Method and apparatus for efficient high-order harmonic generation

Abstract

A high-harmonic radiation source and associated method for generating high-harmonic radiation are disclosed. The high-harmonic radiation source is configured to condition the gaseous medium by irradiating the gaseous medium with a pre-pulse of radiation, thereby generating a plasma comprising a pre-pulse plasma distribution; and irradiating the gaseous medium with a main pulse of radiation to generate the high-harmonic radiation. The conditioning step causes the plasma comprising the pre-pulse plasma distribution to be used to configure the wavefront of the main pulse to improve one or both of the following: the efficiency of the high-harmonic generation process, and the beam quality of the high-harmonic radiation. The high-harmonic radiation source may also include a beam shaping device configured to shape the customized pre-pulse prior to the conditioning.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from European application 19205875.8, filed on October 29, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to the generation of high harmonic radiation and, in particular, to such high harmonic radiation generators in connection with metrology applications in the manufacture of integrated circuits. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. For example, a lithographic apparatus may be used in the manufacture of integrated circuits (ICs). A lithographic apparatus may project a pattern (often 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).

[0005] 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 the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. A lithographic apparatus using extreme ultraviolet (EUV) radiation (having a wavelength in the range of 4-20 nm, such as 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate than a lithographic apparatus using, for example, radiation having a wavelength of 193 nm.

[0006] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD = k1 × λλ / NA, where λλ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature printed, but in this case half the pitch), and k1 is an empirical resolution factor. In general, the smaller k1 is, the more difficult it becomes to reproduce a pattern on the substrate that resembles the shape and dimensions 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 are not limited to: optimization of the 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 techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic equipment can be used to improve the reproduction of the pattern at low k1.

[0007] Current or near-future measurement tools for the semiconductor industry may be configured to use radiation in a wavelength range extending roughly from 1 nm to 100 nm, for example, where visible light begins to provide insufficient spatial resolution for continuously shrinking feature sizes. One proposal for achieving this is to generate such radiation using high harmonic generation (HHG) technology. In HHG, intense laser pulses of visible or infrared (IR) radiation are focused into a gas target or gas target, causing the gas atoms to emit radiation in the desired range due to their interaction with the laser. The HHG-generated light can then be focused onto a target on the wafer by transferring light from the HHG source to the target through a beam of light. The reflected light is detected and processed to infer properties of the target.

[0008] It would be desirable to improve the generation efficiency and / or beam quality of radiation produced by a HHG source. Summary of the Invention

[0009] In a first aspect of the present invention, a high harmonic radiation source is provided, the high harmonic radiation source comprising a gaseous medium and configured to: condition the gaseous medium by irradiating the gaseous medium with a pre-pulse of radiation, thereby generating a plasma comprising a pre-pulse plasma distribution; irradiate the gaseous medium with a main pulse of radiation to generate the high harmonic radiation, wherein the conditioning step causes the plasma comprising the pre-pulse plasma distribution to configure a wavefront of the main pulse to improve one or both of: the efficiency of the high harmonic generation process, and the beam quality of the high harmonic radiation.

[0010] In a second aspect of the present invention, a method of generating high-harmonic radiation in a high-harmonic generation process is provided, comprising: generating a pre-pulse of radiation; conditioning a gaseous medium with the pre-pulse to produce a plasma comprising a pre-pulse plasma profile; irradiating the gaseous medium with a main pulse of radiation to generate the high-harmonic radiation, wherein the pre-pulse plasma profile is configured to configure a wavefront of the main pulse to improve one or both of: efficiency of the high-harmonic generation process, and beam quality of the high-harmonic radiation. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0012] - Figure 1 A schematic overview diagram depicting a lithographic apparatus;

[0013] - Figure 2 A schematic overview depicting a lithography cell;

[0014] - Figure 3 Depicts a schematic representation of overall lithography, showing the collaboration between three key technologies used to optimize semiconductor manufacturing;

[0015] - Figure 4 Schematic illustration of a scatterometer apparatus;

[0016] - Figure 5 Depicting a schematic representation of a metrology apparatus in which EUV and / or SXR radiation is used;- Figure 6 Schematically illustrates a single laser-based high harmonic radiation source according to a first embodiment;

[0017] - Figure 7 schematically illustrates a ring-shaped plasma distribution according to an embodiment;

[0018] - Figure 8 schematically illustrates a dual-laser based high harmonic radiation source according to a second embodiment;

[0019] - Figure 9 schematically illustrates an alternative focusing arrangement comprising two focal planes according to an embodiment;

[0020] - Figure 10 Flowchart including steps in a method of generating high harmonic radiation according to an embodiment. DETAILED DESCRIPTION

[0021] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation and particle radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm), EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 nm to 100 nm), X-ray radiation, electron beam radiation, and other particle radiation.

[0022] As used herein, the terms "reticle," "mask," or "patterning device" should 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, corresponding to the pattern to be created 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-shift, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0023] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also called 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., mask table) T configured to support a patterning device (e.g., mask) MA and connected to a second positioner PW configured to accurately position the patterning device MA according to specific parameters, a substrate support (e.g., 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 specific parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0024] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, diffractive, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0025] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems, including refractive, reflective, diffractive, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate to the exposure radiation used, and / or 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.

[0026] 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 techniques is given in US Pat. No. 6,952,253, which is incorporated herein by reference in its entirety.

[0027] 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 the other substrate support WT is being used to expose a pattern on the other substrate W.

[0028] In addition to the substrate support WT, the lithographic apparatus LA can include a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning devices. The sensors can be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform can hold multiple sensors. The cleaning devices can be arranged to clean a portion of the lithographic apparatus, such as a portion of the projection system PS or a portion of the system for providing immersion liquid. The measurement platform can be moved beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0029] 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 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 a substrate W. With the aid of a second positioning device PW and a position measurement system IF, the substrate support WT can be accurately moved, e.g. in order to position a different target portion C in a focused and aligned position in the path of the radiation beam B. Similarly, the first positioning device PM and possibly another position sensor ( Figure 1 The patterning device MA may be accurately 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 lane alignment marks.

[0030] like Figure 2 As shown, the lithography apparatus LA may form part of a lithography cell LC (sometimes also referred to as a litho cell or (lithography) cluster), which typically also includes equipment for performing pre- and post-exposure processes on a substrate W. Typically, this equipment includes 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, such as for regulating the solvent in the resist layer. A substrate handling device or robot RO picks up substrates W from input ports I / O1 and output ports I / O2, moves them between the various process equipment, and transfers the substrates W to a loading station LB of the lithography apparatus LA. The equipment in the litho cell, also commonly referred to as a coating and developing system or track, is typically under the control of a coating and developing system control unit or track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithography apparatus LA, for example, via a litho control unit LACU.

[0031] During photolithography, it is frequently desirable to measure the structures being created, for example, for process control and verification. The tool used to perform such measurements is often referred to as a metrology tool MT. Different types of metrology tools MT for performing such measurements are well known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow measurements of parameters of the photolithography process to be performed using a sensor in the pupil of the scatterometer objective or in a plane conjugate to the pupil, often referred to as pupil-based measurements, or using a sensor in the image plane or in a plane conjugate to the image plane, often referred to as image-based or field-based measurements. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP 1,628,164 A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure gratings using light from soft x-rays, extreme ultraviolet and visible light to near IR wavelength ranges.

[0032] In order to ensure that the substrates W exposed by the lithographic apparatus LA are correctly and consistently exposed, the substrates need to be inspected to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, inspection tools and / or metrology tools (not shown) may be included in the lithographic cell LC. If an error is detected, for example, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on the substrate W, particularly if the inspection is being performed before other substrates W from the same batch or lot are yet to be exposed or processed.

[0033] An inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of a substrate W, in particular how properties vary between different substrates W, or how properties associated with different layers of the same substrate W vary between layers. The inspection apparatus may alternatively be configured to identify defects on the substrate W and may, for example, be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a separate device. 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), or a developed resist image (where exposed or unexposed portions of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0034] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signals to reconstruct or calculate the properties of the grating. This reconstruction can be generated, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0035] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed to a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile of the target that gave rise to the detected spectrum can be reconstructed, for example, by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0036] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. Ellipsometric scatterometers allow parameters of a lithographic process to be determined by measuring scattered radiation for each polarization state. Such metrology equipment emits polarized light (such as linearly, circularly, or elliptically polarized light) by using, for example, appropriate polarization filters in the illumination section of the metrology equipment. A source suitable for the metrology equipment can also provide polarized radiation. Various embodiments of conventional ellipsometric scatterometers 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, which are incorporated herein by reference in their entirety.

