Radiation spectrum configuration system

Abstract

A radiation source, a metrology tool, a lithographic apparatus, and a method of generating an acoustic mode in a gaseous medium are provided. The radiation spectrum configuration system includes a hollow optical waveguide and one or more transducers. The hollow optical waveguide contains a gaseous medium. The one or more transducers are configured to generate an acoustic mode in a gaseous medium.

Description

Radiation Spectrum Configuration System Cross-references to related applications

[0001] This application claims priority to European Patent Application 23201188.2, filed on 2 October 2023, and European Patent Application 23218665.0, filed on 20 December 2023, the entirety of which is incorporated herein by reference. Technical Field

[0002] This invention relates to a radiation spectral configuration system. This radiation spectral configuration system may be part of a radiation source. This radiation source may be part of a measurement tool. Background Technology

[0003] A lithography apparatus is a machine that creates a desired pattern on a substrate. For example, lithography apparatuses can be used in the manufacture of integrated circuits (ICs). For instance, a lithography apparatus may project a pattern (also commonly referred to as a “design layout” or “design”) from a patterning device (such as a mask) onto a layer of radiation-sensitive material (such as a photoresist) disposed on a substrate (such as a wafer).

[0004] Measurement tools (such as scatterometers), topography measurement systems, or position measurement systems are used to measure the properties of a substrate. This measurement may be performed after the substrate has been exposed. The measurement may also be used to adjust the operation of the photolithography apparatus to facilitate pattern replication of the exposed substrate. Adjustments may include modifying the illumination pattern and / or adjusting the projection optics.

[0005] The radiation characteristics used by a measurement tool can affect the type and quality of the measurements that can be performed. In some applications, it may be advantageous to use multiple radiation wavelengths to measure the substrate. Multiple different wavelengths may be able to propagate, irradiate, and scatter from the measurement target without interfering with or with only minimal interference from other wavelengths. Therefore, different wavelengths may be used, for example, to obtain more measurement data. For example, different wavelengths may be set as a sequence of different wavelengths (i.e., a series of wavelengths). Different radiation wavelengths may also be able to detect and discover different characteristics of the measurement target. Broadband radiation may be used in measurement systems such as, for example, level sensors, alignment mark measurement systems, scattering measurement tools, or inspection tools. Broadband radiation sources may be supercontinuum light sources. Controlled filters may be used to select wavelengths to illuminate the substrate for measurement, and subsequently, another color may be selected to illuminate the substrate for subsequent measurements. A system that receives a given wavelength and provides output radiation at different wavelengths (or provides broadband radiation) may be called a radiation spectral configuration system.

[0006] Generating radiation at the desired wavelength can be difficult and / or expensive.

[0007] Existing radiation spectral configuration systems have the following drawbacks: changing the wavelength of the output radiation is difficult and / or slow. Summary of the Invention

[0008] According to a first aspect of the present invention, a radiation spectrum configuration system is provided, the system comprising: a hollow optical waveguide containing a gaseous medium; and one or more transducers configured to generate an acoustic mode in the gaseous medium.

[0009] Advantageously, the acoustic mode is used to provide modulation of the radiation wavelength from the system output. This allows the desired wavelength to be selected using an optical switch synchronized with the acoustic mode.

[0010] The hollow optical waveguide may be an optical fiber with a hollow core.

[0011] The one or more transducers may include a pair of transducers positioned on opposite sides of the optical fiber.

[0012] The one or more transducers may include transducers wrapped around optical fibers.

[0013] One or more anti-resonant structures may be placed on the outer surface of the optical fiber.

[0014] The hollow optical waveguide may be formed from multiple glass rods within a gas storage chamber. The one or more transducers may include multiple transducers also located within the gas storage chamber.

[0015] It may be possible to provide at least four glass rods.

[0016] The transducer may be located between multiple pairs of glass rods.

[0017] According to a second aspect of the present invention, a radiation source is provided, comprising: a radiation spectrum configuration system of the first aspect of the present invention; and further comprising: a pump radiation source configured to provide pump radiation into a hollow optical waveguide.

[0018] The radiation source may include an optical switch configured to selectively transmit an output radiation beam provided by a radiation spectral configuration system, the optical switch being synchronized with one or more transducers.

[0019] The radiation source may also include an optical switch configured to selectively transmit a pump radiation beam, which is synchronized with one or more transducers.

[0020] According to a third aspect of the present invention, a measuring tool is provided, which includes a radiation source according to a second aspect of the present invention.

[0021] According to a fourth aspect of the invention, a photolithography apparatus is provided that includes any radiation source of the second aspect of the invention and a measurement tool of the third aspect of the invention.

[0022] According to a fifth aspect of the invention, a method is provided, the method comprising: directing a pump radiation beam into a hollow optical waveguide comprising a gas medium configured to provide an output beam with a wavelength different from that of the pump radiation beam; the method further comprising: generating an acoustic mode in the gas medium using one or more transducers, thereby modifying the wavelength of the output beam.

[0023] Advantageously, the acoustic mode is used to provide modulation of the wavelength of the output beam. This allows the desired wavelength to be selected using an optical switch synchronized with the acoustic mode.

[0024] The method may also include using an optical switch synchronized with one or more transducers to select the desired output wavelength.

[0025] The optical switch can be located between the pump radiation beam source and the hollow optical waveguide.

[0026] The optical switch may be located at the output of the hollow optical waveguide.

[0027] Different aspects of the present invention may be combined together. Attached Figure Description

[0028] Embodiments of the present invention will now be described, with reference to the accompanying schematic diagrams, which are for illustrative purposes only:

[0029] Figure 1 is a schematic overview of the photolithography apparatus;

[0030] Figure 2 is a schematic overview diagram showing the photolithography unit;

[0031] Figure 3 is a schematic diagram illustrating overall photolithography, showing the synergistic effect between three key technologies to optimize semiconductor manufacturing;

[0032] Figure 4 is a schematic diagram illustrating a scatterer including a radiation source according to an embodiment of the present invention;

[0033] Figure 5 is a schematic diagram illustrating a horizontal sensor including a radiation source according to an embodiment of the present invention;

[0034] Figure 6 illustrates an alignment sensor including a radiation source according to an embodiment of the present invention;

[0035] Figure 7 is a schematic diagram illustrating a radiation spectrum configuration system according to an embodiment of the present invention;

[0036] Figure 8 is a schematic diagram illustrating a radiation source according to an embodiment of the present invention;

[0037] Figure 9 is a schematic diagram showing the acoustic mode of the optical fiber representing the radiation source;

[0038] Figure 10 is a graph showing the different output spectra provided by the radiation source;

[0039] Figure 11 is a graph showing the time-averaged radiation intensity provided by the radiation source;

[0040] Figure 12 is a schematic diagram illustrating another radiation spectrum configuration system according to an embodiment of the present invention;

[0041] Figure 13 is a schematic diagram illustrating a radiation spectrum configuration system according to another embodiment of the present invention. Detailed Implementation

[0042] Photolithography apparatuses may use electromagnetic radiation to project patterns onto a substrate. The wavelength of this radiation determines the minimum size of a feature that can be formed on the substrate. Typical wavelengths currently used are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Photolithography apparatuses using extreme ultraviolet (EUV) radiation (wavelength range of 4–20 nm, such as 6.7 nm and 13.5 nm) may be used to form smaller features on the substrate than those using, for example, 193 nm wavelength radiation.

[0043] Low-k1 lithography may be used to fabricate features smaller than the classical resolution limit of a lithography apparatus. In such a process, the resolution formula can be expressed as CD = k1 × λ / NA, where λ is the wavelength of radiation used, NA is the numerical aperture of the projection optics in the lithography apparatus, CD is the "critical size" (usually referring to the smallest printed feature size, but in this case, half a pitch), and k1 is an empirical resolution factor. Generally, the smaller k1 is, the more difficult it is to replicate on the substrate a pattern with the shape and size planned by the circuit designer to achieve specific electrical functions and performance. Complex fine-tuning steps may be applied to the lithography projection apparatus and / or the design layout to overcome these difficulties. These steps include, for example, but not limited to: optimizing NA, customizing the illumination scheme, using phase-shifting patterning equipment, various optimizations of the design layout such as optical proximity correction (OPC, sometimes referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques (RET)". Alternatively, a precision control loop for controlling the stability of the lithography apparatus may be used to improve the pattern replication accuracy of low-k1 lithography.

[0044] In the current document, the terms “radiation” and “beam” are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., wavelengths of 365, 248, 193, 157, and 126 nm) and EUV (extreme ultraviolet radiation, e.g., wavelengths with a range of approximately 5–100 nm).

