CN120615181A - Measurement methods and associated measurement tools - Google Patents

Abstract

The present invention discloses a measurement method and a measurement tool, comprising: a first dispersive element, which is operable to receive and scatter source radiation to generate first scattered radiation; a final focusing element, which is configured to: collect at least a portion of the first scattered radiation, the collected first scattered radiation comprising a plurality of first scattered beams that have each been scattered in corresponding different directions; and focus the collected first scattered radiation on a second dispersive element so that each of the first scattered beams is incident on the second dispersive element at a corresponding different angle of incidence; and at least one detector, which is operable to detect the second scattered radiation that has been scattered by the second dispersive element.

Description

Metrology method and associated metrology tool

Cross Reference to Related Applications

The present application claims priority from EP application 23156294.3 filed on day 13 of 2.2023 and EP application 24152531.0 filed on day 18 of 1.2024, and which are incorporated herein by reference in their entireties.

Technical Field

The present invention relates to a metrology method and apparatus which may be used, for example, to determine characteristics of structures on a substrate.

Background

A lithographic apparatus is a machine that is configured to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). The lithographic apparatus may, for example, project a pattern (also commonly 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).

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

Low k ₁ lithography can be used to process features with dimensions less than the classical resolution limit of a lithographic apparatus. In such a process, the resolution formula may be expressed as cd=k ₁ ×λ/NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (typically the minimum feature size of the printing, but in this case half pitch), and k ₁ is the empirical resolution factor. In general, the smaller the k ₁, the more difficult it becomes to reproduce a pattern on a substrate that resembles the shape and size intended by the circuit designer in order to achieve a particular electrical functionality and performance. To overcome these difficulties, complex fine tuning steps may be applied to the lithographic projection apparatus and/or the design layout. These include, for example, but are not limited to, optimization of NA, custom illumination schemes, use of phase shift patterning 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 commonly defined as "resolution enhancement techniques" (RET).

In lithographic processes, as well as other manufacturing processes, it is often desirable to measure a parameter of interest, i.e. to measure the created structure, for example for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are commonly used to measure Critical Dimensions (CDs), as well as specialized tools to measure overlay (alignment accuracy of two layers in a device). Recently, various forms of scatterometers have been developed for use in the field of photolithography.

The fabrication process may be, for example, photolithography, etching, deposition, chemical mechanical planarization, oxidation, ion implantation, diffusion, or a combination of two or more thereof.

Examples of known scatterometers typically rely on providing a dedicated metrology target. For example, the method may require a target in the form of a simple grating that is large enough that the measurement beam generates a spot smaller than the grating (i.e., the grating is underfilled). In the so-called reconstruction method, the properties of the grating can be calculated by modeling the interaction of the scattered radiation with a mathematical model of the target structure. The parameters of the model are adjusted until the simulated interactions produce a diffraction pattern similar to that observed from a real target.

In addition to measuring feature shapes by reconstruction, such devices may also be used to measure diffraction-based overlaps, as described in published patent application US2006066855 A1. Overlay measurement of smaller targets is achieved using diffraction-based overlay metrology of dark field imaging of diffraction orders. These targets may be smaller than the illumination spot and may be surrounded by product structures on the wafer. Examples of dark field imaging metrology can be found in many published patent applications such as, for example, US2011102753A1 and US20120044470 a. Multiple gratings may be measured in one image using a composite grating target. Known scatterometers tend to use light in the visible or near Infrared (IR) wave range, which requires that the pitch of the grating be much coarser than the actual product structure whose properties are actually of interest. Deep Ultraviolet (DUV), extreme Ultraviolet (EUV) or X-ray radiation having much shorter wavelengths may be used to define such product features. Unfortunately, such wavelengths are generally not available or useable for metrology.

On the other hand, modern product structures are so small in size that they cannot be imaged by optical metrology techniques. Small features include, for example, those features formed by multiple patterning processes and/or pitch multiplication. Thus, targets for high volume metrology typically use much larger features than products whose overlay error or critical dimension is a property of interest. The measurement results are only indirectly related to the dimensions of the real product structure and may be inaccurate, as the metrology target does not suffer from the same distortion under optical projection in the lithographic apparatus and/or under different treatments in other steps of the manufacturing process. While Scanning Electron Microscopy (SEM) is able to directly resolve these modern product structures, SEM is much more time consuming than optical measurements. Furthermore, electrons cannot penetrate through too thick a process layer, which makes them less suitable for metrology applications. Other techniques, such as measuring electrical properties using contact pads, are also known, but they only provide indirect evidence of the structure of a real product.

By reducing the wavelength of the radiation used during metrology, it is possible to resolve smaller structures to increase sensitivity to structural changes in the structure and/or to penetrate further into the product structure. One such method of generating suitable high frequency radiation (e.g., hard X-rays, soft X-rays, and/or EUV radiation) may be to excite a generating medium using pump radiation (e.g., infrared IR radiation), thereby generating emission radiation, optionally including higher harmonic generation of the high frequency radiation.

In order to determine a parameter of interest related to a measurement target, it may be desirable to maximize or increase signal diversity in the detection signal.

Disclosure of Invention

In a first aspect of the invention, a metrology tool is provided comprising a first dispersive element operable to receive and scatter source radiation so as to generate first scattered radiation, a final focusing element configured to collect at least a portion of the first scattered radiation, the collected first scattered radiation comprising a plurality of first scattered beams that have each been scattered in a respective different direction, and focus the collected first scattered radiation on a second dispersive element such that each of the first scattered beams is incident on the second dispersive element at a respective different angle of incidence, and at least one detector operable to detect second scattered radiation that has been scattered by the second dispersive element.

In a second aspect of the invention, a focusing mirror element is provided comprising a hollow oblong body portion, having a mirrored inner surface and comprising a first opening at an input end and a second opening at an output end.

In a third aspect of the invention, there is provided a method of manufacturing a focusing mirror element according to the second aspect comprising providing an axisymmetric spindle comprising an oblong portion, forming a focusing mirror element body around the spindle, and releasing the focusing mirror element body from the spindle, whereby the focusing mirror element body has an optical surface defined by the inner surface of the focusing mirror element body.

In a fourth aspect of the invention, a metrology method is provided, comprising generating source radiation, scattering the source radiation so as to generate first scattered radiation, collecting at least a portion of the first scattered radiation, the collected first scattered radiation comprising a plurality of first scattered beams each having been scattered in a respective different direction, focusing the collected first scattered radiation on a target such that each of the first scattered beams is incident on the target at a respective different angle of incidence, and detecting second scattered radiation that has been scattered by the target.

These and other aspects and advantages of the apparatus and methods disclosed herein will be apparent from consideration of the following description and accompanying drawings of exemplary embodiments.

Drawings

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

FIG. 1 depicts a schematic overview of a lithographic apparatus;

fig. 2 depicts a schematic overview of a lithography unit;

FIG. 3 depicts a schematic representation of global lithography representing cooperation between three key technologies to optimize semiconductor manufacturing;

figure 4 schematically illustrates a scatterometry device;

figure 5 schematically illustrates a transmission scatterometry device;

fig. 6 depicts a schematic representation of a metrology apparatus in which EUV and/or SXR radiation is used;

FIG. 7 depicts a simplified schematic of an illumination source;

fig. 8 (a) and 8 (b) each depict a simplified schematic of a metrology arrangement according to an embodiment;

FIG. 9 depicts an exemplary diffraction pattern as may be captured by the metrology arrangement of FIG. 8;

10 (a), 10 (b) and 10 (c) depict an arrangement for providing a virtual grating at a source plane according to an embodiment;

FIG. 11 depicts a proposed refocusing mirror design according to an embodiment and suitable for use in the metrology arrangement of FIG. 8;

Fig. 12 depicts a method for obtaining an origin displacement of a diffracted beam from a diffraction grating for use in a phase-determining embodiment of the concepts disclosed herein;

Figure 13 depicts how such an origin displacement results in mutual wavefront tilting of the diffracted beams of figure 12 and thus in an optical path difference between these diffracted beams;

FIG. 14 is an exemplary interference image obtained from the arrangement depicted in FIGS. 12 and 13;

FIG. 15 (a), FIG. 15 (b), FIG. 15 (c) and FIG. 15 (d) illustrate various SXR interferometry target measurement arrangements according to embodiments, and

Fig. 16 (a) and 16 (b) illustrate other various SXR interferometry target measurement arrangements according to embodiments.

Detailed Description

In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation and particle radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm), EUV (extreme ultra-violet radiation, e.g., having a wavelength in the range of 5 nm to 100 nm), X-ray radiation, electron beam radiation, and other particle radiation.

The terms "reticle," "mask," or "patterning device" as used herein may be broadly interpreted as referring to a generic patterning device that can be used to impart an incoming radiation beam with a patterned cross-section that corresponds to a pattern to be created in a target portion of the substrate. The term "light valve" may also be used in this context. Examples of other such patterning devices, in addition to classical masks (transmissive or reflective, binary, phase-shift, hybrid, etc.), include programmable mirror arrays and programmable LCD arrays.

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

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, such as refractive, reflective, diffractive, magnetic, electromagnetic, electrostatic and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

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

The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system PS and the substrate W, which is also referred to as immersion lithography. Further information about immersion techniques is given in US6952253, which is incorporated herein by reference in its entirety.

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

In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold the sensor and/or the cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning device 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 providing the immersion liquid. When the substrate support WT is remote from the projection system PS, the measurement stage can be moved under the projection system PS.

In operation, the radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the mask support T, and is patterned by a pattern (design layout) present on the patterning device MA. After traversing the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position measurement system IF, the substrate support WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in fig. 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks 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 the target portions C, the substrate alignment marks P1, P2 are referred to as scribe-lane alignment marks.

As shown in fig. 2, the lithographic apparatus LA may form part of a lithographic cell LC, sometimes also referred to as a lithography cell or (lithography) cluster, which typically also includes apparatus to perform pre-exposure and post-exposure processes on the substrate W. Conventionally, these include a spin coater SC to deposit a resist layer, a developer DE to develop the exposed resist, a chill plate CH and a bake plate BK for adjusting the temperature of the substrate W (e.g., for adjusting the solvent in the resist layer), for example. The substrate transport apparatus or robot RO picks up a substrate W from the input/output ports I/O1, I/O2, moves the substrate W between different processing apparatuses, and delivers the substrate W to the load station LB of the lithographic apparatus LA. The apparatus in the lithographic cell (which apparatus is also commonly referred to as a track) may be under the control of a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithographic apparatus LA, e.g. via a lithographic control unit LACU.

In a lithographic process, it may be desirable to frequently measure the created structure, for example, for process control and verification. The tool making such measurements may be referred to as a metrology tool MT. Different types of metrology tools MT for making such measurements are known, including scanning electron microscopy or various forms of scatterometer metrology tools MT. Scatterometers are multipurpose instruments that allow for measurement of parameters of a lithographic process by having a sensor in or near the pupil or a plane conjugate to the pupil of the scatterometer's objective lens, which measurement is commonly referred to as pupil-based measurement, or by having a sensor in or near the image plane or a plane conjugate to the image plane, in which case the measurement is commonly referred to as image-or field-based measurement. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470a, US20110249244, US20110026032 or EP1628164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers may use light from hard X-rays (HXR), soft X-rays (SXR), extreme Ultraviolet (EUV), visible to the near Infrared (IR) and IR wavelength ranges to measure gratings. In the case where the radiation is hard or soft X-rays, the scatterometer may alternatively be a small angle X-ray scatterometry tool.

In order for the substrate W exposed by the lithographic apparatus LA to be correctly and consistently exposed, it may be desirable to inspect the substrate to measure properties of the patterned structure, such as overlay error between subsequent layers, line thickness, critical Dimension (CD), shape of the structure, etc. For this purpose, inspection tools and/or metrology tools (not shown) may be included in the lithography unit LC. If errors are detected, especially if the inspection is completed before other substrates W of the same batch or group remain to be exposed or processed, adjustments may be made, for example, to the exposure of subsequent substrates or other processing steps to be performed on the substrates W.

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

In a first embodiment, the scatterometer MT is an angle resolved scatterometer. In such scatterometers, a reconstruction method may be applied to the measured signal to reconstruct or calculate the properties of the grating. Such a reconstruction may for example result from simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation result with the result of the measurement. The parameters of the mathematical model are adjusted until the simulated interactions produce a diffraction pattern similar to that observed from a real target.

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

In a third embodiment, the scatterometer MT is an ellipsometer. Ellipsometers allow parameters of the lithographic process to be determined by measuring the scattered or transmitted radiation for each polarization state. Such metrology apparatus emit polarized light (such as linear, circular or elliptical) by using, for example, appropriate polarizing filters in the illumination section of the metrology apparatus. Sources suitable for metrology equipment may also provide polarized radiation. Various embodiments of existing ellipsometers 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.

In an embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the spectrum of the reflection and/or detecting an asymmetry in the configuration, the asymmetry being related to the degree of overlap. Two (possibly overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers) and may be formed at substantially the same location on the wafer. The scatterometer may have a symmetric detection configuration as described, for example, in commonly owned patent application ep1,628,164a, such that any asymmetry is clearly distinguishable. This provides a straightforward way to measure misalignment in the grating. Other examples of overlay errors between two layers containing periodic structures when measuring targets by asymmetry of the periodic structures can be found in PCT patent application publication No. WO 2011/012624 or US patent application US 20160161863, which are incorporated herein by reference in their entirety.

Other parameters of interest may be focus and dose. The focus and dose may be determined simultaneously by scatterometry (or alternatively, by scanning electron microscopy), as described in U.S. patent application US2011-0249244, which is incorporated herein by reference in its entirety. 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 known as the focus exposure matrix). If these unique combinations of critical dimensions and sidewall angles are available, then focus and dose values can be uniquely determined from these measurements.