[0037] 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 the reflection spectrum and / or an asymmetry in the detection configuration (the asymmetry being related to the extent of the overlap). The two (typically, superimposed) grating structures can be applied in two different layers (not necessarily consecutive layers), and the two grating structures can be formed to be in substantially the same position on the wafer. The scatterometer can have a symmetrical detection configuration as described, for example, in commonly owned patent application EP1,628,164A, so that any asymmetry is clearly distinguishable. This provides a simple way to measure misalignment in the gratings. Further examples of measuring the overlay error between two layers containing a periodic structure as a target via the asymmetry of the periodic structure can be found in PCT Patent Application Publication No. WO2011 / 012624 or U.S. Patent Application No. US20160161863, both of which are incorporated herein by reference in their entirety.

[0038] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety (or alternatively by scanning electron microscopy). A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM—also known as a focus exposure matrix). If these unique combinations of critical dimension and sidewall angle can be obtained, focus and dose values ​​may be uniquely determined based on these measurements.

[0039] The metrology target can be a collection of composite gratings formed primarily in resist by a photolithography process and also formed after, for example, an etching process. Typically, the pitch and linewidth of the structures in the gratings depend heavily on the measurement optics (specifically, the NA of the optics) to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffraction signal can be used to determine the shift between two layers (also known as "overlay") or to reconstruct at least a portion of the original grating as produced by the photolithography process. This reconstruction can be used to provide guidance on the quality of the photolithography process and can be used to control at least a portion of the photolithography process. The target can have smaller subsections configured to mimic the dimensions of a functional portion of the design layout in the target. Due to these subsections, the target will behave more similarly to the functional portion of the design layout, so that the overall process parameter measurement better resembles the functional portion of the design layout. The target can be measured in either an underfill mode or an overfill mode. In underfill mode, the measurement beam produces a spot that is smaller than the overall target. In overfill mode, the measurement beam produces a spot that is larger than the entire target. In such an overfill mode, it is also possible to measure different targets simultaneously and thus to determine different process parameters simultaneously.

[0040] The overall measurement quality of a lithography parameter performed using a particular target is determined at least in part by the measurement profile used to measure such lithography parameter. The term "substrate measurement profile" may include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement profile is an optical measurement based on diffraction, one or more of the parameters measured may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and so on. One of the criteria for selecting a measurement profile may, for example, be the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016-0161863 and U.S. patent publication US2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0041] Typically, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process, which requires a high degree of accuracy in the sizing and placement of structures on the substrate W. In order to ensure this high accuracy, e.g. Figure 3 As schematically depicted in FIG, three systems can be combined in a so-called "holistic" control environment. One of the systems is the lithography apparatus LA, which is (effectively) connected to the metrology tool MET (the second system) and to the computer system CL (the third system). The key to this "holistic" environment is to optimize the coordination 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 a 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). Variations in process parameters in the lithography process or patterning process can be tolerated within this defined result.

[0042] 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 layouts and lithographic equipment settings achieve the maximum total process window (in terms of resolution enhancement technology) for the patterning process. 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 a metrology tool MET) to predict whether defects may be present due to, for example, suboptimal processing (e.g., in the example of FIG. 1 ). Figure 3 ) as depicted by the arrow pointing to “0” in the second scale SC2.

[0043] 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 possible drifts, e.g. in a calibrated or normalized state of the lithographic apparatus LA (e.g., in a calibrated or normalized state of the lithographic apparatus LA). Figure 3 (depicted by multiple arrows in the third scale SC3).

[0044] During photolithography, it is necessary to frequently measure the structures being created, for example, for process control and verification. Various tools are known for making these measurements, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers. Examples of known scatterometers often rely on the setup of a dedicated metrology target, such as an underfilled target (a target in the form of stacked gratings or simple gratings in different layers that is large enough so that the measurement beam produces a smaller spot than the grating) or an overfilled target (whereby the illumination spot partially or completely encompasses the target). In addition, the use of metrology tools (e.g., angle-resolved scatterometers that illuminate an underfilled target such as a grating) allows the use of so-called reconstruction methods, in which the properties of the grating can be calculated by simulating the interaction of scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0045] Scatterometers are versatile instruments that allow measurements of parameters of a lithographic process to be made by having a sensor in the pupil of the scatterometer objective or in a plane conjugate to the pupil, commonly referred to as pupil-based measurements, or by having a sensor in the image plane or in a plane conjugate to the image plane, commonly referred to as image-based or field-based measurements. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, all of which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure multiple targets from multiple gratings in a single image using light ranging from soft x-rays, extreme ultraviolet, and visible light to near-IR wavelengths.

[0046] exist Figure 4A measurement device such as a scatterometer is depicted in FIG. It comprises a broadband (e.g. white light) radiation projector 2 that projects radiation 5 onto a substrate W. The reflected or scattered radiation 10 is passed to a spectrometer detector 4 which measures the spectrum of the specularly reflected radiation (i.e. a measurement of the intensity as a function of wavelength). From this data, the processing unit PU can generate a spectrum of the specularly reflected radiation (i.e. a measurement of the intensity as a function of wavelength), for example by rigorous coupled wave analysis and nonlinear regression, or by a method such as in FIG. Figure 4 The structure or profile that produced the detected spectrum is reconstructed by comparing it to a library of simulated spectra, as shown at the bottom of the figure. Typically, for the 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. Such a scatterometer can be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0047] As an alternative to optical metrology methods, the use of soft X-rays or EUV radiation is also contemplated, 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. An example of a metrology tool operating in one of the wavelength ranges presented above is transmission small-angle X-ray scattering (T-SAXS, such as that described in US2007224518A, 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-rays (GI-XRS) and extreme ultraviolet (EUV) radiation at grazing incidence are known for measuring properties of films and overlying layers on substrates. Within the general field of reflectometry, goniometric and / or spectroscopic techniques can be applied. In goniometry, the variation in 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 to inspect mask substrates prior to the manufacture of reticles (patterning devices) used in EUV lithography.

[0048] The range of applications may make the use of soft X-rays or wavelengths in the EUV domain insufficient. Therefore, published patent applications US20130304424A1 and US2014019097A1 (Bakeman et al. / KLA) describe hybrid metrology techniques in which measurements using X-rays are combined with optical measurements using wavelengths between 120 nm and 2000 nm to obtain measurements of parameters such as CD. The measurements are obtained by coupling X-ray and optical mathematical models (via one or more common models). The contents of the cited US patent applications are incorporated herein by reference in their entirety.

[0049] Figure 5 Depicted is a schematic representation of a metrology device 302 , in which radiation in a wavelength range from 0.1 nm to 100 nm can be used to measure parameters of structures on a substrate. Figure 5 The metrology apparatus 302 presented in FIG. 3 is suitable for use in the soft X-ray or EUV domain.

[0050] Figure 5 A schematic physical arrangement of a measurement device 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 device may be provided in the form of an angle-resolved scatterometer using radiation at normal incidence (or normal incidence) or near normal incidence (or near normal incidence) similar to conventional scatterometers operating at longer wavelengths.

[0051] 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 .

[0052] In this example, source 310310 comprises a generator of EUV or soft x-ray radiation based on high harmonic generation (HHG) technology. The main components of the radiation source are a drive laser 330 and an HHG gas chamber 332. A gas supply 334 supplies a suitable gas to the gas chamber, where it is optionally ionized by a power supply 336. The drive laser 300 may be, for example, a fiber-based laser with an optical amplifier, thereby generating pulses of infrared radiation 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 may be, for example, about 1 μm (1 micron). The laser pulses are delivered to the HHG gas chamber 332 as a first radiation beam 340, where part of the radiation is converted in the gas into a higher frequency than the first radiation, becoming a beam 342 comprising coherent second radiation having one or more desired wavelengths.

[0053] The second radiation may comprise multiple wavelengths. If the radiation is monochromatic, measurement calculations (e.g., reconstruction) can be simplified, but when utilizing HHG, it is easier to generate radiation having multiple wavelengths. The volume of gas within gas chamber 332 defines the HHG space, but the space does not need to be completely enclosed and a gas flow can be used instead of a static volume. For example, the gas can be an inert gas such as neon (Ne) or argon (Ar). N2, O2, He, Ar, Kr, and Xe gases are all contemplated. These scenarios can even be selectable within the same device. Different wavelengths will provide different levels of contrast, for example, when imaging structures made of different materials. For example, to inspect metal structures or silicon structures, different wavelengths can be selected for imaging features in (carbon-based) resists or for detecting contaminants from these different materials. One or more filter devices 344 may be provided. For example, a filter device such as a thin film of aluminum (Al) or zirconium (Zr) can be used to block the primary IR radiation from further transmission into inspection device 302. A grating (not shown) may be provided to select one or more specific harmonic wavelengths from among the wavelengths generated in the gas cell. Some or all of the beam path may be contained within a vacuum environment, keeping in mind that SXR radiation is absorbed as it travels through 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.

