Radiation source

Abstract

A beam metrology device for determining at least one characteristic of a first radiation and / or at least one characteristic of a second radiation, the second radiation being generated via a first non-linear process upon receipt of a first portion of the first radiation; the beam metrology device comprises: a metrology device non-linear medium configured to receive a second portion of the first radiation and thereby generate a third radiation via a second non-linear process; at least one detector configured to measure at least one characteristic of the third radiation; and a processing unit operable to determine at least one characteristic of the first radiation and / or at least one characteristic of the second radiation based on the at least one characteristic of the third radiation.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22189424.9 filed on August 9, 2022, which is hereby incorporated by reference in its entirety. Technical Field

[0003] The present invention relates to a radiation source, in particular a high harmonic generating radiation source, and to such a radiation source for metrological applications in the manufacture of integrated circuits. Background Art

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

[0005] In order to project a pattern on a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of the radiation determines the minimum size of the 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. Compared to lithographic apparatus using radiation of, for example, 193 nm wavelength, lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4-20 nm, for example, with a wavelength of 6.7 nm or 13.5 nm can be used to form smaller features on a substrate.

[0006] Low k1 lithography can be used to process features with sizes smaller than the classical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case the half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it is to reproduce on a substrate a pattern of shape and size similar to that designed by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase-shifting 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 techniques" (RET). Alternatively, a strict control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] During photolithography, as well as other manufacturing processes, it is often necessary to make measurements of the resulting structures, for example for process control and verification. Various tools are known for making such measurements, including scanning electron microscopes, which are commonly used to measure critical dimensions (CD), and specialized tools for measuring the overlay, alignment accuracy of two layers in a device. More recently, various forms of scatterometers have been developed for use in the field of photolithography.

[0008] The fabrication process may be, for example, photolithography, etching, deposition, chemical mechanical planarization, oxidation, ion implantation, diffusion, or a combination of two or more thereof.

[0009] Examples of known scatterometers generally rely on providing a dedicated measurement target. For example, a method may require a target in the form of a simple grating that is large enough so that the measurement beam produces a spot that is smaller than the grating (i.e., the grating is not filled). In a 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 the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0010] In addition to measuring feature shapes by reconstruction, diffraction-based overlay can also be measured using such a device, as described in published patent application US2006066855A1. Diffraction-based overlay measurement using dark field imaging of diffraction orders enables overlay measurement of smaller targets. These targets can be smaller than the irradiation spot and can be surrounded by product structures on the wafer. Examples of dark field imaging measurements can be found in many published patent applications, such as US2011102753A1 and US20120044470A. Using a composite grating target, multiple gratings can be measured in one image. Known scatterometers tend to use light in the visible or near infrared (IR) wave range, which requires the pitch of the grating to be much coarser than the actual product structure with the properties of actual interest. Such product features can be defined using deep ultraviolet (DUV), extreme ultraviolet (EUV), or X-ray radiation with much shorter wavelengths. Unfortunately, such wavelengths are generally not available or suitable for measurement.

[0011] On the other hand, the size of modern product structures is so small that they cannot be imaged by optical measurement techniques. Small features include, for example, those formed by multiple patterning processes and / or pitch multiplication. Therefore, targets for high-volume measurement usually use features that are much larger than products whose overlay error or critical dimensions are the properties of interest. The measurement results are only indirectly related to the size of the actual product structure and may be inaccurate because the measurement target does not undergo the same distortion under optical projection in the lithography equipment and / or different treatments in other steps of the manufacturing process. Although scanning electron microscopes (SEMs) can directly resolve these modern product structures, SEMs are much more time-consuming than optical measurements. In addition, electrons cannot penetrate thick process layers, which makes them less suitable for measurement applications. Other techniques, such as measuring electrical properties using contact pads, are also known, but they only provide indirect evidence of the true product structure.

[0012] By reducing the wavelength of the radiation used during metrology, smaller structures can be resolved to increase sensitivity to structural changes in structures and / or to penetrate further into the product structure. One method of generating suitable high frequency radiation (e.g. hard X-rays, soft X-rays and / or EUV radiation) may be to use pump radiation (e.g. infrared IR radiation) to excite the generating medium, thereby generating emitted radiation, optionally including high harmonic generation of the high frequency radiation.

[0013] It would be desirable to improve upon current radiation sources used for high harmonic generation. Summary of the invention

[0014] According to a first aspect of the present invention, there is provided a beam measurement device for determining at least one characteristic of a first radiation and / or at least one characteristic of a second radiation, wherein the second radiation is generated via a first nonlinear process upon receiving a first portion of the first radiation; the beam measurement device comprises: a measurement device nonlinear medium configured to receive a second portion of the first radiation and thereby generate a third radiation via a second nonlinear process; at least one detector configured to measure at least one characteristic of the third radiation; and a processing unit operable to determine the at least one characteristic of the first radiation and / or the at least one characteristic of the second radiation based on the at least one characteristic of the third radiation.

[0015] According to a second aspect of the present invention, there is provided a radiation source, comprising: the beam measurement device of the first aspect; a source nonlinear medium, which is configured to generate second radiation via a first nonlinear process when receiving a first part of the first radiation; and a radiation configuration device, which is operable to configure at least one characteristic of the first radiation so as to initiate the control action.

[0016] According to a third aspect of the present invention, a method for controlling a radiation source is provided, comprising: generating second radiation via a first nonlinear process when a first portion of a first radiation is received by a source nonlinear medium; generating third radiation via a second nonlinear process when a second portion of the first radiation is received by a measurement nonlinear medium; measuring at least one characteristic of the third radiation; and controlling the at least one characteristic of the first radiation when generating the second radiation based on the at least one characteristic of the third radiation.

[0017] Other aspects of the invention include a measurement device comprising the radiation source of the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0019] Figure 1 A schematic overview of a lithographic apparatus is depicted;

[0020] Figure 2 A schematic overview of a lithography unit is depicted;

[0021] Figure 3 A schematic diagram of overall lithography is shown, representing the collaboration between the three key technologies to optimize semiconductor manufacturing;

[0022] Figure 4 A scatterometry device is schematically shown;

[0023] Figure 5 A transmission scattering measurement device is schematically shown;

[0024] Figure 6 shows a schematic diagram of a metrology apparatus in which EUV and / or SXR radiation is used;

[0025] Figure 7 A simplified schematic diagram of an illumination source is shown;

[0026] Fig. 8A schematically depicts an exemplary pump radiation suppression scheme preferably implemented in a HHG radiation source;

[0027] Figure 8B A less preferred situation is schematically depicted, where Fig. 8A The exemplary pump radiation suppression scheme of loses its effectiveness in blocking the remaining pump radiation due to beam misalignment;

[0028] Figure 8C Another preferred situation is schematically depicted, wherein: Fig. 8A The exemplary pump radiation suppression scheme of loses its effect of blocking the remaining pump radiation due to the change of the focusing position of the lens;

[0029] Fig. 9 schematically depicts an exemplary HHG radiation source stabilized by a prior art active beam stabilization arrangement;

[0030] Fig.10 schematically depicts an exemplary HHG radiation source stabilized by the proposed beam metrology apparatus according to an embodiment; and

[0031] Fig.11 An exemplary HHG radiation source stabilized by the proposed beam metrology apparatus according to various embodiments is schematically depicted. DETAILED DESCRIPTION

[0032] In this document, the terms "radiation" and "beam" are used to include all types of electromagnetic radiation and particle radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of approximately 5-100 nm), X-ray radiation, electron beam radiation, and other particle radiation.

[0033] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam that corresponds to the pattern to be formed in a target portion of the substrate. The term "light valve" may also be used in this context. In addition to conventional masks (transmissive or reflective, binary, phase-shift, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0034] Figure 1 A lithographic apparatus LA is schematically shown. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation, EUV radiation or X-ray radiation), a mask support (e.g. a mask stage) T configured to support a pattern forming device (e.g. a mask) MA and connected to a first positioning device PM configured to accurately position the pattern forming device MA according to specific parameters, a substrate support (e.g. a wafer stage) WT configured to hold a substrate (e.g. a wafer coated with a resist) W and connected to a second positioning device PW configured to accurately position the substrate support according to specific parameters, and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the pattern forming device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0035] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, diffractive, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in a cross-section of the radiation beam B at the plane of the patterning device MA.

[0036] The term "projection system" PS as used herein should be broadly interpreted as including various types of projection systems, including refractive, reflective, diffractive, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate to the exposure radiation used, and / or to other factors such as the use of immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0037] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, so as to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion techniques is given in US6952253, the entire contents of which are incorporated herein by reference.

[0038] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps in preparation for subsequent exposure of the substrate W may be performed on a substrate W located on one substrate support WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0039] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning means may be arranged to clean a part of the lithographic apparatus, e.g. a part of the projection system PS or a part of the system for providing immersion liquid. The measurement platform may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0040] 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. After passing through the mask MA, the radiation beam B passes through a projection system PS which focuses the radiation beam onto a target portion C of the substrate W. With the aid of a second positioning device PW and a position measurement system IF, the substrate support WT can be precisely moved, for example in order to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the first positioning device PM and possibly another position sensor ( Figure 1 The patterning device MA may be precisely positioned relative to the path of the radiation beam B using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 are shown occupying dedicated target portions, they may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe line alignment marks.

[0041] like Figure 2 As shown, the lithography apparatus LA may form part of a lithography cell LC, which is sometimes also referred to as a lithography cell or (lithography) cluster, and which typically also includes equipment for performing pre-exposure and post-exposure processing on a substrate W. Conventionally, these equipment include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH and a baking plate BK, for example for regulating the temperature of the substrate W, for example for regulating the solvent in the resist layer. A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves them between different processing equipment, and delivers the substrates W to a loading bay LB of the lithography apparatus LA. The devices in the lithography cell, which are also generally referred to as coating and developing systems, may be under the control of a coating and developing system control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithography apparatus LA, for example via a lithography control unit LACU.

[0042] During a lithographic process it is frequently desirable to measure the structures produced, e.g. for process control and verification. The tool that makes such measurements may be referred to as a metrology tool MT. Different types of metrology tools MT for making such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a general purpose instrument that allows parameters of the lithographic process to be measured by arranging a sensor in or near the pupil of an objective of the scatterometer, or in a plane conjugate to the pupil of the scatterometer, in which case the measurement is generally referred to as pupil-based measurement, or by arranging a sensor in or near the image plane, or in a plane conjugate to the image plane, in which case the measurement is generally referred to as image or field-based measurement. Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, the entire contents of which are incorporated herein by reference. The aforementioned scatterometers can measure gratings using light from hard X-ray (HXR), soft X-ray (SXR), extreme ultraviolet (EUV), visible to near infrared (IR) and IR wavelength ranges. In the case where the radiation is hard X-ray or soft X-ray, the aforementioned scatterometer can optionally be a small angle X-ray scattering measurement tool.

