Optical system, measurement device and related method

Abstract

An optical system (400) for focusing a radiation beam (B) onto a region of interest of a substrate in a metrology device is described. The radiation beam comprises radiation in the soft X-ray or extreme ultraviolet spectral range. The optical system comprises a first reflector system (410) and a second reflector system (412). Each of the first reflector system (410) and the second reflector system (412) comprises a finite-to-finite Volta reflector system. The optical system (400) is configured to form a reduced image (414) of an object (416) comprising an apparent source of the radiation beam (B) on the region of interest.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 18195638.4 filed on September 20, 2018, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to optical systems for (but not exclusively) metrology equipment and associated methods. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. A lithographic apparatus may be used, for example, in integrated circuit (IC) manufacturing. A lithographic apparatus may, for example, project a pattern (also often referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

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

[0006] Low k1 lithography can be used to process features with dimensions smaller than the typical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD=k1×λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case it is the half pitch) and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it is to reproduce on a substrate a pattern that resembles the shape and size planned by the circuit designer in order 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 the design layout. These steps include, for example, but are not limited to, optimization of NA, custom illumination schemes, use of phase-shifted pattern forming devices, various optimizations of the design layout, such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement technology" (RET). Alternatively, a strict control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns in low k1 situations.

[0007] During an integrated circuit manufacturing process in which the lithographic apparatus is used, measurements may be performed on structures printed on a substrate for process control and verification purposes. One option for performing measurements on structures may be to use a metrology apparatus or inspection apparatus that applies a beam of soft X-ray (SXR) or EUV radiation that is capable of resolving characteristics of printed structures having multiple sizes, such as may be produced using low-k1 lithography. While various sources may be used to generate beams of SXR or EUV radiation, there are various challenges associated with manipulating the radiation beam in a manner that meets certain criteria. Summary of the invention

[0008] It is an object of the present invention to overcome disadvantages of known systems and / or challenges associated with known systems for steering a radiation beam for (but not exclusively) a metrology device.

[0009] According to one aspect of the present invention, an optical system for focusing a radiation beam on a region of interest of a substrate in a metrology device is provided. The radiation beam may include radiation in the soft X-ray or extreme ultraviolet spectral range. The optical system may include a first reflector system and a second reflector system. Each of the first reflector system and the second reflector system may include a finite-to-finite Walter reflector system. The optical system may be configured to form a reduced image of an object on the region of interest. The object may include an apparent source of the radiation beam.

[0010] The optical system can manipulate the radiation beam in a way that satisfies certain criteria so that measurements can be performed on structures in the area of ​​interest. The finite-to-finite Walter reflector system can utilize the apparent source of the radiation beam at a finite distance from the area of ​​interest to image with imaging performance that satisfies the criteria. This finite-to-finite imaging capability can allow the optical system to be accommodated in a relatively compact volume. In addition, the source of the radiation beam can be provided in a manner very close to the optical system to provide a compact package including the source and the optical system. The Walter reflector system can provide certain benefits in terms of imaging objects that are not optimally aligned relative to the axis of the optical system, and / or when the object being imaged is non-point-shaped. The Walter reflector system can tolerate this non-optimal alignment and / or non-optimal object size to a certain extent, which is greater than the extent that can be achieved by some other reflector systems that utilize imaging SXR or EUV radiation beams. In other words, even if the apparent source is off-axis and / or non-point-shaped (producing a finite (non-zero) field in the object plane), the Walter reflector system can still meet the criteria. Compared to some other optical systems, the use of the Walter reflector system can make strict alignment requirements easier and / or provide greater tolerance for instability or movement of the optical system caused by use or transportation. Therefore, compared to some other reflector systems, the Walter reflector system can be aligned and manufactured with greater stability. The imaging performance of the Walter reflector system can make the aberrations caused by the Walter reflector system less than those produced by some other reflector systems. Compared to some other reflector systems, the Walter reflector system can provide greater flexibility in reducing objects. The reduction ratios provided by each Walter reflector system can be decoupled from each other so that each Walter reflector system can independently provide the desired reduction ratio factor. In an example, the Walter reflector system can be configured to provide isostructural imaging of an object, which may not be possible using some other types of reflector systems.

[0011] Each finite-to-finite Walter reflector system can include a one-dimensional finite-to-finite Walter reflector system, which is configured to reduce the object in one direction acquired along an object plane including the object to form a corresponding reduced image of the object in a corresponding direction acquired along an image plane including the image. The reflector of the type used for the finite-to-finite Walter reflector system can be curved in one direction. Such a reflector can be manufactured in a manner that allows the optical system to provide a surface quality and / or tolerance level that meets the imaging performance criteria. Compared to currently available two-dimensional Walter reflector systems that include reflectors curved in two directions, such a one-dimensional Walter reflector system can provide a lower cost level and / or more satisfactory imaging performance.

[0012] The first reflector system may be configured to reduce the object in a first direction acquired along the object plane to form a corresponding reduced image of the object in a corresponding first direction acquired along the image plane. The second reflector system may be configured to reduce the object in a second direction acquired along the object plane to form a corresponding reduced image of the object in a corresponding second direction acquired along the image plane, the second direction being perpendicular to the first direction.

[0013] The optical system may be configured such that a free working distance defined between components of the optical system and the region of interest is greater than or equal to 10 cm. The free working distance may be such as to allow the reflector to be accommodated without creating volume conflicts with other components and / or radiation beams.

[0014] The optical system may be configured such that the major surfaces of the first reflector system and the second reflector system intersect an axis defined between the object and the image at an intersection point that is closer to the image than to the object. The intersection of the major surfaces with the intersection point that is closer to the image than to the object may provide a reduction factor that may be determined by the distance (s) between the major surfaces and the object. o ) and the distance between the main surface and the image (s i ) is defined by the ratio of s o / s i (s o than i ).

[0015] The major surface of the first reflector system may intersect the axis at substantially the same intersection point as the major surface of the second reflector system. This may provide the same reduction factor for both the first reflector system and the second reflector system, which may form an isomorphic reduced image of the object using the optical system.

[0016] The optical system may be configured such that a demagnification factor of the first reflector system is substantially the same as a demagnification factor of the second reflector system.The demagnification factor may be defined by a ratio between a size of the object and a corresponding size of the image.

[0017] The optical system may be configured to image the object at a desired reduced aspect ratio so as to form a homomorphic image or an anamorphic image of the object.

[0018] The optical system can be configured to reduce at least one dimension of the object by a reduction factor greater than or equal to 5 (optionally greater than or equal to 8, optionally greater than or equal to 10). The Walter reflector system can be capable of achieving relatively high reduction factors while still providing imaging performance that meets the criteria. Reflectors of relatively high curvature can be used to provide high reduction factors. The Walter reflector system configuration can provide acceptable imaging performance even for reflectors of high curvature.

[0019] The first reflector system may be oriented relative to the second reflector system such that at least one plane of incidence that intersects the first reflector system is perpendicular to at least one plane of incidence that intersects the second reflector system.

[0020] The optical system may be configured such that a first incident plane intersects each reflector of the first reflector system and a second incident plane intersects each reflector of the second reflector system, the first incident plane and the second incident plane being perpendicular to each other.

[0021] The optical system can be configured so that the first incident plane intersects the primary reflector of the first reflector system, the second incident plane intersects the primary reflector of the second reflector system, the third incident plane intersects the secondary reflector of the first reflector system, and the fourth incident plane intersects the secondary reflector of the second reflector system, the first incident plane and the third incident plane are not parallel to each other, and the second incident plane and the fourth incident plane are not parallel to each other and are perpendicular to the first incident plane and the third incident plane.

[0022] The optical system can be configured to focus the radiation beam on a region of interest of the substrate so that information about the region of interest can be determined from at least one of radiation reflected, scattered and diffracted by the region of interest, and optionally, the substrate may include an integrated circuit or can be used in the manufacture of an integrated circuit.

[0023] The optical system may include a Walter III type reflector system and one of: an additional Walter III type reflector system, and a Walter I type reflector system. The specific configuration of a Walter type reflector system provided as part of the optical system may be such that space is provided for other elements near the reflector, such as an optional reference detector for measuring a portion of the radiation diffracted from the reflector including the diffractive element.

[0024] The primary reflector of the Walter III reflector system may include an elliptical cylindrical convex reflector.The secondary reflector of the Walter III reflector system may include an elliptical cylindrical concave reflector.

[0025] The primary reflector of the Walter I-type reflector system may include an elliptical cylindrical concave reflector. The secondary reflector of the Walter I-type reflector system may include a hyperbolic cylindrical concave reflector.

[0026] The first reflector system may include a Volta III type reflector system.

[0027] The second reflector system may include an additional Volta III type reflector system.

[0028] The optical system may comprise, in sequence between the object and the image, a Walter Type III reflector system followed by an additional Walter Type III reflector system.

[0029] The optical system may include, in sequence between the object and the image: a primary reflector of a Walter III reflector system, followed by an additional primary reflector of a Walter III reflector system, followed by a secondary reflector of a Walter III reflector system, followed by an additional secondary reflector of a Walter III reflector system.

[0030] The second reflector system may comprise a Volta Type I reflector system. The optical system may comprise, in sequence between the object and the image: a Volta Type III reflector system followed by a Volta Type I reflector system.