[0054] Depending on the material of the structure being inspected, different wavelengths may provide the desired degree of penetration into the underlying layers. In order to resolve the smallest device features and defects among the smallest device features, short wavelengths are likely to be preferred. For example, one or more wavelengths may be selected in the range of 1 nm to 20 nm, or alternatively in the range of 1 nm to 10 nm, or alternatively in the range of 10 nm to 20 nm. Wavelengths shorter than 5 nm have very low critical angles when reflected from materials of general 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, for example to detect contaminants, wavelengths of up to 50 nm may be useful.

[0055] From radiation source 310, filtered beam 342 enters inspection chamber 350, where a substrate W including a structure of interest is held by substrate support 316 for inspection at a measurement position. The structure of interest is designated T. The gas environment within inspection chamber 350 is maintained at a near-vacuum by vacuum pump 352, enabling EUV radiation to pass through the gas environment without undue attenuation. Illumination system 312 has the function of focusing the radiation into a focused beam 356 and may include, for example, a two-dimensional curved mirror or a series of one-dimensional curved mirrors, as described in the aforementioned published U.S. patent application US2017 / 0184981A1 (the contents of which are incorporated herein by reference in their entirety). This focusing is performed to achieve a circular or elliptical spot S with a diameter of less than 10 microns when projected onto the structure of interest. The substrate support 316 includes, for example, an XY translation stage and a rotation stage, by which any portion of the substrate W can be brought to the focus of the beam at a desired orientation. Thus, a radiation spot S is formed on the structure of interest. Alternatively, or in addition, the substrate support 316 includes, for example, a tilting stage that can tilt the substrate W at a certain angle to control the angle of incidence of the focused beam on the structure of interest T.

[0056] 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.

[0057] 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 312 312 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).

[0058] 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 angle of incidence and then relative to the reflected radiation 360. Figure 5In FIG, the plotted diffracted radiation 397 is plotted in a schematic manner, and the diffracted radiation 397 may follow many other paths than the plotted path. The inspection device 302 may also include an additional detection system 398 that detects and / or images at least a portion of the diffracted radiation 397. Figure 5 , a single further detection system 398 is depicted, 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 incident on the target T are detected and / or imaged by the 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.

[0059] To assist in aligning and focusing the spot S with the desired product structure, the inspection apparatus 302 may also be provided with auxiliary optics that utilize auxiliary radiation under the control of the metrology processor 320. The metrology processor 320 may also communicate with a position controller 372, which operates the translation, rotation, and / or tilt stages. The processor 320 receives highly accurate feedback regarding the position and orientation of the substrate via sensors. The sensors 374 may include, for example, interferometers that can provide accuracy on the order of picometers. During operation of the inspection apparatus 302, spectral data 382 captured by the detection system 318 is transmitted to the metrology processing unit 320.

[0060] As mentioned, alternative forms of inspection equipment use soft X-rays and / or EUV radiation at normal incidence or near normal incidence, for example to perform diffraction-based asymmetry measurements. Both types of inspection equipment can be arranged in a hybrid measurement system. The performance parameters to be measured may include overlay (OVL), critical dimension (CD), focus of the lithographic apparatus while the lithographic apparatus prints the target structure, coherent diffraction imaging (CDI) and at-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, optionally in the range of from 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 nature.

[0061] Like optical scatterometers used in today's production facilities, the inspection device 302 can be used to measure structures in resist materials processed in a lithography cell (after development inspection or ADI) and / or to measure structures in harder materials after they have been formed (after etching inspection or AEI). For example, the 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.

[0062] The measurement tool MT, including but not limited to the scatterometer mentioned above, can use radiation from a radiation source to perform measurements. The radiation used by the measurement tool MT can be electromagnetic radiation. The radiation can be optical radiation, such as radiation in the infrared, visible and / or ultraviolet part of the electromagnetic spectrum. The measurement tool MT can use radiation to measure or inspect characteristics and aspects of a substrate, such as a photolithographic exposure pattern on a semiconductor substrate. The type and quality of the measurement may depend on several characteristics of the radiation used by the measurement tool MT. For example, the resolution of the electromagnetic measurement may depend on the wavelength of the radiation, with smaller wavelengths being able to measure smaller features, for example due to the diffraction limit. In order to measure features with small dimensions, it may be preferable 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 of that wavelength / those wavelengths. There are different types of sources for providing radiation of different wavelengths. Depending on the wavelength provided by the source, different types of radiation generation methods can be used. For extreme ultraviolet (EUV) radiation (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 at the desired wavelength. The use of HHG to obtain EUV and / or SXR radiation is known. One of the challenges in the development of these sources is how to improve the efficiency of the HHG process to achieve further power scaling of the output within the desired wavelength range.

[0063] The properties of the radiation used to perform a measurement may affect the quality of the obtained measurement. For example, the shape and size of the transverse beam profile (cross-section) of the radiation beam, the intensity of the radiation, the power spectral density of the radiation, etc. may affect the measurement performed by the radiation. Therefore, it is beneficial to have a source that provides radiation with properties that result in high-quality measurements.

[0064] Described herein are methods, apparatus, and components for HHG to obtain output radiation at high harmonic frequencies of incident radiation (also referred to as drive radiation or pump radiation). The radiation generated by the HHG process can also be provided as radiation for inspection and / or measurement of a substrate in a metrology tool MT. The substrate can be a lithographically patterned substrate. The radiation obtained by the HHG process can also be provided in a lithographic apparatus LA and / or a lithographic cell LC. High harmonic generation uses nonlinear effects to generate radiation at harmonic frequencies of the provided drive radiation. The drive radiation can be pulsed radiation, which can provide high peak intensities for short bursts.

[0065] The existing method that has been adopted for generating high harmonic radiation is to focus the intense laser pulse output from the driving laser onto a HHG gas target or gas target, which can be a gas cell, a gas capillary or a gas jet. The typical peak intensity of the driving laser pulse used in HHG is about 10 14 W / cm 2 Or higher. When gas atoms interact with the strong electric field of the focused laser beam, electrons are stripped from the gas atoms. Once liberated, that is, released, the electrons are accelerated to high energy by the oscillating laser field. The corresponding gas atoms are ionized and the ionization rate has a sensitive dependence on the intensity of the driving laser pulse. After acceleration, the high-energy electrons can collide with their parent ions, i.e., parent ions, again. If such a collision occurs again, the excess kinetic energy obtained during the interaction with the external electric field is emitted as high-energy photons, which have wavelengths within the EUV and / or X-ray (XR) part of the electromagnetic spectrum, for example. The maximum photon energy (or so-called single atom cutoff energy) is proportional to the square of the laser wavelength and the laser intensity. Therefore, in order to produce radiation with high photon energy (or shorter wavelength), a driving laser with higher intensity and longer wavelength is strongly desired. The emitted photons from all interacting gas atoms constitute the final output of the high-harmonic radiation source.

[0066] In order to obtain any significant output power in a high-harmonic radiation source, the individually interacting atoms in the HHG gas target should emit their photons in a coherent manner. In other words, the electromagnetic waves emitted from a large number of individual radiating atoms over an extended region of a highly nonlinear medium should add in phase and form coherent emission. The benefit of producing coherent emission is that the output radiation of a high-harmonic radiation source is a coherent accumulation of the harmonic radiation from all radiating atoms (e.g., N radiating atoms). Therefore, the total power of the output radiation is proportional to the square of the number of radiating atoms (i.e., N 2) is proportional to the power of the radiating atoms. This is in contrast to incoherent emission, in which the output power is proportional to the number of radiating atoms (i.e., N). Therefore, coherent emission is desirable for producing higher output powers. In order to produce such coherent emission, the driving laser and the generated harmonic radiation (EUV and / or XR radiation) should travel at a phase velocity in the highly nonlinear medium (e.g., a gas medium) so that the driving laser can continuously excite multiple gas atoms within the HHG gas target to produce multiple individual harmonics, which, due to being in phase, interfere constructively to form a strong HHG output. Maintaining such phase matching conditions over a sufficient interaction distance, which is determined in part by the characteristics of the driving laser (e.g., beam divergence, intensity, etc.), is important for power scaling of high harmonic radiation sources.

[0067] However, due to the difficulty of continuously power scaling the output without compromising phase matching conditions, most applications of high-harmonic radiation sources are currently limited to relatively low output powers and / or low photon energies. Although high-harmonic radiation sources have been used in various types of metrology tools mentioned above, the throughput of those metrology tools suffers from insufficient power of the high-harmonic radiation sources. Further improvements to existing power scaling methods are highly desirable in order to develop high-harmonic radiation sources that can generate sufficient output power in the EUV and / or XR regions so that the throughput in metrology applications using such high-harmonic radiation sources is at an acceptable level and is therefore cost-effective.