[0043] In order for substrates W exposed by lithographic apparatus LA to be correctly and consistently exposed, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between successive layers, line widths, critical dimensions (CD), shapes of structures, etc. Inspection tools and / or metrology tools (not shown) may be included in lithography cell LC for this purpose. If an error is detected, for example, adjustments may be made to the exposure of subsequent substrates or to other processing steps to be performed on substrate W, particularly if the inspection is completed before other substrates W of the same batch or lot are still to be exposed or processed.

[0044] The inspection apparatus, which may also be referred to as metrology apparatus, is used to determine properties of the substrate W, and in particular, to determine how properties of different substrates W vary or how properties relating to different layers of the same substrate W vary from layer to layer. Alternatively, the inspection apparatus may be configured to identify defects on the substrate W, and may for example be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure) or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB) or a developed resist image (wherein exposed or unexposed parts of the resist have been removed) or even an etched image (after a pattern transfer step such as etching).

[0045] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signals to reconstruct or calculate the properties of the grating. Such a reconstruction can be obtained, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0046] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected, transmitted or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. a measure of the intensity as a function of wavelength). From this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0047] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows the parameters of a lithographic process to be determined by measuring the scattered or transmitted radiation of each polarization state. Such a measurement device emits polarized light (such as linearly polarized light, circularly polarized light or elliptically polarized light) by using, for example, an appropriate polarization filter in the illumination portion of the measurement device. A source suitable for the measurement device can also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. patent applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110 and 13 / 891,410, the entire contents of which are incorporated herein by reference.

[0048] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or an asymmetry in the detection configuration, the asymmetry being related to the degree of overlap. The two (possibly overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers) and may be formed at substantially the same position on the wafer. The scatterometer may have a symmetrical detection configuration, such as described in co-owned patent application EP1,628,164A, so that any asymmetry is clearly distinguishable. This provides a straightforward way to measure misalignment or misalignment in the gratings. Other examples of measuring the overlay error between two layers containing a periodic structure as a target by means of the asymmetry of the periodic structure may be found in PCT patent application publication WO 2011 / 012624 or in US patent application US 20160161863, the entire contents of which are incorporated herein by reference.

[0049] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry (or by scanning electron microscopy), as described in U.S. Patent Application US2011-0249244, the entire contents of which are incorporated herein by reference. A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the Focus Energy Matrix (FEM - also called Focus Exposure Matrix). If these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​may be uniquely determined from these measurements.

[0050] The metrology target can be a collection of composite gratings formed by a lithography process (mainly in resist), but can also be formed after other manufacturing processes, such as after an etching process. The pitch and line width of the structures in the grating can strongly depend on the measurement optics (particularly the NA of the optics) that are able to capture the diffraction orders from the metrology target. As mentioned earlier, the diffraction signal can be used to determine the offset between two layers (also called "overlap"), or can be used to reconstruct at least a part of the original grating produced by the lithography process. This reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least a part of the lithography process. The target can have smaller sub-segments that are configured to simulate the size of a functional part of the design layout in the target. Due to this sub-segmentation, the target can behave more similar to the functional part of the design layout, so that the overall process parameter measurement results better resemble the functional part of the design layout. The target can be measured in an underfill mode or an overfill mode. In the underfill mode, the measurement beam produces a spot that is smaller than the entire target. In the overfill mode, the measurement beam produces a spot that is larger than the entire target. In this overfill mode, different targets can also be measured simultaneously and thus different process parameters can be determined simultaneously.

[0051] The overall measurement quality of this lithography parameter performed using a specific target is determined at least in part by a measurement configuration scheme for measuring the lithography parameter. The term "substrate measurement configuration scheme" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in the substrate measurement configuration scheme is an optical measurement based on diffraction, one or more of the measured parameters may include the wavelength of the radiation, the polarization of the radiation, the incident angle of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting the measurement configuration scheme may be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US 2016-0161863 and published U.S. Patent Application US 2016 / 0370717A1, and the entire contents of the U.S. Patent Applications are incorporated herein by reference.

[0052] The patterning process in the lithographic apparatus LA can be one of the most critical steps in the process, requiring high precision in the dimensioning and positioning of the structures on the substrate W. In order to ensure this high precision, the patterning process can be performed in a manner such as Figure 3Three systems are combined in a so-called "holistic" control environment schematically shown in Figure 1. One of these systems is a lithography apparatus LA, which is (virtually) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning operation performed by the lithography apparatus LA remains within the process window. The process window defines a range of process parameters (e.g. dose, focus, overlap) within which a particular manufacturing process produces a defined result (e.g. a functional semiconductor device), and it may be possible to allow process parameters of the lithography process or the patterning process to vary within this range.

[0053] The computer system CL can use the (partial) 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 (in Figure 3 The resolution enhancement technique may be arranged to match the probability of pattern formation of the lithographic apparatus LA. The computer system CL may also be used to detect (e.g. using input from a metrology tool MET) where within the process window the lithographic apparatus LA is currently operating, to predict whether the lithographic apparatus LA is currently operating due to, for example, suboptimal processing (e.g., Figure 3 There is a defect (indicated by the arrow pointing to "0" in the second measuring disk SC2).

[0054] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drifts, such as drifts in the calibration state of the lithographic apparatus LA (in Figure 3 ), as indicated by the multiple arrows in the third measuring disk SC3.

[0055] Many different forms of metrology tools MT for measuring structures produced using lithographic patterning equipment may be provided. The metrology tool MT may use electromagnetic radiation to probe the structure. The characteristics of the radiation (e.g. wavelength, bandwidth, power) may affect different measurement characteristics of the tool, with shorter wavelengths generally allowing an increase in 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 with small features, a metrology tool MT with a short wavelength radiation source is preferred.

[0056] Another way that the wavelength of the radiation can affect the measurement characteristics is the penetration depth, and the transparency / opacity of the material to be inspected at the wavelength of the radiation. Depending on the opacity and / or penetration depth, the radiation can be used for measurements made in transmission or reflection. The type of measurement can affect whether information is obtained about the surface of the structure / substrate and / or the interior of the body. Therefore, penetration depth and opacity are another factor to consider when selecting a radiation wavelength for a metrology tool.

[0057] In order to achieve higher resolution for measurement of lithographically patterned structures, a metrology tool MT with a short wavelength is preferred. This can include wavelengths shorter than the visible wavelength, such as wavelengths 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), take advantage of the high resolution and high penetration depth of hard X-rays and can therefore operate in transmission mode. On the other hand, soft X-rays and EUV cannot penetrate as deeply into the target, but can induce a rich optical response in the material to be detected. This may be due to the optical properties of many semiconductor materials, and because the structures are comparable in size to the detection wavelength. As a result, the EUV and / or soft X-ray metrology tool MT can operate in reflection mode, for example by imaging, or by analyzing diffraction patterns from lithographically patterned structures.

[0058] For hard X-rays, soft X-rays and EUV radiation, application in high volume manufacturing (HVM) applications may be limited due to the lack of high brightness radiation sources available 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, for example, advanced X-ray tubes based on liquid metal anodes or rotating anodes, may be relatively low cost and compact, but may lack the brightness required for HVM applications. Currently there are high brightness X-ray sources, such as synchrotron light sources (SLS) and X-ray free electron lasers (XFELs), but their size (>100m) and high cost (hundreds of millions of euros) make them too large and expensive for metrology applications. Similarly, there is a lack of available sufficiently bright EUV and soft X-ray radiation sources.

[0059] exist Figure 4 An example of a measurement device (e.g. a scatterometer) is shown in FIG. 1 , which may include a broadband (e.g. white light) radiation projector 2 that projects radiation 5 onto a substrate W. The reflected or scattered radiation 10 is passed to a spectrometer detector 4, which measures the spectrum 6 of the specularly reflected radiation (i.e. a measure of the intensity I as a function of the wavelength λ). From this data, a processing unit PU is used, for example by rigorous coupled wave analysis and nonlinear regression or by comparing with, for example, Figure 4By comparison with the library of simulated spectra shown at the bottom, the structure or profile that produced the detected spectrum can be reconstructed 8. Typically, for the reconstruction, the general form of the structure is known and some parameters are assumed based on knowledge of the process by which the structure was made, leaving only some parameters of the structure to be determined from the scatterometer data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0060] Figure 5 The measurement equipment (e.g. Figure 4 The transmitted radiation 11 is passed to the spectrometer detector 4, which is a transmission form of the example of the scatterometer shown in FIG. Figure 4 The spectrum is measured as discussed above. Such a scatterometer may be configured as a normal incidence scatterometer or an oblique incidence scatterometer. Optionally, the transmission version uses hard X-ray radiation of wavelength <1 nm, optionally <0.1 nm, optionally <0.01 nm.

[0061] As an alternative to optical metrology, the use of hard X-rays, soft X-rays or EUV radiation has also been considered, 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 20 nm, between 5 nm and 20 nm, and between 10 nm and 20 nm. An example of a metrology tool that works in one of the above wavelength ranges is transmissive small angle X-ray scattering (such as T-SAXS in US2007224518A, which is incorporated herein by reference in its entirety). Lemaillet et al. discuss profile (CD) measurements using T-SAXS in the paper "Intercomparison between optical and X-ray scatterometry measurements of FinFET structures", Proc. of SPIE, 2013, 8681. Note that the use of a laser produced plasma (LPP) x-ray source is described in U.S. Patent Publication Nos. 2019 / 003988A1 and 2019 / 215940A1, the entire contents of which are incorporated herein by reference. Reflectometry techniques using grazing incidence X-rays (GI-XRS) and extreme ultraviolet (EUV) radiation can be used to measure the properties of films and stacks on substrates. Within the general field of reflectometry, goniometric and / or spectral techniques can be applied. In goniometric methods, the changes in reflected beams with different angles of incidence can be measured. On the other hand, a spectroscopic reflectometer measures the spectrum of wavelengths reflected at a given angle (using broadband radiation). For example, EUV reflectometry has been used to inspect mask blanks before manufacturing masks (patterning devices) for EUV lithography.

[0062] It may be that the range of applications is such that the use of wavelengths in, for example, hard X-rays, soft X-rays 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 at wavelengths in the range of 120 nm and 2000 nm are combined to obtain measurements of parameters such as CD. X-ray mathematical models and optical mathematical models are coupled through one or more common parts to obtain CD measurements. The contents of the cited US patent applications are incorporated herein by reference in their entirety.

