Equipment for transporting gas and irradiation sources for generating high harmonic radiation.

Abstract

A gas delivery system suitable for a high harmonic generation (HHG) radiation source is disclosed, which can be used to generate measuring radiation for inspection equipment. In such a radiation source, a gas delivery element delivers gas in a first direction. The gas delivery element has an optical input and an optical output, defining an optical path extending in a second direction. The first direction is arranged at a non-perpendicular and non-parallel angle relative to the second direction. A gas delivery element, or a pair of gas delivery elements, with a gas jet shaping device, is also disclosed, one of which delivers a second gas, such that the gas jet shaping device or the second gas is operable to modify the flow profile of the gas to drastically reduce the gas number density.

Description

[0001] This application is a divisional application of patent application No. 201880018587.5 entitled "Apparatus for Transporting Gas and Irradiation Source for Generating High Harmonic Radiation" (International Application Date: 2018-02-15, International Application No.: PCT / EP2018 / 053772), which entered the Chinese national phase on September 16, 2019. Technical Field

[0002] This invention relates to a gas delivery device, and more particularly to a gas delivery device for use in an irradiation or radiation system. Background Technology

[0003] A photolithography apparatus is a machine that applies a desired pattern onto a substrate (typically onto a target portion of the substrate). For example, photolithography apparatus can be used in the fabrication of integrated circuits (ICs). In this case, a patterning apparatus, optionally referred to as a mask or photomask, can be used to generate a circuit pattern to be formed on a single layer of the IC. The pattern can then be transferred onto a target portion (e.g., a portion of a die, one or more dies) on a substrate (e.g., a silicon wafer). Multiple layers, each with a specific pattern and material composition, are applied to define the functional devices and interconnections of the finished product.

[0004] During photolithography, it is desirable to frequently measure the resulting structure, for example, for process control and verification. Various tools are known for performing these measurements, including scanning electron microscopes, often used to measure critical dimensions (CD), and custom tools for measuring overlap (the accuracy of alignment between two layers in a device). Recently, various forms of scattering instruments have been developed for use in the field of photolithography.

[0005] Examples of known scatterers often rely on the setup of a dedicated measurement target. For instance, a method may require a target in the form of a simple grating, large enough that the measurement beam produces a spot smaller than the grating (i.e., the grating is underfilled). In so-called reconstruction methods, the properties of the grating are calculated by simulating the interaction between the scattered radiation and a mathematical model of the target structure. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0006] In addition to reconstructing the shape of the measurement features, diffraction-based overlap can be measured using such a device, as described in published patent application US2006066855A1. Overlap measurements of smaller targets can be achieved using dark-field imaging with diffraction orders. These targets can be smaller than the illumination spot and can be surrounded by product structures on a wafer. Examples of dark-field imaging measurements can be found in numerous published patent applications such as US2011102753A1 and US20120044470A. Multiple gratings can be measured in one image using composite grating targets. Known scatterometers tend to use light in the visible or near-IR wave range, requiring the grating pitch to be much coarser than the actual product structure whose properties are of interest. These product features can be defined using deep ultraviolet (DUV) or extreme ultraviolet (EUV) radiation with much shorter wavelengths. Disadvantageously, these wavelengths are generally unavailable or unsuitable for measurement.

[0007] On the other hand, the dimensions of modern product structures are so small that they cannot be imaged using optical metrology techniques. Small features include those formed, for example, through multiple patterning processes and / or pitch multiplication. Therefore, targets for high-volume metrology often use features much larger than the product's properties of interest, such as overlap error or critical dimensions. Measurement results are only indirectly correlated with the dimensions of the actual product structure and may be inaccurate because the measured target does not undergo the same distortions as optical projection in a lithography apparatus and / or different treatments in other steps of the manufacturing process. While scanning electron microscopy (SEM) can directly resolve these modern product structures, SEM is much more time-consuming than optical measurements. Furthermore, electrons cannot penetrate thick process layers, making them less suitable for metrology applications. Other techniques, such as measuring electrical properties using contact pads, are also known, but they only provide indirect evidence of the actual product structure.

[0008] By reducing the wavelength of the radiation used during measurement (i.e., shifting towards the “soft X-ray” wavelength spectrum), it is possible to resolve smaller structures to increase sensitivity to structural changes and / or further penetrate the product structure. A suitable method for generating high-frequency radiation (e.g., soft X-rays and / or EUV radiation) is to use a high-harmonic generation (HHG) radiation source. Such an HHG radiation source uses laser radiation (e.g., infrared radiation) to excite the HHG generating medium, thereby generating high harmonics that include high-frequency radiation.

[0009] One problem with the generated high-frequency radiation is that it can be absorbed by any particles present in its path. This requires the HHG radiation source to be kept near a vacuum. Since the HHG generating medium is typically a gas, the generating medium must be carefully controlled to prevent it from absorbing the generated radiation.

[0010] Another issue with HHG radiation sources is maintaining a stable output of the generated radiation. Any fluctuations in the supply of the HHG generating medium can negatively affect the temporal stability of the radiation output. Summary of the Invention

[0011] According to a first aspect of the invention, a gas delivery system for an irradiation source is provided, comprising a gas delivery element arranged to guide gas in at least a first direction, wherein the gas delivery element comprises:

[0012] Optical input; and

[0013] Optical output,

[0014] The input and the output define an optical path, which is oriented in a second direction.

[0015] The second direction is neither perpendicular to nor parallel to the first direction.

[0016] According to a second aspect of the invention, a gas delivery system for an irradiation source is provided, comprising: a gas delivery element arranged to guide gas in at least a first direction, wherein the gas delivery element includes: an optical input and an optical output that together define an optical path, the optical path being oriented in a second direction; and a gas jet shaping device operable to modify the flow profile of the gas such that the number density of the gas decreases sharply in the direction of the optical output after a pump radiation interaction region where pump radiation interacts with the gas.

[0017] According to a third aspect of the present invention, an irradiation source for generating high harmonic radiation is provided, comprising:

[0018] Pump radiation source, capable of operating to emit pump radiation; and

[0019] The gas delivery system described above is operable to receive the emitted pump radiation and to generate the high harmonic radiation.

[0020] According to a fourth aspect of the present invention, an inspection apparatus for measuring a target structure on a substrate is provided, comprising:

[0021] The irradiation source described above is used to generate high harmonic radiation; and

[0022] A sensing element for receiving high harmonic radiation scattered by the target structure.

[0023] According to a fifth aspect of the present invention, a photolithography apparatus is provided, comprising: an illumination optical system arranged to illuminate a pattern, and a projection optical system arranged to project an image of the pattern onto a substrate.

[0024] The photolithography apparatus described above includes an irradiation source.

[0025] According to a fifth aspect of the present invention, a photolithography system is provided, comprising:

[0026] A photolithography apparatus includes: an illumination optical system arranged to illuminate a pattern, and a projection optical system arranged to project an image of the pattern onto a substrate; and

[0027] The inspection equipment described above,

[0028] The photolithography apparatus is arranged to use one or more parameters calculated by the inspection apparatus when applying the pattern to another substrate.

[0029] According to a sixth aspect of the invention, a gas delivery system for an irradiation source is provided, comprising at least a first gas delivery element and a second gas delivery element, the first gas delivery element being operable to emit a first gas and the second gas delivery element being operable to emit a second gas, such that the quantity density profile of the first gas is altered by the second gas.

[0030] According to a seventh aspect of the invention, an irradiation source for generating high harmonic radiation is provided, comprising: a pump radiation source operable to emit pump radiation located at a high harmonic generating gas medium, thereby exciting the high harmonic generating gas medium in a pump radiation interaction region to generate the high harmonic radiation; and an ionization radiation source operable to emit ionization radiation located at the high harmonic generating gas medium to ionize the gas in an ionization region between the pump radiation interaction region of the irradiation source and an optical output.

[0031] Further aspects, features, and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will be described in detail below with reference to the accompanying drawings. It should be noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the art based on the teachings contained herein. Attached Figure Description

[0032] Embodiments of the present invention will now be described by way of example only, with reference to the accompanying schematic diagrams, in which corresponding reference numerals denote corresponding components, and wherein:

[0033] Figure 1 A photolithography device is described;

[0034] Figure 2 The image depicts a lithography unit or lithography cluster in which an inspection device according to the present invention can be used;

[0035] Figure 3 (a) and (b) in the figure schematically illustrate an inspection apparatus adapted to perform known dark-field imaging inspection methods;

[0036] Figure 4 A measurement apparatus using an adjustable HHG source according to an embodiment of the present invention is schematically illustrated.

[0037] Figure 5 The details of the HHG gas unit that can be used in an HHG source are schematically shown.

[0038] Figure 6 A first exemplary gas delivery element that can be used in an HHG gas unit is shown;

[0039] Figure 7 Figures (a) to (e) illustrate the principle of the gas delivery element according to the present invention;

[0040] Figure 8 A second exemplary gas delivery element according to the present invention is shown;

[0041] Figure 9 An HHG radiation source according to an embodiment of the present invention is shown; and

[0042] Figure 10 (a) and (b) show filter components according to embodiments of the present invention;

[0043] Figure 11 Images (a) and (b) show a gas delivery element comprising a gas jet forming apparatus according to an embodiment of the present invention;

[0044] Figure 12 In response to Figure 11 A plot of the number density of gas emitted by a gas delivery element and a gas delivery element without a gas jet forming device relative to the distance along the optical path;

[0045] Figure 13 (a) through (d) schematically illustrate four steps of a method for mitigating reabsorption by generating soft X-rays using plasma to measure radiation.