[0045] As used herein, the terms “mask,” “mask,” or “patterning apparatus” are interpreted broadly to refer to a general patterning apparatus that can be used to impart a patterned cross-section to an incident radiation beam, corresponding to a pattern to be created on a target portion of a substrate. The term “optical valve” may also be used in this context. Examples of other such patterning apparatuses, besides classic masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), include programmable mirror arrays and programmable LCD arrays.

[0046] Figure 1 schematically illustrates a lithography apparatus LA. The lithography apparatus LA includes: an irradiation system (also called an irradiator) IL configured to modulate a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask stage) MT configured to support a patterning apparatus (e.g., a mask) MA and connected to a first locator PM configured to precisely position the patterning apparatus MA according to specific parameters; a substrate support (e.g., a wafer stage) WT configured to support a substrate (e.g., a wafer coated with resist) W and connected to a second locator PW configured to precisely position the substrate support according to specific parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted by the radiation beam B by the patterning apparatus MA onto a target portion C (e.g., including one or more dies) of the substrate W.

[0047] In operation, the irradiation system IL receives a radiation beam from the radiation source SO, for example via the beam delivery system BD. The irradiation system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for guiding, shaping, and / or controlling the radiation. The irradiator IL may be used to adjust the radiation beam B to have a desired spatial and angular intensity distribution in the cross-section of the plane in which the patterning device MA is located.

[0048] The term "projection system" (PS) as used herein should be interpreted broadly to encompass multiple types of projection systems, including refractive, reflective, catadioptric, variable, magnetic, electromagnetic, and / or electrostatic optical systems, or any combination thereof, adapted to the type of exposure radiation used and / or other factors such as immersion or vacuum environments. Any term used herein, "projection lens," may be considered equivalent to the more general term "projection system" (PS).

[0049] A lithography apparatus LA can be of the type in which a portion of the substrate is covered by a liquid (such as water) with a relatively high refractive index, such that it fills the space between the projection system PS and the substrate W—this is also known as immersion lithography. More information on immersion technology can be found in US6952253, which is incorporated herein by reference.

[0050] The lithography apparatus LA may also be of the type with two or more substrate supports WT (also known as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or subsequent exposure preparation steps may be performed on a substrate W located on one of the substrate supports WT while another substrate W located on the other substrate support WT is used to expose a pattern on that other substrate.

[0051] In addition to the substrate support WT, the lithography apparatus LA may include a measurement stage. This stage is arranged to support sensors and / or cleaning devices. The sensors may be arranged to measure characteristics of the projection system PS or the radiation beam B. The stage may hold multiple sensors. The cleaning devices may be arranged to clean a part of the lithography apparatus, such as a part of the projection system PS or a system providing immersion solution. The measurement stage may move below the projection system PS as the substrate support WT moves away from the projection system PS.

[0052] In operation, a radiation beam B is incident on a patterning apparatus MA (e.g., a mask) held on a mask support MT and patterned by a pattern (design layout) present on the patterning apparatus MA. After passing through the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of the substrate W. The substrate support WT can be precisely moved, for example, to position different target portions C in a focused and aligned position along the path of the radiation beam B, using a second locator PW and a position measurement system IF. Similarly, a first locator PM and possibly another position sensor (not explicitly shown in Figure 1) may be used for precise positioning of the patterning apparatus MA corresponding to the path of the radiation beam B. The patterning apparatus MA and the substrate W can be aligned with substrate alignment marks P1, P2 using mask alignment marks M1, M2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may also be located in the space between target portions. When substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribing alignment marks.

[0053] The lithography apparatus LA shown in Figure 2 may form part of a lithography unit LC, sometimes referred to as a lithography unit or (lithography) cluster, which typically includes devices for performing pre- and post-exposure processes on a substrate W. These devices typically include: a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chiller CH, and a baking plate BK (e.g., for regulating the temperature of the substrate W, such as for regulating the solvent in the resist layer). A substrate handler or robot RO picks up the substrate W from input / output ports I / O1, I / O2, moves it between different process units, and delivers the substrate W to the feed stage LB of the lithography apparatus LA. The devices in the lithography unit (often collectively referred to as tracks) are typically controlled by a track control unit TCU, which may be controlled by a management control system SCS, which may also control the lithography apparatus LA (e.g., via a lithography control unit LACU).

[0054] To ensure that the substrate W exposed by the lithography unit LA is correctly and consistently exposed, it needs to be inspected to measure characteristics of the patterned structure, such as overlay error between subsequent layers, linewidth thickness, and critical dimension (CD). For this purpose, inspection tools (not shown) may be included within the lithography unit LC. If errors are detected, adjustments can be made to the substrate exposure or other process steps performed on the substrate W, especially if the inspection has been completed before other substrates W in the same production batch or controlled batch still need to be exposed or processed.

[0055] An inspection apparatus (also referred to as a measurement tool) is used to determine the characteristics of a substrate W, particularly the differences in characteristics between different substrates W or how the characteristics associated between different layers of the same substrate W vary layer by layer. This inspection apparatus may alternatively be configured to identify defects in the substrate W and may be, for example, part of a lithography unit LC, or integrated into a lithography apparatus LA, or even as a standalone device. The inspection apparatus may measure the characteristics of a latent image (an image formed in a resist layer after exposure), or a semi-latent image (an image formed in a resist layer after a post-baking step PEB), or a developed resist pattern (where the exposed or unexposed portions of the resist have been removed), or an etched pattern (after a pattern transfer step, such as etching).

[0056] Typically, patterning in a lithography apparatus (LA) is one of the most critical steps in the manufacturing process, requiring extremely high precision in the size and layout of the structure on the substrate (W). To ensure this high precision, three systems may be combined in a so-called “holistic” control environment, as shown in Figure 3. One of these systems is the lithography apparatus (LA) which is (virtually) connected to the metrology tool (MT) (the second system) and the computer system (CL) (the third system). The key to such a “holistic” environment is optimizing the collaboration between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning process performed in the lithography apparatus (LA) remains within the process window. This process window defines the range of process parameters (e.g., dose, focal length, overlay accuracy) within which—typically referring to the range of process parameters allowed to vary in a lithography or patterning process—a particular manufacturing process can produce the desired results (e.g., functional semiconductor devices).

[0057] The computer system CL may use a portion of the design layout to be patterned to predict which resolution enhancement technique to use and perform computer lithography simulations and calculations to determine which mask layout and lithography setup can achieve the maximum overall process window for patterning (as indicated by the double arrows in the first scale SC1 of Figure 3). Typically, resolution enhancement techniques are designed to match the patterning capabilities of the lithography apparatus LA. The computer system CL may also be used to detect the current operating position of the lithography apparatus LA within the process window (e.g., using input from a metrology tool MT) to predict potential defects due to, for example, suboptimal processes (as indicated by the arrow pointing to "0" in the second scale SC2 of Figure 3).

[0058] The measurement tool MT may provide input to the computer system CL, enabling the computer system CL to accurately simulate and predict, and may provide feedback to the lithography apparatus LA to identify possible drifts, such as drifts in the calibration state of the lithography apparatus LA (as shown by the multiple arrows in the third scale SC3 in Figure 3).

[0059] In photolithography, frequent measurements of the created structures are desirable, for example, for process control and verification. The tools used for such measurements are generally called metrology tools (MTs). Several types of metrology tools (MTs) are known for performing such measurements, including scanning electron microscopes (SEMs) or various forms of scatterometer metrology tools (MTs). A scatterometer is a versatile instrument that allows for the measurement of photolithography process parameters by placing a sensor in the pupil plane of the scatterometer objective or in a plane conjugate to the pupil plane; such measurements are generally referred to as pupil-based measurements. Alternatively, the sensor can be placed in the image plane or in a plane conjugate to the image plane; in this case, such measurements are generally referred to as image- or field-of-view based measurements. Such scatterometers and associated measurement techniques are further described in US patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or European patent application EP1628164A, all of which are incorporated herein by reference in their entirety. The aforementioned scattering instrument may use light from soft X-rays and visible light in the near to IR wavelength range to measure the grating.

[0060] In the first embodiment, the scatterer MT is an angle-resolved scatterer. In such a scatterer, a reconstruction method may be applied to the measured signal to reconstruct or calculate the characteristics of the grating. Such reconstruction may be, for example, derived from the results of a simulation interaction between the scattered radiation and a mathematical model of the target, and by comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulation interaction produces a diffraction pattern similar to that observed from a real target.

[0061] In the second embodiment, the scatterer MT is a beam-splitter scatterer MT. In such a beam-splitter scatterer MT, radiation emitted by a radiation source is directed to the target, and reflected or scattered radiation is directed to a spectrometer detector used to measure the spectrum of specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). Based on this data, the structure or profile of the detected spectral target can be reconstructed, possibly through rigorous coupled-wave analysis, nonlinear regression, or by comparison with a simulated spectral library.