The metrology target may be a collection of composite gratings formed by a lithographic process (primarily in a resist), but also after other fabrication processes (e.g., etching processes). The pitch and linewidth of the structures in the grating may depend primarily on the measurement optics (in particular, NA of the optics) to be able to capture the diffraction orders from the metrology targets. As indicated earlier, the diffracted signal may be used to determine a shift (also referred to as an 'overlap') between two layers, or may be used to reconstruct at least a portion of an original grating as produced by a lithographic process. The reconstruction may be used to provide a guide for the quality of the lithographic process and may be used to control at least a portion of the lithographic process. The target may have smaller subsections that are configured to mimic the dimensions of the functional portions of the design layout in the target. Due to this sub-segmentation, the target will behave more like a functional part of the design layout, so that the overall process parameter measurement is better like a functional part of the design layout. The target may be measured in either an underfill mode or an overfill mode. In the underfill mode, the measurement beam produces a spot smaller than the overall target. In the overfill mode, the measurement beam produces a spot that is larger than the overall target. In such an overfill mode, it is also possible to measure different targets simultaneously, thus determining different process parameters simultaneously.

The overall measurement quality of a lithographic parameter using a particular target is determined, at least in part, by the measurement recipe used to measure that lithographic parameter. The term "substrate measurement recipe" may include measuring one or more parameters of itself, one or more parameters of one or more patterns measured, or both. For example, if the measurements used in the substrate measurement recipe are diffraction-based optical measurements, 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 with respect to the substrate, the orientation of the radiation with respect to the pattern on the substrate, and so forth. One of the criteria used to select a measurement option may be, for example, the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US2016/0370717A1, which are incorporated herein by reference in their entirety.

The patterning process in the lithographic apparatus LA may be one of the most critical steps in the process requiring high accuracy in the sizing and placement of structures on the substrate W. To ensure such high accuracy, three systems may be combined in a so-called "overall" control environment, as schematically depicted in fig. 3. One of these systems is a lithographic apparatus LA, which is (virtually) connected to a metrology tool MT (second system) and a computer system CL (third system). The key to such an "overall" environment is to optimize the cooperation between the three systems to enhance the overall process window, and to 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, overlap) within which a particular fabrication process produces defined results (e.g., functional semiconductor devices) -process parameters in a lithographic process or patterning process may be allowed to vary within the range.

The computer system CL can use (a portion of) the design layout to be patterned to predict which resolution enhancement technique to use and perform computational lithography simulation and computation to determine which mask layout and lithographic apparatus set the largest overall process window (depicted in fig. 3 by the double arrow in the first scale SC 1) that implements the patterning process. The resolution enhancement technique may be arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where the lithographic apparatus LA is currently operating within the process window (e.g. using input from the metrology tool MET) to predict whether a defect may exist due to, for example, sub-optimal processing (depicted in fig. 3 by the arrow pointing to "0" in the second scale SC 2).

The metrology tool MT may provide input to the computer system CL to enable accurate simulation and prediction, and may provide feedback to the lithographic apparatus LA to identify possible drift, for example, in a calibrated state of the lithographic apparatus LA (depicted in fig. 3 by the plurality of arrows in the third scale SC 3).

Many different forms of metrology tools MT may be provided for measuring structures created using a lithographic patterning apparatus. The metrology tool MT may use electromagnetic radiation to interrogate structures. The nature of the radiation (e.g., wavelength, bandwidth, power) can affect different measurement characteristics of the tool, with shorter wavelengths generally allowing increased resolution. The wavelength of the radiation has an effect on the resolution that the metrology tool can achieve. Therefore, in order to be able to measure structures with features of small size, metrology tools MT with short wavelength radiation sources are preferred.

Another way in which the radiation wavelength can influence the measurement characteristics is the penetration depth and the transparency/opacity of the material to be inspected at the radiation wavelength. Depending on the opacity and/or penetration depth, the radiation may be used for measurement in transmission or reflection. The type of measurement may influence whether information about the surface of the structure/substrate and/or the interior of the body is obtained. Thus, penetration depth and opacity are another element to be considered when selecting the wavelength of radiation of the metrology tool.

In order to achieve a higher resolution of the measurement of lithographically patterned structures, metrology tools MT with short wavelengths are preferred. This may include, for example, wavelengths shorter than the visible wavelengths in the UV, EUV and X-ray portions of the electromagnetic spectrum. Hard X-ray methods such as transmission small angle X-ray scattering (TSAXS) utilize the high resolution and high penetration depth of hard X-rays and can therefore operate in transmission. Soft X-rays and EUV, on the other hand, do not penetrate as far into the target, but can cause a rich optical response in the material to be detected. This may be due to the optical properties of many semiconductor materials and due to the structure being comparable in size to the detection wavelength. Thus, the EUV and/or soft X-ray metrology tool MT may be operated in a reflective manner, e.g. by imaging or by analyzing a diffraction pattern from a lithographically patterned structure.

For hard X-ray, soft X-ray and EUV radiation, the use in High Volume Manufacturing (HVM) applications may be limited due to the lack of available high intensity radiation sources at the desired wavelengths. In the case of hard X-rays, sources commonly used in industrial applications include X-ray tubes. X-ray tubes, including, for example, advanced X-ray tubes based on liquid metal anodes or rotating anodes, may be relatively inexpensive and compact, but may lack the brightness required for HVM applications. High brightness X-ray sources such as Synchrotron Light Sources (SLS) and X-ray free electron lasers (XFEL) currently exist, but their size (> 100 m) and high cost (hundreds of millions of euros) make them too large and expensive for metrology applications. Similarly, the availability of sufficiently bright EUV and soft X-ray radiation sources is lacking.

One example of a metrology device such as a scatterometer is depicted in fig. 4. It may comprise a broadband (e.g. white light) radiation projector 2 projecting radiation 5 onto a substrate W. The reflected or scattered radiation 10 is passed to a spectrometer detector 4, the spectrometer detector 4 measuring the spectrum 6 of the specularly reflected radiation (i.e. a measurement of the intensity I as a function of the wavelength λ). From this data, the structure or profile 8 yielding the detected spectrum can be reconstructed by the processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra as shown at the bottom of fig. 4. In general, for reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process by which the structure is fabricated, leaving only a few parameters of the structure to be determined from the scatterometry data. Such scatterometers may be configured as normal incidence scatterometers or oblique incidence scatterometers.

A transmissive version of an example of a metrology device such as the scatterometer shown in fig. 4 is depicted in fig. 5. The transmitted radiation 11 is passed to a spectrometer detector 4, the spectrometer detector 4 measuring a spectrum 6 as discussed with respect to fig. 4. Such scatterometers may be configured as normal incidence scatterometers or oblique incidence scatterometers. Alternatively, the transmissive version uses hard X-ray radiation where the wavelength is <1 nm, optionally <0.1 nm, optionally <0.01 nm.

As an alternative to optical metrology methods, the use of hard X-rays, soft X-rays or EUV radiation has also been considered, for example radiation having at least one of the following wavelength ranges, <0.01 nm, <0.1 nm, <1 nm, between 0.01 nm and 100 nm, between 0.01 nm and 50nm, between 1 nm and 50nm, between 1 nm and 20 nm, between 5 nm and 20 nm and between 10 nm and 20 nm. One example of a metrology tool that functions in one of the wavelength ranges presented above is transmissive small angle X-ray scattering (e.g., T-SAXS in US 2007224518a, the contents of which are incorporated herein by reference in its entirety). The use of profile (CD) measurements with T-SAXS is discussed by LEMAILLET et al in the "mutual comparison between optical measurements of FinFET structures and X-ray scatterometry (Intercomparison between optical and X-ray scatterometry measurements of FinFET structures) volume 8681 of SPIE conference, 2013. It is noted that the use of Laser Produced Plasma (LPP) X-ray sources is described in U.S. patent publication No. 2019/003988A1 and U.S. patent publication No. 2019/215940A1, which are incorporated herein by reference in their entirety. Reflectometry techniques using X-ray (GI-XRS) and Extreme Ultraviolet (EUV) radiation at grazing incidence may be used to measure properties of film and layer stacks on substrates. In the general field of reflectometry, angular and/or spectroscopic techniques may be applied. In goniometry, the variation of reflected beams with different angles of incidence can be measured. Spectral reflectometry, on the other hand, measures the spectrum of wavelengths reflected at a given angle (using broadband radiation). For example, EUV reflectometry has been used for inspection of a mask blank prior to manufacturing a reticle (patterning device) for use in EUV lithography.

It is possible that the application range is such that the use of wavelengths in the hard X-ray, soft X-ray or EUV domain, for example, is insufficient. Published patent applications US 20130304424A1 and US 2014019097A1 (Bakeman et al/KLA) describe hybrid metrology techniques in which measurements made using x-rays and optical measurements by means of wavelengths in the range of 120 nm and 2000 nm are combined together to obtain measurements of parameters such as CD. CD measurements are obtained by coupling an x-ray mathematical model and an optical mathematical model by means of one or more common sense. The contents of the referenced U.S. patent application are incorporated herein by reference in their entirety.

Fig. 6 depicts a schematic representation of a metrology apparatus 302 in which the aforementioned radiation may be used to measure parameters of structures on a substrate. The metrology device 302 presented in fig. 6 may be suitable for hard X-ray, soft X-ray and/or EUV fields.

Fig. 6 illustrates by way of example only a schematic physical arrangement of a metrology device 302 comprising a spectroscatterometer using hard X-rays, soft X-rays and/or EUV radiation (optionally at grazing incidence). An alternative form of inspection apparatus may be provided in the form of an angle-resolved scatterometer which may use radiation incident normal or near normal similar to a conventional scatterometer operating at longer wavelengths, and which may also use radiation having a direction greater than 1 ° or 2 ° from the direction parallel to the substrate. An alternative form of inspection apparatus may be provided in the form of a transmission scatterometer to which the arrangement of figure 5 is adapted.

The inspection apparatus 302 includes a radiation source or illumination source 310, an illumination system 312, a substrate support 316, inspection systems 318, 398, and a Metrology Processing Unit (MPU) 320.

In this example, the illumination source 310 is used to generate EUV, hard X-ray or soft X-ray radiation. The illumination source 310 may be based on Higher Harmonic Generation (HHG) technology as shown in fig. 6, and it may also be other types of illumination sources, such as liquid metal injection sources, inverse Compton Scattering (ICS) sources, plasma channel sources, magnetic undulator sources, free Electron Laser (FEL) sources, compact storage ring sources, electrical discharge generated plasma sources, soft X-ray laser sources, rotating anode sources, solid anode sources, particle accelerator sources, micro-focus sources, or laser generated plasma sources.

The HHG source may be a gas jet/nozzle source, a capillary/fiber source, or a plenum source.

For the example of a HHG source, as shown in fig. 6, the main components of the radiation source are a pump radiation source 330 operable to emit pump radiation and a gas delivery system 332. Optionally, the pump radiation source 330 is a laser, optionally the pump radiation source 330 is a pulsed high power infrared or optical laser. The pump radiation source 330 may be, for example, a fiber-based laser with an optical amplifier, so that each pulse is generated with pulses of infrared radiation that may last, for example, less than 1ns (1 nanosecond), with pulse repetition rates as high as several megahertz as desired. The wavelength of the infrared radiation may be in the range of 200 nm to 10 μm, for example in the region of 1 μm (1 micrometer). Optionally, the laser pulses are delivered as first pump radiation 340 to the gas delivery system 332, wherein a portion of the gas, radiation, is converted to higher frequency than the first radiation into emitted radiation 342. The gas supply 334 supplies a suitable gas to the gas delivery system 332 where the gas is optionally ionized by a power supply 336. The gas delivery system 332 may be a cutting tube.

The gas provided by the gas delivery system 332 defines a gas target, which may be a gas flow or a static volume. The gas may be, for example, air, neon (Ne), helium (He), nitrogen (N ₂), oxygen (O ₂), argon (Ar), krypton (Kr), xenon (Xe), carbon dioxide, and combinations thereof. These may be selectable options within the same device. The emitted radiation may contain multiple wavelengths. If the emitted radiation is monochromatic, the measurement calculations (e.g. reconstruction) can be simplified, but radiation with several wavelengths is more easily generated. The emission divergence angle of the emitted radiation may be wavelength dependent. For example, when imaging structures of different materials, different wavelengths will provide different levels of contrast. For inspection of metal or silicon structures, for example, wavelengths may be selected that are different from those used for imaging features of (carbon-based) resists or for detecting contamination of such different materials. One or more filtering devices 344 may be provided. For example, a filter such as a thin film sheet of aluminum (Al) or zirconium (Zr) may be used to cut off the transmission of the primary IR radiation further into the inspection apparatus. A grating (not shown) may be provided to select one or more specific wavelengths from those generated. Optionally, the illumination source comprises a space configured to be evacuated, and the gas delivery system is configured to provide a gas target in the space. Optionally, some or all of the beam path may be contained within a vacuum environment, bearing in mind that SXR and/or EUV radiation is absorbed while traveling in air. The various components of the radiation source 310 and the illumination optics 312 may be adjustable to implement different metrology "options" within the same apparatus. For example, different wavelengths and/or polarizations may be made selectable.

Depending on the material of the structure under inspection, different wavelengths may provide the desired level of penetration into the underlying layers. Short wavelengths may be preferred in order to resolve the smallest device feature and defects among the smallest device features. For example, one or more wavelengths in the range of 0.01 nm to 20 nm, or alternatively in the range of 1 nm to 10 nm, or alternatively in the range of 10 nm to 20 nm, may be selected. Wavelengths shorter than 5nm may suffer from very low critical angles when reflected off materials of interest in semiconductor fabrication. Thus, selecting wavelengths greater than 5nm can provide a stronger signal at higher angles of incidence. On the other hand, wavelengths up to 50 nm may be useful if the inspection task is to detect the presence of a particular material, for example, to detect contamination.

