RADIATION SOURCE FOR METROLOGY

Abstract

Metrology-related radiation source comprising: (a) a gas supply unit comprising an outer nozzle configured to emit a limiting gas cloud and an inner nozzle surrounded by the outer nozzle and configured to emit a gas puff target in the form of an elongated gas jet propagating through a hollow core of the limiting gas cloud; (b) a laser source configured to generate a pair of laser pulses comprising (i) a first laser pulse to convert the gas puff target into plasma and (ii) a second laser pulse to heat the plasma and generate metrology-related radiation; and (c) a skimmer nozzle configured to remove residual gas remaining after the generation of metrology-related radiation.

Description

CROSS-REFERENCE This application claims priority over the preliminary US patent application with serial number 63 / 588,997, filed on October 9, 2023, which is hereby incorporated by reference in its entirety. BACKGROUND OF THE INVENTION Extreme ultraviolet (EUV) radiation has a wavelength in the range of 10 to 121 nanometers. Soft X-rays (SXR) have a wavelength in the range of 20 to 200 angstroms. As semiconductor nodes have now reached the nanometer scale, numerous efforts have been made in recent years to achieve imaging with nanometer-precise spatial resolution in the EUV and SXR spectral ranges using both large-format and compact sources. However, these sources feature large, complex hardware requiring specialized personnel to operate, or they are not robust enough for use in metrology tools. To make SXR metrology, ptychography, and microscopy practical, it is necessary to develop a compact, high-brightness SXR source that can be integrated into semiconductor fabrication. These new SXR sources must be robust, compact, and easy to operate so that they can become part of a metrology tool, rather than being the central component of its function.Furthermore, they must have sufficiently high brightness to ensure short exposure times and adequate analysis throughput. SUMMARY A metrology-related radiation source and procedure will be provided, as described in the description and / or drawings. According to one embodiment, a metrology-related radiation source is provided comprising: (a) a gas supply unit comprising an outer nozzle configured to emit a limiting gas cloud and an inner nozzle surrounded by the outer nozzle and configured to emit a gas puff target in the form of an elongated gas jet propagating through a hollow core of the limiting gas cloud; (b) a laser source configured to generate a pair of laser pulses comprising (i) a first laser pulse to convert the gas puff target into plasma and (ii) a second laser pulse to heat the plasma and generate metrology-related radiation; and (c) a skimmer nozzle configured to remove residual gas remaining after the generation of metrology-related radiation. According to one embodiment, the limiting gas cloud is formed from a first gas, and the gas puff target is formed from a second gas that has a higher Z-factor than the first gas. According to one embodiment, the second gas is a sulfur hexafluoride (SF6). According to one embodiment, the second gas is selected from nitrogen (N2) and carbon dioxide (CO2). According to one embodiment, the second gas is methane (CH4). According to one embodiment, the gas puff target generator is configured to release the first gas before the second gas is released. According to one embodiment, the second gas is a mixture of at least two gases consisting of sulfur hexafluoride (SF6), nitrogen (N2), carbon dioxide (CO2), methane (CH4), xenon (Xe), argon (Ar) or krypton (Kr). According to one embodiment, the second gas is a mixture of (i) at least one gas consisting of sulfur hexafluoride (SF6), nitrogen (N2), carbon dioxide (CO2) or methane (CH4) and (ii) at least one gas consisting of xenon (Xe), argon (Ar) or krypton (Kr). According to one embodiment, the skimmer nozzle is aligned with the inner nozzle. According to one embodiment, the inner nozzle is circular, the outer nozzle is annular, and the inner and outer nozzles are coaxial. According to one embodiment, the metrology-related radiation source is configured to operate in iterations, wherein during each iteration: (a) the gas supply unit is configured to discharge the limiting gas cloud through the outer nozzle; (b) the gas supply unit is configured to discharge the elongated gas jet through the inner nozzle; and (c) the laser source is configured to generate the pair of laser pulses. According to one embodiment, during each iteration the gas supply unit is configured to begin releasing the limiting gas cloud before the elongated gas jet is released. According to one embodiment, during each iteration the laser source is configured to start generating the first laser pulse before starting to generate the second laser pulse. According to one embodiment, the delay between the start of the generation of the first laser pulse and the start of the generation of the second laser pulse is more than one microsecond. According to one embodiment, the first