[0031] At least one of the first reflector system and the second reflector system may include a diffractive element for spatially separating diffracted spectral components of the radiation beam. For the purpose of metrology measurement, the diffractive element may be monitored and / or used with the performance of the source to analyze the spectral composition of the beam.

[0032] The diffractive element may be arranged as part of a secondary reflector of the first reflector system.

[0033] The optical system can be configured such that the path length of the chief ray defined between the object and the image is less than or equal to 3 meters, optionally less than or equal to 1.5 meters. The relatively short path length compared to some other optical systems can allow the optical system to be accommodated in a relatively compact volume and / or footprint.

[0034] According to an aspect of the present invention, a measurement or inspection device is provided. The measurement or inspection device may include a substrate stage for receiving and holding a substrate in a controllable position. The measurement or inspection device may include a radiation source for generating an illuminating radiation beam. The measurement or inspection device may include an optical system according to any aspect or embodiment, the optical system for focusing the illuminating radiation beam on a region of interest on the substrate.

[0035] According to one aspect of the invention, a method of using an optical system for focusing a radiation beam on a region of interest of a substrate in a metrology device is provided. The radiation beam may include radiation in the soft X-ray or extreme ultraviolet spectral range. The method may include providing an optical system including a first reflector system and a second reflector system. Each of the first reflector system and the second reflector system may include a finite-to-finite Volta reflector system. The method may include forming a reduced image of an object on the region of interest. The object may include an apparent source of the radiation beam.

[0036] According to an aspect of the present invention, there is provided a metrology apparatus for determining a property of interest of a portion of a structure fabricated on a substrate. The metrology apparatus may comprise an optical system according to any aspect or embodiment described herein.

[0037] According to an aspect of the present invention, there is provided an inspection apparatus for inspecting a portion of a structure manufactured on a substrate. The inspection apparatus may include an optical system according to any aspect or embodiment described herein.

[0038] According to an aspect of the present invention, there is provided a lithography cell. The lithography cell may include a metrology or inspection apparatus according to any aspect or embodiment described herein.

[0039] At least one feature of any aspect or embodiment described herein may replace any corresponding feature of any aspect or embodiment described herein.At least one feature of any aspect or embodiment described herein may be combined with any other aspect or embodiment described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0041] - Figure 1 A schematic overview diagram depicting a lithographic apparatus;

[0042] - Figure 2 A schematic overview diagram depicting a lithography unit;

[0043] - Figure 3 Depicting a schematic representation of overall lithography showing the collaboration between three key technologies used to optimize semiconductor manufacturing;

[0044] - Figure 4 Describe a schematic representation of the measurement equipment;

[0045] - Figure 5a to Figure 5b depicts schematic representations of optical systems according to embodiments, respectively;

[0046] - Figure 6a to Figure 6b depicts a schematic representation of an optical system according to another embodiment; and

[0047] - Figure 7a to Figure 7b Respectively depicted is a schematic representation of an optical system according to another embodiment. DETAILED DESCRIPTION

[0048] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365, 248, 193, 157 or 126 nm), extreme ultraviolet radiation (EUV, e.g., having a wavelength in the range of about 5 to 100 nm), and soft X-ray radiation (SXR, e.g., having a wavelength in the range of about 5 to 100 nm).

[0049] As used herein, the terms "reticle", "mask" or "patterning device" may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam that corresponds 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 typical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0050] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. a mask stage) MT configured to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. a wafer stage) WT configured to hold a substrate (e.g. a wafer coated with resist) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0051] 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 to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0052] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems suitable for the exposure radiation used and / or for other factors such as the use of immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0053] 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) so as to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion techniques is given in US6952253, incorporated herein by reference.

[0054] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also known as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel and / or a substrate W on one of the substrate supports WT may be used to expose a pattern on another substrate W while a step of preparing the substrate W for subsequent exposure is being performed on the substrate W on another substrate support WT.

[0055] In addition to the substrate support WT, the lithographic apparatus LA may also comprise a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning means may be arranged to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of the system for providing immersion liquid. The measurement platform may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0056] 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) presented on the patterning device MA. After traversing the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example, in order to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the first positioner PM and possibly another position sensor (which is located in the substrate support WT) are provided. Figure 1The patterning device MA may be accurately positioned relative to the path of the radiation beam B using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, 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 line alignment marks.

[0057] like Figure 2 As shown in , the lithography apparatus LA may form part of a lithography cell LC (sometimes also referred to as a lithography cell or (lithography) cluster), which often also includes equipment for performing pre-exposure processes and post-exposure processes on a substrate W. Conventionally, these equipment include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, for example, for regulating the temperature of the substrate W (for example, for regulating the solvent in the resist layer). A substrate transport device or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate W between the different process equipment and transfers the substrate W to a feed table LB of the lithography apparatus LA. The equipment in the lithography cell, which is often also collectively referred to as a coating and developing system, is usually under the control of a coating and developing system control unit TCU, which itself can be controlled by a management control system SCS, which can also control the lithography apparatus LA, for example, via a lithography control unit LACU.

[0058] In order to correctly and consistently expose a substrate W exposed by the lithographic apparatus LA, it is desirable to inspect the substrate to measure properties of the patterned structure, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithography cell LC. If errors are detected, adjustments may be made, for example, to the exposure of subsequent substrates or to other processing steps to be performed on the substrate W, particularly if the inspection is performed before other substrates W of the same batch or lot are yet to be exposed or processed.

[0059] The inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrate W, and in particular, to determine how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary between the different layers. The inspection apparatus may alternatively be configured to identify defects on the substrate W, and may for example be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a separate device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure), or on a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or on a developed resist image (wherein exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0060] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires a high accuracy in the dimensioning and placement of the structures on the substrate W. In order to ensure this high accuracy, the three systems can be combined in a so-called "holistic" control environment, such as Figure 3 Schematically depicted. One of these systems is the lithographic apparatus LA, which is (in practice) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithographic apparatus LA remains within the process window. The process window defines a range of process parameters (e.g. dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g. a functional semiconductor device) - typically within which process parameter variations in the lithographic process or patterning process are allowed.

[0061] The computer system CL can use (portions of) the design layout to be patterned to predict which resolution enhancement techniques to use, and perform computational lithography simulations and calculations to determine which mask layout and lithography equipment settings will achieve the maximum overall process window (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. using input from the metrology tool MT) to predict whether defects may exist (e.g. due to suboptimal processing) Figure 3 ), depicted by an arrow pointing to “0” in the second scale SC2.

[0062] 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 (in Figure 3 SC3).

[0063] During a lithographic process it is desirable to frequently make measurements of the produced structures, for example for process control and verification. The tool used to make such measurements is generally referred to as a metrology tool MT. Different types of metrology tools MT for making such measurements are well known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow measuring parameters of the lithographic process by having a sensor in the pupil or a plane conjugate to the pupil of the scatterometer's objective (the measurements are generally referred to as pupil-based measurements), or by having a sensor in the image plane or a plane conjugate to the image plane, in which case the measurements are generally referred to as image- or field-based measurements. Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164 A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure gratings using light from soft x-rays and visible to near IR wavelength ranges.

[0064] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signals to reconstruct or calculate the properties of the grating. Such a reconstruction can be produced, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured 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.

[0065] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. a measurement of intensity as a function of wavelength). From such data the structure or profile of the target producing the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and non-linear regression or by comparison with a library of simulated spectra.

[0066] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. Ellipsometric scatterometers allow parameters of a lithographic process to be determined by measuring scattered radiation for each polarization state. Such a metrology device emits polarized light (such as linear, circular or elliptically polarized light) by using, for example, appropriate polarization filters in the illumination section of the metrology device. A source suitable for the metrology device may also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. patent applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110 and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0067] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or an asymmetry in the detection configuration (the asymmetry being related to the extent of the overlap). The two (typically superimposed) grating structures may be applied in two different layers (not necessarily consecutive layers) and the two grating structures may be formed to be in substantially the same position on the wafer. The scatterometer may have a symmetrical detection configuration as described, for example, in the commonly owned patent application EP 1,628,164 A, so that any asymmetry is clearly distinguishable. This provides a simple way to measure misalignment in the gratings. Further examples of the measurement of the overlay error between two layers containing a periodic structure as a target via the asymmetry of the periodic structure may be found in PCT Patent Application Publication No. WO 2011 / 012624 or US Patent Application No. US 20160161863, which are incorporated herein by reference in their entirety.

[0068] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety (or alternatively by scanning electron microscopy). A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the Focus Energy Matrix (FEM - also referred to as Focus Exposure Matrix). If these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​may be uniquely determined from these measurements.

[0069] The metrology target may be a collection of composite gratings formed primarily in resist by a lithography process and also formed after, for example, an etching process. Typically, the pitch and line width of the structures in the gratings rely heavily on the measurement optics (specifically the NA of the optics) to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffraction signal may be used to determine the shift between two layers (also referred to as "overlap") or may be used to reconstruct at least a portion of the original grating as produced by the lithography process. Such a reconstruction may be used to provide quality guidance for the lithography process and may be used to control at least a portion of the lithography process. The target may have smaller subsections configured to mimic the size of a functional portion of a design layout in the target. Due to such subsections, the target will behave more similarly to the functional portion of the design layout, so that the overall process parameter measurement better resembles the functional portion of the design layout. The target may be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam produces a spot that is smaller than the overall target. In the overfill mode, the measurement beam produces a spot that is larger than the entire target. In such an overfill mode, it is also possible to measure different targets simultaneously and thus to determine different process parameters simultaneously.

