High-order harmonic generation radiation source

Abstract

The present invention relates to a method and a corresponding device, which are operable to cause an interaction between a driving radiation beam and a medium to produce emitted radiation by high-order harmonic generation, the device comprising: an interaction region, which is positioned in an interaction plane and is configured to receive the medium; a beam stop, which is positioned at a beam blocking plane upstream of the interaction plane and is configured to partially block the driving radiation beam; a beam shaper, which is positioned at an object plane upstream of the beam blocking plane and is configured to control the spatial distribution of the driving radiation beam; and at least one lens, which is positioned upstream of the interaction plane and downstream of the beam blocking plane, wherein the lens is positioned so that an image of the spatial distribution of the driving radiation beam is formed at the interaction plane.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 18189877.6 filed on August 21, 2018, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to methods and apparatus for implementing a radiation source that uses high-order harmonic generation (HHG) to generate radiation. More specifically, the present invention may relate to methods and apparatus for controlling the amount of drive radiation that escapes the radiation source. The present invention may also relate to methods and apparatus for inspection (e.g. metrology) useful in device fabrication, such as by lithography using / including such a radiation source. Background Art

[0004] 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). A lithographic apparatus can, for example, project a pattern (also often referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

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

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

[0007] As explained below, inspection equipment, which may also be referred to as metrology tools, may be used to determine properties of substrates and features fabricated on substrates, and in particular, how properties of different substrates or properties associated with different layers of the same substrate vary from layer to layer. Such inspection equipment may expose the substrate and associated features to radiation and capture the scattered or diffracted radiation to form an image that allows determination of properties of the substrate and / or features. The radiation may include soft x-rays (SXR) and / or EUV radiation.

[0008] Soft x-ray (SXR) and / or EUV radiation has a wavelength extending approximately from 0.1 nm to 100 nm. Applications of SXR and / or EUV include (but are not limited to) existing or near-future metrology tools for the semiconductor industry, for example where visible light begins to give insufficient spatial resolution to the ever-shrinking feature sizes.

[0009] SXR and / or EUV radiation can be generated using HHG, where an intense laser pulse, such as visible or infrared (IR) drive radiation, interacts with a gaseous medium, resulting in SXR and / or EUV emission from the gas atoms due to the interaction of the gas atoms with the drive radiation. The HHG-generated SXR and / or EUV light can then be focused onto a target on a wafer with the aid of an optical column that transfers the light from the HHG source to the target. The reflected light can be detected and processed to infer properties of the target. Summary of the invention

[0010] For the suitability of SXR and / or EUV metrology tools, it is desirable to focus the SXR and / or EUV beam into a very small spot on a custom wafer. This is typically due to the fact that only a very small practical area on the wafer is available for printing metrology targets. For many exemplary use cases, the SXR and / or EUV spot diameter should be less than 5 μm. This is a significant challenge requiring (among other things) a well-behaved and well-focusable SXR and / or EUV beam produced by the HHG source. The focusability of the SXR and / or EUV beam is determined by a variety of properties, including beam divergence, the intensity distribution of the emitted SXR and / or EUV, and beam aberrations, or more generally, by the wavefront of the SXR and / or EUV beam.

[0011] However, the inventors have learned that the atomic HHG mechanism is such that the SXR and / or EUV wavefront is largely determined by the intensity distribution of the drive radiation in the gas target. Therefore, the inventors have realized that control and optimization of the focusability of the SXR and / or EUV beam (which improves the ability to implement SXR and / or EUV metrology tools) relies at least in part on control of the intensity distribution of the drive laser in the gas target.

[0012] Typically, in existing HHG sources, the drive radiation is focused to a spot in the target with a Gaussian intensity distribution. Within the limitations of the Gaussian spot, some coarse control of the SXR and / or EUV wavefront is achieved by optimizing the drive laser focal spot size and / or the target position relative to the focus. However, the inventors have appreciated that more detailed control of the SXR and / or EUV wavefront can be achieved if a custom intensity distribution of the drive radiation can be adapted without being limited to a Gaussian distribution. For example, a different intensity distribution can produce better SXR and / or EUV wavefront properties. Techniques do exist to manipulate the distribution of the focused spot of a laser. In particular, a laser beam can be manipulated by a deformable mirror and a spatial light modulator (SLM) upstream of a lens that focuses the beam to a spot.

[0013] Furthermore, in HHG radiation sources, the SXR and / or EUV beams (transmitting radiation beams) are emitted in the same direction as the drive laser beam. The drive radiation needs to be spaced apart from the SXR and / or EUV beams to prevent them from interfering with the measurement results. In addition, the relatively high power drive radiation needs to be blocked in some way to prevent it from entering the sensitive optical column or being transmitted to the sensitive custom wafer.

[0014] Typically, in existing HHG sources, blocking of the drive laser is performed with the aid of a thin metal foil that partially transmits SXR and / or EUV. However, this approach is not suitable for high-power HHG sources, since such filters cannot withstand the high drive radiation powers. An alternative approach has been proposed (Peatross et al., Opt. Lett. 19, 942 (1994)), in which the central part of the drive laser beam is blocked by a beam stop. The resulting annular beam still generates a roughly Gaussian central spot at the focal point in the target, and therefore does not significantly affect the generation of SXR and / or EUV, but will evolve again into an annular beam downstream of the target. The drive laser can then be blocked by a hole configured to allow the emitted SXR and / or EUV beam to pass through, whereas the annular drive radiation beam is blocked. This reference Figure 5b Then elaborate.

[0015] The inventors recognize that improved solutions to one or more of the problems specified herein or problems otherwise known by a skilled person are desirable. In some arrangements, methods and apparatus may seek to simultaneously solve or mitigate both of the above problems. In an exemplary arrangement, problems associated with separation of drive radiation and emission radiation (SXR and / or EUV beams) may be solved or mitigated by applying beam manipulation techniques (e.g., using a beam shaper such as an SLM) to control the intensity distribution of the drive radiation beam. Problems associated with blocking the drive radiation at the output of the radiation source may be solved or mitigated by blocking a central portion on the laser beam.

[0016] However, the inventors have realised that solutions to both of the above problems may often interfere with each other. That is, the laser beam profile prepared upstream by the SLM, for example, will be improved by a beam stop arranged to form an annular drive radiation beam, resulting in an intensity distribution at the target that is not what is desired. Conversely, the annular beam properties that should be produced by the beam stop will be affected by the beam steering of the SLM, resulting in leakage of the drive radiation via the downstream aperture. An exemplary optical setup is proposed herein that circumvents this problem by exploiting the imaging properties of the lens system.