laser pulse has an optical axis that is oriented relative to an optical axis of the second laser pulse. According to one embodiment, the laser source is configured to direct the first and second laser pulses to a position within the hollow core of the confining gas cloud, each spaced a safe distance from the outer and inner nozzles, thereby reducing exposure of the outer and inner nozzles to ions and neutral particles emanating from the plasma. According to one embodiment, the safety distance exceeds one millimeter. According to one embodiment, the first laser pulse has a wavelength between two hundred and ten nanometers and one thousand two hundred nanometers. According to one embodiment, the second laser pulse has a wavelength between two thousand nanometers and two thousand five hundred nanometers. According to one embodiment, a method for generating metrology-related radiation is provided, comprising: a. Delivering a limiting gas cloud through an outer nozzle of a gas supply unit. b. Delivering a gas puff target in the form of an elongated gas jet propagating through a hollow core of the limiting gas cloud through an inner nozzle of the gas supply unit surrounded by the outer nozzle. c. Generating a pair of laser pulses by a laser source, comprising: (i) a first laser pulse to convert the gas puff target into plasma and (ii) a second laser pulse to heat the plasma and generate metrology-related radiation. d. Delivering the metrology-related radiation. e. Discharge any residual gas remaining after the generation of metrology-related radiation through a skimmer nozzle. According to one embodiment, the method comprises forming the gas cloud from a first gas and forming the gas puff target from a second gas that has a higher Z-factor than the first gas. According to one embodiment, the second gas is a sulfur hexafluoride (SF6). According to one embodiment, the second gas is selected from nitrogen (N2) and carbon dioxide (CO2). According to one embodiment, the second gas is methane (CH4). According to one embodiment, the method includes dispensing the first gas before dispensing the second gas. According to one embodiment, the second gas is a mixture of at least two gases consisting of sulfur hexafluoride (SF6), nitrogen (N2), carbon dioxide (CO2), methane (CH4), xenon (Xe), argon (Ar) or krypton (Kr). According to one embodiment, the second gas is a mixture of (i) at least one gas consisting of sulfur hexafluoride (SF6), nitrogen (N2), carbon dioxide (CO2) or methane (CH4) and (ii) at least one gas consisting of xenon (Xe), argon (Ar) or krypton (Kr). According to one embodiment, the skimmer nozzle is aligned with the inner nozzle. According to one embodiment, the inner nozzle is circular, the outer nozzle is annular, and the inner and outer nozzles are coaxial. According to one embodiment, the method includes performing several iterations of steps (a), (b), (c) and (d). According to one embodiment, during each iteration: a. The gas supply unit is configured to emit the limiting gas cloud through the outer nozzle. b. The gas supply unit is configured to emit the elongated gas jet through the inner nozzle. c. The laser source is configured to generate the pair of laser pulses. According to one embodiment, during each iteration the release of the limiting gas cloud begins before the release of the elongated gas jet begins. According to one embodiment, during each iteration the generation of the first laser pulse begins before the generation of the second laser pulse begins. According to one embodiment, the delay between the start of the generation of the first laser pulse and the start of the generation of the second laser pulse is more than one microsecond. According to one embodiment, the first laser pulse has an optical axis that is oriented relative to an optical axis of the second laser pulse. According to one embodiment, the first and second laser pulses are directed to a position within the hollow core of the limiting gas cloud, each spaced a safe distance from the outer and inner nozzles, thereby reducing the exposure of the outer and inner nozzles to ions and neutral particles emanating from the plasma. According to one embodiment, the safety distance exceeds one millimeter. According to one embodiment, the first laser pulse has a wavelength between two hundred and ten nanometers and one thousand two hundred nanometers. According to one embodiment, the second laser pulse has a wavelength between two thousand nanometers and two thousand five hundred nanometers. BRIEF DESCRIPTION OF THE DRAWINGS The subject matter considered to be the invention is specifically highlighted and clearly claimed in the concluding part of the description. However, the invention, both in terms of its structure and its operation, together with its objectives, features, and advantages, is best understood by reference to the following detailed description, when read in conjunction with the accompanying drawings, in which: Fig. 1 shows examples of confining gas clouds and gas puff targets; Fig. 2 shows an example of a