[0070] The overall measurement quality of a lithography parameter performed using a specific target is at least partially determined by the measurement profile used to measure such lithography parameters. The term "substrate measurement profile" may include one or more parameters of the measurement itself, one or more parameters of one or more patterns measured, or both. For example, if the measurement used in the substrate measurement profile is an optical measurement based on diffraction, one or more of the measured parameters may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the direction of the radiation relative to the pattern on the substrate, and so on. One of the criteria for selecting a measurement profile may, for example, be the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US2016-0161863 and U.S. Patent Publication US 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0071] As an alternative to optical metrology methods, the use of soft X-rays or EUV radiation is also considered, for example, radiation in a wavelength range between 0.1 nm and 100 nm, or alternatively between 1 nm and 50 nm, or alternatively between 5 nm and 100 nm, or alternatively between 10 nm and 20 nm. An example of a metrology tool operating in one of the wavelength ranges presented above is transmission small angle X-ray scattering (T-SAXS in US 2007224518A, the contents of which are incorporated herein by reference in their entirety). Lemaillet et al. discuss profile (CD) measurements using T-SAXS in "Intercomparison between optical and X-ray scatterometry measurements of FinFET structures" (Proc. of SPIE, 2013, 8681). Reflection measurement techniques using X-rays (GI-XRS) and extreme ultraviolet (EUV) radiation at grazing incidence are known for measuring the properties of films and stacked layers on substrates. Within the general reflectometry field, goniometric and / or spectral techniques may be applied. In goniometric techniques, the variation of the reflected beam at different angles of incidence is measured. On the other hand, spectroscopic reflectometry measures the spectrum of wavelengths reflected at a given angle (using broadband radiation). For example, EUV reflectometry has been used for inspection of mask substrates prior to the manufacture of reticles (patterning devices) used in EUV lithography.

[0072] The range of applications may make the use of soft X-rays or wavelengths in the EUV domain insufficient. Therefore, published patent applications US 20130304424A1 and US2014019097A1 (Bakeman et al. / KLA) describe hybrid metrology techniques in which measurements using x-rays and optical measurements using wavelengths in the range of 120nm and 2000nm are combined to obtain measurements of parameters such as CD. The measurements are obtained by coupling (through one or more common models) an x-ray mathematical model and an optical mathematical model. The contents of the cited US patent applications are incorporated herein by reference in their entirety.

[0073] In the context of this document, the term HHG or HHG source is introduced. HHG refers to high-order harmonic generation or is sometimes referred to as high-order harmonic generation. HHG is a nonlinear process in which a target such as a gas, plasma or solid sample is irradiated by a dense laser pulse. Subsequently, the target can emit radiation with a frequency that is multiple times the frequency of the radiation of the laser pulse. Such a frequency that is a multiple is called the harmonic of the radiation of the laser pulse. The generated HHG radiation can be defined as harmonics higher than the fifth harmonic and these harmonics are called high-order harmonics. The physical process that forms the basis of the HHG process is different from the physical process that involves the radiation that produces lower harmonics (usually the 2nd to 5th harmonics). The radiation that produces lower harmonics is related to perturbation theory. The trajectory of the (confined) electrons of the atoms in the target is roughly determined by the Coulomb potential of the matrix ions. In HHG, the trajectory of the electrons that contribute to the HHG process is roughly determined by the electric field of the light introduced into the laser. In the so-called "three-step model" describing HHG, electron tunneling through the Coulomb barrier under the torque is roughly suppressed by the laser field (step 1), follows a trajectory determined by the laser field (step 2), and recombine with some probability when releasing its kinetic energy and ionization energy in the form of radiation (step 3). Another way to phrase the difference between HHG and the generation of lower harmonic radiation is to define all radiation with photon energies above the ionization energy of the target atoms as "higher order harmonic" radiation, such as HHG generating radiation, and to define all radiation with photon energies below the ionization energy as non-HHG generating radiation. If neon is used as the gas target, all radiation with wavelengths shorter than 62 nm (with photon energies above 20.18 eV) is generated by means of the HHG process. For argon as the gas target, all radiation with photon energies above about 15.8 eV is generated by means of the HHG process.

[0074] Figure 4 Depicted is a schematic representation of a metrology device 302 , in which radiation in the wavelength range from 0.1 nm to 100 nm can be used to measure parameters of structures on a substrate. Figure 4 The metrology apparatus 302 presented in FIG. 3 is suitable for use in the soft X-ray or EUV domain.

[0075] Figure 4 A schematic physical arrangement of a measurement apparatus 302 comprising a spectroscopic scatterometer using EUV and / or SXR radiation in grazing incidence is shown by way of example only. An alternative form of inspection apparatus may be provided in the form of an angle-resolved scatterometer using radiation in normal incidence (or normal incidence) or near normal incidence (or near normal incidence) similar to conventional scatterometers operating at longer wavelengths.

[0076] Inspection apparatus 302 includes a radiation source 310 , an illumination system 312 , a substrate support 316 , detection systems 318 , 398 , and a metrology processing unit (MPU) 320 .

[0077] In this example, source 33310310 comprises a generator of EUV or soft x-ray radiation based on high order harmonic generation (HHG) technology. These sources are available, for example, from KMLabs (http: / / www.kmlabs.com / ) of Boulder Colorado, USA. The main components of the radiation source are a drive laser 330 and a HHG gas chamber 332. A gas supply 334 supplies a suitable gas to the gas chamber, where it is optionally ionized by a power supply 336. The drive laser 300 may be, for example, a fiber-based laser with an optical amplifier, thereby generating pulses of infrared radiation that may last, for example, less than 1 ns (1 nanosecond) per pulse, with a pulse repetition rate of up to several megahertz as desired. The wavelength of the infrared radiation may be, for example, about 1 μm (1 micrometer). The laser pulses are delivered as a first radiation beam 340 to the HHG gas cell 332 where a portion of the radiation is converted in the gas to a higher frequency than the first radiation into a beam 342 including coherent second radiation having the desired wavelength or wavelengths.

[0078] The second radiation may include multiple wavelengths. If the radiation is monochromatic, measurement calculations (e.g., reconstruction) can be simplified, but it is easier to generate radiation with multiple wavelengths when using HHG. The volume of the gas in the gas chamber 332 defines the HHG space, but the space does not need to be completely enclosed and a gas flow can be used instead of a static volume. For example, the gas can be an inert gas, such as neon (Ne) or argon (Ar). N2, O2, He, Ar, Kr, Xe gases can all be considered. These situations are design choices and can even be selectable options within the same device. Different wavelengths will, for example, provide different levels of contrast when imaging structures of different materials. For example, in order to inspect metal structures or silicon structures, different wavelengths can be selected as wavelengths for imaging features of (carbon-based) resists or for detecting contaminants of these different materials. One or more filter devices 344 can be provided. For example, a filter device such as an aluminum (Al) film can be used to cut off the basic IR radiation so as not to be further transmitted to the inspection device 302. A grating (not shown) may be provided to select one or more specific harmonic wavelengths from among those generated in the gas cell. Some or all of the beam path may be contained within a vacuum environment, keeping in mind that SXR radiation is absorbed as it travels through air. Various components of the radiation source 33310 and illumination optics 312 may be adjustable to implement different measurement "options" within the same apparatus. For example, different wavelengths and / or polarizations may be made selectable.

[0079] Depending on the material of the structure being inspected, different wavelengths can provide the desired degree of penetration into the lower layer. In order to distinguish the smallest device features and defects among the smallest device features, short wavelengths are likely to be preferred. For example, one or more wavelengths in the range of 1 nanometer to 20 nanometers, or optionally in the range of 1 nanometer to 10 nanometers, or optionally in the range of 10 nanometers to 20 nanometers can be selected. Wavelengths shorter than 5 nanometers have very low critical angles when reflected from materials that are generally concerned in semiconductor manufacturing. Therefore, selecting a wavelength greater than 5 nanometers will provide a stronger signal at a higher angle of incidence. On the other hand, if the inspection task is to detect the presence of a certain material, such as to detect contaminants, wavelengths up to 50 nanometers can be useful.

[0080] From the radiation source 33310, the filtered beam 342 enters an inspection chamber 350 in which a substrate W including a structure of interest is held by a substrate support 316 for inspection at a measurement position. The structure of interest is labeled T. The gas environment within the inspection chamber 350 is maintained as a near vacuum by a vacuum pump 352, so that EUV radiation can pass through the gas environment without undue attenuation. The illumination system 312 has the function of focusing the radiation into a focused beam 356, and may include, for example, a two-dimensional curved mirror, or a series of one-dimensional curved mirrors, as described in the above-mentioned published U.S. patent application US2017 / 0184981A1 (the contents of which are incorporated herein by reference in their entirety). The focusing is performed to achieve a circular or elliptical spot S with a diameter of less than 10 microns when projected onto the structure of interest. The substrate support 316 includes, for example, an XY translation stage and a rotation stage, by which any portion of the substrate W can be brought to the focus of the beam in a desired orientation. Thus, a radiation spot S is formed on the structure of interest. Alternatively, or in addition, the substrate support 316 includes, for example, a tilting stage, which can tilt the substrate W at a certain angle to control the incident angle of the focused beam on the structure of interest T (e.g., at the region of interest).

