TW202507421A - Metrology method and associated metrology tool - Google Patents

Abstract

Disclosed is a metrology method and 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 being configured to: collect at least a portion of said 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 focus said collected first scattered radiation on a second dispersive element such that each of said first scattered beams is incident on said second dispersive element at a respective different angle of incidence; and at least one detector operable to detect second scattered radiation having been scattered by said second dispersive element.

Description

Measurement methods and related measurement tools

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

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

To project a pattern onto a substrate, lithography equipment 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 used are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Lithography equipment using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 nm to 20 nm, such as 6.7 nm or 13.5 nm, can be used to form smaller features on a substrate than lithography equipment using radiation with a wavelength of, for example, 193 nm.

Low- _k1 lithography can be used to process features smaller than the classical resolution limit of the lithography equipment. In this procedure, the resolution formula can be expressed as CD = _k1 × λ/NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithography equipment, CD is the "critical dimension" (usually the smallest feature size printed, but in this case half-pitch), and _k1 is an empirical resolution factor. In general, the smaller _k1 is, the more difficult it is to reproduce on the substrate a pattern that resembles the shape and size planned by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithography projection equipment and/or the design layout. Such steps include, for example, but are not limited to, optimization of the NA, customizing the illumination scheme, using 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 generally defined as "resolution enhancement technology" (RET). Alternatively, a tight control loop for controlling the stability of the lithography equipment can be used to improve the reproduction of the pattern at low _k1 .

In lithography processes, as well as other manufacturing processes, it is frequently necessary to measure parameters of interest, i.e., measurements of the resulting structures, e.g., for process control and verification. Various tools are known for making such measurements, including scanning electron microscopes, which are often used to measure critical dimensions (CD), and specialized tools for measuring overlay (the alignment accuracy of two layers in a device). More recently, various forms of scatterometers have been developed for use in lithography.

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

Examples of known scatterometers usually rely on the deployment of dedicated metrology targets. For example, the method may require a target in the form of a simple grating that is large enough so that the measurement beam produces a light spot that is 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 simulating the interaction of the scattered radiation with a mathematical model of the target structure. The parameters of the model are adjusted until a diffraction pattern similar to the diffraction pattern observed from a real target is produced by the simulated interaction.

In addition to feature shape measurement by reconstruction, this device can also be used to measure diffraction-based overlays as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark field imaging of the diffraction stage enables overlay measurement of smaller targets. These targets can be smaller than the illumination spot and can be surrounded by product structures on the wafer. Examples of dark field imaging metrology can be found in many published patent applications such as US2011102753A1 and US20120044470A. Multiple gratings can be measured in one image using compound grating targets. Known scatterometers tend to use light in the visible or near infrared (IR) wave range, which requires the spacing of the grating to be much coarser than the actual product structure where the properties are actually of interest. Such product features can be defined using deep ultraviolet (DUV), extreme ultraviolet (EUV) or X-ray radiation, which have much shorter wavelengths. Unfortunately, these wavelengths are generally not available or usable for metrology.

On the other hand, the dimensions of modern product structures are so small that they cannot be imaged by optical metrology techniques. Small features include, for example, features formed by multiple patterning processes and/or pitch multiplication. Therefore, targets used for high-volume metrology typically use features that are much larger than the product where the overlay error or critical dimension is the attribute of interest. The measurement results are only indirectly related to the dimensions of the real product structure and may not be accurate because the metrology targets are not subject to the same distortions under optical projection in the lithography equipment and/or different processing in other steps of the manufacturing process. Although scanning electron microscopy (SEM) can directly resolve these modern product structures, SEM is much more time consuming than optical metrology. In addition, electrons cannot penetrate thick process layers, which makes electrons less suitable for metrology applications. Other techniques are known, such as using contact pads to measure electrical properties, but they only provide an indirect indication of the actual product structure.

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 structures and/or to penetrate further into the product structures. One such method of generating suitable high frequency radiation (e.g., hard X-rays, soft X-rays and/or EUV radiation) may use pump radiation (e.g., infrared IR radiation) to excite a production medium, thereby generating emission radiation, optionally including higher order harmonic generation of the high frequency radiation.

In order to determine the parameters of interest associated with the measured targets, it is necessary to maximize or increase the signal diversity in the detected signals.

In a first aspect of the present invention, a metrology tool is provided, which includes: 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 includes a plurality of first scattered beams, each of which has been scattered in a respective different direction; 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 respective different incident angle; and at least one detector, which is operable to detect the second scattered radiation that has been scattered by the second dispersive element.

In a second aspect of the present invention, a focusing mirror element is provided, which includes a hollow elliptical sphere portion having a mirroring inner surface and including: a first opening at an input end and a second opening at an output end.

In a third aspect of the present invention, a method for manufacturing the focusing mirror element of the second aspect is provided, comprising: providing an axially symmetrical spindle comprising a long elliptical 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 present invention, a metrology method is provided, which includes: 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 including a plurality of first scattered beams, each of which has been scattered in a respective different direction; focusing the collected first scattered radiation on a target so that each of the first scattered beams is incident on the target at a respective different incident angle; 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 understood from a consideration of the following description and drawings of exemplary embodiments.

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

As used herein, the term "reduction mask", "mask" or "patterning device" may be broadly interpreted as referring to a general purpose patterning device that can be used to impart a patterned cross-section to an incident radiation beam, which corresponds to the pattern to be produced in a target portion of a substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

FIG1 schematically depicts a lithography apparatus LA. The lithography apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, EUV radiation, or X-ray radiation); a mask support (e.g., a mask stage) T constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; a substrate support (e.g., a wafer stage) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters; and a projection system (e.g., a refractive projection lens system) PS is configured to project the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (eg, comprising one or more dies).

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 to direct, shape and/or control the 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 as used herein should be interpreted broadly as covering various types of projection systems appropriate to the exposure radiation used and/or to other factors such as the use of an immersion liquid or the use of a vacuum, including refractive, reflective, diffractive, catadioptric, synthetic, magnetic, electromagnetic and/or electro-optical systems, or any combination thereof. Any use of the term "projection lens" herein should be considered synonymous with the more general term "projection system" PS.

The lithography apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid, such as water, having a relatively high refractive index, so as to fill the space between the projection system PS and the substrate W - this is also known as immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference in its entirety.

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

In addition to the substrate support WT, the lithography apparatus LA may also comprise a measurement stage. The measurement stage is configured to hold sensors and/or cleaning devices. The sensors may be configured to measure properties of the projection system PS or of the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning device may be configured to clean parts of the lithography apparatus, for example parts of the projection system PS or parts of a system for providing an immersion liquid. The measurement stage may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support T and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example so that different target portions C are positioned at focused and aligned positions in the path of the radiation beam B. Similarly, a first positioner PM and possibly a further position sensor (which is not explicitly depicted in FIG. 1 ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The mask alignment marks M1, M2 and substrate alignment marks P1, P2 may be used to align the patterning device MA and the substrate W. Although the substrate alignment marks P1, P2 as shown occupy dedicated target portions, they may be located in the space between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe line alignment marks.

As shown in FIG. 2 , the lithography apparatus LA may form part of a lithography cell LC (sometimes also referred to as a lithocell or a (lithography) cluster), which often also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. As is known, these equipment include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH, for example for regulating the temperature of the substrate W (for example for regulating the solvent in the resist layer), and a baking plate BK. A substrate handler or robot RO picks up a substrate W from an input/output port I/O1, I/O2, moves the substrate W between the different process equipment and delivers the substrate W to a loading area LB of the lithography apparatus LA. The devices in the lithography unit, which are often collectively referred to as automated photoresist coating and development systems, may be under the control of an automated photoresist coating and development system control unit TCU. The automated photoresist coating and development system control unit itself may be controlled by a supervisory control system SCS, which may also control the lithography equipment LA, for example, via a lithography control unit LACU.

In lithography processes it is frequently necessary to carry out measurements of the produced structures, e.g. for process control and verification. The tool used to carry out such measurements may be referred to as a metrology tool MT. Different types of metrology tools MT for carrying out such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments which allow measuring parameters of the lithography process by having sensors in or near the pupil, or in a plane conjugated to the pupil of the objective of the scatterometer, the measurements being usually referred to as pupil-based measurements, or by having sensors in or near the image plane, or in a plane conjugated to the image plane, in which case the measurements are usually referred to as image- or field-based measurements. Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can use light from the hard X-ray (HXR), soft X-ray (SXR), extreme ultraviolet (EUV), visible to near infrared (IR) and IR wavelength ranges to measure gratings. In the case where the radiation is hard X-ray or soft X-ray, the aforementioned scatterometers can be a small angle X-ray scattering metrology tool as the case may be.

In order to correctly and consistently expose a substrate W exposed by the lithography apparatus LA, it is necessary to inspect the substrate to measure properties of the patterned structure, such as overlay errors between subsequent layers, line thickness, critical dimensions (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 an error is detected, exposure of subsequent substrates or other processing steps to be performed on the substrate W may be adjusted, for example, especially if the inspection is performed before other substrates W of the same batch or lot are still to be exposed or processed.

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

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

In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In this spectroscopic scatterometer 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 which measures the spectrum of the spectroscopically reflected radiation (i.e. a measure of the intensity as a function of wavelength). From this data, the structure or profile of the target which gave rise to the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

In a third embodiment, the scatterometer MT is an elliptical metrology scatterometer. An elliptical metrology scatterometer allows to determine parameters of a lithography process by measuring the scattered or transmitted radiation for each polarization state. This metrology equipment emits polarized light (such as linear, annular or elliptical) by using, for example, appropriate polarization filters in the illumination section of the metrology equipment. A source suitable for the metrology equipment may also provide polarized radiation. Various embodiments of prior art elliptical measurement scatterometers are described in U.S. Patent Applications 11/451,599, 11/708,678, 12/256,780, 12/486,449, 12/920,968, 12/922,587, 13/000,229, 13/033,135, 13/533,110, and 13/891,410, which are incorporated herein by reference in their entirety.

In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the stacking of two misaligned gratings or periodic structures by measuring the reflected spectrum and/or detecting asymmetries in the configuration, which asymmetries are related to the extent of the stacking. The two (overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers) and may be formed to be in substantially the same position on the wafer. The scatterometer may have a symmetric detection configuration such as described in the commonly owned patent application EP1,628,164A, so that any asymmetries are clearly distinguishable. This provides a direct way to measure misalignment in the gratings. Other examples of measuring the overlay error between two layers containing a periodic structure as a target through asymmetry of the periodic structure can be found in PCT Patent Application Publication No. WO 2011/012624 or U.S. Patent Application US 20160161863, which are incorporated herein by reference in their entirety.

Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety (or alternatively by scanning electron microscopy). A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM - also called a focus exposure matrix). If such unique combinations of critical dimensions and sidewall angles are available, focus and dose values may be uniquely determined based on these measurements.

A metrology target can be a collection of composite gratings formed primarily in resist by a lithography process and also formed after other fabrication processes, such as an etching process. The pitch and linewidth of the structures in the grating can depend largely on the measurement optics (specifically, the NA of the optics) to be able to capture the diffraction order from the metrology target. As indicated earlier, the diffraction signal can be used to determine the shift between two layers (also known as "overlap") or can be used to reconstruct at least a portion of the original grating as produced by the lithography process. This reconstruction can be used to provide quality guidance for the lithography process and can be used to control at least a portion of the lithography process. The target can have smaller sub-segments configured to mimic the size of a functional portion of a design layout in the target. Due to this sub-segmentation, the target will behave more like a functional part of the design layout, making the overall process parameter measurement more similar to the functional part of the design layout. The target can be measured in the underfill mode or in the overfill mode. In the underfill mode, the measurement beam produces a spot that is smaller than the overall target. In the overfill mode, the measurement beam produces a spot that is larger than the overall target. In this overfill mode, it is also possible to measure different targets at the same time, and thus determine different processing parameters at the same time.