[0017] According to the present invention, in one aspect, a radiation source device is provided, which is operable to cause interaction between a driving radiation beam and a medium to generate emitted radiation by high-order harmonic generation, the device comprising: an interaction region, the interaction region being positioned at an interaction plane and being configured to receive the medium; a beam stop, the beam stop being positioned at a beam blocking plane upstream of the interaction plane and being configured to partially block the driving radiation beam; a beam shaper, the beam shaper being positioned at an object plane upstream of the beam blocking plane and being configured to control the spatial distribution of the driving radiation beam; and at least one lens, the at least one lens being positioned upstream of the interaction plane and downstream of the beam blocking plane, wherein the lens is positioned so that an image of the spatial distribution of the driving radiation beam is formed at the interaction plane.

[0018] Optionally, the lens is positioned such that the object plane and the interaction plane are conjugate planes.

[0019] Optionally, the apparatus further comprises an aperture positioned at an aperture plane downstream of the interaction plane and configured to allow at least a portion of the emission radiation to pass therethrough and configured to block at least a portion of the drive radiation beam.

[0020] Optionally, the aperture plane is positioned relative to the beam stop plane and the lens such that an image of the beam stop is formed at the aperture plane.

[0021] Optionally, the lens is positioned such that the beam stop plane and the aperture plane are conjugate planes.

[0022] Optionally, a size of the beam stop in the beam stop plane relative to a size of the drive radiation beam in the beam stop plane is such that an image of the beam stop is decoupled from an image of the spatial distribution of the drive radiation beam.

[0023] Optionally, a size of the beam stop in the beam stop plane is 30% or less of a size of the drive radiation beam in the beam stop plane.

[0024] Optionally, the beam stop and the drive radiation beam have a substantially circular cross-section in the beam stop plane, and wherein a dimension of the beam stop and a dimension of the drive radiation beam are diameters.

[0025] Optionally, the depth of focus of the image of the beam stop does not overlap the interaction plane.

[0026] Optionally, the centre of the depth of focus of the image of the beam stop substantially coincides with the aperture plane.

[0027] Optionally, the circle of confusion associated with the depth of focus of the image of the beam stop is larger than the image of the drive radiation beam of the interaction plane.

[0028] Optionally, the depth of focus of the image of the beam stop has a maximum circle of confusion with a diameter of 35 μm or less.

[0029] Optionally, a depth of focus of the spatially distributed image of the drive radiation beam does not overlap the aperture plane.

[0030] Optionally, a centre of the focal depth of the image of the intensity distribution of the drive radiation beam substantially coincides with the interaction plane.

[0031] Optionally, the focal depth of the image of the beam stop and / or the focal depth of the image of the spatial distribution of the drive radiation beam is determined by:

[0032] Depth of focus = 2cN(1+m)

[0033] wherein c is the circle of maximum confusion, N is the ratio of the focal length of the lens to the diameter of the drive radiation beam at the lens, and m is the magnification factor of the image of the beam stop at the aperture plane.

[0034] Optionally, the beam shaper comprises a spatial light modulator positioned in the object plane.

[0035] Optionally, the device also includes a sensor configured to detect a spatial profile of the driving radiation beam at the interaction plane; and a feedback controller configured to feed back data related to the detected spatial profile of the driving radiation beam to the spatial light modulator, wherein the spatial light modulator is configured to control the spatial profile of the driving radiation beam based on the feedback data.

[0036] According to the present invention, in one aspect, there is provided a metrology device comprising an apparatus according to any one of the above-discussed or disclosed herein.

[0037] According to the invention, in one aspect there is provided a lithocell comprising an apparatus according to the above discussion or described elsewhere herein, or a metrology device as mentioned above or described elsewhere herein.

[0038] According to the present invention, in one aspect, a method for causing interaction between a driving radiation beam and a medium is provided for generating emission radiation by high-order harmonic generation, the method comprising: blocking the driving radiation beam by a beam stop so as to block at least a portion of the driving radiation beam; causing the driving radiation beam to propagate through at least one lens, the lens being positioned downstream of the beam stop; causing the driving radiation beam to be incident on an interaction region, the interaction region comprising a medium and being positioned downstream of the beam stop; and causing the driving radiation to be focused at an aperture, the aperture being positioned at an aperture plane downstream of the interaction region so that an image of the beam stop is formed at the aperture plane, the aperture being configured to allow at least a portion of the emission radiation to pass therethrough and being configured to block at least a portion of the driving radiation beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0040] - Figure 1 depicts a schematic overview of a lithographic apparatus;

[0041] - Figure 2 depicts a schematic overview of a lithography cell;

[0042] - Figure 3 Depicts a schematic representation of overall lithography that shows the collaboration between three key technologies to optimize semiconductor manufacturing;

[0043] - Figure 4 Describe a schematic representation of the measurement equipment;

[0044] - Figure 5a Depicting a schematic representation of a HHG radiation source;

[0045] - Figure 5b depicts a schematic representation of an apparatus for use with a HHG radiation source including a beam stop;

[0046] - Figure 6a depicts a schematic representation of an apparatus for use with a HHG radiation source;

[0047] - Figure 6b depicts a schematic representation of an apparatus for use with a HHG radiation source identifying exemplary distances between a beam stop plane, a lens, and an aperture plane;

[0048] - Figure 6c Schematic representation of an apparatus for use with a HHG radiation source that identifies exemplary distances between an object plane, a lens, and an interaction plane;

[0049] - Figure 7depicts an apparatus for use with a HHG radiation source and illustrates a schematic representation of the depth of a beam stop image;

[0050] - Figure 8 depicts a schematic representation of an apparatus for use with a HHG radiation source including a beam shaper;

[0051] - Fig. 9 depicts a schematic representation of an apparatus for use with a HHG radiation source comprising two lenses; and

[0052] - Fig.10 A flow chart showing a method for causing interaction between a drive radiation beam and a medium for producing emission radiation by high order harmonic generation. DETAILED DESCRIPTION

[0053] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365nm, 248nm, 193nm, 157nm or 126nm) and extreme ultraviolet radiation (EUV, e.g., having a wavelength in the range of about 5nm to 100nm).

[0054] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam that corresponds to the pattern to be 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.

[0055] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. mask stage) MT configured to support a patterning device (e.g. 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. wafer stage) WT configured to hold a substrate (e.g. a wafer coated with resist) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. 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.

[0056] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping / controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0057] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems suitable for the exposure radiation used and / or for other factors such as the use of an immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0058] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, so as to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion techniques is given in US6952253, incorporated herein by reference.

[0059] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also known as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps for preparing a substrate W for subsequent exposure may be performed on a substrate W positioned on one of the substrate supports WT while another substrate W on the other substrate support WT is being used to expose a pattern on that substrate W.

[0060] In addition to the substrate support WT, the lithographic apparatus LA may also comprise a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning means may be arranged to clean parts of the lithographic apparatus, for example parts of the projection system PS or parts of the system providing an immersion liquid. The measurement platform may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0061] In operation, a radiation beam B is incident on a patterning device MA (e.g. a mask) held on a mask support MT and is patterned by a pattern (design layout) presented on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, the first positioner PM and possibly a further position sensor (which is located in the substrate support WT) are used to position the radiation beam B. Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, they may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe line alignment marks.