[0068] Similar to other nonlinear optical conversion processes (such as second harmonic generation, Raman effect or optical Kerr effect), the efficiency of the HHG process depends sensitively on the peak intensity of the driving radiation, that is, a higher laser intensity is desired to achieve a higher optical conversion efficiency. The optical efficiency of the HHG process is defined as the power ratio between the power of the generated harmonic radiation and the power of the driving radiation. High driving laser intensity is mainly achieved by focusing the driving laser beam to a small spot size, which is often combined with scaling the driving laser power and shortening the driving laser pulse length depending on the specific requirements / applications. However, it is well known that the axial phase velocity or coaxial phase velocity of the focused laser beam changes along the focus and reaches its maximum value at the diffraction focus. The change in phase velocity leads to a phase mismatch between the driving laser and the generated higher harmonics when traveling through the focal region. As a result, the phase matching distance that fully meets the phase matching condition is significantly shortened and the HHG conversion efficiency is severely impaired.

[0069] Because the phase velocity is a function of the refractive index of the nonlinear medium, it can be adjusted by varying the refractive index of the medium. For gaseous media, the refractive index is proportional to the density of the gas atoms, which can be adjusted by varying the gas pressure. A higher gas pressure results in a higher density of gas atoms, which in turn results in a higher refractive index for the gas volume. At a higher refractive index, the phase velocity of the driving laser is reduced, bringing the driving laser and the generated harmonic magnetic field back into phase. This overcomes the focusing-induced phase mismatch. Therefore, adjusting the gas pressure can be used to adjust the dispersion of the gas medium.

[0070] However, an important complication common to high-harmonic radiation sources is the generation of plasma. As mentioned above, after interacting with the high-intensity drive laser, the gas is partially ionized. The resulting mixture of released electrons, ionized gas atoms, and neutral atoms is a plasma. An optical effect associated with the generation of a plasma is that the effective refractive index of the gas volume in which the plasma is present will be reduced. The effective refractive index of the plasma, n eff Described as: n eff =1–C*n e , where C is a constant and n e is the density of released free electrons. A change (e.g., a decrease) in the effective refractive index in turn changes (e.g., increases) the phase velocity of the laser light as it travels through the plasma. Consequently, the laser light propagates faster at locations with higher electron density (corresponding to high ionization) in the HHG gas target than at locations with low or zero electron density (corresponding to low ionization). The plasma-induced phase velocity change results in a deformed wavefront of the drive laser light. In the case of an uneven ionization distribution, the plasma-induced decrease in the refractive index causes the laser light to diffract away from regions with high ionization and low effective refractive index, a process known as plasma defocusing.

[0071] In the typical case where there is no pre-ionization, the highest degree of ionization occurs where the driving laser beam has the highest intensity, i.e., on the focused beam axis. As such, the effective refractive index is lower in the beam center than at the edge of the laser beam. As the driving laser propagates along the focus and passes through the plasma, plasma defocusing causes the driving laser beam to bend (diffract) away from its beam axis, causing the driving laser beam to be defocused (or expanded). As a result, the wavefront of the driving laser is deformed and thereby the intensity distribution of the driving laser beam is degraded, which in turn degrades the output beam quality of the high harmonic radiation source. Beam quality can be defined as the beam parameter product or M at each wavelength. 2, which, combined with the low chromaticity at the focal plane position, allows chromatic focusing to be minimized.

[0072] Poor beam quality means that the minimum spot size achievable after focusing is much larger than the theoretical limit (or diffraction limit) achievable with a perfect Gaussian beam. Figure 5 When such a high-harmonic radiation source is used in a system (described in ), the optical resolution of the system will be poor. This is clearly undesirable for metrology applications where optical resolution can be maximized for inspecting and measuring semiconductor wafers with ever-shrinking CDs. Furthermore, plasma defocusing significantly reduces the peak intensity of the drive laser and limits the high-intensity region to only the upstream side of the HHG gas target (e.g., along the drive laser propagation direction), where the gas pressure begins to accumulate, i.e., increase, leading to poor HHG efficiency and low output power. Plasma-induced phase velocity (or refractive index) changes negatively impact the phase matching conditions of the HHG process, thereby further degrading HHG efficiency.

[0073] As mentioned above, the pressure of the HHG gas target can be increased to compensate for focusing-induced phase mismatch and maximize the phase matching distance between the fundamental and the generated harmonics. However, higher pressures lead to higher gas densities. At higher gas densities, more gas atoms are ionized by the driver laser, resulting in a more intense plasma, which in turn leads to greater plasma defocusing of the driver laser beam. Plasma defocusing is detrimental to HHG efficiency and output beam quality. However, suppressing such plasma defocusing by keeping the gas pressure low will, on the other hand, weaken the phase matching conditions for the HHG process, leading to the same problem of low HHG efficiency. Therefore, in high-harmonic radiation sources, it is challenging to provide both a sufficiently high gas pressure to ensure good phase matching and a sufficiently low gas pressure to prevent excessive plasma defocusing. The result is often a trade-off, where both—plasma defocus suppression and phase matching—are acceptable compromises, but neither is optimal. Consequently, the useful range of gas pressure is significantly limited by this trade-off.

[0074] Disclosed herein is a method and apparatus for resolving the trade-offs mentioned above. The method and apparatus can reduce the limitations imposed by the plasma defocusing effect on the usable pressure range of the HHG gas target, and thereby enable a high-harmonic radiation source to emit high-harmonic radiation at a higher power. The proposed method includes bombarding the gas target of the high-harmonic radiation source with a pre-pulse that can have a customized spatial distribution before the main drive laser pulse arrives. The pre-pulse is used to condition the gas target by intentionally pre-ionizing a portion of the gas target so as to produce a corresponding spatial distribution (or pattern) of pre-ionization. When the pre-ionization pattern is appropriately selected, it offsets the subsequent adverse plasma defocusing effect suffered by the main pulse. In this way, the high harmonic radiation source can output high harmonic radiation having a desired wavelength in the EUV and / or SXR region with high power and high beam quality (e.g., outputting radiation including one or more wavelengths in the region between 0.01 nm and 100 nm, between 0.1 nm and 100 nm, between 1 nm and 100 nm, between 1 nm and 50 nm, between 1 nm and 20 nm, between 5 nm and 20 nm, between 10 nm and 20 nm, or between 10 nm and 18 nm). Using this output high harmonic radiation as an illumination beam, a metrology system (e.g., such as Figure 5 ) will be able to provide higher throughput and / or better optical resolution.

[0075] Because the distribution of ionization follows the spatial profile of the driving laser beam, to suppress plasma defocusing, the pre-pulse can have a spatial profile that is opposite to that of the driving laser beam, so that when the ionization stimulated by the driving laser is combined with the ionization stimulated by the pre-pulse, a substantially uniform ionization distribution is formed. This is based on the assumption that the degree of ionization induced by the pre-pulse is comparable or identical to the degree of ionization induced by the driving laser. In practice, partial suppression of plasma defocusing may be sufficient to significantly increase the HHG efficiency and thereby increase the power of the high-harmonic radiation, and any pre-pulse spatial profile that at least partially suppresses plasma focusing is included within the scope of the teachings herein.

[0076] Figure 6A high-harmonic radiation source according to a first embodiment is schematically illustrated. A driving laser 610 is used to output a driving laser beam 611, which optionally includes a series or train of short pulses. The driving laser beam 611 can have a Gaussian beam profile and can include radiation having one or more wavelengths higher than one or more wavelengths of the high-harmonic radiation. The driving radiation can include infrared radiation. The driving radiation can include, for example, radiation having a wavelength in the range of 200 nm to 2500 nm, in the range of 800 nm to 1500 nm, in the range of 900 nm to 1300 nm, or in the range of 1000 nm to 1300 nm. The driving radiation can be pulsed radiation. The pulsed driving radiation can include multiple pulses having a duration in the femtosecond range. For example, the pulse duration of the pre-pulse and / or main pulse can be in the range of, for example, 1 fs to 1 ps, 1 fs to 500 fs, or 1 fs to 100 fs. The pulsed drive radiation may have a repetition rate in the range of a few Hertz (Hz) to several hundred Megahertz (MHz).

[0077] After passing through the beam splitter 601 (e.g., a dichroic mirror), the driving laser beam 611 is split into two beams, namely, the pre-pulse beam 611a and the main pulse beam 611b. Correspondingly, the driving laser pulse 612 is split into a pre-pulse 612a and a main pulse 612b. The main pulse 612b is reflected by the first beam splitter 601 into the optical delay line 680, which may include two optical mirrors 602 and 603. The reflectivity of the front surface of the beam splitter (on which the driving laser is first incident) can be flexibly configured by selecting a suitable mirror coating. The optical delay line 680 can be translated so that the delay time between the main pulse 612b and the pre-pulse 612a can be flexibly controlled. In this embodiment, the main pulse is delayed by a predetermined time amount Δt relative to the pre-pulse. The delay time Δt can be in the range of a few fs to several hundred fs. The delayed main pulse beam 611b is then directed via the second beam splitter 604 to spatially overlap with the pre-pulse beam 611a.

