Illumination source and associated metrology device

Abstract

An illumination source is disclosed that includes a gas delivery system having a gas nozzle. The gas nozzle includes an opening in an outlet plane of the gas nozzle. The gas delivery system is configured to provide a gas flow from the opening for generating emitted radiation at an interaction region. The illumination source is configured to receive pump radiation having a propagation direction and to provide the pump radiation in a gas stream. The geometry of the gas nozzle is adapted to shape the profile of the gas flow such that the gas density of the gas flow first increases to a maximum value and then dramatically decreases in the direction of propagation at the cutoff region.

Description

[0001] This application is a divisional application filed on October 7, 2020, the international application date of which entered the Chinese national phase on April 15, 2022, with application number 202080072661.9 and invention name “Irradiation Source and Related Measurement Equipment”.

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to EP application 19203732.3 filed on October 17, 2019 and EP application 19216363.2 filed on December 16, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0004] The present invention relates to an illumination source, a measurement device and a method of delivering a gas in an illumination source. Background Art

[0005] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus can project a pattern (often also referred to as a "design layout" or "design") from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0006] To project a pattern onto a substrate, a lithographic apparatus uses electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently used are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Compared to lithographic apparatus using radiation with a wavelength of, for example, 193 nm, lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the 4-20 nm range, such as 6.7 nm or 13.5 nm, can be used to form smaller features on a substrate.

[0007] Low-k1 lithography can be used to process features smaller than the classical resolution limit of the lithographic apparatus. In such processes, the resolution equation can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the minimum feature size printed, but in this case, half the pitch), and k1 is an empirical resolution factor. Generally, the smaller k1 is, the more difficult it is to replicate a pattern on a substrate with a shape and size similar to that planned by the circuit designer in order to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These include, for example, but are not limited to, optimization of the NA, customized illumination schemes, the use of phase-shifting patterning devices, various optimizations of the design layout (such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout), or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve pattern reproduction at low k1.

[0008] During photolithography, it is often desirable to measure the resulting structure, for example, for process control and verification. Various tools are known for making such measurements, including scanning electron microscopes, commonly used to measure critical dimensions (CDs), and specialized tools for measuring overlay—the alignment accuracy of two layers in a device. More recently, various forms of scatterometers have been developed for use in the field of photolithography.

[0009] Known examples of scatterometers typically rely on the provision of a dedicated measurement target. For example, one approach might require a target in the form of a simple grating, large enough so that the measurement beam produces a spot smaller than the grating (i.e., the grating is underfilled). In so-called reconstruction methods, 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 that observed from a real target.

[0010] In addition to measuring feature shapes by reconstruction, such equipment can be used to measure diffraction-based overlay, 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 illumination point and can be surrounded by product structures on the wafer. Examples of dark-field imaging metrology can be found in many published patent applications, such as US2011102753A1 and US20120044470A. Multiple gratings can be measured in one image using a composite grating target. Known scatterometers tend to use light in the visible or near-IR wave range, which requires the pitch of the grating to be much coarser than the actual product structure, whose characteristics are actually of interest. Such product features can be defined using X-ray radiation, extreme ultraviolet (EUV), or deep ultraviolet (DUV) with much shorter wavelengths. Unfortunately, such wavelengths are often not available or cannot be used for measurement.

[0011] On the other hand, the dimensions of modern product structures are so small that they cannot be imaged by optical metrology techniques. Small features include, for example, those formed by multiple patterning processes and / or pitch multiplication. Therefore, targets for high-volume metrology typically use features that are much larger than the product whose overlay error or critical dimension is the characteristic of interest. The measurement results are only indirectly related to the dimensions of the actual product structure and may be inaccurate because the measurement target does not suffer the same distortion under different processing during optical projection in the lithography equipment and / or 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 processing layers, which makes them less suitable for metrology applications. Other techniques, such as measuring electrical properties using contact pads, are also known, but they only provide indirect evidence of the actual product structure.

[0012] By reducing the wavelength of the radiation used during the measurement (i.e., moving towards the "soft X-ray" wavelength spectrum), smaller structures can be resolved, to increase sensitivity to structural changes in the structure and / or to penetrate further into the product structure. One such method of generating suitable high-frequency radiation (e.g., soft X-rays and / or EUV radiation) can be to use pump radiation (e.g., infrared radiation) to excite the generation medium, thereby generating emitted radiation, optionally including higher harmonics of the high-frequency radiation. Summary of the Invention

[0013] According to a first aspect of the present invention, an illumination source is provided that includes a gas delivery system, the gas delivery system including a gas nozzle. The gas nozzle includes an opening in an outlet plane of the gas nozzle. The gas delivery system is configured to provide a gas flow from the opening for generating emitted radiation at an interaction region. The illumination source is configured to receive pump radiation having a propagation direction and provide the pump radiation in the gas flow. The geometry of the gas nozzle is adapted to shape the profile of the gas flow such that the gas density of the gas flow initially increases to a maximum value and then decreases sharply in a cutoff region along the propagation direction.

[0014] Optionally, the gas flow is profiled to suppress energy divergence of the pump radiation within the gas flow, wherein the energy divergence is caused by the fact that part of the gas flow is ionized by the pump radiation.

[0015] Optionally, the maximum value is above the phase matching pressure.

[0016] Optionally, the profile of the gas flow has a cutoff region length of less than 100 μm in the propagation direction of the pump radiation.

[0017] Optionally, the shape of the opening in the exit plane is asymmetric with respect to a plane perpendicular to the propagation direction of the pump radiation.

[0018] Optionally, the width of the opening gradually increases along the propagation direction of the pump radiation.

[0019] Optionally, the shape of the opening is trapezoidal.

[0020] Optionally, the shape of the opening is an isosceles trapezoid.

[0021] Optionally, for at least a portion of the gas nozzle, an internal cross-sectional area of ​​the gas nozzle in a plane parallel to the outlet plane increases in the direction of gas flow.

[0022] Optionally, the illumination source comprises an adjustable element for varying the profile of the gas flow.

[0023] Optionally, an adjustable element is used to change the profile of the air flow during production of the emitted radiation.

[0024] Optionally, the adjustable element is configured to be positioned at least partially in the gas flow.

[0025] Optionally, the illumination source comprises a pump radiation source operable to emit pump radiation.

[0026] Optionally, the illumination source is used for high harmonic generation.

[0027] Optionally, the gas flow is provided by a gas delivery system into the evacuated or nearly evacuated space.

[0028] Optionally, the emitted radiation has a wavelength in the X-ray or EUV range, wherein the wavelength is in the range of 0.01 nm to 100 nm, optionally 0.1 nm to 100 nm, optionally 1 nm to 100 nm, optionally 1 nm to 50 nm or optionally 10 nm to 20 nm.

[0029] Optionally, in operation, the emitted radiation is directed to a target on the wafer.

[0030] Optionally, the emitted radiation is used for metrology measurements.

[0031] Optionally, the radiation source includes a temperature control assembly.

[0032] According to another aspect of the present invention, a method for delivering a gas in an illumination source is provided, comprising providing a gas flow from an opening in an outlet plane of a gas nozzle for receiving pump radiation having a propagation direction and for producing emitted radiation at an interaction region. The geometry of the gas nozzle is adapted to shape the profile of the gas flow such that the gas density of the gas flow initially increases to a maximum value and then sharply decreases in a cut-off region along the propagation direction.

[0033] According to another aspect of the present invention, there is provided a metrology device comprising the illumination source as described above.

[0034] According to another aspect of the present invention, an inspection device is provided, comprising the illumination source as described above.

[0035] According to another aspect of the present invention, there is provided a lithographic apparatus comprising an illumination source as described above.

[0036] According to another aspect of the present invention, there is provided a lithography cell comprising the illumination source as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0038] - Figure 1 shows a schematic diagram of a lithographic apparatus;

[0039] - Figure 2 shows a schematic diagram of a lithography unit;

[0040] - Figure 3 Depicted is a schematic representation of overall lithography, showing the collaboration between three key technologies used to optimize semiconductor manufacturing;

[0041] - Figure 4 Schematic representation of the scatterometry setup;

[0042] - Figure 5shows a schematic representation of a metrology apparatus in which EUV and / or SXR radiation is used;

[0043] - Figure 6 A simplified schematic diagram of an illumination source is shown;

[0044] - Figure 7 Depicted are (a) a schematic diagram of an asymmetric opening, (b) a plot of gas density distribution, and (c) a contour plot of the gas flow;

[0045] - Figure 8 Depicted are (a) gas density distribution diagram, (b) schematic diagram of the gas nozzle, and (c) contour diagram of the gas flow;

[0046] - Figure 9 The radiation intensity distribution and contour of the gas flow are depicted in (a) and (b);

[0047] - Figure 10 Depicting (a) a schematic diagram of the gas profile and (b) a plot of the output power of the emitted radiation;

[0048] - Figure 11 schematically illustrates a gas delivery system with a temperature control assembly;

[0049] Figure 12 Flowchart including steps in a method of generating emitted radiation. DETAILED DESCRIPTION

[0050] As used herein, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation and particle radiation, including ultraviolet radiation (e.g., having a wavelength of 365, 248, 193, 157 or 126 nm), EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5-100 nm), X-ray radiation, electron beam radiation, and other particle radiation.