The filtered beam 342 may enter the inspection chamber 350 from the radiation source 310, wherein the substrate W including the structure of interest is held at a measurement position by the substrate support 316 for inspection. The structure of interest is labeled T. Optionally, the environment within the inspection chamber 350 may be maintained near vacuum by a vacuum pump 352 such that SXR and/or EUV radiation may pass through the environment without excessive attenuation. The illumination system 312 has the function of focusing 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-referenced published U.S. patent application US2017/0184981A1 (the contents of which are incorporated herein by reference in their entirety). Focusing is performed to achieve a circular or elliptical spot S below 10 μm in diameter when projected onto a structure of interest. The substrate support 316 includes, for example, an X-Y translation stage and a rotation stage by which any portion of the substrate W may be brought to the focal point 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 tilt stage that can tilt the substrate W at a particular angle to control the angle of incidence of the focused beam on the structure of interest T.

Optionally, the illumination system 312 provides a reference radiation beam to the reference detector 314, and the reference detector 314 may be configured to measure spectra and/or intensities of different wavelengths in the filtered beam 342. The reference detector 314 may be configured to generate a signal 315 that is provided to the processor 320, and the filter may include information about the spectrum of the filtered beam 342 and/or the intensity of different wavelengths in the filtered beam.

The reflected radiation 360 is captured by the detector 318 and the spectrum is provided to the processor 320 for calculating properties of the target structure T. The illumination system 312 and the detection system 318 thus form an inspection apparatus. The inspection apparatus may include a hard X-ray, soft X-ray and/or EUV spectral reflectometer of the kind described in US2016282282A1, the contents of US2016282282A1 being incorporated herein by reference in its entirety.

The radiation of the focused beam 356 may also be partially diffracted if the target Ta has a certain periodicity. Diffracted radiation 397 follows another path at a well-defined angle relative to the angle of incidence, followed by reflected radiation 360. In fig. 6, the diffracted radiation 397 is depicted in a schematic manner, and the diffracted radiation 397 may follow many other paths than the depicted path. Inspection apparatus 302 may also include an additional detection system 398 that detects and/or images at least a portion of diffracted radiation 397. In fig. 6, a single further detection system 398 is depicted, but embodiments of inspection apparatus 302 may also include more than one further detection system 398 arranged at different positions to detect and/or image diffracted radiation 397 in multiple diffraction directions. In other words, the (higher) diffraction order of the focused radiation beam impinging on the target Ta is detected and/or imaged by one or more further detection systems 398. One or more inspection systems 398 generate signals 399 that are provided to the metrology 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.

To aid in alignment and focusing of the spot S with the desired product structure, the inspection apparatus 302 may also use auxiliary radiation to provide auxiliary optics under the control of the metrology processor 320. The metrology processor 320 may also be in communication with a position controller 372 that operates the platen, the rotating and/or tilting stage. The processor 320 receives highly accurate feedback regarding the position and orientation of the substrate via the sensors. The sensor 374 may include, for example, an interferometer that may give accuracy in the picometer region. In operation of the inspection apparatus 302, the spectral data 382 captured by the detection system 318 is delivered to the metrology processing unit 320.

As mentioned, alternative forms of inspection apparatus optionally use hard X-rays, soft X-rays and/or EUV radiation at or near normal incidence, for example to perform diffraction-based asymmetry measurements. Another alternative form of inspection apparatus uses hard X-rays, soft X-rays and/or EUV radiation having a direction greater than 1 ° or 2 ° from the direction parallel to the substrate. Two types of inspection equipment may be provided in a hybrid metrology system. The performance parameters to be measured may include Overlay (OVL), critical Dimension (CD), focus of the lithographic apparatus when the lithographic apparatus prints the target structure, coherent Diffraction Imaging (CDI), and overlay At Resolution (ARO) measurements. The hard X-ray, soft X-ray and/or EUV radiation may for example have a wavelength of less than 100nm, for example radiation in the range of 1 nm to 50nm, 1 nm to 30 nm, 5nm to 30 nm or optionally in the range of from 10 nm to 20 nm is used. The radiation may be narrowband or wideband in nature. The radiation may have discrete peaks in a particular wavelength band or may have more continuous characteristics.

Similar to optical scatterometers used in today's production facilities, inspection apparatus 302 may be used to measure structures within resist material processed within a lithography unit (after a development inspection or ADI), and/or to measure structures after structures have been formed in a harder material (after an etching inspection or AEI). For example, inspection apparatus 302 may be used to inspect a substrate after the substrate has been processed by a developing apparatus, an etching apparatus, an annealing apparatus, and/or other apparatus.

A metrology tool MT including, but not limited to, the scatterometer mentioned above may use radiation from a radiation source to perform measurements. The radiation used by the metrology tool MT may be electromagnetic radiation. The radiation may be optical radiation, for example, radiation in the infrared, visible, and/or ultraviolet portions of the electromagnetic spectrum. The metrology tool MT may use radiation to measure or inspect properties and aspects of a substrate, for example, a lithographically exposed pattern on a semiconductor substrate. The type and quality of the measurement may depend on several properties of the radiation used by the metrology tool MT. For example, the resolution of electromagnetic measurement may depend on the wavelength of the radiation, where smaller wavelengths are able to measure smaller features, for example due to diffraction limits. For measuring features with small dimensions, it may be preferable to perform the measurement using radiation with short wavelengths, e.g. EUV, hard X-ray (HXR) and/or soft X-ray (SXR) radiation. In order to perform measurements at a particular wavelength or range of wavelengths, the metrology tool MT needs to access the source providing radiation at that wavelength/those wavelengths. There are different types of sources for providing radiation of different wavelengths. Depending on the wavelength(s) provided by the source, different types of radiation generation methods may be used. For Extreme Ultraviolet (EUV) radiation (e.g., 1 nm to 100 nm) and/or soft X-ray (SXR) radiation (e.g., 0.1 nm to 20 nm, 1 nm to 20 nm, or 10 nm to 20 nm), the source may use Higher Harmonic Generation (HHG) or any other type of source mentioned above to obtain radiation at the desired wavelength(s).

Fig. 7 shows a simplified schematic diagram of an embodiment 600 of an illumination source 310. The illumination source 310 may be an illumination source for Higher Harmonic Generation (HHG). One or more of the features of the illumination source in the metrology tool described with respect to fig. 6 may also be optionally present in the illumination source 600. The illumination source 600 comprises a chamber 601 and is configured to receive pump radiation 611 having a propagation direction indicated by an arrow. The pump radiation 611 shown here is an example of pump radiation 340 from the pump radiation source 330, as shown in fig. 6. The pump radiation 611 may be directed into the chamber 601 through a radiation input 605, which radiation input 605 may be a viewport, optionally made of fused silica or equivalent material. The pump radiation 611 may have a gaussian or hollow (e.g., annular) transverse cross-sectional profile and may be incident (optionally focused) on the gas flow 615 within the chamber 601, the gas flow 615 having a flow direction indicated by the second arrow. The gas flow 615 includes a small volume called a gas volume or gas target (e.g., a few cubic millimeters) of a particular gas (e.g., air, neon (Ne), helium (He), nitrogen (N ₂), oxygen (O ₂), argon (Ar), krypton (Kr), xenon (Xe), carbon dioxide, and combinations thereof), wherein the gas pressure is above the particular value. The airflow 615 may be a steady flow. Other media may also be used, such as a metal plasma (e.g., aluminum plasma).

The gas delivery system of the illumination source 600 is configured to provide a gas flow 615. The illumination source 600 is configured to provide pump radiation 611 in a gas stream 615 to drive the generation of emitted radiation 613. The region in which at least the majority of the emitted radiation 613 is generated is referred to as the interaction region. The interaction area can vary from tens of micrometers (for tightly focused pump radiation) to a few millimeters or centimeters (for moderately focused pump radiation) or even up to a few meters (for extremely loosely focused pump radiation). The gas delivery system is configured to provide a gas target for generating emitted radiation at an interaction region of the gas target, and optionally the illumination source is configured to receive pump radiation and provide pump radiation at the interaction region. Optionally, a gas stream 615 is provided by the gas delivery system into the evacuated or nearly evacuated space. The gas delivery system may comprise a gas nozzle 609, as shown in fig. 6, the gas nozzle 609 comprising an opening 617 in the exit plane of the gas nozzle 609. The air flow 615 is provided from an opening 617. The gas trap is used to confine the gas flow 615 to a specific volume by extracting a residual gas flow and maintaining a vacuum or near-vacuum atmosphere inside the chamber 601. Alternatively, the gas nozzle 609 may be made of a thick-walled tube and/or a high thermal conductivity material to avoid thermal deformation due to the high power pump radiation 611.

The size of the gas nozzles 609 is also contemplated for use in scaled-up or scaled-down versions from micron-sized nozzles to meter-sized nozzles. This wide range of sizing comes from the fact that the settings can be scaled such that the intensity of the pump radiation at the air flow is eventually within a certain range, which may be beneficial for the emitted radiation, which requires different sizing for different pump radiation energies, which may be pulsed lasers, and the pulse energy may vary from tens of microjoules to joules. Optionally, the gas nozzle 609 has a thicker wall to reduce nozzle distortion caused by thermal expansion effects, which can be detected by, for example, a camera. A gas nozzle with thicker walls may produce a stable gas volume with reduced variation. Optionally, the illumination source includes a gas trap proximate to the gas nozzle to maintain the pressure of the chamber 601.

Due to the interaction of the pump radiation 611 with the gas atoms of the gas flow 615, the gas flow 615 converts a portion of the pump radiation 611 into emitted radiation 613, which emitted radiation 613 may be an example of the emitted radiation 342 shown in fig. 6. The central axis of the emitted radiation 613 may be collinear with the central axis of the incident pump radiation 611. The emitted radiation 613 may have a wavelength in the X-ray or EUV range, hereinafter SXR radiation, wherein the wavelength is in the range from 0.01 nm to 100 nm, optionally from 0.1 nm to 100 nm, optionally from 1 nm to 100 nm, optionally from 1 nm to 50 nm, or optionally from 10nm to 20 nm.

In operation, the emitted radiation 613 beam may pass through the radiation output 607 and may then be steered and directed by the illumination system 603 to a substrate to be inspected for metrology measurements, the illumination system 603 may be an example of the illumination system 312 in fig. 6. The emitted radiation 613 can be directed (optionally focused) onto a structure on the substrate.

Because air (and virtually any gas) substantially absorbs SXR or EUV radiation, the volume between the gas flow 615 and the wafer to be inspected may be evacuated or nearly evacuated. Since the central axis of the emitted radiation 613 may be collinear with the central axis of the incident pump radiation 611, it may be desirable to block the pump radiation 611 from passing through the radiation output 607 and into the illumination system 603. This may be accomplished by incorporating the filtering means 344 shown in fig. 6 into the radiation output 607, the radiation output 607 being placed in the path of the emitted beam and being opaque or almost opaque to the pump radiation (e.g. opaque or almost opaque to infrared or visible light), but at least partly transparent to the emitted radiation beam. The filter may be fabricated using zirconium or a variety of materials combined in multiple layers. When the pump radiation 611 has a hollow, optionally annular, transverse cross-sectional profile, the filter may be a hollow, optionally annular block. Optionally, the filter is non-perpendicular and non-parallel to the propagation direction of the emitted radiation beam to have efficient filtering of the pump radiation. Optionally, the filter device 344 includes a hollow block and a membrane filter, such as an aluminum (Al) or zirconium (Zr) membrane filter. Alternatively, the filtering means 344 may also comprise a mirror that effectively reflects the emitted radiation but poorly reflects the pump radiation, or a wire mesh that effectively transmits the emitted radiation but poorly transmits the pump radiation.

Methods, apparatus, and components are described herein for selectively obtaining emitted radiation at higher harmonic frequencies of pump radiation. Radiation generated by the process (optionally, HHG, which uses nonlinear effects to generate radiation optionally at the harmonic frequency of the provided pump radiation) may be provided as radiation in the metrology tool MT for inspecting and/or measuring substrates. If the pump radiation comprises short pulses (i.e. several periods), the generated radiation does not necessarily have to be at exactly the harmonics of the pump radiation frequency. The substrate may be a lithographically patterned substrate. The radiation obtained by the process may also be provided in the lithographic apparatus LA and/or the lithographic cell LC. The pump radiation may be pulsed radiation that may provide a high peak intensity in a short burst time.

The pump radiation 611 may comprise radiation having one or more wavelengths higher than the one or more wavelengths of the emitted radiation. The pump radiation may comprise infrared radiation. The pump radiation may comprise radiation having a wavelength(s) in the range 500nm to 1500 nm. The pump radiation may comprise radiation having a wavelength(s) in the range 800 nm to 1300 nm. The pump radiation may comprise radiation having a wavelength(s) in the range 900 nm to 1300 nm. The pump radiation may be pulsed radiation. The pulsed pump radiation may comprise pulses having a duration in the femtosecond range.

For some embodiments, the emitted radiation (optionally higher harmonic radiation) may include one or more harmonics of the pump radiation wavelength(s). The emitted radiation may include wavelengths in the extreme ultraviolet, soft X-ray, and/or hard X-ray portions of the electromagnetic spectrum. The emitted radiation 613 may include wavelengths in one or more of the ranges of less than 1 nm, less than 0.1 nm, less than 0.01 nm, 0.01 nm to 100 nm, 0.1 nm to 100 nm, 0.1 nm to 50 nm, 1 nm to 50 nm, and 10 nm to 20 nm.