[0062] like Figure 2 As shown in , the lithography apparatus LA may form part of a lithography cell LC (sometimes also referred to as a lithography cell or (lithography) cluster), which often also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these equipment include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, for example, for regulating the temperature of the substrate W (for example, for regulating the solvent in the resist layer). A substrate transport device or robot RO picks up the substrate W from the input / output terminal ports I / O1, I / O2, moves them between the different process equipment and transfers the substrate W to a feed table LB of the lithography apparatus LA. The equipment in the lithography cell, which is also generally referred to as a coating and developing system, is usually under the control of a coating and developing system control unit TCU, which itself may be controlled by a management control system SCS, which may also control the lithography apparatus LA, for example via a lithography control unit LACU.

[0063] In order for substrates W exposed by lithographic apparatus LA to be correctly and consistently exposed, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in lithography cell LC. If an error is detected, then, for example, adjustments may be made to the exposure of subsequent substrates or to other processing steps to be performed on substrate W, particularly if the inspection is completed before other substrates W of the same batch or lot are yet to be exposed or processed.

[0064] An inspection apparatus (which may also be referred to as a metrology apparatus) may be used to determine properties of a substrate W, and in particular how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be configured to identify defects on the substrate W, and may for example be part of a lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a separate device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure), or on a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or on a developed resist image (where exposed or non-exposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0065] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires a high accuracy in the dimensioning and placement of the structures on the substrate W. In order to ensure this high accuracy, the three systems can be combined in a so-called "holistic" control environment, such as Figure 3 Schematically depicted. One of these systems is the lithographic apparatus LA, which is (virtually) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithographic apparatus LA remains within the process window. The process window defines 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) - typically the process parameters of the lithographic process or patterning process are allowed to vary within the range.

[0066] The computer system CL can use (part of) the 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 lithography equipment settings achieve the maximum overall process window (in Figure 3 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from the metrology tool MT) to predict whether defects may be present due to, for example, suboptimal processing (e.g., in the process window in which the lithographic apparatus LA is currently operating). Figure 3 ), depicted by an arrow pointing to “0” in the second scale SC2.

[0067] The metrology tool MT may provide input to the computer system CL to enable accurate simulation and prediction, and may provide feedback to the lithographic apparatus LA to identify, for example, possible drift in the calibration state of the lithographic apparatus LA (in Figure 3 SC3 is depicted by multiple arrows in the third scale SC3).

[0068] As an alternative to optical metrology methods, the use of soft X-rays or EUV radiation, for example radiation in a wavelength range between 0.1 nm and 100 nm, or alternatively between 1 nm and 50 nm, or alternatively between 10 nm and 20 nm, is also contemplated. An example of a metrology tool operating in one of the wavelength ranges presented above is transmissive small angle X-ray scattering (T-SAXS as in US 2007224518A, the contents of which are incorporated herein by reference in their entirety). Lemaillet et al. discuss profile (CD) measurements using T-SAXS in "Intercomparison between optical and X-ray scatterometry measurements of FinFET structures" (Proc. of SPIE, 2013, 8681). Reflectometry techniques using X-ray (GI-XRS) and extreme ultraviolet (EUV) radiation at grazing incidence are known for measuring the properties of films and stacked layers on substrates. Within the general field of reflectometry, goniometric and / or spectroscopy techniques may be applied. In goniometry, the variation of the reflected beam at different angles of incidence is measured. Spectral reflectometry, on the other hand, measures the spectrum of wavelengths reflected at a given angle (using broadband radiation). For example, EUV reflectometry has been used for inspection of mask substrates prior to the manufacture of reticles (patterning devices) used in EUV lithography.

[0069] The range of applications may make the use of soft X-rays or wavelengths in the EUV domain insufficient. Therefore, published patent applications US 20130304424A1 and US2014019097A1 (Bakeman et al. / KLA) describe hybrid metrology techniques in which measurements using x-rays and optical measurements using wavelengths in the range of 120 nm and 2000 nm are combined to obtain measurements of parameters such as CD. CD measurements can be obtained using x-ray mathematical models and / or optical mathematical models. The contents of the cited US patent applications are incorporated herein by reference in their entirety.

[0070] Figure 4 A schematic representation of a metrology device 302 is depicted, wherein radiation in the wavelength range of 0.1 nm to 100 nm may be used to measure parameters of structures on a substrate. Figure 4The metrology device 302 presented in FIG. 3 is suitable for use in the soft X-ray or EUV domain.

[0071] Figure 4 A schematic physical arrangement of a metrology apparatus 302 comprising a spectroscopic scatterometer using EUV and / or SXR radiation at grazing incidence is shown by way of example only. An alternative form of inspection apparatus may be provided in the form of an angle resolved scatterometer using radiation at normal or near normal incidence similar to conventional scatterometers operating at longer wavelengths.

[0072] Inspection apparatus 302 includes a radiation source 310 , an illumination system 312 , a substrate support 316 , detection systems 318 , 398 , and a metrology processing unit (MPU) 320 .

[0073] In this example, source 310 comprises a generator of EUV or soft x-ray radiation based on high-order harmonic generation (HHG) technology. The main components of the radiation source are a drive laser 330 (which is used to generate the drive radiation) and a HHG gas cell 332. A gas supply 334 supplies a suitable gas target (or medium) to the gas cell, in which the suitable gas is optionally ionized by a power supply 336. The drive laser 300 can be, for example, a fiber-based laser with an optical amplifier, which generates pulses of infrared radiation that can last, for example, less than 1 nanosecond (1 ns) per pulse, with a pulse repetition rate of up to several megahertz as required. The wavelength of the infrared radiation can be, for example, about 1 micron (1 μm). The laser pulse is delivered to the HHG gas cell 332 as a first radiation beam 340, where in the gas, a portion of the radiation is converted to a beam 342 of coherent second (or emission) radiation of a higher frequency than the first radiation to include one or more desired wavelengths.

[0074] The second radiation may contain multiple wavelengths. If the radiation is monochromatic, the measurement calculations (e.g., reconstruction) may be simplified, but it is easier to generate radiation with several wavelengths using HHG. The volume of gas within the gas chamber 332 defines the HHG space, but the space does not need to be completely sealed and a gas flow may be used instead of a static volume. For example, the gas may be an inert gas, such as neon (Ne) or argon (Ar). N2, O2, He, Ar, Kr, Xe gases may all be considered. These are matters of design choice and may even be optional options within the same device. Different wavelengths will provide different levels of contrast, for example, when imaging structures of different materials. For example, in order to inspect metal structures or silicon structures, different wavelengths may be selected as wavelengths for imaging features of (carbon-based) resists or for detecting contamination of these different materials. One or more optical filter devices 344 may be provided. For example, an optical filter such as an aluminum (Al) film may be used to cut off fundamental IR radiation to prevent further transmission into the inspection device. A grating (not shown) may be provided to select one or more specific harmonic wavelengths from the wavelengths generated in the gas chamber. Some or all of the beam path may be contained within a vacuum environment, keeping in mind that SXR and / or EUV radiation is absorbed when traveling in air. Various components of the radiation source 310 and illumination optics 312 may be adjustable to implement different measurement "options" within the same device. For example, different wavelengths and / or polarizations may be selectable.