[0062] In the third embodiment, the scatterer MT is an elliptically polarized scatterer. This elliptically polarized scatterer allows the determination of lithography process parameters by measuring the scattered radiation in each polarization state. Such a measurement device emits polarized light (e.g., linear, circular, or elliptical) by, for example, using a suitable polarization filter in its irradiation section. A light source suitable for the measurement device may also provide polarized radiation. Several existing embodiments of elliptically polarized scatterers are described in U.S. Patent Applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0063] In one embodiment of a scattering instrument (MT), the MT is adapted to measure asymmetry in the reflection spectrum and / or to measure the overlay accuracy of two misaligned gratings or periodic structures using appropriate detection configurations, the asymmetry being related to the degree of overlay accuracy. The two (typically overlapping) grating structures may be applied in two different layers (not necessarily adjacent layers) and may be formed approximately at the same location on the wafer. The scattering instrument may have a symmetric detection configuration, for example, as described in the commonly owned patent application EP1628164A, to clearly distinguish any asymmetry. This provides an obvious method for measuring misalignment in gratings. Further examples of measuring overlay accuracy errors between two layers containing a periodic structure as the measurement target by means of asymmetry in a periodic structure can be found in PCT patent application publication WO 2011 / 012624 or U.S. patent application US20160161863, all of which are incorporated herein by reference.

[0064] Other relevant parameters may be focal length and dose. Focal length and dose may be determined simultaneously by scattering measurements (or selectively by scanning electron microscopy), as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure may be used for points in a focusing energy matrix (FEM—also known as a focusing exposure matrix), which has a unique combination of critical dimensions and sidewall angle measurements. If such a unique combination of critical dimensions and sidewall angles is obtained, the focal length and dose values ​​may be uniquely determined from these measurements.

[0065] The measurement target may be formed by a photolithography process, mostly within the resist, but also within a set of composite gratings, for example, after an etching process. Typically, the pitch and linewidth of the structures within the grating are strongly dependent on the measurement optics (particularly the NA in the optics), capable of capturing the diffraction order from the measurement target. As previously mentioned, the diffraction signal may be used to determine the shift between two layers (also referring to "overlap error") or to reconstruct at least a portion of the original grating produced by the photolithography process. This reconstruction can guide the photolithography process or may also be used to control at least a portion of the process. The target may have finer sub-segmentation structures configured to simulate the dimensions of functional components in the design layout within the target. Due to such sub-segmentation structures, the target's behavior will be more similar to the functional portions of the design layout, allowing for a better match between overall process parameter measurements and the functional portions of the design layout. These targets may be measured in underfill or overfill modes. In underfill mode, the spot formed by the measurement beam is smaller than the overall target. In overfill mode, the spot formed by the measurement beam is larger than the overall target. In such an overfill mode, it is also possible to measure different targets simultaneously, thereby determining different process parameters at the same time.

[0066] The overall measurement quality of lithography parameters used for a specific target depends at least in part on the measurement formulation used to measure that lithography parameter. The term "substrate measurement formulation" may include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement formulation is a diffraction-based optical measurement, one or more parameters of that measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence relative to the substrate, the radiation orientation relative to the pattern on the substrate, and so on. For example, one criterion for selecting a measurement scheme may be the sensitivity of a measurement parameter to process deviations. Further examples are described in U.S. Patent Application US2016-0161863 and published U.S. Patent Application US2016 / 0370717A, which are incorporated herein by reference in their entirety.

[0067] The measurement device (e.g., a scatterer SM1) is shown in Figure 4. It includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. The reflected or scattered radiation is transmitted to a spectrometer detector 4, which measures the spectrum 10 of the specular reflected radiation (i.e., measures the intensity In1 as a function of wavelength λ). Based on this data, the processing unit PU may reconstruct the structure or profile that caused the detected spectrum, for example, through rigorous coupled-wave analysis and linear regression, or by comparing it with a simulated spectral library shown at the bottom of Figure 4. Typically, for this reconstruction, the basic form of the structure is known and some parameters are estimated based on experience with the process of manufacturing the structure, leaving only a few structural parameters that need to be determined by the scattering measurement data. Such a scatterer may be configured as a vertically incident scatterer or an obliquely incident scatterer.

[0068] In photolithography, frequent measurements of the fabricated structure are typically required, for purposes such as process control and verification. Several tools are known for performing these measurements, including scanning electron microscopes (SEMs) and various forms of metrology devices, such as scatterometers. Examples of known scatterometers often depend on the specific measurement target being measured, such as an underfilled target (a target in the form of a simple grating or overlapping gratings in different layers, and whose size is large enough that the spot generated by the measurement beam is smaller than the grating) or an overfilled target (i.e., the target is partially or completely irradiated by the beam). Furthermore, the use of metrology tools (e.g., an angle-resolved scatterometer irradiating an underfilled target, such as a grating) allows for the use of so-called reconstruction methods, which calculate the characteristics of the grating by simulating the interaction of scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. Model parameters are adjusted until the simulated interaction produces a diffraction pattern similar to that observed on the real target.

[0069] A scatterometer is a general-purpose measurement device that allows for the measurement of lithography process parameters by placing a sensor in the pupil plane of the scatterometer objective or in a plane conjugate to the pupil plane; such measurements are typically referred to as pupil-based measurements. Alternatively, measurements can be performed by placing a sensor in the image plane or a plane conjugate to the image plane; such measurements are typically referred to as image- or field-of-view measurements. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1628164A, all of which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure multiple targets originating from multiple gratings using soft X-rays and the visible to near-IR wavelength range within a single image.

[0070] A topography measurement system, a horizontal sensor, or a height sensor (possibly integrated into a photolithography apparatus) is arranged to measure the topography of the upper surface of a substrate (or wafer). These measurements may generate a topography map of the substrate (also called a height map), which indicates how the substrate height varies as a function of its position on its surface. This height map may be used to correct the substrate's position during the transfer of patterns onto the substrate in order to correctly position the spatial image of the patterning apparatus at a focused location on the substrate. It should be understood that "height" in this context refers generally to a dimension (also called the Z-axis) that deviates approximately from the plane of the substrate. Typically, the horizontal or height sensor performs measurements in a fixed position (relative to its own optical system), and the relative movement between the substrate and the horizontal or height sensor's optical system enables height measurements throughout the substrate's location.

[0071] Figure 5 schematically illustrates an example of a level sensor or height sensor LS known in the art, for illustrative purposes only. In this example, the level sensor includes an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB, which is supplied by a projection grating PGR of the projection unit LSP. For example, the radiation source LSP may be a narrowband radiation source (e.g., bandwidth not exceeding 10 nm) or a broadband radiation source (e.g., bandwidth greater than 10 nm, such as greater than 100 nm—such sources may also be referred to as supercontinuum sources). The supercontinuum source may be polarized or unpolarized, pulsed or continuous, such as a polarized or unpolarized laser beam. The radiation source LSO may include several radiation sources with different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but may additionally or selectively cover UV and / or IR radiation as well as any wavelength range suitable for reflection from the substrate surface.

[0072] The projection grating PGR is a periodic grating comprising a periodic structure, such that the radiation beam BE1 has a periodically varying intensity. The radiation beam BE1 with its periodically varying intensity is guided to a measurement position MLO on the substrate W, which has an incident angle ANG between 0° and 90° (typically 70° to 80°) relative to the axis (Z-axis) perpendicular to the surface of the substrate. At this measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and guided towards the detection unit LSD.

[0073] To determine the height level of the measurement location MLO, the level sensor also includes a detection system comprising a detection grating DGR, a detector DET, and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR may be consistent with a projection grating PGR. The detector DET generates a detector output signal indicating the received light, such as indicating the intensity of the received light (e.g., a photodetector), or indicating the spatial distribution of the received light intensity (e.g., a camera). The detector DET may comprise any combination of one or more detector types.

[0074] The height level of the measurement location MLO can be determined using triangulation techniques. The detected height level is typically related to the signal strength measured by the detector DET, which exhibits periodicity. This periodicity depends particularly on the design of the projection grating PGR and the (oblique) incident angle ANG.

[0075] The projection unit LSP and / or detection unit LSD may also include optical elements, such as lenses and / or mirrors, along the path of a patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0076] In some embodiments, the detection grating DR may be omitted, and the detector DET may be placed where the detection grating DGR is located. This configuration provides more direct detection of the image of the projection grating PGR.