[0051] The terms "reticle," "mask," or "patterning device," as used herein, should be broadly interpreted to refer to a general patterning device that can be used to impart an incident radiation beam with a patterned cross-section that corresponds to the pattern to be produced in a target portion of the substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0052] Figure 1A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, EUV radiation, or X-ray radiation); a mask support (e.g., a mask table) T configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; a substrate support (e.g., a wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0053] 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 so that it has a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0054] The term "projection system" PS as used herein should be broadly interpreted as encompassing various types of projection systems, including refractive, reflective, diffractive, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate depending on the exposure radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0055] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g., water) so as to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US 6,952,253, the entire contents of which are incorporated herein by reference.

[0056] 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 for preparing a substrate W for subsequent exposure may be performed on a substrate W on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0057] In addition to the substrate support WT, the lithographic apparatus LA can include a measurement stage. The measurement stage is configured to hold sensors and / or cleaning equipment. The sensors can be arranged to measure characteristics of the projection system PS or characteristics of the radiation beam B. The measurement stage can accommodate multiple sensors. The cleaning equipment can be configured to clean a portion of the lithographic apparatus, such as a portion of the projection system PS or a portion of a system for providing immersion liquid. The measurement stage can be moved beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0058] 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 beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, a first positioner 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-lane alignment marks.

[0059] like Figure 2 As shown, the lithography apparatus LA may form part of a lithocell LC, sometimes also referred to as a lithocell or a (lithography) cluster, which typically also includes equipment for performing pre- and post-exposure processing on a substrate W. Conventionally, these include a spin coater SC to deposit a resist layer, a developer DE to develop the exposed resist, a chill plate CH, and a bake plate BK, for 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 the substrates W between the different processing equipment, and transfers the substrates W to a loading bay LB of the lithography apparatus LA. The equipment in the lithocell, which is also often collectively referred to as a track, may be under the control of a track control unit TCU, which itself may be controlled by a monitoring system SCS, which in turn may control the lithography apparatus LA, for example via the litho control unit LACU.

[0060] In photolithography processes, it is frequently desirable to measure the resulting structures, for example, for process control and verification. The tool used to perform such measurements may be referred to as a metrology tool MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow measurement of photolithography process parameters by having a sensor in the pupil or in a plane conjugate to the pupil of the scatterometer objective (the measurement is often referred to as pupil-based measurement), or by having a sensor in the image plane or in a plane conjugate to the image plane (in which case the measurement is often referred to as image-based 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, which are hereby incorporated by reference in their entirety. The scatterometers described above can measure gratings using light ranging from soft X-rays, deep UV and visible light to near IR wavelengths.

[0061] In order to ensure that substrates W exposed by the lithographic apparatus LA are correctly and consistently exposed, it is desirable to inspect the substrates to measure characteristics of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CDs), etc. For this purpose, inspection tools and / or metrology tools (not shown) may be included in the lithography cell LC. If errors are detected, adjustments may be made to the exposure of subsequent substrates or to other processing steps to be performed on the substrate W, particularly if the inspection is performed before other substrates W from the same batch or stack are yet to be exposed or processed.

[0062] The inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrate W, in particular how properties vary from one substrate W to another or how properties associated with 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, for example, may be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a standalone apparatus. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure), a semi-latent image (the image in the resist layer after a post-exposure bake step (PEB), a developed resist image (in which exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0063] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measurement signals to reconstruct or calculate the properties of the grating. For example, such a reconstruction can be obtained by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0064] In a second embodiment, the scatterometer MT is a spectral scatterometer MT. In such a spectral scatterometer MT, radiation emitted by a radiation source is directed onto a target, and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e., a measure of intensity as a function of wavelength). From this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0065] In a third embodiment, the scatterometer MT is an ellipsometer. Ellipsometers allow parameters of a photolithographic process to be determined by measuring scattered radiation in each polarization state. Such metrology equipment emits polarized light (e.g., linear, circular, or elliptical) by using, for example, appropriate polarization filters in the illumination portion of the metrology equipment. A source suitable for the metrology equipment can also provide polarized radiation. Various embodiments of conventional ellipsometers are described in U.S. patent applications Ser. Nos. 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, the entire contents of which are incorporated herein by reference.

[0066] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned gratings or periodic structures by measuring an asymmetry in the reflection spectrum and / or the detection configuration, which asymmetry is related to the degree of overlay. The two (possibly overlaid) grating structures may be applied in two different layers (not necessarily continuous 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 can be clearly distinguished. This provides a direct method of measuring misalignment in the gratings. Further examples of measuring the overlay error between two layers comprising a periodic structure when the target is measured by the asymmetry of the periodic structure can be found in PCT patent application publication number WO2011 / 012624 or in U.S. patent application US 20160161863, the entire contents of which are incorporated herein by reference.

[0067] Other parameters of interest may be focus and dose. Focus and dose can be determined simultaneously by scatterometry (or by scanning electron microscopy), as described in U.S. Patent Application No. US2011-0249244, the entire contents of which are incorporated herein by reference. A single structure can be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the focus energy matrix (FEM—also known as the focus exposure matrix). If these unique combinations of critical dimensions and sidewall angles are available, focus and dose values ​​can be uniquely determined from these measurements.

[0068] The metrology target can be a collection of composite gratings formed by a lithography process, primarily in resist, but also after, for example, an etching process. The pitch and linewidth of the structures in the grating can be strongly dependent on the measurement optics (particularly the NA of the optics) to be able to capture the diffraction orders from the metrology target. As previously described, the diffraction signal can be used to determine the offset between two layers (also known as "overlay"), or to reconstruct at least a portion 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 to control at least a portion of the lithography process. The target can have smaller sub-segments configured to mimic the dimensions of functional portions of the design layout in the target. Due to this sub-segmentation, the target will behave more similarly to the functional portions of the design layout, allowing the overall process parameter measurement to better resemble that of the functional portions of the design layout. The target can be measured in either underfill mode or overfill mode. In underfill mode, the measurement beam produces a spot that is smaller than the entire target. In 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, thereby determining different process parameters simultaneously.

[0069] The overall measurement quality of a lithography parameter using a particular target is determined at least in part by the measurement recipe used to measure the lithography parameter. The term "substrate measurement recipe" can include: one or more parameters of the measurement itself, one or more parameters of the one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, the one or more parameters being measured can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and the like. One of the criteria for selecting a measurement recipe can, for example, be the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0070] The patterning process in the lithographic apparatus LA can be one of the most critical steps in the process, requiring high precision in the dimensional design and placement of the structures on the substrate W. To ensure this high precision, three systems can be combined in a Figure 3 In the so-called "holistic" control environment schematically shown in Figure 1 , one of these systems is a lithography apparatus LA that is (virtually) connected to a metrology tool MET (a second system) and a computer system CL (a third system). The key to this "holistic" environment is to optimize the collaboration between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device)—perhaps within which variations in process parameters in the lithography process or patterning process are permitted.

[0071] The computer system CL can use the (partial) design layout to be patterned to predict which resolution enhancement technology to use and perform computational lithography simulations and calculations to determine which mask layout and lithographic equipment settings achieve the maximum total process window for the patterning process (in Figure 3 The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from a metrology tool MET) to predict whether resolution enhancement techniques may be employed due to, for example, suboptimal processing (e.g., in the process window). Figure 3 There is a defect (depicted by the arrow pointing to "0" in the second level plate SC2).

[0072] The metrology tool MET may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify, for example, possible drifts in the calibration state of the lithographic apparatus LA (e.g., Figure 3 (depicted by multiple arrows in the third balance plate SC3).

[0073] In photolithographic processes, it is frequently desirable to perform measurements on the structures produced, for example for process control and verification. Various tools are known for performing such measurements, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers. Examples of known scatterometers typically rely on providing dedicated metrology targets, such as underfilled targets (targets in the form of simple gratings or overlaid gratings in different layers, which are large enough so that the measurement beam produces a spot smaller than the grating) or overfilled targets (whereby the illumination spot partially or completely encompasses the target). Furthermore, the use of metrology tools, such as angle-resolved scatterometers, which illuminate underfilled targets such as gratings, allows the use of so-called reconstruction methods, in which the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0074] Scatterometers are versatile instruments that allow measurement of parameters of a lithographic process by having a sensor in the pupil or in a plane conjugated to the pupil of the scatterometer's objective (measurements often referred to as pupil-based measurements), or by having a sensor in the image plane or in a plane conjugated to the image plane (in which case measurements are often referred to as image-based or field-based measurements). Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US2011024924, US20110026032, or EP1,628,164A, all of which are incorporated herein by reference. The aforementioned scatterometers can measure multiple targets from multiple gratings in a single image using light from soft X-rays, the far ultraviolet, and the visible to near IR wave range.