metrology-related radiation source; Fig. 3 shows an example of a section of the metrology-related radiation source from Fig. 2; Fig. 4 shows an example of a section of the metrology-related radiation source from Fig. 2; and Fig. 5 shows an example of a method. DETAILED DESCRIPTION OF THE DRAWINGS Any reference to a hollow core (of the limiting gas cloud) shall accordingly be applied to an interior space and / or an interior channel and / or an interior stream that is formed in the limiting gas cloud. Any reference to a limiting gas cloud is to be applied accordingly to a limiting gas volume of a different shape surrounding the gas puff target. X-rays can be generated using an X-ray tube in which high-energy electrons strike a solid anode, producing bremsstrahlung and characteristic emission. This method is limited by the amount of power that can be delivered to the target, of which less than 1% is converted into SXR radiation. One of the most efficient methods for generating SXR is to use high-intensity pulsed lasers (~1E+10 - 1E+14 W / cm2) that interact with matter to create a highly ionized plasma that produces a broadband SXR spectrum. One challenge with laser-generated plasma (LPP) source systems is the generation of large quantities of residues (ions and particles) that need to be handled. Another important challenge in the development of high-brightness soft X-ray sources is that achieving high resolution (2–50 nm) requires a coherent beam (a beam in which photons oscillate at the same frequency and wavelengths are in phase) of synchrotron radiation or line emission originating from highly ordered harmonics, plasma soft X-ray lasers, and incoherent emission in laser-generated low-Z plasmas. These light sources can only be found in large facilities such as synchrotrons in national laboratories. LPP sources, which emit shortwave radiation in the spectral range of soft X-rays (SXR) and extreme ultraviolet (EUV), offer an important alternative as sources for compact imaging systems, overcoming the limited accessibility of large facilities and maintaining comparable spatial resolution. Plasma generated from a laser-irradiated gas puff target, created by pulsed gas injection via a solenoid valve, offers the possibility of developing a residue-free, laser-generated X-ray source. Studies of soft X-ray emission from the gas puff target irradiated with nanosecond laser pulses from an Nd:YAG laser have demonstrated high X-ray conversion efficiency. Indeed, a high conversion efficiency of laser light to 1-keV X-rays can be beneficial for proximity-printing X-ray lithography. However, achieving such high X-ray generation requires the laser focus to be very close to the nozzle exit, leading to degradation of the nozzle by the plasma. To prevent nozzle degradation and optimize target characteristics (higher gas density at greater distances from the nozzle outlet), we use a dual-nozzle arrangement to create a gas puff target. This target is formed by pulsed injection of a small amount of gas into a gas cloud surrounding the nozzle outlet. Due to the confinement of the gas flow through the nozzle into the gas cloud, the gas should form an elongated, high-density gas jet, creating a gas puff target. The dual-nozzle arrangement comprises two coaxial nozzles. The outer nozzle has the form of an annular opening and creates a hollow cylinder of low-Z gas (helium). In one embodiment, the inner nozzle is a circular opening through which high-Z gas (xenon, argon, krypton, SF6, CO2, N2, CH4, and mixtures of these gases) is injected into the outer gas stream. Each nozzle is supplied via a separate valve, which is powered by a separate power supply and triggered at a suitable time relative to the laser pulse irradiating the gas puff target. According to one embodiment, the target is created by injecting gas at high pressure from a nozzle into a gas cloud surrounding the nozzle outlet. The surrounding gas confines the gas flowing through the nozzle and prevents spherical expansion of the gas outside the nozzle, as occurs in a conventional gas puff target created by injecting gas into a vacuum. This arrangement forms an elongated gas jet with high core density, even at a relatively large distance from the nozzle outlet, as shown in Fig. 1. The used gas is collected with a skimmer nozzle, which leads to a discharge device such as a Roots blower. Fig. 2 presents an example of a metrology-related radiation source 10 comprising: a. A gas supply unit 20 comprising an outer nozzle (N1 21) configured to emit a limiting gas cloud 12, and an inner nozzle (N2 22) surrounded by the outer nozzle and configured to emit a gas puff target 11 in the form of an elongated gas jet propagating through a hollow core of the limiting gas cloud. In Fig. 2, the gas puff target 11 is shown to not reach the skimmer nozzle 31; however, this is merely an example, as in one embodiment the gas puff target 11 does reach the skimmer nozzle 31.Figure 2 shows the gas puff target 11 with a curved cross-section—even a cross-section in the shape of a truncated