[0077] To more efficiently cover the surface of substrate W, the horizontal sensor LS may be configured to project an array of measurement beams BE1 onto the surface of substrate W, thereby generating an array of measurement regions MLO or spots that cover a larger measurement range.

[0078] For example, several general-purpose height sensors are disclosed in US7265364 and US7646471, both of which are incorporated herein by reference. The height sensor disclosed in US2010233600A1 uses UV radiation instead of visible or infrared radiation; this patent is also incorporated herein by reference. The compact height sensor described in WO2016102127A1, which is incorporated herein by reference, uses a multi-element detector instead of requiring a detection grating to detect and identify the position of the grating image.

[0079] A position measurement system (PMS) may include any type of sensor suitable for determining the position of a substrate stage WT. A position measurement system (PMS) may include any type of sensor suitable for determining the position of a mask support MT. This sensor may be, for example, an interferometer or an encoder. A position measurement system (PMS) may include a combined system of interferometers and encoders. The sensor may be another type of sensor, such as a magnetic sensor, a capacitive sensor, or an inductive sensor. The position measurement system (PMS) may determine the position relative to a reference object, such as a measurement frame MF or a projection system PS. The position measurement system (PMS) may determine the position by measuring the position of the substrate stage WT and / or the mask support MT or by measuring the time derivative of that position (e.g., velocity or acceleration).

[0080] A position measurement system (PMS) may include an encoder system. Known encoder systems, such as U.S. Patent Application US2007 / 0058173A1, filed September 7, 2006, are incorporated herein by reference. This encoder system includes an encoder readhead, a grating, and a sensor. The encoder system may receive a primary radiation beam and a secondary radiation beam. Both the primary and secondary radiation beams originate from the same radiation beam, i.e., the original radiation beam. At least one of the primary and secondary radiation beams is generated by diffracting the original radiation beam through a grating. If both the primary and secondary radiation beams are generated by diffracting the original radiation beam through a grating, the primary radiation beam needs to have a different diffraction order than the secondary radiation beam. Different diffraction orders are, for example, +1, -1, +2, and -2. The encoder system optically combines the primary and secondary radiation beams to form a combined radiation beam. A sensor in the encoder readhead determines the phase and phase difference of the combined radiation beam. The sensor generates a signal based on the phase or phase difference. This signal indicates the encoder readhead position relative to the grating. One of the encoder read heads and the grating may be disposed on the substrate structure WT. The other encoder read heads and gratings may be disposed on the measurement frame MF or the base frame BF. For example, multiple encoder read heads may be disposed on the measurement frame MF, while the grating is disposed on the top surface of the substrate support WT. In other examples, the grating is disposed on the bottom surface of the substrate support WT, and the encoder read heads are disposed below the substrate support WT.

[0081] A position measurement system (PMS) may include an interferometer system. For example, a known interferometer system is derived from U.S. Patent 6020964, filed July 13, 1998, which is incorporated herein by reference. This interferometer system may include a beam splitter, a mirror, a reference mirror, and a sensor. A radiation beam is split into a reference beam and a measurement beam by the beam splitter. The measurement beam propagates to the mirror and is reflected back to the beam splitter. The reference beam propagates to the reference mirror and is reflected back to the beam splitter. At the beam splitter, the measurement beam and the reference beam are combined into a combined radiation beam. The combined radiation beam is incident on the sensor. The sensor determines the phase or frequency of the combined radiation beam. The sensor generates a signal based on the phase or frequency. This signal represents the displacement of the mirror. In one embodiment, the mirror is connected to a substrate support WT. The reference mirror may be connected to a measurement frame MF. In another embodiment, the measurement beam and the reference beam are combined into a combined radiation beam by additional optical components instead of a beam splitter.

[0082] In the fabrication of complex devices, multiple photolithographic patterning steps are typically required to form functional features in successive layers on a substrate. Therefore, a key aspect of the performance of this photolithography apparatus lies in its ability to correctly and accurately position the applied pattern relative to features already formed in previous layers (whether by the same apparatus or different photolithography apparatuses). For this purpose, one or more sets of markers are placed on the substrate. Any marker is a structure whose position can subsequently be measured using a position sensor (typically an optical position sensor). This position sensor may be called an "alignment sensor," and these markers may be called "alignment marks." The markers may also be referred to as measurement targets.

[0083] Photolithography apparatuses may include one or more alignment sensors that can precisely measure the position of alignment marks formed on a substrate. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information from the alignment marks formed on the substrate. Examples of alignment sensors used in existing photolithography apparatuses are based on the self-reference interferometer described in US6961116. Several enhancements and improvements to this position sensor have been developed, such as those disclosed in US2015261097A1. All published content is incorporated herein by reference.

[0084] The markings or alignment marks may comprise a series of gratings formed on or within a layer disposed on a substrate, or (directly) formed within the substrate. The gratings may be arranged at regular intervals and serve as grating lines, thus the marking can be considered a diffraction grating with a defined spatial period (pitch). Depending on the orientation of these grating lines, the markings are designed to allow measurement of position along the X-axis or along the Y-axis (which is oriented substantially perpendicular to the X-axis). Markings comprising gratings arranged at +45° and / or -45° relative to both the X-axis and Y-axis allow for combined X- and Y-measurements using the techniques described in US2009 / 195768A, which is incorporated herein by reference.

[0085] The alignment sensor optically scans each mark using a radiated spot of light to acquire a periodically changing signal, such as a sine wave. The phase of this signal is analyzed to determine the position of the mark and the position of the substrate relative to the alignment sensor, which is fixed relative to the reference frame of the photolithography apparatus. So-called coarse and fine marks may be set with different (coarse and fine) mark sizes, allowing the alignment sensor to distinguish different periods of the periodic signal and the specific position (phase) within a period. Marks with different pitches may also be used for this purpose.

[0086] Measuring the location of the markings can also provide information about the deformation of the substrate where the markings are set, for example, in the form of a wafer network. Substrate deformation can be caused by a number of factors, such as the substrate being electrostatically held to a substrate stage and / or the substrate heating up when it is exposed to radiation.

[0087] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as the alignment sensor described in US6961116, which is incorporated herein by reference. A radiation source RSO provides a radiation beam RB having one or more wavelengths, which is deflected by a diffusing optics onto a mark (e.g., mark AM) on a substrate W as an illumination spot SP. In this example, the diffusing optics include a point mirror SM and an objective lens OL. The illumination spot SP illuminated by the mark AM may be slightly smaller in diameter than the width of the mark itself.

[0088] The radiation diffracted by the marker AM (via the objective lens OL in this example) is aligned to the information-carrying beam IB. The term "diffraction" is intended to include zero-order diffraction from the marker (which may be referred to as reflection). A self-referenced interferometer SRI (such as the type disclosed in US6961116 mentioned above) self-interferes with the beam IB, after which the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide a separate beam if the radiation source RSO generates more than one wavelength. The photodetector may be a single element, or it may include multiple pixels as desired. The photodetector may include a sensor array.

[0089] This example includes a diffusing optics device for the point reflector SM, which may also be used to block the zero-order radiation reflected by the marker, so that the information-carrying beam IB only includes higher-order diffraction radiation from the marker AM (this is not necessary for measurement, but improves the signal-to-noise ratio).

[0090] The intensity signal SI is supplied to the processing unit PU. Through optical processing in the combination module SRI and computational processing in the processing unit PU, the values ​​of the X- and Y-positions on the substrate relative to the reference frame are output.

[0091] A single measurement of this type limits the mark to a specific range corresponding to the mark's pitch. Coarse measurement techniques are used in conjunction with this to identify which period of the sine wave contains the mark's location. The same coarse or fine process may be repeated at different wavelengths to improve accuracy and / or reliable mark detection regardless of the material used to manufacture the mark or the materials placed above and / or below it. The wavelength may be optically multiplexed and demultiplexed for simultaneous processing and / or multiplexed via time-division multiplexing or frequency-division multiplexing.

[0092] In this example, the alignment sensor and the light spot SP remain stationary while the substrate W moves. The alignment sensor can be rigidly and precisely mounted to the reference frame as it effectively scans the marker AM in a direction opposite to the direction of substrate W's movement. This movement of the substrate W is controlled by its mounting on the substrate support and by a positioning system that controls the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) markers are disposed on the substrate support. Measuring the position of the markers disposed on the substrate support allows calibration of the substrate support position (e.g., relative to a frame to which the alignment system is attached) determined by the position sensor. Measuring the position of the markers disposed on the substrate allows determination of the substrate's position relative to the substrate support.