[0075] Figure 4 A measurement device such as a scatterometer is shown. It comprises a broadband (e.g. white light) radiation projector 2 which projects radiation 5 onto a substrate W. The reflected or scattered radiation 10 is transmitted to a spectrometer detector 4 which measures the spectrum 6 of the specularly reflected radiation (i.e. a measure of the intensity as a function of wavelength). From this data, the spectrum can be processed by a processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression or by comparing the spectrum with the spectrum of the substrate. Figure 4 The library of simulated spectra shown at the bottom is compared to reconstruct the structure or profile that produced the detected spectrum. Typically, for reconstruction, the general form of the structure is known, and some parameters are assumed based on knowledge of the process used to fabricate the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. This scatterometer can be configured as either a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0076] As an alternative to optical metrology methods, the use of soft X-rays or EUV radiation, for example, with a wavelength range between 0.1 nm and 100 nm, or alternatively between 1 nm and 50 nm, or alternatively between 10 nm and 20 nm, is also contemplated. An example of a metrology tool operating in one of the aforementioned wavelength ranges is transmission small-angle X-ray scattering (T-SAXS, such as US 2007224518A, the contents of which are incorporated herein by reference in their entirety). Lemaillet et al., “Intercomparison between optical and X-ray scatterometry measurements of finfet structures,” Proc. of SPIE, 2013, 8681, discuss profile (CD) measurements using T-SAXS. Reflectometry techniques using grazing-incidence X-rays (GI-XRS) and extreme ultraviolet (EUV) radiation are known for measuring properties of layer stacks and films on substrates. Within the general field of reflectometry, goniometric and / or spectroscopy techniques can be applied. In goniometrics, the variation in the reflected beam with different angles of incidence is measured. Spectral reflectometers, on the other hand, measure the spectrum of wavelengths reflected at a given angle (using broadband radiation). For example, EUV reflectometers have been used to inspect mask blanks (patterning devices) before manufacturing them for EUV lithography.

[0077] It's possible that the range of applications makes the use of soft X-rays or wavelengths in the EUV domain insufficient. Consequently, published patent applications US20130304424A1 and US2014019097A1 (Bakeman et al. / KLA) describe hybrid metrology techniques in which X-ray measurements and optical measurements at wavelengths between 120 nm and 2000 nm are combined to obtain measurements of parameters such as CD. CD measurements are obtained by coupling one or more common components and X-ray and optical mathematical models. The contents of the cited US patent applications are incorporated herein by reference in their entirety.

[0078] Figure 5 A schematic diagram of a metrology device 302 is shown, wherein radiation with a wavelength ranging from 0.1 nm to 100 nm can be used to measure parameters of structures on a substrate. Figure 5 The metrology device 302 shown is suitable for use in the soft X-ray or EUV domain.

[0079] Figure 5 A schematic physical arrangement of a metrology apparatus 302 comprising a spectral scatterometer using EUV and / or SXR radiation at grazing incidence is shown by way of example only. Another form of inspection apparatus may be provided in the form of an angle-resolved scatterometer using radiation at normal or near normal incidence, similar to conventional scatterometers operating at longer wavelengths.

[0080] The inspection apparatus 302 comprises a radiation source or so-called illumination source 310 , an illumination system 312 , a substrate holder 316 , detection systems 318 , 398 and a metrology processing unit (MPU) 320 .

[0081] In this example, the radiation source 310 is used to generate EUV or soft x-ray radiation, which can be based on high harmonic generation (HHG) technology. 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 generates infrared radiation pulses that can last, for example, less than 1 ns (1 nanosecond) per pulse, with a pulse repetition rate of up to several megahertz as required. The wavelength of the infrared radiation can, for example, be in the range of 1 μm (1 micrometer). Optionally, the laser pulses are transmitted as first pump radiation 340 to the gas delivery system 332, where a portion of the radiation is converted into a higher frequency than the first radiation in the gas, becoming emitted radiation 342. A gas supply 334 supplies a suitable gas to a gas delivery system 332 where the gas is optionally ionized by a power source 336. The gas delivery system 332 may be a cutting tube as will be discussed below.

[0082] 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 several wavelengths. The emission divergence angle of the emitted radiation can be wavelength-dependent. The gas provided by the gas delivery system 332 defines the gas target, which can be a gas flow or a static volume. The gas can be, for example, an inert gas such as neon (Ne), helium (He), or argon (Ar). N2, O2, Ar, Kr, and Xe gases are contemplated. These can be selectable within the same device. For example, when imaging structures of different materials, different wavelengths will provide different levels of contrast. For example, to inspect metal structures or silicon structures, different wavelengths can be selected for imaging features in (carbon-based) resists or for detecting contamination of such different materials. One or more filtering devices 344 can be provided. For example, filters such as aluminum (Al) or zirconium (Zr) thin films can be used to cut off the fundamental IR radiation, preventing it from entering the inspection device. A grating (not shown) can be provided to select one or more specific harmonic wavelengths from the generated harmonic wavelengths. Some or all of the beam path can be contained in a vacuum or near-vacuum environment, keeping in mind that SXR radiation is absorbed when traveling in air. Various components of the radiation source 310 and illumination optics 312 can be adjustable to achieve different measurement 'recipes' in the same device. For example, different wavelengths and / or polarizations can be selected.

[0083] Depending on the material of the structure being inspected, different wavelengths may provide the desired level of penetration into the underlying layers. In order to distinguish 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 1-20 nm, or alternatively in the range of 1-10 nm, or alternatively in the range of 10-20 nm may be selected. Wavelengths shorter than 5 nm may suffer from very low critical angles when reflected from materials of interest in semiconductor manufacturing. Therefore, selecting a wavelength greater than 5 nm will 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.

[0084] Filtered beam 342 travels from radiation source 310 into inspection chamber 350, where a substrate W including a structure of interest is held in a measurement position for inspection by substrate holder 316. The structure of interest is labeled T. The atmosphere within inspection chamber 350 is maintained at or near a vacuum by vacuum pump 352, allowing EUV radiation to pass through the atmosphere without excessive attenuation. Illumination system 312 focuses 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 aforementioned US patent application US2017 / 0184981A1 (the contents of which are incorporated herein by reference in their entirety). When projected onto the structure of interest, focusing is performed to obtain a circular or elliptical spot S with a diameter of less than 10 μm. Substrate holder 316 includes, for example, an X-Y translation stage and a rotation stage, by which any portion of substrate W can be brought into the beam's focal point and into a desired orientation. Consequently, a radiation spot S is formed on the structure of interest. Alternatively or additionally, the substrate holder 316 comprises, for example, a tilt stage, which can tilt the substrate W at a certain angle to control the incident angle of the focused light beam on the structure T of interest.

[0085] 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 310, and the filter can include information about the spectrum of filtered beam 342 and / or the intensity of different wavelengths in the filtered beam.

[0086] The reflected radiation 360 is captured by the detector 318 and the spectrum is provided to the processor 320 for use in calculating the properties of the target structure T. The illumination system 312 and the detection system 318 thus form an inspection device. The inspection device may include a soft X-ray and / or EUV spectral reflectometer of the type described in US2016282282A1, the contents of which are incorporated herein by reference in their entirety.

[0087] If the target T has a certain periodicity, the radiation of the focused beam 356 can 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, the drawn 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 a further detection system 398 that detects and / or images at least a portion of the diffracted radiation 397. Figure 5398, but embodiments of the inspection apparatus 302 may also include more than one further detection system 398, which are arranged at different locations to detect and / or image the diffracted radiation 397 in a plurality of diffraction directions. In other words, the (higher) diffraction orders of the focused radiation beam impinging on the target T 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 about the diffracted light 397 and / or may include an image obtained from the diffracted light 397.

[0088] To help align and focus the spot S with the desired product structure, the inspection apparatus 302 may also use auxiliary radiation to provide auxiliary optics under the control of the metrology processor 320. The metrology processor 320 may also communicate with a position controller 372 that operates the translation, rotation, and / or tilt stages. The processor 320 receives high-precision feedback regarding the position and orientation of the substrate via sensors. For example, the sensor 374 may include an interferometer that can provide accuracy in the picometer range. During operation of the inspection apparatus 302, spectral data 382 captured by the detection system 318 is transmitted to the metrology processing unit 320.

[0089] As described above, alternative forms of inspection devices use soft X-rays and / or EUV radiation at normal incidence or near normal incidence, for example to perform diffraction-based asymmetry measurements. Two types of inspection devices may be provided in a hybrid measurement system. The performance parameters to be measured may include overlay (OVL), critical dimension (CD), the focus of the lithography device when printing target structures, coherent diffraction imaging (CDI) and resolution overlay (ARO) measurements. The soft X-ray and / or EUV radiation may, for example, have a wavelength of less than 100 nm, for example using radiation in the range of 5-30 nm, or alternatively radiation in the range of 10 nm to 20 nm. The radiation may be narrowband or broadband in nature. The radiation may have discrete peaks in a particular wavelength band, or may have a more continuous characteristic.

[0090] Similar to light scatterometers used in current production equipment, inspection device 302 can be used to measure structures within resist materials processed within a lithography cell (after development inspection or ADI) and / or to measure structures after they are formed in harder materials (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.