ellipsoid—but this is not necessarily the case, and the gas puff target 11 may have other cross-sections and / or shapes—for example, it may gradually expand as it propagates away from the gas supply unit. The gas supply unit also includes a first valve V1 23 for controlling a supply of the first gas to the first nozzle, a second valve V2 24 for controlling the supply of the second gas to the second nozzle, a first gas supply unit (SP1) 25 for supplying the first gas to the first valve, and a second gas supply unit (SP2) 26 for supplying the second gas to the second valve.A laser source 40 configured to generate a pair of laser pulses comprising (i) a first laser pulse 51 to convert the gas puff target into plasma and (ii) a second laser pulse 52 to heat the plasma and generate metrology-related radiation; c. A skimmer nozzle 31 configured to discharge the residual gas after the generation of metrology-related radiation. According to one embodiment, the residual gas also includes confining gas from the confining gas cloud that reaches the skimmer nozzle. According to one embodiment, the limiting gas cloud is formed by a first gas (labeled 18 in Fig. 2), and the gas puff target is formed from a second gas (labeled 19 in Fig. 2) which has a higher Z-factor than the first gas. Fig. 1 also depicts the metrology-related radiation source 10, comprising: a. A controller 70 for controlling the operation of the metrology-related radiation source—for example, for determining the timing of laser pulse emission and / or the timing of valve opening. b. A vacuum system 62. c. A chamber 63 for keeping most or all components of the metrology-related radiation source within a controlled vacuum level (using the vacuum system 62) and reducing contamination. d. A window 61 (such as a beryllium window) for emitting the metrology-related radiation 53 from the chamber. Fig. 2 also shows the laser source 40 comprising a first laser source unit (LSU1) 41 for generating the first laser pulse 51 and a second laser source unit (LSU2) 42 for generating the second laser pulse 52. Both Fig. 3 and Fig. 4 provide an example of the nozzles and the first and second laser beams and represent the point 17 where the laser pulses interact with the gas puff target. Fig. 4 differs from Fig. 3 by including a dielectric mirror 81, which is wavelength-selective, i.e., transmits the second laser pulse 52 and reflects the first laser pulse 51. According to one embodiment, and in order to achieve a high photon flux (>1E+12 photons / (sr*line*pulse)), it is advantageous to control the spectral behavior by reducing self-absorption and increasing spectral purity in the spectral range of interest. According to one embodiment, this is achieved by using two laser pulses to obtain the optimal spectral structure and the most efficient energy coupling. According to one embodiment, the pulsed laser or main laser beam (i.e., Nd:YAG, diode) with a wavelength range of 210-1200 nm, a pulse energy range of 0.2-0.8 J, a pulse duration in the femtosecond to nanosecond range, a frequency of 10 Hz to 50 kHz, a spot size between 0.2-20 mm and a maximum intensity range between 1E+10 and 1E+14 W / cm2 and higher is used to irradiate the gas puff targets. According to one embodiment, the laser pulse is focused onto the vacuum chamber (~20–500 mm), enabling the use of a dual-stream gas puff target source as described above to increase the gas density along the emission direction. The system consists of two circularly concentric nozzles. The inner nozzle is circular, has a diameter of 0.2–0.6 mm, and injects a small amount of gas with a high Z-value (xenon, argon, krypton, SF6, CO2, N2, CH4, and mixtures of these gases) into the vacuum. The concentric outer annular nozzle, with a diameter of 0.4–2 mm, injects a gas with a low Z-value (helium) to reduce the gradient of the working gas density along the normal to the nozzle axis.One of the main advantages of using this compact source is the ability to generate thousands of pulses daily without residue formation, thus avoiding laser absorption by clusters as occurs with solid targets, and achieving a very high plasma density (~1E+17 - 1E+18 atoms / cm3). Efficient generation of SXR / EUV radiation requires precise timing synchronization of the valves between the formation of the gas puff target and the arrival of the laser beam. Valve control is synchronized with a synchronization output signal from the pump laser, and for a system operating at a repetition rate of, for example, 10 Hz, the cycle duration is 100 ms. The external gas pressure (for the low Z-value gas) is set to 5–7 bar, while the working gas pressure is in the range of 1–12 bar to achieve the highest possible gas flow. According to one embodiment, the use of two laser pulses represents an efficient way to control the plasma dynamics. This approach allows the energy of the main laser pulse to be effectively used to heat a pre-generated plasma, which would otherwise be wasted ionizing the target material. Selecting a suitable delay between the two laser pulses could