[0046] Figure 14 It shows the method for execution Figure 13 The arrangement of the method shown;

[0047] Figure 15 (a) through (c) schematically illustrate three steps of an alternative method for mitigating reabsorption by generating soft X-rays using plasma to measure radiation; and

[0048] Figure 16 (a) Another arrangement for mitigating the reabsorption of the resulting measurement radiation is shown; and (b) a plot of the gas density relative to the pump radiation propagation direction x, illustrating... Figure 16 The working principle upon which the arrangement of (a) is based. Detailed Implementation

[0049] Before describing the embodiments of the present invention in detail, it is helpful to provide an example environment in which the embodiments of the present invention can be implemented.

[0050] Figure 1 A lithography apparatus LA is schematically depicted. The apparatus includes: an irradiation system (irradiator) IL configured to modulate a radiation beam B (e.g., ultraviolet (UV) radiation or deep ultraviolet (DUV) radiation); a patterning apparatus support or support structure (e.g., a mask stage) MT configured to support a patterning apparatus (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning apparatus according to specific parameters; two substrate stages (e.g., wafer stages) WTa and WTb, each configured to hold a substrate (e.g., a resist-coated wafer) W and each connected to a second positioner PW configured to accurately position the substrate according to specific parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted by the radiation beam B by the patterning apparatus MA onto a target portion C (e.g., comprising one or more dies) of the substrate W. A reference frame RF connects the various components and serves as a reference for setting and measuring the positions of the patterning apparatus and the substrate, as well as the positions of features on the patterning apparatus and the substrate.

[0051] Irradiation systems may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, to guide, shape, or control radiation.

[0052] The patterning apparatus support holds the patterning apparatus in a manner dependent on the orientation of the patterning apparatus, the design of the lithography equipment, and other conditions such as whether the patterning apparatus is kept in a vacuum environment. The patterning apparatus support can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning apparatus. The patterning apparatus support MT can be a frame or a stage, and for example, it can be fixed or movable as needed. The patterning apparatus support ensures that the patterning apparatus is positioned in a desired location, such as relative to a projection system.

[0053] The term "patterning apparatus" as used herein should be broadly understood to refer to any apparatus capable of imparting a pattern across the cross-section of a radiation beam to produce a pattern on a target portion of a substrate. It should be noted that the pattern imparted by the radiation beam may not perfectly correspond to the desired pattern in the target portion of the substrate, for example, if the pattern includes phase-shifting features or so-called auxiliary features. Typically, the pattern imparted by the radiation beam will correspond to a specific functional layer (such as an integrated circuit) in the device produced in the target portion.

[0054] As described herein, the apparatus is transmissive (e.g., employing a transmissive pattern forming apparatus). Alternatively, the apparatus may be reflective (e.g., employing a programmable mirror array of the type mentioned above, or employing a reflective mask). Examples of pattern forming apparatuses include masks, programmable mirror arrays, and programmable LCD panels. Any term "mask" or "mask" as used herein may be considered synonymous with the more general term "pattern forming apparatus." The term "pattern forming apparatus" may also be understood to refer to means of storing, in digital form, pattern information for controlling such a programmable pattern forming apparatus.

[0055] The term "projection system" as used herein should be interpreted broadly to encompass any type of projection system, including refractive, reflective, reflective-refractive, magnetic, electromagnetic, and electrostatic optical systems, or any combination thereof, as suitable for the exposure radiation used, or for other factors such as the use of immersion fluids or vacuum. Any term "projection lens" as used herein may be considered synonymous with the more general term "projection system."

[0056] The photolithography apparatus can also be of this type, wherein at least a portion of the substrate can be covered by a liquid (e.g., water) with a relatively high refractive index to facilitate filling the space between the projection system and the substrate. Immersion fluid can also be applied to other spaces within the photolithography apparatus, such as between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

[0057] In operation, the irradiator IL receives a radiation beam from a radiation source SO. The radiation source and the lithography apparatus can be separate entities (e.g., when the source is an excimer laser). In this case, the source is not considered part of the lithography apparatus, and the radiation beam is transmitted from the source SO to the irradiator IL by means of a beam delivery system BD, including, for example, suitable directional mirrors and / or beam expanders. In other cases, the source can be an integral part of the lithography apparatus (e.g., when the source is a mercury lamp). The source SO, the irradiator IL, and the beam delivery system BD, if necessary, can be collectively referred to as the radiation system.

[0058] The irradiator IL may, for example, include an adjuster AD, an integrator IN, and a concentrator CO for adjusting the angular intensity distribution of the radiation beam. The irradiator can be used to adjust the radiation beam to have a desired uniformity and intensity distribution in its cross-section.

[0059] The radiation beam B is incident on the pattern forming apparatus MA held on the pattern forming apparatus support MT, and a pattern is formed by the pattern forming apparatus. After passing through the pattern forming apparatus (e.g., a mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto the target portion C of the substrate W. The substrate stage WTa or WTb can be accurately moved by means of a second positioner PW and a position sensor IF (e.g., an interferometer, linear encoder, 2D encoder, or capacitive sensor), for example, to position different target portions C within the path of the radiation beam B. Similarly, for example, after mechanical acquisition from a mask library, or during scanning, the first positioner PM and another position sensor (… Figure 1 (Not explicitly shown) for accurately positioning the pattern forming apparatus (e.g., mask) MA relative to the path of the radiation beam B.

[0060] The patterning apparatus (mask) MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks shown occupy dedicated target portions, they can be located in the space between the target portions (these are known as scribing alignment marks). Similarly, in cases where more than one die is disposed on the patterning apparatus (e.g., mask) MA, the mask alignment marks can be located between the dies. Small alignment marks can also be included within the die, within device features, in which case it is desirable that the markings be as small as possible and do not require any different imaging or process conditions compared to adjacent features. The alignment system for detecting the alignment marks is further described below.

[0061] The described apparatus can be used in various modes. In scanning mode, a pattern imparted by the radiation beam is projected onto a target portion C while the patterning apparatus support (e.g., mask stage) MT and substrate stage WT are scanned synchronously (i.e., a single dynamic exposure). The velocity and direction of the substrate stage WT relative to the patterning apparatus support (e.g., mask stage) MT can be determined by the (reduced) magnification and image inversion characteristics of the projection system PS. In scanning mode, the maximum size of the exposure field limits the width of the target portion (along the non-scanning direction) in a single dynamic exposure, while the length of the scanning motion determines the height of the target portion (along the scanning direction). Other types and operating modes of lithography apparatus are known in the art. For example, stepping mode is known. In so-called "maskless" lithography, the programmable patterning apparatus is kept stationary but has a changing pattern, and the substrate stage WT is moved or scanned.

[0062] You can also use combinations and / or variations of the above usage patterns, or completely different usage patterns.

[0063] The lithography apparatus LA is a so-called dual-platform type, featuring two substrate stages WTa and WTb and two stations—an exposure station EXP and a measurement station MEA—the substrate stages can be interchanged between the exposure and measurement stations. While one substrate on one stage is being exposed at the exposure station, another substrate can be loaded onto the other substrate stage at the measurement station and various preparatory steps can be performed. This enables a substantial increase in the apparatus's throughput. The preparatory steps may include mapping the surface height profile of the substrate using a level sensor LS and measuring the position of alignment marks on the substrate using an alignment sensor AS. If the position sensor IF cannot measure the position of the substrate stage when it is at both the measurement and exposure stations, a second position sensor can be provided so that the position of the substrate stage relative to the reference frame RF can be tracked at both stations. Other arrangements are known and available instead of the illustrated dual-platform arrangement. For example, other lithography apparatuses with substrate stages and measurement stages are known. These substrate stages and measurement stages are mated together during preparatory measurements and then not mated together when the substrate stage is being exposed.

[0064] like Figure 2As shown, the lithography equipment LA constitutes part of the lithography unit LC (sometimes also called a lithography cell or lithography cluster), which also includes equipment for performing pre-exposure and post-exposure processes on the substrate. Typically, these devices include a spin coater SC for depositing a resist layer, a developer DE for developing the resist after exposure, a chiller CH, and a baking plate BK. A substrate processor or robot RO picks up substrates from input / output ports I / O1 and I / O2, moves them between different process devices, and delivers them to the lithography equipment's feed stage LB. These devices, often collectively referred to as the coating and developing system (track), are under the control of the coating and developing system control unit TCU, which in turn is controlled by a management control system SCS, which in turn controls the lithography equipment via the lithography control unit LACU. The management control system can also control one or more inspection devices MET, which perform measurements on the substrate W to ensure the quality and consistency of the lithography process and to determine any necessary corrections. Therefore, different devices can be operated to maximize throughput and processing efficiency. The substrate, which has been processed by the coating and developing system, is then transferred to other processing tools for etching and other chemical or physical treatments during the device manufacturing process.

[0065] The lithography equipment control unit (LACU) controls all movements and measurements of the various actuators and sensors described. The LACU also includes signal processing and data processing capabilities for performing the desired calculations related to the operation of the equipment. In the terminology of the specification and claims, this combination of processing and control functions is simply referred to as the "controller." In practice, the control unit (LACU) will be implemented as a system of many sub-units, each handling real-time data acquisition, processing, and control of subsystems or components within the equipment. For example, one processing subsystem may be dedicated to the servo control of the substrate positioner (PW). Discrete units may even manipulate coarse and fine actuators, or different axes. Another unit may be dedicated to the readout of the position sensor (IF). Overall control of the equipment can be controlled by a central processing unit (CPU) that communicates with these subsystem processing units, with the operator, and with other equipment involved in the lithography process.

[0066] Figure 3 (a) (or Figure 3 (a) schematically illustrates key components of an inspection apparatus (MET) for performing so-called dark-field imaging measurements. The apparatus can be a standalone device or integrated into, for example, a lithography apparatus (LA) or lithography unit (LC) at a measurement station. The dotted line O represents an optical axis with several branches running through the apparatus. Figure 3 (b) (or Figure 3(b) illustrates the target grating structure T and the diffraction rays in more detail.