[0091] The measurement tool MT, including but not limited to the scatterometer described above, 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 parts of the electromagnetic spectrum. The measurement tool MT can use radiation to measure or inspect properties and aspects of a substrate, such as a photolithographic exposure pattern on a semiconductor substrate. The type and quality of the measurement can depend on several characteristics 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 smaller wavelengths are able to measure smaller features, for example due to the diffraction limit. In order to measure features with small dimensions, it is preferred to use radiation with a short wavelength, such as EUV 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 access to a source that provides radiation at that / these 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 (e.g., 1 nm to 100 nm) and / or soft X-ray (SXR) radiation (e.g., 0.1 nm to 10 nm), sources can use high harmonic generation (HHG) to obtain radiation of the desired wavelength. One of the challenges faced in the development of these sources is how to effectively couple the emitted radiation out of the generation device and separate the emitted radiation from the radiation used to drive the process.

[0092] Figure 6 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. Figure 5 One or more of the features of the illumination source in the metrology tool described above may also be present in the illumination source 600 as appropriate. The illumination source 600 comprises a chamber 601. The illumination source 600 is configured to receive pump radiation 611 having a propagation direction indicated by an arrow. The pump radiation 611 shown here is an example of pump radiation 340 from the pump radiation source 330, as shown in FIG. Figure 5 As shown. Pump radiation 611 can be directed into chamber 601 via radiation input 605, which can be a viewport made of fused silica or a comparable material. Pump radiation 611 can have a Gaussian or hollow (e.g., annular) cross-sectional profile and can be incident on (optionally focused on) a gas flow 615 within chamber 601, gas flow 615 having a flow direction indicated by a second arrow. Gas flow 615 comprises a small volume (e.g., a few cubic millimeters) of a specific gas (e.g., an inert gas, optionally helium, argon or neon, nitrogen, oxygen or carbon dioxide), where the gas pressure is above a certain value. Gas flow 615 can be a steady flow. Other media can also be used, such as metal plasma (e.g., aluminum plasma).

[0093] The gas delivery system of the radiation source 600 is configured to provide a gas flow 615. The radiation source 600 is configured to provide pump radiation 611 in the gas flow 615 to drive the generation of emitted radiation 613. The region where at least most of the emitted radiation 613 is generated is called the interaction region. The interaction region can vary from tens of microns (for tightly focused pump radiation) to several millimeters or centimeters (for moderately focused pump radiation), or even up to several meters (for very loosely focused pump radiation). Optionally, the gas flow 615 is provided by the gas delivery system into an evacuated or nearly evacuated space. The gas delivery system includes a gas nozzle 609, such as Figure 6 As shown, it includes an opening 617 in the outlet plane of the gas nozzle 609. The gas flow 615 is provided from the opening 617. In almost all prior art, the gas nozzle has a cut-tube geometry, which is a uniform cylindrical internal geometry, and the shape of the opening in the outlet plane is circular. As described in patent application CN101515105B, elongated openings are also used. The entire content of CN101515105B is incorporated herein by reference.

[0094] An example of such a gas delivery system is described, for example, in SJ Goh et al., "Single-shot fluctuations in waveguided high-harmonic generation", Opticsexpress, 23(19), 24888, compared to the use of other gas delivery systems (e.g., gas-filled capillaries). The advantage of using a gas nozzle (optionally with a cut tube or an adapted geometry) is that the alignment of the pump radiation and the gas target / stream is easier. When using a gas-filled capillary, if the alignment is not accurate enough, the gas delivery may burn due to the high power of the pump radiation.

[0095] The size of the gas nozzle 609 can also be used in scaled-up or scaled-down form, ranging from micrometer-sized nozzles to meter-sized nozzles. This wide size range comes from the fact that the setup should be scaled so that the intensity of the pump radiation at the gas flow ends up in a specific range that can be beneficial for the emitted radiation, which requires different size designs for different pump radiation energies, which can be pulsed lasers, and the pulse energy can vary from tens of microjoules to several joules.

[0096] Due to the interaction of the pump radiation 611 with the gas atoms of the gas stream 615, the gas stream 615 converts part of the pump radiation 611 into emitted radiation 613, which may be Figure 5An example of emitted radiation 342 is shown. The central axis of the emitted radiation 613 can be collinear with the central axis of the incident pump radiation 611. The 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 from 0.1 nm to 100 nm, optionally from 1 nm to 100 nm, optionally from 1 nm to 50 nm, or optionally from 10 nm to 20 nm.

[0097] In operation, the emitted beam of radiation 613 may pass through the radiation output 607 and may then be illuminated by the illumination system 603 (which may be Figure 5 The emitted radiation 613 can be directed, optionally focused, to a target on the wafer.

[0098] 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 can be evacuated or nearly evacuated. Since the central axis of the emitted radiation 613 can be collinear with the central axis of the incident pump radiation 611, it may be necessary to block the pump radiation 611 from passing through the radiation output 607 and into the illumination system 603. This can be accomplished by placing Figure 5 The filter arrangement 344 shown is implemented by incorporating it into the radiation output 607, which is placed in the emitted beam path and is opaque or nearly opaque to the drive radiation (e.g., opaque or nearly opaque to infrared or visible light), but at least partially transparent to the emitted radiation beam. The filter can be made of zirconium. When the pump radiation 611 has a hollow (optionally annular) cross-sectional profile, the filter can be a hollow (optionally annular) block.

[0099] Methods, apparatus, and assemblies are described herein for obtaining emitted radiation, optionally at high harmonic frequencies of pump radiation. The radiation generated by this process—optionally HHG, which uses nonlinear effects to generate radiation at harmonic frequencies of the provided pump radiation—can be provided as radiation in a metrology tool MT for inspecting and / or measuring a substrate. The substrate can be a lithographically patterned substrate. The radiation obtained by this process can also be provided in a lithographic apparatus LA and / or a lithographic cell LC. The pump radiation can be pulsed radiation, which can provide high peak intensity with short pulse durations.

[0100] 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 800 nm to 1500 nm. The pump radiation may include radiation having a wavelength in the range of 900 nm to 1300 nm. The pump radiation may include radiation having a wavelength in the range of 100 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.

[0101] 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 (EUV), soft X-ray (SXR), and / or hard X-ray portions of the electromagnetic spectrum. The emitted radiation 613 may include wavelengths in the range of 0.01 nm to 100 nm. The emitted radiation 613 may include wavelengths in the range of 0.1 nm to 100 nm. The emitted radiation 613 may include wavelengths in the range of 0.1 nm to 50 nm. The emitted radiation 613 may include wavelengths in the range of 1 nm to 50 nm. The emitted radiation 613 may include wavelengths in the range of 10 nm to 20 nm.

[0102] The illumination source may be arranged in, for example, the metrology apparatus MT, the inspection apparatus, the lithographic apparatus LA and / or the lithocell LC.

[0103] The properties of the emitted radiation used to perform the measurement can affect the quality of the obtained measurement. For example, the shape and size of the transverse beam profile (cross-section) of the radiation beam, the intensity of the radiation, the power spectral density of the radiation, etc. can affect the measurement performed by the radiation. Therefore, it is beneficial to have a source of radiation that provides properties that result in high-quality measurements.

[0104] When the emitted radiation is generated by irradiating a radiation generating target (e.g. a gas, plasma or solid sample) with pump radiation, typically the conversion efficiency, which may be the ratio of the number of photons of the emitted radiation to the number of photons of the pump radiation, is small. It is therefore challenging to develop illumination sources that generate high output powers in order to enable metrology measurements with acceptably high output powers. Therefore, in order to achieve high output powers it is necessary to ensure that at least some of the atoms in the radiation generating target emit their radiation at least partially coherently, meaning that the electromagnetic waves constituting the emitted radiation are in phase, which is known as coherent emission. A benefit may be that in case of coherent emission the total intensity of the emitted radiation caused by N radiating atoms is equal to N 2 This can lead to a much higher total power than in the case of atomic incoherent emission, for which the total intensity is proportional to N.

[0105] A condition for coherent emission can be that the electromagnetic waves constituting the pump radiation and the emitted radiation propagate through the radiation generating target with the same phase velocity (i.e., crest velocity), which is called phase matching. A focused beam naturally acquires a higher phase velocity than an unfocused beam, so the pump radiation, which can be focused radiation, naturally has a higher phase velocity than the emitted radiation, thereby preventing coherent emission. In order to compensate for this difference in natural wave velocities, the pump radiation can be slowed down by generating a sufficiently high refractive index in the target. This can be achieved by ensuring a sufficiently high gas pressure (called phase matching pressure). The maximum value of the gas density of the gas flow may need to be above the phase matching pressure to achieve high output power.

[0106] During the generation of emitted radiation (optional HHG), the gas flow may be partially ionized by the pump radiation and a plasma may be generated. The effect of the plasma is to deteriorate the intensity distribution of the pump radiation through an effect known as plasma defocusing, thereby reducing the peak intensity that may be required for high output power and preventing the intensity distribution of the pump radiation that is required for optimal emitted radiation quality. The intensity of the plasma defocusing effect may increase with the gas pressure in the gas flow. However, as mentioned above, the gas pressure may be determined by the requirements of phase matching and cannot be freely selected. Therefore, it is often a challenge to provide a gas pressure that is high enough to ensure phase matching and a gas pressure that is low enough to prevent plasma defocusing effects. The typical result is often a compromise, where both are more or less acceptable, but neither is optimal, which means that the gas pressure can be reduced from the optimal phase matching pressure to limit plasma defocusing, resulting in reduced output power.