effectively control the electron density of the plasma and the scale of its density gradient with which the main laser pulse interacts. Because the main laser pulse interacts with a pre-plasma whose density is lower than that of the target, the electron temperature is lower compared to that of a single laser pulse. According to one embodiment, a dual-laser system is provided that uses a low-energy laser pre-pulse and a high-energy laser main pulse. The main pulse is the previously defined pulsed laser. Similar to the main pulse beam, the pre-pulse beam is generated by pulsed lasers (i.e., Nd:YAG, diode) with a wavelength range of 210–1200 nm (optionally, lasers with 2000–5000 nm can be added), a pulse energy range of a few mJ (0.2–0.8 mJ), a pulse duration in the femtosecond to nanosecond range, a frequency of 10 Hz to 50 kHz, a spot size between 0.2 and 20 mm, and a maximum intensity between 1E+10 and 1E+14 W / cm² and higher. According to one embodiment, the pre-pulse (also referred to as the first laser pulse) could also utilize the fundamental wave remaining after the extraction of the main pulse (also referred to as the second laser pulse). The low-energy pre-pulse reaches the target range 0.2–500 ns before the main pulse to achieve maximum output intensity. Using different wavelengths for the pre- and main pulses facilitates beam superposition using wavelength-selective dielectric mirrors. Furthermore, avoiding the fundamental IR laser wavelength eliminates the risk of laser damage from back-reflected light, e.g., from the plasma. After the variable delay of the pre-impulse beam, the two beams are spatially superimposed on a quartz glass substrate using a dielectric mirror (labeled 81 in Fig. 4). The dispersion in the 50–100 mm focusing lens could be compensated for by a lens with a long focal length (500–1000 mm) inserted into the pre-impulse beam path, thus allowing the two focal points to be precisely superimposed on the target. The entire source would be housed in a separate chamber, pumped by a turbomolecular pump (TMP), and the output of the SXR source would be routed to the outside through a beryllium window. This keeps the source isolated from the tool for contamination prevention and maintenance purposes. According to one embodiment, a double nozzle arrangement is provided to generate a dual-stream gas puff target. The double nozzle has two circularly concentric nozzles. The inner nozzle is circular, has a diameter of 0.2–0.6 mm, and injects a small amount of gas with a high z-value (xenon, argon, krypton, SF6, CO2, N2, CH4, and mixtures of these gases) into the vacuum. The concentric outer annular nozzle has a diameter of 0.4–2 mm and injects a gas with a low z-value (helium). According to one embodiment, a suitable delay between the two circularly concentric nozzles (0 - 400 nanoseconds) is selected to generate an elongated gas jet with a high gas density area even at a relatively large distance (for example, up to two millimeters) from the outlet of the double nozzle. According to one embodiment, the two laser pulses achieve the optimal spectral structure and the most efficient energy coupling, and effective use of the energy of the main laser pulse to heat a pre-plasma. According to one embodiment, a pre-pulse beam is provided that is similar to the pulsed main laser but has a lower energy. A pulsed laser (i.e., Nd:YAG diode) with a wavelength range of 210–1200 nm, a pulse energy range of a few mJ (0.2–0.8 mJ), a pulse duration in the femtosecond to nanosecond range, frequencies from 10 Hz to 50 kHz, a spot size between 0.2 and 20 mm, and a maximum intensity range between 1E+10 and 1E+14 W / cm² and higher is used. Examples of options for the main pulse laser include: a. UV lasers (190–550 nm or specifically 254, 266, or 355 nm) and IR lasers in the range of 900 nm–1.2 µm or infrared (IR) lasers in the range of 2000 nm–5 µm. b. For example (but not limited to): Nd:YAG diodes with a wavelength range of 210–1200 nm, pulse duration in the femtosecond to nanosecond range, frequency range of 10 Hz to 50 kHz, intensity range of 1E+14 W / cm² and higher are used to irradiate the gas puff target and are focused onto the vacuum chamber (~20–500). c. UV lasers and Vis lasers 190–550 nm. d. Fiber laser 2.4-5 µm, According to one embodiment, the spent gas is collected with a skimmer nozzle leading to an evacuation unit such as, but not limited to, a Roots blower. This makes it possible to generate thousands of pulses per day without residue formation, thus avoiding laser absorption by clusters, as occurs with solid targets. According to one embodiment, a suitable delay between the two laser pulses (main and pre-pulses with a delay range of 0.2-500 ns) is selected to control the plasma density (~1E+17 - 1E+18 atoms / cm3). According to one embodiment, a small X-ray source with a spot size of up to a few micrometers is provided - for example, with a spot diameter between two and five micrometers. According to one embodiment, sources for compact SRX imaging are provided for inspection and metrology