[0078] In an alternative embodiment, a small portion of the spatially combined beams 611a, 611b can be directed by a third beam splitter 605 to a timing detection device 618 (e.g., a photodetector) to accurately measure the delay time between the two pulses. The measured delay time can be compared with a target delay time to generate a timing error. Such a timing error can be fed back to the control system 617 of the optical delay line 680 so that the delay time can be adjusted to minimize the received timing error.

[0079] After being emitted through the beam splitter 601, the pre-pulse beam 611a can pass through a beam shaping device 620, which modifies the phase distribution (wavefront) and / or intensity distribution of the pre-pulse beam in a customized manner. Modifying the phase distribution can involve modifying the phase at one or more points within the beam (relative to the phase at any (e.g., arbitrarily selected) reference point). Such a modified phase distribution will affect the subsequent propagation of the beam. Such a beam shaping device 620 can shape the spatial distribution of the pre-pulse in terms of intensity and / or phase. The beam shaping device 620 can include, for example, one or a combination of the following: a phase modulation component or an intensity modulation component; for example, a phase plate with a fixed phase pattern, a spatial light modulator (SLM) with a programmable / configurable phase pattern, or an aperture with a fixed or programmable / configurable pattern. Then, after passing through the optical mirror 604, the modified pre-pulse beam is spatially superimposed with the delayed main pulse beam. Note that even if the pre-pulse beam and the main pulse beam overlap spatially, the pre-pulse and the main pulse contained in their respective laser beams are separated in time by a delay time Δt.

[0080] The spatially overlapped but temporally separated laser pulses (pre-pulse 612a and main pulse 612b) may then be focused by an optical lens 630 into a vacuum or near-vacuum vessel 690. The vacuum vessel 690, connected to a vacuum pump (not shown), includes an input optical window 640 configured to allow high optical transmission of the input laser pulse (or beam) and an output optical window configured to optionally block (reflect or absorb) residual (or unconverted) fundamental radiation, i.e., residual pre-pulse beam 611a' and residual main pulse beam 611b', while allowing high optical transmission of the generated harmonic radiation. Within the vacuum vessel 690, the focal plane of the pre-pulse beam 611a may overlap with the focal plane of the main pulse beam 611b. The overlapped focal planes may be located at the center of the gas target 650 and perpendicular to the propagation axis of the laser pulse, which may overlap with the axis of the vacuum vessel (e.g., Figure 6 ). In embodiments, the gas target 650 may be a gas chamber of various shapes. Alternatively, the gas target 650 may be a gas jet or a fiber / capillary filled with gas. In some embodiments, the two focal planes, namely the focal plane of the pre-pulse beam 611a and the focal plane of the main pulse beam 611b, may not overlap and may instead be generated at different locations within the gas target 650. This will be described in detail below.

[0081] After being focused onto the gas target 650, the pre-pulse beam 611a, including a predetermined (e.g., customized) phase and / or intensity distribution (wavefront), pre-ionizes the gas target 650. Because the delay time between the pre-pulse and the subsequent main pulse is on the femtosecond timescale, the liberated, or released, electrons (after pre-ionization) may not have sufficient time to diffuse elsewhere within the gas target 650. Thus, the spatial distribution of the plasma can follow the spatial distribution of the pre-pulse beam 611a. In other words, the pre-pulse beam 611a imprints its spatial distribution on the electron / plasma distribution, which can remain substantially the same until the subsequent main pulse arrives and generates the HHG radiation.

[0082] In an embodiment, the beam shaping device 620 shapes the spatial distribution of the pre-pulse to have an off-axis intensity distribution. Such an off-axis intensity distribution causes the applied intensity distribution to be concentrated outside a central or axial region; for example, such spatial distribution is concentrated in an outer or peripheral region relative to the central region (for example, immediately around the central region). The result of the pre-pulse shaped by such a beam shaping device is therefore the generation of an off-axis plasma with a greater plasma density outside the central region; for example, there is more plasma generated outside the plasma region than inside the plasma region. Note that this is only the effect of the pre-pulse; the effect of the combination of the pre-pulse and the main pulse can be to substantially homogenize the plasma distribution, at least to the extent that the effect of the pre-pulse is a more uniform plasma after the main pulse relative to the case without the shaped pre-pulse.

[0083] In an embodiment, the beam shaping device may apply a substantially annular intensity distribution to the pre-pulse. After pre-ionization, the resulting spatial distribution of the plasma will also show an annular shape profile 700, as shown in FIG. Figure 7 . Such an annular shaped profile 700 includes a central region or axial region (hollow core) 710 having a negligible or at least relatively low plasma density relative to a peripheral region or outer region 720 immediately outside this axial region (e.g., in the annular region 720 and possibly having an annular distribution). A plasma having such an annular shaped distribution may also be referred to as a plasma tube. Such an annular shaped plasma distribution is roughly opposite to the plasma distribution produced by the main pulse, in which the plasma is concentrated in the central (axial) region (where the intensity is highest). In some other embodiments, the intensity and / or phase distribution (wavefront) of the pre-pulse can be modified in different ways so that different spatial distributions of ionization / plasma can be produced.

[0084] In certain embodiments, the beam shaping device 620 may include a phase modulator or phase plate / SLM that modifies the wavefront of the pre-pulse beam 611a to obtain a customized pre-pulse, as described.

[0085] In other embodiments, the beam shaping device can be omitted entirely. Although in the above description, the pre-pulse is actively customized by the beam shaping device or wavefront modification, it should be understood that this is optional. In other embodiments, the pre-pulse is not actively shaped. Such embodiments may use a long pre-pulse / thermal shock-driven pre-pulse that utilizes a conventional, i.e., ordinary Gaussian pre-pulse. Such shock-driven pre-pulse techniques are described, for example, in the publication "Light Pipe for High Intensity Laser Pulses" by CG Durfee III and HM Milchberg (Physical Review Letters, Vol. 71, No. 15), which is incorporated herein by reference.

[0086] It will be appreciated that a customized pre-pulse may be obtained without a separate beam shaping device; the pre-pulse laser may be configured to emit a shaped pre-pulse (eg, a ring pre-pulse).

[0087] Because both pulses originate from the same drive laser pulse 612, the pre-pulse 612a and the main pulse 612b have approximately the same pulse length. Similarly, the pre-pulse beam and the main pulse beam can both have approximately the same beam diameter. This is based on the assumption that the optical components in the two optical arms cause negligible or equal temporal / spatial deformations to both beams. Alternatively, optical components can be included in one or both arms to enable adjustment of one (or both) beam diameters; for example, using a telescoping lens configuration. To counteract the aforementioned effects, namely plasma defocusing and plasma-induced phase mismatch, the ionization level produced by the pre-pulse can be sufficiently close to that produced by the main pulse. In this way, the toroidal plasma distribution produced by the ionization induced by the pre-pulse can compensate for and (at least partially) homogenize the non-uniform plasma distribution produced by the ionization induced by the main pulse. As a result, the overall plasma distribution that the main pulse subsequently sees and interacts with during the HHG process is significantly more uniform. A more uniform distribution of plasma results in a more uniform distribution of the refractive index, causing less wavefront deformation on the main pulse beam 611b and thus weaker plasma defocusing and phase mismatch effects. In this way, both the HHG efficiency and the beam quality of the generated high-harmonic radiation are improved.

[0088] The generated high-harmonic radiation may include one or more harmonics of the drive radiation wavelength, such as second, third, fourth, ... nth harmonics of the drive radiation wavelength. The high-harmonic radiation may include wavelengths within the extreme ultraviolet (EUV), soft X-ray (SXR), and / or hard X-ray portions of the electromagnetic spectrum. The high-harmonic radiation may include wavelengths in the range of 0.01 nm to 100 nm, in the range of 0.1 nm to 100 nm, in the range of 0.1 nm to 50 nm, in the range of 1 nm to 50 nm, in the range of 5 nm to 20 nm, in the range of 10 nm to 20 nm, or in the range of 10 nm to 18 nm.