Radiation such as the higher harmonic radiation described above may be provided as source radiation in the metrology tool MT. The metrology tool MT may use source radiation to perform measurements on a substrate exposed by a lithographic apparatus. The measurements may be used to determine one or more parameters of a structure on the substrate. The use of radiation at shorter wavelengths (e.g., EUV, SXR, and/or HXR wavelengths, for example, in the wavelength ranges as included above) may allow smaller features of the structure to be resolved by the metrology tool as compared to the use of longer wavelengths (e.g., visible radiation, infrared radiation). Radiation having a shorter wavelength (such as EUV, SXR and/or HXR radiation) may also penetrate deeper into materials such as patterned substrates, which means that metrology of deeper layers on the substrate is possible. These deeper layers may not be accessible to radiation having longer wavelengths.

In the metrology tool MT, source radiation may be emitted from a radiation source and directed onto a target structure (or other structure) on a substrate. The source radiation may include EUV, SXR and/or HXR radiation. The target structure may reflect, transmit, and/or diffract source radiation incident on the target structure. The metrology tool MT may comprise one or more sensors for detecting diffracted radiation. For example, the metrology tool MT may comprise a detector for detecting positive (+1) and negative (-1) first diffraction orders. The metrology tool MT may also measure specularly reflected or transmitted radiation (0 th order diffracted radiation). Additional sensors for metrology may be present in the metrology tool MT, for example, to measure additional diffraction orders (e.g., higher diffraction orders).

In an exemplary lithographic metrology application, HHG generated radiation may be focused onto a target on a substrate using an optical column, which may be referred to as an illuminator, which transfers the radiation from the HHG source to the target. HHG radiation may then be reflected, detected, and processed from the target, for example, to measure and/or infer properties of the target.

Gas target HHG configurations can be broadly divided into three independent categories, gas injection, gas cell (gas cell), and gas capillary. Fig. 7 depicts an exemplary gas injection configuration in which a gas volume is introduced into a drive/pump radiation laser beam. In the gas injection configuration, the interaction of the driving radiation with the solid part is kept to a minimum. The gas volume may for example comprise a gas flow perpendicular to the driving radiation beam, wherein the gas volume is enclosed inside the gas chamber. In a gas capillary arrangement, the dimensions of the capillary structure holding the gas are small in the lateral direction such that it significantly affects the propagation of the driving radiation laser beam. The capillary structure may be, for example, a hollow core fiber, wherein the hollow core is configured to hold a gas.

The gas jet HHG configuration may provide a relative degree of freedom to form a spatial profile of the driving radiation beam in the far field, as it is not constrained by the limitations imposed by the gas capillary structure. The gas injection arrangement may also have less stringent alignment tolerances. On the other hand, gas capillaries can provide increased interaction areas of driving radiation and gaseous media, which can optimize HHG processes.

To use HHG radiation in metrology applications, for example, it is separated from the driving radiation downstream of the gas target. The separation of HHG and driving radiation may be different for gas jet and gas capillary configurations. In both cases, the driving radiation blocking scheme may include a metal transmissive filter for filtering out any remaining driving radiation from the short wavelength radiation. However, before such a filter can be used, the intensity of the driving radiation should be significantly reduced from its intensity at the gas target in order to avoid damage to the filter. Methods that can be used for such intensity reduction are different for gas injection and capillary configurations. For gas jet HHG, this may be designed such that it has a low intensity in the far field along the direction of short wavelength radiation propagation, due to the relative degrees of freedom of the shape and spatial profile (which may also be referred to as spatial distribution and/or spatial frequency) of the driving radiation beam focused onto the gas target. This spatial separation in the far field means that an aperture can be used to block the driving radiation and reduce its intensity.

In contrast, in a gas capillary structure, the spatial profile of the beam as it passes through the gaseous medium can be largely determined by the capillary. The spatial profile of the driving radiation may be determined by the shape and material of the capillary structure. For example, where hollow core fibers are used as the capillary structure, the shape and material of the fiber structure determines which driving radiation patterns are supported for propagation through the fiber. For most standard fibers, the supported propagation modes result in spatial profiles in which the high intensity of the driving radiation overlaps with the high intensity of the HHG radiation. For example, the driving radiation intensity may be centered in the far field with a gaussian profile or a near gaussian profile.

In the aforementioned SXR sensor/metrology configuration, the beam of SXR light may be focused on the target at a fixed single angle by means of a (e.g., single) focusing mirror or mirror system. One or more sensors in close proximity to the target may be used to detect the intensity of the diffraction orders scattered by the target. Algorithms that infer profile parameters or parameter values of interest associated with the illuminated target may be used to analyze the detected light. In the context of the present disclosure, the terms "parameter of interest" and/or "profile parameter" may include, inter alia, overlay and/or other (e.g., 3D) profile parameters, such as etch depth, sidewall angle (SWA), layer thickness Critical Dimension (CD), or any other dimension of any feature of a structure, for example. As such, in the context of the present disclosure, the target parameter or profile parameter describes a physical profile parameter of the target itself, rather than, for example, its location. More complex targets may include more than 10 or even more than 20 profile parameters.

A subset of profile parameters may be more or less correlated within the detected signal, meaning that a change in each of two or more such parameters results in a similar change in the detected signal. This means that it may be difficult to understand the effects of these parameters and determine which parameter(s) are responsible for a particular signal effect. Moreover, in such a configuration, some parameters may result in very weak signal contributions, making it difficult to obtain an accurate estimate of the parameter from the measured data.

Theoretically, measurements at multiple angles of incidence can be used to create greater signal diversity (see, e.g., ellipsometry). However, at broadband SXR or EUV wavelengths, it is difficult and/or expensive to manufacture optics to achieve this. A simple solution may (in principle) consist in providing multiple sources and illuminators in order to provide independent sources of incident light. However, this would result in a large cost and would be difficult to accommodate in a compact tool design.

To address these issues, it is proposed herein to provide a (e.g., first) dispersive element (e.g., grating) in the conjugate (field/image) plane of the wafer-side focal plane, which effectively multiplies the source beam so as to optionally simultaneously provide multiple angles of incidence on the target.

In a first embodiment, the proposed (e.g., SXR) metrology tool may comprise only a simple finite-finite conjugated illuminator system (e.g., a single annular, ellipsoidal, or kokepatrick-betz (KB) mirror system). In such a case, the only available conjugate field plane is the plane of the (virtual) source. In another embodiment, the illuminator has an intermediate focus at the second conjugate field plane. The conjugate field plane at the source is hereinafter referred to as the Source Field Plane (SFP), and the conjugate field plane at the intermediate focus is referred to as the Intermediate Field Plane (IFP).

In either case, the first scattered radiation that has been scattered by the first dispersive element may be captured by a (e.g. single) final focusing mirror element. The first scattered radiation comprises a plurality of scattered beams, i.e. both diffracted radiation and specularly reflected radiation. The final focusing mirror element re-images the plane including the first dispersive element (e.g., SFP or IFP depending on the arrangement) onto the target or the second diffractive element by capturing as much of the first scattered radiation as possible. In this way, the final focusing mirror element may capture and re-image the reflected radiation and at least one and preferably a plurality (e.g. as many as possible) of the diffraction orders comprised in the first scattered radiation. Thus, due to the various diffraction angles from the first dispersive element, the re-imaged first scattered radiation will be incident on the target at various corresponding angles of incidence. In the case where the dispersed radiation is broadband or comprises multiple wavelengths, each "diffraction order" from the first dispersive element will actually comprise multiple monochromatic diffraction orders with a color-dependent angle of incidence on the target. The second scattered radiation that has been scattered from the target or the second diffractive element may comprise specular reflection and multiple diffraction orders from the target (i.e. second scattered beams), each of which may be detected and will comprise additional information when compared to a conventional measurement using a single beam at a single angle of incidence.

Fig. 8 shows (a) a conceptual diagram and (b) a simplified schematic diagram of an embodiment. In fig. 8 (a), all beams are drawn in transmission for clarity. In fig. 8 (b), the focusing element and dispersive element are reflective, as may be more suitable for SXR measurements (at least for focusing elements, dispersive elements may operate in reflection or transmission). Fig. 8 shows a source 800, a first focusing element or first focusing mirror 805, which first focusing element or first focusing mirror 805 re-images the source 800 at an intermediate field plane comprising a first dispersive element or grating 810. Scattered radiation from the grating 810 may be captured by a second focusing element or final focusing mirror element 815, which re-images the mid-field plane onto a second dispersive element or target 820 (optionally, which is the final focusing element before the target/wafer). The second refocusing element may comprise an ellipsoidal mirror comprising a particular ellipsoidal configuration, as will be described later. Second scattered radiation from the target 820 may be captured on a detector or camera 825.

In this specification, the numbering convention for the captured diffraction orders will describe each captured order (m, n), where m describes the diffraction order from the first dispersive element/grating 810 (e.g., including at least the zeroth order and at least one higher diffraction order), and n describes the diffraction order from the second dispersive element/target 820. In conventional SXR measurements, only m=0 steps are detected. Thus, where the captured diffraction order is a +1 diffraction order from the target 820 (resulting from incident radiation on the target including a-1 diffraction order from the grating 810), the diffraction order may be described as a (-1, +1) diffraction order. Note that the captured diffraction orders may optionally include diffraction orders higher than the first order (e.g., m≥2 and/or n≥2).

In fig. 8 (a), only m=0 and m= -1 orders from the grating 810 are plotted for a single wavelength only for clarity, other orders (both positive and negative) may also be captured by the final focusing mirror. It will be appreciated that the diffraction orders for different wavelengths are angularly displaced (i.e. the diffraction angle and hence the angle of incidence on the target are different for different wavelengths). Again, for clarity, the only diffraction orders shown to be captured are (-1, -1), (0, 0), (-1, +1). More diffraction orders can be captured. In fig. 8 (b), more diffraction orders are shown as labeled, but other diffraction orders may be captured again.

Note that in such an arrangement, the diffraction orders incident on the second dispersive element may comprise at least two non-complementary diffraction orders (e.g. and thus at least two non-complementary angles of incidence). The complementary diffraction orders may include positive and negative orders of the same magnitude (e.g., +1 and-1 orders), while the at least two non-complementary orders may include, for example, a zeroth order having any other order.

Fig. 9 is an exemplary captured image including a plurality of captured diffraction orders as labeled, for example, illustrating where various orders may be detected on the detector. Note that this example assumes a 1D line grating for the target, which may also include a 2D grating. Each individual point of the diffraction order may relate to a different wavelength.

The first dispersive element may comprise, for example, a reflective grating or a transmissive grating. It may comprise a 1D (line) grating, a 2D grating or a more general dispersive element creating multiple stages at various diffraction angles. In the case of a grating, the first dispersive element may comprise a varying pitch and/or groove depth, for example, to correct for optical parameters such as wavelength dependent defocus of the wavelength dependent diffraction angle. Moreover, its orientation can be varied to optimize the angle of outgoing diffraction.

In its simplest form, the grating at IFP may comprise a flat 1D reflective line grating with a constant pitch, and its lines oriented parallel to the plane of reflection (i.e. in a conical diffraction configuration). Both grating pitch and angle of incidence can be selected to optimize the diffraction angle. The angle of incidence may be chosen such that the zeroth order reflection has a large reflection coefficient due to total external reflection (this may include glancing incidence angles below 20 degrees). The groove profile, and in particular the groove depth of the grating, may be chosen such that the diffraction efficiency of the +1 and-1 orders is maximized, or the combined energy (specular reflection and diffraction efficiency) of all beams incident on the second dispersive element is maximized. The azimuth angle between the grating line and the reflection plane may be zero, but may be made non-zero to optimize the position of the stage on the detector.

In an embodiment, some of these measured parameters (e.g., angle of incidence, azimuth angle, grating pitch) may be varied to optimize the diffracted light for a specific target. For example, in the recipe generation, the configuration may be optimized to create the most favorable angle of incidence on the target to obtain the maximum signal divergence/information content. The pitch of the gratings may be varied, for example by providing line-space gratings with a variable/selectable pitch or by providing individual gratings (e.g. grating selectors or grating wheels comprising a plurality of gratings of different respective pitches) arranged to be selectively (individually) switched into the mid-field plane.

Some of these parameter variation examples may be equally applicable to transmissive and reflective gratings, e.g., azimuthal and pitch variations. In general, higher diffraction efficiency can be achieved by means of a transmission grating.

In one embodiment, the first dispersive element is a physical first dispersive element. In one embodiment, the first dispersive element is a virtual first dispersive element. It is also possible to provide the first dispersive element at the SFP instead of at the IFP, although not a physical first dispersive element (i.e. a virtual first dispersive element). At the SFP, SXR radiation may be generated by focusing a high energy pulsed IR laser (pump/drive radiation, optionally pump/drive laser) into the gas jet or gas target. If the local field strength in the gas is sufficiently large, photons are generated optionally at the SXR wavelength. Some source configuration parameters will determine whether all photons emitted in the gas will constructively or destructively interfere in a certain direction and, thus, whether the emitted radiation (optionally SXR radiation) is emitted from the source in a particular direction.

In an embodiment, it is proposed to provide a plurality of driving radiation beams configured to overlap (in space and time) in the gas injection. Optionally, the plurality of driving radiation beams are coherent. Because of their coherence, they will interfere and, depending on the angle between the two incoming wavefronts, an interference pattern will be generated. For example, when two beams overlap at an angle, an intensity grating may be created in the gas. Since SXR photons are generated only above a certain field intensity threshold, the SXR source itself is actually a grating or virtual first dispersive element. As a result, the phases of SXR radiation will match only in a specific direction after the gas, resulting in multiple outgoing beams at various (diffraction) angles. This process has been described, for example, in "non-collinear enhancement cavity for high outcoupling efficiency by the invasive recording of EUV frequency combs" (Noncollinear ENHANCEMENT CAVITY for recovery-high out-coupling efficiency of an EUV frequency comb) "by Zhang et al, incorporated herein by reference.