[0107] When a typical gas nozzle is used for gas delivery, which means that the gas nozzle has a cut-tube geometry with a uniform cylindrical internal geometry and the shape of the opening in the outlet plane is circular, the gas flow has a profile with a typical symmetrical or nearly symmetrical shape along the propagation direction of the pumping radiation, such as Figure 9 (a) Visible. Figure 9(a) shows a simulated intensity distribution 901 in grayscale of pump radiation, in this example a laser beam, propagating from left to right (in the direction of arrow 921) through neon gas with a typical symmetrical gas flow profile. This means that the gas profile 911 is such that the gas density of the gas flow, represented by the P-axis, is symmetrical along the propagation direction, represented by the X-axis. For gas profile 911, the P-axis represents the gas density, while for the grayscale intensity distribution, the P-axis represents the radial direction of the pump radiation perpendicular to the propagation direction. Dark colors in plot 901 represent low pump radiation intensity, while light colors represent high pump radiation intensity. Arrow 921 indicates the central axis and propagation direction of the pump radiation. Two vertical dashed lines 931 represent the region of gas flow 615 along the propagation direction of the pump radiation, which contributes the majority of the emitted radiation. The gas flow 615 between the two dashed lines 931 can be referred to as the interaction region.

[0108] With a symmetrical gas distribution 911, plasma defocusing shifts the majority of the pump radiation (optionally pump laser radiation) away from the axis 921, thereby limiting the highest pump radiation intensity to only the first part of the gas flow (white area). In addition, plasma defocusing results in a reduction in intensity in the gas region between the dashed lines, which contributes most significantly to the output power. This simulation example shows that plasma formation in the gas flow significantly reduces the peak laser intensity and limits the high-intensity region of pump radiation to only the upstream side of the gas flow, in Figure 9 In the example shown, the upstream side is the left side of the gas flow. Therefore, plasma defocusing affects the output power.

[0109] The characteristics of the emitted radiation can be tailored by varying the profile of the gas flow or the so-called gas profile or the so-called gas flow profile - optionally varying the gas density of the gas flow along the propagation direction. The profile of the gas flow can suppress energy divergence of the pump radiation within the gas flow, wherein the energy divergence is caused by the fact that part of the gas flow is ionized by the pump radiation (optionally by plasma defocusing effects). The profile of the gas flow can be shaped by adjusting the geometry of the gas nozzle. For example, the shape of the opening in the outlet plane is typically symmetrical with respect to a plane perpendicular to the propagation direction of the pump radiation, which results in a gas flow with a symmetrical or approximately symmetrical distribution along the propagation direction, e.g. Figure 9 Gas profile in (a) 911. The shape of the opening in the exit plane can also be made asymmetric with respect to the plane perpendicular to the propagation direction of the pump radiation, which is called an asymmetric opening, e.g. Figure 7 The opening 701 in (a) will be discussed later in the text. Asymmetric openings can produce airflow with an asymmetric shape along the propagation direction, e.g. Figure 7(c) Gas profiles 719, 729, and 739, which will also be discussed later in the text, may be due to the asymmetric opening resulting in a gas flow of asymmetric gas density profile can eliminate or suppress the plasma defocusing effect at sufficiently high gas pressure (optionally above the phase matching pressure) and lead to higher output power, optionally with better radiation quality.

[0110] In addition to modifying the shape of the opening in the outlet plane, other parameters of the gas nozzle geometry can also be adapted to shape the gas flow profile. Optionally, the internal cross-sectional area of ​​the gas nozzle in a plane parallel to the outlet plane can be adjusted. Optionally, for at least a portion of the gas nozzle, the internal cross-sectional area can increase, optionally linearly, along the direction of gas flow. Optionally, the radiation source can include an adjustable element for modifying the gas flow profile. Optionally, the adjustable element can be used to modify the gas flow profile during the generation of the emitted radiation. Optionally, the aforementioned parameters of the gas nozzle geometry can be adaptively combined.

[0111] exist Figure 10 An example of the effect of the gas flow profile on the output power of the emitted radiation is simulated in . The gas flow length and the pump radiation are fixed. Figure 10 (a) shows a schematic diagram of the gas pressure profile along the propagation direction X. The vertical axis P represents the pressure or gas density of the airflow. The two vertical lines represent the gas density at two specific positions on the gas profile, which are respectively referred to as the first pressure and the second pressure. Along the propagation direction X, the gas density gradually increases before the first pressure 1011 and gradually decreases after the second pressure 1013. The gas density changes linearly between the first pressure 1011 and the second pressure 1013. Note that the linear change between the first pressure 1011 and the second pressure 1013 is only for the purpose of simplifying the simulation, and in practice the change in gas density between the two vertical lines does not need to be linear. Note that the shape of the simulated gas profile is only an example, and in practice the profile of the airflow may have other shapes.

[0112] During the simulation, the first pressure and the second pressure are independently varied over a set of different values, and the resulting output power of the emitted radiation as a function of the first pressure and the second pressure, represented by axes P1013 and P1011, is shown in a grayscale graph of output power 1003, as shown in FIG. Figure 10 (b) is shown. Dark colors in the plot 1003 represent low output power, while light colors represent high output power. Solid line 1025 represents a first pressure pair and a second pressure pair, where the first pressure and the second pressure are equal, for example, with Figure 9 The airflow of the typical symmetrical shape of the profile 911 in the propagation direction in (a). The dotted line 1027 represents the case where the first pressure is half of the second pressure. Figure 9The two examples 901 and 903 in (a) and (b) are represented by crosses 1023 and 1021 respectively. Figure 9 From the simulation results in (b), it can be seen that the highest output power is obtained in the region close to the y-axis, that is, in the region where the first pressure is relatively low.

[0113] Figure 9 (b) shows Figure 9 A comparative simulation of the simulation in (a) shows a pump radiation intensity distribution 903 with an asymmetric gas flow profile 913, where the gas density of the gas flow initially increases to a maximum and then sharply decreases in a cutoff region along the propagation direction. The interaction region may be followed by a sharp cutoff region along the propagation direction to reduce reabsorption of the emitted radiation by the gas flow. Optionally, the cutoff region length of the gas flow profile in the propagation direction of the pump radiation is less than 500 μm. Optionally, the cutoff region length of the gas flow profile in the propagation direction of the pump radiation is less than 200 μm. Optionally, the cutoff region length of the gas flow profile in the propagation direction of the pump radiation is less than 100 μm. Optionally, the cutoff region length of the gas flow profile in the propagation direction of the pump radiation is less than 50 μm. Arrow 923 indicates the central axis and propagation direction of the pump radiation. Two vertical dashed lines 933 indicate the region of gas flow 615 along the propagation direction of the pump radiation that contributes most to the emitted radiation. The gas flow 615 between the two dashed lines 933 may be referred to as the interaction region. The maximum gas pressure here is Figure 9 Same as in (a).

[0114] It should be understood that the airflow distribution 913 is only exemplary to illustrate the principles of the present disclosure. In practice, any similar airflow profile, such as having convex or twist, can have similar effects, and many specific embodiments can be envisioned based on the principles of the present disclosure.

[0115] exist Figure 9 (b) and Figure 9 (a), the plasma defocusing effect is smaller because the upstream gas pressure is lower. As a result, the high intensity region (bright area) extends to the downstream part of the gas flow profile. Figure 9 In the example shown, this downstream portion is the right part of the airflow, resulting in a higher output power of the emitted radiation. Figure 9 Compared with the case with symmetrical gas profile in (a), Figure 9 The asymmetric gas profile in (b) results in an increase in the total output power by approximately 50%.

[0116] The effects of plasma defocusing may be most significant when it occurs in the upstream portion, as the pump radiation may subsequently be affected along the entire length of the gas flow. In contrast, good phase matching may be most important in the downstream portion of the gas, as the emitted radiation there contributes the majority of the total output power, and the emitted radiation from the downstream portion of the gas profile is least reabsorbed. In one embodiment, the gas flow profile is shaped so that the pressure on the downstream side is optimized to achieve optimal phase matching and, therefore, optimal output power, while the pressure on the upstream side is reduced to reduce the effects of plasma defocusing. In this way, the conflicting requirements of high pressure for phase matching, on the one hand, and low pressure for reducing plasma defocusing, on the other hand, can be separated, at least to some extent, by applying them to different portions of the gas flow profile.

[0117] As described above, by using a gas nozzle with a tuned geometry, the aforementioned gas flow profile can be achieved, wherein the gas density of the gas flow initially increases to a maximum value and then decreases sharply in the cutoff region along the propagation direction. In one embodiment, the width of the opening gradually increases along the propagation direction of the pump radiation. Optionally, the opening has a trapezoidal shape.

[0118] Figure 7 (a) shows a schematic diagram of an example 701 in which the shape of the opening 711 is a trapezoid, more specifically, an isosceles trapezoid. Axis X represents the propagation direction of the pump radiation. Axis Y represents one of the other directions in the exit plane perpendicular to X. Axis Z represents a direction perpendicular to the exit plane and forms an acute angle with the gas flow direction. In one embodiment, the length of first side 725 is 50 μm, and the length of second side 723 is one of 400 μm, 200 μm, and 100 μm. In this case, the shape of the opening in the exit plane is asymmetric with respect to a plane perpendicular to the propagation direction of the pump radiation. A second side 723 of 50 μm was also simulated as a reference. The length of opening 721 along the propagation direction of the pump radiation is 500 μm.