[0093] Measurement and / or inspection tools (also known as measuring instruments), as described above, typically use radiation to acquire measurement data. Different types of radiation may be used depending on the target being measured and the characteristics to be measured. A distinct characteristic of radiation is the wavelength used to acquire the measurement, as different wavelengths may provide different information about the target being measured. Some measuring instruments may use their desired wavelength for a specific measurement. Some measuring instruments may use a range of different desired wavelengths for a specific measurement. A light source may provide the desired wavelength (e.g., with a correlated bandwidth of a few nanometers—e.g., at least 0.1 nm, and at most 10 nm). A light source may provide the desired radiation spectrum (e.g., a spectrum centered on the desired wavelength with a width up to 100 nm or higher). These light sources are all examples of providing radiation with a desired configuration (the configuration of radiation may be the wavelength of the radiation and may be the spectrum of the radiation).

[0094] An example of a source configured to provide radiation with a desired configuration uses a seed laser beam coupled to an optical fiber containing a gas. This seed excitation beam may have a broadband spectrum (e.g., 10 nm or wider). A pump laser beam is also coupled into the fiber. The pump laser beam provides energy to the fiber, and this energy amplifies the seed laser beam. The wavelength amplified by the seed laser beam is determined by the gas in the fiber and will change with variations in gas pressure. Therefore, the optical fiber can be used to provide an output beam with a desired wavelength. Simultaneously, the optical fiber can be used to provide an output beam with a spectrum centered on the desired wavelength (e.g., a broadband spectrum). The specific operation of the source will be described in more detail below.

[0095] In some cases, nonlinear broadening techniques may be used to obtain a desired spectral configuration of radiation. Various known apparatuses and methods exist for achieving nonlinear broadening. Typically, these methods rely on confinement of the pump radiation to achieve the desired high intensity that withstands significant nonlinear effects. Known methods for achieving nonlinear broadening by confining radiation involve confining the laser pump radiation outside the fiber to generate broadband radiation. The laser may be an ultrashort pulse laser (e.g., pulse widths on the order of picoseconds to femtoseconds). The nonlinear propagation dynamics of this radiation within the fiber may result in broadband radiation as a consequence of soliton self-squeezing and / or modulation instabilities. For example, this could be used to generate supercontinuum radiation spanning the wavelength range from IR to UV.

[0096] In other radiation spectral configurations, such as when the source is used to provide the desired wavelength, nonlinear effects may not be required. In such examples, the pump laser beam may provide longer pulses or continuous radiation and may operate at a single wavelength (with an associated bandwidth, e.g., 0.1 nm to 5 nm).

[0097] The optical fiber may be a gas-filled hollow fiber, such as hollow-core photonic crystal fiber (42-PCF). The gas mixture filling this fiber (which may also be referred to as the gas composition) includes one or more components, of which at least one is the working gas. The working gas is a gas that may exhibit significant nonlinear effects when interacting with high-intensity radiation. The performance and lifetime of the light source may depend on the composition of the gas mixture filling the optical fiber.

[0098] In some implementations, photonic crystal fibers (PCFs) can generate output radiation. In several embodiments, such photonic crystal fibers have microstructures surrounding the fiber core that help confine the radiation traveling through the fiber within the core.

[0099] In some embodiments, as further discussed below with reference to Figure 8, the methods and apparatus for configuring the spectrum of radiation may use optical fibers to confine both the input and output radiation. The optical fiber may be hollow-core and may include internal structures to achieve efficient guidance and confinement of radiation within the fiber. The optical fiber may be hollow photonic crystal fiber (42-PCF), which is particularly suitable for strong radiation confinement (primarily within the hollow core of the fiber), thereby achieving high radiation intensity.

[0100] The hollow core of an optical fiber may be filled with a gas, which is pumped by pump radiation, serving as a medium. Such an arrangement of the fiber and gas may be used to generate a desired radiation spectrum configuration (e.g., the desired wavelength of the output radiation). The radiation input to the optical fiber may be electromagnetic radiation, such as radiation in one or more of the infrared, visible, ultraviolet, and extreme ultraviolet spectra.

[0101] The optical fiber may be a hollow photonic crystal fiber that includes an antiresonant structure for confining radiation. Such fibers, including an antiresonant structure, are referred to in the art as antiresonant fibers, tubular fibers, single-loop fibers, negative curvature fibers, or suppression-coupled fibers. Several different designs of such fibers are known in the art. Alternatively, the fiber may also be a photonic bandgap fiber (42-PBF, such as Kagome fiber).

[0102] Several types of 42-PCFs can be designed, each based on a different physical guidance mechanism. Two such 42-PCFs include: hollow-core photonic bandgap fiber (42-PBF) and hollow-core anti-resonant reflective fiber (42-ARF). Design and manufacturing details of the 42-PCF can be found in US Patent US2004 / 015085A1 (for 42-PBF) and International PCT Patent Application WO2017 / 032454A1 (for hollow-core anti-resonant reflective fiber), which are incorporated herein by reference.

[0103] According to an embodiment of the present invention, an example of a radiation spectrum configuration system 22 will now be described with reference to FIG7. FIG7 is a schematic cross-sectional view showing an optical fiber 24 in a transverse plane, with transducers 26 located on opposite sides of the optical fiber.

[0104] Optical fiber 24 comprises an elongated body that is longer in one dimension than the other two dimensions of the fiber. This longer dimension may be referred to as the axial direction and may define the axis of optical fiber 24. The planes defined by the other two dimensions may be referred to as the transverse plane. Figure 7 shows a cross-sectional view of optical fiber 24 in this transverse plane (i.e., perpendicular to the axis), labeled as the xy plane. The cross-section of optical fiber 24 may remain substantially constant along the fiber axis.

[0105] It should be understood that the optical fiber 24 and the transducer 26 (which may be a piezoelectric transducer) may have a degree of flexibility. Therefore, the optical fiber 24 and the transducer 26 may be curved rather than straight (although straight optical fibers and transducers may also be used). Consequently, in some cases, the orientation of the axis may not be uniform along the length of the optical fiber. For example, terms such as optical axis and cross-section should be understood as referring to local optical axis, local cross-section, etc. Furthermore, if these components are described as cylindrical or tubular, it should be understood that these terms encompass such shapes that may have been distorted due to the bending of the optical fiber 24.

[0106] Fiber optic cable 24 may have any length and it should be understood that its length depends on the application. Fiber optic cable 24 may have a length between 1 cm and 10 m, for example, fiber optic cable 24 may have a length between 10 cm and 100 cm.

[0107] Optical fiber 24 includes a hollow core 28, a cladding portion 30 surrounding the hollow core, and a sheath portion 32 surrounding and supporting the cladding portion. The cladding portion 30 is used to guide radiation that propagates primarily within the hollow core 28 through the optical fiber 24. The guided radiation may exhibit a Gaussian distribution pattern. The hollow core 28 of the optical fiber 24 may be located approximately in the central region of the fiber, and therefore the axis of the fiber may also define the axis of the hollow core.

[0108] According to an embodiment of the invention, FIG8 shows a radiation source 40. A pump radiation source 41 and a seed radiation source 43 are provided. The pump radiation source 41 may be pulsed and may be a laser capable of generating short pulses of desired length and energy level. The seed radiation source 43 may provide broadband radiation (e.g., bandwidth of 10 nm or greater, such as 100 nm or greater—which may be referred to as supercontinuum radiation). FIG8 also shows an optical fiber 24 with a hollow core 28 (described in conjunction with FIG7 above) and a gas working medium 42 disposed within the hollow core. Transducers 26 are located on opposite sides of the optical fiber 24. As shown, transducers 26 may extend to the end of the optical fiber 24. In some examples, the transducers may be shorter than the optical fiber 24. The operation of the transducers 26 is controlled by a controller 44.

[0109] Pump radiation source 41 is configured to provide pump radiation beam 46. Seed radiation source 43 is configured to provide seed radiation beam 47. Pump radiation beam 46 and seed radiation beam 47 are combined by a dichroic mirror 48 (or other beam combining device). Pump radiation beam 46 and seed radiation beam 47 propagate coaxially and are both coupled into the hollow core 28 of optical fiber 24.

[0110] The gas working medium 42 within the optical fiber receives energy from the pump radiation beam 46 (i.e., is pumped by the pump radiation beam). The gas working medium 42 releases this energy by emitting radiation. The wavelength of the radiation emitted by the gas working medium 42 is determined by the type and pressure of the gas. Metastable energy states of the gas decay to lower energy states and emit photons, the wavelength of which corresponds to the energy difference between the energy states. The energy difference of the photon emission transition depends on the type and pressure of the gas.

[0111] The seed radiation beam 47 has a broadband spectrum and includes the wavelength at which the gaseous working medium 42 will emit photons. At this wavelength, the seed radiation beam 47 excites the gaseous working medium 42 to emit photons. At this wavelength, the emitted photons excite the gaseous working medium 42 to emit more photons. Thus, an output beam 50 at a certain wavelength is generated, which depends on the type and pressure of the gas.