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

The patterning process in the lithography apparatus LA may be one of the most critical steps in the processing, requiring a high accuracy of the dimensioning and placement of the structures on the substrate W. In order to ensure this high accuracy, three systems may be combined in a so-called "holistic" control environment, as schematically depicted in FIG3 . One of these systems is the lithography apparatus LA, which is (actually) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and to provide a tight control loop, thereby ensuring that the patterning performed by the lithography apparatus LA remains within the process window. A process window defines a range of process parameters (e.g., dose, focus, overlay) within which a particular fabrication process produces a defined result (e.g., a functional semiconductor device) - variations in process parameters in a lithography process or patterning process may be permitted within the process parameter 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 simulations and calculations to determine which mask layout and lithography equipment settings achieve the maximum overall process window for the patterning process (depicted in FIG. 3 by the double arrows in the first scale SC1). The resolution enhancement technique can be configured to match the patterning possibilities of the lithography equipment LA. The computer system CL can also be used to detect where within the process window the lithography equipment LA is currently operating (e.g., using input from a metrology tool MET) to predict whether defects may be present due to, for example, suboptimal processing (depicted in FIG. 3 by the arrow pointing to "0" in the second scale SC2).

The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithography apparatus LA to identify possible drifts in the calibration state of the lithography apparatus LA, for example (depicted in FIG. 3 by arrows in the third scale SC3).

Many different forms of metrology tools MT are available for measuring structures produced using lithographic patterning equipment. The metrology tools MT can use electromagnetic radiation to interrogate structures. The properties 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 influence on the resolution that can be achieved by the metrology tool. Therefore, in order to be able to measure structures using features with small dimensions, metrology tools MT with short wavelength radiation sources are preferred.

Another way that radiation wavelength can affect the measured properties is the penetration depth, and transparency/opacity of the material being inspected at the radiation wavelength. Depending on the opacity and/or penetration depth, radiation can be used for transmission or reflection measurements. The type of measurement can affect whether information is obtained about the surface and/or the interior of the bulk of the structure/substrate. Therefore, penetration depth and opacity are another factor to consider when selecting a radiation wavelength for a metrology tool.

To achieve higher resolution for the measurement of lithographically patterned structures, metrology tools MT with short wavelengths are preferred. This may include wavelengths shorter than the wavelength of visible light, for example in the UV, EUV and X-ray parts of the electromagnetic spectrum. Hard X-ray methods such as transmission small angle X-ray scattering (TSAXS) exploit the high resolution and high penetration depth of hard X-rays and can therefore operate in transmission. On the other hand, soft X-rays and EUV do not penetrate the target so far, but can induce rich optical responses in the material to be probed. This can be attributed to the optical properties of many semiconductor materials and to the fact that the size of the structures is comparable to the probe wavelength. Thus, an EUV and/or soft X-ray metrology tool MT may operate in reflection, for example by imaging or by analyzing diffraction patterns from lithographically patterned structures.

For hard X-rays, soft X-rays, and EUV radiation, application in high volume manufacturing (HVM) applications can be limited by the unavailability of high-intensity radiation sources at the required wavelengths. In the case of hard X-rays, sources commonly used in industrial applications include X-ray tubes. X-ray tubes, including advanced X-ray tubes (e.g., based on liquid metal anodes or rotating anodes), can be relatively affordable and compact, but may lack the brightness required for HVM applications. Currently there are high-intensity X-ray sources such as synchrotron light sources (SLS) and X-ray free electron lasers (XFELs), but their size (>100 m) and high cost (more than 100 million Euros) make them too large and expensive for metrology applications. Similarly, there is a lack of availability of sufficiently bright EUV and soft X-ray radiation sources.

An example of a metrology apparatus, such as a scatterometer, is depicted in Fig. 4. It may comprise a broadband (e.g. white light) radiation projector 2 which projects radiation 5 onto a substrate W. The reflected or scattered radiation 10 is transmitted to a spectrometer detector 4 which measures the spectrum 6 of the radiation reflected from the mirror (i.e. a measure of the intensity I as a function of the wavelength λ). From this data, the structure or profile 8 which produced the detected spectrum may be reconstructed by a processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra shown at the bottom of Fig. 4. Generally, for reconstruction, the general form of the structure is known and some parameters are assumed based on knowledge of the process used to make the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. The scatterometer can be configured as either a normal-incidence scatterometer or an oblique-incidence scatterometer.

A transmission version of an example of a metrology apparatus is depicted in FIG5, such as the scatterometer shown in FIG4. Transmitted radiation 11 is transmitted to a spectrometer detector 4, which measures spectrum 6 as discussed with respect to FIG4. This scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer. Optionally, a transmission version of hard X-ray radiation is used with a wavelength < 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, such as 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 50 nm, between 1 nm and 50 nm, between 1 nm and 20 nm, between 5 nm and 20 nm, and between 10 nm and 20 nm, has also been considered. An example of a metrology tool that functions in one of the wavelength ranges presented above is transmission small angle X-ray scattering (such as T-SAXS in US 2007224518A, the contents of which are incorporated herein by reference in their entirety). Lemaillet et al. discuss profile (CD) measurements using T-SAXS in "Intercomparison between optical and X-ray scatterometry measurements of FinFET structures" (Proc. of SPIE, 2013, 8681). It should be noted that the use of laser generated 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. Reflection measurement techniques using X-rays at grazing incidence (GI-XRS) and extreme ultraviolet (EUV) radiation can be used to measure the properties of films and layer stacks on substrates. Within the general field of reflection measurement, goniometric and/or spectroscopy techniques can be applied. In goniometric measurement, the variation of the reflected beam with different angles of incidence can be measured. Spectroscopic 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 mask substrates prior to manufacturing reticles (patterning devices) used in EUV lithography.

It is possible that the scope of applicability is such that the use of wavelengths in, for example, the hard X-ray, soft X-ray, or EUV domains is insufficient. Published patent applications US 20130304424A1 and US2014019097A1 (Bakeman et al./KLA) describe hybrid metrology techniques in which measurements using x-rays and optical measurements using wavelengths in the range of 120 nm and 2000 nm are combined to obtain measurements of parameters such as CD. The CD measurement is obtained by coupling the x-ray mathematical model and the optical mathematical model through one or more common parts. The contents of the listed U.S. patent applications are incorporated herein by reference in their entirety.

Figure 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 apparatus 302 presented in Figure 6 may be applicable in the hard X-ray, soft X-ray and/or EUV domains.

Figure 6 shows a schematic physical configuration of a metrology apparatus 302 comprising a spectroscopic scatterometer using hard X-rays, soft X-rays and/or EUV radiation at grazing incidence as appropriate, purely as an example. An alternative form of detection apparatus may be provided in the form of an angle-resolved scatterometer which, like conventional scatterometers operating at longer wavelengths, may use radiation at normal or near normal incidence, and which may also use radiation having a direction which makes an angle greater than 1° or 2° with respect to a direction parallel to the substrate. An alternative form of detection apparatus may be provided in the form of a transmission scatterometer, for which the configuration in Figure 5 applies.

The testing apparatus 302 includes a radiation source or illumination source 310 , an illumination system 312 , a substrate support 316 , a detection system 318 , 398 and a metrology processing unit (MPU) 320 .

The illumination source 310 in this example is used to generate EUV, hard X-ray or soft X-ray radiation. The illumination source 310 may be based on the high order harmonic generation (HHG) technology as shown in FIG6 , and it may also be other types of illumination sources, such as a liquid metal jet source, an inverse Compton scattering (ICS) source, a plasma channel source, a magnetic undulator source, a free electron laser (FEL) source, a compact storage ring source, a discharge generated plasma source, a soft X-ray laser source, a rotating anode source, a solid anode source, a particle accelerator source, a microfocus source or a laser generated plasma source.

The HHG source can be a gas jet/nozzle source, a capillary/fiber source, or an air cell source.

For an example of a HHG source, as shown in FIG6 , 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, and 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-optic based laser with an optical amplifier, thereby generating pulses of infrared radiation that may last, for example, less than 1 nanosecond (1 ns) per pulse, with a pulse repetition rate of up to 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 micrometer (1 μm). Optionally, the laser pulse is delivered as first pump radiation 340 to a gas delivery system 332, where a portion of the radiation is converted in the gas to a higher frequency than the first radiation as emission radiation 342. A gas supply 334 supplies a suitable gas to the gas delivery system 332, where the suitable gas is optionally ionized by a power source 336. The gas delivery system 332 may be a cutting tube.

The gas provided by the gas delivery system 332 defines the gas target, which can be a gas stream or a static volume. The gas can 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. Such gases can be selectable options within the same device. The emitted radiation can contain multiple wavelengths. If the emitted radiation is monochromatic, measurement calculations (e.g., reconstruction) can be simplified, but it is easier to produce radiation with several wavelengths. The emission divergence angle of the emitted radiation can be wavelength dependent. Different wavelengths will provide different levels of contrast, for example when imaging structures of different materials. For example, to detect metal structures or silicon structures, different wavelengths may be selected as wavelengths for imaging features of (carbon-based) resists or for detecting contamination of these different materials. One or more filter devices 344 may be provided. For example, filters such as thin membranes of aluminum (Al) or zirconium (Zr) may be used to cut off fundamental IR radiation from further transmission into the detection equipment. A grating (not shown) may be provided to select one or more specific wavelengths from the wavelengths generated. Optionally, the illumination source includes a space configured to be evacuated, and the gas delivery system is configured to provide a gas target in the space. As appropriate, some or all of the beam path may be contained within a vacuum environment, keeping in mind that SXR and/or EUV radiation is absorbed when traveling in air. The various components of the radiation source 310 and illumination optics 312 may be adjustable to implement different metrology "recipes" 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 degree of penetration into underlying layers. In order to resolve the smallest device features and defects within the smallest device features, short wavelengths are likely to be preferred. For example, one or more wavelengths may be selected in the range of 0.01 to 20 nm, or optionally in the range of 1 to 10 nm, or optionally in the range of 10 to 20 nm. Wavelengths shorter than 5 nm may suffer from extremely low critical angles when reflected from materials of interest in semiconductor manufacturing. Therefore, selecting wavelengths greater than 5 nm may provide stronger signals at higher angles of incidence. On the other hand, if the inspection task is to detect the presence of a certain material, for example to detect contamination, wavelengths up to 50 nm may be useful.

From the radiation source 310, the filtered light beam 342 may enter a detection chamber 350 in which a substrate W including a structure of interest is held by a substrate support 316 for detection at a measurement location. The structure of interest is labeled T. Optionally, the atmosphere within the detection chamber 350 may be maintained as a near vacuum by a vacuum pump 352 so that SXR and/or EUV radiation may pass through the atmosphere without undue attenuation. The illumination system 312 has the function of focusing the radiation into a focused beam 356 and may include, for example, a two-dimensional curved mirror or a series of one-dimensional curved mirrors, as described in the above-mentioned published U.S. patent application US2017/0184981A1 (the contents of which are incorporated herein by reference in their entirety). Focusing is performed to achieve a circular or elliptical spot S with a diameter of less than 10 μm when projected onto the structure of interest. The substrate support 316 includes, for example, an X-Y translation stage and a rotation stage, by which any part of the substrate W can be brought to the focus of the beam in a desired orientation. Thus, the radiation spot S is formed on the structure of interest. Alternatively or additionally, the substrate support 316 includes, for example, a tilt stage, which can tilt the substrate W at a certain angle to control the incident angle of the focused beam on the structure of interest T.

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

Reflected radiation 360 is captured by detector 318 and the spectrum is provided to processor 320 for use in calculating properties of target structure T. Illumination system 312 and detection system 318 thus form a detection apparatus. This detection apparatus may include a hard X-ray, soft X-ray and/or EUV spectroscopic reflectometer of the type described in US2016282282A1, the contents of which are incorporated herein by reference in their entirety.