[0063] Figure 6A schematic diagram of a metrology device 302 is shown, wherein the above-mentioned radiation can be used to measure parameters of structures on a substrate. Figure 6 The metrology device 302 shown in FIG. 3 may be applicable to hard X-ray, soft X-ray and / or EUV domains.

[0064] Figure 6 A schematic physical arrangement of a metrology apparatus 302 comprising a spectroscopic scatterometer using hard X-ray, soft X-ray and / or EUV radiation, optionally at grazing incidence, is shown by way of example only. An alternative form of inspection apparatus may be provided in the form of an angle resolved scatterometer, which may use radiation at normal or near normal incidence similar to conventional scatterometers operating at longer wavelengths, and which may also use radiation at an angle greater than 1° or 2° from parallel to the substrate. An alternative form of inspection apparatus may be provided in the form of a transmission scatterometer, Figure 5 The configuration in is used for this transmission scatterometer.

[0065] The inspection apparatus 302 includes a radiation source or so-called illumination source 310 , an illumination system 312 , a substrate support 316 , detection systems 318 , 398 and a metrology processing unit (MPU) 320 .

[0066] In this example, the radiation source 310 is used to generate EUV, hard X-ray or soft X-ray radiation. The radiation source 310 can be based on Figure 6 The high harmonic generation (HHG) technology shown, and it can also be other types of irradiation sources, such as liquid metal jet sources, inverse Compton scattering (ICS) sources, plasma channel sources, magnetic undulator sources, free electron laser (FEL) sources, compact storage ring sources, discharge-generated plasma sources, soft X-ray laser sources, rotating anode sources, solid-state anode sources, particle accelerator sources, microfocus sources or laser-generated plasma sources.

[0067] The HHG source can be a gas jet / nozzle source, a capillary / fiber source, or a gas chamber source.

[0068] For example, for HHG sources, Figure 6As shown, 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 can be, for example, a fiber-based laser with an optical amplifier, which produces pulses of infrared radiation, which can last, for example, less than 1 ns (1 nanosecond) / pulse, with a pulse repetition rate of up to several megahertz as required. The wavelength of the infrared radiation can be in the range of 200nm to 10μm, for example in the range of 1μm (1 micrometer). Optionally, the laser pulse is delivered to the gas delivery system 332 as the first pump radiation 340, wherein a portion of the radiation is converted to a higher frequency than the first radiation and becomes the emission radiation 342. The gas supply 334 supplies a suitable gas to the gas delivery system 332, in which the gas is optionally ionized by the power supply 336. The gas delivery system 332 can be a cutting tube.

[0069] The gas provided by the gas delivery system 332 defines a gas target, which can be a gas flow or a static gas mass. The gas can be, for example, air, neon (Ne), helium (He), nitrogen (N2), oxygen (O2), argon (Ar), krypton (Kr), xenon (Xe), carbon dioxide, and combinations thereof. These 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 generate radiation with multiple wavelengths. The emission divergence angle of the emitted radiation can be wavelength-dependent. For example, when imaging structures of different materials, different wavelengths will provide different contrast levels. For example, in order to inspect metal structures or silicon structures, different wavelengths can be selected for imaging the features of (carbon-based) resists, or for detecting contamination of these different materials. One or more filtering devices 344 can be provided. For example, filtering devices such as thin films of aluminum (Al) or zirconium (Zr) can be used to prevent basic IR radiation from further entering the inspection device. A grating (not shown) can be provided to select one or more specific wavelengths from the generated wavelengths. 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. Optionally, some or all of the beam path may be contained in a vacuum environment, taking into account that SXR and / or EUV radiation is absorbed when traveling in air. Various components of the radiation source 310 and the illumination optical device 312 may be adjustable to achieve different measurement "configurations" within the same device. For example, different wavelengths and / or polarizations may be implemented as selectable.

[0070] Depending on the material of the structure being inspected, different wavelengths can provide the desired level of penetration into the underlying layers. In order to address the smallest device features and defects in the smallest device features, short wavelengths may be preferred. For example, one or more wavelengths in the range of 0.01-20 nm, or alternatively in the range of 1-10 nm, or optionally in the range of 10-20 nm can be selected. Wavelengths shorter than 5 nm may have very low critical angles when reflected off the material of interest in semiconductor manufacturing. Therefore, selecting a wavelength greater than 5 nm can provide a stronger signal at higher angles of incidence. On the other hand, if the inspection task is to detect the presence of a certain material, such as detecting contamination, wavelengths up to 50 nm may be useful.

[0071] From the radiation source 310, the filtered beam 342 may enter an inspection chamber 350, in which a substrate W including a structure of interest is held in a measurement position by a substrate support 316 for inspection. The structure of interest is marked as T. Optionally, the gas environment within the inspection chamber 350 may be maintained close to vacuum by a vacuum pump 352, so that SXR and / or EUV radiation may pass through the gas environment without undue attenuation. The irradiation 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 XY translation stage and a rotation stage, by which any portion of the substrate W may be brought to the focus of the beam to be in a desired orientation. Thus, a radiation spot S is formed on the structure of interest. Alternatively, or additionally, the substrate support 316 includes, for example, a tilting platform, 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.

[0072] Optionally, illumination system 312 provides a reference radiation beam to reference detector 314, which can be configured to measure the spectrum and / or intensity of different wavelengths in filtered beam 342. Reference detector 314 can be configured to generate signal 315 that is provided to processor 320, and the filter can include information about the spectrum of filtered beam 342 and / or the intensity of different wavelengths in the filtered beam.

[0073] The reflected radiation 360 is captured by the detector 318, and the spectrum is provided to the processor 320 for calculating the properties of the target structure T. The illumination system 312 and the detection system 318 then form an inspection device. The inspection device 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.

[0074] If the target Ta has some periodicity, the radiation of the focused beam 356 may also be partially diffracted. The diffracted radiation 397 follows another path at a well-defined angle relative to the angle of incidence, followed by the reflected radiation 360. Figure 5 In FIG. 3 , the diffracted radiation 397 is drawn in a schematic manner, and the diffracted radiation 397 may follow many other paths than the drawn path. The inspection device 302 may also include an additional detection system 398 for detecting and / or imaging at least a portion of the diffracted radiation 397. Figure 5 , a single further detection system 398 is depicted, but embodiments of the inspection device 302 may also include more than one further detection system 398 arranged at different positions to detect and / or image the diffracted radiation 397 in multiple diffraction directions. In other words, the (higher) diffraction orders of the focused 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.

[0075] To help align and focus the spot S with the desired product structure, the inspection device 302 may also provide auxiliary optics that use auxiliary radiation under the control of the measurement processor 320. The measurement processor 320 may also communicate with a position controller 372 that operates a translation platform, a rotation and / or a tilt platform. The processor 320 receives highly accurate feedback about the position and orientation of the substrate through sensors. The sensor 374 may include, for example, an interferometer that can give an accuracy in the picometer range. In operation of the inspection device 302, the spectral data 382 captured by the detection system 318 is transmitted to the measurement processing unit 320.

[0076] As described above, alternative forms of inspection equipment use hard X-rays, soft X-rays and / or EUV radiation, which are optionally incident vertically or nearly vertically, 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, whose direction is greater than 1° or 2° from the direction parallel to the substrate. Both types of inspection equipment can be set in a hybrid measurement system. The performance parameters to be measured may include overlay (OVL), critical dimension (CD), focus or focal length of the lithography device when the lithography device prints the target structure, coherent diffraction imaging (CDI) and resolution overlay (ARO) measurement. Hard X-rays, soft X-rays and / or EUV radiation can have a wavelength less than 100 nm, for example using radiation in the range of 5nm to 30nm, optionally in the range of 10nm to 20nm. The radiation can be narrowband or broadband in nature. The radiation can have discrete peaks in a specific wavelength band, or it can have a more continuous characteristic.

[0077] Like optical scatterometers used in today's production facilities, inspection device 302 can be used to measure structures in resist materials processed in a lithography cell (after development inspection or ADI), and / or to measure structures in harder materials after they have been formed (after etching inspection or AEI). For example, inspection device 302 can be used to inspect a substrate after it has been processed by a development device, an etching device, an annealing device, and / or other devices.

[0078] The measurement tool MT, including but not limited to the above-mentioned scatterometer, can use radiation from a radiation source to perform measurements. The radiation used by the measurement tool MT can be electromagnetic radiation. The radiation can be optical radiation, such as radiation in the infrared, visible and / or ultraviolet part of the electromagnetic spectrum. The measurement tool MT can use radiation to measure or inspect the properties and morphology of a substrate (such as a lithographic exposure pattern on a semiconductor substrate). The type and quality of the measurement can depend on multiple properties of the radiation used by the measurement tool MT. For example, the resolution of the electromagnetic measurement can depend on the wavelength of the radiation, where, for example, due to the diffraction limit, smaller wavelengths can measure smaller features. In order to measure features with small sizes, 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 measurements at a specific wavelength or wavelength range, the measurement tool MT needs to access a source that provides radiation of 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 can be used. For extreme ultraviolet (EUV) radiation (eg, 1 nm to 100 nm) and / or soft X-ray (SXR) radiation (eg, 0.1 nm to 10 nm), the source may use high harmonic generation (HHG) or any other type of source described above to obtain radiation of the desired wavelength.

[0079] Figure 7 A simplified schematic diagram of an embodiment 600 of an illumination source 310 is shown, which may be an illumination source for high harmonic generation (HHG). Figure 6 One or more of the features of an illumination source in the described metrology tool. Illumination source 600 includes cavity 601 and is configured to receive pump radiation 611 (or drive radiation) having a propagation direction indicated by an arrow. Pump radiation 611 shown here is an example of pump radiation 340 from pump radiation source 330, such as Figure 6As shown. Pump radiation 611 can be directed into cavity 601 through radiation input end 605, which can be a window, optionally made of fused silica or similar material. Pump radiation 611 can have a Gaussian or hollow, such as annular, transverse cross-sectional profile, and can be incident, optionally focused, on a gas flow 615 having a flow direction indicated by a second arrow in chamber 601. Gas flow 615 includes a small volume of a specific gas (e.g., air, neon (Ne), helium (He), nitrogen (N2), oxygen (O2), argon (Ar), krypton (Kr), xenon (Xe), carbon dioxide, and combinations thereof), which is called a gas mass or gas target (e.g., a few cubic millimeters), in which the gas pressure is above a certain value. Gas flow 615 can be a steady gas flow. Other media, such as metal plasma (e.g., aluminum plasma) can also be used.