[0075] Depending on the material of the structure under inspection, different wavelengths can provide the desired level of penetration into the lower layer. In order to distinguish the smallest device features and defects among the smallest device features, short wavelengths are likely to be preferred. For example, one or more wavelengths in the range of 1 to 20nm or optionally in the range of 1 to 10nm or optionally in the range of 10 to 20nm can be selected. Wavelengths shorter than 5nm suffer from very low critical angles when reflected from materials of general interest in semiconductor manufacturing. Therefore, selecting a wavelength greater than 5nm will provide a stronger signal at a higher angle of incidence. On the other hand, if the inspection task is to detect the presence of a certain material (for example) to detect contamination, wavelengths up to 50nm may be useful.

[0076] The filtered beam 342 enters the inspection chamber 350 from the radiation source 310, wherein the substrate W including the structure of interest is held by the substrate support 316 for inspection at the measurement position. The structure of interest is marked as T. The atmosphere in the inspection chamber 350 is maintained as a near vacuum by the vacuum pump 352, so that the EUV radiation can pass through the atmosphere without excessive attenuation. The irradiation system 312 has the function of focusing the radiation into the focused beam 356, and may include, for example, a two-dimensional curved mirror or a series of one-dimensional curved mirrors, as described in the above-mentioned published US patent application US2017 / 0184981A1 (the contents of which are incorporated herein by reference in their entirety). The focusing is performed to achieve a circular or elliptical spot S with a diameter of less than 10 μm when projected onto the structure of interest. The substrate support 316 includes, for example, an XY translation stage and a rotation stage, by which any part of the substrate W can be brought to the focus of the beam in the desired direction. Thus, the radiation spot S is formed on the structure of interest. Alternatively or additionally, the substrate support 316 includes, for example, a tilting platform that can tilt the substrate W at an angle to control the angle of incidence of the focused beam on the structure T of interest.

[0077] Optionally, the illumination system 312 provides a reference radiation beam to a reference detector 314, which can be configured to measure the spectrum and / or intensity of different wavelengths in the filtered beam 342. The reference detector 314 can be configured to generate a signal 315 that is provided to the processor 310, and the filter can include information about the spectrum of the filtered beam 342 and / or the intensity of different wavelengths in the filtered beam.

[0078] The reflected radiation 360 is captured by the detector 318, and the spectrum is provided to the processor 320 for calculating the properties of the target structure T. The illumination system 312 and the detection system 318 thus form an inspection device. Such an inspection device may include a soft X-ray and / or EUV spectroscopic reflectometer of the type described in US2016282282A1, the contents of which are incorporated herein by reference in their entirety.

[0079] If the target T has a certain periodicity, the radiation of the focused beam 356 may also be partially diffracted. The diffracted radiation 397 follows another path at a well-defined angle relative to the incident angle and then relative to the reflected radiation 360. Figure 4 In the example, the extracted diffracted radiation 397 is extracted in a schematic manner, and the diffracted radiation 397 may follow many other paths besides the extraction path. The inspection device 302 may also include an additional detection system 398 for detecting and / or imaging at least a portion of the diffracted radiation 397. Figure 4, a single further detection system 398 is depicted, but embodiments of the inspection device 302 may also include more than one further detection system 398, which are arranged at different positions to detect and / or image the diffracted radiation 397 in multiple diffraction directions. In other words, the (higher) diffraction orders of the focused radiation beam incident on the target T are detected and / or imaged by one or more further detection systems 398. The one or more detection systems 398 generate a signal 399 which is provided to the metrology processor 320. The signal 399 may include information of the diffracted light 397 and / or may include an image obtained from the diffracted light 397.

[0080] To assist in the alignment and focusing of the spot S with the desired product structure, the inspection device 302 may also provide auxiliary optics that use auxiliary radiation under the control of the metrology processor 320. The metrology processor 320 may also communicate with a position controller 372 that operates the translation stage, the rotation stage, and / or the tilt stage. The processor 320 receives highly accurate feedback of the position and orientation of the substrate via sensors. The sensor 374 may include, for example, an interferometer that can give an accuracy of about picometers. In operation of the inspection device 302, the spectral data 382 captured by the detection system 318 is transmitted to the metrology processing unit 320.

[0081] As mentioned, alternative forms of inspection equipment use soft X-rays and / or EUV radiation at normal or near normal incidence, for example to perform diffraction-based measurements of asymmetries. Both types of inspection equipment can be provided in a hybrid measurement system. The performance parameters to be measured can include overlay (OVL), critical dimension (CD), coherent diffraction imaging (CDI), and overlay at resolution (ARO) measurements. The soft X-ray and / or EUV radiation can, for example, have a wavelength of less than 100 nm, for example using radiation in the range of 5 to 30 nm, optionally in the range of 10 nm to 20 nm. The radiation can be narrowband or broadband in nature. The radiation can have discrete peaks in a particular wavelength band or can have a more continuous nature.

[0082] Like optical scatterometers used in today's production facilities, inspection device 302 can be used to measure structures in resist materials processed in a lithography unit (after development inspection or ADI), and / or to measure structures after they have been 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.

[0083] Figure 5aA schematic illustration of a radiation source 500 for producing emission radiation by HHG is shown. The source 500 includes a vacuum vessel 502 and a vacuum optical system 504, which includes the optical column mentioned above to direct the emission radiation beam onto the substrate. The vacuum vessel 502 includes an interaction region 506 for receiving a medium 508 used as a target (such as a gas target). The vacuum vessel 502 includes a viewport 510 or another entrance to the vacuum vessel 502, through which drive radiation 512 enters the vacuum vessel 502. At the interaction region 506, the drive radiation 512 interacts with the medium 508 to produce emission radiation 514 by HHG. It can be seen in FIG. 5 that the drive radiation 512 continues to propagate outside the interaction region 506 in the emission direction of the emission radiation 514. For the reasons described above, a blocking filter 516 can be used to block at least a portion of the drive radiation 512 and allow at least a portion of the emission radiation 514 to pass. Typically such a filter may attenuate the drive radiation by many orders of magnitude (eg only 10-11 of the incident drive radiation may pass through the filter), and typically may pass several tens of percent (eg 20%) of the HHG radiation.

[0084] Figure 5b An exemplary apparatus for use with source 500 and for blocking drive radiation 512 from being emitted from source 500 is shown. Figure 5b The characteristics can be appropriately Figure 5a feature combination. Figure 5b The device includes a beam stop 518, which is positioned upstream of the interaction region 506 (i.e., closer to the source of the driving radiation). The beam stop 518 blocks at least a portion of the driving radiation and Figure 5b An annular drive radiation beam is generated in an arrangement of the type described herein. The term "annular" in this sense encompasses any shape of a beam having a blocked inner region. The beam stop may be a physical stop placed in the propagation path of the drive radiation 512 or may be provided by other means such as a perforated mirror or a pair of axicons.