[0119] exist Figure 7 (b) and Figure 7 (c) shows the Figure 7 The corresponding simulated gas density distribution plot 715 and contour plot 703 of the opening 711 in (a) along the airflow propagation direction. Figure 7 In (b), the schematic block 713 represents a portion of the gas nozzle 609, wherein the opening 701 is in its outlet plane, which is the bottom side of the block. The length of the second side 723 is 400 μm in this example.

[0120] Plot 703 includes gas profiles 719, 729, 739, and 749 with second side lengths of 400 μm, 200 μm, 100 μm, and 50 μm, respectively. The vertical axis P represents gas density. It can be seen that when the second side 723 is larger than the first side 725, the gas profile has an asymmetric shape along the propagation direction, and the gas density of the gas flow first increases to a maximum value and then sharply decreases in the cutoff region along the propagation direction indicated by X.

[0121] It should be understood that the trapezoidal shape is merely exemplary to illustrate the principles of the present disclosure. In practice, any shape with an opening width that gradually increases along the propagation direction of the pump radiation can produce a similar profile, and many specific implementations can be envisioned based on the principles of the present disclosure.

[0122] As mentioned above, the internal cross-sectional area of ​​the gas nozzle in a plane parallel to the outlet plane can be adapted, optionally together with an adjustable element, to change the profile of the gas flow, e.g. Figure 8 The embodiment shown. Figure 8 As can be seen from the schematic diagram of the gas nozzle 803 in (b), the inner surface 815 of the gas nozzle varies in such a way that for at least a portion of the gas nozzle, the inner cross-sectional area increases, optionally linearly, in the direction of gas flow. The portion of the gas nozzle with an increased inner cross-sectional area (which may be referred to as diverging) may result in gas expansion, a decrease in gas density and / or an increase in gas flow rate. For example, Figure 8 With the adapted geometry of the gas nozzle shown in (b), the volume of the gas flow has sharper boundaries than a non-divergent gas flow, which can help, for example, to produce a relatively small cut-off region length. Figure 8 In the schematic diagram of the gas nozzle 803 in (b), the internal cross-sectional area of ​​the gas nozzle is symmetrical along the gas flow direction before leaving the opening, but in practice the internal cross-sectional area of ​​the gas nozzle can also be asymmetrical with respect to a plane perpendicular to the propagation direction of the pump radiation, which can produce asymmetrically shaped gas flow profiles, e.g. Figure 9 (b) The gas flow profile 913. In one embodiment, the internal cross-section of the gas nozzle has a Figure 7 The embodiment of the opening 711 in (a) has the same shape, which is asymmetric with respect to a plane perpendicular to the propagation direction of the pump radiation, such as a plane including the Y and Z axes.

[0123] Examples of gas nozzles 803 may include adjustable elements 813 . Figure 8 (a) and Figure 8 (b) shows a cross section of the adjustable element 813. The adjustable element 813 is, for example, wedge-shaped. It should be noted that the adjustable element 813 may also have other shapes. Figure 8As shown in (b), an adjustable element 813 positionable between the exit plane and the pump radiation can further change the profile of the gas flow. The adjustable element 813 can be configured to be positioned at least partially in the gas flow. After leaving the exit plane, the gas flow can impact the adjustable element, one of the surfaces of the adjustable element being inclined in the direction of the gas flow. The gas flow exits through the gas nozzle, optionally with an increased internal cross-sectional area, and can be compressed by the adjustable element 813 arranged after the exit plane of the gas nozzle. The adjustable element can shape the gas flow profile and can generate shock waves, which are caused by Figure 8 The white arrows and dashed lines in (b) indicate that the shock wave can compress the airflow into a high-density region near the adjustable element 813. Figure 8 (b) is represented by the area within the dashed ellipse. The region between adjustable element 813 and the shock wave can have a higher density, and there can be a sharp change in density across the shock wave. Because the shock wave's length along the pump radiation propagation direction is tens of microns, a relatively small cutoff length along the pump radiation propagation direction can be achieved, optionally less than 100 μm. After impacting the adjustable element, the airflow with a modified profile will further expand into the surroundings, which can be a vacuum or near-vacuum.

[0124] Optionally, the position of the adjustable element relative to the airflow is adjustable. The adjustable element can be tuned in rotation and translation to optimize the emitted radiation during its generation. Tuning the adjustable element can shape the profile of the airflow, which can further change, and optionally further optimize, the characteristics of the emitted radiation.

[0125] exist Figure 8 In (a) and (b), the adjustable element is placed after the outlet plane along the gas flow direction, but in practice it can also be placed before leaving the outlet plane and be part of the inner surface of the gas nozzle, which can shape the gas flow profile so that the gas density of the gas flow first increases to a maximum value and then drops sharply in the cut-off region along the propagation direction, similar to Figure 8 (c) shown.

[0126] Figure 8(a) A simulated gas density distribution plot 801 with a gas nozzle 811 and an adjustable element 813 is shown in grayscale. The X-axis represents the propagation direction of the pump radiation. The Y-axis represents one of the other directions in the outlet plane perpendicular to X. The Z-axis represents the direction perpendicular to the outlet plane and at an acute angle to the gas flow direction. The gas density distribution is shown in grayscale, with dark colors in the plot representing low gas density and light colors representing high gas density. The location of the shock wave is shown as a sharp change in gas density. In this embodiment, the divergence is in the Y-direction. Note that the divergence does not need to have a symmetrical cross-sectional area in a plane parallel to the outlet plane.

[0127] An example of pump radiation 817 is as follows Figure 8 The propagation direction indicated by the black arrow in (b) propagates through the changing airflow. The gas density measured along the propagation direction can have a profile in which the gas density of the airflow first increases to a maximum value and then drops sharply in the cutoff region along the propagation direction. Figure 8 (c) shows the simulated gas distribution along the propagation direction at varying distances from the adjustable element. For gas profile examples 821, 823, and 825, the distances from the adjustable element are 300 μm, 400 μm, and 500 μm, respectively. It can be seen that the gas density of the gas flow first increases to a maximum value and then drops sharply in the cutoff region along the propagation direction. In one embodiment, there may be a bulge or so-called twist in the gas profile on the upstream and / or downstream sides of the gas flow, which forms a zigzag shape in the gas profile. The bulge may be caused by boundary effects caused by the adjustable element.

[0128] It should be understood that the shapes of the above gas profiles are merely exemplary in order to illustrate the principles of the present disclosure, and many specific embodiments can be conceived based on the principles of the present disclosure.

[0129] Figure 8 Another benefit of this embodiment is that it is able to produce an optimal gas profile at some significant distance from the nozzle (optionally 400 μm), rather than being forced to make a suboptimal compromise between being as close to the nozzle as possible for, e.g., good distribution, on the one hand, and being far enough away from the nozzle to avoid, e.g., damage and laser clipping, on the other hand.

[0130] with the use of a gas shaping element comprising a hole for transmitting the generated measuring radiation (one embodiment of which is described in patent application WO 2018 / 166741 A1) Figure 11Compared to the embodiment of WO2018166741A1 (shown in FIG), the advantage of using an adjustable element 813 is that alignment of the pump radiation is easier when the adjustable element 813 is positioned between the outlet plane and the pump radiation. When using the embodiment of WO2018166741A1, if the challenging precise alignment is not achieved, the gas-shaping element may easily burn due to the high power of the pump radiation. WO2018 / 166741A1 is incorporated herein by reference in its entirety.

[0131] In addition to the above-described embodiments, a system with two gas jets of two different gases can also produce a similar gas profile, an example of which is shown in FIG16 of patent application WO2018 / 166741A1. Compared to a system with two gas nozzles, the advantage of using an adapted gas nozzle is that the emitted radiation is more easily optimized due to the uniform gas composition of the gas flow, and a vacuum or near-vacuum atmosphere in the inspection chamber 350 is easier to maintain.

[0132] During the generation of the emitted radiation, the gas nozzle may be subjected to both cooling and heating effects. Due to its acceleration and rapid expansion, the gas flow exiting the opening can have a low temperature, optionally around 200 K, which can cool the gas nozzle. For a 3 mm gas nozzle made of nickel, the contraction along the gas nozzle can be approximately 20 μm, which is not negligible compared to the distance between the pump radiation and the gas nozzle (e.g., 50-100 μm) and the focal spot diameter of the pump radiation (e.g., 20-30 μm). On the other hand, the pump radiation can have high intensity near / in the interaction region and can heat or even burn the gas nozzle, depending on the distance between the pump radiation and the gas nozzle. During the generation of the emitted radiation, the relative position between the pump radiation and the gas nozzle can be adjusted to optimize the emitted radiation. The combination of heating and cooling effects can introduce additional thermal expansion / contraction of the gas nozzle along the direction of gas flow, which can lead to drift in the emitted radiation, as the gas profile can vary along the gas flow direction. Furthermore, turning the gas flow on and off during the process can also introduce drift.