[0080] The gas delivery system of the irradiation source 600 is configured to provide a gas flow 615. The irradiation source 600 is configured to provide pump radiation 611 in the gas flow 615 to drive the generation of emission radiation 613. The area where at least most of the emission radiation 613 is generated is called the interaction area. The interaction area can vary from tens of microns (for strictly focused pump radiation) to several millimeters or centimeters (for moderately focused pump radiation) or even up to several 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 area of ​​the gas target, and optionally, the irradiation source is configured to receive the pump radiation and provide the pump radiation at the interaction area. Optionally, the gas flow 615 is provided by the gas delivery system into a space that is evacuated or nearly evacuated. The gas delivery system may include, for example, Figure 6 The gas nozzle 609 shown includes an opening 617 in the exit plane of the gas nozzle 609. A gas flow 615 is provided from the opening 617. A gas trap is used to confine the gas flow 615 within a certain volume by extracting the residual gas flow and maintaining a vacuum or near-vacuum gas environment inside the chamber 601. Optionally, the gas nozzle 609 can be made of a thick-walled tube and / or a high thermal conductivity material to avoid thermal deformation due to the high-power pump radiation 611.

[0081] It is conceivable that the size of the gas nozzle 609 can also be used in a scaled-up or scaled-down form ranging from a micrometer-sized nozzle to a meter-sized nozzle. This wide range of dimensioning comes from the fact that the device can be scaled so that the intensity of the pump radiation at the gas flow ends up in a specific range that may be beneficial for emitting radiation, which requires different dimensioning for different pump radiation energies, which can be pulsed lasers and the pulse energy can vary from tens of microjoules to 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 irradiation source includes a gas trap that is close to the gas nozzle to maintain the pressure of the chamber 601.

[0082] Due to the interaction of the pump radiation 611 with the gas atoms of the gas stream 615, the gas stream 615 converts a portion of the pump radiation 611 into emission radiation 613, which may be Figure 6 In the example of emitted radiation 342 shown in , the central axis of emitted radiation 613 can be co-linear with the central axis of incident pump radiation 611. Emitted radiation 613 can have a wavelength in the X-ray or EUV range, wherein the wavelength is in the range from 0.01 nm to 100 nm, optionally in the range from 0.1 nm to 100 nm, optionally in the range from 1 nm to 50 nm, or optionally in the range from 10 nm to 20 nm.

[0083] In operation, the emitted radiation beam 613 may pass through the radiation output terminal 607 and may then be transmitted by the Figure 6 The illumination system 603 of the example of the illumination system 312 in FIG. 6 is manipulated and directed to a substrate to be inspected for metrology measurements. Emitted radiation 613 may be directed, and optionally focused, to structures on the substrate.

[0084] Because air (and indeed any gas) absorbs SXR or EUV radiation significantly, the volume between the gas flow 615 and the wafer to be inspected may be evacuated or nearly evacuated. Since the central axis of the emitted radiation 613 may be collinear with the central axis of the incident pump radiation 611, 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 accomplished by placing Figure 6The filter device 344 shown in is incorporated into the radiation output end 607, which is placed in the emission beam path and is opaque or almost opaque to the pump radiation (for example, opaque or almost opaque to infrared or visible light), but is at least partially transparent to the emission radiation beam. The filter device can be made of zirconium or a plurality of materials combined in multiple layers. When the pump radiation 611 has a hollow, optionally annular transverse cross-sectional profile, the filter device can be a hollow, optionally annular block. Optionally, the filter device is not perpendicular to and is 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 film filter device, such as an aluminum (Al) or zirconium (Zr) film filter device. Optionally, the filter device 344 can also include a reflector that effectively reflects the emission radiation but poorly reflects the pump radiation, or the filter device 344 can include a wire mesh that effectively transmits the emission radiation but poorly transmits the pump radiation.

[0085] Methods, devices and assemblies for obtaining emitted radiation are described herein, the emitted radiation optionally being at a high harmonic frequency of the pump radiation. The radiation generated by the process (optionally utilizing nonlinear effects to generate HHG of radiation optionally at a harmonic frequency of the provided pump radiation) can be provided as radiation in a metrology tool MT for inspecting and / or measuring a substrate. If the pump radiation comprises short pulses (i.e., a few cycles), the radiation generated is not necessarily exactly at a harmonic of the pump radiation frequency. The substrate may be a lithographically patterned substrate. The radiation obtained by the process may also be provided in a lithographic apparatus LA and / or a lithographic cell LC. The pump radiation may be pulsed radiation, which may provide a high peak intensity in a short burst time.

[0086] The pump radiation 611 may include radiation having one or more wavelengths higher than the wavelength or wavelengths of the emitted 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. The pulsed pump radiation may include pulses having a duration in the femtosecond range.

[0087] For some embodiments, the emitted radiation (optionally high harmonic radiation) may include one or more harmonics of the pump radiation wavelength. 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 a plurality of ranges of less than 1 nm, less than 0.1 nm, less than 0.01 nm, 0.01 nm to 100 nm, 0.1 nm to 50 nm, 1 nm to 50 nm, and 10 nm to 20 nm.

[0088] Radiation, such as the above-mentioned high-harmonic radiation, can be provided as source radiation in the measurement tool MT. The measurement tool MT can use the source radiation to perform measurements on the substrate exposed by the lithographic equipment. The measurements can be used to determine one or more parameters of the structure on the substrate. Compared with the use of longer wavelengths (e.g., visible light radiation, infrared radiation), the use of shorter wavelengths (e.g., EUV, SXR and / or HXR wavelengths included in the above-mentioned wavelength range) of radiation allows smaller features of the structure to be resolved by the measurement tool. 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 measurement of deeper layers on the substrate is possible. Radiation with longer wavelengths may not be able to reach these deeper layers.

[0089] In the measurement 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 measurement tool MT may include one or more sensors for detecting diffracted radiation. For example, the measurement tool MT may include detectors for detecting positive (+1) and negative (-1) first diffraction orders. The measurement tool MT may also measure radiation that is specularly reflected or transmitted (0th order diffracted radiation). Additional sensors for measurement may be present in the measurement tool MT, for example to measure additional diffraction orders (e.g., higher diffraction orders).

[0090] In an exemplary lithography metrology application, HHG-generated radiation can be focused onto a target on a substrate using an optical column, which can be referred to as an illuminator, which transmits radiation from the HHG source to the target. The HHG radiation can then be reflected from the target, detected, and processed, for example, to measure and / or infer properties of the target.

[0091] Gas-targeted HHG configurations can be broadly divided into three separate categories: gas injectors, gas chambers, and gas capillaries. Figure 7An exemplary gas jet configuration is depicted, in which a gas volume or air mass is introduced into the driving radiation laser beam. In the gas jet configuration, the interaction of the driving radiation with the solid component is kept to a minimum. The gas volume or air mass may, for example, include a gas flow perpendicular to the driving radiation beam, wherein the gas volume or air mass is enclosed inside a gas chamber. In a gas capillary configuration, the size of the capillary structure holding the gas is small in the lateral direction so that it significantly affects the propagation of the driving radiation laser beam. The capillary structure may, for example, be a hollow fiber, in which the hollow core is configured to hold the gas.

[0092] The gas jet HHG configuration can provide a relative degree of freedom for shaping the spatial distribution of the drive radiation beam in the far field because it is not limited by the constraints imposed by the gas capillary structure. The gas jet configuration can also have a less stringent alignment tolerance. On the other hand, the gas capillary can provide an increased interaction area of ​​the drive radiation and the gaseous medium, which can optimize the HHG process. In order to use HHG radiation, for example, in measurement applications, the HHG radiation is separated from the drive radiation downstream of the gas target. The separation of HHG radiation and drive radiation can be different for gas jet and gas capillary structures. In both cases, the drive radiation suppression scheme can include a metal transmissive filter device for filtering out any remaining drive radiation from the short wavelength radiation. However, before such a filter device can be used, the intensity of the drive radiation may need to be significantly reduced from its intensity at the gas target to avoid damage to the filter device. The methods that can be used for this intensity reduction are different for gas jet 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 drive radiation beam focused onto the gas target, this can be designed so that in the far field the gas jet HHG has low intensity along the direction of short wavelength radiation propagation. This spatial separation in the far field means that an aperture can be used to block the drive radiation and reduce its intensity.

[0093] In contrast, in a gas capillary structure, the spatial distribution of the radiation beam as it passes through the gas medium may be determined primarily by the capillary. The spatial distribution 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 fiber is used as the capillary structure, the shape and material of the fiber structure determine which modes of driving radiation are supported for propagation through the fiber. For most standard optical fibers, the supported propagation modes result in a spatial distribution 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 in the far field with a Gaussian or near-Gaussian distribution.

[0094] Return to reference Figure 7An important aspect of any HHG source is that the HHG radiation is emitted into the same direction as the powerful pump radiation. This is why, as mentioned above, a pump radiation suppression scheme must be employed to separate the remaining unconverted pump radiation from the emitted HHG radiation. In the case of using HHG in a metrology tool MT (e.g. Figure 6 In the case of a laser (as shown), good separation of pump radiation and HHG radiation can: 1) protect sensitive HHG optics used downstream, for example, for refocusing the HHG radiation onto the target, 2) prevent laser damage to the measurement target, and 3) prevent the emitted weak HHG radiation from being drowned out by the strong background of the remaining pump radiation.

[0095] Existing pump radiation suppression schemes typically rely on the use of a radiation block to block most of the remaining pump radiation. In some exemplary schemes, the radiation blocking device may include an aperture (or a pinhole mirror, i.e., a mirror with a hole in the center) that preferentially blocks (or deflects, in the case of a pinhole mirror) a more divergent pump laser beam while allowing HHG radiation to pass through the hole without being attenuated. In some exemplary schemes, the radiation block may include a Brewster plate that preferentially reflects HHG radiation while allowing the remaining pump radiation to pass through the Brewster plate and be blocked, for example, by the beam block. In other exemplary schemes, the radiation block may be a wire mesh that preferentially transmits HHG radiation. Typical blocking ratios for any of the above-mentioned pump radiation suppression schemes are in the range of 10 3 and 10 6 Here, the blocking ratio is the ratio of the power of the total pump radiation along the HHG radiation path to the power of the leaked pump radiation. In general, a single pump radiation suppression scheme may not provide a high blocking ratio (e.g., 10) that is desired for certain applications (e.g., metrology). 8 of the order of magnitude). Therefore, additional separation steps or schemes are needed to further improve the blocking ratio. For example, a second separation step can be performed by inserting one or more thin film metal filter devices (e.g., Al or Zr film filter devices) into the HHG radiation beam. Such a film metal filter device exerts strong attenuation on the remaining pump radiation that is not blocked by the first separation step, while allowing the HHG radiation to pass with relatively low or negligible attenuation. The film metal filter device (e.g., Al or Zr film filter device) can have a thickness of, for example, several hundred nanometers.