[0085] The lens 520 is positioned downstream of the beam stop 518 and upstream of the interaction zone 506 and is arranged to focus the drive radiation 512 at the interaction zone 506. With this arrangement, a roughly Gaussian intensity distribution of the drive radiation 512 is visible at the interaction zone 506, after which the drive radiation 512 continues to propagate downstream of the interaction zone 506 as an annular beam. The blocking filter 516 includes an aperture 522. The size of the aperture 522 is designed to allow a large amount of the drive radiation 512 to be blocked and a large amount of the emission radiation 514 to pass through. Typically, the aperture may be selected so that at least 99% or 99.9% of the drive radiation is blocked, leaving the remainder of sufficiently low intensity so as not to damage any additional filters that may be placed downstream. In addition, it is desirable to limit the loss of HHG radiation by the aperture, so the aperture may be selected to allow at least a few tens of percent (e.g., 20%) to pass through.

[0086] Figure 6a Schematic diagram showing the device for use in a radiation source. Figure 6a The characteristics can be appropriately Figure 5a feature combination. Figure 6a The device includes an interaction region 606, which is positioned at an interaction plane and is configured to receive a medium (such as a gas target) for generating emission radiation by HHG. The device also includes a beam stop 618, which is positioned upstream of the interaction region 606 at the beam stop plane and is positioned to block the drive radiation beam ( Figure 6a 606 and the beam stop 618. The device also includes a lens 620 positioned upstream of the interaction zone 606 and downstream of the beam stop 618. The device also includes an aperture 622 positioned at the aperture plane downstream of the interaction zone 606. It should be noted here that although the aperture plane is generally perpendicular to the direction in which the driving radiation propagates through the source, the aperture 622 itself can be transverse to the direction in which the driving radiation propagates through the source. The term "transverse" used in this context encompasses perpendicular to, but is not necessarily limited to the definition. In an exemplary device, the aperture 622 can be angled relative to the aperture plane to reflect the driving radiation away from the propagation axis of the driving radiation. For example, the aperture can be formed in a reflector that is angled relative to the propagation axis. In such a device, at least a portion of the aperture 622 will coincide with the aperture plane. As with other devices described herein, the aperture 622 is configured to allow at least a portion of the emitted radiation to pass through and block at least a portion of the driving radiation beam. Typically SXR and / or EUV efficient designs will allow a significant fraction (eg, greater than 50%, and ideally >90%) to pass through.

[0087] The drive laser blocking efficiency can depend on the sensitivity of the downstream optics, sample and measurement to the drive radiation. Typically, this sensitivity is very high, and the residual drive radiation that passes through the blocking member can be several orders of magnitude smaller than the input beam. Therefore, typically a metal filter can be used downstream of the blocking aperture to filter out any residual drive radiation that passes through the aperture. Assuming this situation, the blocking efficiency of the aperture can be set according to the damage threshold of the downstream filter. Typically, such a filter can withstand no more than 1W of laser, corresponding to a blocking efficiency of around 99%.

[0088] It should be noted that within the present invention, imaging means that the image plane is positioned within the depth of focus (DOF) of the lens. Imaging the beam stop is defined as when the distance between the aperture plane and the conjugate plane of the beam stop plane is less than the depth of focus. Imaging the spatial distribution of the drive radiation beam is defined as when the distance between the interaction plane and the conjugate plane of the object plane is less than the depth of focus.

[0089] The definition of forming a clear image is that the image plane is a conjugate plane.

[0090] exist Figure 6a In the arrangement of , the aperture plane is positioned relative to the beam blocking plane and the lens 620 so that an image of the beam blocking member is formed at the aperture plane. The beam blocking plane and the aperture plane are mounted so that they can be conjugate planes. This results in a driving radiation beam cross-section having a well-defined hole in the center when it arrives at the aperture plane (i.e. the laser beam will be annular at the aperture plane), thereby allowing the driving laser beam to be effectively blocked by the aperture while transmitting the emitted (e.g. SXR and / or EUV) beam.

[0091] like Figure 6a As can be seen in FIG. 1 , the device also includes an object plane. The object plane may be a conjugate plane of the gas target plane, i.e., the lens forms a sharp image of the object plane in the gas target plane. In an exemplary device (e.g. Figure 6c ), a desired intensity distribution of the driving radiation is formed in the object plane. Figure 8 In some exemplary arrangements, and as explained below, the object plane and the interaction plane may also be positioned so that a clear image of the intensity of the drive radiation at the object plane is formed at the interaction plane, i.e., they are conjugate planes.

[0092] Figure 6b A schematic representation of the beam stop 618, the lens 620, the interaction region 606 and the aperture 622 is shown. The relative distances between the beam stop plane and the lens 620, and between the lens 620 and the aperture plane can be calculated using the formula:

[0093]

[0094] d lens-aperture =(1+m b )f

[0095] Where m b is the magnification factor of the beam stop to the image of the beam stop provided by lens 620 , and f is the focal length of lens 620 .

[0096] The driving radiation intensity distribution at both the beam blocking plane 624 and the aperture plane 626 is shown in Figure 6b , and shows that the intensity in the central region of the drive radiation 626 at the aperture plane is zero.

[0097] As described above, in some exemplary arrangements, the object plane and the interaction plane are also conjugate planes, so that drive radiation having a desired intensity distribution at the interaction plane 630 is imaged at the object plane 628. This is shown in Figure 6c The relative distances between the object plane and the lens 620, and between the lens 620 and the interaction plane can be calculated using the formula:

[0098]

[0099] d lens-inter =(1+m s )f

[0100] Where m s is the magnification factor of the object plane to the image of the object plane provided by the lens 620 , and f is the focal length of the lens 620 .

[0101] Such an arrangement can ensure that in addition to the beam stop plane being imaged at the aperture plane, the object plane (with the correct intensity distribution of the driving radiation) is imaged at the interaction plane. In some such exemplary arrangements, the image of the object plane can be decoupled from the image of the beam stop plane so that they do not interfere with each other, or at least their mutual interference is small enough that the operation of the source is not adversely affected. In such a context, the term "decoupled" can cover the following situations: the presence of the beam stop does not significantly affect the intensity distribution of the driving radiation at the object plane, and the presence of a non-Gaussian beam at the object plane does not significantly affect the intensity distribution of the driving radiation at the aperture plane.

[0102] In an exemplary arrangement, the size of the beam stop 618 at the beam stop plane relative to the size of the driving radiation at the beam stop plane may be configured to ensure that an image of the beam stop 618 is not present in the interaction region. In some exemplary arrangements, the size of the beam stop 618 may be at least 20%, at least 40%, at least 50%, or at least 70% smaller than the corresponding size of the driving radiation beam. Furthermore, in an exemplary arrangement, the size of the beam stop 618 may be 10% to 70%, or 20% to 60%, or 30% to 50% smaller than the corresponding size of the driving radiation beam.