[0089] Figure 8 A high-harmonic radiation source according to a second embodiment is schematically illustrated. In this embodiment, a driving laser 810 is used to output a driving laser beam 811, which includes a series or a string of short pulses, such as a main pulse 812. The pulse 812 is used as the main driving pulse for generating high-harmonic radiation. The aforementioned characteristics of the main pulse (e.g., pulse duration, wavelength, and reception rate) also apply to this embodiment. Unlike the first embodiment, this embodiment uses a separate laser (or pre-pulse laser) 817 instead of the same driving laser to provide the pre-pulse. Since the degree of ionization is proportional to the peak intensity of the pulsed radiation, to achieve the same degree of ionization, a separate pre-pulse laser can output pulsed radiation with a peak intensity similar to that of the main pulse. The use of a separate pre-pulse laser allows for flexible selection of pulse characteristics. For example, when a shorter pulse length is selected, the required pulse energy can be lower (to maintain the same peak intensity). Laser sources that produce low-energy pulses are more cost-effective than their high-energy counterparts. This is due to the fact that the generation of high-energy pulses typically requires a more complex laser design, such as a larger amplifier stage. The wavelength of the pre-pulse laser can be selected to be close to the wavelength of the main pulse laser. The pre-pulse laser can be a femtosecond passively mode-locked solid-state laser, such as a femtosecond Nd:YAG laser or a femtosecond Yb:YAG laser.

[0090] The pre-pulse laser 817 emits a pre-pulse laser beam 871 comprising a series or train of short pulses, such as pre-pulses 872. The pre-pulses 872 are guided by an optical mirror 801 to pass through a beam shaping device 820, which tailors the intensity and / or phase distribution of the pre-pulses 872. After being shaped, the pre-pulses are then directed by another optical mirror 802 toward a beam splitter 803, where they are spatially overlapped with the main pulse 812. A portion of the two spatially overlapped but temporally separated pulses can be directed by a beam splitter 805 to an optional timing detection device 818.

[0091] The separate laser 817 can have a repetition rate equal to that of the driver laser or a repetition rate that is a harmonic of the driver laser. This allows the two pulse trains reflected from the driver laser and the pre-pulse laser, respectively, to be synchronized in the time domain. Synchronization means that the temporal positions of the pulses from the two separate lasers coincide in time or have a fixed delay time. Such timing synchronization can be achieved and subsequently maintained using, for example, a timing synchronization system. The timing synchronization system can include a control system 817 and a timing detection device 818, the control system 817 controlling the timing synchronization system, and the timing detection device 818 accurately detecting the timing difference between the main pulse 812 and the pre-pulse 872 and sending a corresponding timing error to the control system 817. After receiving the timing error from the timing detection device 818, the control system 817 then commands a locking mechanism (not shown) to adjust the timing of one or both laser pulses, for example by adjusting the chamber length. In this way, the timing error signal received from the timing detection device is minimized. In addition, the delay time Δt between the pre-pulse and the main pulse can be controlled by an additional optical delay line 880, which includes the above-mentioned optical mirrors 801 and 802 and the beam shaping device 820. The delay time Δt can be in the range of several fs to several hundred fs (e.g., 1 fs to 1 ps, 1 fs to 500 fs, or 1 fs to 100 fs).

[0092] In extension of the above, the pre-pulse concept described herein can be combined with the concept of two-color HHG. In such an embodiment, the pre-pulse can be selected to have a different wavelength than the main pulse. It is known that driving HHG with a main pulse (the main pulse is superimposed with its second harmonic) can increase conversion efficiency and modify the output radiation beam shape. Therefore, an optional embodiment can include the use of a second color laser to generate the pre-pulse, so that the pre-pulse has a wavelength different from that of the main pulse. In such an embodiment, the second color laser can also be used as a second color driver in addition to generating the pre-pulse; for example, to generate a second harmonic pulse that is superimposed with the main pulse during HHG generation.

[0093] Similar to the first embodiment, a beam shaping device 820 (e.g., a phase plate) is used to shape or customize the intensity and / or phase distribution of the pre-pulse 872 so that, after being focused onto the gas target 850, the pre-pulse 872 pre-ionizes the gas and produces a desired spatial distribution of plasma. After passing through the beam shaping device 820, the pre-pulse 872 is guided by optical mirrors 802 and 803 to spatially overlap with the main pulse 812 from the drive laser 810. Moreover, even though the laser beam 871 including the pre-pulse 872 and the laser beam 811 including the main pulse 812 are spatially overlapped, the two pulses 872 and 812 are separated in time by a predetermined delay time of Δt. Subsequently, the spatially overlapped but temporally delayed pulses 872 and 812 are focused by an optical lens into a container 890.

[0094] The container 890 is essentially the same as that described in the first embodiment. It includes an input optical window 840, which is configured to be highly transmissive to the pre-pulse and the main pulse, and an output optical window 860, which is configured to be highly transmissive to the generated high-harmonic radiation but highly absorptive / reflective to the residual fundamental radiation. After being admitted into the vacuum container, similar to the first embodiment, the two input pulses 872 and 812 can be sequentially focused onto the same focal plane. In this way, the plasma distribution excited by the pre-pulse can fully counteract the plasma distribution excited by the main pulse, resulting in a more uniform plasma distribution and a less distorted main pulse wavefront. Consequently, higher HHG efficiency and better HHG beam quality can be achieved.

[0095] Return Reference Figure 6 and Figure 8, both embodiments may include additional optical components / devices to provide additional functionality and / or better optical control. For example, two or more optical mirrors may be added between the beam splitter 605, 805 and the optical lens 630, 830 to increase flexibility and accuracy in controlling the laser beam before it enters the vacuum vessel 690, 890. Moreover, one or more beam measurement devices may be included in the apparatus to monitor and stabilize the pointing of the two laser beams 611a, 871 and 611b, 811. Since the HHG process is polarization dependent, the use of a circularly polarized pre-pulse can effectively eliminate the high-order harmonic radiation induced by the pre-pulse. Therefore, an additional wave plate, i.e., a wave plate, may be added to the beam path of the pre-pulse in both embodiments 600, 800 so that the polarization of the pre-pulse can be converted into circular polarization. Alternatively, for Figure 8 In the second embodiment, the pre-pulse laser 870 can directly emit a circularly polarized laser beam.

[0096] In addition to the additional components in the common beam path, additional components can also be added to one of the arms. This enables, for example, control of one beam relative to the other, thereby enabling, for example, individual control of beam size, lateral focal length, and / or focal plane position.

[0097] Due to spatiotemporal aberrations, for example, during the HHG process, the wavefront of the high-harmonic radiation can experience chromatic focusing effects. Because high-harmonic radiation often covers a wide spectral range, the highly deformed wavefront will lead to a strong chromatic focusing effect; a consequence of this chromatic focusing effect is that different wavelengths of the high-harmonic radiation will be focused to different locations along the optical axis when focused by a lens. Each color can individually have a perfectly behaved wavefront while still being focused into different planes, leading to this chromatic focusing effect. In this regard, it should be understood that aberrations (i.e., poor beam parameter products) and chromatic focusing are two different things. A perfect wavefront is spherical in shape, and aberrations (characterized by large beam parameter products) are essentially deviations from a perfect, or ideal, spherical shape. In contrast, the chromatic focusing effect is the result of different radii of curvature of the otherwise spherical shape for different colors. The chromatic distribution of focal lengths, or chromatic focusing effects, significantly increases the focal volume and negatively impacts the resolution, particularly the vertical (or depth) resolution, of an imaging system (eg, a metrology system / inspection system).

[0098] As disclosed herein, a method for tailoring the main pulse intensity and phase distribution using a suitably prepared pre-pulse plasma can be used to suppress either or both the chromatic focusing effect and aberration-induced wavefront deformation of high-harmonic radiation. This can be achieved by creating an effective phase plate within the gas target, so that the wavefront of the main pulse can be pre-shaped by this effective phase plate before the HHG process begins. When the pre-shaped wavefront interacts with the HHG gas target, the spatiotemporal aberrations generated during the HHG process are appropriately compensated, thereby suppressing the chromatic focusing effect. This effective phase plate is created by imprinting the spatial pattern of the pre-pulse onto the gas target, resulting in a desired plasma distribution. Because the effective refractive index of the plasma differs from that of the neutral gas, the plasma distribution induced by the pre-pulse can thus serve as a phase plate. The effective phase plate can be created before focusing the main pulse. This means that the focal plane of the pre-pulse beam can be positioned before the focal plane of the main pulse beam. The position of the pre-pulse focal plane can be adjusted by varying the divergence of the pre-pulse beam. This can be achieved by passing the pre-pulse beam through an additional lens.

[0099] The degree of suppression of aberrations and / or chromatic focus will depend on how well the spatiotemporal aberrations are compensated. It should be noted that the focal plane of the pre-pulse beam can be located at a location within the gas target where sufficient pre-ionization can occur and where the ionization produced by the delayed main pulse is significantly weaker. The ionization stimulated by the main pulse can be 10, 20, 50, or 100 times weaker than the ionization stimulated by the pre-pulse, i.e., 1 / 10, 1 / 20, 1 / 50, or 1 / 100 of the ionization stimulated by the pre-pulse.