Fig. 10 (a) illustrates this principle. It schematically shows two infrared laser pulses 1000a, 1000b, the two infrared laser pulses 1000a, 1000b overlapping to form an intensity grating 1010, resulting in the generation of SXR radiation comprising zeroth order 1020 and +1, -1 diffraction orders 1030+, 1030-. A blocking aperture or pinhole 1040 may be provided to block unwanted (e.g., infrared) radiation.

Fig. 10 (b) and 10 (c) illustrate variations of this embodiment in which half-wave plate 1050 has been added to one or both arms of infrared laser pulses 1000a, 1000 b. By rotating half-wave plate 1050, the polarization of each arm 1000a, 1000b can be controlled. When both arms have the same polarization, an interference pattern is formed and SXR is generated at the zeroth and diffraction orders 1030+, 1030-angles (fig. 10 (b)). When the polarizations are orthogonal, there is no interference and a regular circular intensity pattern is created, thus, a single SXR beam 1020 is generated (fig. 10 (c)). Half-wave plate 1050 may be rotated to select between illumination angles, and thus between each of the illustrated modes of operation. This effectively provides control to vary the NA of SXR illumination.

It is also possible to add a delay between the IR beams 1000a and 1000b so that the pulses do not interact at all. In this case, two completely independent SXR sources are created along the direction of 1000a and 1000 b.

It will be appreciated that more than two infrared drive radiation beams may be overlapped in the same manner, enabling more complex intensity patterns to be generated, for example to optimise the diffraction pattern of the outgoing SXR beam. Alternatively, the three infrared drive lasers overlap in the same way. Alternatively, the four infrared drive lasers overlap in the same way.

Wherever the first diffractive element is located, the first scattered radiation (i.e. comprising one or more non-zero diffraction orders) should be captured by the final focusing mirror, which may be the final mirror before the target (or the only mirror of the SFP diffraction embodiment). The captured diffraction angle from the first diffractive element should be properly re-imaged without too much aberration, e.g. in order to under fill the target grating. The mirror should also have a large capture NA to capture all the desired diffraction angles.

Thus, the final focusing mirror may comprise an ellipsoidal surface. Such a surface ensures (at least nominally) almost aberration-free re-imaging. While it may be the case that there may be some magnification at some angle of incidence and thus there is a non-matching object and image distance, this does not immediately cause problems when the target is underfilled. In any case, the configuration of the IFP grating and ellipsoid can be selected to match all the magnifications, if desired. To ensure a large NA, the ellipsoidal surface can extend a significant distance.

Fig. 11 schematically illustrates a proposed focusing mirror element according to an embodiment. The final focusing mirror may comprise an ellipsoidal surface described by an elliptical portion rotating about its long axis. Thus, such a final focusing mirror may include a hollow oblong body portion 1100 with a mirrored inner surface 1110. In one embodiment, the hollow prolate spheroid portion may comprise a hollow prolate spheroid portion having a first (input) opening larger than a second (output) opening. In another embodiment, the hollow prolate spheroid portion can comprise a hollow prolate spheroid portion having a first (input) opening smaller than a second (output) opening. The hollow oblong body portion may, for example, include a half or less portion having an opening at each end. As such, the final focusing mirror may include a hollow prolate spheroid portion having a larger first opening 1120 at the input end and a smaller second opening 1130 at the output end. The smaller opening at the output end may be defined by a cut parallel to the minor axis plane near one end of the hollow prolate spheroid. The larger opening at the input end may be defined by a cutout along the minor axis plane or a cutout parallel to the minor axis plane between the minor axis plane and the output end (e.g., at a point near the minor axis plane). Such mirrors need not be entirely rotationally symmetric and may include a surface that rotates only partially about the long axis. However, such mirrors may be manufactured using electroforming techniques, which would result in a fully rotationally symmetric mirror. This also provides additional mechanical stability.

The proposed method for manufacturing such mirrors may be based on the disclosure of WO2019/238382, which is incorporated herein by reference. The configuration of the actual mirror described therein is quite different from the configuration presented herein, but the basic concept of the manufacturing method described therein can be used to manufacture the proposed final focusing mirror.

The proposed method may thus comprise a method of manufacturing a focusing mirror element comprising providing an axisymmetric spindle comprising an oblong body portion, forming a focusing mirror element body around said spindle, and releasing said focusing mirror element body from said spindle, whereby said focusing mirror element body has an optical surface defined by said inner surface of said focusing mirror element body. The mandrel may include a superfinishing surface, such as a superfinishing surface resulting from a superfinishing step using a process selected from the group consisting of magnetorheological fluid finishing (MRF), fluid Jet Polishing (FJP), elastic Emission Machining (EEM), ion beam machining, and floating polishing. Such a superfinishing step may be performed such that the mandrel surface has a surface roughness of less than 100 pm Root Mean Square (RMS), optionally less than 50 pm (RMS), optionally less than 35 pm RMS. Alternatively or additionally, a reflection enhancing single or multi-layer coating may be applied to the optical surface of the focusing mirror element body. Alternatively or additionally, the electrode and release layer may be applied to the mandrel prior to the forming step.

It can be appreciated that ellipsoids are very sensitive to misalignment and that sensitivity increases rapidly with increasing beam divergence. However, the proposed way of using the final focusing mirror and ellipsoidal surface proposed herein does not result in a larger beam divergence, but rather in multiple beams with small divergence over a large angular spread. The sensitivity of the individual low divergence beams may still be small.

In another embodiment, it is proposed to modify the illumination configuration using the same basic concepts as described above to achieve direct phase detection. The proposed direct phase detection can be performed in a similar way as shear interferometry. In shearing interferometry, the phase of a (probing) radiation beam is detected by (partially) overlapping the beam with a reference beam that is coherent with the probing beam. The wavefronts of the two beams are tilted relative to each other, thereby causing an interference pattern with fringes to be formed.

In order to create two mutually coherent beams, SXR radiation from a source may be initially focused at an intermediate focus. A diffractive element (e.g., the first diffractive element or grating previously described) may be located at the intermediate focus. The first dispersive element or the intermediate focusing grating may be oriented in a conical orientation (a line parallel to the plane of reflection) or in a planar orientation (a line perpendicular to the plane of reflection). In other embodiments, the lines may also be tilted (e.g., not perfectly parallel or perfectly perpendicular) with respect to the plane of reflection, but oriented at a particular azimuth angle. As before, the first diffractive element may comprise a reflective element (e.g., a reflective grating) or a transmissive element (e.g., a transmissive grating). In another specification, it will be assumed that the orientation of the first dispersive element is purely conical. As with the previous embodiment, the first dispersive element will generate a first scattered radiation comprising a plurality of beams, a diffraction order (±m, λ), where m is the order of the diffraction peak and λ is the wavelength, as the grating disperses the radiation at various angles. A special case of m=0 can be identified as a specular reflection or zeroth order beam. The beams m=0, m= -1, λ and m= +1, λ (and higher orders if configuration allows) are collected by an ellipsoidal mirror or final focusing mirror (e.g. as already described) which refocuses the beam on the second diffractive element or target under inspection.

It will now be assumed that the orientation of the second dispersive element is also conical. The beam incident on the second dispersive element will again diffract into diffraction orders (±n, λ), and for simplicity only the zeroth and first diffraction orders of the incident beams m= -1 and m= +1 will be discussed below (wavelength parameter values are also omitted for the sake of symbol simplicity, wherein it is implicitly assumed that the beam is monochromatic at a specific wavelength in the spectrum, the wavelength dependent formalization (formalism) is presented further below). Thus, the diffracted beams from the second dispersive element may be classified as (m, n) = (-1, -1), (m, n) = (-1, 0), (m, n) = (-1, +1), (m, n) = (+1, -1), (m, n) = (+1, 0), (m, n) = (+1, +1).

It can be demonstrated that there are configuration parameters for which some of these beams will spatially overlap on the detector such that they interfere. The two logical combinations are using the zeroth order reflection from the second dispersive element as reference beam and using the appropriate first order diffracted beam as probe beam, e.g. the beam pair may comprise using (m, n) = (-1, 0) as reference beam and (m, n) = (+1, -1) as probe beam, or (m, n) = (+1, 0) as reference beam and (m, n) = (-1, +1) as probe beam. These two beam pairs are illustrated as overlapping in fig. 8 (b). These are merely examples and other combinations may be made to overlap.

In order to create (fringe) interference patterns using overlapping beam pairs, their wavefronts need to be tilted with respect to each other. This can be achieved by providing two beams with origins that are (slightly) displaced from each other, such that each beam in a beam pair has a slightly different origin.

Although the target is typically very small without much space to accommodate the origin of such displacement, the required tilt (and thus displacement) is also very small. For example, as a function of angle α within the beam divergence, the optical path length difference (OPD) calculated in waves can be determined as:

Where μ is the displacement of the origin on the target and θ is the tilt angle of the outgoing beam from the target surface normal.

For example, assuming a beam 1/e ² divergence of about 2 mrad and at least two fringes are required over a full angle beam divergence of θ=0, the required displacement would be μ=6.8 μm (assuming λ=13.5 nm). Thus, the focal points of beams m= -1 and m= +1 on the target should be laterally displaced by at least 6.8 μm, or more generally more than 4 μm, more than 5 μm or more than 6 μm (e.g. between 4 μm and 10 μm, between 5 μm and 9 μm, between 5.5 μm and 9 μm, between 6 μm and 8 μm, between 6 μm and 7 μm or between 6.5 μm and 7 μm).

To achieve this, it is proposed to provide a first tilt for the beam m= -1 in the illumination pupil and a second tilt for the beam m= +1 in the illumination pupil. The illumination pupil may comprise a pupil plane or fourier plane in the illumination branch before the target/second diffractive element. In an embodiment, the first tilt and the second tilt may be equal in magnitude and opposite in direction. This corresponds to a uniform aberration across the illumination pupil.

Fig. 12 schematically illustrates an exemplary (partial) configuration for achieving this. The simplest uniform aberration is defocus and the simplest way to insert defocus in a configuration is by positioning the first dispersive element slightly out of focus (e.g., slightly displaced from the mid-field plane (i.e., the conjugate plane of the target and source)). Thus, the first dispersive element 810 is located at an defocus dZ displacement away from the mid-field plane IFP. By placing the first dispersive element 810 slightly out of focus, the apparent foci of the stages are shifted by μ relative to each other. The conjugate plane of the source is imaged as before the final focused reflected image.

The required defocus dZ can be calculated as:

Where θ _G1 is an incident angle on the first dispersive element, and p1 is a pitch of the first dispersive element.

Fig. 13 is a schematic diagram illustrating how non-zero values of μ lead to (spherical) wavefronts 1365 (+1, 0), 1365 (-1, -1) that are related to the (+1, 0) and (-1, -1) diffraction orders, respectively, being slightly tilted with respect to each other, which in turn leads to OPD (dependent on angle α). Plane 1370 is the conjugate plane of the first diffractive element.

Fig. 14 shows an exemplary (simulated) fringe pattern as can be seen on a detector (monochrome) when performing the method of this embodiment. The figure shows an intensity I plot of X detector pixel position DPX relative to Y detector pixel position DPY (where a lighter area indicates a higher intensity). The (left/right) position of the stripes 1480 will depend on, for example, the phase difference between beams (m, n) = (-1, 0) and (m, n) = (+1, -1), which is phase information providing additional information about the target. In this way, the phase difference between the beams can be determined from the position of the fringes on the detector.

The phase difference may be used to infer one or more profile parameters. For example, a swing curve describing phase differences as a function of wavelength may be determined and used to infer one or more profile parameters. Such a wobble curve may be shown to vary significantly with variations in some profile parameters (e.g., CD and grating height).

So far, it has been assumed that the reference beam and the probe beam can be superimposed on the detector. The following is a description of what conditions do so. Starting with the conical grating equation:

Where θ _i and θ _f are the incoming and outgoing tilt angles, respectively, phi _i and phi _f are the incoming and outgoing azimuth angles, respectively, p _x and p _y are the grating pitches in the x and y directions, respectively, and (h, k, l) is the reciprocal lattice coordinate (reciprocal lattice coordinate). The Z component can be ignored because that is mainly balanced energy conservation. Furthermore, in conical diffraction, h=0 and k=m, as defined above.

To simplify notation, the equation for grating j in a conical configuration will be denoted as:

Wherein the method comprises the steps of Is the incoming beam of light which is directed,Is an outgoing beam, andIs a raster vector.

After the first grating, the outgoing beam is re-imaged and is incident on the second grating at a different angle. Let θ _j be the zeroth order tilt angle on grating j and θ=θ ₂-θ₁ be the tilt angle difference between the first dispersive element and the second dispersive element, then the outgoing beam after the second dispersive element can be written as a combination of two equations:

when Δθ=0, the equation becomes much simpler:

Wherein the method comprises the steps of Simply equal toAnd the beam overlap condition is written as:

It is equal to:

Where ε is the ratio between the pitch on the first dispersive element and the pitch on the second dispersive element, and p is the pitch of the second dispersive element (e.g., the target). For ε=2, the above conditions are satisfied for the combinations (m, n) = (-1, 0) and (m ', n')= (+1, -1) and for the combinations (m, n) = (+1, 0) and (m ', n')= (-1, +1). Note that the wavelength dependence has decreased and thus beam matching is achieved for all wavelengths in the spectrum at the same time.