[0103] In other exemplary arrangements, the dimensions of the beam stop 618 and the drive radiation at the beam stop plane may be configured to achieve a Strehl ratio of 0.8 or greater. In arrangements where the drive radiation beam and the beam stop 618 have circular cross-sections, the dimensions may be diameters.

[0104] Decoupling may also be defined in terms of the depth of focus (DOF) of one or both of the image of the beam stop at the aperture plane and the image of the drive radiation beam at the interaction plane. The DOF in this context may be defined as the range from the lens within which an image may be formed, as explained in more detail below.

[0105] For purposes of the methods and apparatus disclosed herein, the interaction plane may be positioned outside the DOF of the beam stop image. That is, in some exemplary arrangements where the aperture plane coincides with the center of the DOF of the beam stop image, the aperture plane may be sufficiently far from the interaction plane that the interaction plane falls outside the DOF of the beam stop image. An image of the beam stop is not formed at the interaction plane.

[0106] Similarly, the aperture plane may be positioned such that it lies outside the DOF of the image of the driving radiation intensity distribution formed at the interaction plane. That is, in some exemplary arrangements where the interaction plane coincides with the center of the DOF of the image of the driving radiation intensity distribution, the interaction plane may be sufficiently far from the aperture plane such that the aperture plane falls outside the DOF of the image of the driving radiation intensity distribution. The image of the driving radiation intensity distribution at the object plane is formed at the aperture plane.

[0107] Figure 7A schematic diagram illustrating the principle of DOF is shown. As can be seen, the radiation rays originating from the beam stop 618 are focused by the lens 620 to a point that coincides with the aperture plane. This point is the point at which the image of the beam stop 618 can be in sharp focus and can be the center of the DOF. At locations upstream and downstream of the center of the DOF, the degree to which the image of the beam stop 618 is out of focus increases with increasing distance away from the center of the DOF. At these locations away from the center of the DOF, the radiation rays from the beam stop 618 are distributed on a blurred spot, which is called the circle of confusion.

[0108] The DOF of the beam stop image is the distance on either side of the aperture plane within which the circle of confusion is less than or equal to the maximum diameter c. The diameter c can be set to a size whereby the beam stop image is blurred over the relevant radial domain. The interaction plane can be positioned outside the DOF at a distance from the aperture plane that is further than the distance that would result in a circle of confusion of diameter c. That is, at the gas target plane, the circle of confusion should be larger than the size of the field distribution in the gas target plane, which is typically tens of microns. Half of the DOF should be less than the distance between the gas target plane and the aperture plane.

[0109] DOF can be calculated using the following formula:

[0110] DOF=2cN(1+m)

[0111] Where N is the ratio of the focal length to the beam diameter at the lens 620 (typically about 100), and m is the magnification of the beam stop image by the lens 620 (typically about 1). Using those typical features and assuming c=30 μm, the DOF is approximately 10 mm. Features of the devices disclosed herein can be configured so that the DOF of the beam stop image is in the range of 7 mm to 13 mm, 8 mm to 12 mm, and in specific examples 10 mm to 11 mm. In some devices, the diameter c of the circle of confusion of the beam stop image is 35 μm or less or 30 μm or less.

[0112] An exemplary device for a radiation source may include a beam shaper. The beam shaper may be any device capable of controlling the intensity distribution of the driving radiation, which may be generated by a laser as discussed above. Examples of beam shapers include a spatial light modulator (SLM), a flat-top beam shaper, and a deformable mirror. The beam shaper may be placed significantly closer to the beam stop than the actual object plane, so that the overall system size may be significantly reduced. The beam shaper may be used to generate a driving radiation beam having a desired intensity distribution at the object plane. In other devices, the beam shaper may also provide a desired intensity distribution at any other plane upstream of the beam stop. In exemplary devices using a beam shaper, there may be no actual object plane to the left of the lens that reproduces the desired distribution at the target plane, in which case the object plane may be considered a virtual object plane.

[0113] Figure 8 An exemplary device for use with a radiation source is shown, wherein a beam shaper 800 is positioned upstream of the beam stop 618. The beam shaper 800 receives driving radiation 802 from a driving radiation source (not shown) and having, for example, a broad Gaussian intensity distribution, and produces driving radiation 804 having a specific intensity distribution. The device also includes a sensor and a feedback controller 806. The sensor senses the intensity distribution of the driving radiation at the interaction plane and transmits it to the feedback controller 806. The feedback controller 806 transmits data related to the sensed intensity distribution to the beam shaper 804, which controls the intensity distribution of its output based thereon. In an embodiment, a 1% beam splitter is used to project a portion of the driving radiation toward a CCD camera used as the sensor indicated above. The position of such a beam splitter can be seen in the transmission direction of the driving radiation only before or after the lens.

[0114] Figure 8 The device may include one or more features of the device disclosed above, and may be used with Figure 5a An appropriate combination of features.

[0115] Fig. 9 An exemplary arrangement is shown comprising two lenses 620a, 620b. This allows the overall length of the system to be reduced. Fig. 9 It may also include one or more of the features disclosed with respect to the other devices above, and / or may be combined with Figure 5a An appropriate combination of features.

[0116] Fig. 9The two lens system of does not provide an intermediate focus of the driving radiation, as this would lead to degradation of the driving radiation beam due to ionization of the air. The second lens 620b is a negative lens, which forms a virtual intermediate image 900 instead of an actual intermediate image of the intensity distribution of the driving radiation at the object plane. The second lens 620b is a negative lens. The second lens 620b is upstream of the beam stop 618.

[0117] The distance of the second lens 620b from the object plane can be determined by the following equation:

[0118]

[0119] Where m_ is the magnification of the image of the intensity of the driving radiation beam produced by the second lens 620b, and f_ is the focal length of the second lens 620b. The distance of the second lens 620b from the virtual intermediate image 900 can be determined by the following equation:

[0120] d lens2-virtual =(1-m - )|f - |

[0121] The distance between the first lens 620a and the virtual intermediate image 900 may be determined by the following equation:

[0122]

[0123] Where m + is the magnification of the virtual image 900 of the intensity of the driving radiation beam produced by the first lens 620a, and f + is the focal length of the first lens 620a. The distance between the first lens 620a and the virtual intermediate image 900 can be determined by the following equation:

[0124] d lens1-inter =(1+m + )f +

[0125] Fig.10 A flow chart showing a method for causing interaction between a drive radiation beam and a medium for producing emission radiation by high order harmonic generation.

[0126] The driving radiation generated by a driving radiation source such as a laser is propagated 1000 to the vacuum container, such as Figure 5a1002. At least a portion of the drive radiation is blocked 1002 by a beam stop 618. The beam stop 618 may be as described above or have any other form known to those skilled in the art. The partially blocked drive radiation propagates through a lens 1004, which focuses it onto an interaction plane. The focused drive radiation is incident on a medium 1006, such as a gas target positioned downstream of the beam stop and the lens. The lens also focuses the drive radiation originating from the beam stop 618 at an aperture plane positioned downstream of the interaction plane, such that an image of the beam stop 618 is formed at the aperture plane.