[0112] In another embodiment, the seed radiation source 43 is omitted. In such an embodiment, the nonlinear interaction between the pump radiation and the gas working medium can generate photons with different energy ranges (corresponding to different wavelengths). These nonlinear interactions may occur at or immediately adjacent to the inlet of the hollow core 28 of the optical fiber 24. Photons with energies corresponding to photon emission transitions in the gas working medium 42 excite photons to be emitted from the gas. These photons excite the emission of more photons with that energy (corresponding to the wavelength). This produces an output beam 50 with a wavelength that depends on the type and pressure of the gas.

[0113] The radiation source 40 may also include a storage chamber 52. An optical fiber 24 is disposed within the storage chamber 52. The storage chamber 52 may also be referred to as a housing, container, or gas pool. The storage chamber 52 is configured to contain a gaseous working medium 42. The storage chamber 52 may include one or more features in the art for controlling, regulating, and / or monitoring the composition of the working medium 42 within the storage chamber 52. The storage chamber 52 is provided with a first transparent window 54. In use, the optical fiber 24 is disposed within the storage chamber 52 such that the first transparent window 54 is located near the input end 56 of the optical fiber 24. The first transparent window 54 may form part of the wall of the storage chamber 52. The first transparent window 54 is transparent at least to the frequency of the received input radiation, such that the received input radiation 46 (or at least a majority thereof) may be coupled into the optical fiber 24 located within the storage chamber 52. It should be understood that optics (not shown) may be provided to couple the input radiation 46 into the optical fiber 24.

[0114] Storage chamber 52 is provided with a second transparent window 56, forming part of the wall of storage chamber 52. In use, when optical fiber 24 is disposed inside storage chamber 52, the second transparent window 56 is located near the output end 58 of optical fiber 24. The second transparent window 56 is transparent to the frequency of the output radiation 50 of at least device 120.

[0115] In this context, a window is transparent to a given frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of the radiation incident on the window is transmitted through the window.

[0116] Both the first transparent window 54 and the second transparent window 56 may form an airtight seal within the cavity wall of the storage chamber 52, thereby enclosing the gaseous working medium 42 inside the storage chamber 52. It should be understood that the gas 48 may be contained inside the storage chamber 52 at a pressure different from the ambient pressure of the storage chamber 52.

[0117] The working gas medium 42 may include rare gases (e.g., argon, krypton, xenon), Raman-active gases (e.g., deuterium, nitrogen), or gas mixtures (e.g., argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures, or nitrogen / hydrogen mixtures). Depending on the type of gas filled, possible nonlinear optical processing may include modulation instability (MI), soliton self-compression, soliton fission, Kerr effect, Raman effect, and dispersive wave generation, details of which are described in WO20-18 / 127266A1 and US9160137B1 (both incorporated herein by reference).

[0118] The advantage of hollow-core fiber 24 is that it can achieve high-intensity radiation due to the strong spatial confinement of radiation propagating through the fiber (providing high local radiation intensity). For example, the radiation intensity inside fiber 24 may be high due to the higher received input radiation and / or due to the strong spatial confinement of radiation within fiber 24. This may promote a strong interaction between pump radiation and the gaseous working medium 42. The advantage of hollow-core fiber is that it can guide radiation with a wider wavelength range than solid-core fiber, and in particular, hollow-core fiber can guide radiation in the ultraviolet and infrared bands.

[0119] The advantage of using hollow fiber 24 is that the majority of the radiation guided into the fiber 24 is confined within the hollow core 24. Therefore, the radiation inside the fiber 24 primarily interacts with the gaseous working medium 42 disposed within the hollow core 28 of the fiber 24. As a result, the spectral configuration effect of the radiation achieved via the gaseous working medium 42 may be increased.

[0120] The received pump radiation 46 may be electromagnetic radiation. The pump radiation 46 may be received in the form of pulsed radiation. For example, the pump radiation 46 may include ultrafast pulses, which are generated, for example, by a laser.

[0121] Pump radiation 46 may be coherent radiation. Pump radiation 46 may also be collimated radiation, which has the advantage of potentially promoting and improving the efficiency of coupling pump radiation 46 into fiber 24. Pump radiation 46 may include a single frequency or a narrow range of frequencies. Pump radiation 46 may be generated by a laser. Similarly, output radiation 50 may be collimated and / or coherent.

[0122] In some embodiments, the output beam 50 may also have a desired wavelength (e.g., an associated bandwidth of a few nanometers, such as at least 0.1 nm, or up to 10 nm). The output beam 50 may have a desired radiation spectrum (e.g., a spectrum with wavelengths of 10 nm or higher, such as up to 100 nm, centered at the desired wavelength). These examples all provide radiation with a desired configuration (which may be the wavelength of the radiation and may also be the spectrum of the radiation).

[0123] Output beam 50 may be a broadband output beam (e.g., with a spectral width greater than 10 nm). The broadband range of output beam 50 may be a substantially continuous range, including a substantially continuous range of radiation frequencies. In this context, a frequency range that includes a large number of extremely fine discrete spectral lines invisible to conventional detectors may be considered substantially continuous.

[0124] The output beam 50 may include supercontinuum radiation. Supercontinuum radiation may be beneficial for a variety of applications, such as in measurement applications. For example, a continuous frequency range may be used to probe a large number of characteristics. For example, a continuous frequency range may be used to determine and / or eliminate the frequency dependence of the measured characteristic. The supercontinuum output beam radiation 50 may include, for example, electromagnetic radiation with a wavelength range of 100 nm or greater.

[0125] The wavelength of the output beam 50 can be tuned using the acoustic mode generated by transducer 26, as further described below.

[0126] The pump radiation 46 provided by the pulsed pump radiation source 41 may be pulsed. The pump radiation 46 may include electromagnetic radiation of one or more frequencies with wavelengths between 200 nm and 2 µm. For example, the pump radiation 46 may include electromagnetic radiation with a wavelength of 1.03 µm. The repetition frequency of the pulsed pump radiation 46 may be on the order of 1 kHz to 100 MHz. The pulse energy may be on the order of 0.1 µJ to 100 µJ, for example, 1–10 µJ. The pulse duration of the input radiation 46 may be between 10 fs and 10 ps, ​​for example, 300 fs. The average power of the pump radiation 46 may be between 100 mW and several hundred W. The average power of the pump radiation 46 may be, for example, between 20–50 W.

[0127] The wavelength of the radiation output from optical fiber 24 may be determined by certain characteristics of the fiber, such as its length, size, and the shape of the hollow core 101. For a given optical fiber 24, these characteristics are fixed and therefore cannot be modified to change the wavelength of the radiation output from the fiber. It is possible to adjust the wavelength of the radiation output from optical fiber 24 by adjusting the gas type of optical fiber 24. However, changing the gas type is a slow process.

[0128] The output radiation 50 provided by the radiation source 40 has a minimum average output power of 1 W. This average output power may be at least 5 W. This average output power may be at least 10 W. The output radiation 50 may be pulsed radiation. The power spectral density of the output radiation 50 across the entire wavelength band may be at least 0.01 mW / nm. This power spectral density of the output radiation across the entire wavelength band may be at least 3 mW / nm.

[0129] The wavelength of the output radiation 50 provided by the radiation source 40 can be adjusted by changing the pressure of the gas working medium 42 in the hollow core 28 of the optical fiber 24. In embodiments of the invention, the pressure of the gas working medium is changed by using an acoustic mode. In the embodiments shown in Figures 7 and 8, this is achieved by generating an acoustic mode within the gas working medium 42 in the hollow core 28.

[0130] Figure 9 shows the fundamental acoustic mode of the hollow core 28 of optical fiber 24 in cross-sectional form. The hollow core of this fiber is an example of a hollow optical waveguide, within which acoustic modes in a gaseous working medium may be generated. Embodiments of the invention may use other forms of hollow optical waveguides. Since Figure 9 is a grayscale image rather than a color image, labels have been added to identify the minimum and maximum gas pressures. Figure 9 shows the gas pressure in the hollow core 28 when the fundamental acoustic mode is excited. In Figure 9, the gas pressure is highest at the center of the hollow core 28 and lowest at the periphery of the hollow core.