[0081] Optionally, the illumination system 312 provides a reference radiation beam to a reference detector 314, which can be configured to measure the spectrum and / or intensity of different wavelengths in the filtered beam 342. The reference detector 314 can be configured to generate a signal 315 that is provided to the processor 310, and the filter can include information about the spectrum of the filtered beam 342 and / or the intensity of different wavelengths in the filtered beam.

[0082] The reflected radiation 360 is captured by the detector 318 and the spectrum is provided to the processor 320 for calculating the properties of the target structure T. The illumination system 312312 and the detection system 318 thus form an inspection device. Such an inspection device may include a soft X-ray and / or EUV spectroscopic reflectometer of the type described in US2016282282A1 (the entire text of which is incorporated herein by reference).

[0083] If the target T has a certain periodicity, the radiation of the focused beam 356 may also be partially diffracted. The diffracted radiation 397 follows another path at a well-defined angle relative to the angle of incidence and then relative to the reflected radiation 360. Figure 4 In FIG. 3 , the plotted diffracted radiation 397 is plotted in a schematic manner, and the diffracted radiation 397 may follow many other paths than the plotted path. The inspection device 302 may also include an additional detection system 398 that detects and / or images at least a portion of the diffracted radiation 397. Figure 4 , a single further detection system 398 is drawn, but embodiments of the inspection device 302 may also include more than one further detection system 398, which are arranged at different positions to detect and / or image the diffracted radiation 397 in multiple diffraction directions. In other words, the (higher) diffraction orders of the focused radiation beam incident on the target T are detected and / or imaged by one or more further detection systems 398. One or more detection systems 398 generate a signal 399 that is provided to the measurement processor 320. The signal 399 may include information of the diffracted light 397 and / or may include an image obtained from the diffracted light 397.

[0084] To assist in the alignment and focusing of the spot S with the desired product structure, the inspection apparatus 302 may also be provided with auxiliary optics using auxiliary radiation under the control of the metrology processor 320. The metrology processor 320 may also communicate with a position controller 372, which operates a translation stage, a rotation stage, and / or a tilt stage. The processor 320 receives very accurate feedback about the position and orientation of the substrate via sensors. The sensor 374 may include, for example, an interferometer, which may give an accuracy of approximately the order of picometers. In operation of the inspection apparatus 302, the spectral data 382 captured by the detection system 318 is transmitted to the metrology processing unit 320.

[0085] As mentioned, alternative forms of inspection equipment use soft X-rays and / or EUV radiation at normal incidence or near normal incidence, for example to perform diffraction-based asymmetry measurements. Both types of inspection equipment can be arranged in a hybrid measurement system. The performance parameters to be measured may include overlay (OVL), critical dimension (CD), coherent diffraction imaging (CDI) and at-resolution overlay (ARO) measurements. The soft X-ray and / or EUV radiation may, for example, have a wavelength of less than 100 nm, for example using radiation in the range of 5-30 nm, optionally in the range of from 10 nm to 20 nm. The radiation may be narrowband or broadband in nature. The radiation may have discrete peaks in a particular wavelength band or may have a more continuous nature.

[0086] Like optical scatterometers used in today's production facilities, the inspection device 302 can be used to measure structures in resist materials that have been processed in a lithography unit (after development inspection or ADI), and / or to measure structures in harder materials after they have been formed (after etching inspection or AEI). For example, the inspection device 302 can be used to inspect a substrate after it has been processed by a development device, an etching device, an annealing device, and / or other devices.

[0087] The embodiments of the present disclosure relate to a method for providing Figure 4 An optical system that illustrates the functionality of the depicted illumination system 312. The metrology device 302 is designed to meet the required specifications for a particular application of the metrology device 302. For example, the metrology device 302 may be configured to inspect substrates to measure properties of patterned structures produced in those substrates by the lithography device LA. If the metrology device 302 meets the required specifications for that particular application, only certain properties of the patterned structures may be determined. The optical system may therefore be configured to meet certain criteria that allow the metrology device 302 to meet the required specifications. For example, the optical system may be designed to provide a focused beam 356 having one or more properties such that the interaction of the beam 356 with the structure of interest allows information about the structure of interest to be determined according to the specifications using the metrology device 302. The degrees of freedom in designing an optical system depend on one or more boundary conditions. Thus, the ability of the optical system to meet certain criteria depends on the one or more boundary conditions.

[0088] The optical system may be configured to focus the radiation beam on a region of interest of the substrate such that information about the region of interest may be determined from radiation that is at least one of: radiation reflected, scattered, and diffracted from the region of interest. Thus, the metrology apparatus 302 may be configured to determine such information using parameters of the metrology apparatus 302 appropriately selected for determining information about the region of interest of the integrated circuit.

[0089] Examples of boundary conditions include, but are not limited to: spectral composition of the filtered beam 342; size and / or aspect ratio of the beam 342 (e.g., at the apparent source or apparent source of the beam 342 that defines an object imaged by the optical system); quality of the beam 342 (e.g., M squared value); spatial and / or spectral variation of the beam 342; energy and / or power of the beam 342; any other operating characteristics of the radiation source 310; transmittance of the optical system; available space for components of the optical system; and volume conflicts between components of the metrology device 302 (such as the reference detector 314) and / or conflicts of these components with the radiation beam. In addition, one or more boundary conditions may vary during use depending on the particular application. For example, one or more components of the metrology device 302 may be moved or replaced during use, which may affect the available space for components of the optical system. Another example is that the radiation source 310 may be configured with different operating schemes, for example, to provide a beam 342 with a particular spectral composition and / or other operating characteristics of the beam may vary.

[0090] One or more embodiments of the optical system can be configured so that the measurement device 302 meets the required specifications according to one or more boundary conditions. In one example specification, the beam 356 is focused by the optical system to achieve a circular or elliptical spot S with a diameter less than 10 μm when projected onto the structure of interest. In this example, the boundary condition can be defined by at least one dimension (e.g., radius, diameter, etc.) of the apparent source of the focused beam 356. When observed from the far field, the apparent source can define the object from which the source of the beam 356 appears to originate. The optical system thus projects an image of the object onto the structure of interest, wherein the corresponding at least one dimension of the image can depend on one or more of the following: (i) at least one dimension of the object; (ii) imaging properties of the optical system; and (iii) the quality of the beam 342 imaged by the optical system.

[0091] In an example, at least one dimension of the object may be such that the optical system can be configured to measure the object by the size of the object (d o ) corresponds to the size of the image (d i ) to reduce at least one dimension of the object to meet certain criteria, where the reduction factor = d o / d iFor a Gaussian beam, the maximum intensity in the object or image can be calculated based on 1 / e 2 The size is defined by an enclosing radius of the object or image in the case of a reduction factor of greater than 1 (or optionally greater than or equal to: 2, 5, 8, 10, 15, 20 or 25).

[0092] In an example, the spot size S at the region of interest may be calculated based on 1 / e of the maximum intensity in the image. 2 (i.e., 13.5%), the spot radius may be equal to or lower than at least one of the following: 5, 4, 3, 2, 1.5, 1.25, 1, 0.75, 0.66, and 0.5 microns. It should be understood that the spot S size may not be equal in all directions (e.g., if the beam spot is non-circular), and therefore, the beam spot radius as defined above may refer to the maximum radius of the beam spot.

[0093] In an example, at least one dimension of the object may be such that the optical system satisfies certain criteria by imaging the object with a desired reduced aspect ratio so as to form an isomorphic image or a deformed image of the object. The aspect ratio of the object or image may be defined by a ratio between a first dimension of the object or image and a second dimension of the object or image. The first dimension of the object may be measured in a first direction taken along an object plane including the object. The second dimension of the object may be measured in a second direction taken along the object plane, the second direction being perpendicular to the first direction. Similarly, the first dimension of the image may be measured in a first direction taken along an image plane including the image, wherein the first direction of the image corresponds to the first direction of the object. Additionally, the second dimension of the image may be measured in a second direction taken along the image plane, wherein the second direction of the image corresponds to the second direction of the object.

[0094] Thus, the optical system may be configured to reduce the object in a first direction acquired along the object plane to form a corresponding reduced image of the object in a corresponding first direction acquired along the image plane. In addition, the optical system may be configured to reduce the object in a second direction acquired along the object plane to form a corresponding reduced image of the object in a corresponding second direction acquired along the image plane.

[0095] The reduction factor in the first direction and the second direction may affect the aspect ratio of the image formed by the object. If the reduction factor is the same in both the first direction and the second direction, the image may be an isomorphic image having the same aspect ratio as the object. On the other hand, if the reduction factor is different in the first direction and the second direction, the image may be a deformed image having a different aspect ratio than the object being imaged. The shape of the object in the object plane may be circular or non-circular (e.g., elliptical). Depending on the criteria to be met, the optical system may be configured such that a circular or non-circular object may be reduced to form any of the following: a circular and a non-circular image of the circular or non-circular object.

