, including, but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1110. Volatile media include dynamic memory, such as main memory 1106. Transmission media include coaxial cables, copper wire, and optical fiber, including the wires comprising bus 1102. Transmission media can also take the form of sound or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. For example, common forms of computer-readable media can include floppy disks, flexible disks, hard disks, magnetic tape, any other magnetic medium, CD-ROMs, DVDs, any other optical medium, punch cards, paper tape, any other physical medium with a pattern of holes, RAM, PROMs and EPROMs, FLASH-EPROMs, any other memory chip or cartridge, a carrier wave as described below, or any other medium from which a computer can read.

[0124] Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to the processor 1104 for execution. For example, the instructions may initially be carried on a disk of a remote computer. The remote computer may load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to the computer system 1100 may receive the data on the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to the bus 1102 may receive the data carried in the infrared signal and place the data on the bus 1102. The bus 1102 carries the data to the main memory 1106, from which the processor 1104 retrieves and executes the instructions. The instructions received by the main memory 1106 may optionally be stored on the storage device 1110 before or after execution by the processor 1104.

[0125] The computer system 1100 also preferably includes a communication interface 1118 coupled to the bus 1102. The communication interface 1118 provides a two-way data communication coupling to a network link 1120 connected to a local area network 1122. For example, the communication interface 1118 can be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, the communication interface 1118 can be a local area network (LAN) card to provide a data communication connection to a compatible LAN. A wireless link can also be implemented. In any such implementation, the communication interface 1118 sends and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information.

[0126] Network link 1120 typically provides data communication through one or more networks to other data devices. For example, network link 1120 may provide a connection through local network 1122 to a host computer 1124 or to data equipment operated by an Internet Service Provider (ISP) 1126. ISP 1126, in turn, provides data communication services through the global packet data communication network, now commonly referred to as the "Internet" 1128. Both local network 1122 and Internet 1128 use electrical, electromagnetic, or optical signals to carry digital data streams. The signals passing through the various networks and the signals on network link 1120 and through communication interface 1118, which carry the digital data to and from computer system 1100, are exemplary forms of carrier waves transporting the information.

[0127] Further embodiments are disclosed in the following numbered item lists:

[0128] 1. A Fourier transform spectrometer, comprising:

[0129] a beam splitting arrangement operable to define a first radiation source and a second radiation source from a common radiation source,

[0130] at least one detector operable to detect interferogram data as a function of detection position in at least a first detection plane direction of a detection plane, the interferogram data resulting from interference of a first diverging beam emitted from the first radiation source and a second diverging beam emitted from the second radiation source; and

[0131] A processor, the processor being operable to:

[0132] performing a linearization correction on the interferogram data to obtain linearized interferogram data; and

[0133] The linearized interferogram data is Fourier transformed to obtain spectral characteristic data associated with the common radiation source.

[0134] 2. The Fourier transform spectrometer of clause 1 comprising no intervening optics between the beam splitting arrangement and the at least one detector, such that the detector is arranged to directly detect the first diverging beam and the second diverging beam.

[0135] 3. A Fourier transform spectrometer according to clause 1 or 2, wherein the processor is operable to perform a linearization correction by transforming the interferogram data into the linearized interferogram data, so that the linearized interferogram data has a linearized relationship with the optical path difference between the first divergent beam and the second divergent beam.

[0136] 4. A Fourier transform spectrometer according to any preceding clause, wherein the beam splitting arrangement is configured such that the divergence of the first and second diverging beams is greater in the direction of the first detection plane relative to a second detection plane direction of the detection plane.

[0137] 5. The Fourier transform spectrometer of clause 4, wherein the beam splitting arrangement is configured such that a divergence of the first diverging beam and the second diverging beam is minimized in the direction of the second detection plane.

[0138] 6. A Fourier transform spectrometer according to clause 4 or 5, wherein the at least one detector comprises a rectangular or linear detector having a larger dimension extending substantially in the direction of the first detection plane.