[0133] Figure 11 A gas delivery system, optionally a radiation source, having a temperature control assembly is schematically shown. The temperature control assembly may include a temperature control element 1109 that can be used to change the temperature of at least a portion of the gas nozzle 609 or maintain the temperature near a desired set point. Alternatively, the temperature control element 1109 may be a source that emits electromagnetic radiation, optionally a laser, or a heating element, optionally a heating filament. Alternatively, the temperature control element 1109 may be a cooling element, such as a thermoelectric cooler or a liquid cooling system. Alternatively, the temperature control element 1109 may be a thermostat that maintains at least a portion of the gas nozzle near a desired set point.

[0134] The temperature control assembly may include a temperature control detector 1107 that can detect changes in the temperature or geometry of the gas nozzle. Alternatively, the temperature control detector 1107 may be a camera. The camera may use inferred measurements for temperature detection and / or detection of shape differences. Alternatively, the temperature control detector 1107 may be a temperature sensor such as a thermometer. Signals received by the temperature control detector 1107 may be transmitted to a temperature control processor 1111. Based on the received signals, the temperature control processor 1111 may communicate with the temperature control element 1109 to control the temperature of at least a portion of the gas nozzle, which may be referred to as a feedback control loop.

[0135] All of the above embodiments may be used in methods of generating radiation using an irradiation source, such as Figure 12 As shown. The first step 200 is to deliver a gas flow 615 in the illumination source, which includes providing a gas flow from an opening 617 in the outlet plane of the gas nozzle 609 for receiving pump radiation having a propagation direction and for generating emitted radiation at the interaction region. In the method, the geometry of the gas nozzle can be adapted to shape the profile of the gas flow so that the gas density of the gas flow first increases to a maximum value and then drops sharply in the cut-off region along the propagation direction. Optionally, there can be a second step 202, which provides pump radiation 611 having a propagation direction in the gas flow 615. Optionally, there can be a third step 204, which generates the emitted radiation in the interaction region. More details on the method of generating radiation using an illumination source are provided in the description of the embodiments described above in conjunction with the figures and text.

[0136] Further embodiments are disclosed in the following numbered clauses:

[0137] 1. An irradiation source comprising:

[0138] - a gas delivery system comprising a gas nozzle, wherein said gas nozzle comprises an opening in an outlet plane of said gas nozzle,

[0139] wherein the gas delivery system is configured to provide a gas flow from the opening for producing the emitted radiation at the interaction region,

[0140] - wherein the illumination source is configured to receive pump radiation having a propagation direction and to provide the pump radiation in the gas flow,

[0141] The geometry of the gas nozzle is adapted to shape the profile of the gas flow such that the gas density of the gas flow first increases to a maximum and then drops sharply in a cut-off region along the propagation direction.

[0142] 2. The illumination source according to clause 1, wherein the profile of the gas flow is used to suppress energy divergence of the pump radiation within the gas flow, wherein the energy divergence is caused by the fact that a portion of the gas flow is ionized by the pump radiation.

[0143] 3. The illumination source of clause 1 or 2, wherein the maximum value is above the phase matching pressure.

[0144] 4. The illumination source of any preceding clause, wherein the profile of the gas flow has a cut-off region length of less than 100 μm in the propagation direction of the pump radiation.

[0145] 5. The illumination source of any preceding clause, wherein the shape of the opening in the exit plane is asymmetric with respect to a plane perpendicular to the propagation direction of the pump radiation.

[0146] 6. The illumination source of any preceding clause, wherein the width of the opening gradually increases along the propagation direction of the pump radiation.

[0147] 7. The illumination source of clause 5 or 6, wherein the opening is trapezoidal in shape.

[0148] 8. The illumination source of clause 7, wherein the opening is in the shape of an isosceles trapezoid.

[0149] 9. The illumination source according to any of the preceding clauses, wherein an internal cross-sectional area of ​​the gas nozzle in a plane parallel to the outlet plane increases for at least a portion of the gas nozzle in the direction of the gas flow.

[0150] 10. An illumination source according to any preceding clause, wherein the illumination source comprises an adjustable element for varying the profile of the gas flow.

[0151] 11. The illumination source of clause 10, wherein the adjustable element is used to change the profile of the gas flow during generation of the emitted radiation.

[0152] 12. The illumination source of clause 10 or 11, wherein the adjustable element is configured to be positioned at least partially in the air flow.

[0153] 13. The illumination source of any preceding clause, wherein the illumination source comprises a pump radiation source operable to emit the pump radiation.

[0154] 14. The illumination source of any preceding clause, wherein the illumination source is used for high harmonic generation.

[0155] 15. The illumination source of any preceding clause, wherein the gas flow is provided by the gas delivery system into an evacuated or nearly evacuated space.

[0156] 16. An illumination source according to any preceding clause, wherein the emitted radiation has a wavelength in the X-ray or EUV range, wherein the wavelength is in the range of 0.01 nm to 100 nm, optionally 0.1 nm to 100 nm, optionally 1 nm to 100 nm, optionally 1 nm to 50 nm, or optionally 10 nm to 20 nm.

[0157] 17. An illumination source according to any preceding clause, wherein in operation, the emitted radiation is directed towards a target on a wafer.

[0158] 18. An illumination source according to any preceding clause, wherein the emitted radiation is used for metrological measurements.

[0159] 19. An illumination source according to any preceding clause, wherein the illumination source comprises a temperature control component.

[0160] 20. A metrology apparatus comprising an illumination source according to any one of clauses 1 to 19.

[0161] 21. A lithocell comprising an illumination source according to any of clauses 1 to 19.

[0162] 22. A method of delivering a gas in an irradiation source, comprising:

[0163] - providing a gas flow from an opening in an outlet plane of the gas nozzle for receiving pump radiation having a propagation direction and for generating emitted radiation at the interaction region;

[0164] The geometry of the gas nozzle is adapted to shape the profile of the gas flow such that the gas density of the gas flow first increases to a maximum and then drops sharply in a cut-off region along the propagation direction.

[0165] 23. An illumination source comprising:

[0166] a gas delivery system comprising a gas nozzle, wherein the gas nozzle comprises an opening in an outlet plane of the gas nozzle, wherein the gas delivery system is configured to provide a gas flow from the opening for producing the emitted radiation at the interaction region, and

[0167] an adjustable element for changing the profile of the airflow,

[0168] wherein the illumination source is configured to receive pump radiation having a propagation direction and to provide the pump radiation in the gas flow,

[0169] wherein the adjustable element is positioned between the exit plane and the pump radiation.

[0170] 24. The illumination source according to clause 23, wherein the profile of the gas flow is such that the gas density of the gas flow first increases to a maximum value and then drops sharply in a cut-off region along the propagation direction.

[0171] 25. The illumination source of clause 24, wherein the maximum value is above a phase matching pressure.

[0172] 26. The illumination source of any preceding clause, wherein the profile of the gas flow is adapted to suppress energy divergence of the pump radiation within the gas flow.

[0173] 27. The illumination source according to clause 26, wherein the energy divergence is caused by the fact that a portion of the gas flow is ionized by the pump radiation.

[0174] 28. An illumination source according to any preceding clause, wherein the adjustable element is used to change the profile of the gas flow during the generation of the emitted radiation.

[0175] 29. The illumination source of any preceding clause, wherein the adjustable element is configured to be positioned at least partially in the air flow.

[0176] 30. The illumination source of any preceding clause, wherein the profile of the gas flow has a cut-off region length of less than 100 μm in the propagation direction of the pump radiation.

[0177] 31. The illumination source of any preceding clause, wherein the shape of the opening in the exit plane is asymmetric with respect to a plane perpendicular to the propagation direction of the pump radiation.

[0178] 32. The illumination source of any preceding clause, wherein the width of the opening gradually increases along the propagation direction of the pump radiation.

[0179] 33. The illumination source of clause 31 or 32, wherein the shape of the opening is trapezoidal.

[0180] 34. The illumination source of clause 33, wherein the shape of the opening is an isosceles trapezoid.

[0181] 35. The illumination source of any preceding clause, wherein an internal cross-sectional area of ​​the gas nozzle in a plane parallel to the outlet plane increases for at least a portion of the gas nozzle in the direction of the gas flow.

[0182] 36. The illumination source of any preceding clause, wherein the illumination source comprises a pump radiation source operable to emit the pump radiation.

[0183] 37. An illumination source according to any preceding clause, wherein the illumination source is used for high harmonic generation.

[0184] 38. The illumination source of any preceding clause, wherein the gas flow is provided by the gas delivery system into an evacuated or nearly evacuated space.

[0185] 39. An illumination source according to any preceding clause, wherein the emitted radiation has a wavelength in the X-ray or EUV range, wherein the wavelength is in the range of 0.01 nm to 100 nm, optionally 0.1 nm to 100 nm, optionally 1 nm to 100 nm, optionally 1 nm to 50 nm, or optionally 10 nm to 20 nm.

[0186] 40. An illumination source according to any preceding clause, wherein in operation, the emitted radiation is directed towards a target on a wafer.

[0187] 41. An illumination source according to any preceding clause, wherein the emitted radiation is used for metrological measurements.