systems. Fig. 5 shows an example of method 500 for generating metrology-related radiation. According to one embodiment, method 500 comprises steps 510, 520, 530 and 540. According to one embodiment, step 510 includes the release of a limiting gas cloud through an external nozzle of a gas supply unit. According to one embodiment, step 520 comprises delivering a gas puff target in the form of an elongated gas jet, propagating through a hollow core of the bounding gas cloud, through an inner nozzle of the gas supply unit, which is surrounded by the outer nozzle. According to one embodiment, the inner nozzle is circular, the outer nozzle is annular, and the inner and outer nozzles are coaxial. According to one embodiment, step 510 comprises forming the gas cloud from a first gas, and step 520 comprises forming the gas puff target from a second gas that has a higher Z-factor than the first gas. According to one embodiment, the second gas is sulfur hexafluoride (SF6) or nitrogen (N2) or carbon dioxide (CO2) or methane (CH4) or xenon (Xe) or argon (Ar) or krypton (Kr) - or a mixture of two or more of the gases mentioned. According to one embodiment, the second gas is a combination of (a) at least one gas consisting of sulfur hexafluoride (SF6), nitrogen (N2), carbon dioxide (CO2) or methane (CH4) and (b) at least one gas consisting of xenon (Xe), argon (Ar) or krypton (Kr). According to one embodiment, step 530 comprises generating a pair of laser pulses by a laser source, comprising (i) a first laser pulse to convert the gas puff target into plasma and (ii) a second laser pulse to heat the plasma and generate metrology-related radiation. According to one embodiment, step 530 comprises step 531 of generating the first laser pulse and step 532 of generating the second laser pulse. According to one embodiment, step 530 is followed by step 535 of emitting the metrology-related radiation. According to one embodiment, step 540 comprises the removal of residual gas remaining after the generation of metrology-related radiation through a skimmer nozzle. According to one embodiment, the skimmer nozzle is aligned with the inner nozzle. According to one embodiment, steps 510 and 520 begin simultaneously. According to one embodiment, step 510 begins before the start of step 520. According to one embodiment, step 530 begins after the start of steps 510 and 520. According to another embodiment, step 530 overlaps at least partially with step 520. According to one embodiment, step 540 overlaps at least partially with steps 510-530. For example, evacuation can begin before the start of step 510 and continue while either step 520 or step 530 continues. According to one embodiment, the method 500 is carried out multiple times - during several iterations - and steps 510, 520 and 530 are repeated to generate multiple plasma pulses that result in the emission of pulses of metrology-related radiation. According to one embodiment, step 540 can be repeated multiple times, but can also be performed between one iteration and the next. According to one embodiment, during each iteration: a. The gas supply unit is configured to discharge the limiting gas cloud through the outer nozzle. b. The gas supply unit is configured to discharge the elongated gas jet through the inner nozzle. c. The laser source is configured to generate the pair of laser pulses. According to one embodiment, during each iteration the release of the limiting gas cloud begins before the release of the elongated gas jet begins. According to one embodiment, during each iteration the generation of the first laser pulse begins before the generation of the second laser pulse begins. According to one embodiment, the delay between the start of the generation of the first laser pulse and the start of the generation of the second laser pulse is more than one microsecond. According to one embodiment, the first laser pulse has an optical axis that is oriented relative to an optical axis of the second laser pulse. According to one embodiment, the first laser pulse has an optical axis that is parallel and optionally coaxial to an optical axis of the second laser pulse. According to one embodiment, the first and second laser pulses are directed to a position within the hollow core of the limiting gas cloud, which is spaced a safe distance from the outer and inner nozzles, respectively, thereby reducing the exposure of the outer and inner nozzles to ions and neutral particles emanating from the plasma. According to one embodiment, the safety distance exceeds one millimeter. According to one embodiment, the first laser pulse has a wavelength between two hundred and ten nanometers and one thousand two hundred nanometers. According to one embodiment, the second laser pulse has a wavelength between two thousand nanometers and two thousand five hundred nanometers. The preceding detailed description presents numerous specific details to provide a comprehensive understanding of the invention. However, it will be clear to those skilled in the art that the present invention can also be implemented