[0096] It has been found that the operation of any existing pump radiation suppression scheme is sensitive to misalignment of the radiation beam. If everything is perfectly aligned, the remaining unconverted pump radiation can be effectively blocked. Otherwise, any small misalignment can potentially lead to a significant reduction in the effectiveness of blocking the remaining pump radiation. Fig. 8A An exemplary pump radiation suppression scheme preferably implemented in a HHG radiation source is schematically described. Fig. 8A As shown, input pump radiation IRD having an annular transverse cross-sectional profile is focused onto the gas target GT by a lens LEN. The position and tilt (or pointing) of the focused pump radiation IRD are optimized relative to the gas target to obtain the best HHG efficiency. The remaining unconverted pump radiation R-IRD and the emitted HHG radiation ORD propagate in the same direction, wherein the central axis of the emitted HHG radiation ORD is collinear with the central axis of the remaining pump radiation R-IRD. The annular beam profile of the pump radiation IRD is superior to, for example, a Gaussian beam profile, because it allows the remaining pump radiation R-IRD to be spatially separated from the co-propagating HGG radiation, thereby facilitating effective blocking of the remaining pump radiation R-IRD. The pump blocking aperture BLA is preferably positioned so as to effectively block the remaining pump radiation R-IRD while allowing the HHG radiation to pass through the aperture without being clipped. This is typically achieved by initial optimization of the HHG source at low pump laser power (e.g., <1 Watt (W)). Fig. 8A Preferred beam alignment shown. Once the initial optimization is completed, the power of the pump radiation is then increased incrementally from a lower power to a higher power (eg 100 W) to reach the desired power level of the HHG radiation.

[0097] As the power of the pump radiation increases, Fig. 8A The initially optimized beam alignment shown in is changed or degraded due to one or more thermal effects inside the pump radiation source and in the beam path. Thermal effects can negatively affect the beam alignment in several different ways. For example, absorption of a small portion of the pump radiation may lead to local heating of components located inside the pump radiation source or in the beam path (e.g., mirrors), which leads to drift of the components and thus to beam misalignment. This negative effect is caused by Figure 8B . In this exemplary case, due to beam misalignment, the input pump radiation IRD is both tilted and offset with respect to the initial preferred beam path to the gas target GT. The deviation from the initial preferred beam path means that both the conversion efficiency of the HHG process and the blocking ratio of the blocking aperture BLA become suboptimal. As a result, part IRD-LK of the remaining pump radiation R-IRD manages to leak through the holes of the blocking aperture BLA and the simultaneously emitted HHG radiation ORD is clipped by the hole edges.

[0098] Furthermore, absorption of a small portion of the pump radiation can also lead to thermal lensing of transmissive components (e.g. focusing lenses) located inside the pump radiation source or in the beam path. Thermal lensing can change the divergence of the pump radiation beam and thus cause direct or indirect beam alignment changes. This negative effect is caused by Figure 8C. In this example case, the thermal lens effect changes the effective focal length of the lens LEN. This change in the effective focal length of the lens LEN causes the focusing of the pump radiation IRD to shift from the initial preferred position, resulting in a suboptimal HHG conversion efficiency. The change in the effective focal length of the lens LEN also changes the divergence of the emitted HHG radiation as well as the remaining unconverted drive radiation, which in turn reduces the blocking ratio of the blocking aperture and allows part IRD-LK of the remaining pump radiation R-IRD to leak through the holes of the blocking aperture.

[0099] In addition to the thermal effects described above, external disturbances (e.g. vibrations) may also lead to beam misalignment in a HHG source due to, for example, drift of one or more components of the source. Beam misalignment may be corrected, for example, by realigning the pump radiation IRD to the gas target GT, and / or by reoptimizing the gas target GT, and / or by realigning the blocking aperture BLA with respect to the remaining pump radiation and the HHG radiation. Currently, active pump beam stabilization techniques have been implemented in existing HHG radiation sources to deal with the misalignment problem. Fig. 9 An example HHG radiation source stabilized by a prior art active beam stabilization arrangement is schematically depicted. Fig. 9 As shown, two actuatable mirrors AM1, AM2 (actuated, for example, by piezoelectric transducers or electromagnetic motors) are used to controllably manipulate the input pump radiation IRD through a negative axis prism NAX and a positive axis prism PAX. The axis prism pair is used to convert, for example, a Gaussian beam profile into an annular beam profile. The annular pump beam AID is then focused on the gas target GT by a focusing lens LEN. Between the lens LEN and the gas target GT, there is a first interceptor (pick off) PO1, which is configured to reflect off a small portion AIRD-2 of the pump radiation AIRD for beam stabilization (see below), while transmitting most of the AIRD-1 of the HHG process to the gas target GT.

[0100] The emitted HHG radiation ORD and the remaining pump radiation R-AIRD propagate collinearly towards the blocking aperture BLA. Since the remaining pump radiation R-AIRD has an annular beam profile, the remaining pump radiation R-AIRD can be spatially separated from the emitted HHG radiation by selecting suitable axicons NAX, PAX and / or focusing lenses LEN. For example, the lens combination can be selected such that the remaining pump radiation R-AIRD has a larger divergence than the emitted HHG radiation, so that the emitted HHG radiation is surrounded by the remaining pump radiation R-AIRD and is spatially separated from the remaining pump radiation R-AIRD, as Fig. 9 As shown, a blocking aperture BLA is then used to completely block the remaining pump radiation R-AIRD so as to allow only the HHG radiation ORD to be output from the HHG radiation source.

[0101] The portion AIRD-2 of the pump radiation AIRD reflected by the first interceptor PO1 is directed to the first position sensitive detector PSD1. Between the first interceptor PO1 and the first position sensitive detector PSD1, there is a second interceptor PO2, which is configured to direct a small portion AIRD-3 of the pump radiation AIRD-2 to the second position sensitive detector PSD2, while transmitting most of the pump radiation AIRD-2 AIRD-4. The position of the pump radiation AIRD can be measured using only the first position sensitive detector PSD1, while the pointing (or direction) of the pump radiation AIRD can be determined based on the first position POS1 and the second position POS2 measured by the first position sensitive detector PSD1 and the second position sensitive detector PSD2, respectively. The measurement data is sent to a processing unit, which is configured to process the data (e.g., calculate a pointing change), generate a control signal based on the determined position and pointing change of the pump radiation AIRD, and send the control signal to the actuatable mirrors AM1, AM2. Upon receiving the control signal, the actuatable mirrors AM1, AM2 will move (e.g. rotate and / or tilt) accordingly to correct any drift (e.g. position and pointing changes) of the misalignment of the pump radiation AIRD. This beam stabilization process will be repeated until the position and pointing changes are minimized.

[0102] The above beam stabilization method has a number of disadvantages. First, for non-Gaussian beams, such as annular beams, the beam stabilization method is unreliable. As mentioned above, annular beam profiles are preferred over Gaussian beam profiles because they facilitate efficient blocking of the remaining pump radiation R-AIRD. The unreliability is mainly due to the fact that the position sensitive detectors (e.g. PSD1, PSD2) determine the position of the beam by estimating the centroid of the beam, and therefore it is difficult for the position sensitive detectors to reliably determine the centroid of annular beams that have no intensity in the center of the beam.

[0103] Second, the position sensitive detectors (e.g., PSD1, PSD2) are insensitive to any changes in the divergence and wavefront of the beam. As is known, the output performance of the HHG radiation ORD also depends on the divergence and wavefront of the pump radiation, both of which can be changed without disturbing the pump beam alignment (e.g., Figure 8C ) may reduce HHG output (e.g., reduce conversion efficiency and / or beam alignment).

[0104] Third, the beam stabilization method does not correct for misalignment with any components located between the second actuated mirror AM2 and the gas target GT (e.g., negative axicon NAX, positive axicon PAX, interceptor PO3). Correcting for misalignment of any of these components is crucial to maintain the colinearity of the residual pump radiation R-AIRD and the HHG radiation ORD, which in turn is crucial to achieve a high blocking ratio of the pump radiation suppression scheme.

[0105] It may be advantageous to use the position, orientation and intensity of the HHG radiation ORD to generate a feedback control signal for optimizing the alignment of the pump gas jet. However, it is difficult to measure the position and orientation of the HHG radiation with a position sensitive detector due to sensitivity limitations in the desired wavelength region (e.g., 0.1 nm to 10 nm). Furthermore, in order to measure the characteristics of the HHG radiation, a beam splitter or interceptor is required. However, it is difficult or even impossible to adapt the beam splitter to the desired wavelength region, e.g., 0.1 nm to 10 nm. Furthermore, the use of a beam splitter results in a power loss of the HHG radiation ORD.

[0106] In order to alleviate some or all of the above problems, a beam measurement device and a method for controlling a radiation source are proposed below. According to the general concept of the present disclosure, a beam measurement device for determining at least one characteristic of a first radiation and / or at least one characteristic of a second radiation is provided, wherein the second radiation is generated via a first nonlinear process when a first portion of the first radiation is received; the radiation beam measurement device comprises: a measurement device nonlinear medium, which is configured to receive a second portion of the first radiation and thereby generate a third radiation via a second nonlinear process; at least one detector, which is configured to measure at least one characteristic of the third radiation; and a processing unit, which is operable to determine the at least one characteristic of the first radiation and / or the at least one characteristic of the second radiation based on the at least one characteristic of the third radiation.

[0107] In an embodiment, the third radiation may have a higher frequency than the first radiation. Accordingly, in an embodiment, the second nonlinear process may be a second harmonic generation (SHG) process. In an embodiment, the measurement device nonlinear medium may be a nonlinear crystal. In an embodiment, the measurement device nonlinear medium may be, for example, a beta barium borate or metaborate (BBO) crystal, a lithium triborate (LBO) crystal, potassium dideuterium phosphate (KDP) or one of its isomorphs (KD*P (e.g., DKDP)), potassium titanyl phosphate (KTP) or cesium lithium borate (CLBO). Please note that the above list of nonlinear crystals is non-exclusive. Other nonlinear crystals may also be applicable. In other embodiments, the second nonlinear process may be any nonlinear process other than the SHG process. For example, in an embodiment, the second nonlinear process is a third harmonic generation (THG) process. In an embodiment, the second nonlinear process may be a sum frequency generation (SFG) process. In an embodiment, the second nonlinear process may be a supercontinuum generation process. In an embodiment, the second nonlinear process may be a HHG process. In an embodiment, the second nonlinear process may be a difference frequency generation (DFG) process. In an embodiment, the second non-linear process may be a supercontinuum generation process.