[0067] As described in the prior application cited in the background introduction, Figure 3 (a) The dark-field imaging apparatus can be a part of a multipurpose angle-resolved scatterometer that can replace a spectral scatterometer or be used in addition to a spectral scatterometer. In this type of inspection apparatus, the radiation emitted by the radiation source 11 (an HHG radiation source in this disclosure) is modulated by the illumination system 12. For example, the illumination system 12 may include a collimating lens system, a color filter, a polarizer, and an aperture device. The modulated radiation follows an illumination path in which the radiation is reflected by a partially reflective surface 15 and focused by a microscope objective 16 onto a spot S on the substrate W. The measurement target T can be formed on the substrate W. The lens 16 has a high numerical aperture (NA), preferably at least 0.9 and more preferably at least 0.95. If desired, a wetting fluid can be used to obtain a numerical aperture greater than 1. The multipurpose scatterometer may have two or more measurement branches. In addition, additional optical systems and branches will be included in the actual apparatus, for example, to collect reference radiation for intensity normalization, for capturing coarse imaging of the target, for focusing, etc. These details can be found in the prior publications mentioned above. For the purposes of this disclosure, only the measurement branches of interest used in dark-field imaging measurements will be described and illustrated in detail.

[0068] In the collection path used for dark-field imaging, the imaging optics system 21 forms an image of the target on the substrate W on the sensor 23 (e.g., a CCD or CMOS sensor). An aperture stop 20 is disposed in the plane P' of the collection path. The plane P' is a plane conjugate to the pupil plane P” of the objective lens 16. The aperture stop 20 may also be referred to as the pupil stop. The aperture stop 20 can take different forms, just as the illumination aperture can take different forms. The combination of the aperture stop 20 and the effective aperture of the lens 16 determines which portion of the scattered radiation will produce an image on the sensor 23. Typically, the aperture stop 20 is used to block the zeroth-order diffraction beam, so that the image of the target formed on the sensor 23 is formed only by the first-order beam. In an example where the first-order beams are combined to form an image, this would be a so-called dark-field image. This is equivalent to dark-field microscopy. However, in this application, only one of the first-order measurements is imaged at a time, as explained below. The image captured by sensor 23 is output to image processor and controller 40, the functionality of which will depend on the specific type of measurement being performed. For the purposes of this invention, asymmetry measurements of the target structure are performed. Asymmetry measurements can be combined with knowledge of the target structure to obtain measurement results of performance parameters for the photolithography process used to form said target structure. Performance parameters that can be measured in this way include, for example, overlap, focus, and dose.

[0069] When the measurement target T is set on the substrate W, this can be a 1D grating, which is printed such that, after development, the grating strips are formed by solid resist lines. The target can be a 2D grating, which is printed such that, after development, the grating is formed by solid resist pillars or vias in the resist. The grating strips, pillars, or vias can alternatively be etched into the substrate. Each of these gratings is an example of a target structure whose properties can be investigated using inspection equipment.

[0070] Various components of the illumination system 12 can be adjusted to implement different measurement "recipes" within the same device. Besides selecting wavelength (color) and polarization as specific characteristics, the illumination system 12 can also be adjusted to implement different illumination profiles. Because the plane P" is conjugate to the pupil plane P' of the objective lens 16 and the plane of the detector 23, the illumination profile in the plane P" defines the angular distribution of light incident on the upper substrate W in the spot S. To implement different illumination profiles, aperture devices can be positioned in the illumination path. Aperture devices can include different apertures mounted on movable sliders or wheels. Alternatively, they can include a programmable spatial light modulator. As another alternative, optical fibers can be positioned at different locations in the plane P" and selectively used to transmit light or not transmit light at their respective locations. These variations are discussed and illustrated in the documents cited above.

[0071] In the first example illumination mode, ray 30a is provided such that the angle of incidence is as shown at “I”, and the path of the zeroth-order ray reflected by the target T is marked “0” (not to be confused with the optical axis “O”). In the second illumination mode, ray 30b can be provided, in which case the angle of incidence and the angle of reflection are interchanged. Both of these illumination modes will be considered off-axis illumination modes. Many different illumination modes can be implemented for different purposes.

[0072] like Figure 3As shown in more detail in (b), the target grating T, as an example of the target structure, is positioned such that the substrate W is perpendicular to the optical axis O of the objective lens 16. In the case of off-axis illumination profile, the illumination ray I strikes the grating T from an angle deviating from the axis O, producing a zeroth-order ray (solid line 0) and two first-order rays (dotted chain line +1 and double dotted chain line -1). It should be remembered that, in the case of an overfilled small target grating, these rays are only one of many parallel rays covering the area of ​​the substrate including the measurement target grating T and other features. Since the beam of illumination ray 30a has a finite width (to allow for a useful amount of light), the incident ray I will in fact occupy an angular range, and the diffracted rays 0 and +1 / -1 will spread out slightly. According to the point spread function of the small target, each +1 and -1 will spread out further over an angular range, rather than a single ideal ray as shown.

[0073] See also Figure 3 (a) In the first illumination mode of ray 30a, the +1st order diffracted ray from the target grating enters the objective lens 16 and contributes to the image recorded at the sensor 23. When using the second illumination mode, ray 30b is incident at the opposite angle to ray 30a, and therefore the -1st order diffracted ray enters the objective lens and contributes to the SUOS image. When using off-axis illumination, the aperture stop 20 blocks the zeroth order radiation. As described in previously published materials, the illumination mode can be defined by off-axis illumination in the X and Y directions.

[0074] Asymmetry measurement results can be obtained by comparing images of the target grating under these different illumination modes. Alternatively, asymmetry measurement results can be obtained by maintaining the same illumination mode but rotating the target. Although off-axis illumination is shown, coaxial illumination of the target can be used instead, and a modified off-axis aperture 20 can be used to deliver essentially only one first-order diffracted beam to the sensor. In another example, a prism is used instead of aperture stop 20, the prism having the ability to direct +1 and -1 orders to different locations on sensor 23 so that they can be detected and compared without two sequential image capture steps. This technology is disclosed in the published patent application US2011102753A1 mentioned above, the contents of which are incorporated herein by reference. Instead of the first-order beam or in addition to the first-order beam, second-order, third-order, and higher-order beams can be used ( Figure 3 (Not shown in the image) is used in the measurement. As another variation, the off-axis illumination mode can be kept constant while the target itself is rotated 180 degrees below objective 16 to capture the image using the opposite diffraction order.

[0075] The above techniques are typically performed using radiation with visible light wavelengths. Therefore, the scattering measurement target has a pitch larger than that of the product structure on the substrate. As an example, the scattering measurement target may have a target grating pitch measured in micrometers (μm), while the product structure on the same substrate may have a pitch measured in nanometers (nm).

[0076] The pitch difference causes an offset between the measured overlap and the actual overlap on the product structure. This offset is at least partly due to optical projection distortion from the lithography equipment and / or different processing in other steps of the manufacturing process. Currently, the offset contributes significantly to the overall measured overlap. Reducing or eliminating the offset will therefore improve the overall overlap performance.

[0077] Measurement tools can be developed using sources that emit radiation in the “soft X-ray” or EUV range (e.g., with wavelengths ranging from 0.1 nm to 100 nm, or optionally from 1 nm to 50 nm, or optionally from 10 nm to 20 nm). Examples of such sources include discharge-generated plasma sources, laser-generated plasma sources, or high-harmonic generation (HHG) sources. HHG sources are known to provide high-flux (high-brightness) collimated photons in the emitted light.

[0078] HHG sources for metrological applications are described and further described in European patent applications EP152020301, EP16168237, and EP16167512, the entire contents of which are incorporated herein by reference. In metrological applications, such HHG sources can be used, for example, at normal incidence, very close to normal incidence (e.g., within 10 degrees of normal incidence), at grazing incidence (e.g., within 20 degrees of the surface), at any angle, or at multiple angles (to obtain more measurement information in a single capture).

[0079] Figure 4The measurement arrangement of radiation source 430 is illustrated in more detail. Radiation source 430 is an HHG source for generating “soft X-ray” / EUV (high harmonic radiation) based on high harmonic generation (HHG) technology. The main components of radiation source 430 are pump radiation source 431 (e.g., pump laser or oscillator) and HHG medium such as HHG gas unit 432. Gas supply device 434 supplies a suitable gas to the gas unit, which is optionally ionized by a power source (not shown). Pump radiation source 431 can be, for example, a fiber-based laser with an optical amplifier, thereby generating radiation pulses of infrared radiation with a duration of less than 1 nanosecond (1 ns) per pulse, with a pulse repetition rate as high as several megahertz as needed. The wavelength of the pump radiation can be, for example, about 1 micrometer (1 μm). The radiation pulse is delivered as a pump radiation beam 440 to HHG gas unit 432, where a portion of the radiation is converted to a higher frequency. A beam of measuring radiation 442, comprising coherent radiation of the desired wavelength, is emitted from the HHG gas unit 432.

[0080] The measured radiation 442 can include multiple wavelengths. If the radiation is also monochromatic, the measurement calculations (reconstruction) can be simplified, but with an HHG, it is easier to generate radiation with several wavelengths. These are design choices and can even be optional options within the same device. When imaging structures of different materials, different wavelengths will, for example, provide different levels of contrast. For example, to examine metallic or silicon structures, different wavelengths can be selected to characterize (carbon-based) resists or to detect contamination in these different materials.

[0081] One or more filter devices 444 may be provided. For example, filters such as aluminum (Al) thin films may be used to cut off the basic IR radiation to prevent further propagation into the inspection apparatus. Gratings may be provided to select one or more specific harmonic wavelengths from the wavelengths generated in the gas element 432. Some or all of the beam path may be contained within a vacuum environment, noting that EUV radiation is absorbed as it travels through air. The radiation source 430 and various components of the illumination optics may be adjustable to implement different measurement “options” within the same apparatus. For example, different wavelengths and / or polarizations may be selectable.