[0096] In an example, the boundary condition may be defined by a potential volume collision between the optical system and the substrate W. For example, a free working distance may be defined between a component of the optical system and a region of interest of the substrate W so as to prevent a volume collision between the component and the substrate and / or the focused beam 356. In an example, a component of the optical system may include a reflector that is closer to the region of interest than any other reflector of the optical system. In this example, the free working distance may be defined between a surface of the reflector and the region of interest or between a surface of a component associated with the reflector (such as a mounting component) and the region of interest. The free working distance between a component of the optical system and the region of interest of the substrate W may be greater than or equal to: 10 cm, 15 cm, or 20 cm. The free working distance may be measured between the region of interest and the surface of the component closest to the region of interest.

[0097] In an example, the boundary condition can be defined by the path length of the main ray between the object and the image. Compared with other examples of optical systems, one or more optical systems described herein can be configured to have a relatively short path length between the object and the image. The relatively short path length between the object and the image can allow the optical system to be arranged in a relatively compact volume or footprint or to allow the optical system to be accommodated in an available space. One or more optical systems described herein with a relatively short path length between the object and the image can have a path length less than or equal to the following: 3, 2.5, 2, 1.5, 1.25, 1.1, or 1 meter. It has been recognized that in the case of such a short path length between the object and the image, it is difficult to obtain a relatively large (which can be greater than or equal to 10cm) free working distance between the components of the optical system and the area of ​​interest. The inventor has realized that the following embodiments herein can provide such a relatively large working distance.

[0098] In an example, the boundary conditions may be defined by the numerical aperture (NA) of the optical system relative to the radiation beam focused on the region of interest. Such numerical aperture may be greater than or equal to at least one of the following: 0.005, 0.01, 0.05, and 0.1. If a certain numerical aperture range of the optical system meets certain criteria, such numerical aperture range may affect or be affected by other boundary conditions, such as the available range of path lengths for the optical system and at least one size of the object.

[0099] In an example, the boundary condition can be defined by a range of grazing angles of incidence (GAOI) that can be accepted by one or more reflectors of the optical system. At least one reflector of any optical system described herein can be oriented so that the GAOI of the main ray of the radiation beam on the reflector is greater than or equal to at least one of the following: 1 degree, 2 degrees, 3 degrees, 4 degrees, 5 degrees, 6 degrees, 7 degrees, 8 degrees, 9 degrees and 10 degrees. In another example, an acceptable GAOI can be in the range of 1 degree to 20 degrees. In another example, an acceptable GAOI can be greater than 10 degrees, for example, in the range of 10 degrees to 20 degrees. GAOI can be defined as the angle between the surface of the reflector and the incident ray. Grazing incidence optics can be used to provide total external reflection of radiation beams including, for example, EUV, SXR and / or X-ray wavelengths.

[0100] In an example, the boundary condition can be defined by the divergence of the radiation beam at the object. At least one reflector of the optical system can be configured to receive a radiation beam, wherein the radiation beam has a divergence half-angle greater than or equal to at least one of the following: 1, 2, 3, 5, 10, 15, 20 and 25 mrad. For a Gaussian beam, the divergence half-angle of the radiation beam B can be defined by the expression θ≈λ0 / (πnw0), where λ0 is the wavelength, n is the refractive index and w0 is the beam waist radius. The divergence is related to the numerical aperture or acceptable numerical aperture of the optical system. The numerical aperture can be approximately twice the expected (Gaussian) beam divergence half-angle θ. In an example, the optical system can be designed so that 99.99% of the energy of the radiation beam can be within the imaging spot at the area of ​​interest. More generally, the divergence or convergence can be defined as the divergence at 1 / e 2(13.5%) intensity level. The beam may have the same or different divergence half-angles in a first direction and a second direction, which may be perpendicular directions defined along the object plane. Although the divergence half-angles or numerical aperture values ​​for the first direction and the second direction may be different, they may still be of similar order of magnitude. If a certain divergence half-angle or numerical aperture range is used to meet the criterion, such a divergence half-angle or numerical aperture range may affect or be affected by other boundary conditions, such as transmittance of the optical system, volume interference, etc.

[0101] Throughout this disclosure and where appropriate, the term "divergence" may refer to the divergence half-angle as defined above. However, where appropriate, the term "numerical aperture" may be more appropriate if reference is made to the properties of the optical system itself. The numerical aperture NA can be calculated using the maximum half-angle that can propagate through the optical system. Since the maximum half-angle of the optical system is 1 / e 2 The divergence half angle is defined in the context of the intensity level, so the maximum half angle can be larger than the divergence half angle depending on the imaging criteria, for example so that 99.99% of the energy of the radiation beam can be within the imaging spot of the area of ​​interest. Generally, it can be assumed that the maximum half angle that can be propagated through the system is slightly larger than about twice the divergence half angle. However, the ratio between the maximum half angle and the divergence half angle can be different, for example less than or greater than 2.

[0102] In an example, the boundary condition may be defined by the transmittance of the optical system. The transmittance of the optical system may be greater than or equal to: 0.1, 0.2, 0.3, or 0.4. If a certain transmittance range is used to meet the criteria, such a transmittance range may affect or be affected by other boundary conditions, such as the available range of beam divergence for the optical system, etc.

[0103] In an example, the boundary condition can be defined by the size of one or more reflectors of the optical system. The reflector size can be such as to reflect substantially the entire radiation beam (e.g., reflect at least one of greater than or equal to 50%, 60%, 70%, 80%, 90%, 95%, 99% of the energy of the beam), as long as there are no volume conflicts between the reflector and other reflectors or other components.

[0104] In an example, the boundary conditions may be defined by the manufacturability of one or more reflectors of the optical system. In order to meet certain criteria, one or more reflectors may be manufactured within a certain design and / or tolerance considerations. Although certain reflector designs may be desired in order to meet certain criteria, those designs may not be allowed if the manufacturability of those reflector designs exceeds one or more boundary conditions. For example, it may be impossible to manufacture a reflector with a certain design that may otherwise be desired to meet certain criteria. Therefore, depending on the specific application, the availability / freedom of manufacturing certain reflectors of the optical system may depend on the criteria to be met.

[0105] One or more boundary conditions may affect at least one other boundary condition, which may result in the criteria being satisfied, or may not result in the criteria being satisfied. In addition, certain boundary conditions may vary depending on the specific application of the measurement device. Therefore, certain configurations of the optical system may satisfy the criteria within the defined boundary conditions. However, if one or more of the boundary conditions changes, the configuration of the optical system may also change in order to satisfy the criteria.

[0106] As described in more detail below, the inventors have identified several optical systems that satisfy the criteria for one or more boundary conditions. Each optical system described herein is configured to focus a radiation beam on a region of interest of a substrate in a metrology apparatus. The radiation beam includes radiation in the soft X-ray (SXR) and / or extreme ultraviolet (EUV) spectral range.

[0107] One or more optical systems described herein may meet certain criteria when configured according to one or more of the following examples. In an example, the optical system may be configured so that a radiation beam may be incident on one or more reflectors of the optical system at a grazing angle of incidence. Thus, the optical system may provide acceptable imaging performance across a wide spectral range (e.g., for a radiation beam having a spectral band between 10 and 20 nm). In an example, the optical system may be configured to provide a relatively high reduction factor (e.g., about 10) between an object and an image. In an example, the optical system may be configured to focus the radiation beam on a region of interest using a relatively large numerical aperture (e.g., about 0.1 to, for example, 0.05). In an example, the optical system may be configured with a relatively short path length (e.g., about 1.5 meters) between an object and an image. Therefore, the optical system may be accommodated in a relatively compact volume or footprint. In an example, the optical system may include a reflector having a very large curvature in order to provide a relatively short path length and / or a high numerical aperture. A specific type of reflector for an optical system can allow for providing imaging performance that satisfies certain criteria while providing a relatively short path length and / or a high numerical aperture. In an example, the surface profile of the reflector of the optical system can have a shape and / or surface quality that can be manufactured so that the optical system can provide imaging performance that satisfies certain criteria. In an example, the components of the optical system can be configured to align with a stable configuration so that a certain deviation from the optimal alignment still results in imaging performance that satisfies certain criteria. Thus, the optical system can have a relatively loose alignment tolerance compared to other optical systems that can additionally meet certain other criteria described herein. In an example, if the apparent source is not a point source and / or if the apparent source is off-axis, the optical system can still provide imaging performance that satisfies certain criteria due to correctly imaging the off-axis and on-axis portions of the object (i.e., even if the apparent source is not a precise point and / or if at least a portion of the apparent source is off-axis, the optical system can still meet the Abbe sine condition). That is, the optical systems described herein can provide improved imaging performance compared to other optical systems with respect to situations where the Abbe sine condition is satisfied even though the apparent source has a finite (non-zero) field in the object plane to some extent (where those other optical systems would not satisfy the sine condition).