[0108] The beam profile of the third radiation may be primarily determined by the beam profile of the second portion of the first radiation. In an embodiment, the third radiation may include a central portion, and the at least one detector is configured to measure the at least one characteristic of the third radiation from the central portion. In an embodiment, the beam measurement device may further include a third radiation block, the third radiation block being configured to block a peripheral portion of the third radiation while transmitting the central portion, the peripheral portion surrounding the central portion of the generated third radiation.

[0109] In an embodiment, the at least one detector may include at least one position sensitive detector for determining at least a first position of the third radiation. In an embodiment, the at least one detector may include a first position sensitive detector and a second position sensitive detector. The first position sensitive detector and the second position sensitive detector may be separated by a distance and configured to measure the first position of the third radiation and the second position of the third radiation, respectively. In an embodiment, the at least one detector may include a wavefront detector configured to measure a wavefront of the third radiation. The wavefront detector may be used alone or in combination with one or more position sensitive detectors.

[0110] In an embodiment, the first nonlinear process may be a high harmonic generation process. Thus, the second radiation may be HHG radiation emitted from a HHG radiation source. Fig.10 The application of the proposed beam measurement device in a HHG radiation source is described in detail. In other embodiments, the first nonlinear process may be any nonlinear process other than a HHG process. For example, in an embodiment, the first nonlinear process may be a SHG process. In an embodiment, the first nonlinear process may be a third harmonic generation (THG) process. In an embodiment, the first nonlinear process may be a sum frequency generation (SFG) process. In an embodiment, the first nonlinear process may be a difference frequency generation (DFG) process. In an embodiment, the first nonlinear process may be a supercontinuum generation process.

[0111] In an embodiment, the first portion of the first radiation and the second portion of the first radiation may each comprise a substantially annular beam.

[0112] In an embodiment, the processing unit may be further operable to determine, based on the at least one characteristic of the third radiation, a control action on at least one characteristic of the first radiation when generating the second radiation. The control action may relate to a position and / or orientation of the first radiation relative to a source nonlinear medium for generating the second radiation. Alternatively or additionally, the control action may relate to a wavefront of the first radiation.

[0113] According to the overall concept of the present disclosure, there is provided a radiation source, comprising: a beam measurement device as described above; a source nonlinear medium, which is configured to generate a second radiation via a first nonlinear process when receiving a first portion of a first radiation; and a radiation configuration device, which is operable to configure at least one characteristic of the first radiation so as to initiate the control action.

[0114] Fig.10 Schematically shows an example HHG radiation source stabilized by the proposed beam measurement device BMD (in the dashed line) according to an embodiment. Fig.10 As shown, the source nonlinear medium can be a gas target GT, and the first nonlinear process can be a high harmonic generation HHG process. In an embodiment, the radiation source may include a gas delivery system configured to provide a gas medium flow. In an embodiment, the radiation source may include a gas chamber configured to at least partially surround the gas medium. In an embodiment, the radiation source may include a hollow core optical fiber configured to at least partially surround the gas medium. The gas medium can be any one of the following: air, Ne, He, N2, O2, Ar, Kr, Xe, carbon dioxide, and combinations thereof.

[0115] The first radiation IRD may have a Gaussian beam distribution, and the radiation source may further include a profile configuration device, which is operable to impose a substantially annular distribution on the first radiation. In an embodiment, the profile configuration device includes a negative axicon NAX and a positive axicon PAX. In an embodiment, the first radiation IRD may have an annular beam profile, and thus the radiation source may not include an axicon. In an embodiment, the radiation source may also include a focusing device (e.g., a lens) LEN, which is used to focus a first part AIRD-1 of the first radiation AIRD onto a source nonlinear medium (e.g., a gas target GT) so as to produce a second radiation ORD via a first nonlinear process.

[0116] In an embodiment, the radiation source may further include a first radiation block BLA1, which is operable to substantially transmit or transmit only the second radiation ORD without transmitting or transmitting any of the first radiation ORD. In an embodiment, the first radiation block BLA1 may include an aperture configured to block the remaining portion R-AID of the first radiation AID in an annular shape, while allowing the second radiation ORD to pass through the hole of the aperture without being attenuated. In an embodiment, the radiation blocking device BLA may include a pinhole reflector, which includes a central hole and is configured to reflect the remaining portion R-AIRD of the first radiation AIRD in an annular shape, while allowing the second radiation ORD to pass through the central hole without being attenuated. In an embodiment, the radiation source may also include one or more additional radiation blocking devices for improving the suppression of the remaining portion of the first radiation AIRD in an annular shape. In an implementation scheme, the one or more additional radiation blocking devices may include one or more film-type metal filter devices (e.g., Al and / or Zr film-type filter devices).

[0117] In an embodiment, the processing unit is operable to determine a control action on at least one characteristic of the first radiation AIRD so as to minimize or reduce the amount of the remaining portion R-AIRD of the annular first radiation AIRD transmitted or transmitted via the first radiation block BLA1, and / or maximize or increase the amount of the second radiation ORD transmitted or transmitted via the first radiation block BLA1.

[0118] The radiation source may also include an interceptor PO1, which is located upstream of the source nonlinear medium GT and is configured to guide the second part AIRD-2 of the first radiation AIRD to the measurement device nonlinear medium NLE of the beam measurement device BMD. The second part AIRD-2 of the first radiation AIRD may account for no more than 10%, no more than 5% or no more than 1% of the total power of the first radiation AIRD. The measurement device nonlinear medium NLE may be located at the focus of the second part AIRD-2 of the first radiation AIRD. In an embodiment, the second part AIRD-2 of the first radiation AIRD may be a part scattered or reflected from a component in the beam path of the first radiation (e.g., a positive axis prism PAX or a lens LEN). Since the second part AIRD-2 of the first radiation has an annular beam distribution, the third radiation NRD generated by the SHG process includes a central part NRD-1 and a peripheral part (not shown) surrounding the central part NRD-1. The central part NRD-1 of the third radiation NRD is generated due to the conservation of momentum of photons (the same principle is used in the intensity autocorrelator). As described above, the position sensitive detector is unreliable for measuring the position of the annular beam. Therefore, it is desirable to use only the central portion NRD-1 of the third radiation NRD. Fig.10In the illustrated embodiment, the beam measurement device BMD may include a third radiation block BLA3 (eg, an aperture) configured to substantially transmit only a central portion NRD-1 and not a peripheral portion of the third radiation NRD.

[0119] The beam measurement device BMD may include a third position sensitive detector PSD 3 and a fourth position sensitive detector PSD 4. The two detectors PSD 3, PSD 4 may be separated by a certain distance and configured to measure a third position POS 3 and a fourth position POS 4 of the central portion NRD- 1 of the third radiation NRD, respectively. Based on the measured third position and fourth position of the central portion NRD- 1 of the third radiation NRD, the processing unit may determine the pointing direction or orientation of the central portion NRD- 1 of the third radiation NRD. Fig.10 As shown, the third interceptor PO3 can be placed in the beam path of the central portion NRD-1 of the third radiation NRD. The third interceptor PO3 can be configured to reflect a predetermined portion of the central portion NRD-1 of the third radiation NRD toward the fourth position sensitive detector PSD4, while allowing the remaining portion of the central portion NRD-1 of the third radiation NRD to pass through to reach the third position sensitive detector PSD3. Similar to the above reference Fig. 9 The method described can measure the position of the central part NRD-1 of the third radiation NRD by using the third position sensitive detector PSD3 alone, and the pointing or direction of the central part NRD-1 of the third radiation NRD can be determined based on the two positions POS3, POS4 measured by the third position sensitive detector PSD3 and the fourth position sensitive detector PSD4 respectively. Alternatively or additionally, the radiation source may include a wavefront detector configured to measure the wavefront of the central part NRD-1 of the third radiation NRD. Information from this measurement can be used as input in a feedback loop of a wavefront configuration device for wavefront correction (see below).

[0120] Periodically or as needed, measurement data collected by at least one detector of the beam metrology device BMD may be sent to a processing unit of the radiation source for data processing and subsequent control of the radiation configuration device. In an embodiment, the radiation configuration device may comprise one or more beam steering components configured to adjust the position and / or direction of the first radiation relative to the source nonlinear medium. In an embodiment, the one or more beam steering components may comprise one or more actuatable mirrors, e.g. Fig.10 Actuable mirrors AM1, AM2 are shown. In an embodiment, the radiation configuring means may comprise a wavefront configuring means configured to configure a wavefront of the first radiation. In an embodiment, the wavefront configuring means may comprise a spatial light modulator, a digital micromirror device or a deformable mirror.

[0121] The processing unit may be operable to control two actuatable reflectors AM1, AM2 (e.g. actuated by a piezoelectric transducer or an electromagnetic motor) based on the measured position and orientation changes of the central portion NRD-1 of the third radiation NRD to adjust the position and / or orientation of the first radiation AIRD so as to minimize the changes in the position and / or orientation of the second radiation ORD. The processing unit may be operable to: receive a signal indicating the position and / or orientation of the central portion NRD-1 of the third radiation NRD measured by the position sensitive detectors PSD 3, PSD4; determine the changes in the position and / or orientation of the central portion NRD-1 of the third radiation NRD measured by the position sensitive detectors PSD 3, PSD4; and generate a control signal for controlling the radiation configuration device so as to counteract the determined changes in the measured position and / or orientation of the central portion NRD-1 of the third radiation NRD. According to another general concept of the present disclosure, a method for controlling a radiation source is provided, comprising: generating second radiation via a first nonlinear process when a first portion of a first radiation is received by a source nonlinear medium; generating third radiation via a second nonlinear process when a second portion of the first radiation is received by a measuring device nonlinear medium; measuring at least one characteristic of the third radiation; and controlling the at least one characteristic of the first radiation when generating the second radiation based on the at least one characteristic of the third radiation.

[0122] The proposed concept of using the output of the second nonlinear process to generate a feedback control signal for controlling or stabilizing the output of the first nonlinear process is superior to conventional stabilization methods that use the input of the first nonlinear process to generate a feedback control signal (e.g., Fig. 9 An active beam stabilization arrangement implemented in the HHG source shown).