The above process assumes that the angle of incidence on the first dispersive element is equal to the angle of incidence on the second dispersive element. Although this is not a priori a problem, for an angle of incidence of θ ₂ =30° on the target, for example, this would mean a significant transmission loss on the first dispersive element. An alternative is to have a larger glancing incidence angle on both the first dispersive element and the second dispersive element. Until now, this was not considered because (1) for small target pitches, the stage no longer propagates, and (2) under-filling the target becomes more difficult due to the projection of the focal spot onto the wafer plane. Nevertheless, embodiments are conceivable in which such an 'equal angle of incidence' geometry is applied, for example when using a range of shorter wavelengths or when designing a first dispersive element grating with sufficient diffraction efficiency at larger angles of incidence by, for example, applying a broadband (multilayer) coating. Moreover, such equal angle of incidence geometry may be suitable for some applications (e.g., because most larger pitches are of interest).

In another embodiment, a larger glancing incidence angle is selected on the first dispersive element to maintain high diffraction efficiency and reflectivity by means of a simple coating while maintaining near normal incidence angles on the target. In such an arrangement, Δθ will be non-zero in the above equation. Due toCan no longer be equal toAnd should therefore be evaluated unambiguously from the input beam coordinates:

By means of this expression, it is possible to evaluateTo obtain similar conditions as above (forFirst order approximation of (a):

since the expression now includes There is thus a wavelength dependence in the direction of the outgoing beam present. Furthermore, the pitch of the first grating is no longer a simple multiple of the target pitch. Wavelength dependence is not a direct problem as long as the difference in outgoing beam direction of the two overlapping beams is assumed to be small with respect to the divergence of the beams. Furthermore, the pitch p1 can always be found such that the overlap is perfect for at least one wavelength. Given the angles of incidence θ ₁ and θ ₂ on the first and second dispersive elements and the target pitch p ₂ of the second dispersive element for a single wavelength, the recurrence relation for finding the best matching pitch p ₁ is given by:

For this pitch, the overlap at wavelength λ is perfect, with the overlap decreasing for more distant wavelengths.

In addition to those parameters mentioned above, there are also many parameters that can be tuned to optimize beam overlap. For example, in an embodiment, one or both of a first azimuth angle of incidence on a first dispersive element and a second azimuth angle of incidence on a second dispersive element may be tuned. The pitch of the first dispersive element may be variable (e.g. a variable line-to-line grating) to provide wavelength dependent defocus and tilt aberrations. The first dispersive element may comprise a transmission grating or a more complex diffractive element. The optical parameters (e.g., angle of incidence, image distance, object distance) of the ellipsoidal final focusing mirror between the first dispersive element and the second dispersive element can be optimized, for example, such that all stages are reflected at a plane of symmetry intersecting the prolate ellipsoidal axis. As already stated, beam pairs different from those explicitly described may be made to overlap.

The pitch p ₁ of the first dispersive element is associated with a target pitch p ₂. For small target pitch variations (e.g., pitch walk (walk)), the angle of incidence on the first dispersive element may be modified to retrieve the overlap condition according to the above recurrence relation. For large changes in the target pitch, a different first dispersive element pitch is required. In an embodiment, this may be implemented by providing a grating selector or grating wheel as already described.

In embodiments, various exposure schemes may be used. For example, by subsequently 1) simultaneously exposing the object by means of two beams m= -1 and m= +1 (resulting in interference fringes) and 2) sequentially exposing by means of the beams m= -1 and m= +1. The resulting data can be analyzed by determining the differences in these exposures, whereby only a phase response and a much cleaner signal are obtained due to the scattered and non-interfering radiation being removed.

In an embodiment, color selection may be achieved by blocking some wavelengths in the diffraction order m after the first dispersive element. In most of the above, only the m+.0 level is considered, but the m=0 beam from the first dispersive element can still be used for conventional SXR measurements, or the (m, n) = (0, +/-n) beam can be made to interfere with the m+.0 beam.

A number of interferometric structure measurement arrangements that enable direct phase detection of radiation scattered from a structure being measured will now be described. Such arrangements may be similar to those already described in the Wen Xiangwei detection section above.

The method may include generating detection radiation (e.g., a detection beam) and reference radiation (e.g., a reference beam) such that at least one structure on the substrate is illuminated with the detection radiation and the reference radiation to obtain scattered detection radiation and scattered reference radiation, and interfering the scattered detection radiation and the scattered reference radiation on the detector. One or more structural parameters and/or profile parameters may be determined from the interfered scattered detection radiation and the scattered reference radiation.

In many of the arrangements described in this section, the probe beam may illuminate a first structure or structure of interest, and the reference beam may illuminate a reference structure (e.g., comprising substantially the same pitch as the first structure). In this way, a focusing mirror arrangement comprising two foci may be provided to focus each of these beams on their respective structures. Alternatively, the illumination radiation may be split before the mirror arrangement (e.g. the driving laser radiation may be split). However, the reference beam may also illuminate the first structure, or may be reflected from another reflector (e.g., a mirror).

The method described in this section may be implemented in combination with, or entirely independent of, the multiple angle of incidence embodiments described in the Wen Xiangwei detection section above (e.g., using the first dispersive element generation). In this way, it is also within the scope of this section to use the first dispersive element to generate a detection beam and a reference beam and to irradiate the first structure by means of the detection beam and the second structure by means of the reference beam to perform interferometry as disclosed herein. However, this section will now describe alternative methods of generating the detection and reference beams and interfering with the respective scattered radiation from at least one structure on the detector.

Conventional metrology detects only the diffraction amplitude, while valuable information is also present in the absolute phase of the diffracted light. For example, the frequency in the signal corresponds to the layer height without phase, whereas with the aid of phase information it is possible to infer the absolute z-coordinate (absolute position in the direction perpendicular to the substrate plane).

In this section, a number of arrangements and/or methods for performing interferometry on a structure (e.g., a metrology target) will be described. For performing interferometry, the illumination beam is split into a reference beam and a detection beam. After the probe beam interacts with the metrology target (first structure), it is recombined with the reference beam, e.g. such that the beams interfere at the detection plane (i.e. on the detector).

Fig. 15 illustrates three different embodiments of interferometric structure measurement arrangements according to this section.

Fig. 15 (a) includes an exemplary arrangement in which the division into the detection beam and the reference beam is performed before SXR generation, i.e. by dividing the driving radiation beam (infrared IR radiation). The driving laser radiation 1500 (e.g. IR radiation) is split by a beam splitting device 1505 (the beam splitting device 1505 may differ in detail from the simplified arrangement illustrated) into a first driving radiation beam 1510a and a second driving radiation beam 1510b. Each of the drive radiation beams 1510a, 1510b is used to generate a respective SXR beam 1520a, 1520b by exciting the HHG module 1515 (e.g., comprising a HHG medium, such as a suitable gas, as already described), e.g., via a HHG process. One of the SXR beams (i.e., probe beam 1520 a) irradiates a first structure 1535 or first target, i.e., a structure or target of interest (e.g., a structure on which profilometry and/or overlay metrology is being performed). The other beam (i.e., reference beam 1520 b) illuminates the reference structure 1530 or the reference target. The first structure 1535 and the reference structure 1530 may be adjacent to each other on the substrate. A focusing mirror or other optical element 1525 may be used to illuminate each structure 1535, 1530 by way of its respective SXR beam 1520a, 1520b. In this way, two beams (detection and reference) 1520a, 1520b are imaged adjacent to each other on the wafer.

Similar to normal profilometry measurements, scattered probe radiation 1540A carries profilometry information. This information is present in the diffraction orders. In contrast to the normal profilometry measurements, however, there is now scattered reference radiation 1540b caused by the reference SXR beam 1520b diffracted from the reference structure 1530. The reference structure 1530 may include a simple 1D grating that acts as a reference mirror. The planar reference mirror will not provide the same diffraction order as the profilometry target, and thus, the reference structure 1530 may comprise a grating structure having the same pitch (or a sufficiently similar pitch) as the first structure 1535.

The scattered detection radiation 1540a and the scattered reference radiation 1540b each comprise diffraction patterns which may overlap on the detector 1550. This overlap may occur directly on detector 1550 without any beam combiner.

Fig. 15 (b) illustrates the superposition of scattered detection radiation 1540a and scattered reference radiation 1540b, resulting in an image 1555a of scattered detection radiation and an image 1555b of scattered reference radiation as detected by the detector. An interference pattern 1560 is formed in the overlapping region of images 1555a and 1555b. For all diffracted harmonics, the overlapping of the beams occurs, not just a single specularly reflected beam (this is symbolically reflected by the shadows of beams 1540a, 1540b in fig. 15 (a), 15 (c), 15 (d)). Note that fig. 15 (b) shows the detection arrangement of all the embodiments illustrated in fig. 15.

Depending on the interference pattern (e.g. each harmonic or wavelength), the absolute phase (phase data or absolute phase data) of the scattered detection radiation can be extracted in addition to the detected intensity. Such phase information may be determined, for example, in dependence on the distance between the fringes and/or the position of the fringes. One or more profile parameters (e.g., including overlap) of the first structure may be determined from the determined absolute phase data (e.g., and intensity data). For example, a full contour determination may be performed on the first structure to determine a plurality of contour parameters thereof.

It will be appreciated that due to the short coherence length, the two path lengths of each beam path 1520a, 1520b should match. Since HHG is a coherent process, this results in SXR of the two beams being mutually coherent. Such matching may be implemented on the IR side (e.g., within beam splitting device 1505), for example, by providing path length adjustments in the path of one or both of the driving radiation beams 1510a, 1510 b. The beam is only displaced by a small amount, e.g. on the order of the focal spot size. Since the divergence overlaps the two beams, only a lateral displacement is required.

The IR beams 1500 should be split and tightly combined with their relative path lengths accurately controlled so that they match adequately. In the case where the SXR wavelength is 1/50 to 1/100 of the IR wavelength, the challenge may be significantly greater than for a normal interferometer (depending on the sensitivity to the parameters for SXR, this may be advantageous in terms of wavelength compared to visible light). The beam travels via the same optics after the beam splitter arrangement 1505, so only this beam splitter arrangement 1505 needs to be stable. The beam splitter may comprise a monolithic assembly having only a minimum non-common path length (i.e., not shared by the two beams).

For example, in the context of fourier transform spectroscopy, in Jansen et al (SPATIALLY RESOLVED FOURIER TRANSFORM SPECTROSCOPY IN THE EXTREME ULTRAVIOLET) in extreme ultraviolet spatially resolved fourier transform spectroscopy (p.v.) at optics (Optica), volume 3, stage 10, pages 1122 to 1125 (2016), which is incorporated herein by reference, methods for generating and interfering with the two SXR beams 1520a, 1520b are described. The fourier transform spectroscopy is actually a measurement of only the spectral amplitude, and does not describe a beam-splitting path to measure the phase response of the target.

To provide some examples for illustration only, the SXR beam divergence is approximately 2 mrad, and the detector may be approximately 40 mm from the SXR source. Thus, when the spot falls on the detector, the spot may have diverged up to about 80 μm. This will provide a large overlap on the detector if the two spots are separated by, for example, about 20 μm. When the spots overlap at the detector, they can arrive at an angle of 0.5 mrad. At a distance of 80 μm on the detector, this would be about 40 nm, which is 4 waves for a 10 nm wavelength. This requires a 10 μm pixel size on the detector to sample at the nyquist frequency, however smaller pixels may be preferred.

Fig. 15 (c) illustrates another example in which the difficulty of splitting the IR-driven radiation beam is overcome. In this embodiment, the focusing mirror arrangement (or illuminator) is divided into two mirror portions 1525a, 1525b, such that it divides SXR beam 1522 into reference beam 1524b and probe beam 1524a. Note that in this example, a single HHG beam 1522 is generated by HHG module 1515, HHG beam 1522 comprising a first beam portion (illustrated by beam edge 1522a in the figure) incident on mirror portion 1525a and a second beam portion (illustrated by beam edge 1522b in the figure) incident on mirror portion 1525 b.

From the perspective of the beam splitting mirrors 1525a, 1525b, the reference beam 1524b and the probe beam 1524a are split in the far field. Thus, the far field profile of each of the beams will be (approximately) semicircular or "D" shaped in opposite orientations, respectively. This may reduce the overlap area between the two beams on the detector, but the general principle is not changed. Because of the semicircular far field profile, the two beams will overlap only when a small difference in incidence angle is introduced between the beams 1524a, 1524b, otherwise the two semicircular profiles will be adjacent to each other, with only the diffraction effect causing the overlap. In this way, one of the mirror portions may be tilted relative to the other (e.g., adjustable mirror portion 1525b may be tilted) such that there is a small angular tilt difference between the two mirror portions.

If the overlap area is so small that there are no complete fringes to distinguish, the phase may be scanned by moving the mirror segments (e.g., by providing at least one movable mirror portion 1525b as illustrated by the arrow) to obtain an absolute phase. In this way, if there is some (even small) overlap, the phase can be retrieved.

The mirror is the most sensitive element of this interferometer example, since the two beam paths fall onto two different optical surfaces. Since the mirror portions 1525a, 1525b are very close together, they may be mechanically and thermally coupled for stability in terms of vibration and thermal differences. The two targets 1535, 1530 are also each part of a different optical path, but are located in close proximity (e.g., within tens of μm) on the same substrate, and as such vibration or thermal effects should not be a problem.

The mirror portions 1525a, 1525b may be manufactured by using two separate mirror blanks in the shape of a D, after which they may be mounted adjacent to each other.

Fig. 15 (d) illustrates a variation on the arrangement of fig. 15 (c), whereby one of the mirror portions 1525b' is controllably deformable. This can be achieved by, for example, local thermal expansion or an actuator (e.g., piezoelectric) underneath the mirror coating. Alternatively, as illustrated in the inset, the mirror may comprise a monolithic mirror 1525' having a controllably deformable portion (e.g., that focuses only one of the beams 1524a, 1524 b). As in the fig. 15 (c) example, the two individual mirror portions 1525a, 1525b may be mechanically and thermally coupled to effectively act as a single mirror and thus (virtually) be immune to vibration or drift.