without these specific details. In other instances, known methods, processes, and components have not been described in detail in order to avoid obscuring the present invention. The subject matter considered to be the invention is specifically highlighted and clearly claimed in the concluding part of the description. However, the invention, both in terms of its structure and its function, together with its objectives, features, and advantages, is best understood by reference to the following detailed description in conjunction with the accompanying drawings. It is understood that, for the sake of simplicity and clarity, the elements depicted in the figures are not necessarily drawn to scale. For example, the dimensions of some elements may be exaggerated compared to others for better clarity. Furthermore, where deemed appropriate, reference symbols may be repeated in the figures to indicate corresponding or analogous elements. Since the embodiments of the present invention are largely realized using electronic components and circuits known to those skilled in the art, details are not explained beyond what is deemed necessary, as explained above, for understanding and appreciating the underlying concepts of the present invention, and in order not to obscure or distract from the teachings of the present invention. Any reference in the description to a procedure shall be applied accordingly to a system capable of executing the procedure. Each reference in the description to a system shall be applied accordingly to a procedure that can be executed by the system. In the foregoing description, the invention has been described with reference to specific embodiments of the invention. However, it will be obvious that various modifications and changes can be made to it without deviating from the broader meaning and scope of the invention as set out in the appended claims. Furthermore, the terms “front”, “back”, “top”, “bottom”, “above”, “below”, and the like are used in the description and, where applicable, in the claims for descriptive purposes and not necessarily to describe permanent relative positions. It is understood that the terms used in this way are interchangeable under suitable circumstances, so that the embodiments of the invention described herein may, for example, be operated in orientations other than those shown herein or otherwise described. Any arrangement of components that achieves the same functionality is effectively "connected" to achieve the desired functionality. Therefore, any two components combined here to achieve a specific functionality can be considered "connected" to each other, thus achieving the desired functionality, regardless of architectures or intervening components. Likewise, two such connected components can also be considered "functionally connected" or "functionally coupled" to achieve the desired functionality. Furthermore, it will be clear to those skilled in the art that the boundaries between the process steps described above are merely exemplary. The several process steps can be combined into a single process step, a single process step can be distributed across further process steps, and process steps can be executed with at least partial temporal overlap. Moreover, alternative embodiments can include multiple instances of a particular process step, and the sequence of the process steps can be changed in various other embodiments. However, other modifications, variations, and alternatives are also possible. The descriptions and drawings should therefore be considered illustrative rather than restrictive. Reference numerals in parentheses in claims are not to be interpreted as limiting the claim. The word "comprising" does not preclude the presence of elements or steps other than those listed in a claim. Furthermore, the terms "a" or "an," as used here, are defined as one or more than one. Likewise, the use of introductory phrases such as "at least one" and "one or more" in claims should not be interpreted as limiting a particular claim containing such an introduced feature to inventions containing only one such feature, even if the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an." The same applies to the use of definite articles.Unless otherwise stated, terms such as "first" and "second" are used arbitrarily to distinguish between the elements they describe. Thus, these terms are not necessarily intended to indicate any temporal or other prioritization of such elements. The mere fact that certain measures are listed in independent claims does not preclude the possibility of advantageously combining these measures. Although certain features of the invention have been presented and described here, the person skilled in the art will now think of many modifications, substitutions, changes, and equivalents. It is therefore understood that the appended claims are intended to cover all such modifications and changes that are within the true meaning of the invention. QUOTES INCLUDED IN THE DESCRIPTION This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature US 63 / 588,997