[0139] 7. The Fourier transform spectrometer of any preceding clause, wherein the at least one detector comprises two or more detectors, each for a respective different wavelength range.

[0140] 8. The Fourier transform spectrometer of any preceding clause, wherein the distance between the first radiation source and the second radiation source in a plane parallel to the detection plane is between 1 mm and 10 mm.

[0141] 9. The Fourier transform spectrometer of any preceding clause, wherein the distance between the first radiation source and the second radiation source in a direction parallel to the detection plane is between 2 mm and 6 mm.

[0142] 10. The Fourier transform spectrometer of any preceding clause, wherein the distance between the beam splitting arrangement and the at least one detector in a direction perpendicular to the detection plane is between 100 mm and 400 mm.

[0143] 11. A Fourier transform spectrometer according to any preceding clause, wherein the beam splitting arrangement comprises a waveguide beam splitter.

[0144] 12. A Fourier transform spectrometer according to any preceding clause, wherein the spectral characteristic data comprises a central wavelength value and a spectral shape of the radiation emitted by the common radiation source.

[0145] 13. A Fourier transform spectrometer according to any preceding clause, wherein the processor is operable to perform the linearisation correction by interpolating the detected interferogram data.

[0146] 14. A Fourier transform spectrometer comprising:

[0147] a beam splitting arrangement operable to define a first radiation source and a second radiation source from a common radiation source,

[0148] at least one detector operable to detect interferogram data describing an interference pattern as a function of detection position in at least a first detection plane direction of a detection plane, the interferogram data resulting from interference of a first diverging beam emitted from the first radiation source and a second diverging beam emitted from the second radiation source;

[0149] wherein the Fourier transform spectrometer comprises no intervening optics between the beam splitting arrangement and the at least one detector, such that the detector is arranged to directly detect the interference pattern in an overlapping region of the first diverging beam and the second diverging beam.

[0150] 15. The Fourier transform spectrometer of clause 14, comprising a processor operable to:

[0151] performing a linearization correction on the interferogram data to obtain linearized interferogram data; and

[0152] The linearized interferogram data is Fourier transformed to obtain spectral characteristic data associated with the common radiation source.

[0153] 16. A Fourier transform spectrometer according to item 15, wherein the processor is capable of performing a linearization correction by transforming the interferogram data into the linearized interferogram data, so that the linearized interferogram data has a linearized relationship with the optical path difference between the first divergent beam and the second divergent beam.

[0154] 17. A Fourier transform spectrometer according to clause 15 or 16, wherein the processor is operable to perform the linearisation correction by interpolating the detected interferogram data.

[0155] 18. A Fourier transform spectrometer according to any of clauses 14 to 17, wherein the beam splitting arrangement is configured such that the divergence of the first and second diverging beams is greater in the direction of the first detection plane relative to in the direction of a second detection plane of the detection plane.

[0156] 19. The Fourier transform spectrometer of clause 18, wherein the beam splitting arrangement is configured such that a divergence of the first diverging beam and the second diverging beam is minimized in the direction of the second detection plane.

[0157] 20. A Fourier transform spectrometer according to clause 18 or 19, wherein the at least one detector comprises a rectangular or linear detector having a larger dimension extending substantially in the direction of the first detection plane.

[0158] 21. The Fourier transform spectrometer of any of clauses 14 to 20, wherein the at least one detector comprises two or more detectors, each for a respective different wavelength range.

[0159] 22. The Fourier transform spectrometer of any of clauses 14 to 21, wherein the first radiation source and the second radiation source are separated by a distance in a plane parallel to the detection plane of between 1 mm and 10 mm.

[0160] 23. The Fourier transform spectrometer of any of clauses 14 to 22, wherein the distance between the first radiation source and the second radiation source in a direction parallel to the detection plane is between 2 mm and 6 mm.

[0161] 24. A Fourier transform spectrometer according to any of clauses 14 to 23, wherein the distance between the beam splitting arrangement and the at least one detector in a direction perpendicular to the detection plane is between 100 mm and 400 mm.