[0082] A filtered measurement beam enters the inspection chamber from radiation source 430, in which a substrate W of the structure of interest or target structure is held by a substrate support 414 for inspection. The target structure is labeled T. The atmosphere within the inspection chamber is maintained at near-vacuum by a vacuum pump 452, allowing soft X-ray radiation to pass through the atmosphere without excessive attenuation. The irradiation system includes one or more optical elements 454 for focusing the radiation onto the focused beam 456, and may include, for example, two-dimensional curved mirrors or a series of one-dimensional curved mirrors, as described in the aforementioned prior applications. Diffraction gratings, such as spectral gratings, may be combined with these mirrors if desired. When projected onto the structure of interest, focusing is performed to achieve a circular or elliptical spot with a diameter less than 10 μm. The substrate support 414 includes, for example, an XY translation platform and a rotation platform, by which any portion of the substrate W can be brought to the focal point of the beam in the desired orientation. Thus, a radiation spot S is formed on the structure of interest. Next, the scattered radiation 408 from the structure of interest is detected by detector 460.

[0083] Figure 5 It shows that, for example, it can be in Figure 4 A more detailed illustration of the exemplary HHG gas unit implemented in the system described herein. For ease of reference... Figure 4 Comparison, with Figure 4 The corresponding components are similar Figure 5 The components are used with Figure 4 The reference markers in the text are similar but have the prefix "5" instead of "4" and are used for marking.

[0084] The incident pump radiation 540 (e.g., IR radiation) is shown focused onto the HHG gas unit 532 (or other HHG generating medium). The generated HHG radiation (measuring radiation or soft X-ray / EUV radiation) 542 and residual pump radiation 544 are shown outside the HHG gas unit 532, the residual pump radiation 544 of which needs to be filtered out from the generated measuring radiation 542. Since there is typically an overlap area between the generated measuring radiation 542 and the residual pump radiation 544, a filter that substantially transmits the measuring radiation 542 but blocks the pump radiation 544 is required. In some examples, this can be achieved by using an ultrathin metal film filter. Within the HHG gas unit, a capillary 570 delivers the gas (i.e., the HHG medium) into the examination chamber (as indicated by arrow 572). Once the gas exits the capillary, it expands into the examination chamber depending on many characteristics (e.g., fluid velocity, material properties, and capillary properties), thereby forming a gas “cloud” 574. Pump radiation propagates through the gas, thereby generating measurement radiation. As described above, the examination chamber is maintained at a near-vacuum level to prevent the generated soft X-ray radiation from being absorbed by the present atmospheric particles.

[0085] One problem with known exemplary arrangements is that once a gas particle exits the capillary, it substantially propagates in all directions. While gas particles are necessary for generating the measurement radiation, as described above, any gas particles present in the path of the measurement radiation will absorb it. Since a significant proportion of gas particles may propagate along or through the path of the measurement radiation, a substantial portion of the measurement radiation may be absorbed in known systems. In other words, gas particles in the path of the measurement radiation can negatively affect the intensity and temporal stability of the measurement radiation.

[0086] The inventors have realized that it is possible to provide a device for transporting gas while minimizing the amount of radiation absorbed.

[0087] Figure 6 The illustration depicts a gas delivery system 600 for use in an irradiation source according to an aspect of the invention. The gas delivery system can be implemented in suitable radiation or irradiation systems, such as those described above. Figure 4 The radiation or irradiation system shown.

[0088] The system includes a gas delivery element 602 arranged to guide gas in at least a first direction 603. The gas delivery element includes an optical input 606. Additionally, the gas delivery element includes an optical output 608. The optical input and optical output define an optical path 610 oriented in a second direction that is neither perpendicular to nor parallel to the first direction. In some examples, the optical input and / or optical output may depend on the characteristics of the pump radiation beam. In one example, the optical input and optical output are matched to substantially conform to the beam shape of the pump radiation.

[0089] The gas delivery element can have any suitable or convenient shape. In this example, the gas delivery element includes two pairs of opposing walls; a first wall 612 including an optical input 606, a second wall 614 including an optical output, a first sidewall 616, and a second sidewall 618.

[0090] During operation, gas 604 enters the gas delivery element and flows substantially in the first direction 603. A first portion 620 of the gas escapes through optical input 606, and a second portion 622 of the gas escapes through optical output 608. As explained above, the second portion of the gas is used to absorb the generated radiation, reducing its intensity. A third portion 605 of the gas is directed toward the output of the gas delivery element.

[0091] The gas delivery system is arranged such that the pump radiation beam propagates along the optical path 610 and is focused at a point substantially inside the gas delivery system (i.e., located on the optical path 610 and between the first wall 612 and the second wall 614). In other words, the pump radiation beam is arranged to maximize the intensity of the pump radiation within the gas flow located inside the gas delivery element (i.e., having a focal point or "waist"). However, it should be understood that this is for illustrative purposes only. In some examples, the focal point is located on the optical path but not inside the gas delivery element. In one example, the focal point is located on the input side of the gas delivery element. In another example, the focal point is located on the output side of the gas delivery element.

[0092] Of course, it should be understood that multiple specific cross-sections and / or shapes can be conceived for the gas delivery element. In this example, the gas delivery element includes two pairs of opposing walls defining a specific cross-section for gas flow within it. It should be understood that many specific cross-sections (including, but not limited to, circular, elliptical, or rectangular) can be conceived. In some examples, the cross-section of the gas delivery element may be adapted to provide one or more specific effects, such as a specific flow profile of the gas.

[0093] Figure 7 The principle of the invention is illustrated schematically. For ease of understanding... Figure 6Comparison, with Figure 6 The corresponding components are similar Figure 7 Components used for Figure 6 The reference markers in the text are similar, but the reference markers are marked with the prefix "7" instead of "6".

[0094] Gas delivery element 702 delivers gas 704 in a first direction 703. In this example, the gas delivery element is... Figure 7 The gas delivery element shown is essentially the same, comprising a first wall 712 having an optical input 706 and a second wall 714 having an optical output 708. A first portion 720 of the gas will escape through the optical input 706, and a second portion 722 of the gas will escape through the optical output 708. As explained above, the second portion of the gas will be used to absorb the generated radiation, which reduces the intensity of the generated radiation. A third portion 705 of the gas will propagate toward the output of the gas delivery element.

[0095] Now specifically turning to Figure 7 (a) (or Figure 7 (a) illustrates the known situation: the first direction 703 is perpendicular to the second direction 710, that is, the angle 724a between the first direction and the second direction is 90 degrees. Due to this perpendicularity, the portion of gas escaping from the optical input is substantially the same as the portion of gas escaping from the optical output.

[0096] Figure 7 (b) (or Figure 7 (b) illustrates an exemplary scenario according to an aspect of this disclosure. In this exemplary scenario, the first direction 703 is arranged at an angle 724b that is neither perpendicular nor parallel to the second direction 710. Similar to... Figure 7 The situation described in (a) Figure 7 In the exemplary case of (b), a first portion of the gas will escape through the optical input and a second portion of the gas will escape through the optical output. Due to non-perpendicularity, the first portion of the gas is larger than the second portion of the gas.

[0097] Figure 7 (c) (or Figure 7 (c))- Figure 7 (e) (or Figure 7 (e) illustrates many exemplary scenarios, which are essentially similar to Figure 7The exemplary scenario illustrated in (b) applies, but for different values ​​of angles 724c, 724d, and 724e between the first and second directions. It will be understood that as the angle between the first direction 703 and the second direction 710 increases, the airflow through the optical input (i.e., the first portion) increases, and the airflow through the optical output (i.e., the second portion) decreases. It will be understood that the values ​​described below are merely exemplary to illustrate the principles of this disclosure.

[0098] 0 1 1 10 1.09 0.93 30 1.67 0.70 45 2.25 0.52 50 2.83 0.46

[0099] The increased airflow at the input side of the gas delivery element does not substantially affect the output of the measured radiation. However, by reducing the amount of gas flowing out of the gas delivery element through the optical output, the absorption of soft X-ray radiation due to the presence of particles at the optical output is reduced. Accordingly, this increases the radiation output of the irradiation source.

[0100] Figure 8 A second exemplary embodiment of a gas delivery system for an irradiation source according to a second aspect of the present invention is shown. For ease of reference... Figure 6 Comparison, with Figure 6 The corresponding components are similar Figure 8 Components used for Figure 6 The reference markers in the text are similar but have the prefix "8" instead of "6".

[0101] Similar to the above, see also Figure 6 In the described example, the gas delivery element 802 includes an optical input 806 and an optical output 808 defining an optical path. In this example, the optical path forms a portion of a generally cylindrical radiation guide that defines a second direction 810. The gas delivery element further includes a generally annular gas delivery component 807. The annular gas delivery component is connected to the radiation guide via at least one gas delivery passage 809, thereby delivering gas to the radiation guide in at least a first direction 803. The gas delivery passage can have any suitable shape. In this example, the gas delivery passage has a generally truncated conical shape such that the gas is delivered generally symmetrically about the second direction 810. In other words, the gas delivery passage delivers gas 804 at multiple angles that are neither perpendicular to nor parallel to the second direction.

[0102] It should be understood that the implementation schemes discussed above are merely exemplary, and many specific implementation schemes can be conceived based on the principles of this disclosure.

[0103] As discussed above, maximizing the intensity and stability of the measured radiation is desirable. To ensure stability, it is necessary to maintain a substantially constant supply of gas to the HHG gas unit. Any variation or instability in the characteristics of the supplied gas, such as (but not limited to) gas velocity or flow rate, will cause the characteristics of the measured radiation to change over time. For example, a reduced airflow will decrease the amount of measured radiation, which in turn reduces its perceived intensity. This, in turn, can affect the quality of measurements performed using the measured radiation.