[0188] 42. An illumination source according to any preceding clause, wherein the illumination source comprises a temperature control component.

[0189] 43. The illumination source of any preceding clause, wherein the pump radiation comprises pulses.

[0190] 44. A metrology apparatus comprising an illumination source according to any of clauses 23 to 43.

[0191] 45. A lithocell comprising an illumination source according to any of clauses 23 to 43.

[0192] 46. ​​A method of delivering a gas in an irradiation source, comprising:

[0193] providing a gas flow from an opening in an outlet plane of the gas nozzle for receiving pump radiation having a propagation direction and for generating emitted radiation at the interaction region, and

[0194] Changing the profile of the airflow with adjustable elements,

[0195] wherein the adjustable element is positioned between the exit plane and the pump radiation.

[0196] 47. An irradiation source comprising:

[0197] - a gas delivery system comprising a gas nozzle, wherein said gas nozzle comprises an opening in an outlet plane of said gas nozzle,

[0198] - wherein the gas delivery system is configured to provide a gas flow from the opening for producing the emitted radiation at the interaction region,

[0199] - wherein the illumination source is configured to receive pump radiation having a propagation direction and to provide the pump radiation in the gas flow,

[0200] - wherein the shape of the opening in the exit plane is asymmetric with respect to a plane perpendicular to the propagation direction of the pump radiation.

[0201] 48. The illumination source according to clause 47, wherein the geometry of the gas nozzle is adapted to shape the profile of the gas flow such that the gas density of the gas flow first increases to a maximum value and then drops sharply in a cut-off region along the propagation direction.

[0202] 49. The illumination source of clause 48, wherein the profile of the gas flow is configured to suppress energy divergence of the pump radiation within the gas flow.

[0203] 50. The illumination source of clause 49, wherein the energy divergence is caused by the fact that a portion of the gas flow is ionized by the pump radiation.

[0204] 51. The illumination source of any of clauses 48 to 50, wherein the maximum value is above the phase matching pressure.

[0205] 52. The illumination source of any of clauses 48 to 51, wherein the profile of the gas flow has a cut-off region length of less than 100 μm in the propagation direction of the pump radiation.

[0206] 53. The illumination source of any preceding clause, wherein the width of the opening gradually increases along the propagation direction of the pump radiation, optionally the shape of the opening is a trapezoid, optionally the shape of the opening is an isosceles trapezoid.

[0207] 54. An illumination source according to any preceding clause, wherein an internal cross-sectional area of ​​the gas nozzle in a plane parallel to the outlet plane increases for at least a portion of the gas nozzle in the direction of the gas flow.

[0208] 55. An illumination source according to any preceding clause, wherein the illumination source comprises an adjustable element for varying the profile of the gas flow.

[0209] 56. The illumination source of clause 55, wherein the adjustable element is used to change the profile of the gas flow during generation of the emitted radiation.

[0210] 57. The illumination source of clause 54 or 55, wherein the adjustable element is configured to be positioned at least partially in the air flow.

[0211] 58. The illumination source of any preceding clause, wherein the gas flow is provided by the gas delivery system into an evacuated or nearly evacuated space.

[0212] 59. An illumination source according to any preceding clause, wherein the emitted radiation has a wavelength in the X-ray or EUV range, wherein the wavelength is in the range of 0.01 nm to 100 nm, optionally 0.1 nm to 100 nm, optionally 1 nm to 100 nm, optionally 1 nm to 50 nm, or optionally 10 nm to 20 nm.

[0213] 60. An illumination source according to any preceding clause, wherein the illumination source comprises a temperature control component.

[0214] 61. A metrology apparatus or lithocell comprising an illumination source according to any of clauses 47 to 60.

[0215] 62. A method of delivering a gas in an irradiation source, comprising:

[0216] providing a gas flow from an opening in an outlet plane of a gas nozzle for receiving pump radiation having a propagation direction and for generating emission radiation at the interaction region;

[0217] The shape of the opening in the exit plane is asymmetrical with respect to a plane perpendicular to the propagation direction of the pump radiation.

[0218] Although specific reference may be made herein to the use of lithographic apparatus in IC fabrication, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the fabrication of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0219] Although specific reference may be made herein to embodiments in the context of lithographic apparatus, embodiments may be used in other apparatus. Embodiments may form part of mask inspection equipment, metrology equipment, or any apparatus for measuring or processing objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatuses are generally referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0220] Although specific reference may be made herein to embodiments in the context of inspection or metrology apparatus, embodiments may be used in other apparatuses. Embodiments may form part of mask inspection apparatus, lithographic apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other pattern forming device). The term "metrology apparatus" (or "inspection apparatus") may also refer to an inspection apparatus or an inspection system (or metrology apparatus or metrology system). That is, for example, an inspection apparatus including an embodiment 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.

[0221] Although specific reference has been made above to the use of embodiments in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0222] While the targets or target structures (more generally, structures on a substrate) described above are metrology target structures that are specifically designed and formed for measurement purposes, in other embodiments, the characteristic of interest may be measured on one or more structures that are functional components 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 be provided specifically for the measurement being made. Furthermore, 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 typical non-target structures (optionally product structures) that are made in the target portion C by a photolithographic process. In practice, the lines and / or spaces of the overlaid grating within the target structure may be made to include smaller structures of similar size to the non-target structures.

[0223] Although specific embodiments have been described above, it should be understood that the present invention may be implemented in other ways than those described. The above description is intended to be illustrative and not limiting. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

[0224] Although specific reference is made to a "metrology device / tool / system" or an "inspection device / tool / system," these terms may refer to the same or similar types of tools, devices, or systems. For example, an inspection or metrology device including embodiments of the present invention may be used to determine characteristics of structures on a substrate or on a wafer. For example, an inspection or metrology device including embodiments of the present invention may be used to detect defects in a substrate or a structure on a substrate or on a wafer. In such embodiments, the characteristic of interest in 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 or on the wafer.

[0225] Although specific reference is made to SXR and EUV electromagnetic radiation, it will be understood that the present 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. As an alternative to optical metrology methods, the use of X-rays, optionally hard X-rays, for example radiation having a wavelength in the range of between 0.01 nm and 10 nm, or alternatively between 0.01 nm and 0.2 nm, or alternatively between 0.1 nm and 0.2 nm, is also contemplated for metrology measurements.

Claims

1. An irradiation source comprising: a gas delivery system comprising a gas nozzle, wherein the gas nozzle comprises an opening in an outlet plane of the gas nozzle; wherein the gas delivery system is configured to provide a gas flow from the opening to produce the emitted radiation at the interaction region; wherein the illumination source is configured to receive pump radiation having a propagation direction and to provide the pump radiation in the gas flow; The gas nozzle is designed such that, for at least a portion of the gas nozzle, an inner cross-sectional area of ​​the gas nozzle increases in the direction of the gas flow.

2. The illumination source of claim 1, wherein the inner cross-sectional area increases linearly.

3. The irradiation source according to claim 1 or 2, The geometry of the gas nozzle is adapted to shape the profile of the gas flow such that the gas density of the gas flow first increases to a maximum value and then decreases sharply in the cut-off region along the propagation direction. The illumination source of claim 3 , wherein the maximum value is above a phase matching pressure. 5 . The illumination source according to claim 3 , wherein the length of the cut-off region of the profile of the gas flow along the propagation direction of the pump radiation is less than 100 μm.

6. The illumination source according to any of the preceding claims, wherein the width of the opening gradually increases along the propagation direction of the pump radiation, optionally the shape of the opening is a trapezoid, optionally the shape of the opening is an isosceles trapezoid.

7. An illumination source according to any one of the preceding claims, wherein the illumination source comprises an adjustable element for varying the profile of the gas flow.

8. The illumination source of claim 7, wherein the adjustable element is used to change the profile of the gas flow during generation of the emitted radiation.

9. The illumination source of claim 7 or 8, wherein the adjustable element is configured to be positioned at least partially in the air flow.

10. The illumination source of any preceding claim, wherein the gas flow is provided by the gas delivery system into an evacuated or nearly evacuated space.

11. The radiation source of any one of the preceding claims, wherein the wavelength of the emitted radiation is in the X-ray or EUV range, wherein the wavelength is in the range of 0.01 nm to 100 nm, optionally in the range of 0.1 nm to 100 nm, optionally in the range of 1 nm to 100 nm, optionally in the range of 1 nm to 50 nm, or optionally in the range of 10 nm to 20 nm.

12. The illumination source of any preceding claim, wherein the illumination source comprises a temperature control assembly.

13. A metrology apparatus or lithography cell comprising an illumination source according to any one of claims 1 to 12.

14. A method of delivering a gas in an irradiation source, comprising: - providing a gas flow from an opening in an outlet plane of the gas nozzle for receiving pump radiation having a propagation direction and generating emitted radiation at the interaction region; The gas nozzle is designed such that, for at least a portion of the gas nozzle, an inner cross-sectional area of ​​the gas nozzle increases in the direction of the gas flow. 15 . The method according to claim 14 , wherein the geometry of the gas nozzle is adapted to shape the profile of the gas flow such that the gas density of the gas flow first increases to a maximum value and then drops sharply in a cut-off region along the propagation direction.

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