In either case, each of the beams 1524a, 1524b may still be focused on the respective structure 1535, 1530 by a different mirror portion 1525a, 1525b 'or a different region of the mirror 1525'. For example, the deformable or active region can focus reference beam 1524b on reference structure 1530, wherein the normal portion of the mirror can focus probe beam 1524a on first structure 1535. Thus, the mirror will have two foci, as before. Shaping the monolithic optic 1525 'to have two foci can be challenging and thus the two mirror portions 1525a, 1525b' arrangement can be easier to implement.

In an embodiment, focusing can be achieved using a standard mirror portion 1525a for the entire beam, with the desired tilting of half of the beam being applied by a deformable mirror portion 1525 b'.

It is appreciated that the two mirror portions 1525a, 1525b 'or different regions of the mirror 1525' may each be deformable.

Fig. 16 (a) illustrates another embodiment that does not require changing the measurement setup, but rather depends on the specific nature of the measured targets or structures 1630, 1635. In particular, it is proposed to apply a height difference between the first target 1635 and the reference structure(s) 1630. By focusing the incoming beam 1624 (note that in this embodiment this is a single SXR beam, which has not been split before the substrate) on the edges of the dividing reference structure 1630 and the first structure 1635, the beam 1624 is split at the substrate level. Upon further propagation, the beams diverge and overlap again at detector 1550. This approach requires a dedicated edge between the reference structure 1630 and the first structure 1635 for each phase difference. To scan the phases, different heights/phase offsets of the reference structure 1635 may be placed adjacent to the first target 1630.

The advantage of this arrangement is that the only optical elements that are not combined in the beam path are the reference structure and the first structure within a few tens of micrometers of each other on the same substrate. This makes such interferometer arrangements immune to vibrations or thermal drift.

Fig. 16 (b) includes a method including a pair of EUV beam splitters 1660 (typically pellicle) arranged to split and combine SXR radiation beams into a detection beam and a reference beam, respectively. This is similar to the Mach-Zehnder interferometer setup. Note that this arrangement combines only specularly reflected (reference) beams (e.g., as reflected from reflector or mirror 1639). The method is mechanically simple and has no restrictions on the structure to be measured.

Another embodiment is an apparatus having SXR illumination for simultaneous metrology measurements at multiple angles of incidence on a target. The metrology measurements are based on phase measurements and/or intensity measurements made by the detector/sensor.

Another embodiment is an apparatus for phase information measurement with SXR illumination. The phase information measurements may be measured at multiple angles of incidence or at a single angle of incidence on the target.

Embodiments may include a computer program containing one or more sequences of machine-readable instructions describing an optical metrology method and/or a method of analyzing measurements to obtain information about a lithographic process. Embodiments may include computer code comprising one or more sequences or data of machine-readable instructions describing a method. The computer program or code may be executed, for example, in the unit MPU in the device of fig. 6 and/or the control unit CL of fig. 3. A data storage medium (e.g., semiconductor memory, magnetic or optical disk, etc.) may also be provided in which such a computer program or code is stored. Where existing metrology equipment of the type shown in fig. 6, for example, is already in production and/or use, embodiments of the invention may be implemented by providing an updated computer program product for causing a processor to perform one or more of the methods described herein. The computer program or code may optionally be arranged to control an optical system, a substrate support and the like to perform a method of measuring parameters of a lithographic process on a suitable plurality of targets. The computer program or code may update the lithography and/or metrology recipes for the measurement of additional substrates. The computer program or code may be arranged to control (directly or indirectly) a lithographic apparatus for patterning and processing a further substrate.

The illumination source may be provided in, for example, the metrology device MT, the inspection device, the lithographic device LA and/or the lithographic cell LC.

Note that the term "diffraction order" encompasses specular radiation or zeroth order.

The nature of the emitted radiation used to perform the measurement may affect the quality of the obtained measurement. For example, the shape and size of the transverse beam profile (cross section) of the radiation beam, the intensity of the radiation, the power spectral density of the radiation, etc. may affect the measurements performed by the radiation. It is therefore beneficial to have a source that provides radiation with properties that lead to high quality measurements.

In all of the above aspects, the term "profile parameters" may include, for example, one or more of overlap, etch depth, sidewall angle, layer thickness, critical Dimension (CD), and/or any other dimension.

Further embodiments are disclosed in the following numbered aspects:

1. A metrology tool for measuring a profile parameter of a second dispersive element on a substrate, the metrology tool comprising:

a first dispersive element operable to receive and scatter source radiation so as to generate first scattered radiation;

a final focusing element configured to:

Collecting at least a portion of the first scattered radiation, the collected first scattered radiation comprising a plurality of first scattered beams each having been scattered in a respective different direction, and

Focusing the collected first scattered radiation on a second dispersive element such that each of the first scattered beams is incident on the second dispersive element at a respective different angle of incidence, and

At least one detector operable to detect second scattered radiation that has been scattered by the second dispersive element.

2. The metrology tool of aspect 1, wherein the first dispersive element comprises a grating.

3. The metrology tool of aspect 1 or 2, comprising a first focusing element operable to focus the source radiation at a mid-field plane.

4. The metrology tool of aspect 3, wherein the first focusing element comprises a first focusing mirror element.

5. The metrology tool of any one of aspects 2-4, wherein the first dispersive element is located at the mid-field plane.

6. The metrology tool of aspects 3 or 4, wherein the metrology tool is configured such that:

At least one pair of second scattered beams comprised in said second scattered radiation overlap on said detector to form interference fringes.

7. The metrology tool of aspect 6, wherein the metrology tool is configured to determine, for each pair of second scattered beams, a phase difference between the beams of each pair of second scattered beams as a function of the position of the respective interference fringe on the detector.

8. The metrology tool of aspect 6 or 7, configured such that at least one pair of the first scattered beams comprises respective different effective origins in the mid-field plane.

9. The metrology tool of aspect 8, wherein the displacement between the respective different effective origins is greater than 4 μιη.

10. The metrology tool of aspect 8, wherein the displacement between the respective different effective origins is between 4 μιη and 10 μιη.

11. The metrology tool of aspect 8, wherein the displacement between the respective different effective origins is between 5 μιη and 9 μιη.

12. The metrology tool of any one of aspects 6 to 11, wherein the first dispersive element is located at a defocus distance from the mid-field plane.

13. The metrology tool of aspect 1, wherein the first dispersive element comprises a virtual first dispersive element located at a source plane.

14. The metrology tool of aspect 13, comprising a radiation source for generating the source radiation, the radiation source comprising a laser source device operable to generate a pair of radiation beams towards a gas target such that the pair of radiation beams overlap in time and space in the gas target and at respective different angles so as to generate an interference pattern forming the virtual first dispersive element.

15. The metrology tool of aspect 14, comprising a rotatable wave plate in the path of at least one of the pair of radiation beams.

16. The metrology tool of aspects 14 or 15, wherein the radiation source is operable to generate source radiation comprising one or more wavelengths in the range of 1 nm to 20nm.

17. The metrology tool of any one of aspects 1 to 12, further comprising a radiation source for generating the source radiation, wherein the radiation source is operable to generate source radiation comprising one or more wavelengths in the range of 1nm to 20 nm.

18. The metrology tool of any preceding claim, wherein the final focus element comprises a final focus mirror element.

19. The metrology tool of aspect 18, wherein the final focusing mirror element comprises a hollow prolate spheroid portion having a reflective inner surface.

20. The metrology tool of claim 19, wherein the hollow oblong body portion comprises a first opening at an input end and a second opening at an output end.

21. The metrology tool of aspect 19, wherein the first opening and the second opening are each defined by a cut parallel to a minor axis plane of the hollow prolate spheroid.

22. The metrology tool of aspects 20 or 21, wherein the first opening is larger than the second opening.

23. The metrology tool of claim 23, wherein the first opening is defined by a cut along the minor axis plane or a cut parallel to the minor axis plane between the minor axis plane and the output end.

24. The metrology tool of any preceding aspect, wherein the final focusing element is fully rotationally symmetric.

25. A metrology tool according to any preceding claim, wherein the second dispersive element comprises a target being measured.

26. A metrology tool according to any preceding claim, wherein the profile parameter comprises a physical parameter of the second dispersive element other than its position.

27. A metrology tool according to any preceding aspect, comprising a processor operable to determine the profile parameter from at least the detected second scattered radiation.

28. The metrology tool of any preceding claim, wherein the collected first scattered radiation focused on the second dispersive element comprises a zeroth order component of the first scattered radiation.

29. The metrology tool of any preceding claim, wherein the first scattered beam incident on the second dispersive element comprises at least two non-complementary diffraction orders that have been scattered from the first dispersive element.

30. A metrology tool according to any preceding aspect, comprising determining a value of at least one parameter of interest associated with the second dispersive element from the second scattered radiation.

31. A focusing mirror element includes a hollow prolate spheroid portion having a reflective inner surface and including a first opening at an input end and a second opening at an output end.

32. The focusing mirror element of claim 31, wherein the first opening and the second opening are each defined by a cut parallel to a minor axis plane of the hollow prolate spheroid.

33. The focusing mirror element of aspect 30, wherein the first opening is larger than the second opening.

34. The focusing mirror element of claim 33, wherein the first opening is defined by a cutout along the minor axis plane or a cutout parallel to the minor axis plane between the minor axis plane and the output end.

35. The focusing mirror element of any one of claims 31 to 34, wherein the focusing element is fully rotationally symmetric.

36. A method of manufacturing a focusing mirror element according to any one of aspects 31 to 35, comprising:

Providing an axisymmetric mandrel comprising an oblong body portion;

Forming a focusing mirror element body around the spindle, and

Releasing the focusing mirror element body from the spindle;

Whereby the focusing mirror element body has an optical surface defined by the inner surface of the focusing mirror element body.

37. The method of aspect 36, comprising performing a super-polishing step on the mandrel using a process selected from the group consisting of magnetorheological fluid finishing (MRF), fluid Jet Polishing (FJP), elastomeric Emitter Machining (EEM), ion beam trimming, and floating polishing.

38. The method of claim 37, wherein the superfinishing step is performed such that the mandrel surface has a surface roughness of less than 200 pm Root Mean Square (RMS), optionally less than 100 pm (RMS), optionally less than 50 pm (RMS), optionally less than 35 pm RMS.

39. The method of any of aspects 36-38, comprising applying a reflection enhancing single-layer or multi-layer coating to the mirrored surface of the focusing mirror element body.

40. The method of any one of aspects 36 to 39, comprising applying an electrode and a release layer to the mandrel prior to the forming step.

41. A method of measuring, comprising:

generating source radiation;

scattering the source radiation to generate first scattered radiation;

Collecting at least a portion of the first scattered radiation, the collected first scattered radiation comprising a plurality of first scattered beams each having been scattered in a respective different direction;

Focusing the collected first scattered radiation on a target such that each of the first scattered beams is incident on the target at a respective different angle of incidence;

Detecting second scattered radiation that has been scattered by said object, and

A value of at least one profile parameter of the object is determined from the second scattered radiation.

42. The metrology of aspect 41 comprising focusing the source radiation at a mid-field plane.

43. The metrology of aspect 42, wherein the source radiation is scattered at the mid-field plane.

44. A metrology method according to aspect 42, comprising overlapping at least one pair of second scattered beams comprised within the second scattered radiation at a detector to form interference fringes on the detector.

45. The metrology method of aspect 44, comprising, for each pair of second scattered beams, determining a phase difference between the beams of the each pair of second scattered beams as a function of a position of the respective interference fringe on the detector.

46. The metrology method of aspects 42 or 45, wherein at least one pair of the first scattered beams comprises a respective different effective origin in the mid-field plane.

47. The metrology of aspect 46, wherein the displacement between the respective different effective origins is greater than 4 μm.

48. The metrology of aspect 46, wherein the displacement between the respective different effective origins is between 4 μιη and 10 μιη.

49. The metrology of aspect 46, wherein the displacement between the respective different effective origins is between 5 μιη and 9 μιη.

50. The metrology method of any one of aspects 44 through 49, comprising focusing the source radiation at a defocus distance from the mid-field plane.

51. The metrology of aspect 41 comprising generating a virtual first dispersive element at a source plane.

52. The metrology method of aspect 51, comprising generating a pair of radiation beams and directing them at a gas target such that the pair of radiation beams overlap in time and space in the gas target and at respective different angles so as to generate an interference pattern forming the virtual first dispersive element.

53. The metrology method of aspect 52, comprising controlling polarization of at least one of the pair of radiation beams.

54. The method of metrology of any one of aspects 41 to 53, wherein the source radiation comprises one or more wavelengths in the range of 1 nm to 20 nm.

55. The metrology method of any preceding claim, wherein the collecting step and the step of focusing the collected first scattered radiation on a target are performed using a focusing mirror element according to any one of aspects 31 to 35.

56. The method of metrology of any one of aspects 41 to 55, wherein the profile parameters comprise physical parameters of the target other than its position.

57. The metrology method of any one of aspects 41 through 56, wherein the collected first scattered radiation focused upon the target comprises a zeroth order component of the first scattered radiation.

58. The metrology method of any one of aspects 41 to 57, wherein the first scattered beam incident on the target comprises at least two non-complementary diffraction orders comprised within the first scattered radiation.

59. The metrology method of any one of aspects 41 to 58, comprising optimizing one or more measurement parameters to optimize the first scattered radiation for the target.

60. A method, comprising:

Generating source radiation, wherein the source radiation comprises soft X-ray radiation;

scattering the source radiation to generate a plurality of first scattered beams;

exposing a substrate to the plurality of first scattered beams such that each of the first scattered beams is incident on the substrate at a respective different angle of incidence to generate second scattered radiation;

detecting the second scattered radiation, and

The detected second scattered radiation is used to determine a parameter of interest.