[0104] Therefore, it is advantageous to maintain the stability of the gas supplied to the HHG gas unit at a constant and high level. Many exemplary embodiments will now be discussed, the intention of which is to improve the stability of the gas supply to the gas delivery system.

[0105] Figure 9 The illustration shows a radiation source 930 including an exemplary gas delivery system. For ease of reference... Figure 4 Comparison, with Figure 4 The corresponding components are similar Figure 9 Components used for Figure 4 The reference markers in the text are similar but have the prefix "9" instead of "4" to indicate reference markers.

[0106] The gas delivery system includes a gas buffer element 960 positioned between a gas source 934 and an HHG gas unit 932. The gas buffer element has an input connected to the output of the gas source and an output connected to the input of the HHG gas unit. In operation, gas is delivered from the gas source (which may be a high-pressure gas cylinder fitted with a simple valve) to the gas buffer element. Typically, the gas source includes an output with high levels of gas flow variation and instability. The gas buffer element is operable to absorb variations in the gas flow from the gas source and deliver a substantially constant gas flow to the HHG gas unit.

[0107] The gas buffer element can have any suitable shape or form. In some examples, it may be provided with one or more control devices 962 for controlling the properties of the gas. In a particular example, the gas buffer element includes a temperature control element. This allows for control of the gas temperature, which can reduce changes in the specific mass of the gas due to temperature variations in the environment of the gas delivery system itself. It should be understood that although the temperature control element is discussed in this example as being included as part of the gas buffer element, the temperature control element may be located externally and used in conjunction with the gas buffer element in some examples. In other examples, additional or alternative control devices may be provided. In some examples, the gas buffer element or gas delivery system may include a gas composition control element. This is particularly relevant where the gas includes a mixture of a particular gas or compound. In still other examples, the gas buffer element or gas delivery system may include a purity control element. This is relevant where gas quality or purity control is of concern. In a particular example, the purity control element is coupled to a scrubber or gas replacement element.

[0108] Figure 10 Several exemplary gas delivery elements for improving gas flow profiles are schematically illustrated. For ease of reference... Figure 6 Comparison, with Figure 6 The corresponding components are similar Figure 10 Components used for Figure 6 The reference markers in the text are similar but have the prefix "10" instead of "6".

[0109] Figure 10 (a) (or Figure 10 (a) shows a known gas delivery element 1002, such as see [reference] Figure 5 The capillary discussed. It is known that in a gas delivery element such as the one shown, the moving gas 1004 has a specific flow profile (as indicated by dotted line 1024). Typically, the gas velocity and pressure are highest near the middle of the gas delivery element and lowest near the wall of the gas delivery element. At the outlet of the gas delivery element, the flow profile 1026a is substantially the same as the flow profile within the gas delivery element. Because the flow profile is not laterally uniform, the gas diffusivity at the outlet of the gas delivery element is higher than that would occur if the gas flow profile were substantially laterally uniform (e.g., if the gas flow in the gas delivery element were laminar). Therefore, a gas “cloud” forms in the region where the gas interacts with the incident pump radiation. To maximize the conversion of pump radiation to the generated radiation and to minimize the absorption of measured radiation by gas particles, it is desirable to ensure that the gas flow is laminar.

[0110] Figure 10 (b) (or Figure 10 (b)) An exemplary gas delivery element 1002 according to this disclosure is illustrated. It should be understood that, although illustrated substantially with... Figure 10 The capillary shown in (a) is the same capillary, but the principle of this example can be applied to many specific implementations.

[0111] Figure 10 The gas delivery element shown in (b) includes a filter element 1028. The filter element is operable to modify the flow profile of the gas flowing through the gas delivery element. In some examples, the filter element is operable to provide gas with a laminar flow profile 1026b at the output of the gas delivery element. By reducing the amount of turbulence in the gas, the gas diffusivity decreases once the gas exits the gas delivery element. This, in turn, maintains a higher concentration of gas in the region where the gas interacts with pump radiation (which increases the conversion efficiency and resulting intensity of the generated radiation). In this example, the filter element 1028 includes a first filter element 1030 and a second filter element 1032. In this example, both the first and second filter elements include multiple pores. The first filter element equalizes the pressure and velocity of the gas within the filter element. The second filter element provides a uniform flow distribution of gas exiting the gas delivery element. It should be understood that it is possible to control a specific flow distribution by controlling the gas distribution and pore size, for example, to optimize the generation of measured radiation. It will be further understood that this example is merely exemplary, and multiple specific implementations can be envisioned to perform the desired function, namely, providing laminar fluid flow in a gas delivery element. It will be further understood that, although in Figure 10 The illustration shows a capillary, but the principle of this example can be readily applied to other specific exemplary embodiments of this disclosure.

[0112] To generate (e.g., soft X-rays) measurement radiation with reasonable energy conversion efficiency, several physical parameters can be fine-tuned. One such parameter affecting the generation of HHG measurement radiation is the gas number density within the gas cell. Preferably, the number density should be high in the pump-radiation interaction region (where the pump radiation interacts with the gas / excites the gas) for phase matching and ionization, but low in the region immediately outside this region to prevent absorption of the measurement radiation. Furthermore, for efficient generation of measurement radiation, the high-density region should extend for a minimum distance (e.g., a few millimeters) and then drop sharply to a low density, for example, within 10% of the length of the pump-radiation interaction region along the direction of the pump radiation beam.

[0113] Therefore, it is proposed to use a gas jet shaping device to shape the gas jet such that the descent length from high number density to low number density is reduced. The gas jet shaping device can further shape the gas jet such that the length of the pump-radiation interaction region (the length of the region with high number density) is increased relative to the case without the gas jet shaping device. In an embodiment, the gas jet shaping device can make the short descent length less than 10% of the length of the pump-radiation interaction region. In this case, "low density" can include the background pressure in the vacuum container, which should be low enough to avoid absorbing significantly large amounts of measurement radiation, for example, typically 1-10 Pa. In an embodiment, the descent length can describe the distance the gas density travels from a density drop of at least ten times from the pump-radiation interaction region to the low-density region. The actual length of the pump-radiation interaction region can vary by an order of magnitude or more; the optimal length of the pump-radiation interaction region will depend on the gas type and the intensity and focusing of the pump-radiation beam.

[0114] In an embodiment, the gas jet shaping element may include an angled wall element located below the gas delivery element. The angled wall element may be attached to the gas delivery element at a point adjacent to an output (e.g., a nozzle output) such that it extends at an angle (e.g., a few millimeters – less than 10 mm) below that output toward the emitted gas jet. The angle (relative to the vertical axis or z-axis) may be, for example, between 20 and 60 degrees, between 20 and 50 degrees, or between 30 and 40 degrees. In an embodiment, the gas jet shaping element may include an angled horizontal cylindrical segment (e.g., a horizontally cut cylindrical segment) that opens or is open at its bottom end. More specifically, the gas jet shaping element may include a semi-cylindrical element. The gas jet shaping element may be positioned such that the wall of the gas jet shaping element is located between the gas jet and the optical output of the gas unit, and not between the gas jet and the optical input of the gas unit. The gas shaping element may include an aperture for transmitting the generated measurement radiation.

[0115] Figure 11The diagram shows (a) an isometric projection and (b) a cross-section of a gas delivery element 1170 and a gas jet shaping element 1180 according to an embodiment. The gas delivery element 1170 includes a nozzle outlet 1184. The gas jet shaping element 1180 in this embodiment is semi-cylindrical, with its walls located between the gas jet 1104 and an optical outlet (not shown). The gas shaping element 1180 includes an aperture 1182 for transmitting the generated measurement radiation. The shape and location of the gas shaping element 1180 result in a sharp decrease in the number density of the gas jet 1104, indicated by dotted line 1186. Therefore, a very low number density (e.g., background pressure in a vacuum container) is present in the region immediately following this dotted line 1186 along the optical path in the direction toward the optical outlet. This serves to reduce the absorption of the generated measurement radiation by the gas jet 1104. Due to the high speed and kinetic energy of the gas molecules in the vertical direction, relatively few gas molecules pass through the aperture 1182.

[0116] In this embodiment, the wall of the gas jet forming element 1180 may be thin, for example, less than 0.2 mm, or in the range of 0.1 mm. It may comprise any suitable material, such as a weldable material (e.g., such that it can be welded to a disk 1187 surrounding the nozzle outlet 1184). The material may be wear-resistant and have a high melting temperature. For example, suitable materials may include tungsten, molybdenum, aluminum, or stainless steel.

[0117] The function of the gas jet forming element 1180 is to provide a high-density gas on the gas delivery side of the gas jet forming element 1180 and to provide a steep or abrupt density drop on the outlet side of the gas jet forming element 1180 near the orifice 1182. More specifically, the gas jet forming element 1180 provides a relatively long (compared to the case without a gas jet forming element) pump-radiation interaction region on its gas delivery side and a density drop region on its outlet side that is less than 10% of the length of the pump-radiation interaction region.

[0118] Figure 12The effect is illustrated. It shows two gas density profiles: a first gas density profile 1291 (solid line) for a gas delivery system without a gas forming element, and a second gas density profile 1292 (dotted line) for a gas delivery system with a gas forming element as disclosed herein. In each case, the gas density profile includes a plot of the molecular gas density on a logarithmic scale (on the y-axis) relative to a distance along the optical path (x-axis), where the origin 0 is at the center of the output nozzle. A first gas density level line 1293 is also marked on the y-axis, showing the first gas density level at which the pump radiation will excite the gas. This corresponds to a region of relatively high density (e.g., at least 50% of the peak) at a specific distance below the nozzle output; the pump radiation does not pass through the density peak because it becomes zero after the laser interaction length. The second gas density level line 1294 shows a gas density level that is an order of magnitude (i.e., 10 times) lower than the first gas density level. The second gas density profile 1292 shows a longer pump-radiative interaction length 1295 relative to the pump-radiative interaction length 1296 of the first gas density profile 1291. The drop length 1297 of the second gas density profile 1292 and the drop length 1298 of the first gas density profile 1291 are also shown. Notably, the gas density drop length 1297 is less than 10% of the pump-radiative interaction length 1295 for the second gas density profile 1292, while the gas density drop length 1298 is greater than 10% of the pump-radiative interaction length 1296 for the first gas density profile 1291.