[0162] 25. A Fourier transform spectrometer according to any of clauses 14 to 24, wherein the beam splitting arrangement comprises a waveguide beam splitter.

[0163] 26. A Fourier transform spectrometer according to any of clauses 14 to 25, wherein the spectral characteristic data comprises a central wavelength value and a spectral shape of the radiation emitted by the common radiation source.

[0164] 27. A measurement device comprising a Fourier transform spectrometer according to any of the preceding clauses, the measurement device being operable to measure spectral characteristic data of measurement radiation used by the measurement device when performing measurements.

[0165] 28. The measurement device according to clause 27, comprising a digital holographic microscope.

[0166] 29. A measurement device according to clause 27, comprising a scatterometer.

[0167] 30. A measuring device according to clause 27, comprising an alignment sensor.

[0168] 31. A measuring device according to clause 27, comprising a levelling sensor.

[0169] 32. A lithographic apparatus comprising a metrology device according to clause 30 or 31.

[0170] 33. A lithographic apparatus comprising a Fourier transform spectrometer according to any preceding clause, the lithographic apparatus being operable to measure spectral characteristic data of exposure radiation used by the metrology device when performing an exposure.

[0171] 34. A method comprising:

[0172] detecting interferogram data as a function of a detection position in at least a first detection plane direction of a detection plane, the interferogram data resulting from interference of a first diverging beam emitted from a first radiation source and a second diverging beam emitted from a second radiation source, the first radiation source and the second radiation source originating from a common radiation source; performing a linearization correction on the interferogram data to obtain linearized interferogram data; and

[0173] The linearized interferogram data is Fourier transformed to obtain spectral characteristic data associated with the common radiation source.

[0174] 35. A method according to item 34, wherein the performing linearization correction includes transforming the interference pattern data into the linearized interference pattern data so that the linearized interference pattern data has a linear relationship with the optical path difference between the first divergent beam and the second divergent beam.

[0175] 36. A method according to clause 34 or 35, comprising performing the linearisation correction by interpolating the detected interferogram data.

[0176] 37. A method according to any one of clauses 34 to 36, comprising the initial steps of:

[0177] dividing the common radiation source into the first radiation source and the second radiation source;

[0178] emitting the first diverging beam from the first radiation source;

[0179] The second diverging beam is emitted from the second radiation source.

[0180] 38. A method according to clause 37, comprising propagating the first diverging beam and the second diverging beam directly onto the detection plane through free space only.

[0181] 39. A method according to any of clauses 34 to 38, comprising diverging the first and second diverging beams by a greater amount in the first detection plane direction than in a second detection plane direction of the detection plane.

[0182] 40. The method according to clause 39, comprising minimizing the divergence of the first diverging beam and the second diverging beam in the direction of the second detection plane.

[0183] 41. A method according to any of clauses 34 to 40, wherein the first radiation source and the second radiation source are separated by a distance in a plane parallel to the detection plane of between 1 mm and 10 mm.

[0184] 42. A method according to any of clauses 34 to 40, wherein the distance between the first radiation source and the second radiation source in a direction parallel to the detection plane is between 2 mm and 6 mm.

[0185] 43. A method according to any of clauses 34 to 42, wherein the distance between the beam splitting arrangement and the detection plane in a direction perpendicular to the detection plane is between 100 mm and 410 mm.

[0186] 44. A method according to any one of clauses 34 to 43, wherein the spectral characteristic data includes a central wavelength value and a spectral shape of the radiation emitted by the common radiation source.

[0187] 45. A computer program comprising computer-readable instructions operable to perform the method of clause 34, 35 or 36 when executed on a computer.

[0188] 46. ​​A non-transitory computer medium comprising a computer program according to clause 45.

[0189] The computer system 1100 can send messages and receive data, including process code, via the network, network link 1120, and communication interface 1118. In the Internet example, a server 1130 can transmit the requested code for an application program via the Internet 1128, ISP 1126, local area network 1122, and communication interface 1118. For example, such a download application can be provided for one or more of the techniques described herein. The received code can be executed by the processor 1104 upon receipt and / or stored in the storage device 1110 or other non-volatile storage device for later execution. In this manner, the computer system 1100 can obtain application code in the form of a carrier wave.