If the target Ta has a certain periodicity, the radiation of the focused beam 356 may also be partially diffracted. The diffracted radiation 397 follows another path at a well-defined angle relative to the incident angle and then relative to the reflected radiation 360. In Figure 6, the absorbed diffracted radiation 397 is absorbed in a schematic manner, and the diffracted radiation 397 can follow many other paths besides the absorbed path. The detection device 302 may also include another detection system 398 that detects and/or images at least a portion of the diffracted radiation 397. In Fig. 6, a single further detection system 398 is depicted, but embodiments of the detection device 302 may also include more than one further detection system 398, which are arranged at different positions to detect the diffracted radiation 397 in a plurality of diffraction directions and/or to image the diffracted radiation 397. In other words, (higher) diffraction orders of the focused radiation beam impinging on the target Ta are detected and/or imaged by one or more further detection systems 398. The one or more detection systems 398 generate a signal 399 which is 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 assist in aligning and focusing the light spot S with the desired product structure, the inspection apparatus 302 may also provide auxiliary optics using auxiliary radiation under the control of a metrology processor 320. The metrology processor 320 may also communicate with a position controller 372 which operates a translation stage, a rotation and/or a tilt stage. The processor 320 receives highly accurate feedback about the position and orientation of the substrate via sensors. The sensor 374 may include, for example, an interferometer which may give an accuracy of approximately a few picometers. In operation of the inspection apparatus 302, spectral data 382 captured by the detection system 318 is delivered to the metrology processing unit 320.

As mentioned, an alternative form of inspection equipment uses hard X-rays, soft X-rays and/or EUV radiation at normal incidence or near normal incidence, as appropriate, for example to perform diffraction-based asymmetry measurements. Another alternative form of inspection equipment uses hard X-rays, soft X-rays and/or EUV radiation having a direction that is at an angle greater than 1° or 2° from a direction parallel to the substrate. Both types of inspection equipment can be provided in a hybrid metrology system. Performance parameters to be measured may include overlay (OVL), critical dimension (CD), focus of the lithography equipment when the lithography equipment is printing a target structure, coherent diffraction imaging (CDI), and resolution-based overlay (ARO) metrology. Hard X-ray, soft X-ray and/or EUV radiation may, for example, have a wavelength less than 100 nm, for example using radiation in the range of 1 to 50 nm, 1 to 30 nm, 5 to 30 nm, or, as appropriate, in the range of 10 nm to 20 nm. The radiation may be narrowband or broadband in character. The radiation may have discrete peaks in a particular wavelength band or may have a more continuous character.

Similar to optical scatterometers used in current production facilities, the inspection apparatus 302 can be used to measure structures in resist materials processed in a lithography cell (post-development inspection or ADI) and/or to measure structures after they have been formed in harder materials (post-etch inspection or AEI). For example, the inspection apparatus 302 can be used to inspect a substrate after it has been processed by a developer, etcher, annealer, and/or other apparatus.

The metrology tool MT, including but not limited to the scatterometer mentioned above, can 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, such as radiation in the infrared, visible and/or ultraviolet part of the electromagnetic spectrum. The metrology tool MT can use radiation to measure or detect properties and states of a substrate, such as a lithographically exposed pattern on a semiconductor substrate. The type and quality of the measurement may depend on certain properties of the radiation used by the metrology tool MT. For example, the resolution of the electromagnetic measurement may depend on the wavelength of the radiation, wherein smaller wavelengths enable smaller features to be measured, for example due to diffraction limitations. In order to measure features with small dimensions, it may be preferable to use radiation with a short wavelength, such as EUV, hard X-ray (HXR) and/or soft X-ray (SXR) radiation, to perform the measurement. In order to perform metrology at a specific wavelength or wavelength range, the metrology tool MT needs access to a source providing radiation at that/those wavelengths. There are different types of sources for providing radiation of different wavelengths. Depending on the wavelength 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 high-order harmonic generation (HHG) or any other type of source mentioned above to obtain radiation of the desired wavelength.

FIG7 shows a simplified schematic diagram of an embodiment 600 of an illumination source 310, which may be an illumination source for high order harmonic generation (HHG). One or more of the features of the illumination source in the metrology tool described with respect to FIG6 may also be present in illumination source 600, as appropriate. Illumination source 600 includes 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 pump radiation source 330, as shown in FIG6. Pump radiation 611 may be directed into chamber 601 via radiation input 605, which may be a viewing region optionally made of molten silica or a comparable material. The pump radiation 611 may have a Gaussian or hollow (e.g., annular) transverse cross-sectional profile and may be incident (focused as appropriate) on a gas stream 615 within the chamber 601, the gas stream having a flow direction indicated by a second arrow. The gas stream 615 comprises a small volume of a specific 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) with a gas pressure above a certain value, the small volume being referred to as a gas volume or gas target (e.g., a few cubic mm). The gas stream 615 may be a steady flow. Other media such as metal plasma (e.g., aluminum plasma) may also be used.

The gas delivery system of the illumination source 600 is configured to provide a gas stream 615. The illumination source 600 is configured to provide pump radiation 611 in the gas stream 615 to drive the generation of emission radiation 613. The region in which at least a large portion of the emission radiation 613 is generated is referred to as the interaction region. The interaction region can vary from tens of microns (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 emission radiation at the 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 flow 615 is provided by a gas delivery system into the evacuated or nearly evacuated space. The gas delivery system may include a gas nozzle 609, as shown in FIG6 , which includes an opening 617 in the exit plane of the gas nozzle 609. The gas flow 615 is provided from the opening 617. A gas trap is used to confine the gas flow 615 to a certain volume by extracting the residual gas flow and maintaining a vacuum or near-vacuum atmosphere inside the chamber 601. Optionally, the gas nozzle 609 may be made of thick-walled tubing and/or highly thermally conductive materials to avoid thermal deformation due to high-power pump radiation 611.

The dimensions of the gas nozzle 609 can conceivably also be used in scaled-up or scaled-down versions ranging from micrometer-sized nozzles to meter-sized nozzles. This wide range of sizing results from the fact that the settings can be scaled so that the intensity of the pump radiation at the gas stream is ultimately within a specific range that can be beneficial for emitting radiation, which requires different sizing for different pump radiation energies that can be pulsed lasers, and the pulse energies can vary from tens of microjoules to several joules. Optionally, the gas nozzle 609 has thicker walls to reduce nozzle deformation caused by thermal expansion effects that can be detected by, for example, a camera. A gas nozzle with thicker walls can produce a stable gas volume with reduced variation. Optionally, the illumination source includes a gas trap close to the gas nozzle to maintain the pressure of the chamber 601.

Due to the interaction of pump radiation 611 with gas atoms of gas stream 615, gas stream 615 will convert part of pump radiation 611 into emission radiation 613, which can be an example of emission radiation 342 shown in Figure 6. The central axis of emission radiation 613 can be collinear with the central axis of incident pump radiation 611. Emission radiation 613 can have a wavelength in the X-ray or EUV range, hereinafter referred to as SXR radiation, wherein the wavelength is in the range of 0.01 nm to 100 nm, optionally 0.1 nm to 100 nm, optionally 1 nm to 100 nm, optionally 1 nm to 50 nm, or optionally 10 nm to 20 nm.

In operation, a beam of emitted radiation 613 may pass through radiation output 607 and may then be manipulated and directed to a substrate to be inspected for metrology measurement by illumination system 603, which may be an example of illumination system 312 in Figure 6. Emitted radiation 613 may be directed (optionally focused) to structures on the substrate.

Because air (and indeed any gas) absorbs SXR or EUV radiation to a great extent, the volume between the gas stream 615 and the wafer to be inspected may be evacuated or nearly evacuated. Since the central axis of the emission radiation 613 may be collinear with the central axis of the incident pump radiation 611, the pump radiation 611 may need to be blocked to prevent it from passing through the radiation output 607 and entering the illumination system 603. This may be done by incorporating the filter device 344 shown in FIG. 6 into the radiation output 607, which is placed in the emission beam path and is opaque or nearly opaque to the pump radiation (e.g., opaque or nearly opaque to infrared or visible light) but at least partially transparent to the emission radiation beam. The filter may be made of zirconium or multiple 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 not perpendicular and not parallel to the propagation direction of the emission radiation beam to have efficient pump radiation filtering. Optionally, the filter device 344 includes a hollow block and a thin membrane filter such as an aluminum (Al) or zirconium (Zr) membrane filter. Optionally, the filter device 344 may also include a mirror that effectively reflects the emitted radiation but poorly reflects the pump radiation, or a metal mesh that effectively transmits the emitted radiation but poorly transmits the pump radiation.

Methods, apparatus and assemblies are described herein for obtaining emitted radiation, optionally at a high-order harmonic frequency of a pump radiation. The radiation generated by the procedure (optionally using nonlinear effects to produce HHG of radiation, optionally at a harmonic frequency of the provided pump radiation) can be provided as radiation in a metrology tool MT for inspection and/or measurement of substrates. If the pump radiation comprises short pulses (i.e. a few cycles), the radiation generated does not have to be exactly at the harmonics of the pump radiation frequency. The substrate can be a lithographically patterned substrate. The radiation obtained by the procedure can also be provided in a lithography apparatus LA and/or a lithography cell LC. The pump radiation may be pulsed radiation which provides high peak intensity in short bursts.

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

For some embodiments, the emitted radiation (and optionally higher order harmonic radiation) may include one or more harmonics having a wavelength of the pump radiation. 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 following ranges: 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 high order harmonic radiation as described above may be provided as source radiation in the metrology tool MT. The metrology tool MT may use the source radiation to perform measurements on a substrate exposed by a lithography apparatus. The measurements may be used to determine one or more parameters of a structure on the substrate. Using radiation at shorter wavelengths (e.g. at EUV, SXR and/or HXR wavelengths included in the wavelength range described above) may allow smaller features of a structure to be resolved by the metrology tool compared to using longer wavelengths (e.g. visible radiation, infrared radiation). Radiation with shorter wavelengths, such as EUV, SXR and/or HXR radiation, can 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 by radiation with 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 the source radiation incident on the target structure. The metrology tool MT may include one or more sensors for detecting diffracted radiation. For example, the metrology tool MT may include detectors for detecting positive one (+1) and negative one (-1) diffraction orders. The metrology tool MT may also measure mirror reflected or transmitted radiation (0-order diffraction radiation). Other sensors used for metrology may be present in the metrology tool MT, for example to measure other diffraction orders (eg higher diffraction orders).

In an exemplary lithography 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 radiation from the HHG source to the target. The HHG radiation may then be reflected from the target, detected, and processed, for example, to measure and/or infer properties of the target.

Gas target HHG configurations can be broadly divided into three separate categories: gas jets, gas cells, and gas capillaries. FIG. 7 depicts an exemplary gas jet configuration in which a gas volume is introduced into the driving/pump radiation laser beam. In the gas jet configuration, the interaction of the driving radiation with solid components 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 an air cell. In a gas capillary arrangement, the size of the capillary structure containing the gas is so small in the lateral direction that it significantly affects the propagation of the driving radiation laser beam. The capillary structure may, for example, be a hollow core optical fiber in which the hollow core is configured to contain the gas.

The gas jet HHG configuration can provide a relative degree of freedom to shape the spatial profile of the driving radiation beam in the far field, since it is not subject to the defined constraints imposed by the gas capillary structure. The gas jet configuration can also have less stringent alignment tolerances. On the other hand, the gas capillary can provide an increased interaction zone of the driving radiation with the gaseous medium, which can optimize the HHG process.

In order to use the HHG radiation, for example in metrology applications, the HHG radiation is separated from the driver radiation downstream of the gas target. The separation of HHG radiation from the driver radiation may be different for gas jet configurations and gas capillary configurations. In both cases, the driver radiation suppression scheme may include a metal transmission filter for filtering out any remaining driver radiation from the shortwave radiation. However, before this filter can be used, the intensity of the driver radiation should be significantly reduced from its intensity at the gas target to avoid damage to the filter. The methods that can be used for this intensity reduction are different for gas jet configurations and capillary configurations. For gas jet HHG, due to the relative 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, it can be engineered so that in the far field it has low intensity along the direction of short wavelength radiation propagation. This spatial separation in the far field means that apertures 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 may be primarily dictated by the capillaries. The spatial profile of the driving radiation may be determined by the shape and material of the capillary structure. For example, in the case where a hollow core optical fiber is used as the capillary structure, the shape and material of the optical fiber structure determine which driving radiation modes are supported to propagate through the optical fiber. For most standard optical fibers, the supported propagation modes produce a spatial profile in which high intensities of the driving radiation overlap with high intensities of the HHG radiation. For example, the driving radiation intensity may be centered on a Gaussian or near-Gaussian profile in the far field.