[0119] It should be noted that the actual length of the pump-radiative interaction region 1295 is variable depending on the gas type, pressure, etc. Therefore, a purely exemplary operating point could have a pump-radiative interaction length 1295 of 1.5 mm and a drop length 1297 of 0.1 mm. However, this can be scaled down by a factor of 10, for example, at higher gas densities. Thus, another (purely exemplary) operating point could have a pump-radiative interaction length 1295 of 0.2 mm and a drop length 1297 of less than 0.02 mm. The gas density profile can be scaled down in both width (x-axis: mm) and height (y-axis: gas density).

[0120] It should be noted that the gas jet forming element disclosed herein can be used in conjunction with other disclosed embodiments, i.e., in conjunction with a gas delivery element having an optical path that is not perpendicular to or parallel to the gas jet delivery direction, or it can be incorporated into a more conventional gas delivery element with an optical path perpendicular to the gas jet delivery direction.

[0121] As already described, there are advantages to reducing the gas density drop length. Where the gas density drop in the gaseous medium has a sharp edge in the propagation direction x of the pump radiation such that the gas density drops sharply immediately after the pump radiation interaction length, reabsorption of the generated measurement radiation by the gas can be substantially prevented. More specifically, the sharp edge may be located at position x=x0 along the propagation axis x of the pump radiation beam, where the gas density drops sharply from a specific phase matching density at x<x0 to a density at x>x0 that is sufficiently low for the gas to be substantially transparent to soft X-ray radiation. Soft X-ray radiation intensity generally reaches a high peak near the focus of the pump radiation beam. Assuming that the gas density profile decreases slowly after this maximum (as is usual), the soft X-ray intensity will be reduced to a low value due to absorption by the gas in the region x>x0. In contrast, where the gas density has a sharp edge near x=x0, absorption in the region x>x0 is substantially avoided and the soft X-ray intensity remains high in the region x>x0. This desired behavior results from the "sharpness" of the gas density profile when the density decreases over a length comparable to or smaller than the absorption length of the gas, which is a characteristic length scale dependent on the gas type. Generally, the absorption length of a gas suitable for generating soft X-rays at wavelengths below 20 nm is on the order of a few tenths of a millimeter or less. The gas density is typically 10 23 –10 26 m -3 in the range of.

[0122] It is proposed to use a second high-intensity (e.g., laser) radiation pulse to generate highly or fully ionized plasma in the region x>x0 at the time point when the soft X-ray pulse passes through the region. The plasma comprises ions having a much smaller absorption cross-section than the original atoms (or molecules) in the gas phase, and therefore absorbs significantly less of the measurement soft X-ray radiation. This eliminates the need for a steep particle density gradient and provides additional control over the precise region where measurement radiation is generated and reabsorbed.

[0123] Conventional laser intensities can be used to generate highly ionized plasma. To generate such highly ionized plasma over a length of approximately 3 mm (the length over which gas density decreases significantly in a gas jet configuration), a laser pulse with a wavelength of, for example, 1 μm can be used, focused into a spot with a diameter of approximately 30 μm. To achieve an intensity of 10 18 W / m 2 , the energy of a 50 fs laser pulse will be approximately 0.5 mJ, which is comparable to the pump laser pulse used to generate the measurement radiation.

[0124] Figure 13Possible configurations including backpropagating laser pulses are shown, illustrating (a)-(d) pump radiation pulse 1300 and backpropagating ionizing radiation pulse 1310 at different times during the measurement of the radiation generation process. Figure 13 (a) (or Figure 13 (a) shows the pump radiation pulse 1300 and ionization radiation pulse 1310 at the first moment, each pulse propagating toward the gas target 1320 at x = x0. Figure 13 (b) (or Figure 13 (b) shows pump radiation pulse 1300 and ionization radiation pulse 1310 at a later time, each pulse becoming closer to the focal point as the intensity increases. Figure 13 (c) (or Figure 13 (c) illustrates a plasma 1330 generated by ionizing radiation pulses, which ionize a gas target 1320 in a region immediately following x = x0 (e.g., x > x0) relative to the direction of pump radiation propagation. The delay between pulses is finely tuned so that they overlap at (close to) x = x0, where the resulting measurement radiation intensity reaches its maximum. Therefore, the resulting soft X-ray measurement radiation 1340 propagates through the plasma 1330 instead of the inert gas. Figure 13 (d) (or Figure 13 (d) shows the generated soft X-ray measurement radiation 1340 propagating together with the pump radiation after it has propagated through plasma 1330.

[0125] The ionizing radiation pulse can be synchronized with the pump radiation pulse. This would be automatic if the ionizing radiation pulse is generated from the same oscillator as the pump radiation pulse (e.g., a pump laser). The delay of the ionizing radiation pulse can be finely tuned so that plasma forms as soon as the pump radiation pulse reaches x = x0, thus generating the measurement radiation. Those skilled in the art will recognize that this is quite evident in micrometer and sub-picosecond precision. Accurate timing also means that the location where the measurement radiation pulse encounters the gas / plasma boundary can be actively selected. The backpropagating ionizing pulse can have the same wavelength as the pump laser pulse or a different wavelength. The focused size (and therefore divergence) of the ionizing radiation pulse can be similar to, but not necessarily the same as, the focused size (and therefore divergence) of the pump radiation pulse. The focused size and divergence of the ionizing radiation can be matched to the size of the resulting measurement radiation beam so that the measurement radiation beam propagates through highly ionized plasma rather than unionized gas.

[0126] Figure 14A possible arrangement for generating backpropagating pump radiation pulses 1400 and ionization radiation pulses 1410 from a single radiation source (oscillator) 1405 is shown. The output radiation 1407 is separated by a beam splitter 1415 into a pump radiation beam path 1425 and an ionization radiation beam path 1427. The pump radiation is directed to the gas target 1420 by an optical element 1435. The ionization radiation is directed to the gas target 1420 by an optical element 1450 and via a delay platform 1430. Although the delay platform 1430 is shown here in the ionization radiation beam path 1427, it can be located in either beam path or even in both beam paths.

[0127] To ensure that the returning ionized radiation pulses are prevented from re-entering the laser source system 1405, several methods of separating the laser pulses are possible. An arrangement using polarization to separate the pulses is shown here. A half-wave plate 1455 is located in one of the beam paths (which beam path is not important), followed by polarization beam splitters 1460, 1475 and beam collectors 1470, 1485 in each beam path. Due to this arrangement, the two pulses 1400, 1410 will have orthogonal polarization, and thus the incident radiation pulse can be separated from the returning radiation pulse by means of polarization beam splitters 1460, 1475, where the incident pulse is guided to beam collectors 1470, 1485. Alternatively, the ionized radiation pulse can be introduced into the system at a small angle to separate the beam path of the returning pulse from that of the incident pulse. As another alternative, different wavelengths can be used for the pulses, allowing separation by means of dichroic mirrors or filters.

[0128] Figure 15 Figures (a)-(c) show alternative possible configurations for generating plasma at three different times. This uses a cylindrical lens 1550, a cylindrical mirror, or other suitable optical element to focus the ionizing radiation pulse 1560 into a line focus. This line is oriented along the path of the measuring radiation pulse 1540 (e.g., in the region immediately following x = x0 relative to the propagation direction of the pump radiation pulse 1500), such that the resulting measuring radiation pulse propagates through the plasma 1530 rather than the inert gas atoms. The advantage of this configuration is that, relative to... Figure 13 The arrangement can generate relatively long plasma durations. Additionally, there is significant decoupling in timing based on location. Plasma can be generated some time before the arrival of the measured radiation pulse (approximately several picoseconds, down to nanoseconds). This avoids the need for precise timing.

[0129] The ionization radiation pulse arrives from a direction relative to or orthogonal to the propagation direction of the pump radiation pulse (e.g., in...). Figure 13 and Figure 15(As specifically shown in the text) is not necessary. In other embodiments, the propagation directions of the ionization radiation pulse and the pump radiation pulse may be separated at different angles. Relatedly, the generated plasma is adjacent to the region that generates the generated measurement radiation pulses, such that these measurement radiation pulses have only (at most) a short distance to propagate through the high harmonic generation gas before encountering the plasma.

[0130] Another advantage of the plasma generation method disclosed above lies in dose control. By accurately timing the ionized radiation pulse relative to the pump radiation pulse and / or by controlling the intensity of the ionized radiation pulse, it is possible to increase or decrease the absorption by the inert gas. Therefore, the proposed irradiation source may include a dose control element that controls the timing of the ionized radiation pulse relative to the pump radiation pulse and / or controls the intensity of the ionized radiation pulse. This dose control element can also be used to control the dose level and / or to effectively and temporarily “stop” the measurement radiation pulse by ensuring that no plasma is generated. If no plasma is generated and the gas configuration is appropriately selected, all or almost all of the measurement radiation will be reabsorbed by the gas. Dose control can be implemented in any suitable manner, for example, by suitable software running on a processor that controls the ionized radiation pulse and its generation. Such a processor can be a processor that more generally controls the operation of the irradiation source.

[0131] The plasma generated by the ionizing radiation source will refract the pump radiation pulse, while the measurement radiation pulse will be only negligibly affected. This is because the frequency of the measurement radiation pulse is much higher than the plasma frequency in the relevant typical density range. This means that it will be easier, for example, to separate the pump radiation pulse from the soft X-ray measurement radiation by means of a pinhole in the beam path that is aligned with the measurement radiation beam but blocks the refracted and (therefore) deflected pump radiation pulse.