61. An apparatus having an SXR source configured to measure a parameter of interest at multiple angles of incidence simultaneously during a manufacturing process.

62. An apparatus having an SXR source configured to simultaneously measure a parameter of interest at multiple angles of incidence on a substrate, wherein the measurement is based on phase measurements and/or intensity measurements using a detector.

63. An apparatus having an SXR source configured to measure a parameter of interest on a substrate, wherein the measurement is based on a phase measurement using a detector.

Further additional embodiments are disclosed in the following numbered aspects:

1. A metrology apparatus for measuring a profile parameter of a first structure on a substrate, the metrology tool comprising:

An illumination device operable to generate detection radiation and reference radiation;

a mirror arrangement operable to focus the detection radiation onto the first structure to generate scattered detection radiation and to focus the reference radiation onto the first structure or reference structure to generate scattered reference radiation, and

A detector operable to receive the scattered detection radiation and the scattered reference radiation at least partially overlapping on the detector so as to generate interference fringes.

2. The metrology apparatus of aspect 1 wherein the mirror arrangement is operable to focus the reference radiation onto the reference structure to generate the scattered reference radiation.

3. The metrology apparatus of aspect 2, wherein the reference structure comprises a periodic structure or grating.

4. The metrology apparatus of aspect 3, wherein the reference structure has substantially the same pitch as the first structure.

5. A metrology apparatus according to any preceding claim, wherein the illumination means comprises:

a beam splitting device operable to split the drive radiation beam into a first drive radiation beam and a second drive radiation beam, and

A higher harmonic generation module operable to receive the first drive radiation beam to generate the detection radiation and to receive the second drive radiation beam to generate the reference radiation.

6. The metrology apparatus of claim 5, wherein the illumination device comprises a path length adjustment arrangement for at least one of the first and second drive radiation beams.

7. The metrology apparatus of any one of aspects 1 to 4, comprising a higher harmonic generation module operable to receive the driving radiation to generate higher harmonic generated radiation, and

Beam splitting means operable to split the higher harmonic generated radiation into the detection radiation and the reference radiation.

8. The metrology apparatus of claim 7, wherein the beam splitting means comprises the mirror means comprising a first focus for focusing the probe radiation onto the first structure and a second focus for focusing the reference radiation onto the reference structure.

9. The metrology apparatus of claim 8, wherein the mirror arrangement comprises a first mirror portion operable to receive and focus a first portion of the higher harmonic generated radiation to generate the probe radiation, and a second mirror portion operable to receive and focus a second portion of the higher harmonic generated radiation to generate the reference radiation.

10. The metrology apparatus of aspect 9, wherein a tilt angle difference between the tilt angle of the first mirror portion and the tilt angle of the second mirror portion is included.

11. The metrology apparatus of claim 9 or 10, wherein at least the second mirror portion is displaceable to vary a path length of the reference radiation.

12. The metrology apparatus of aspects 9, 10 or 11, wherein at least the second mirror portion is deformable.

13. The metrology apparatus of any one of aspects 9 to 12, wherein the first mirror portion and the second first mirror portion are thermally and/or mechanically coupled.

14. The metrology apparatus of claim 9, wherein the mirror arrangement comprises a monolithic focusing mirror comprising a first mirror region operable to receive and focus a first portion of the higher harmonic generated radiation to generate the probe radiation, and a second mirror region operable to receive and focus a second portion of the higher harmonic generated radiation to generate the reference radiation, and wherein the second mirror region is deformable.

15. The metrology apparatus of aspect 7 wherein the beam splitting device comprises a first thin film beam splitter and a second thin film beam splitter.

16. The metrology apparatus of aspect 7 wherein the beam splitting device comprises a dispersive element.

17. The metrology apparatus of any one of aspects 5 to 16, wherein the higher harmonic generated radiation comprises one or more wavelengths in the range of 1 nm to 20 nm.

18. A metrology apparatus according to any preceding aspect comprising a processor operable to determine phase data from the interference fringes.

19. The metrology apparatus of aspect 18 wherein the phase data describes an absolute phase of the scattered probe radiation.

20. A metrology apparatus according to claim 18 or 19 wherein the processor is operable to determine the at least one profile parameter from at least the phase data.

21. A method of metrology according to any preceding aspect, wherein the profile parameter comprises a physical parameter of the first structure other than its position.

22. The metrology of any preceding claim, wherein the profile parameters comprise one or more of an overlap of the first structure, an etch depth, a sidewall angle, a layer thickness, a Critical Dimension (CD), and/or any other dimension.

23. A method for measuring a profile parameter of a first structure on a substrate, the metrology tool comprising:

Generating detection radiation and reference radiation;

focusing the detection radiation onto the first structure to generate scattered detection radiation;

Focusing the reference radiation onto the first structure or reference structure to generate scattered reference radiation, and

Interfering said scattered detection radiation and said scattered reference radiation on a detector to generate interference fringes, and

Phase data describing the phase of the detected radiation is determined from the interference fringes.

24. The method of aspect 23, comprising focusing the reference radiation onto the reference structure to generate scattered reference radiation.

25. The method of aspect 24, wherein the reference structure comprises a periodic structure or a grating.

26. The method of aspect 25, wherein the reference structure has substantially the same pitch as the first structure.

27. The method of any one of aspects 23 to 26, comprising:

dividing the drive radiation beam into a first drive radiation beam and a second drive radiation beam;

the detection radiation is generated from the first drive radiation via a higher harmonic generation process, and the reference radiation is generated from the second drive radiation via a higher harmonic generation process.

28. The method of aspect 27, comprising matching path lengths of the first and second drive radiation beams.

29. The method of any one of aspects 23 to 26, comprising:

generating radiation generated by higher harmonics, and

Dividing the higher harmonic generated radiation into the detection radiation and the reference radiation.

30. The method of claim 29, wherein the splitting is performed using a mirror arrangement comprising a first focus for focusing the detection radiation onto the first structure and a second focus for focusing the reference radiation onto the reference structure.

31. The method of aspect 30, comprising:

Receiving a first portion of the higher harmonic generated radiation on the first mirror portion and causing the first mirror portion to focus the first portion of the higher harmonic generated radiation onto the first structure, and

A second portion of the higher harmonic generated radiation is received on the second mirror portion such that the second mirror portion focuses the second portion of the higher harmonic generated radiation onto the reference structure.

32. The method of aspect 31, comprising applying a tilt angle difference between the tilt angle of the first mirror portion and the tilt angle of the second mirror portion.

33. The method of aspect 31 or 32, comprising displacing the second mirror portion to match the path lengths of the detection radiation and reference radiation.

34. The method of aspects 31, 32 or 33, comprising controllably deforming the second mirror portion.

35. The method of any of aspects 31 to 34, wherein the first mirror portion and the second mirror portion are thermally and/or mechanically coupled.

36. The method of aspect 31, wherein the mirror device comprises a monolithic focusing mirror, and the method comprises:

Receiving a first portion of the higher harmonic generated radiation on a first mirror region of the monolithic focusing mirror such that the first mirror region focuses the first portion of the higher harmonic generated radiation onto the first structure, and

A second portion of the higher harmonic generated radiation is received on the second mirror region of the monolithic focusing mirror such that the second mirror region focuses the second portion of the higher harmonic generated radiation onto the reference structure.

37. The method of aspect 36, wherein the second mirror region is deformable.

38. The method of aspect 29, comprising performing the beam splitting using a first thin film beam splitter and recombining the probe beam and the reference beam using a second thin film beam splitter.

39. The method of aspect 29, comprising performing the splitting using a dispersive element.

40. The method of any of claims 27 to 39, wherein the higher harmonic generated radiation comprises one or more wavelengths in the range of 1 nm to 20 nm.

41. The method of aspect 29, comprising performing the beam splitting by illuminating the first structure with a first portion of the higher harmonic generated radiation and illuminating the reference structure with a second portion of the higher harmonic generated radiation;

Wherein the first structure and the reference structure have been formed with a height offset.

42. The method of any of claims 23 to 40, wherein the phase data describes an absolute phase of the scattered detection radiation.

43. A method according to any one of claims 23 to 42, comprising determining said at least one profile parameter in dependence on at least said phase data.

44. A method of metrology according to any preceding aspect, wherein the profile parameter comprises a physical parameter of the first structure other than its position.

45. The metrology of any preceding claim, wherein the profile parameters comprise one or more of an overlap of the first structure, an etch depth, a sidewall angle, a layer thickness, a Critical Dimension (CD), and/or any other dimension.

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

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

Although embodiments may be specifically referred to herein in the context of inspection or metrology equipment, embodiments may be used in other equipment. Embodiments may form part of a mask inspection apparatus, a lithographic apparatus, or any apparatus that measures or processes an object, such as a wafer (or other substrate) or a mask (or other patterning device). The term "metrology apparatus" (or "inspection apparatus") may also refer to an inspection apparatus or inspection system (or metrology apparatus or metrology system). Inspection apparatus including embodiments, for example, may be used to detect defects in a substrate or defects in structures on a substrate. In such embodiments, a property of interest of a structure on a substrate may relate to a defect in the structure, the absence of a specific portion of the structure, or the presence of an unwanted structure on the substrate.

While the foregoing has been with specific reference to the use of embodiments 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, for example imprint lithography, where the context allows.

While the targets or target structures described above (more generally, structures on a substrate) are metrology target structures specifically designed and formed for measurement purposes, in other embodiments, properties of interest may be measured on one or more structures that are functional portions of devices formed on a substrate. Many devices have a regular grating-like structure. The terms structure, target grating and target structure as used herein do not require that structures have been provided specifically for the measurements being performed. Furthermore, the pitch of the metrology target, while approaching the resolution limit of the scatterometer's optical system, may be smaller, but may be much larger than the size of a typical non-target structure (optionally a product structure made by a lithographic process in target portion C). In practice, the lines and/or spaces of overlapping gratings within a target structure may be made to include smaller structures similar in size to non-target structures.

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

Although specific reference is made to "metrology apparatus/tool/system" or "inspection apparatus/tool/system," these terms may refer to the same or similar type of tool, apparatus or system. Inspection or metrology equipment, including for example embodiments of the present invention, can be used to determine characteristics of structures on a substrate or on a wafer. For example, an inspection apparatus or metrology apparatus including embodiments of the present invention may be used to detect defects in a substrate or in structures on a substrate or wafer. In such embodiments, a feature of interest of a structure on a substrate may relate to a defect in the structure, the absence of a specific portion of the structure, or the presence of an unwanted structure on the substrate or on a wafer.

Although specific reference is made to HXR, SXR and EUV electromagnetic radiation, it will be appreciated that the invention may be practiced with all electromagnetic radiation including radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays and gamma rays, as well as with other radiation including electron beam radiation and (charged) particle radiation, where the context allows.

Additional objects, advantages, and features of the application are set forth in the description which follows, and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the application. The application disclosed in this application is not limited to any particular set or combination of objects, advantages, and features. Various combinations of the stated objects, advantages and features are contemplated to constitute the application disclosed in this application.

Claims (15)

1. A measuring tool comprising: a first dispersive element operable to receive and scatter source radiation to generate first scattered radiation; final focusing element; and at least one detector, The final focusing element is configured to: collecting at least a portion of the first scattered radiation, the collected first scattered radiation comprising a plurality of first scattered beams that have each been scattered in respective different directions; and The collected first scattered radiation is focused on a second dispersive element so that each of the first scattered beams is incident on the second dispersive element at a corresponding different angle of incidence, and the at least one detector is operable to detect the second scattered radiation that has been scattered by the second dispersive element. 2 . The metrology tool of claim 1 , comprising a first focusing element operable to focus the source radiation at an intermediate field plane. The metrology tool of claim 2 , wherein the first dispersive element is located at the mid-field plane. 4. A metrology tool according to any one of the preceding claims, wherein the metrology tool is configured such that: At least one pair of second scattered beams included in the second scattered radiation overlap on the detector to form interference fringes. 5. The metrology tool of claim 4, wherein the metrology tool is configured to determine, for each pair of second scattered beams, a phase difference between the beams in the pair of second scattered beams based on positions of the corresponding interference fringes on the detector. 6. A metrology tool according to any one of claims 2 to 5, configured such that at least one pair of the first scattered beams comprise respective different effective origins in the intermediate field plane. The metrology tool according to claim 6 , wherein the displacement between the corresponding different effective origins is greater than 4 μm. 8. The metrology tool according to any one of claims 2 to 7, wherein the first dispersive element is located at a defocus distance from the intermediate field plane. 9. The metrology tool of claim 1, wherein the first dispersive element comprises a virtual first dispersive element located at a source plane. 10. The metrology tool of claim 9 , comprising a radiation source for generating the source radiation, the radiation source comprising a laser source device operable to generate a pair of radiation beams toward a gas target such that the pair of radiation beams overlap temporally and spatially and at respective different angles in the gas target so as to generate an interference pattern forming the virtual first dispersive element. 11. The metrology tool of claim 10, comprising a rotatable wave plate in the path of at least one of the pair of radiation beams. 12. A metrology tool according to any preceding claim, wherein the final focusing element comprises a final focusing mirror element, and wherein the final focusing mirror element comprises a hollow prolate spheroid portion having a reflective inner surface. 13. The metrology tool of any preceding claim, wherein the second dispersive element comprises a target being measured. 14. A method comprising: generating source radiation, wherein the source radiation comprises SXR; scattering the source radiation to generate a plurality of first scattered beams; exposing a substrate to the plurality of first scattered beams such that each of the first scattered beams is incident on the substrate at a respective different angle of incidence to generate second scattered radiation; detecting the second scattered radiation; and The detected second scattered radiation is used to determine a parameter of interest. 15. An apparatus having an SXR source configured to measure a parameter of interest simultaneously at multiple angles of incidence during a manufacturing process.