[0131] Due to the nature of the acoustic mode, the pressure of the gas 42 in the hollow fiber 28 will vary with the acoustic frequency (potentially on the order of MHz in this embodiment). Therefore, at the center of the hollow fiber 28, the pressure will vary between a maximum and a minimum with respect to the acoustic frequency. Similarly, at the periphery of the hollow fiber 28, the pressure will vary between a minimum and a maximum with respect to the acoustic frequency. The midpoint between the center and the periphery is the node of the acoustic mode. At this midpoint, the gas pressure does not change but remains constant. The gas pressure at this node may be equivalent to the pressure present when the acoustic mode is not excited. This pressure corresponds to the pressure of the gas 42 in the storage chamber 52 and may be referred to as the background gas pressure.

[0132] Figure 10 illustrates an example of the wavelength of the output radiation 50 provided from radiation source 50. The horizontal axis of the graph in Figure 10 represents the wavelength in nm, and the vertical axis represents the normalized parametric gain (i.e., the power of the output radiation 50 relative to the power of the input pump radiation 46 (see Figure 8)). The vertical axis is scaled logarithmically. Five outputs 60-64 are shown. The full width at half maximum (FWHM) of each output 60-64 is approximately 80 nm. In other embodiments, the outputs may have an FWHM no greater than 10 nm (e.g., 0.1 nm or wider). In some embodiments, the outputs may have an FWHM greater than 10 nm (e.g., 100 nm or wider).

[0133] When the acoustic mode has no effect on the pressure of the gas 42 in the hollow fiber 28, the center output is 62. In other words, the gas pressure throughout the hollow fiber is essentially flat (at the background gas pressure). The peak 64 on the right shows the output when the acoustic mode has increased the gas pressure at the center of the hollow fiber 28 to 20% higher than the background gas pressure. It can be seen that this increase in gas pressure has increased the center wavelength of the output from approximately 880 nm to approximately 1120 nm.

[0134] Output 60 on the left is the output when the gas pressure in the hollow core 28 of the optical fiber is 20% lower than the background gas pressure. It can be seen that the center wavelength of this output is approximately 660nm.

[0135] Additional outputs 61 and 63 also show that the gas pressure at the center of the optical fiber hollow core 28 is 10% lower and 10% higher than the background pressure.

[0136] As shown in Figure 10, the acoustic mode causes the center wavelength of the output radiation 50 to vary within a wavelength range of approximately 450 nm. As mentioned above, this variation in output occurs at the frequency of the acoustic mode (e.g., in the MHz range).

[0137] As shown in Figure 9, when the acoustic mode provides the maximum gas pressure at the center of the fiber hollow 29, the minimum gas pressure occurs at the periphery of the fiber hollow. The effect of the maximum gas pressure at the center and the minimum gas pressure at the periphery causes the output radiation to be unequal. The pump radiation 46 (see Figure 8) has a Gaussian distribution (or other peak distribution) inside the fiber hollow 28. As a result, the radiation intensity at the center of the fiber hollow 28 is much higher than the radiation intensity at the periphery of the fiber hollow. At least a portion of the radiation spectrum configuration provided by the gas working medium 42 is a nonlinear process driven by the pump radiation. As a result, the output radiation 50 provided by the radiation source 40 depends on the gas pressure near the center of the fiber hollow 28 and is not significantly affected by the gas pressure at the periphery of the fiber hollow.

[0138] Referring again to Figure 8, the controller 44 provides a modulated output signal that causes the transducer 26 to vibrate at the frequency that generates the fundamental acoustic mode. That is, the frequency at which the transducer is modulated corresponds to the frequency of the fundamental acoustic mode. The controller 44 also provides a signal to an acousto-optic modulator 57 disposed at the output of the radiation source 40. The acousto-optic modulator 57 is configured to selectively transmit or block the output radiation beam 50 according to the signal received from the controller 44. The signal provided by the controller 44 is synchronized with the signal driving the transducer 26. The acousto-optic modulator 57 is used to select the spectrum of the output radiation 50, a function achieved by the controller 44 (or other electronic components) adjusting the phase of the signal driving the acousto-optic modulator 57 (relative to the signal driving the transducer 26). In this way, by using the acousto-optic modulator 57, it is possible to select any of the outputs 60-64 shown in Figure 10.

[0139] In another embodiment, an acousto-optic modulator 59 may be positioned between the pump radiation source 41 and the optical fiber 24. The acousto-optic modulator 59 may receive an input signal from a controller 44 synchronized with the signal driving the transducer 26. The phase of the signal supplied to the acousto-optic modulator 59 may be selected such that when the acoustic mode within the hollow core 28 of the optical fiber provides the desired output radiation spectrum, the pump radiation passes only through the acousto-optic modulator and enters the optical fiber 24. This type of embodiment may result in a longer lifespan for the optical fiber 24.

[0140] Typically, a modulation optical switch may be used to select the output from radiation source 40. The modulation optical switch may be an acousto-optic modulator, an electro-optic modulator, or other form of modulator. The modulation optical switch may be located at the output of radiation source 40 or positioned to block the pump radiation beam 46. Although Figure 8 shows two modulation optical switches, only one is needed in practice.

[0141] In an alternative configuration (not shown), a filter may be used to select a desired wavelength or wavelength range from the output radiation beam 50. This filter may be adjustable (e.g., a rotatable reflective grating).

[0142] In another alternative configuration, the pump source 41 may include an optical switch that forms part of the pump source, and the optical switch is synchronized with the modulation of the transducer 26.

[0143] Typically, the pulsed pumped radiation beam 46 may be synchronized with the modulation of the transducer.

[0144] As further described above, the frequency of the acoustic mode of the hollow capillary may be in the MHz range. In such a case, the signal provided by the controller 44 is in the MHz range. The repetition frequency of the pulsed pump beam 46 provided by the pump radiation source 41 may be much higher (e.g., tens of MHz or higher). As a result, multiple pump radiation pulses may pass through the hollow fiber 28 within a single turn-on cycle of the acousto-optic modulators 57, 59.

[0145] Figure 11 is a graph showing the time-averaged relative intensity of the output radiation beam 50 when the acoustic mode amplitude is selected to provide a wavelength range of 100 nm (peak-to-peak). As can be seen from Figure 11, this time-averaged spectrum is described as an arcsine distribution. Because the gas pressure changes more slowly near its extreme values, the output spectrum remains at the extreme pressure values ​​for a longer period. That is, more radiation occurs near the extreme values ​​of the modulation band. It can be understood from Figure 11 that if a specific wavelength is desired, the acoustic modulation amplitude can be selected such that the wavelength appears at the maximum or minimum value of the acoustic mode.

[0146] In Figures 7 and 8, a pair of transducers 26 are disposed on opposite sides of an optical fiber 24. The transducers 26 may be piezoelectric transducers. The transducers 26 may be fixed to the optical fiber 24 (e.g., by adhesive bonding). The transducers 26 may be pressed onto the optical fiber 24 by a clamp (not shown).

[0147] In another configuration, a transducer (e.g., a piezoelectric transducer) may be deposited on optical fiber 24. The transducer may surround the optical fiber.

[0148] Typically, the transducer may be arranged such that it causes at least a portion of the optical fiber 24 to contract and expand (along a direction perpendicular to the fiber's axis). The contraction and expansion excite acoustic modes of the gas within the optical fiber 24. The contraction and expansion may overlap with mechanical modes of the gas inside the optical fiber, such as drum-shaped modes.

[0149] The fundamental acoustic mode of fiber 24 described above is excited. Other acoustic modes may also be excited. When a higher order of magnitude acoustic mode is excited, the pressure distribution of that acoustic mode becomes more complex. As a result, when using an optical switch, the spectrum of the output radiation 50 may be broader and / or less tunable. A wavelength-selectable filter may be used to select the desired wavelength or wavelength range.

[0150] According to another embodiment of the present invention, FIG12 shows a cross-sectional view of a radiation spectral configuration system 122. The radiation spectral configuration system includes an optical fiber 124 comprising: a hollow core 128, a cladding portion 130 surrounding the hollow core, and an anti-resonant structure 180. The anti-resonant structure may also be referred to as an anti-resonant optical fiber. Radiation propagates through the hollow core 128 in a guided mode. This guided radiation may be in the form of a Gaussian distribution. Transducers 126 are disposed on opposite sides of the optical fiber 124. The transducers are configured to excite an acoustic mode of a gas disposed within the hollow core 128. The radiation spectral configuration system may form part of a radiation source (e.g., formed as shown in FIG8).

[0151] Figure 13 schematically shows a cross-sectional view of a radiation spectral configuration system 222 according to an alternative embodiment of the present invention. In this embodiment, a storage chamber 252 is formed by a housing 253. A gaseous working medium 242 is disposed inside the storage chamber. Windows (not shown) are located at both ends of the storage chamber. Five glass rods 270 extend through the storage chamber 252. The spacing between the glass rods may be on the order of micrometers.