[0190] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0191] Although embodiments of the present invention may be specifically referenced herein in the context of lithographic equipment, embodiments of the present invention may be employed in other equipment. Embodiments of the present invention may form mask inspection equipment, metrology equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These equipment may generally be referred to as lithographic tools. Such lithographic tools may utilize vacuum conditions or ambient (non-vacuum) conditions.

[0192] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications such as imprint lithography where the context permits.

[0193] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in other ways than as described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be clear to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A Fourier transform spectrometer, comprising: a beam splitting arrangement operable to define a first radiation source and a second radiation source from a common radiation source, at least one detector operable to detect interference pattern data as a function of detection position in at least a first detection plane direction of a detection plane, the interference pattern data resulting from interference of a first diverging beam emitted from the first radiation source and a second diverging beam emitted from the second radiation source; and A processor, the processor being operable to: performing a linearization correction on the interferogram data to obtain linearized interferogram data; and The linearized interference pattern data is Fourier transformed to obtain spectral characteristic data associated with the common radiation source.

2. The Fourier transform spectrometer of claim 1 comprising no intervening optics between the beam splitting arrangement and the at least one detector, such that the detector is arranged to directly detect the first diverging beam and the second diverging beam.

3. The Fourier transform spectrometer according to claim 1 or 2, wherein the processor is operable to perform a linearization correction by transforming the interference pattern data into the linearized interference pattern data, so that the linearized interference pattern data has a linearized relationship with the optical path difference between the first divergent beam and the second divergent beam.

4. A Fourier transform spectrometer according to any one of the preceding claims, wherein the beam splitting arrangement is configured such that the divergence of the first diverging beam and the second diverging beam is greater in the direction of the first detection plane relative to in the direction of a second detection plane of the detection plane. 5 . The Fourier transform spectrometer of claim 4 , wherein the beam splitting arrangement is configured such that divergences of the first diverging beam and the second diverging beam are minimized in the direction of the second detection plane.

6. A Fourier transform spectrometer according to claim 4 or 5, wherein the at least one detector comprises a rectangular or linear detector having a larger dimension extending substantially in the direction of the first detection plane.

7. A Fourier transform spectrometer according to any one of the preceding claims, wherein the at least one detector comprises two or more detectors, each for a respective different wavelength range.

8. The Fourier transform spectrometer according to any of the preceding claims, wherein the distance between the first radiation source and the second radiation source in a plane parallel to the detection plane is between 1 mm and 10 mm.

9. The Fourier transform spectrometer according to any one of the preceding claims, wherein the distance between the first radiation source and the second radiation source in a direction parallel to the detection plane is between 2 mm and 6 mm.

10. A Fourier transform spectrometer according to any of the preceding claims, wherein the distance between the beam splitting arrangement and the at least one detector in a direction perpendicular to the detection plane is between 100 mm and 400 mm.

11. A Fourier transform spectrometer according to any one of the preceding claims, wherein the beam splitting arrangement comprises a waveguide beam splitter.

12. A Fourier transform spectrometer according to any preceding claim, wherein the processor is operable to perform the linearisation correction by interpolating the detected interferogram data.

13. A measurement device comprising a Fourier transform spectrometer according to any one of the preceding claims, the measurement device being operable to measure data of spectral characteristics of measurement radiation used by the measurement device when performing measurements.

14. A lithographic apparatus comprising the metrology device according to claim 13.

15. A method comprising: detecting interference pattern data as a function of detection position in at least a first detection plane direction of a detection plane, the interference pattern data resulting from interference of a first diverging beam emitted from a first radiation source and a second diverging beam emitted from a second radiation source, the first radiation source and the second radiation source originating from a common radiation source; performing a linearization correction on the interferogram data to obtain linearized interferogram data; as well as The linearized interference pattern data is Fourier transformed to obtain spectral characteristic data associated with the common radiation source.

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