[0127] Further embodiments are defined in subsequently numbered aspects:

[0128] 1. A radiation source device operable to cause interaction between a drive radiation beam and a medium to produce emitted radiation by high-order harmonic generation, the radiation source device comprising:

[0129] an interaction region positioned at the interaction plane and configured to receive the medium;

[0130] a beam stop positioned at a beam stop plane upstream of the interaction plane and configured to partially block the drive radiation beam; and

[0131] A beam shaper is positioned at the object plane upstream of the beam block plane and is configured to control a spatial distribution of the drive radiation beam.

[0132] 2. An apparatus according to aspect 1, wherein at least one lens is positioned upstream of the interaction plane and downstream of the beam blocking plane, wherein the lens is positioned such that an image of the spatial distribution of the drive radiation beam is formed at the interaction plane.

[0133] 3. An apparatus according to clause 1 or 2, wherein the lens is positioned so that the object plane and the interaction plane are conjugate planes.

[0134] 4. A device according to any of the preceding aspects, wherein the hole is positioned at an hole plane downstream of the interaction plane and is configured to allow at least a portion of the emitted radiation to pass through and is configured to block at least a portion of the drive radiation beam, wherein the hole plane is positioned relative to the beam blocking plane and the lens so that an image of the beam blocking member is formed at the hole plane.

[0135] 5. The apparatus of clause 4, wherein the lens is positioned so that the beam blocking plane and the aperture plane are conjugate planes.

[0136] 6. An apparatus according to aspect 4 or 5, wherein the relationship between the size of the beam stop in the beam blocking plane and the size of the driving radiation beam in the beam blocking plane is such that the image of the beam stop is decoupled from the image of the spatial distribution of the driving radiation beam.

[0137] 7. Apparatus according to clause 6, wherein a dimension of the beam stop in the beam blocking plane is 30% or less of a dimension of the drive radiation beam in the beam blocking plane.

[0138] 8. Apparatus according to clause 7, wherein the beam stop and the drive radiation beam have substantially circular cross-sections in the beam stop plane, and wherein the dimensions of the beam stop and the drive radiation beam are diameters.

[0139] 9. Apparatus according to any preceding clause, wherein the depth of focus of the image of the beam stop does not overlap the interaction plane.

[0140] 10. The apparatus of clause 9, wherein the center of the depth of focus of the image of the beam stop substantially coincides with the aperture plane.

[0141] 11. Apparatus according to aspects 9 or 10 when appended directly or indirectly to aspect 4, wherein the circle of confusion associated with the depth of focus of the image of the beam stop is larger than the image of the drive radiation beam at the interaction plane.

[0142] 12. The apparatus of clause 11, wherein the depth of focus of the image of the beam stop has a maximum circle of confusion with a diameter of 35 μm or less.

[0143] 13. An apparatus according to any preceding clause, wherein the focal depth of the spatially distributed image of the drive radiation beam does not overlap with the aperture plane.

[0144] 14. Apparatus according to clause 13, wherein the centre of the depth of focus of the image of the intensity distribution of the drive radiation beam substantially coincides with the interaction plane.

[0145] 15. Apparatus according to any one of clauses 9 to 14, wherein the depth of focus of the image of the beam stop and / or the depth of focus of the image of the spatial distribution of the drive radiation beam is determined by:

[0146] Depth of focus = 2cN (1+m)

[0147] wherein c is the circle of maximum confusion, N is the ratio of the focal length of the lens to the diameter of the drive radiation beam at the lens, and m is the magnification factor of the image of the beam stop at the aperture plane.

[0148] 16. Apparatus according to any one of clauses 4 to 15 when appended directly or indirectly to clause 3, wherein the beam shaper comprises a spatial light modulator positioned at the object plane.

[0149] 17. The apparatus of clause 16, the radiation source arrangement further comprising: a sensor configured to detect a spatial profile of the drive radiation beam at the interaction plane; and

[0150] a feedback controller configured to feed back data relating to the detected spatial profile of the drive radiation beam to the spatial light modulator,

[0151] Wherein the spatial light modulator is configured to control the spatial profile of the drive radiation beam based on feedback data.

[0152] 18. A measurement device, comprising the apparatus according to any one of the preceding aspects.

[0153] 19. A lithocell comprising the apparatus according to any one of the preceding aspects or the metrology equipment according to aspect 18.

[0154] 20. A method of causing interaction between a drive radiation beam and a medium for producing emitted radiation by high-order harmonic generation, the method comprising:

[0155] blocking the drive radiation beam by a beam stop such that at least a portion of the drive radiation beam is blocked;

[0156] causing the drive radiation beam to propagate through at least one lens positioned downstream of the beam stop;

[0157] causing the drive radiation beam to be incident on an interaction region, the interaction region comprising the medium and positioned downstream of the beam stop; and

[0158] The drive radiation is focused at an aperture positioned at an aperture plane downstream of the interaction zone so that an image of the beam stop is formed at the aperture plane, the aperture being configured to allow at least a portion of the emitted radiation to pass therethrough and being configured to block at least a portion of the drive radiation beam.

[0159] In the context of the present document, the term HHG or HHG source is introduced. HHG refers to High Harmonic Generation, which is sometimes also called high order harmonic generation. HHG is a nonlinear process in which a target (e.g., a gas, plasma, or solid sample) is irradiated at the interaction zone by an intensive laser pulse of a driving radiation. Subsequently, the target can emit radiation with a frequency that is a multiple of the frequency of the driving radiation of the laser pulse. Such a frequency that is a multiple is called a harmonic of the radiation of the laser pulse. The generated HHG radiation can be defined as harmonics above the fifth order harmonic, and these harmonics are called high order harmonics.

[0160] The physical process that forms the basis of the HHG process is different from the physical process involving the generation of radiation of lower harmonics (usually 2nd to 5th order harmonics). The generation of radiation of lower harmonics involves perturbation theory. The trajectory of the (bound) electrons of the atoms in the target is largely determined by the Coulomb potential of the matrix ions.

[0161] In HHG, the trajectory of the electrons contributing to the HHG process is generally determined by the electric field of the incident driving laser. In the so-called "three-step model" describing HHG, the electrons tunnel through the Coulomb barrier, which is now generally suppressed by the laser field (step 1), along a trajectory determined by the laser field (step 2), and recombine with a certain probability while releasing their kinetic energy and ionization energy in the form of radiation (step 3). Another way to express the difference between HHG and the generation of lower harmonic radiation in words is to define all radiation with photon energies above the ionization energy of the target atoms as "higher-order harmonic" radiation, such as HHG-generated radiation, and to define all radiation with photon energies below the ionization energy as non-HHG-generated radiation. If neon is used as a gas target, all radiation with wavelengths shorter than 62 nm (with photon energies above 20.18 eV) is generated by means of the HHG process. For argon as a gas target, all radiation with photon energies above about 15.8 eV is generated by means of the HHG process.