[0152] Five acoustic transducers are disposed inside storage chamber 252. To avoid complicating the diagram, not all glass rods and transducers are shown. An acoustic transducer 226 is disposed between any opposing pair of rods 270. In other embodiments, the number of transducers may differ from the number of glass rods. For example, the number of transducers may be less than the number of rods. A single transducer may be inefficient. Two or more transducers may provide efficient modulation of gas 242.

[0153] The radiation spectrum configuration system 222 may form part of a radiation source (e.g., as shown in Figure 8).

[0154] In use, the pump radiation beam is coupled into storage chamber 252 and guided through the storage chamber by glass rod 270. This glass rod 280 thus forms a hollow optical waveguide. An example of this configuration is described in more detail in APL Photonics 6, 061301 (2021). Acoustic transducer 226 is modulated at a selected frequency to generate acoustic modes within the portion of storage chamber 252 surrounded by glass rod 270.

[0155] The wavelength selection of the output radiation provided in the embodiment shown in Figure 13 is achieved in the same manner as described in Figure 8 above (e.g., by modulating the pump radiation beam or modulating the output radiation beam).

[0156] The embodiment shown in Figure 13 has the advantage that the storage chamber 152, acting as an acoustic resonator, has a larger cross-section than the hollow fiber core. Therefore, the transducer 226 can excite a wider range of acoustic modes. Furthermore, it is possible to excite acoustic modes with a relatively uniform cross-section through which the pump radiation beam passes. This may provide a relatively uniform pressure distribution (which varies with acoustic frequency) and potentially a more uniform output radiation spectrum.

[0157] Although five optical rods 270 and five transducers 226 are shown in Figure 13, other numbers of rods may be used. For example, four optical rods may be used, or five, six, seven, or eight optical rods may be used. Using more than eight optical rods may generate more complex acoustic patterns and may therefore reduce the cross-section of the gas 242 at the desired pressure.

[0158] The embodiments described in this invention relate to pulsed pump radiation sources. However, the pump radiation source does not necessarily have to be pulsed. A continuous beam of radiation may be used. The radiation source may be a pulsed laser, a continuous-wave laser, a plasma source, etc. Pulsed radiation sources may be preferred because they provide a higher peak radiation intensity.

[0159] The acoustic modes excited by one or more transducers may be transverse acoustic modes.

[0160] One or more transducers may be the same length as the optical waveguide. One or more transducers may be shorter than the optical waveguide.

[0161] Embodiments of the present invention may advantageously provide a radiation source capable of rapidly switching between desired wavelengths (e.g., by using one or more automatic filters positioned at the output of the radiation source).

[0162] Further embodiments are disclosed in the following numbered clauses: 1. A radiation spectral configuration system comprising: a hollow optical waveguide containing a gaseous medium; and one or more transducers configured to generate acoustic modes in the gaseous medium. 2. The radiation spectral configuration system of claim 1, wherein the hollow optical waveguide is an optical fiber with a hollow core. 3. The radiation spectral configuration system of claim 2, wherein the one or more transducers comprise a pair of transducers provided on opposite sides of the optical fiber. 4. The radiation spectral configuration system of claim 2, wherein the one or more transducers comprise transducers surrounding the optical fiber. 5. The radiation spectral configuration system of claims 2 and 3, wherein one or more anti-resonant structures are provided on the outer surface of the optical fiber. 6. The radiation spectral configuration system of claim 1, wherein the hollow waveguide is formed of a plurality of glass rods located within a gas storage chamber, and wherein the one or more transducers comprise a plurality of transducers also located within the gas storage chamber. 7. The radiation spectral configuration system of claim 6, wherein at least four glass rods are provided. 8. The radiation spectral configuration system according to items 6 and 7, wherein transducers are located between pairs of rods. 9. A radiation source comprising: the radiation spectral configuration system according to any of the preceding items; and further comprising: a pump radiation source configured to provide pump radiation into the hollow optical waveguide. 10. The radiation source according to item 9, further comprising: an optical switch configured to selectively transmit an output radiation beam provided from the radiation spectral configuration system, the optical switch being synchronized with the one or more transducers. 11. The radiation source according to item 9, further comprising: an optical switch configured to selectively transmit a pump radiation beam, the optical switch being synchronized with the one or more transducers. 12. A measurement tool comprising the radiation source according to any of items 9 to 11. 13. A photolithography apparatus comprising the radiation source according to any of items 9 to 11 or the measurement tool according to item 12. 14. A method comprising: directing a pump radiation beam into a hollow optical waveguide, the hollow optical waveguide comprising a gas medium configured to provide an output beam having a different wavelength from the pump radiation beam; the method further comprising: generating an acoustic mode in the gas medium using one or more transducers, and thus modifying the wavelength of the output beam. 15. The method of claim 14, further comprising: using an optical switch synchronized with one or more transducers to select a desired output wavelength. 16. The method of claim 15, wherein the optical switch is located between a source of the pump radiation beam and the hollow optical waveguide. 17. The method of claim 15, wherein the optical switch is located at the output of the hollow optical waveguide.

[0163] While this article may specifically cite applications of photolithography equipment in IC manufacturing, it should be understood that the photolithography equipment described herein may also have other applications. Other possible applications include: the manufacturing of integrated optical systems, the guiding and detection of patterns in magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0164] While specific embodiments of the invention may be referenced herein in the section concerning lithography apparatus, these embodiments may also be used in other devices. Embodiments of the invention may form part of mask inspection equipment, metrology tools, or any apparatus for measuring or processing objects such as wafers (or other substrates), masks (or other patterning apparatus). These apparatuses may be commonly referred to as lithography tools. Such lithography tools may be used in vacuum or atmospheric pressure (non-vacuum) environments.

[0165] Although specific embodiments of the invention have been described above, it should be understood that the invention may be practiced in ways other than those described. The above description is intended to be illustrative and not limiting. Therefore, those skilled in the art will understand that modifications may be made to the invention without departing from the scope of the claims set forth below.

[0166] Although this document specifically refers to "measuring apparatus / tool / system" or "inspection apparatus / tool / system," these terms may refer to tools, apparatus, or systems of the same or similar type. For example, an inspection or measuring apparatus including embodiments of the present invention may be used to determine the characteristics of a structure on a substrate or wafer. For example, an inspection or measuring apparatus, including embodiments of the present invention, may be used to detect substrate defects, structural defects on a substrate, or on a wafer. In such embodiments, the target characteristics of a structure on a substrate may be related to structural defects, missing portions of a structure, or the presence of undesired structures on the substrate or wafer.

Claims

1. A radiation spectrum configuration system, comprising: Hollow optical waveguide containing a gaseous medium; and one or more transducers configured to generate acoustic modes in the gaseous medium.

2. The radiation spectrum configuration system according to claim 1, wherein the hollow optical waveguide is an optical fiber with a hollow core.

3. The radiation spectral configuration system of claim 2, wherein the one or more transducers comprise a pair of transducers provided on opposite sides of the optical fiber.

4. The radiation spectral configuration system of claim 2, wherein the one or more transducers comprise transducers surrounding the optical fiber.

5. The radiation spectral configuration system according to claim 2 or claim 3, wherein one or more anti-resonant structures are provided on the outer surface of the optical fiber.

6. The radiation spectral configuration system of claim 1, wherein the hollow waveguide is formed of a plurality of glass rods located within a gas storage chamber, and wherein the one or more transducers comprise a plurality of transducers also located within the gas storage chamber.

7. The radiation spectral configuration system of claim 7, wherein at least four glass rods are provided.

8. The radiation spectral configuration system according to claim 6 or claim 7, wherein the transducers are located between pairs of rods.

9. A radiation source, comprising: The radiation spectrum configuration system according to any of the preceding claims; It also includes a pump radiation source configured to provide pump radiation into the hollow optical waveguide.

10. The radiation source according to claim 9, further comprising: An optical switch is configured to selectively transmit an output radiation beam provided from the radiation spectral configuration system, the optical switch being synchronized with the one or more transducers.

11. The radiation source according to claim 9, further comprising: An optical switch is configured to selectively transmit the pump radiation beam, and the optical switch is synchronized with the one or more transducers.

12. A measuring instrument comprising a radiation source according to any one of claims 9 to 11.

13. A photolithography apparatus, comprising: The radiation source according to any one of claims 9 to 11 or the measuring tool according to claim 12.

14. A method comprising: A pump radiation beam is directed into a hollow optical waveguide containing a gas medium configured to provide an output beam with a different wavelength from the pump radiation beam. The method further includes: using one or more transducers to generate an acoustic pattern in the gas medium, and thereby modifying the wavelength of the output beam.

15. The method of claim 14, further comprising: An optical switch synchronized with the one or more transducers is used to select the desired output wavelength.

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