In the aforementioned SXR sensor/metrology configuration, the SXR light beam may be focused on a target at a fixed single angle by means of a (e.g., single) focusing mirror or mirror system. The intensity of the diffraction order scattered by the target may be detected using one or more sensors close to the target. The detected light may be analyzed using algorithms that infer profile parameters or parameter-of-interest values associated with the illuminated target. In the context of the present disclosure, the terms "parameter-of-interest" and/or "profile parameter" may include, for example, overlay and/or other (e.g., 3D) profile parameters such as etch depth, side wall angle (SWA), layer thickness, critical dimension (CD), or any other dimension of any feature of a structure. Thus, in the context of the present disclosure, target parameters or profile parameters describe the physical profile parameters 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 the profile parameters may be correlated to a greater or lesser extent in the detected signal, meaning that changes in each of two or more such parameters result in similar changes in the detected signal. This means that it may be difficult to disentangle the effects of these parameters, and it may be difficult to determine which parameter(s) are responsible for a particular signal effect. Also, some parameters may cause very weak signal contributions in this configuration, making it difficult to obtain an accurate estimate of that parameter from the measured data.

In theory, measurements at multiple incident angles can be used to generate greater signal diversity (see, for example, elliptical polarimetry). However, at wideband SXR or EUV wavelengths, it is difficult and/or expensive to manufacture optics to achieve this. A simple solution (in principle) could consist in providing multiple sources and illuminators in order to provide independent incident light sources. However, this would result in greater costs and would be difficult to accommodate in compact tool designs.

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

In a first embodiment, the proposed (e.g. SXR) metrology tool may comprise only a simple finite-finite conjugate illuminator system (e.g. a single annular, elliptical or Kirkpatrick-Baez (KB) mirror system). In this case, the only available conjugate field plane is the plane of the (virtual) source. In another embodiment, the illuminator comprises a middle focus at a 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 middle 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 can be captured by a (e.g., single) final focusing mirror element. The first scattered radiation includes a plurality of scattered beams, namely both diffracted radiation and mirror reflected radiation. This final focusing mirror element re-images the plane (e.g., SFP or IFP, depending on the configuration) containing the first dispersive element onto the target or the second diffraction element by capturing as much of the first scattered radiation as possible. Thus, the final focusing mirror element can capture and re-image the reflected radiation and at least one and preferably a plurality of (e.g., as many as possible) diffraction orders contained in the first scattered radiation. Due to the various diffraction angles from the first dispersive element, this re-imaged first scattered radiation will therefore be incident on the target at a variety of different angles of incidence. In the case where the dispersed radiation is broadband or contains multiple wavelengths, each "diffraction order" from the first dispersive element will actually include multiple monochromatic diffraction orders with color-dependent angles of incidence on the target. Second scattered radiation that has been scattered from the target or the second diffraction element may include specular reflections and multiple diffraction orders from the target (i.e., second scattered beams); each of which can be detected and will contain additional information when compared to known measurements using a single beam at a single angle of incidence.

FIG8 shows (a) a conceptual diagram and (b) a simplified schematic diagram of an embodiment. In FIG8(a), all beams are drawn in transmission for clarity. In FIG8(b), the focusing element and the dispersive element are reflective, which may be more suitable for SXR metrology (at least for the focusing element, the dispersive element can operate in reflection or transmission). FIG8 shows a source 800, a first focusing element or first focusing mirror 805, which re-images the source 800 at the intermediate field plane including a first dispersive element or grating 810. The scattered radiation from the grating 810 can be captured by a second focusing element or final focusing mirror element 815, which re-images the intermediate field plane onto a second dispersive element or target 820 (which is the final focusing element before the target/wafer, as the case may be). This second refocusing element can include an elliptical mirror, which includes a specific elliptical configuration, as will be described later. The second scattered radiation from the target 820 can be captured on a detector or camera 825.

In this description, the numbering convention for captured diffraction orders will describe each captured order as (m, n), where m describes the diffraction order (e.g., including at least the zero order and at least one higher diffraction order) from the first dispersive element/grating 810, and n describes the diffraction order from the second dispersive element/target 820. In known SXR metrology, only the m=0 order is detected. Thus, in the case 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, this diffraction order can be described as a (-1, +1) diffraction order. It should be noted that the captured diffraction order visual case includes diffraction orders higher than one (eg, m ≥ 2 and/or n ≥ 2).

In FIG8( a ), only the m=0 order and the m=-1 order from the grating 810 are plotted for a single wavelength for the sake of clarity; other orders (both positive and negative) may also be captured by the final focusing mirror. It will be appreciated that the diffraction orders with different wavelengths are angularly displaced (i.e., the diffraction angles, and therefore the angle of incidence on the target, are different for different wavelengths). Again, for the sake of clarity, the only captured diffraction orders shown are (-1,-1), (0,0), (-1,0), (-1,+1). More diffraction orders may be captured. In FIG8( b ), more captured diffraction orders are shown as labeled, but again other diffraction orders may also be captured.

It should be noted that in this configuration, the diffraction orders incident on the second dispersive element may include at least two non-complementary diffraction orders (e.g., and thus at least two non-complementary incidence angles). The complementary diffraction orders may include positive and negative orders having the same order magnitude (e.g., +1 and -1 orders), while the at least two non-complementary orders may include, for example, the zero order as well as any other order.

FIG9 is an exemplary captured image including a plurality of captured diffraction orders labeled, which illustrates, for example, the positions at which various orders may be detected on a detector. Note that this example assumes a 1D grating of the target; the target may also include a 2D grating. Each individual point of the diffraction order may be associated with 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 which produces multiple steps at various diffraction angles. If a grating, the first dispersive element may comprise different spacings and/or groove depths, for example to correct optical parameters such as wavelength-dependent defocus for wavelength-dependent diffraction angles. Also, its orientation may be varied to optimize the exit diffraction angle.

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

In one embodiment, some of these measurement parameters (e.g., angle of incidence, azimuth, grating spacing) may be varied to optimize the diffracted light for a particular target. For example, in recipe generation, the configuration may be optimized to produce the most favorable angle of incidence on the target to obtain maximum signal divergence/information content. For example, the spacing of the gratings may be varied by providing a variable/selectable spacing to the line-space gratings or by providing individual gratings configured to be selectively (individually) switched to the mid-field plane (e.g., a grating selector or grating wheel comprising multiple gratings with different individual spacings).

Some of these parameter variation examples are equally applicable to transmissive and reflective gratings; for example, azimuth and spacing variations. In general, higher diffraction efficiency can be achieved with transmissive gratings.

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 can be generated by focusing a high-energy pulsed IR laser (pump/drive radiation, as the case may be, pump/drive laser) into a gas jet or a gas target. If the local field strength in the gas is large enough, photons are generated, as the case may be, at the SXR wavelength. Certain source configuration parameters will determine whether all photons emitted in the gas will interfere constructively or destructively in a certain direction, and therefore whether emitted radiation, in the case of SXR radiation, is emitted from the source in a particular direction.

In one embodiment, it is proposed to provide a plurality of drive radiation beams, which are configured to overlap (in space and time) in a gas jet. Optionally, the plurality of drive radiation beams are coherent. Due to their coherence, they will interfere and, depending on the angle between the two incident wavefronts, an interference pattern will result. For example, when the two beams overlap at an angle, an intensity grating can be generated in the gas. Since SXR photons are only generated above a certain field intensity threshold, the SXR source itself is actually a grating or a virtual first dispersive element. Therefore, only in certain directions, after the gas, the phase of the SXR radiation will be matched, resulting in a plurality of outgoing beams at various (diffraction) angles. This procedure has been described, for example, in “Noncollinear enhancement cavity for record-high out-coupling efficiency of an EUV frequency comb” - Zhang et al. 2020, which is incorporated herein by reference.

Figure 10(a) illustrates this principle. It schematically shows two infrared laser pulses 1000a, 1000b superimposed to form an intensity grating 1010, thereby generating generated SXR radiation including a zero 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.

Figure 10 (b) and Figure 10 (c) illustrate the variation of this embodiment, wherein half-wave plate 1050 has been added to one or two arms of infrared laser pulse 1000a, 1000b. By rotating half-wave plate 1050, the polarization of each arm 1000a, 1000b can be controlled. When two arms have the same polarization, interference pattern is formed, and SXR (Figure 10 (b)) is produced under zero order 1020 and diffraction order 1030+, 1030- angle. When polarization is orthogonal, there is no interference, and regular circular intensity pattern is produced; Therefore, single SXR beam 1020 (Figure 10 (c)) is produced. Half-wave plate 1050 can be rotated to select between illumination angles and therefore select between each in the illustrated operating mode. This effectively provides control over changing the NA of the 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 that case, two completely independent SXR sources are created along the directions of 1000a and 1000b.

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

Regardless of where the first diffraction element is located, the first scattered radiation (i.e., containing one or more non-zero diffraction steps) should be captured by the final focusing mirror, which can be the final mirror before the target (or the only mirror for SFP diffraction embodiments). The captured diffraction angles from the first diffraction element should be properly re-imaged without excessive aberrations, for example, in order to underfill the target grating. The mirror should also have a large capture NA to capture all desired diffraction angles.

Thus, the final focusing mirror may comprise an ellipsoidal surface. This surface ensures (at least nominally) nearly aberration-free re-imaging. Although there may be some magnification at certain angles of incidence, and thus mismatched object and image distances, this does not immediately cause problems when the target is underfilled. In any case, the configuration of the IFP grating and the ellipsoid may be chosen as desired to match all magnifications. To ensure a large NA, the ellipsoidal surface may extend over a considerable distance.

FIG. 11 schematically illustrates a proposed focusing mirror element according to an embodiment. The final focusing mirror may comprise an elliptical surface, which is described by an elliptical portion rotated about its long axis. This final focusing mirror may therefore comprise a hollow ellipsoidal portion 1100 having a mirroring inner surface 1110. In one embodiment, the hollow ellipsoidal portion may comprise a hollow ellipsoidal portion having a first (input) opening, which is larger than the second (output) opening. In another embodiment, the hollow ellipsoidal portion may comprise a hollow ellipsoidal portion having a first (input) opening, which is smaller than the second (output) opening. The hollow ellipsoidal portion may, for example, comprise a half portion or smaller portion having an opening at each end. Thus, the final focusing mirror may include a hollow ellipsoid 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 ellipsoid. The larger opening at the input end may be 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 (for example, at a point near the minor axis plane). This mirror does not need to be completely rotationally symmetric and may include a surface that is only partially rotated about the major axis. However, such a mirror can be manufactured using electroforming techniques, which will produce a completely rotationally symmetric mirror. This also provides additional mechanical stability.

The proposed method for manufacturing this mirror can 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 proposed 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 axially symmetrical mandrel comprising an elongated elliptical spherical portion; forming a focusing mirror element body around the mandrel; and releasing the focusing mirror element body from the mandrel, whereby the focusing mirror element body has an optical surface defined by an inner surface of the focusing mirror element body. The mandrel may comprise a super-polished surface, which is produced, for example, by a super-polishing step using a process selected from the group consisting of: magnetorheological fluid finishing (MRF), fluid jet polishing (FJP), elastic emission machining (EEM), ion beam shaping and float polishing. This super polishing 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, electrodes and a release layer may be applied to the mandrel prior to the forming step.

It is understood that an ellipsoid is quite sensitive to misalignment, and the sensitivity increases rapidly as the beam divergence increases. However, the proposed approach of using the final focusing mirror and the ellipsoidal surface proposed herein does not produce a larger beam divergence, but actually produces a plurality of beams with a smaller divergence within a larger angular spread. The sensitivity of individual low divergence beams may be still smaller.