[0100] Figure 9A focusing configuration for suppressing chromatic focusing effects according to another embodiment is schematically illustrated. In this embodiment, the focal plane 951 of the pre-pulse beam 971 and the focal plane 952 of the main pulse beam 911 are intentionally generated at two different locations within the gas target 950 and along the propagation axis 908. In this way, the plasma induced by the pre-pulse acts as an effective phase plate 953 based on the spatial profile of the pre-pulse beam 911, modifying the wavefront of the subsequent main pulse beam 911 in a controlled manner. After being modified by such an effective phase plate 953, the main pulse beam 911 with a pre-shaped wavefront will subsequently interact with the gas target to generate high-harmonic radiation. In this way, the spatiotemporal aberrations of the generated high-harmonic radiation can be minimized and the chromatic focusing effect can be suppressed. It should be noted that this focusing configuration can be applied to any of the embodiments mentioned above. It is also possible to use two different gas targets: a first gas target for shaping and a second gas target for HHG, each gas target being optimized separately according to one or more of the following: gas type, geometry, pressure, etc., as appropriate for the two different functions.

[0101] The high harmonic radiation sources 600, 800 may be operated in a method for efficiently generating high harmonic radiation, such as Figure 10As depicted in FIG. In step 1010, the intensity and / or phase distribution of the pre-pulse 612a, 872 can be shaped in a customized manner using a beam shaper 620, 820. In step 1020, a target delay time can be set between the main pulse 612b, 812 and the pre-pulse 612a, 872. In step 1030, once the delay time is set, the pre-pulse 612a, 872 can be used to condition a gas target 650, 850, 950 that may be contained in a container 690, 890, 990. This conditioning of the gas target by the pre-pulse 612a, 872 can be configured to occur before the main pulse 612b, 812 arrives. The gas target 650, 850, 950 may include a gas medium suitable for high harmonic generation (HHG). The pre-pulse 612a, 872 can define a customized plasma distribution in the gas target 650, 850, 950 that compensates for the plasma distribution initiated by the main pulse 612b, 812. In step 1040, after conditioning the gas target 650, 850, 950 by the pre-pulse 612a, 872, the main pulse is emitted to the gas target 650, 850, 950 for generating high harmonic radiation. At least some of the high harmonic radiation obtained by the HHG process can exit the container 690, 890, 990 through the optical filter 660, 860, 960. Figures 6 to 9 Further details of the method of generating high harmonic radiation are provided in the description of the described components.

[0102] The high harmonic radiation sources 600, 800 may be provided in, for example, a metrology apparatus MT, an inspection apparatus, a lithography apparatus LA, and / or a lithography cell LC. Although specific reference may be made herein to the use of lithography apparatus in IC manufacturing, it should be understood that the lithography apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0103] Additional embodiments are disclosed in the following list of numbered aspects:

[0104] 1. A high harmonic radiation source comprising a gas medium and configured to:

[0105] conditioning the gaseous medium by irradiating the gaseous medium with a pre-pulse of radiation, thereby generating a plasma comprising a pre-pulse plasma profile;

[0106] irradiating the gaseous medium with a main pulse of radiation to generate the high harmonic radiation,

[0107] wherein the adjusting step causes the plasma, including the pre-pulse plasma profile, to be used to configure the wavefront of the main pulse to improve one or both of:

[0108] the efficiency of the high harmonic generation process, and

[0109] The beam quality of the high harmonic radiation.

[0110] 2. The high harmonic radiation source according to aspect 1, wherein the high harmonic radiation source further comprises a beam shaping device, wherein the beam shaping device is configured to shape the pre-pulse before the adjustment so that the pre-pulse comprises a customized pre-pulse of radiation.

[0111] 3. The apparatus according to clause 2, wherein the beam shaping means comprises a phase modulation means and / or an intensity modulation means.

[0112] 4. The apparatus of clause 2 or 3, wherein the beam shaping device comprises one or any combination of a phase plate, a spatial light modulator, or an aperture.

[0113] 5. A high harmonic radiation source according to any one of aspects 2, 3 or 4, wherein the beam shaping device is capable of shaping the customized pre-pulse so that the customized pre-pulse produces the pre-pulse plasma distribution, which compensates the main pulse plasma distribution subsequently produced by the main pulse so that the main pulse experiences a more uniform plasma distribution in a post-main pulse plasma produced by the combination of the pre-pulse and the main pulse.

[0114] 6. A high harmonic radiation source according to any one of aspects 2 to 5, wherein the beam shaping device is capable of operating to shape the customized pre-pulse to have an off-axis spatial intensity distribution, and the off-axis spatial intensity distribution is configured to produce an off-axis pre-pulse plasma distribution.

[0115] 7. A high harmonic radiation source according to any one of aspects 2 to 6, wherein the beam shaping device is capable of shaping the customized pre-pulse so that the pre-pulse plasma distribution has a lower plasma density in the axial region than the plasma density in the region outside the axial region.

[0116] 8. A high harmonic radiation source according to any one of aspects 2 to 7, wherein the customized pre-pulse is configured to shape the customized pre-pulse to include a substantially annular spatial distribution, and thereby generate the pre-pulse plasma distribution having a substantially annular spatial distribution.

[0117] 9. A high harmonic radiation source according to any one of clauses 2 to 8, wherein the beam shaping device is operable to shape the tailored pre-pulse to minimize plasma defocusing effects originating from the main pulse plasma distribution.

[0118] 10. A high harmonic radiation source according to any one of aspects 2 to 9, wherein the beam shaping device is capable of operating to shape the customized pre-pulse to maximize the phase matching distance, within which the phases of the main pulse and high harmonic radiation are approximately phase matched.

[0119] 11. The high harmonic radiation source of aspect 10, wherein the high harmonic radiation source is configured such that each of the main pulse and the customized pre-pulse is focused at a respective different focal plane.

[0120] 12. The high harmonic radiation source of clause 10 or clause 11, wherein the pre-pulse plasma profile is configured to correct the wavefront of the main pulse such that beam quality of the high harmonic radiation is improved.

[0121] 13. The high harmonic radiation source of any one of aspects 2 to 10, configured such that each of the main pulse and the pre-pulse is focused at a common focal plane.

[0122] 14. The high harmonic radiation source according to any preceding aspect, wherein the main pulse is configured to be delayed in time relative to the pre-pulse by a delay time.

[0123] 15. The high harmonic radiation source of clause 14, wherein the delay time is in the range of 1 fs to 1 ps, and both the main pulse and the pre-pulse are configured to comprise a pulse duration in the range of 1 fs to 1 ps.

[0124] 16. A high harmonic radiation source according to clause 14 or 15, comprising an optical delay line which delays the main pulse in time relative to the pre-pulse.

[0125] 17. A high harmonic radiation source according to any preceding aspect, operable to generate the pre-pulse as a circularly polarized pre-pulse.

[0126] 18. A high harmonic radiation source according to any preceding aspect, comprising a common driving laser for generating the main pulse and the pre-pulses.

[0127] 19. The high harmonic radiation source according to any one of aspects 1 to 17, wherein the high harmonic radiation source comprises a main pulse laser for generating the main pulse and a pre-pulse laser for generating the pre-pulse.

[0128] 20. The high harmonic radiation source of clause 19, wherein the pre-pulse laser comprises a passively mode-locked solid-state laser.

[0129] 21. A high harmonic radiation source according to aspect 19 or 20, wherein the main pulse laser and the pre-pulse laser are configured to emit respective temporally synchronized pulse trains or pulse sequences using a timing synchronization system.

[0130] 22. A high harmonic radiation source according to aspect 19, 20 or 21, wherein the main pulse laser and the pre-pulse laser are configured to generate radiation of different wavelengths; and the pre-pulse laser is further configured to generate an additional pulse overlapping with the main pulse.

[0131] 23. A high harmonic radiation source according to aspect 22, wherein the additional pulses are at a second harmonic frequency relative to the main pulse.

[0132] 24. The high harmonic radiation source of any preceding aspect, wherein each of the main pulse and the pre-pulse comprises a wavelength in the range of 200 nm to 2500 nm.

[0133] 25. The high harmonic radiation source according to any preceding aspect, wherein the repetition rate of both the main pulse and the pre-pulse comprises a repetition rate in the range of 1 Hz to 10 MHz.

[0134] 26. A high harmonic radiation source according to any preceding aspect, wherein the high harmonic radiation comprises wavelengths in the range of 1 nm to 100 nm.

[0135] 27. A measurement device comprising the high-harmonic radiation source according to any preceding aspect.

[0136] 28. The metrology apparatus of clause 27, comprising a scatterometer metrology device, a level sensor or an alignment sensor.

[0137] 29. A method for generating high-order harmonic radiation during a high-order harmonic generation process, comprising:

[0138] generating a pre-pulse of radiation;

[0139] conditioning a gaseous medium using the pre-pulse to produce a plasma comprising a pre-pulse plasma profile;

[0140] irradiating the gaseous medium with a main pulse of radiation to generate the high harmonic radiation,

[0141] The pre-pulse plasma profile is configured to configure a wavefront of the main pulse so as to improve one or both of: efficiency of the high harmonic generation process, and beam quality of the high harmonic radiation.