[0108] Figure 5a to Figure 5b A schematic representation of an optical system 400 satisfying certain criteria for one or more boundary conditions is respectively depicted. Figure 5a is a projection of the optical path of the optical system 400 in the first direction. Figure 5b is the projection of the optical path of the optical system 400 in a second direction perpendicular to the first direction. Therefore, Figure 5a and Figure 5b Respectively represent orthogonal views of the optical system 400.

[0109] The optical system 400 includes a first reflector system 410 and a second reflector system 412. Each of the first reflector system 410 and the second reflector system 412 includes a finite-to-finite Volta reflector system. The optical system 400 is configured to reflect the target T (see Figure 4 ) forms a reduced image 414 of an object 416 including an apparent source of radiation beam B on a region of interest on the image 414. Each finite-to-finite Walter reflector system has a finite-to-finite conjugate distance relationship, wherein the image 414 is positioned a finite distance away from the object 416. The Walter reflector system can meet certain criteria that other reflector configurations may not meet based on one or more boundary conditions. For example, even if the apparent source is somewhat not a precise point and / or when at least a portion of the apparent source is off-axis, the Walter reflector system can still meet the Abbe sine condition to provide imaging performance that meets the criteria.

[0110] The first reflector system 410 and the second reflector system 412 each include a one-dimensional finite-to-finite Volta reflector system. Figure 5a As depicted, first reflector system 410 is configured to demagnify object 416 in a first direction 418 taken along object plane 420 including object 416 to form a corresponding demagnified image 414 of object 416 in a corresponding first direction 418 taken along image plane 422 including image 414. Figure 5b As depicted, the second reflector system 412 is configured to reduce the object 414 in a second direction 424 taken along the object plane 420 to form a corresponding reduced image 414 of the object 416 in a corresponding second direction 424 taken along the image plane 422, the second direction 424 being perpendicular to the first direction 418. A one-dimensional Walter reflector system can be manufactured within a desired tolerance level to provide imaging performance that meets the criteria. In contrast, a two-dimensional Walter reflector system (i.e., having a doubly curved reflector surface) may not provide imaging performance that meets the criteria because the doubly curved reflector surface may not be manufacturable and / or may not have an acceptable surface quality for providing imaging performance that meets the criteria.

[0111] The optical system 400 is configured such that the major surface 426 of the first reflector system 410 and the major surface 428 of the second reflector system 412 intersect an axis 430 defined between the object 416 and the image 424 at an intersection point 432 that is closer to the image 414 than the object 416. The location of each of the major surfaces 426, 428 closer to the image 414 than the object 416 results in a zoomed-out configuration of the optical system 400. The Walter reflector system is configured such that the major surface 426 of the first reflector system 410 intersects the axis 430 at substantially the same intersection point 432 as the major surface 428 of the second reflector system 412.

[0112] The demagnification factors in the first direction 418 and the second direction 424 may be made substantially the same or the same by ensuring that the intersection point 432 is the same for both the first reflector system 410 and the second reflector system 412. Beneficially, providing the same demagnification factor in both the first direction 418 and the second direction 424 may allow the optical system 400 to form a homogeneous image 414 of the object 416. Alternatively, and if preferred, different demagnification factors may be provided for the first direction 418 and the second direction 424 to form a deformed image of the object (e.g., such that there is no longer a single intersection point 432 that includes both major surfaces 426, 428).

[0113] The reduction factor may be defined by a ratio between a size of the object 416 acquired along the first direction 418 or the second direction 424 and a corresponding size of the image 414 acquired along the corresponding first direction 418 or the second direction 424. For example, the reduction factor may be defined by 1 / e of the maximum intensity of the object 416 along the first direction 418. 2 (ie 13.5%), the enclosing radius (r o ) and 1 / e of the maximum intensity of the image 414 along the corresponding first direction 418 2 (ie, 13.5%), the enclosing radius (r i ). Thus, the reduction factor in the first direction 418 is equal to r o / r i (r o R i ). A similar relationship applies to the reduction factor in the second direction 424.

[0114] The orientation of the respective components of the optical system 400 is now described in more detail. The first reflector system 410 is oriented relative to the second reflector system 412 such that at least one incident plane intersecting the first reflector system 410 is perpendicular to at least one incident plane intersecting the second reflector system 412. The incident plane of the reflector includes the incident and reflected chief rays of the radiation beam B reflected by the reflector. As can be seen by comparing Figure 5a and Figure 5b It can be seen that the optical system 400 is configured such that the first incident plane intersects each reflector M1, M2 of the first reflector system 410. The first incident plane is parallel to the first direction 418 and the axis 430. In addition, the second incident plane intersects each reflector M3, M4 of the second reflector system 412. The second incident plane is parallel to the second direction 424 and the axis 430. Since the reflectors M1, M2 of the first reflector system 410 are oriented in a perpendicular relationship to the reflectors M3, M4 of the second reflector system 412, it can be seen that the first incident plane and the second incident plane are also perpendicular to each other.

[0115] A specific type of Walter reflector system for the optical system 400 is now described in more detail. The optical system 400 includes a Walter III type reflector system configured as a first reflector system 410 and a Walter I type reflector system configured as a second (subsequent) reflector system 412. As previously mentioned, each of these Walter reflector systems is configured in a finite-to-finite configuration. Additionally, each of the reflectors is referred to as a one-dimensional reflector, where each reflector surface is curved in only a single direction (i.e., the reflectors do not include a doubly curved reflector surface).

[0116] In its finite-to-finite configuration, the primary reflector of the Walter III reflector system comprises an elliptical cylindrical convex reflector and the secondary reflector of the Walter III reflector system comprises an elliptical cylindrical concave reflector. In its finite-to-finite configuration, the primary reflector of the Walter I reflector system comprises an elliptical cylindrical concave reflector and the secondary reflector of the Walter I reflector system comprises a hyperbolic cylindrical concave reflector.

[0117] Thus, the sequence of reflectors provided in the optical system 400 from the object 416 to the image 414 is as follows. The first reflector M1 of the optical system 400 (i.e., the primary reflector of the first reflector system 410) includes an elliptical cylindrical convex reflector. The second reflector M2 of the optical system 400 (i.e., the secondary reflector of the first reflector system 410) includes an elliptical cylindrical concave reflector. The third reflector M3 of the optical system 400 (i.e., the primary reflector of the second reflector system 412) includes an elliptical cylindrical convex reflector. The fourth reflector M4 of the optical system 400 (i.e., the secondary reflector of the second reflector system 412) includes a hyperbolic cylindrical concave reflector.

[0118] Optionally, the optical system 400 comprises a diffraction element (not shown) for diffracting at least a portion of the radiation beam B to produce spatially separated spectral components of the radiation beam B away from the optical path. Figure 5a In the depicted example, the diffraction element is arranged as part of the second reflector M2 of the first reflector system 410. In other examples, the diffraction element and the corresponding detector may be arranged at another part of the optical system, for example as part of one or more reflectors. The diffraction element may be a separate element arranged on a reflector of the optical system 400 or provided as part of the reflector. Alternatively, the diffraction element may be formed as part of the surface of the reflector. In such an alternative configuration, the diffraction element is configured to reflect a portion RP of the radiation beam B in a mirrored manner for projection through the remainder of the optical system 400. The diffraction element is also configured to reflect the radiation beam B towards the reference detector 314 (see Figure 4 ) diffracts the remaining part DP of the radiation beam B for analyzing the spectral composition of the diffracted part of the radiation beam B.

[0119] Figure 6a to Figure 6b A schematic representation of an optical system 500 satisfying certain criteria for one or more boundary conditions is respectively depicted. Figure 6a is a projection of the optical path of the optical system 500 in the first direction. Figure 6b is the projection of the optical path of the optical system 500 in a second direction perpendicular to the first direction. Therefore, Figure 6a and Figure 6b Respectively represent orthogonal views of the optical system 500.

[0120] Like optical system 400, optical system 500 includes two Walter reflector systems, albeit with different configurations as described in more detail below. Features of optical system 500 that correspond to features of optical system 400 are indicated with reference numerals that are incremented by 100. Optical system 500 is thus similar to optical system 400 and can provide imaging performance that meets certain criteria.

[0121] The optical system 500 includes a first reflector system 510 and a second reflector system 512. Each of the first reflector system 510 and the second reflector system 512 includes a one-dimensional finite-to-finite Volta reflector system. Figure 5a to Figure 5bIn contrast to the optical system 400, the optical system 500 includes two Walter III reflector systems (i.e., a Walter III reflector system followed by an additional Walter III reflector system). Therefore, the sequence of reflectors arranged in the optical system 500 from the object 516 to the image 514 is as follows. The first reflector M1' of the optical system 500 (i.e., the primary reflector of the first reflector system 510) includes an elliptical cylindrical convex reflector. The second reflector M2' of the optical system 500 (i.e., the secondary reflector of the first reflector system 510) includes an elliptical cylindrical concave reflector. The third reflector M3' of the optical system 500 (i.e., the primary reflector of the second reflector system 512) includes an elliptical cylindrical convex reflector. The fourth reflector M4' of the optical system 500 (i.e., the secondary reflector of the second reflector system 512) includes an elliptical cylindrical concave reflector. Figure 5a to Figure 5b Compared to the optical system 400 of the embodiment of the present invention, the fourth reflector M4' of the optical system 500 may be positioned farther away from the area of ​​interest due to the second reflector system 512 comprising a Walter III type reflector system rather than a Walter I type reflector system. Thus, the configuration of the optical system 500 may provide a larger free working distance between the fourth reflector M4' and the area of ​​interest compared to the optical system 400. Such a configuration may represent an easing of at least one boundary condition (e.g., free working distance) compared to the optical system 400. However, while at least one boundary condition may be eased, such a particular configuration may affect at least one other boundary condition.