In another embodiment, it is proposed to use the same basic concept as described above to modify the illumination configuration so as to achieve direct phase detection. The proposed direct phase detection can be employed in a manner similar to shearing interferometry. In shearing interferometry, the phase of a radiation beam is detected (probed) by (partially) overlapping the beam with a reference beam that is coherent with the probe beam. The wavefronts of the two beams are tilted with respect to each other such that an interference pattern with fringes is formed.

In order to produce two mutually coherent beams, the SXR radiation from the source may initially be focused at an intermediate focus. A diffraction element (e.g., the first diffraction element or grating described previously) may be located at this intermediate focus. The first dispersive element or intermediate focus grating may be oriented in a conical orientation (lines parallel to the reflection plane) or in a planar orientation (lines perpendicular to the reflection plane). In other embodiments, the lines may also be tilted relative to the reflection plane (e.g., not completely parallel or not completely perpendicular), but oriented at a certain azimuth angle. As previously described, the first diffraction element may include a reflective element (e.g., a reflective grating) or a transmissive element (e.g., a transmissive grating). In the further description, it will be assumed that the orientation of the first dispersive element is purely conical. As in the previous embodiment, when the grating disperses the radiation at various angles, the first dispersive element will produce a first scattered radiation comprising a plurality of beams: diffraction orders (±m,λ), where m is the order of the diffraction peak and λ is the wavelength. The special case of m=0 can be recognized as a mirror-reflected beam or a zero-order beam. The beams m=0, m=-1,λ and m=+1,λ (and higher orders if the configuration permits) are collected by an ellipsoidal mirror or final focusing mirror (such as has been described), which refocuses the beam onto a second diffraction element or target to be detected.

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 be diverted again to diversion orders (±n,λ), and for simplicity, only the zero diversion order and the one diversion order of the incident beam m=-1 and m=+1 will be discussed below (the wavelength argument is also omitted for simplicity of notation, where it is implicitly assumed that the beam is monochromatic at a certain wavelength in the spectrum; the wavelength-dependent form is also presented further below). The diffracted beams from the second dispersive element can therefore be classified as follows: (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 shown that there are configuration parameters where some of these beams will overlap spatially at the detector so that they interfere. Two logical combinations would be to use the zeroth order reflection from the second dispersive element as the reference beam and the appropriate first order diffraction beam as the probe beam: for example, a beam pair could include using (m,n)=(-1,0) as the reference beam and (m,n)=(+1,-1) as the probe beam or using (m,n)=(+1,0) as the reference beam and (m,n)=(-1,+1) as the probe beam. Two of these beam pairs are shown overlapping in FIG8( b ). This is just an example and other combinations can be made to overlap as well.

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

Although there is not much room to accommodate the origin of such a shift because the target is usually very small, the required tilt (and therefore shift) is also very small. For example, the optical path-length difference (OPD) for the wave as a function of the angle α within the beam divergence 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 with respect to the normal to the target surface.

For example, assume that the 1/e ² divergence of the beam is about 2 mrad, and that If at least two stripes are present over the full angular beam divergence, then the required displacement will be (assumed ). Therefore, the focus of beam m=-1 and beam m=+1 on the target should be displaced laterally by at least 6.8 μm, or more generally greater than 4 μm, greater than 5 μm, or greater than 6 μm (e.g., between 4 μm and 10 μm, between 5 μm and 10 μm, between 5 μm and 9 μm, between 5.5 μm and 9 μm, between 6 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 have the beam m=-1 with a first tilt in the illumination pupil and the beam m=+1 with a second tilt in the illumination pupil. The illumination pupil may comprise a pupil plane or a Fourier plane in the illumination branch before the target/second diffraction element. In one embodiment, the first tilt and the second tilt may be equal in magnitude and opposite in direction. This corresponds to a uniform aberration throughout the entire illumination pupil.

FIG12 schematically illustrates an exemplary (partial) configuration for achieving this. The simplest uniform aberration is defocus, and the simplest way to insert defocus in this 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 a 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 order are displaced relative to each other. As mentioned above, the final focusing mirror forms an image in a plane conjugate with the source.

The required defocus dZ can be calculated as: in is the incident angle on the first dispersive element, and p1 is the spacing of the first dispersive elements.

Figure 13 shows Schematic diagram of how a non-zero value of produces (spherical) wavefronts 1365(+1,0), 1365(-1,-1) associated with diffraction orders (+1,0) and (-1,-1), respectively, which are slightly tilted relative to each other; this in turn produces (angle Plane 1370 is a concentric plane of the first diffraction element.

FIG. 14 shows an exemplary (simulated) stripe pattern that may be seen on a detector (monochrome) when the method of this embodiment is performed. The figure shows a plot of intensity I for X detector pixel locations DPX relative to Y detector pixel locations DPY (where brighter areas indicate higher intensity). The (left/right) position of the stripe 1480 will depend on the phase difference between, for example, beams (m,n)=(-1,0) and (m,n)=(+1,-1); this is phase information that provides additional information about the target. Thus, the phase difference between the beams can be determined from the position of the stripe on the detector.

This phase difference can be used to infer one or more profile parameters. For example, a wobble curve describing the wavelength-dependent phase difference can be determined and used to infer one or more profile parameters. It can be shown that such a wobble curve varies significantly with changes 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 made to overlap at the detector. Below is a formal description of the conditions under which this is indeed the case. Starting from the equation for the conical grating: where θ _i and θ _f are the incident and exit tilt angles, φ _i and φ _f are the incident and exit azimuth angles, _px and _py are the grating spacings in the x and y directions, and (h, k, l) are the inverse lattice coordinates. The Z component can be neglected because it mainly balances energy conservation. In addition, in conical diffraction, h=0 and k=m, as defined above.

To simplify the notation, the grating The equation in the pyramidal configuration would be written as: in is the incident beam, is the outgoing beam, and is the grating vector.

After the first grating, the outgoing beam is re-imaged and incident on the second grating at a different angle. Grating The zero-order tilt above makes is 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 , the equation becomes much simpler: in Simply equal to , and the condition for beam overlap is written as: It is equivalent to: in is the ratio between the spacing on the first dispersive element and the spacing on the second dispersive element, and is the spacing of the second dispersive element (e.g., target). , which is satisfied for the combination (m,n)=(-1,0) and (m',n')=(+1,-1) as well as for the combination (m,n)=(+1,0) and (m',n')=(-1,+1). Note that the wavelength dependence has vanished and thus beam matching is achieved simultaneously for all wavelengths in the spectrum.

The above treatment 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, it would mean that, for example, the angle of incidence on the target An alternative is to have larger grazing incidence angles on both the first and second dispersive elements. To date, this has not been considered because (1) the order no longer propagates for small target spacings and (2) underfilling the targets becomes more difficult due to the focus being projected on the wafer plane. Nevertheless, embodiments can be envisioned in which this “equal incidence angle” geometry is applied, for example when using a shorter wavelength range or by designing a first dispersive element grating with sufficient diffraction efficiency at larger incidence angles, for example by applying a broadband (multi-layer) coating. Again, this equal incidence angle geometry may be suitable for some applications (e.g., because most larger spacings are of concern).

In another embodiment, a larger grazing incidence angle is selected on the first dispersive element to maintain high diffraction efficiency and reflectivity with a simple coating while maintaining a near normal incidence angle on the target. In this configuration, in the above equation, will be non-zero. It is no longer equal to , then it should be explicitly evaluated based on the incident beam coordinates :

Using this expression, we can evaluate , to obtain the above (with A similar condition to the first-order approximation in :

Since this expression now contains , so there is a wavelength dependency in the direction of the exit beams that exist. Furthermore, the spacing of the first grating is no longer a simple multiple of the target spacing. Wavelength dependency is not a pressing issue as long as it is assumed that the difference in the exit beam directions of the two overlapping beams is small compared to the divergence of the beams. Moreover, a spacing p1 can always be found such that the overlap is perfect for at least one wavelength. For the angle of incidence on the first dispersive element and the second dispersive element and And the recursive relationship for finding the best matching spacing _p1 for the target spacing _p2 of the second dispersion element for a single wavelength is given by:

For this spacing, the wavelength The overlap is perfect below 100 nm and decreases for further wavelengths.

In addition to the parameters mentioned above, there are many more parameters that can be tuned to optimize beam overlap. For example, in one embodiment, one or both of a first azimuth angle incident on a first dispersive element and a second azimuth angle incident on a second dispersive element can be tuned. The spacing of the first dispersive element can be variable (e.g., variable line space gratings) to provide wavelength-dependent defocus and tilt aberrations. The first dispersive element can include a transmissive grating or a more complex diffraction element. The optical parameters of the elliptical final focusing mirror between the first dispersive element and the second dispersive element (e.g., angle of incidence, image distance, object distance) can be optimized, for example, so that all orders are reflected at symmetry planes that intersect the long elliptical axis. As already stated, beam pairs other than those explicitly described can be superimposed.

The spacing _p1 of the first dispersive element is related to the target spacing _p2 . For small target spacing changes (e.g. spacing play), the angle of incidence on the first dispersive element can be modified to capture the overlap condition according to the above recursive relationship. For larger changes in target spacing, a different first dispersive element spacing is required. In one embodiment, this can be implemented by providing a grating selector or grating wheel as already described.

In one embodiment, various exposure schemes may be used. For example, by subsequently 1) exposing the target with m=-1 and m=+1 beams simultaneously (thus obtaining interference fringes) and 2) exposing sequentially with m=-1 and m=+1 beams. The resulting data may be analyzed by determining the difference between these exposures, thereby obtaining only the phase response and a cleaner signal due to the removal of scattered and non-interfering radiation.

In one embodiment, color selection can be achieved by blocking some of the wavelengths in the diffraction order m after the first dispersive element. In most of the above text, only the wavelengths in the order m are considered. However, the m=0 beam from the first dispersive element can still be used for the known SXR measurement, or the (m,n)=(0,+/-n) beam can be interferometrically Beam.

A number of interferometric structure measurement configurations will now be described which enable direct phase detection based on radiation scattered from the measurement structure. Such configurations may be similar to those already described in the phase detection section above.

The methods may include generating probe radiation (e.g., a probe beam) and reference radiation (e.g., a reference beam), illuminating at least one structure on a substrate with the probe radiation and the reference radiation to obtain scattered probe radiation and scattered reference radiation, and interfering the scattered probe radiation and the scattered reference radiation at a detector. One or more structural parameters and/or profile parameters may be determined based on the interfering scattered probe radiation and the scattered reference radiation.

In many of the arrangements described in this paragraph, 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 spacing as the first structure). Thus, a focusing mirror arrangement comprising two focal points may be provided to focus each of these beams on its respective structure. Alternatively, the illuminating radiation may be split before the mirror arrangement (e.g. the drive 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 methods described in this paragraph may be implemented in conjunction with the multiple incident angle embodiments described in the phase detection paragraph above (e.g., generated using a first dispersive element), or implemented entirely independently of such multiple incident angle embodiments. Thus, it is also within the scope of this paragraph to use a first dispersive element to generate a probe beam and a reference beam and illuminate a first structure with the probe beam and illuminate a second structure with the reference beam to perform the interferometry disclosed herein. However, this paragraph will now describe an alternative method of generating a probe beam and a reference beam and interfering respective scattered radiation from at least one structure at a detector.

Conventional metrology only detects the diffraction amplitude, but valuable information also exists in the absolute phase of the diffracted light. For example, the frequency of the signal corresponds to the layer height without phase, while using the phase information, the absolute z-coordinate (absolute position in the direction perpendicular to the substrate plane) can be inferred.

In this section, a number of configurations and/or methods for performing interferometry on a structure (e.g., a metrology target) will be described. To perform interferometry, an illumination beam is split into a reference beam and a probe 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 a detection plane (i.e., at the detector).

FIG. 15 shows three different implementations of the interferometric structure measurement configuration according to this paragraph.