[0142] 30. The method of aspect 29, further comprising shaping the phase and / or intensity of the pre-pulse prior to the adjusting such that the pre-pulse comprises a customized pre-pulse of radiation.

[0143] 31. A method according to aspect 30, wherein the pre-pulse plasma distribution compensates for the main pulse plasma distribution subsequently produced by the main pulse, so that the main pulse experiences a more uniform plasma distribution in the post-main pulse plasma produced by the combination of the pre-pulse and the main pulse.

[0144] 32. The method of clause 30 or 31, wherein the tailored pre-pulse has an off-axis spatial intensity distribution that produces an off-axis pre-pulse plasma distribution.

[0145] 33. The method of aspect 30, 31 or 32, wherein the pre-pulse plasma distribution has a lower plasma density in an axial region than a plasma density at regions outside the axial region.

[0146] 34. The method of any one of aspects 30 to 33, comprising generating the customized pre-pulse with a substantially annular spatial distribution, such that the pre-pulse plasma distribution has a substantially annular spatial distribution.

[0147] 35. The method of any one of aspects 31 to 34, wherein the pre-pulse is shaped to minimize plasma defocusing effects originating from the main pulse plasma distribution.

[0148] 36. A method according to any one of aspects 30 to 35, wherein the pre-pulse is shaped to maximize a phase matching distance within which the phases of the main pulse and high harmonic radiation are approximately phase matched.

[0149] 37. The method of clause 36, wherein each of the main pulse and the customized pre-pulse is focused at a respective different focal plane.

[0150] 38. The method of clause 36 or 37, wherein the pre-pulse plasma profile corrects the wavefront of the main pulse such that the beam quality of the high harmonic radiation is improved.

[0151] 39. The method of any one of aspects 30 to 36, wherein each of the main pulse and the customized pre-pulse is focused at a common focal plane.

[0152] 40. The method according to any one of aspects 29 to 39, wherein the main pulse is configured to be delayed in time relative to the pre-pulse by a delay time.

[0153] 41. The method of clause 40, wherein the delay time is in the range of 1 fs to 1 ps, and both the main pulse and the pre-pulse are configured to include a pulse duration in the range of 1 fs to 1 ps.

[0154] 42. The method of any one of aspects 29 to 41, wherein the high harmonic radiation comprises a wavelength in the range of 1 nm to 100 nm.

[0155] 43. A method according to any one of aspects 29 to 42, wherein the pre-pulse is generated as a circularly polarized pre-pulse.

[0156] 44. The method of any one of aspects 29 to 43, wherein the main pulse and the pre-pulse are generated at different wavelengths; and the method further comprises:

[0157] An additional pulse is generated overlapping the main pulse.

[0158] 45. A method according to aspect 44, wherein the further pulses are at a second harmonic frequency relative to the main pulse.

[0159] Although specific reference may be made herein to embodiments in the context of lithographic apparatus, embodiments may be used with other apparatus. Embodiments may form part of mask inspection equipment, metrology equipment, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0160] Although specific reference may be made herein to embodiments of the invention in the context of inspection or metrology equipment, embodiments may be used in other equipment. Embodiments may form part of mask inspection equipment, lithography equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or masks (or other pattern forming devices). The term "metrology equipment" or "inspection equipment" may also refer to an inspection equipment or detection system (or metrology equipment or metrology system). For example, the inspection equipment including embodiments may be used to detect defects in a substrate or defects in a structure on a substrate. In such embodiments, the characteristic of interest of the structure on the substrate may relate to a defect in the structure, the absence of a particular part of the structure, or the presence of an unwanted structure on the substrate.

[0161] Although the above may specifically refer to the use of embodiments in the context of optical lithography, it will be appreciated that the invention may not be limited to optical lithography and may be used in other applications (eg, imprint lithography) where the context permits.

[0162] While the targets or target structures (more generally, structures on the substrate) described above are metrology target structures specifically designed and formed for the purpose of measurement, in other embodiments, the property of interest may be measured on one or more structures that are functional parts of a device formed on the substrate. Many devices have regular grating-like structures. The terms structure, target grating, and target structure as used herein do not require that the structure be set up specifically for the measurement being performed. Additionally, the spacing of the metrology targets may be close to the resolution limit of the optical system of the scatterometer or possibly smaller, but may be much larger than the size of typical non-target structures (optionally product structures produced by a photolithographic process in the target portion C). In practice, the lines and / or spaces of the overlapping gratings within the target structure may be manufactured to include smaller structures similar in size to the non-target features.

[0163] Although specific embodiments 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. Thus, those skilled in the art will appreciate that modifications may be made to the described invention without departing from the scope of the claims set forth below.

[0164] Although specific reference is made to "measurement equipment / tools / systems" or "inspection equipment / tools / systems," these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or on a wafer. For example, an inspection 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 the structure on the 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.

[0165] Although specific reference is made to SXR and EUV electromagnetic radiation, it will be appreciated 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 in the wavelength range 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, has also been considered for metrology measurements.

Claims

1. A high harmonic radiation source comprising a gas medium and configured to: performing a high harmonic generation step of irradiating the gaseous medium with a main pulse of radiation to generate high harmonic radiation; performing a conditioning step of irradiating the gaseous medium with a pre-pulse of radiation to generate a plasma comprising a pre-pulse plasma distribution prior to arrival of the main pulse of radiation in the high harmonic generation step, the conditioning step being performed by intentionally pre-ionizing a portion of the gaseous medium such that a corresponding spatial distribution or pattern of pre-ionization is generated to counteract plasma defocusing effects experienced by the main pulse, The pre-pulse of radiation is used to condition the gaseous medium during the conditioning step.

2. The high harmonic radiation source according to claim 1, wherein: The conditioning step is such that the pre-pulse is used to condition the gaseous medium by intentionally pre-ionizing a portion of the gaseous medium.

3. The high harmonic radiation source according to claim 1 or 2, wherein: The regulating step results in generation of a plasma comprising a pre-pulse plasma profile.

4. The high harmonic radiation source according to claim 3, wherein: The adjusting step causes the plasma including the pre-pulse plasma profile to be used to configure a wavefront of the main pulse.

5. The high harmonic radiation source according to claim 4, wherein: The pre-pulse plasma profile is configured to correct the wavefront of the main pulse such that The beam quality of the high harmonic radiation is improved.

6. A high harmonic radiation source according to any preceding claim, wherein: The high harmonic radiation source further comprises a beam shaping device configured to shape the pre-pulse into a customized pre-pulse before the adjusting step.

7. The high harmonic radiation source according to claim 6, wherein: The beam shaping means comprises a phase modulation means and / or an intensity modulation means.

8. The high harmonic radiation source according to claim 6 or 7, wherein: The beam shaping device includes one or any combination of a phase plate, a spatial light modulator or an aperture.

9. A high harmonic radiation source according to any one of claims 6, 7 or 8 when dependent on claim 3, 4 or 5, wherein: The beam shaping device is operable to shape the customized pre-pulse such that the customized pre-pulse generates the pre-pulse plasma distribution that compensates for a main pulse plasma distribution subsequently generated by the main pulse.

10. The high harmonic radiation source according to claim 6 when referring to claim 3, 4 or 5, wherein: The beam shaping device is operable to shape the customized pre-pulse to have an off-axis spatial intensity distribution configured to produce an off-axis pre-pulse plasma distribution.

11. A high harmonic radiation source according to any preceding claim, wherein: The main pulse is configured to be delayed in time with respect to the pre-pulse by a delay time.

12. A high harmonic radiation source according to any preceding claim, operable to generate the pre-pulse as a circularly polarised pre-pulse.

13. A high harmonic radiation source according to any preceding claim, wherein: The high-harmonic radiation source includes a main pulse laser for generating the main pulse and a pre-pulse laser for generating the pre-pulse.

14. A measurement device comprising a high harmonic radiation source according to any preceding claim.

15. A method for generating high-order harmonic radiation during a high-order harmonic generation process, comprising: receiving a pre-pulse of radiation; Before the arrival of a main pulse of radiation in a subsequent high-harmonic generation step, a conditioning step is performed of irradiating a gaseous medium with the pre-pulse to generate a plasma comprising a pre-pulse plasma distribution, wherein a portion of the gaseous medium is intentionally pre-ionized so as to generate a corresponding spatial distribution or pattern of pre-ionization to counteract a plasma defocusing effect experienced by the main pulse; as well as performing said high harmonic generation step of irradiating said gaseous medium to generate said high harmonic radiation using said main pulse of radiation, Therein, the pre-pulse of radiation is used in the conditioning step to condition the gaseous medium.

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