[0123] As an example and reference Fig.10 , in the case where the first nonlinear process is a HHG process and the second nonlinear process is a SHG process, the emission angle of the central part NRD-1 of the third radiation NRD is similar to the emission angle of the second radiation ORD. If the annular shaped first radiation AIRD is completely symmetrical, it is expected that the central part NRD-1 of the third radiation NRD will be completely located in the center. If the symmetry of the annular shaped first radiation AIRD changes, the emission angle of the central part NRD-1 of the third radiation NRD also changes. Since the emission angle of the second radiation ORD changes with the annular shaped first radiation AIRD in a similar manner to the third radiation, the relative position of the central part NRD-1 of the third radiation NRD with respect to the peripheral part of the third radiation NRD can be a measure of the symmetry of the annular first radiation AIRD, and therefore an indication of the emission angle of the second radiation ORD. By comparison, existing stabilization methods (e.g., Fig. 9) cannot measure or determine the symmetry of the annular shaped first radiation AIRD.

[0124] exist Fig.10 The stabilization method implemented in the HHG source shown also provides information about the degree of alignment of the first radiation IRD on the axicon NAX, PAX (or pinhole mirror), as well as information about the alignment of the first radiation on the gas target GT. If the focus position of the annular shaped first radiation AIRD-2 moves forward or backward due to misalignment or thermal lensing, the conversion efficiency of the SHG process decreases and the divergence of the third radiation NRD changes. Since the second radiation ORD will respond to changes in the focus position of the annular shaped first radiation AIRD-1 in a similar manner to the third radiation, a stabilization method can be used to monitor the relative divergence changes of the second radiation ORD, which is a problem with existing stabilization methods (e.g., as Fig. 9 The change in the focus position can be corrected, for example, by moving the focus lens LEN.

[0125] It should be understood that the above embodiments are provided to illustrate the proposed concept and are not intended to limit the scope of the present disclosure. Of course, other embodiments are contemplated. Fig.11 In an embodiment, the interceptor PO1 may be located downstream of the source nonlinear medium GT and configured to direct a small portion R-AIRD-2 of the remaining first radiation R-AIRD toward the measurement device nonlinear medium NLE of the beam measurement device BMD. The remaining R-AIRD-1 of the remaining first radiation R-AIRD passing through the interceptor PO1 may be directed in a manner similar to Fig.10 In the same manner as in the embodiment shown in FIG. 1 , the beam measurement device can be blocked by the first radiation block BLA1. Fig.10 The interceptor PO1 is configured and operated in substantially the same manner as shown. Fig.10 Compared to the arrangement of the beam measurement device BMD (not shown), placing the interceptor PO1 after the gas target GT allows any instabilities of the gas target GT to be detected by the beam measurement device BMD and to be taken into account when generating the feedback control signal. For example, the gas target GT may be provided by a gas nozzle (not shown), which may drift or move over time. The drift of the gas nozzle may reduce the spatial overlap between the focus of the annular shaped first radiation and the gas target GT and thus cause a variation in the divergence and power of the second radiation ORD. Both variations will be captured by the beam measurement device BMD and a corresponding feedback control signal will be generated for controlling the radiation configuration device (e.g. one or more actuators for position control mounted on the gas nozzle) in order to counteract such variations.

[0126] It should be understood that the characteristics of the third radiation measured for the purpose of controlling the corresponding characteristics of the second radiation may not be limited to beam alignment (beam position and beam pointing), divergence and wavefront, as described in the aforementioned embodiments. Other different characteristics of the third radiation, such as spot size, centroid wavelength, bandwidth, pulse width, average power, peak power, etc. may also be useful. As an example, since the intensity of the third radiation (e.g., the SHG of the first radiation) is a measure of the peak intensity of the first radiation, changes in the measured intensity of the third radiation can be used to determine changes in the following pump beam parameters: (1) Beam alignment, which can be achieved by, for example, actuating a mirror (e.g., Fig.10 and 11 ) to compensate for beam alignment; (2) the pulse duration can be adjusted, for example, by changing the (dispersion) settings of a pump laser emitting the first radiation; (3) beam size and divergence, which can be adjusted, for example, by changing the settings of a focusing device; and (4) pulse energy, which can be adjusted, for example, by the diode current of the laser. Thus, the measured intensity of the third radiation can be used to determine changes in the pulses of the first radiation and to initiate corresponding control actions. For example, if it is the pulse energy that changes, the control action can be to adjust, for example, the diode current. And if it is the pulse duration that changes, the control action can be to adjust the settings of the pump laser.

[0127] Embodiments may include a computer program comprising one or more machine-readable instruction sequences describing a method of optical metrology and / or a method of analyzing measurement results to obtain information about a lithography process. Embodiments may include a computer code comprising one or more machine-readable instruction sequences or data sequences describing the method. The computer program may be implemented in, for example, Figure 6 The unit MPU and / or Figure 3 The computer program or code is executed in the control unit CL of the computer. A data storage medium (e.g., a semiconductor memory, a magnetic disk or an optical disk, etc.) in which such a computer program or code is stored may also be provided. Figure 6 Where existing measurement equipment of the type shown is already in production and / or in use, embodiments of the present invention may be implemented by providing an updated computer program product for causing a processor to perform one or more of the methods described herein. Optionally, the computer program or code may be arranged to control an optical system, a substrate support, etc. to perform a method of measuring parameters of a lithography process on a suitable plurality of targets. The computer program or code may update a lithography and / or measurement configuration scheme for measuring additional substrates. The computer program or code may be arranged to control (directly or indirectly) a lithography apparatus for patterning and processing of additional substrates.

[0128] The illumination source may be arranged, for example, in the metrology apparatus MT, the inspection apparatus, the lithographic apparatus LA and / or the lithocell LC.

[0129] The properties of the emitted radiation used to perform the measurement may affect the quality of the measurement results obtained. For example, the shape and size of the lateral beam profile (cross-section) of the radiation beam, the intensity of the radiation, the power spectral density of the radiation, etc. may affect the measurement performed by the radiation. Therefore, it is beneficial to have a source that provides radiation with properties that result in high quality measurements.

[0130] Further embodiments are disclosed in the following sequentially numbered aspects:

[0131] 1. A beam measurement device for determining at least one characteristic of a first radiation and / or at least one characteristic of a second radiation, the second radiation being generated via a first non-linear process upon receiving a first portion of the first radiation; the beam measurement device comprising:

[0132] a measurement device nonlinear medium configured to receive a second portion of the first radiation and to generate third radiation therefrom via a second nonlinear process;

[0133] at least one detector configured to measure at least one characteristic of the third radiation; and

[0134] A processing unit operable to determine the at least one characteristic of the first radiation and / or the at least one characteristic of the second radiation based on the at least one characteristic of the third radiation.

[0135] 2. A beam metrology device according to aspect 1, wherein the third radiation has a higher frequency than the first radiation.

[0136] 3. A beam metrology apparatus according to aspect 1 or 2, wherein the second nonlinear process is a second harmonic generation process.

[0137] 4. A beam metrology device according to aspects 1, 2 or 3, wherein the metrology device nonlinear medium is a nonlinear crystal.

[0138] 5. The beam measurement device according to aspect 3 or 4, wherein the nonlinear medium of the measurement device is a BBO crystal or a LBO crystal.

[0139] 6. The beam metrology apparatus according to the preceding aspect, wherein the third radiation comprises a central portion, and the at least one detector is configured to measure the at least one characteristic of the third radiation from the central portion.

[0140] 7. The beam measurement device according to aspect 6, further comprising a third radiation block, the third radiation block being configured to block a peripheral portion of the third radiation and transmit the central portion, the peripheral portion surrounding the central portion of the generated third radiation.

[0141] 8. The beam metrology apparatus according to any preceding aspect, wherein the at least one detector comprises at least one position sensitive detector for determining at least a first position of the third radiation.

[0142] 9. The beam measurement device according to aspect 8, wherein the at least one detector includes a first position sensitive detector and a second position sensitive detector, the first position sensitive detector and the second position sensitive detector are separated by a distance, and the first position sensitive detector and the second position sensitive detector are configured to measure the first position of the third radiation and the second position of the third radiation, respectively.

[0143] 10. The beam metrology apparatus of any preceding aspect, wherein the at least one detector comprises a wavefront detector configured to measure a wavefront of the third radiation.

[0144] 11. The beam metrology apparatus according to any preceding aspect, wherein the first nonlinear process is a high harmonic generation process.

[0145] 12. The beam metrology device according to any preceding aspect, wherein the first portion of the first radiation and the second portion of the first radiation each comprise a substantially annular beam.

[0146] 13. The beam metrology apparatus of any preceding aspect, wherein the processing unit is further operable to determine, based on the at least one characteristic of the third radiation, a control action on the at least one characteristic of the first radiation when generating the second radiation.

[0147] 14. Beam metrology apparatus according to clause 13, wherein the control action relates to the position and / or direction of the first radiation relative to a source nonlinear medium used to generate the second radiation.

[0148] 15. A beam metrology device according to clause 13 or 14, wherein the control action relates to a wavefront of the first radiation.

[0149] 16. A radiation source comprising:

[0150] A beam metrology apparatus according to aspects 13, 14 or 15;

[0151] a source nonlinear medium configured to generate second radiation via a first nonlinear process upon receiving the first portion of the first radiation; and

[0152] Radiation configuration means operable to configure at least one characteristic of said first radiation so as to enable said control action.

[0153] 17. A radiation source according to aspect 16, wherein the processing unit is operable to determine the control action of the at least one characteristic of the first radiation so as to minimise a change in at least one characteristic of the second radiation.

[0154] 18. The radiation source of aspect 17, wherein the processing unit is further configured to:

[0155] receiving a signal indicative of the at least one characteristic of the third radiation measured by the at least one detector;

[0156] determining a change in the at least one characteristic of the third radiation measured by the at least one detector; and

[0157] A control signal is generated for controlling the radiation configuration device so as to counteract the determined change of the at least one measured characteristic of the third radiation.

[0158] 19. The radiation source of any of clauses 16 to 18, further comprising a first radiation block operable to transmit substantially only the second radiation and not any of the first radiation.

[0159] 20. The radiation source of aspect 19, wherein the first radiation block comprises an aperture or a pinhole reflector.

[0160] 21. A radiation source according to aspect 19 or 20, wherein the processing unit is capable of operating to determine the control action on the at least one characteristic of the first radiation so as to minimize or reduce the amount of the first radiation transmitted through the first radiation block and / or maximize or increase the amount of the second radiation transmitted through the first radiation block.

[0161] 22. The radiation source of any one of aspects 16 to 21, further comprising an interceptor located upstream of the source nonlinear medium and configured to direct a second portion of the first radiation toward the vector sensing device nonlinear medium.