FIG15( a) includes an exemplary configuration in which splitting into a probe beam and a reference beam is performed prior to SXR generation, i.e., by splitting a drive radiation beam (infrared IR radiation). Drive laser radiation 1500 (e.g., IR radiation) is split into a first drive radiation beam 1510a and a second drive radiation beam 1510b by a beam splitting configuration 1505 (which may differ in detail from the simplified configuration shown). Each of the drive radiation beams 1510a, 1510b is used to generate a respective SXR beam 1520a, 1520b, for example, via a HHG process, by exciting a HHG module 1515 (e.g., a HHG medium comprising a suitable gas as already described). One of the SXR beams, the probe beam 1520a, illuminates a first structure 1535 or a first target, i.e., a structure or target of interest (e.g., a structure on which profilometry and/or overlay metrology is performed). The other beam, the reference beam 1520b, illuminates a reference structure 1530 or a reference target. The first structure 1535 and the reference structure 1530 may be adjacent to each other on a substrate. A focusing mirror or other optical element 1525 may be used to illuminate each structure 1535, 1530 with its respective SXR beam 1520a, 1520b. Thus, the two beams (probe and reference) 1520a, 1520b are imaged close to each other on the wafer.

Similar to normal profilometry, the scattered probe radiation 1540a carries the profilometry information. This information is present in the diffraction order. However, compared to normal profilometry, there is now scattered reference radiation 1540b produced by the reference SXR beam 1520b diffracted from the reference structure 1530. The reference structure 1530 may comprise a simple 1D grating, which acts as a reference mirror. A planar reference mirror will not provide the same diffraction order as the profilometry target; therefore, the reference structure 1530 may comprise a grating structure having the same spacing as the first structure 1535 (or a spacing sufficiently similar).

The scattered detection radiation 1540a and the scattered reference radiation 1540b each include a diffraction pattern, which may be superimposed at the detector 1550. This superimposition may occur directly at the detector 1550 without the need for any beam combiner.

FIG. 15( b) shows the superposition of scattered probe radiation 1540a and scattered reference radiation 1540b, resulting in an image 1555a of the scattered probe radiation and an image 1555b of the scattered reference radiation detected by the detector. An interference pattern 1560 is formed in the overlapping region of images 1555a and 1555b. The superposition of beams occurs for all diffraction harmonics and not just a single mirror-reflected beam (this is symbolically reflected by the shading of beams 1540a, 1540b in FIG. 15( a), FIG. 15( c), and FIG. 15( d)). It should be noted that FIG. 15( b) shows the detection configuration used for all embodiments shown in FIG. 15.

From this interference pattern (e.g., each harmonic or wavelength), in addition to the detected intensity, the absolute phase of the scattered detection radiation (phase data or absolute phase data) can also be extracted. This phase information can be determined based on, for example, the distance between the stripes and/or the position of the stripes. One or more profile parameters (e.g., including the overlay) of the first structure can be determined based on the determined absolute phase data (e.g., and the intensity data). For example, a complete profilometry can be performed on the first structure in order to determine a plurality of its profile parameters.

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

The IR beams 1500 should be separated and tightly combined, with their relative path lengths accurately controlled so that they are well matched. With SXR wavelengths 50 to 100 times shorter than IR wavelengths, this challenge can be significantly greater than with normal interferometers (depending on the sensitivity to SXR parameters, which can be favorable in wavelength compared to visible light). The beams travel through the same optics after the beam splitter arrangement 1505, so only this beam splitter arrangement 1505 needs to be stabilized. This splitter can include a monolithic assembly with only minimal non-common path lengths (i.e., not shared by both beams).

For example, a method for generating and interfering two SXR beams 1520a, 1520b is described in the context of Fourier transform spectroscopy in Jansen et al., Spatially resolved Fourier transform spectroscopy in the extreme ultraviolet; Optica, Vol. 3, No. 10, pp. 1122-1125 (2016), which is incorporated herein by reference. This Fourier transform spectroscopy is actually only a spectral amplitude measurement and does not describe a split path for measuring the phase response of a target.

To provide some examples purely for illustration, 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, it may diverge approximately 80 μm. If the two spots are, for example, approximately 20 μm apart, this will provide a larger overlap at the detector. When the spots overlap at the detector, they may reach an angle of 0.5 mrad. Over a distance of 80 μm on the detector, this will be approximately 40 nm, which is 4 waves for a 10 nm wavelength. This requires a pixel size of 10 μm on the detector to sample at the Nyquist frequency; however, smaller pixels may be preferred.

FIG15( c ) shows another example that overcomes the difficulty of splitting the IR driven radiation beam. In this embodiment, the focusing mirror arrangement (or illuminator) is split into two mirror portions 1525 a, 1525 b so that it splits the SXR beam 1522 into a reference beam 1524 b and a detection beam 1524 a. It should be noted that in this example, a single HHG beam 1522 is generated by the HHG module 1515, and the HHG beam 1522 includes a first beam portion incident on the mirror portion 1525 a (shown in the figure by the beam edge 1522 a) and a second beam portion incident on the mirror portion 1525 b (shown in the figure by the beam edge 1522 b).

From the perspective of the splitting mirrors 1525a, 1525b, the reference beam 1524b and the detection beam 1524a are split in the far field. The far field profile of each of the beams will therefore be a (nearly) semicircular or "D" shape, respectively, in opposite orientations. This can reduce the overlap area between the two beams at the detector, but the general principle remains the same. Due to the semicircular far field profiles, the two beams will overlap only when a small angle of incidence difference is introduced between the beams 1524a, 1524b, otherwise the two semicircular profiles will be adjacent to each other and only diffraction effects will cause the overlap. Thus, one of the mirror portions can be tilted relative to the other (eg, adjustable mirror portion 1525b can be tilted) such that there is a small angle tilt difference between the two mirror portions.

If the overlap is too small to distinguish complete stripes, the phase can be scanned by moving the mirror segments (e.g., by providing at least one movable mirror portion 1525b, as indicated by the arrow) to obtain the absolute phase. Thus, if there is some (even small) overlap, the phase can be captured.

The mirror is the most sensitive element of this interferometer example because the two beam paths fall onto two different optical surfaces. Because the mirror portions 1525a, 1525b are in close proximity together, they can be coupled mechanically and thermally for stability with respect to vibration and thermal differences. The two targets 1535, 1530 are also parts of different optical paths, but are in close proximity (e.g., within tens of μm) on the same substrate, and therefore, vibration or thermal effects should not be a problem.

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

FIG15( d) shows a variation of the configuration of FIG15( c) in which one of the mirror portions 1525b' can be controllably deformed. This can be achieved, for example, by localized thermal expansion or a (e.g., piezoelectric) actuator beneath the mirror coating. As an alternative to that shown in the illustration, the mirror can include a single mirror 1525' having a portion that can be controllably deformed (e.g., which only focuses one of the beams 1524a, 1524b). As with the example of FIG15( c), the two individual mirror portions 1525a, 1525b can be mechanically and thermally coupled to effectively act as a single mirror, and thereby be (almost) unaffected by vibration or drift.

In either case, each of the beams 1524a, 1524b may still be focused on respective structures 1535, 1530 by different mirror portions 1525a, 1525b' or different regions of the mirror 1525'. For example, the deformable or active region may focus the reference beam 1524b on the reference structure 1530, where the normal portion of the mirror may focus the probe beam 1524a on the first structure 1535. The mirror would thus have two focal points as previously described. Shaping a monolithic optic 1525' to have two focal points may be challenging, and therefore, a two mirror portion 1525a, 1525b' configuration may be easier to implement.

In one embodiment, focusing may be achieved using a standard mirror portion 1525a for the entire beam, with the desired tilt for half the beam being imposed by a deformable mirror portion 1525b'.

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

Fig. 16(a) shows another embodiment which does not require a change in the metrology setup, but relies on specific properties of the targets or structures 1630, 1635 being measured. In detail, it is proposed to impose a height difference between a first target 1635 and a reference structure 1630. By focusing the incident beam 1624 (note that this is in this embodiment a single SXR beam that has not been split before the substrate) on the edge that separates the reference structure 1630 and the first structure 1635, the beam 1624 is split at the substrate level. After further propagation, the beam diverges and overlaps again at the detector 1550. This approach requires a dedicated edge between the reference structure 1630 and the first structure 1635 per phase difference. To scan the phase, a reference structure 1635 of different height/phase offsets may be placed adjacent to the first target 1630 .

An advantage of this configuration is that the only optical elements not joined in the beam path are the reference structure and the first structure, which are located on the same substrate and within tens of microns of each other. This makes this interferometer configuration immune to vibration or thermal drift.

FIG. 16( b ) includes a method that includes a pair of EUV beam splitters 1660, typically pellicles, configured to split the SXR radiation beam into a probe beam and a reference beam, respectively, and to combine the probe beam and the reference beam. This is similar to, for example, a Mach-Zehnder interferometer setup. It should be noted that this configuration only combines mirror-reflected (reference) beams (e.g., reflected from a reflector or mirror 1639). This method is mechanically simple and has no restrictions on the structure being measured.

Another embodiment is an apparatus with SXR illumination for performing metrological measurements at multiple incident angles on a target simultaneously. The metrological measurements are based on phase measurements and/or intensity measurements performed by a detector/sensor.

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

Embodiments may include a computer program containing one or more machine-readable instruction sequences that describe methods of optical metrology and/or methods of analyzing measurements to obtain information about lithographic processes. Embodiments may include computer program code containing one or more sequences of machine-readable instructions or data describing the methods. For example, the computer program or program code may be executed in the unit MPU in the apparatus 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.) having the computer program or program code stored therein may also be provided. Where existing metrology equipment (e.g., of the type shown in FIG. 6 ) is already in production and/or in use, embodiments of the invention may be implemented by providing an updated computer program product to cause a processor to perform one or more of the methods described herein. The computer program or program code may be configured to control an optical system, a substrate support, and the like, as appropriate, to perform methods of measuring parameters of a lithography process for a plurality of targets. The computer program or program code may update lithography and/or metrology recipes for measurement of additional substrates. The computer program or program code may be configured to control (directly or indirectly) lithography equipment for patterning and processing of additional substrates.

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

It should be noted that the term "diffraction order" covers both specular radiation or zero order.

The properties of the emitted radiation used to perform measurements can affect the quality of the measurements obtained. For example, the shape and size of the transverse beam profile (cross-section) of the radiation beam, the intensity of the radiation, the power spectrum density of the radiation, etc. can affect the measurements performed by the radiation. Therefore, it is beneficial to have a source that provides radiation that has properties that produce high quality measurements.