[0162] Although specific reference may be made herein to the use of lithographic equipment in IC manufacturing, it should be understood that the lithographic equipment described herein may have other applications. Possible other applications include manufacturing integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0163] Although reference may be made herein specifically to embodiments of the invention in the context of lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other pattern forming device). These apparatuses may be generally referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0164] Although specific reference is made to "measurement equipment / tool / system" or "inspection equipment / tool / system", these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of a structure on a substrate or on a wafer. For example, an inspection equipment or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or on a wafer. In such embodiments, the characteristic of interest of a structure on a substrate may be related to a defect in the structure, the absence of a particular portion of a structure, or the presence of an unwanted structure on a substrate or on a wafer.

[0165] Although specific reference may be made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0166] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be appreciated by those skilled in the art that modifications may be made to the present invention as described without departing from the scope of the claims set forth below.

Claims

1. A radiation source device operable to cause interaction between a drive radiation beam and a medium to produce emitted radiation by high-order harmonic generation, the radiation source device comprising: an interaction region positioned at the interaction plane and configured to receive the medium; a beam stop positioned at a beam stop plane upstream of the interaction plane and configured to partially block the drive radiation beam; a beam shaper positioned at the object plane upstream of the beam block plane and configured to control a spatial distribution of the drive radiation beam; as well as at least one lens positioned upstream of the interaction plane and downstream of the beam stop plane, wherein the lens is positioned such that an image of the spatial distribution of the drive radiation beam is formed at the interaction plane, The beam shaper comprises a spatial light modulator, a flat-top beam shaper, a deformable mirror, or a combination thereof, and Therein, the beam shaper is configured to control the spatial distribution of the driving radiation beam such that the driving radiation beam is a non-Gaussian beam located at the object plane.

2. The radiation source arrangement of claim 1, wherein the lens is positioned such that the object plane and the interaction plane are conjugate planes.

3. A radiation source device according to any of the preceding claims, wherein an aperture is positioned at an aperture plane downstream of the interaction plane and is configured to allow at least a portion of the emitted radiation to pass therethrough and is configured to block at least a portion of the drive radiation beam, wherein the aperture plane is positioned relative to the beam blocking plane and the lens so that an image of the beam blocking member is formed at the aperture plane.

4. The radiation source arrangement of claim 3, wherein the lens is positioned such that the beam blocking plane and the aperture plane are conjugate planes.

5. A radiation source device according to claim 3, wherein the relationship between the size of the beam stop in the beam blocking plane and the size of the driving radiation beam in the beam blocking plane is such that the image of the beam stop is decoupled from the image of the spatial distribution of the driving radiation beam. 6 . The radiation source arrangement of claim 5 , wherein a dimension of the beam stop in the beam blocking plane is 30% or less of a dimension of the drive radiation beam in the beam blocking plane.

7. The radiation source arrangement of claim 6, wherein the beam stop and the drive radiation beam have substantially circular cross-sections in the beam stop plane, and wherein the dimensions of the beam stop and the drive radiation beam are diameters.

8. The radiation source arrangement of claim 3, wherein a depth of focus of an image of the beam stop does not overlap the interaction plane.

9. The radiation source arrangement of claim 8, wherein the center of the depth of focus of the image of the beam stop substantially coincides with the aperture plane.

10. The radiation source arrangement of claim 8, wherein a circle of confusion associated with a depth of focus of an image of the beam stop is larger than an image of the drive radiation beam at the interaction plane. 11 . The radiation source arrangement of claim 10 , wherein a depth of focus of the image of the beam stop has a maximum circle of confusion with a diameter of 35 μm or less.

12. The radiation source arrangement of claim 1 or 2, wherein a depth of focus of the spatially distributed image of the drive radiation beam does not overlap with an aperture plane of an aperture positioned downstream of the interaction plane.

13. The radiation source arrangement of claim 12, wherein a center of the depth of focus of the image of the spatial distribution of the drive radiation beam substantially coincides with the interaction plane.

14. The radiation source arrangement of claim 8, wherein at least one of the depth of focus of the image of the beam stop and the depth of focus of the image of the spatial distribution of the drive radiation beam is determined by: Depth of focus = 2cN(1+m) wherein c is the circle of maximum confusion, N is the ratio of the focal length of the lens to the diameter of the drive radiation beam at the lens, and m is the magnification factor of the image of the beam stop at the aperture plane.

15. The radiation source device according to claim 1 or 2, further comprising: a sensor configured to detect a spatial profile of the drive radiation beam at the interaction plane; and a feedback controller configured to feed back data relating to the detected spatial profile of the drive radiation beam to the beam shaper, Wherein the beam shaper is configured to control the spatial profile of the drive radiation beam based on fed-back data.

16. A measuring device, comprising: An arrangement operable to cause interaction between a drive radiation beam and a medium to produce emitted radiation by high-order harmonic generation, the arrangement comprising: an interaction region positioned at the interaction plane and configured to receive the medium; a beam stop positioned at a beam stop plane upstream of the interaction plane and configured to partially block the drive radiation beam; a beam shaper positioned at the object plane upstream of the beam block plane and configured to control the spatial distribution of the drive radiation beam; and at least one lens positioned upstream of the interaction plane and downstream of the beam stop plane, wherein the lens is positioned such that an image of the spatial distribution of the drive radiation beam is formed at the interaction plane, The beam shaper comprises a spatial light modulator, a flat-top beam shaper, a deformable mirror, or a combination thereof, and Therein, the beam shaper is configured to control the spatial distribution of the driving radiation beam such that the driving radiation beam is a non-Gaussian beam located at the object plane.

17. A lithography cell comprising a metrology apparatus according to claim 16.

18. A method of causing interaction between a drive radiation beam and a medium for producing emitted radiation by high order harmonic generation, the method comprising: shaping the drive radiation beam using a beam shaper to control a spatial distribution of the drive radiation beam, wherein the beam shaper comprises a spatial light modulator (SLM), a flat-top beam shaper, a deformable mirror, or a combination thereof, and wherein the beam shaper is configured to control the spatial distribution of the drive radiation beam such that the drive radiation beam is a non-Gaussian beam at an object plane; blocking the drive radiation beam by a beam stop positioned downstream of the beam shaper such that at least a portion of the drive radiation beam is blocked; causing the drive radiation beam to propagate through at least one lens positioned downstream of the beam stop; causing the drive radiation beam to be incident on an interaction region, the interaction region comprising the medium and positioned downstream of the beam stop; and The drive radiation is focused at an aperture positioned at an aperture plane downstream of the interaction zone so that an image of the beam stop is formed at the aperture plane, the aperture being configured to allow at least a portion of the emitted radiation to pass therethrough and being configured to block at least a portion of the drive radiation beam.

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