[0122] If Figure 6a As depicted, first reflector system 510 is configured to demagnify object 516 in a first direction 518 taken along object plane 520 including object 516 to form a corresponding demagnified image 514 of object 516 in a corresponding first direction 518 taken along image plane 522 including image 514. Figure 6b As depicted, second reflector system 512 is configured to demagnify object 514 in a second direction 524 taken along object plane 520 to form a corresponding demagnified image 514 of object 516 in a corresponding second direction 524 taken along image plane 522 , the second direction 524 being perpendicular to the first direction 518 .

[0123] The optical system 500 can be configured such that the major surface 526 of the first reflector system 510 and the major surface 528 of the second reflector system 512 intersect an axis 530 defined between the object 516 and the image 524 at an intersection point 532 that is closer to the image 514 than to the object 516. In this example, the Walter reflector system is configured such that the major surface 526 of the first reflector system 510 intersects the axis 530 at approximately the same intersection point 532 as the major surface 528 of the second reflector system 512.

[0124] Optionally, the optical system 500 comprises a diffraction element (not shown) for spatially separating the diffracted spectral components of the radiation beam B. Figure 6a In the depicted example, the diffractive element is arranged in the form of part of a second reflector M2 ′ of the first reflector system 510 .

[0125] Figure 7a to Figure 7b A schematic representation of an optical system 600 satisfying certain criteria for one or more boundary conditions is respectively depicted. Figure 7a is a projection of the optical path of the optical system 600 in the first direction. Figure 7b is the projection of the optical path of the optical system 600 in a second direction perpendicular to the first direction. Therefore, Figure 7a and Figure 7b Respectively represent orthogonal views of the optical system 600.

[0126] Like optical systems 400 and 500, optical system 600 includes two Walter reflector systems, albeit with different configurations as described in more detail below. Features of optical system 600 that correspond to features of optical system 500 are indicated with reference numerals that are incremented by 100. Optical system 600 is similar to optical system 500 and can provide imaging performance that meets certain criteria.

[0127] The optical system 600 includes a first reflector system 610 and a second reflector system 612. Each of the first reflector system 610 and the second reflector system 612 includes a one-dimensional finite-to-finite Volta reflector system. Figure 6a to Figure 6b Optical system 500, optical system 600 includes two Walter III reflector systems. Optical system 600 can have technical benefits similar to those provided by optical system 500. In contrast to the sequence of reflectors provided in optical system 500, the reflectors of the two Walter III reflector systems of optical system 600 are closely connected or entangled as described by the following sequence between object 616 and image 614.

[0128] The first reflector M1" of the optical system 600 (i.e., the primary reflector of the first reflector system 610) includes an elliptical cylindrical convex reflector. The first reflector M1" is followed by the second reflector M2" of the optical system 600 (i.e., the primary reflector of the second reflector system 612) including an elliptical cylindrical convex reflector. The second reflector M2" is followed by the third reflector M3" of the optical system 600 (i.e., the secondary reflector of the first reflector system 610) including an elliptical cylindrical concave reflector. The third reflector M3" is followed by the fourth reflector M4" of the optical system 600 (i.e., the secondary reflector of the second reflector system 612) including an elliptical cylindrical concave reflector.

[0129] If Figure 7a As depicted, first reflector system 610 is configured to demagnify object 616 in a first direction 618 taken along object plane 620 including object 616 to form a corresponding demagnified image 614 of object 616 in a corresponding first direction 618 taken along image plane 622 including image 614. Figure 7b As depicted, second reflector system 612 is configured to demagnify object 614 in a second direction 624 taken along object plane 620 to form a corresponding demagnified image 614 of object 616 in a corresponding second direction 624 taken along image plane 522 , the second direction 624 being perpendicular to the first direction 618 .

[0130] refer to Figure 7a and Figure 7b , it can be seen that the optical system 600 is configured such that:

[0131] - the first plane of incidence intersects the primary reflector M1″ of the first reflector system 610,

[0132] - the second incident plane intersects the primary reflector M2″ of the second reflector system 612,

[0133] - the third incident plane intersects the secondary reflector M3" of the first reflector system 610 and

[0134] - The fourth incident plane intersects the secondary reflector M4″ of the second reflector system 612.

[0135] The first incident plane and the third incident plane are not parallel to each other (see Figure 7b The second incident plane and the fourth incident plane are also not parallel to each other (see Figure 7a ) but perpendicular to the first and third incident planes (see Figure 7a and Figure 7b Both).

[0136] Although the embodiments described herein recognize that about 1 / e of the maximum intensity at, for example, 2 In some embodiments, the optical system may be designed so that 99.99% of the energy of the radiation beam may be within the imaging spot of the area of ​​interest. However, the optical system may be designed so that greater than or equal to 90%, 95%, 97.5%, 99%, 99.9% of the energy of the radiation beam may be within the imaging spot at the area of ​​interest.

[0137] Further embodiments are provided in subsequent numbered aspects:

[0138] 1. An optical system for focusing a radiation beam onto a region of interest of a substrate in a metrology apparatus, the radiation beam comprising radiation in the soft X-ray or extreme ultraviolet spectral range, the optical system comprising:

[0139] a first reflector system and a second reflector system, each of the first reflector system and the second reflector system comprising a finite-to-finite Volta reflector system,

[0140] The optical system is configured to form a reduced image of an object including an apparent source of the radiation beam over the region of interest.

[0141] 2. An optical system according to aspect 1, wherein each finite-to-finite Walter reflector system includes a one-dimensional finite-to-finite Walter reflector system, which is configured to reduce the object in a direction acquired along an object plane including the object to form a corresponding reduced image of the object in a corresponding direction acquired along an image plane including the image.

[0142] 3. An optical system according to aspect 2, wherein the first reflector system is configured to reduce the object in a first direction acquired along the object plane to form a corresponding reduced image of the object in a corresponding first direction acquired along the image plane, and the second reflector system is configured to reduce the object in a second direction acquired along the object plane to form a corresponding reduced image of the object in a corresponding second direction acquired along the image plane, the second direction being perpendicular to the first direction.

[0143] 4. An optical system according to any one of aspects 1 to 3, wherein the optical system is configured such that a free working distance defined between a component of the optical system and the area of ​​interest is greater than or equal to 10 cm.

[0144] 5. An optical system according to any one of Aspects 1 to 4, wherein the optical system is configured so that the major surfaces of the first reflector system and the second reflector system intersect with an axis defined between the object and the image at an intersection point closer to the image than to the object.

[0145] 6. The optical system of clause 5, wherein a major surface of the first reflector system intersects the axis at substantially the same intersection point as a major surface of the second reflector system.

[0146] 7. An optical system according to any one of Aspects 1 to 6, wherein the optical system is configured so that a reduction factor of the first reflector system is approximately the same as a reduction factor of the second reflector system, the reduction factor being defined by the ratio between a size of the object and a corresponding size of the image.

[0147] 8. An optical system according to any one of aspects 1 to 7, wherein the optical system is configured to image the object at a desired reduced aspect ratio so as to form a homomorphic image or an anamorphic image of the object.

[0148] 9. An optical system according to any one of aspects 1 to 8, wherein the optical system is configured to reduce at least one dimension of the object by a reduction factor greater than or equal to 5, optionally greater than or equal to 8, optionally greater than or equal to 10.

[0149] 10. An optical system according to any one of Aspects 1 to 9, wherein the first reflector system is oriented relative to the second reflector system such that at least one incident plane intersecting the first reflector system is perpendicular to at least one incident plane intersecting the second reflector system.

[0150] 11. An optical system according to aspect 10, wherein the optical system is configured so that a first incident plane intersects each reflector of the first reflector system and a second incident plane intersects each reflector of the second reflector system, the first incident plane and the second incident plane being perpendicular to each other.

[0151] 12. An optical system according to aspect 10, wherein the optical system is configured such that a first incident plane intersects with the primary reflector of the first reflector system, a second incident plane intersects with the primary reflector of the second reflector system, a third incident plane intersects with the secondary reflector of the first reflector system, and a fourth incident plane intersects with the secondary reflector of the second reflector system, the first incident plane and the third incident plane are not parallel to each other, the second incident plane and the fourth incident plane are not parallel to each other and are perpendicular to the first incident plane and the third incident plane.

[0152] 13. An optical system according to any one of Aspects 1 to 12, wherein the optical system is configured to focus the radiation beam on the area of ​​interest of the substrate so that information about the area of ​​interest can be determined from radiation that is at least one of reflected, scattered and diffracted from the area of ​​interest, and optionally wherein the substrate includes an integrated circuit or can be used in the manufacture of an integrated circuit.

[0153] 14. The optical system of any one of aspects 1 to 13, wherein the optical system comprises a Walter Type III reflector system and one of: an additional Walter Type III reflector system, and a Walter Type I reflector system.