[0132] It should be noted that the gas ionization method disclosed herein can be used in conjunction with other disclosed embodiments, i.e., in conjunction with a gas delivery element having an optical path that is not perpendicular to or parallel to the gas jet delivery direction, and / or as... Figure 11 The gas jet forming element shown is used with a delivery element and / or two or more gas nozzles as described below.

[0133] Figure 16 The illustration depicts an alternative arrangement for mitigating the reabsorption of measurement radiation by a gas, from which the measurement radiation is generated. In this arrangement, it is proposed to use at least two gas jets containing two different gases (e.g., different gas species). Figure 16 (a) (or Figure 16(a) illustrates this arrangement, which has a first gas nozzle 1600a and a second gas nozzle 1600b. The gas can be selected such that the first gas jet 1610a emitted by the first gas nozzle 1600a comprises a gaseous medium that effectively generates the desired (e.g., soft X-ray) measurement radiation. Therefore, the first gas can be the gas used in other embodiments described above. The second gas jet 1610b emitted by the second gas jet nozzle 1600b can comprise a second gas with a much lower absorptivity at the desired (e.g., soft X-ray) wavelength. For example, if the generated measurement radiation includes soft X-ray radiation in the 10-20 nm range, neon can be a suitable first gas and argon can be a suitable second gas. Argon has a much lower absorptivity than neon in this wavelength range.

[0134] The flow of the second gas jet 1610b is used to shape the first gas jet 1610a to obtain a steep density descent gradient immediately following the x = x0 position. This is in Figure 16 (b) (or Figure 16 (b) is illustrated in the figure, which is a plot of gas density relative to a distance along x (the direction of pump radiation propagation). A first plot 1620 (a thinner line) is the density profile for the first gas, and a second plot 1630 (a thicker line) is the density profile for the second gas. A plot of the overall gas density 1640 within the target gas region (HHG gas cell) is also shown (dotted line). It can be seen that the density profile 1620 for the first gas shows a very steep drop at the position x = x0. Outside this position, it is essentially only the second gas that will not absorb the measured radiation.

[0135] exist Figure 16 In the embodiment shown in (a), the gas jets are adjacent to each other. Optionally, the second gas jet 1610b may be inclined toward the first gas jet 1610a. In another possible embodiment, the gas jets may be arranged coaxially; for example, the first gas nozzle is located inside the second gas nozzle.

[0136] More gas jets with different gas profiles can be added to shape the gas profile of the first gas. The gas jets can be operated at similar or different gas pressures. They can also have different shapes and sizes.

[0137] It should be noted that two or more gas nozzle embodiments disclosed herein can be used in combination with other disclosed embodiments, i.e., in combination with gas delivery elements having optical paths that are not perpendicular to or parallel to the gas jet delivery direction and / or as... Figure 11 The gas jet forming element shown is used to deliver the element and / or ionization pulse to obtain the plasma method described above.

[0138] Further embodiments are defined in the subsequent numbering:

[0139] 1. A gas delivery system for an irradiation source, comprising:

[0140] A gas delivery element, the gas delivery element being arranged to guide gas in at least a first direction, wherein the gas delivery element comprises:

[0141] Optical input; and

[0142] Optical output,

[0143] The input and the output define an optical path, which is oriented in a second direction.

[0144] The second direction is neither perpendicular to nor parallel to the first direction.

[0145] 2. The gas delivery system according to aspect 1, wherein the first direction forms an obtuse angle with respect to the second direction.

[0146] 3. The gas delivery system according to aspect 1 or 2, wherein the optical input and optical output are arranged to allow pump radiation to pass through the gas to generate high harmonic radiation.

[0147] 4. The gas delivery system according to aspect 3, wherein the optical input and the optical output are arranged concentrically with the optical path.

[0148] 5. The gas delivery system according to aspect 3 or 4, wherein the optical input includes an opening in a first wall of the gas delivery system, and wherein the optical output includes an opening in an opposite wall of the gas delivery system.

[0149] 6. The gas delivery system according to any of the foregoing aspects, wherein at least one of the optical input or the optical output has a cross-section substantially corresponding to the beam cross-section of the pump radiation.

[0150] 7. The gas delivery system according to any of the foregoing aspects further includes at least one pumping element connected to the gas delivery element.

[0151] 8. A gas delivery system according to any of the foregoing aspects, wherein the gas delivery element has a cross-section in the first direction that is one of the following: rectangular, circular, or elliptical.

[0152] 9. The gas delivery system according to any one of aspects 1-7, wherein the gas delivery element includes an annular gas delivery member arranged to deliver gas to the optical path in at least the first direction.

[0153] 10. The gas delivery system according to aspect 9, wherein the annular gas delivery component is arranged to deliver gas to the optical path in a plurality of first directions.

[0154] 11. The gas delivery system according to any of the foregoing aspects further includes a gas buffer element.

[0155] 12. The gas delivery system according to aspect 11, wherein the gas buffer element includes a temperature control element.

[0156] 13. The gas delivery system according to any of the foregoing aspects further includes a filter element operable to modify the flow profile of the gas.

[0157] 14. The gas delivery system according to aspect 13, wherein the filter element is operable to provide a laminar flow profile of the gas.

[0158] 15. The gas delivery system according to any of the foregoing aspects further includes a gas jet shaping device operable to modify the flow profile of the gas such that the number density of the gas decreases sharply in the direction of the optical output after the pump radiation interaction region where the pump radiation interacts with the gas.

[0159] 16. The gas delivery system according to aspect 15, wherein the quantity density of the gas is reduced by at least 10 times relative to the quantity density of the pump-radiation interaction region in a descending region immediately following the pump-radiation interaction region in the direction toward the output, the length of the descending region being 10% or less of the length of the pump-radiation interaction region.

[0160] 17. The gas delivery system according to aspect 15 or 16, wherein the modification of the flow profile is further operable to extend the length of the pump-radiation interaction region relative to the absence of a gas jet forming device.

[0161] 18. The gas delivery system according to aspect 17, wherein the length of the pump-radiation interaction region is extended by more than 50% relative to the case where no gas jet forming device is present.

[0162] 19. The gas delivery system according to any one of aspects 15-18, wherein the gas jet forming apparatus includes an angled wall element located below the gas delivery element and at an obtuse angle relative to the first direction.

[0163] 20. The gas delivery system according to aspect 19, wherein the angled wall element is attached to a point adjacent to the gas output of the gas delivery element, such that the angled wall element extends below the gas output and at an angle toward the emitted gas.

[0164] 21. The gas delivery system according to aspect 20, wherein the gas jet forming element includes an angled horizontal cylindrical segment that opens at its bottom end.

[0165] 22. The gas delivery system according to aspect 21, wherein the gas jet forming element comprises a semi-cylindrical element.

[0166] 23. The gas delivery system according to any one of aspects 19-22, wherein the gas jet forming element is positioned such that the wall of the gas jet forming element is located between the pump-radiation interaction region and the optical output, and the wall of the gas jet forming element is not located between the gas pump-radiation interaction region and the optical input.

[0167] 24. The gas delivery system according to aspect 23, wherein the gas forming element includes an aperture in the wall for transmitting the generated measurement radiation to the optical output.

[0168] 25. A gas delivery system for an irradiation source, comprising:

[0169] A gas delivery element is arranged to guide gas in at least a first direction, wherein the gas delivery element comprises:

[0170] Optical input and optical output together define an optical path, which is oriented in a second direction; and

[0171] A gas jet shaping apparatus is operable to modify the flow profile of the gas such that the number density of the gas decreases sharply in the direction of the optical output after the pump radiation interaction region where the pump radiation interacts with the gas.

[0172] 26. The gas delivery system according to aspect 25, wherein the quantity density of the gas is reduced by at least 10 times relative to the quantity density of the pump-radiation interaction region in a descending region immediately following the pump-radiation interaction region in the direction toward the output, the length of the descending region being 10% or less of the length of the pump-radiation interaction region.

[0173] 27. The gas delivery system according to aspect 25 or 26, wherein the modification of the flow profile is further operable to extend the length of the pump-radiation interaction region relative to the absence of a gas jet forming device.

[0174] 28. The gas delivery system according to aspect 27, wherein the length of the pump-radiation interaction region is extended by more than 50% relative to the case where no gas jet forming device is present.

[0175] 29. The gas delivery system according to any one of aspects 25-28, wherein the gas jet forming apparatus includes an angled wall element located below the gas delivery element and at an obtuse angle relative to the first direction.

[0176] 30. The gas delivery system according to aspect 29, wherein the angled wall element is attached to a point adjacent to the gas output of the gas delivery element, such that the angled wall element extends below the gas output and at an angle toward the emitted gas.

[0177] 31. The gas delivery system according to aspect 30, wherein the gas jet forming element includes an angled horizontal cylindrical segment that opens at its bottom end.

[0178] 32. The gas delivery system according to aspect 31, wherein the gas jet forming element comprises a semi-cylindrical element.

[0179] 33. The gas delivery system according to any one of aspects 29-32, wherein the gas jet forming element is positioned such that the wall of the gas jet forming element is located between the pump-radiation interaction region and the optical output, and the wall of the gas jet forming element is not located between the gas pump-radiation interaction region and the optical input.

[0180] 34. The gas delivery system according to aspect 33, wherein the gas forming element includes an aperture in the wall for transmitting the generated measuring radiation to the optical output.

[0181] 35. An irradiation source for generating high harmonic radiation, comprising:

[0182] Pump radiation source, capable of operating to emit pump radiation; and

[0183] The gas delivery system according to any one of aspects 1-34 or 39-43 is operable to receive emitted pump radiation and generate the high harmonic radiation.

[0184] 36. An inspection apparatus for measuring a target structure on a substrate, comprising:

[0185] The irradiation source according to aspect 35 is used to generate high harmonic radiation; and

[0186] A sensing element for receiving high harmonic radiation scattered by the target structure.