[0001]

Claims

Metrology-related radiation source, comprising: a gas supply unit, comprising: an outer nozzle configured to emit a limiting gas cloud, and an inner nozzle surrounded by the outer nozzle and configured to emit a gas puff target in the form of an elongated gas jet propagating through a hollow core of the limiting gas cloud; a laser source configured to generate a pair of laser pulses, comprising (i) a first laser pulse to convert the gas puff target into plasma, and (ii) a second laser pulse to heat the plasma and generate metrology-related radiation; and a skimmer nozzle configured to remove residual gas remaining after the generation of metrology-related radiation. Metrology-related radiation source according to claim 1, wherein the limiting gas cloud is formed from a first gas and the gas puff target is formed from a second gas having a higher Z-factor than the first gas. Metrology-related radiation source according to claim 2, wherein the second gas is sulfur hexafluoride (SF6). Metrology-related radiation source according to claim 2, wherein the second gas is selected from nitrogen (N2) and carbon dioxide (CO2). Metrology-related radiation source according to claim 2, wherein the second gas is methane (CH4). Metrology-related radiation source according to claim 2, wherein the gas puff target generator is configured to release the first gas before the second gas is released. Metrology-related radiation source according to claim 2, wherein the second gas is a mixture of at least two gases of sulfur hexafluoride (SF6), nitrogen (N2), carbon dioxide (CO2), methane (CH4), xenon (Xe), argon (Ar) or krypton (Kr). Metrology-related radiation source according to claim 2, wherein the second gas is a mixture of (i) at least one gas of sulfur hexafluoride (SF6), nitrogen (N2), carbon dioxide (CO2) or methane (CH4) and (ii) at least one gas of xenon (Xe), argon (Ar) or krypton (Kr). Metrology-related radiation source according to claim 1, wherein the skimmer nozzle is aligned with the inner nozzle. Metrology-related radiation source according to claim 1, wherein the inner nozzle is circular, the outer nozzle is annular, and the inner and outer nozzles are coaxial. Metrology-related radiation source according to claim 1, which is configured to operate in iterations, wherein during each iteration: the gas supply unit is configured to emit the limiting gas cloud through the outer nozzle; the gas supply unit is configured to emit the elongated gas jet through the inner nozzle; and the laser source is configured to generate the pair of laser pulses. Metrology-related radiation source according to claim 11, wherein during each iteration the gas supply unit is configured to begin emitting the limiting gas cloud before the emission of the elongated gas jet begins. Metrology-related radiation source according to claim 11, wherein during each iteration the laser source is configured to start generating the first laser pulse before starting generating the second laser pulse. Metrology-related radiation source according to claim 13, wherein the delay between the start of the generation of the first laser pulse and the start of the generation of the second laser pulse is more than one microsecond. Metrology-related radiation source according to claim 1, wherein the first laser pulse has an optical axis oriented relative to an optical axis of the second laser pulse. Metrology-related radiation source according to claim 1, wherein the laser source is configured to direct the first and second laser pulses to a position within the hollow core of the limiting gas cloud, each spaced a safe distance from the outer nozzle and the inner nozzle, thereby reducing exposure of the outer nozzle and the inner nozzle to ions and neutral particles emanating from the plasma. Metrology-related radiation source according to claim 16, wherein the safety distance exceeds one millimeter. Metrology-related radiation source according to claim 1, wherein the first laser pulse has a wavelength between two hundred and ten nanometers and one thousand two hundred nanometers. Metrology-related radiation source according to claim 1, wherein the second laser pulse has a wavelength between two thousand nanometers and two thousand five hundred nanometers. A method for generating metrology-related radiation, comprising: dispensing a limiting gas cloud through an outer nozzle of a gas supply unit; dispensing a gas puff target in the form of an elongated gas jet propagating through a hollow core of the limiting gas cloud through an inner nozzle of the gas supply unit surrounded by the outer nozzle; generating a pair of laser pulses by a laser source, comprising (i) a first laser pulse to convert the gas puff target into plasma and (ii) a second laser pulse to heat the plasma and generate metrology-related radiation; dispensing the metrology-related radiation; and venting residual gas remaining after the generation of metrology-related radiation through a skimmer nozzle.

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