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

Other embodiments are disclosed in subsequent numbered clauses: 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 to produce 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, each of which has 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 the second scattered radiation that has been scattered by the second dispersive element. 2. A metrology tool as in claim 1, wherein the first dispersive element comprises a grating. 3. A metrology tool as in claim 1 or 2, comprising a first focusing element operable to focus the source radiation at a mid-field plane. 4. A metrology tool as in claim 3, wherein the first focusing element comprises a first focusing mirror element. 5. A metrology tool as in any of claims 2 to 4, wherein the first dispersive element is located at the mid-field plane. 6. A metrology tool as in claim 3 or 4, wherein the metrology tool is configured such that: At least one pair of second scattered beams contained in the second scattered radiation overlap on the detector to form interference fringes. 7. The metrology tool of clause 6, wherein the metrology tool is configured to determine a phase difference between the beams in each pair of second scattered beams based on the position of the respective interference fringes on the detector. 8. The metrology tool of clause 6 or 7, configured so that at least one pair of the first scattered beams includes respective effective origins in the intermediate field plane. 9. The metrology tool of clause 8, wherein the displacement between the respective effective origins is greater than 4 μm. 10.   The metrology tool of clause 8, wherein the displacement between the respective effective origins is between 4 μm and 10 μm. 11.   The metrology tool of clause 8, wherein the displacement between the respective effective origins is between 5 μm and 9 μm. 12.   The metrology tool of any of clauses 6 to 11, wherein the first dispersive element is positioned a defocus distance from the intermediate field plane. 13.   The metrology tool of clause 1, wherein the first dispersive element comprises a virtual first dispersive element located at a source plane. 14.   The metrology tool of clause 13, comprising a radiation source for generating the source radiation, the radiation source comprising a laser source configuration operable to generate a pair of radiation beams toward a gas target such that the pair of radiation beams overlap in time and space at respective different angles in the gas target to generate an interference pattern forming the virtual first dispersive element. 15.   The metrology tool of clause 14, comprising a rotatable wave plate in the path of at least one of the pair of radiation beams. 16.   The metrology tool of clause 14 or 15, wherein the radiation source is operable to generate source radiation of one or more wavelengths in the range of 1 nm to 20 nm. 17.   The metrology tool of any of clauses 1 to 12, further comprising a radiation source for generating the source radiation, wherein the radiation source is operable to generate source radiation of one or more wavelengths in the range of 1 nm to 20 nm. 18.   The metrology tool of any of the preceding clauses, wherein the final focusing element comprises a final focusing mirror element. 19.   The metrological instrument of clause 18, wherein the final focusing mirror element comprises a hollow ellipsoid portion having a reflective inner surface. 20.   The metrological instrument of clause 19, wherein the hollow ellipsoid portion comprises a first opening at an input end and a second opening at an output end. 21.   The metrological instrument of clause 19, wherein the first opening and the second opening are each defined by a cut parallel to the minor axis plane of the hollow ellipsoid. 22.   The metrological instrument of clause 20 or 21, wherein the first opening is larger than the second opening. 23.   The metrological instrument of clause 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.   A metrology tool as in any preceding clause, wherein the final focusing element is completely rotationally symmetric. 25.   A metrology tool as in any preceding clause, wherein the second dispersive element comprises a target being measured. 26.   A metrology tool as in any preceding clause, wherein the profile parameter comprises a physical parameter of the second dispersive element other than its position. 27.   A metrology tool as in any preceding clause, comprising a processor operable to determine the profile parameter based on at least the detected second scattered radiation. 28.   A metrology tool as in any preceding clause, wherein the collected first scattered radiation focused on the second dispersive element comprises a zero-order component of the first scattered radiation. 29.   A metrology tool as in any preceding clause, wherein the first scattered radiation incident on the second dispersive element comprises at least two non-complementary diffraction orders having been scattered from the first dispersive element. 30.   A metrology tool as in any preceding clause, comprising determining a value for at least one parameter of interest associated with the second dispersive element based on the second scattered radiation. 31.   A focusing mirror element comprising a hollow ellipsoid portion having a reflective interior surface and comprising a first opening at an input end and a second opening at an output end. 32.   A focusing mirror element as in clause 31, wherein the first opening and the second opening are each defined by a cut in a plane parallel to the minor axis of the hollow ellipsoid. 33.   The focusing mirror element of clause 30, wherein the first opening is larger than the second opening. 34.   The focusing mirror element of clause 33, 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. 35.   The focusing mirror element of any one of clauses 31 to 34, wherein the focusing element is completely rotationally symmetric. 36.   A method of manufacturing a focusing mirror element as in any one of clauses 31 to 35, comprising: providing an axially symmetrical mandrel comprising a long elliptical portion; forming a focusing mirror element body around the mandrel; and releasing the focusing mirror element body from the mandrel; whereby the focusing mirror element body has an optical surface defined by the inner surface of the focusing mirror element body. 37.   The method as in clause 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), elastic emission machining (EEM), ion beam shaping and float polishing. 38.   The method of clause 37, wherein the super polishing step is performed so that the mandrel surface has a surface roughness of less than 200 pm root mean square (RMS), less than 100 pm (RMS), less than 50 pm (RMS), less than 35 pm RMS. 39.   The method of any of clauses 36 to 38, comprising applying a reflection enhancing single or multi-layer coating to the mirror surface of the focusing mirror element body. 40.   The method of any of clauses 36 to 39, comprising applying an electrode and a release layer to the mandrel prior to the forming step. 41.   A metrology method 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 of which has been scattered in a respective different direction; focusing the collected first scattered radiation on a target so 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 the target; and determining a value of at least one profile parameter for the target based on the second scattered radiation. 42.   The metrology method of clause 41, comprising focusing the source radiation at a mid-field plane. 43.   The metrology method of clause 42, wherein the source radiation is scattered at the intermediate field plane. 44.   The metrology method of clause 42, comprising superimposing at least one pair of second scattered beams contained in the second scattered radiation at a detector to form interference fringes on the detector. 45.   The metrology method of clause 44, comprising determining a phase difference between the beams in each pair of second scattered beams according to the position of the respective interference fringes on the detector for each pair of second scattered beams. 46.   The metrology method of clause 42 or 45, wherein at least one pair of the first scattered beams comprises respective different effective origins in the intermediate field plane. 47.   The metrology method of clause 46, wherein the displacement between the respective effective origins is greater than 4 μm. 48.   The metrology method of clause 46, wherein the displacement between the respective effective origins is between 4 μm and 10 μm. 49.   The metrology method of clause 46, wherein the displacement between the respective effective origins is between 5 μm and 9 μm. 50.   The metrology method of any of clauses 44 to 49, comprising focusing the source radiation at a defocus distance from the intermediate field plane. 51.   The metrology method of clause 41, comprising generating a virtual first dispersive element at a source plane. 52.   The metrology method of clause 51, comprising generating a pair of radiation beams and directing them at a gas target so that the pair of radiation beams overlap in time and space at respective different angles in the gas target to generate an interference pattern forming the virtual first dispersive element. 53.   The metrology method of clause 52, comprising controlling the polarization of at least one of the pair of radiation beams. 54.   The metrology method of any of clauses 41 to 53, wherein the source radiation comprises one or more wavelengths in the range of 1 nm to 20 nm. 55.   A metrology method as in any of the preceding clauses, wherein the collecting step and the step of focusing the collected first scattered radiation on a target are performed using a focusing mirror element as in any of clauses 31 to 35. 56.   A metrology method as in any of clauses 41 to 55, wherein the profile parameter comprises a physical parameter of the target other than its position. 57.   A metrology method as in any of clauses 41 to 56, wherein the collected first scattered radiation focused on the target comprises a zero-order component of the first scattered radiation. 58.   A metrology method as in any of clauses 41 to 57, wherein the first scattered radiation incident on the target comprises at least two non-complementary diffraction orders contained in the first scattered radiation. 59.   A metrology method as in any of clauses 41 to 58, comprising optimizing one or more measurement parameters to optimize the first scattered radiation for the target. 60.   A method comprising: generating a source radiation, wherein the source radiation comprises soft X-ray radiation; scattering the source radiation to produce 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 produce a second scattered radiation; detecting the second scattered radiation; and using the detected second scattered radiation to determine a parameter of interest. 61.   An apparatus having an SXR source configured to measure a parameter of interest at multiple incident angles simultaneously during a manufacturing process. 62.   An apparatus having an SXR source configured to measure a parameter of interest at multiple incident angles simultaneously on a substrate, wherein the measurement is based on phase measurement and/or intensity measurement 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 phase measurement using a detector.

Other additional embodiments are disclosed in subsequent numbered clauses: 1. A metrology apparatus for measuring a profile parameter of a first structure on a substrate, the metrology apparatus comprising: an illumination arrangement operable to generate probe radiation and reference radiation; a mirror arrangement operable to focus the probe radiation onto the first structure to generate scattered probe radiation, and to focus the reference radiation onto the first structure or a reference structure to generate scattered reference radiation; and a detector operable to receive the scattered probe radiation and the scattered reference radiation at least partially overlapped on the detector to generate an inferred fringing. 2. A metrological apparatus as claimed in claim 1, wherein the mirror configuration is operable to focus the reference radiation onto the reference structure to produce the scattered reference radiation. 3. A metrological apparatus as claimed in claim 2, wherein the reference structure comprises a periodic structure or grating. 4. A metrological apparatus as claimed in claim 3, wherein the reference structure has substantially the same spacing as the first structure. 5. A metrology apparatus as in any of the preceding clauses, wherein the illumination configuration comprises: a beam splitting configuration operable to split a drive radiation beam into a first drive radiation beam and a second drive radiation beam; and a high-order harmonic generation module operable to receive the first drive radiation beam to generate the detection radiation; and receive the second drive radiation beam to generate the reference radiation. 6. A metrology apparatus as in clause 5, wherein the illumination configuration comprises a path length adjustment configuration for at least one of the first drive radiation beam and the second drive radiation beam. 7. A metrology device as in any one of clauses 1 to 4, the metrology device comprising: a high-order harmonic wave generating module operable to receive the drive radiation to generate high-order harmonic wave generating radiation; and a beam splitting configuration operable to split the high-order harmonic wave generating radiation into the detection radiation and the reference radiation. 8. A metrology device as in clause 7, wherein the beam splitting configuration comprises the mirror configuration, the mirror configuration comprising a first focus for focusing the detection radiation to the first structure and a second focus for focusing the reference radiation to the reference structure. 9.   The metrology apparatus of clause 8, wherein the mirror configuration comprises: a first mirror portion operable to receive and focus a first portion of the high-order harmonic generating radiation to generate the detection radiation; and a second mirror portion operable to receive and focus a second portion of the high-order harmonic generating radiation to generate the reference radiation. 10.   The metrology apparatus of clause 9, wherein a tilt angle difference is included between a tilt angle of the first mirror portion and a tilt angle of the second mirror portion. 11.   The metrology apparatus of clause 9 or 10, wherein at least the second mirror portion is displaceable to change a path length of the reference radiation. 12.   A metrological apparatus as claimed in clauses 9, 10 or 11, wherein at least the second mirror portion is deformable. 13.   A metrological apparatus as claimed in any one of clauses 9 to 12, wherein the first mirror portion and the second first mirror portion are thermally coupled and/or mechanically coupled. 14.   A metrological apparatus as claimed in clause 9, wherein the mirror configuration comprises a monolithic focusing mirror, the monolithic focusing mirror comprising: a first mirror region operable to receive and focus a first portion of the high-order harmonic generating radiation to generate the detection radiation; and a second mirror region operable to receive and focus a second portion of the high-order harmonic generating radiation to generate the reference radiation, and wherein the second mirror region is deformable. 15.   The metrology apparatus of clause 7, wherein the beam splitting arrangement comprises a first pellicle beam splitter and a second pellicle beam splitter. 16.   The metrology apparatus of clause 7, wherein the beam splitting arrangement comprises a dispersive element. 17.   The metrology apparatus of any of clauses 5 to 16, wherein the high-order harmonic generating radiation comprises one or more wavelengths in the range of 1 nm to 20 nm. 18.   The metrology apparatus of any of the preceding clauses, comprising a processor operable to determine phase data based on the inferred fringes. 19.   The metrology apparatus of clause 18, wherein the phase data describes an absolute phase of the scattered probe radiation. 20.   A metrology apparatus as in clause 18 or 19, wherein the processor is operable to determine the at least one profile parameter based on at least the phase data. 21.   A metrology method as in any preceding clause, wherein the profile parameter comprises a physical parameter of the first structure other than its position. 22.   A metrology method as in any preceding clause, wherein the profile parameter comprises one or more of: overlay, etch depth, sidewall angle, layer thickness, critical dimension (CD) and/or any other dimension of the first structure. 23.   A method for measuring a profile parameter of a first structure on a substrate, the metrology tool comprising: generating probe radiation and reference radiation; focusing the probe radiation onto the first structure to generate scattered probe radiation; focusing the reference radiation onto the first structure or a reference structure to generate scattered reference radiation; and interfering the scattered probe radiation and the scattered reference radiation at a detector to generate inferred fringes; and determining phase data describing a phase of the probe radiation based on the inferred fringes. 24.   The method of clause 23, comprising focusing the reference radiation onto the reference structure to generate scattered reference radiation. 25.   The method of clause 24, wherein the reference structure comprises a periodic structure or grating. 26.   The method of clause 25, wherein the reference structure has substantially the same spacing as the first structure. 27.   The method of any one of clauses 23 to 26, comprising: splitting a drive radiation beam into a first drive radiation beam and a second drive radiation beam; generating the detection radiation from the first drive radiation via a high-order harmonic generation process; and generating the reference radiation from the second drive radiation via a high-order harmonic generation process. 28.   The method of clause 27, comprising matching a path length of the first drive radiation beam and the second drive radiation beam. 29.   The method of any one of clauses 23 to 26, comprising: generating high-order harmonics to generate radiation; and splitting the high-order harmonics to generate radiation into the detection radiation and the reference radiation. 30.   The method of clause 29, wherein the splitting is performed using a mirror configuration, the mirror configuration comprising a first focus for focusing the detection radiation to the first structure and a second focus for focusing the reference radiation to the reference structure. 31.   The method of clause 30, comprising: receiving a first portion of the high-order harmonic wave generating radiation on the first mirror portion and causing the first mirror portion to focus the first portion of the high-order harmonic wave generating radiation onto the first structure; and receiving a second portion of the high-order harmonic wave generating radiation on the second mirror portion and causing the second mirror portion to focus the second portion of the high-order harmonic wave generating radiation onto the reference structure. 32.   The method of clause 31, comprising applying a tilt angle difference between a tilt angle of the first mirror portion and a tilt angle of the second mirror portion. 33.   The method of clause 31 or 32, comprising displacing the second mirror portion to match the path lengths of the detection radiation and the reference radiation. 34.   The method of clause 31, 32 or 33, comprising controllably deforming the second mirror portion. 35.   The method of any one of clauses 31 to 34, wherein the first mirror portion and the second first mirror portion are thermally and/or mechanically coupled. 36.   The method of clause 31, wherein the mirror configuration comprises a monolithic focusing mirror, and the method comprises: receiving a first portion of the high-order harmonic generating radiation on a first mirror region of the monolithic focusing mirror, such that the first mirror region focuses the first portion of the high-order harmonic generating radiation onto the first structure; and receiving a second portion of the high-order harmonic generating radiation on the second mirror region of the monolithic focusing mirror, such that the second mirror region focuses the second portion of the high-order harmonic generating radiation onto the reference structure. 37.   The method of clause 36, wherein the second mirror region is deformable. 38.   The method of clause 29, comprising performing the splitting using a first pellicle beam splitter and recombining the probe beam and the reference beam using a second pellicle beam splitter. 39.   The method of clause 29, comprising performing the splitting using a dispersive element. 40.   The method of any of clauses 27 to 39, wherein the high-order harmonic generating radiation comprises one or more wavelengths in the range of 1 nm to 20 nm. 41.   The method of clause 29, comprising performing the splitting by illuminating the first structure with a first portion of the high-order harmonic generating radiation and illuminating the reference structure with a second portion of the high-order harmonic generating radiation; wherein the first structure and the reference structure have been formed with a height offset. 42.   A method as in any of clauses 23 to 40, wherein the phase data describes an absolute phase of the scattered probe radiation. 43.   A method as in any of clauses 23 to 42, comprising determining the at least one profile parameter based on at least the phase data. 44.   A metrology method as in any of the preceding clauses, wherein the profile parameter comprises a physical parameter of the first structure other than its position. 45.   A metrology method as in any of the preceding clauses, wherein the profile parameter comprises one or more of: overlay, etch depth, sidewall angle, layer thickness, critical dimension (CD) and/or any other dimension of the first structure.