[0162] 23. The radiation source of any one of aspects 16 to 21, further comprising an interceptor located downstream of the source nonlinear medium and configured to direct a second portion of the first radiation toward the vector sensing device nonlinear medium.

[0163] 24. A radiation source according to aspect 22 or 23, the second part of the first radiation accounting for no more than 5% of the total power of the first radiation.

[0164] 25. A radiation source according to aspects 22 to 24, the second portion of the first radiation accounting for no more than 1% of the total power of the first radiation.

[0165] 26. A radiation source according to any of clauses 16 to 25, wherein the radiation configuration device comprises one or more beam steering components configured to adjust the position and / or direction of the first radiation relative to a source nonlinear medium.

[0166] 27. A radiation source according to aspect 26, wherein the one or more beam steering components include one or more actuatable mirrors.

[0167] 28. A radiation source according to any of clauses 16 to 27, wherein the radiation configuring means comprises a wavefront configuring means configured to configure a wavefront of the first radiation.

[0168] 29. A radiation source according to aspect 28, wherein the wavefront configuration device includes a spatial light modulator, a digital micromirror device or a deformable mirror.

[0169] 30. A radiation source according to any of clauses 16 to 29, wherein the source nonlinear medium is a gas medium.

[0170] 31. The radiation source of aspect 30, further comprising:

[0171] a gas delivery system configured to provide a flow of the gaseous medium; and

[0172] A gas chamber and / or a hollow core optical fiber is configured to at least partially surround the gas medium.

[0173] 32. A radiation source according to any of aspects 16 to 31, further comprising a contour configuring device operable to apply a substantially annular contour to the first radiation.

[0174] 33. A radiation source according to aspect 32, wherein the contour configuration device includes a negative axis prism and a positive axis prism.

[0175] 34. The radiation source according to any of clauses 16 to 33, further comprising focusing means for focusing a first part of the first radiation onto a source nonlinear medium so as to generate the second radiation via the first nonlinear process.

[0176] 35. The radiation source of any of aspects 16 to 34, further comprising a pump radiation source configured to emit the first radiation.

[0177] 36. A radiation source according to any of aspects 16 to 35, wherein the third radiation has a higher frequency than the first radiation.

[0178] 37. A measurement device comprising a radiation source according to any one of aspects 16 to 36.

[0179] 38. The metrology apparatus according to clause 37, comprising a scatterometer metrology device, a holographic microscope, a level sensor or an alignment sensor.

[0180] 39. A method for controlling a radiation source, comprising:

[0181] upon receiving a first portion of the first radiation by the source nonlinear medium, generating second radiation via a first nonlinear process;

[0182] When a second portion of the first radiation is received by the measurement nonlinear medium, a third radiation is generated via a second nonlinear process;

[0183] measuring at least one characteristic of the third radiation; and

[0184] Based on the at least one characteristic of the third radiation, the at least one characteristic of the first radiation is controlled when generating the second radiation.

[0185] 40. A method according to clause 39, wherein the measuring step comprises measuring at least a first position and / or direction of the third radiation.

[0186] 41. A method according to clause 40, wherein the controlling step comprises controlling the position and / or direction of the first radiation relative to a source nonlinear medium.

[0187] 42. A method according to any one of aspects 39 to 41, wherein the measuring step comprises measuring at least a wavefront of the third radiation.

[0188] 43. A method according to aspect 42, wherein the controlling step includes controlling the wavefront of the first radiation.

[0189] 44. A method according to any one of aspects 39 to 43, comprising:

[0190] receiving a signal indicative of the at least one characteristic of the third radiation measured by the at least one detector;

[0191] determining a change in the at least one characteristic of the third radiation measured by the at least one detector; and

[0192] A control signal is generated for controlling the at least one characteristic of the first radiation so as to counteract the determined change in the at least one measured characteristic of the third radiation.

[0193] 45. A method according to any one of clauses 39 to 44, comprising, after generating the second radiation, transmitting substantially only the second radiation and not any of the first radiation.

[0194] 46. ​​A method according to aspect 45, wherein controlling the at least one characteristic of the first radiation includes: controlling the at least one characteristic of the first radiation so as to minimize or reduce the amount of the first radiation transmitted and / or maximize or increase the amount of the second radiation transmitted.

[0195] 47. A method apparatus according to any one of aspects 39 to 46, wherein the second radiation and / or the third radiation has a higher frequency than the first radiation.

[0196] 48. A method apparatus according to any one of aspects 39 to 47, wherein the second nonlinear process is a second harmonic generation process.

[0197] 49. A method apparatus according to any one of aspects 39 to 48, wherein the first nonlinear process is a high harmonic generation process.

[0198] 50. A method apparatus according to any of clauses 39 to 49, wherein the third radiation comprises a central portion, and the measuring step comprises measuring the at least one characteristic of the third radiation from the central portion.

[0199] 51. A method according to aspect 50, wherein the third radiation further includes a peripheral portion surrounding the central portion, and the method further includes blocking the peripheral portion of the generated third radiation.

[0200] Although specific reference is made herein to the use of lithographic equipment in the manufacture of ICs, it should be understood that the lithographic equipment described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns of magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc. Although specific reference is made herein to embodiments in the context of lithographic equipment, embodiments may be used in other equipment. Embodiments may form part of mask inspection equipment, measurement equipment, or any equipment for measuring or processing objects such as wafers (or other substrates) or masks (or other pattern forming devices). These devices may be generally referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0201] Although specific reference may be made herein to embodiments in the context of inspection or measurement equipment, embodiments may be used in other equipment. Embodiments may form part of mask inspection equipment, lithography equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or masks (or other pattern forming devices). The term "measurement equipment" (or "inspection equipment") may also refer to inspection equipment or inspection systems (or measurement equipment or measurement systems). For example, inspection equipment including embodiments may be used to detect defects in a substrate or defects in a structure on a substrate. In such embodiments, the characteristic of interest of a structure on a substrate may relate 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.

[0202] Although specific reference has been made above to the use of the embodiments in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications such as imprint lithography where the context permits.

[0203] While the targets or target structures described above (more generally, structures on a substrate) are metrology target structures specifically designed and formed for measurement purposes, in other embodiments, the properties of interest may be measured on one or more structures that are functional parts of a device formed on a substrate. Many devices have a regular, grating-like structure. The terms "structure," "target grating," and "target structure" as used herein do not require that the structure has been specifically provided for the measurement being performed. In addition, the pitch of the metrology target may be close to the resolution limit of the optical system of the scatterometer, or may be smaller, but may be much larger than the size of a typical non-target structure, which may alternatively be a product structure manufactured by a photolithography process in the target portion C. In practice, the lines and spaces of the overlapping gratings within the target structure may be made to include smaller structures that are similar in size to the non-target structure.

[0204] Although specific embodiments have been described above, it should be understood that the present invention may be implemented in a manner different from that described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be readily apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the appended claims.

[0205] Although specific reference is made to "measurement equipment / tool / system" or "inspection equipment / tool / system", these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, an inspection equipment or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or wafer. In such embodiments, the characteristic of interest of a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of a structure, or the presence of an undesirable structure on the substrate or wafer.

[0206] Although specific reference is made to HXR, SXR and EUV electromagnetic radiation, it will be appreciated that the invention may be practiced with all electromagnetic radiation, including radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays and gamma rays, where the context permits.

[0207] Although specific embodiments have been described above, it will be appreciated that one or more features in one embodiment may also be present in a different embodiment, and features in two or more different embodiments may also be combined.

Claims

1. A beam measurement device for determining at least one characteristic of a first radiation and / or at least one characteristic of a second radiation, the second radiation being generated via a first non-linear process upon receiving a first portion of the first radiation; The beam measurement device comprises: a measurement device nonlinear medium configured to receive a second portion of the first radiation and to generate third radiation therefrom via a second nonlinear process; at least one detector configured to measure at least one characteristic of the third radiation; and A processing unit operable to determine the at least one characteristic of the first radiation and / or the at least one characteristic of the second radiation based on the at least one characteristic of the third radiation.

2. The beam measurement device according to claim 1, wherein: The third radiation has a higher frequency than the first radiation.

3. The beam measurement device according to claim 1 or 2, wherein: The second nonlinear process is a second harmonic generation process.

4. A beam measurement device according to claim 1, 2 or 3, wherein: The nonlinear medium of the measuring device is a nonlinear crystal.

5. Beam measurement device according to the preceding claim, wherein: The third radiation comprises a central portion, and the at least one detector is configured to measure the at least one characteristic of the third radiation from the central portion.

6. A beam measurement device according to any preceding claim, wherein: The at least one detector comprises at least one position sensitive detector for determining at least a first position of the third radiation.

7. The beam measurement device according to claim 6, wherein: The at least one detector includes a first position sensitive detector and a second position sensitive detector, the first position sensitive detector and the second position sensitive detector are separated by a distance, and the first position sensitive detector and the second position sensitive detector are configured to measure the first position of the third radiation and the second position of the third radiation, respectively.

8. A beam measurement device according to any preceding claim, wherein: The at least one detector comprises a wavefront detector configured to measure a wavefront of the third radiation.

9. A beam measurement device according to any preceding claim, wherein: The first nonlinear process is a high-order harmonic generation process.

10. A beam metrology device according to any preceding claim, wherein: The first portion of the first radiation and the second portion of the first radiation each comprise a substantially annular beam.

11. A beam measurement device according to any preceding claim, wherein: The processing unit is further operable to determine, based on the at least one characteristic of the third radiation, a control action on the at least one characteristic of the first radiation when generating the second radiation.

12. The beam measurement device according to claim 11, wherein: The control action relates to the position and / or direction of the first radiation relative to a source nonlinear medium used to generate the second radiation.

13. The beam measurement device according to claim 11 or 12, wherein: The controlling action relates to a wavefront of the first radiation.

14. A radiation source comprising: A beam measurement device according to claim 11, 12 or 13; a source nonlinear medium configured to generate second radiation via a first nonlinear process upon receiving the first portion of the first radiation; as well as Radiation configuration means operable to configure at least one characteristic of said first radiation so as to enable said control action.

15. A method for controlling a radiation source, comprising: upon receiving a first portion of the first radiation by the source nonlinear medium, generating second radiation via a first nonlinear process; When a second portion of the first radiation is received by the measurement nonlinear medium, a third radiation is generated via a second nonlinear process; measuring at least one characteristic of the third radiation; as well as Based on the at least one characteristic of the third radiation, the at least one characteristic of the first radiation is controlled when generating the second radiation.

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