[0154] 15. The optical system of clause 14, wherein the primary reflector of the Walter III-type reflector system comprises an elliptical cylindrical convex reflector and the secondary reflector of the Walter III-type reflector system comprises an elliptical cylindrical concave reflector.

[0155] 16. The optical system of aspect 14, wherein the primary reflector of the Walter I-type reflector system comprises an elliptical cylindrical concave reflector and the secondary reflector of the Walter I-type reflector system comprises a hyperbolic cylindrical concave reflector.

[0156] 17. The optical system of claim 14 or 15, wherein the first reflector system comprises the Volta III type reflector system.

[0157] 18. The optical system of clause 14, 15 or 17, wherein the second reflector system comprises the additional Volta III type reflector system.

[0158] 19. The optical system of clause 18, wherein the optical system comprises, in sequence between the object and the image: the Walter Type III reflector system followed by the additional Walter Type III reflector system.

[0159] 20. An optical system according to aspect 18, wherein the optical system includes, in sequence between the object and the image: a primary reflector of the Walter III reflector system, followed by a primary reflector of the additional Walter III reflector system, followed by a secondary reflector of the Walter III reflector system, and followed by a secondary reflector of the additional Walter III reflector system.

[0160] 21. An optical system according to any one of aspects 14 to 17, wherein the second reflector system includes the Walter Type I reflector system, and the optical system includes, in sequence between the object and the image: the Walter Type III reflector system followed by the Walter Type I reflector system.

[0161] 22. An optical system according to any one of Aspects 1 to 21, wherein at least one of the first reflector system and the second reflector system comprises a diffraction element for spatially separating diffracted spectral components of the radiation beam.

[0162] 23. An optical system according to clause 22, wherein the diffractive element is arranged as part of a secondary reflector of the first reflector system.

[0163] 24. An optical system according to any one of aspects 1 to 23, wherein the optical system is configured such that the path length of the main ray defined between the object and the image is less than or equal to 3 meters, optionally less than or equal to 1.5 meters.

[0164] 25. A measuring or inspection device comprising:

[0165] a substrate table for receiving and holding a substrate at a controllable position;

[0166] a radiation source for generating an illuminating radiation beam; and

[0167] An optical system according to any one of clauses 1 to 24, the optical system being used to focus the illumination radiation beam at a region of interest on the substrate.

[0168] 26. A metrology apparatus for determining a property of interest of a portion of a structure fabricated on a substrate, the metrology apparatus comprising an optical system according to any one of clauses 1 to 24.

[0169] 27. An inspection apparatus for inspecting a portion of a structure fabricated on a substrate, the inspection apparatus comprising an optical system according to any one of clauses 1 to 24.

[0170] 28. A lithocell comprising a metrology or inspection apparatus according to clause 25, a metrology apparatus according to clause 26 or an inspection apparatus according to clause 27.

[0171] 29. A method of using an optical system for focusing a radiation beam onto a region of interest of a substrate in a metrology apparatus, the radiation beam comprising radiation in the soft X-ray or extreme ultraviolet spectral range, the method comprising:

[0172] providing an optical system comprising a first reflector system and a second reflector system, each of the first reflector system and the second reflector system comprising a finite-to-finite Volta reflector system; and

[0173] A reduced image of the object including the apparent source of the radiation beam is formed over the region of interest.

[0174] Although specific reference may be made herein to the use of lithographic equipment in IC manufacturing, it should be understood that the lithographic equipment described herein may have other applications. Possible other applications include manufacturing integrated optical systems, guidance and detection for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0175] Although detailed reference may be made herein to embodiments of the invention in the context of lithographic equipment, embodiments of the invention may be used in other equipment. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0176] Although specific reference is made to "measurement equipment / tools / systems" or "inspection equipment / tools / systems," these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or on a wafer. For example, an inspection equipment or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate, or defects in a structure on a substrate or on a wafer. In such embodiments, the characteristic of interest of a structure on a substrate may be related to a defect in the structure, the absence of a particular portion of a structure, or the presence of an unwanted structure on a substrate or on a wafer.

[0177] Although detailed reference has been made 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.

[0178] Although specific embodiments of the present invention have been described above, it should be understood that the present invention can be practiced in a manner different from that described above. The above description is intended to be illustrative rather than restrictive. Therefore, it will be understood by those skilled in the art that the described invention can be modified without departing from the scope of the claims set forth below.

Claims

1. An optical system for focusing a radiation beam onto a region of interest of a substrate in a metrology apparatus, the radiation beam comprising radiation in the soft X-ray or extreme ultraviolet spectral range, the optical system comprising: a first reflector system and a second reflector system, each of the first reflector system and the second reflector system comprising a finite-to-finite Volta reflector system, The optical system is configured to form a reduced image of an object including an apparent source of the radiation beam over the region of interest.

2. The optical system of claim 1 , wherein each finite-to-finite Volta reflector system comprises a one-dimensional finite-to-finite Volta reflector system configured to demagnify the object in one direction taken along an object plane including the object to form a corresponding demagnified image of the object in a corresponding direction taken along an image plane including the image; And among them, The first reflector system is configured to reduce the object in a first direction acquired along the object plane to form a corresponding reduced image of the object in a corresponding first direction acquired along the image plane, and the second reflector system is configured to reduce the object in a second direction acquired along the object plane to form a corresponding reduced image of the object in a corresponding second direction acquired along the image plane, the second direction being perpendicular to the first direction.

3. The optical system according to any one of claims 1 to 2, wherein the optical system is configured such that a free working distance defined between components of the optical system and the region of interest is greater than or equal to 10 cm.

4. The optical system according to any one of claims 1 to 3, wherein the optical system is configured so that the major surfaces of the first reflector system and the second reflector system intersect an axis defined between the object and the image at an intersection point closer to the image than to the object; And among them, The major surface of the first reflector system intersects the axis at substantially the same intersection point as the major surface of the second reflector system.

5. An optical system according to any one of claims 1 to 4, wherein the optical system is configured so that a reduction factor of the first reflector system is substantially the same as a reduction factor of the second reflector system, the reduction factor being defined by a ratio between a size of the object and a corresponding size of the image.

6. The optical system according to any one of claims 1 to 5, wherein the first reflector system is oriented relative to the second reflector system such that at least one incident plane intersecting the first reflector system is perpendicular to at least one incident plane intersecting the second reflector system, And among them, One of the following exists: the optical system is configured such that a first incident plane intersects each reflector of the first reflector system and a second incident plane intersects each reflector of the second reflector system, the first incident plane and the second incident plane being perpendicular to each other, -The optical system is configured such that a first incident plane intersects with the primary reflector of the first reflector system, a second incident plane intersects with the primary reflector of the second reflector system, a third incident plane intersects with the secondary reflector of the first reflector system, and a fourth incident plane intersects with the secondary reflector of the second reflector system, the first incident plane and the third incident plane are not parallel to each other, the second incident plane and the fourth incident plane are not parallel to each other and are perpendicular to the first incident plane and the third incident plane.

7. The optical system of any one of claims 1 to 6, wherein the optical system comprises a Walter III-type reflector system and one of: an additional Walter III-type reflector system, and a Walter I-type reflector system, And among them, One of the following exists: - the primary reflector of the Walter III reflector system comprises an elliptical cylindrical convex reflector and the secondary reflector of the Walter III reflector system comprises an elliptical cylindrical concave reflector, - The primary reflector of the Walter I-type reflector system comprises an elliptical cylindrical concave reflector and the secondary reflector of the Walter I-type reflector system comprises a hyperbolic cylindrical concave reflector.

8. The optical system of claim 7, wherein the first reflector system comprises the Volta III type reflector system.

9. An optical system according to claim 7 or 8, wherein the second reflector system comprises the additional Volta III type reflector system.

10. The optical system of claim 9, wherein one of the following is present: - the optical system comprises, in sequence between the object and the image: the Volta III reflector system, followed by the additional Volta III reflector system, - the optical system comprises, in sequence between the object and the image: a primary reflector of the Walter III reflector system, followed by a primary reflector of the additional Walter III reflector system, followed by a secondary reflector of the Walter III reflector system, followed by a secondary reflector of the additional Walter III reflector system.

11. An optical system according to any one of claims 1 to 10, wherein at least one of the first reflector system and the second reflector system comprises a diffractive element for spatially separating diffracted spectral components of the radiation beam, And among them, The diffractive element is arranged as part of a secondary reflector of the first reflector system.

12. The optical system according to any one of claims 1 to 11, wherein the optical system is configured such that a path length of a chief ray defined between the object and the image is less than or equal to 3 meters.

13. A metrology apparatus for determining a property of interest of a portion of a structure fabricated on a substrate, the metrology apparatus comprising an optical system according to any one of claims 1 to 12.

14. A lithography cell comprising a metrology apparatus according to claim 13.

15. A method of using an optical system for focusing a radiation beam onto a region of interest of a substrate in a metrology apparatus, the radiation beam comprising radiation in the soft X-ray or extreme ultraviolet spectral range, the method comprising: providing an optical system comprising a first reflector system and a second reflector system, each of the first reflector system and the second reflector system comprising a finite-to-finite Volta reflector system; and A reduced image of the object including the apparent source of the radiation beam is formed over the region of interest.

Images