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

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

Although specific reference may be made herein to embodiments in the context of inspection or metrology equipment, embodiments may be used in other equipment. Embodiments may form part of a mask inspection equipment, a lithography equipment, or any equipment that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterned device). The term "metrology equipment" (or "inspection equipment") may also refer to an inspection equipment or an inspection system (or a metrology equipment or a metrology system). For example, an inspection equipment including an embodiment may be used to detect defects in a substrate or defects in a structure on a substrate. In this embodiment, the characteristic of interest of the structure on the substrate may be related to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate.

Although the above may specifically refer to the use of embodiments in the context of photolithography, it will be appreciated that the invention is not limited to photolithography and may be used in other applications (eg, imprint lithography) where the context permits.

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

Although specific embodiments have been described above, it will be appreciated that the invention may be practiced in other ways than those described. The above description is intended to be illustrative rather than restrictive. Thus, it will 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 forth below.

Although specific reference is made to "metrology equipment/tool/system" or "testing equipment/tool/system", these terms may refer to the same or similar types of tools, equipment or systems. For example, a test or metrology equipment incorporating embodiments of the present invention may be used to determine characteristics of a structure on a substrate or on a wafer. For example, a test or metrology equipment incorporating embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or on a wafer. In this embodiment, the characteristic of interest of the structure on the substrate may be related to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate or on the wafer.

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

Additional objects, advantages, and features of the invention are described in this specification and will become apparent in part to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The invention disclosed in this application is not limited to any particular set or combination of objects, advantages, and features. It is considered that various combinations of the objects, advantages, and features constitute the invention disclosed in this application.

2: Broadband radiation projector 4: Spectrometer detector 5: Radiation 6: Spectrum 8: Structure or contour 10: Reflected or scattered radiation 11: Transmitted radiation 302: Metrology equipment 310: Radiation source/illumination source 312: Illumination system 314: Reference detector 315: Signal 316: Substrate support 318: Detection system 320: Metrology processing unit 330: Pump radiation Source 332: Gas delivery system 334: Gas supply 336: Power supply 340: Pump radiation 342: Filtered beam 344: Filter device 350: Detection chamber 352: Vacuum pump 356: Focused beam 360: Reflected radiation 372: Position controller 374: Sensor 382: Spectral data 397: Diffracted radiation 398: Detection system 399: Signal 6 00: illumination source 601: chamber 603: illumination system 605: radiation input 607: radiation output 609: gas nozzle 611: pump radiation 613: emission radiation 615: gas flow 617: opening 800: source 805: first focusing element or first focusing mirror 810: first dispersive element or grating 815: second focusing element or final focusing mirror element 820: Target 825: Detector or camera 1000a: Infrared laser pulse 1000b: Infrared laser pulse 1010: Intensity grating 1020: Zero order 1030+: +1 diffraction order 1030-: -1 diffraction order 1040: Aperture or pinhole 1050: Half-wave plate 1100: Hollow elliptical spherical part 1110: Mirror inner surface 1120: Larger first opening Port 1130: Smaller second opening 1365 (-1, -1): Wavefront 1365 (+1, 0): Wavefront 1370: Plane 1480: Stripe 1500: Drive laser radiation 1505: Beam splitting arrangement 1510a: First drive radiation beam 1510b: Second drive radiation beam 1515: HHG module 1520a: SXR beam 1520 b: SXR beam 1522: SXR beam 1522a: beam edge 1522b: beam edge 1524a: detection beam 1524b: reference beam 1525: focusing mirror or other optical element 1525': single-piece mirror 1525a: mirror portion 1525b: mirror portion 1525b': mirror portion 1530: reference structure 1535: A structure 1540a: scattered detection radiation 1540b: scattered reference radiation 1550: detector 1555a: image 1555b: image 1560: interference pattern 1624: incident beam 1630: reference structure 1635: first target 1639: reflector or mirror 1660: EUV beam splitter B: radiation beam BD: beam delivery system BK: baking plate C: target part CL: computer system CH: cooling plate DE: display DPX: X detector pixel position DPY: Y detector pixel position dZ: defocus IF: position measurement system IFP: intermediate field plane IL: illumination system I/O1: input/output port I/O2: input/output port LA: lithography equipment LACU: lithography control unit LB: loading area M ₁ : Mask alignment mark M ₂ : Mask alignment mark MA: Patterning device MPU: Unit MT: Metrology tool P ₁ : Substrate alignment mark P ₂ : Substrate alignment mark PM: First positioner PW: Second positioner PS: Projection system PU: Processing unit p ₁ : Pitch p ₂ : Target pitch RO: Substrate handler or robot S: Light spot SC: Spin coater SC1: First scale SC2: Second scale SC3: Third scale SCS: Supervisory control system SO: Radiation source T: Mask support Ta: Target TCU: Automated photoresist coating and development system control unit W: Substrate WT: Substrate support

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 lithography apparatus; - FIG. 2 depicts a schematic overview of a lithography unit; - FIG. 3 depicts a schematic representation of overall lithography showing the cooperation between three key technologies used to optimize semiconductor manufacturing; - FIG. 4 schematically depicts a scatterometry apparatus; - FIG. 5 schematically depicts a transmissive scatterometry apparatus; - 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 diagram of an illumination source; - FIG. 8(a) and FIG. 8(b) each depict a simplified schematic diagram of a metrology configuration according to an embodiment; - FIG. 9 depicts an exemplary diffraction pattern that may be captured by the metrology configuration of FIG. 8; - FIG. 10(a), FIG. 10(b) and FIG. 10(c) depict a configuration for providing a virtual grating at a source plane according to an embodiment; - FIG. 11 depicts a proposed refocusing mirror design suitable for use in the metrology configuration of FIG. 8 according to an embodiment; - FIG. 12 depicts a method for obtaining an origin displacement of a diffraction beam from a diffraction grating for use in a phase determination embodiment of the concepts disclosed herein; - FIG. 13 depicts how this origin displacement results in mutual wavefront tilts of the diffraction beams of FIG. 12 and, therefore, optical path differences between such diffraction beams; - FIG. 14 is an exemplary interferometric image obtained from the configuration depicted in FIG. 12 and FIG. 13; - FIG. 15(a), FIG. 15(b), FIG. 15(c) and FIG. 15(d) illustrate various SXR interferometric target measurement configurations according to an embodiment; and - FIG. 16(a) and FIG. 16(b) illustrate other various SXR interferometric target measurement configurations according to an embodiment.

800: Source

805: first focusing element or first focusing mirror

810: first dispersion element or grating

815: Second focusing element or final focusing mirror element

820: Target

825: Detector or camera

Claims (15)

A metrology tool comprising: a first dispersive element operable to receive and scatter a source radiation to produce a 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, each of which has 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 a second scattered radiation that has been scattered by the second dispersive element. A metrology tool as claimed in claim 1, comprising a first focusing element operable to focus the source radiation at an intermediate field plane. A metrology tool as claimed in claim 2, wherein the first dispersion element is located at the mid-field plane. A metrology tool as claimed in any one of claims 1 to 3, wherein the metrology tool is configured so that: At least one pair of second scattered beams contained in the second scattered radiation overlap on the detector to form interference fringes. A metrology tool as claimed in claim 4, wherein the metrology tool is configured to determine a phase difference between the beams in each pair of second scattered beams based on the positions of the respective interference fringes on the detector. A metrology tool as claimed in claim 2 or 3, configured so that at least one pair of said first scattered beams include respective different effective origins in said intermediate field plane. A metrological tool as claimed in claim 6, wherein the displacement between the respective effective origins is greater than 4 μm. A metrology tool as claimed in claim 2 or 3, wherein the first dispersive element is positioned a defocus distance from the mid-field plane. A metrology tool as claimed in claim 1, wherein the first dispersive element comprises a virtual first dispersive element located at a source plane. A metrological tool as claimed in claim 9, comprising a radiation source for generating the source radiation, the radiation source comprising a laser source configuration, the laser source configuration being operable to generate a pair of radiation beams directed toward a gas target so that the pair of radiation beams overlap in time and space at respective different angles in the gas target so as to generate an interference pattern forming the virtual first dispersion element. A metrological tool as claimed in claim 10, comprising a rotatable wave plate in the path of at least one radiation beam in the pair of radiation beams. A metrological tool as in any one of claims 1 to 3, wherein the final focusing element comprises a final focusing mirror element, and wherein the final focusing mirror element comprises a hollow ellipsoidal portion having a reflective inner surface. A metrology tool as claimed in any one of claims 1 to 3, wherein the second dispersive element comprises a target to be measured. A method comprising: generating a source radiation, wherein the source radiation comprises soft X-rays; and measuring a parameter of interest at multiple incident angles simultaneously. An apparatus having a soft X-ray source configured to measure a parameter of interest at multiple incident angles simultaneously.