[0187] 37. A photolithography apparatus, comprising: an illumination optical system arranged to illuminate a pattern, and a projection optical system arranged to project an image of the pattern onto a substrate.

[0188] The lithography apparatus thereunder includes an irradiation source according to any one of aspect 35 or aspects 44-55.

[0189] 38. A photolithography system, comprising:

[0190] A photolithography apparatus includes: an illumination optical system arranged to illuminate a pattern, and a projection optical system arranged to project an image of the pattern onto a substrate; and

[0191] The inspection equipment according to aspect 36

[0192] The photolithography apparatus is arranged to use one or more parameters calculated by the inspection apparatus when applying the pattern to another substrate.

[0193] 39. A gas delivery system for an irradiation source, comprising at least a first gas delivery element and a second gas delivery element, the first gas delivery element being operable to emit a first gas and the second gas delivery element being operable to emit a second gas, such that the quantity density profile of the first gas is altered by the second gas.

[0194] 40. The gas delivery system according to aspect 39, wherein the first gas is a high harmonic generation gas medium for generating high harmonic radiation, and the second gas has a lower absorption rate for high harmonic radiation compared to the first gas.

[0195] 41. The gas delivery system according to aspect 39 or 40, wherein the second gas is operable to modify the flow profile of the first gas such that the number density of the gas decreases sharply in the direction of propagation of the pump radiation pulse after the pump radiation interaction region where the pump radiation pulse interacts with the first gas.

[0196] 42. The gas delivery system according to aspect 39, 40 or 41, wherein the first gas delivery element is adjacent to the second gas delivery element.

[0197] 43. The gas delivery system according to aspects 39, 40 or 41, wherein the first gas delivery element and the second gas delivery element are arranged concentrically.

[0198] 44. An irradiation source for generating high harmonic radiation, the irradiation system comprising a gas delivery system according to any one of aspects 39-43 and including a pump radiation source operable to emit pump radiation located at the first gas.

[0199] 45. An irradiation source for generating high harmonic radiation, comprising:

[0200] A pump radiation source, operable to emit pump radiation located at a high harmonic generating gas medium, thereby exciting the high harmonic generating gas medium within a pump radiation interaction region to generate the high harmonic radiation; and

[0201] An ionization radiation source is operable to emit ionization radiation located at the high harmonic generating gas medium to ionize the gas in the ionization region between the pump radiation interaction region of the irradiation source and the optical output.

[0202] 46. ​​The irradiation source according to aspect 45, wherein the ionization region is adjacent to the pump radiation interaction region.

[0203] 47. An irradiation source according to aspect 45 or 46, wherein the irradiation source is arranged such that the ionization radiation and the pump radiation excite the high harmonic generating gas medium substantially simultaneously to ionize the gas at the ionization region.

[0204] 48. The irradiation source according to aspect 45, 46 or 47, wherein the propagation direction of the ionization radiation is opposite to the propagation direction of the pump radiation.

[0205] 49. The irradiation source according to aspect 48, wherein the pump radiation source and the ionization radiation source are located on opposite sides of the high harmonic generation gas medium.

[0206] 50. The irradiation source according to aspect 49 is operable such that at least one of the wavelength, polarization, or propagation angle of the pump radiation is different from at least one of the wavelength, polarization, or propagation angle of the ionization radiation, thereby enabling separation of the returned pump radiation and / or ionization radiation.

[0207] 51. The irradiation source according to aspect 45, 46 or 47, wherein the propagation direction of the ionization radiation is orthogonal to the propagation direction of the pump radiation.

[0208] 52. The irradiation source according to aspect 51, comprising an optical element operable to focus the ionizing radiation onto a line focus at the ionization region.

[0209] 53. The irradiation source according to any one of aspects 45-52, comprising a common oscillator operable to provide both the pump radiation and the ionization radiation.

[0210] 54. The irradiation source according to any one of aspects 45-53, wherein the optical output includes an aperture arranged to allow the pump radiation to pass through and to block the high harmonic radiation, the pump radiation having the property of suffering a greater deflection than the high harmonic radiation due to refraction within the ionizing radiation.

[0211] 55. The irradiation source according to any one of aspects 45-54, comprising a dose control element that controls the timing of the ionizing radiation relative to the pump radiation and / or the intensity of the ionizing radiation, thereby controlling the absorption characteristics within the ionization region.

[0212] If the term "measuring device" is used herein, those skilled in the art will also read the term "detection device" in the same location, and vice versa. In the context of this document, the device can be used to determine a characteristic of interest of a structure on a substrate. The characteristic of interest can be a measured value or a deviation from an expected pattern, such as the absence of a structure, the presence of an unexpected structure, or a change in the expected pattern.

[0213] While this document provides detailed reference to the use of lithography equipment in the manufacture of ICs (integrated circuits), it should be understood that the lithography equipment described herein can have other applications, such as the manufacture of integrated optical systems, the guidance and detection of patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in these alternative applications, any terms “wafer” or “die” used herein can be considered synonymous with the more general terms “substrate” or “target portion,” respectively. Substrates referenced herein can be processed before or after exposure, for example in coating and developing systems (tools that typically apply a layer of resist to a substrate and develop the exposed resist), measurement tools, and / or inspection tools. Where appropriate, the disclosure herein can be applied to these and other substrate processing tools. Furthermore, the substrate can be processed more than once, for example to produce multilayer ICs, such that the term “substrate” used herein can also refer to a substrate that has contained multiple processed layers.

[0214] Although detailed reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be understood that the invention can be used in other applications (e.g., imprint lithography) and is not limited to optical lithography where permitted. In imprint lithography, the morphology in a patterning apparatus defines a pattern produced on a substrate. The morphology of the patterning apparatus can be pressed into a resist layer supplied to the substrate, where the resist is cured by applying electromagnetic radiation, heat, pressure, or a combination thereof. After the resist has cured, the patterning apparatus is removed from the resist, thereby leaving a pattern therein.

[0215] The terms “radiation” and “beam” as used herein encompass all types of electromagnetic radiation, including: ultraviolet (UV) radiation (e.g., having wavelengths of 365, 355, 248, 193, 157, or 126 nm or approximately) and extreme ultraviolet (EUV) radiation (e.g., having wavelengths in the range of 5–20 nm), as well as particle beams, such as ion beams or electron beams.

[0216] Where permitted, the term "lens" may refer to any one or a combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components.

[0217] While specific embodiments of the invention have been described above, it will be understood that the invention may be practiced in ways other than those described. For example, the invention may take the form of a computer program comprising one or more sequences of machine-readable instructions describing the methods disclosed above; or a data storage medium (e.g., semiconductor memory, magnetic disk, or optical disk) in which such a computer program is stored.

[0218] The above description is intended to be illustrative and not restrictive. Therefore, it will be understood by those skilled in the art that modifications can be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A high harmonic generation source, comprising: - A gas delivery system for supplying gas. The high harmonic generation source is configured to receive pump radiation having a propagation direction to provide the pump radiation in the gas at a pump radiation interaction region for generating high harmonic generation radiation, and to shape the density of the gas along the propagation direction such that, during operation, the density of the gas decreases sharply in a descending region immediately following the pump radiation interaction region.

2. The high harmonic generation source according to claim 1, wherein, The density of the gas decreases by at least 10 times in the descent region relative to the density in the pump-radiation interaction region along the propagation direction.

3. The high harmonic generation source according to claim 1 or 2, wherein, The length of the descending region along the propagation direction is at most 10% of the length of the pump-radiation interaction region along the propagation direction.

4. The high harmonic generation source according to claim 1 or 2, wherein, The density of the gas is further shaped to be operable to extend the length of the pump-radiation interaction region relative to the case where no gas shaping occurs.

5. The high harmonic generation source according to claim 4, wherein, The length of the pump-radiation interaction region along the propagation direction is extended by more than 50% relative to the case where no gas shaping occurs.

6. The high harmonic generation source according to claim 1 or 2, wherein, The high harmonic generation radiation is in the soft X-ray and / or extreme ultraviolet wavelength region.

7. The high harmonic generation source according to claim 1 or 2, wherein, The pump radiation is in the infrared wavelength region.

8. The high harmonic generation source according to claim 1 or 2, wherein, The pump radiation is pulsed.

9. The high harmonic generation source according to claim 1 or 2, comprising a gas jet shaping device for shaping the density of the gas along the propagation direction such that, during operation, the density of the gas decreases sharply in the descent region immediately following the pump-radiation interaction region.

10. The high harmonic generation source according to claim 1 or 2, wherein, The density of the gas after the descent region is low enough that it does not absorb a significant amount of the high harmonics that generate radiation.

11. A high harmonic generation source, comprising: - A gas delivery system for supplying gas. The high harmonic generation source is configured to receive pump radiation having a propagation direction to provide the pump radiation in the gas at a pump radiation interaction region for generating high harmonic generation radiation, and to shape the density of the gas along the propagation direction such that the amount of absorption of the high harmonic generation radiation by the gas is reduced during operation.

12. A measuring device comprising a high harmonic generation source according to any one of claims 1 to 11.

13. A photolithography unit comprising a high harmonic generation source according to any one of claims 1 to 11, or comprising a measurement device according to claim 12.

14. A method for delivering a gas in an irradiation source, comprising: A gas is provided for receiving pump radiation having a propagation direction and for generating emitted radiation at the pump radiation interaction region, and The density of the gas is shaped along the propagation direction so that the density of the gas decreases sharply in the descending region immediately following the pump-radiation interaction region.

15. A method for delivering gas in an irradiation source, comprising: A gas is provided for receiving pump radiation having a propagation direction and for generating emitted radiation at the pump radiation interaction region, and The density of the gas is shaped along the propagation direction to reduce the amount of radiation absorbed by the gas from the emitted radiation.

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