Method and apparatus for spatially filtering optical pulses

Abstract

The optical filter device comprises a light diverging device operable to receive light pulses and spatially distribute the light pulses on an optical surface according to the pulse energy of each light pulse, and a spatial filter disposed on the optical surface and operable to apply spatial filtering to the light pulses based on the position of each light pulse on the optical surface resulting from the spatial distribution.

Description

[Technical Field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to U.S. Application No. 63 / 145,736, filed February 4, 2021, and European Application No. 21165646.7, filed March 29, 2021, which are incorporated herein by reference in their entireties.

[0002] [Technical field] The present invention relates to methods and apparatus for spatially filtering optical pulses, and in particular to such methods and apparatus in connection with noise reduction applications in pulsed radiation sources. [Background technology]

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern (also called a "design layout" or "design") in a patterning device (e.g., mask) onto a layer of radiation-sensitive material (resist) provided on the substrate (e.g., wafer).

[0004] Lithography systems can use electromagnetic radiation to project patterns onto a substrate. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Lithography systems using extreme ultraviolet (EUV) radiation with wavelengths in the 4-20 nm range, e.g., 6.7 nm or 13.5 nm, can be used to form smaller features on a substrate than lithography systems using, for example, 193 nm radiation.

[0005] Low-k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of the lithography tool. In such processes, the resolution equation can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the lithography tool's projection optics, CD is the "critical dimension" (usually the smallest feature size to be printed, in this case the half pitch), and k1 is an empirical resolution factor. Generally, the smaller k1, the more difficult it is to reproduce on the substrate a pattern similar to the shape and dimensions planned by the circuit designer to achieve a specific electrical function and performance. To overcome these difficulties, advanced fine-tuning steps can be applied to the lithography projection tool and / or the design layout. These include, but are not limited to, optimizing the NA, customized illumination schemes, the use of phase-shifting patterning devices, various optimizations of the design layout such as optical proximity correction (OPC), also known as "optical and process correction," or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, tight control loops for controlling the stability of the lithographic apparatus can be used to improve pattern reproduction at low k1. Summary of the Invention [Problem to be solved by the invention]

[0006] Metrology tools are used in many aspects of the IC manufacturing process, for example, as alignment tools to properly position the substrate before exposure, as leveling tools to measure the surface topology of the substrate, as focus control and scatterometry-based tools to inspect / measure the exposed and / or etched product in process control. In all cases, a radiation source is required. For various reasons, including measurement robustness and accuracy, broadband or white light radiation sources are increasingly being used for such metrology applications. It is desirable to improve current equipment for broadband radiation generation. [Means for solving the problem]

[0007] In a first aspect of the present invention, there is provided an optical filter device comprising: a light diverging device operable to receive light pulses and spatially distribute the light pulses on an optical surface according to the pulse energy of each light pulse; and a spatial filter disposed on the optical surface and operable to apply spatial filtering to the light pulses based on the position of each light pulse on the optical surface resulting from the spatial distribution.

[0008] Preferably, the light diverging device comprises a deflecting mirror configured to receive and subsequently reflect the light pulses, the deflecting mirror being further configured to deflect upon each reflection of the light pulse, the magnitude of the deflection being dependent on the pulse energy of each of the light pulses, and the deflection resulting in a spatial distribution of said light pulses on the optical surface.

[0009] More preferably, the light diverging device further comprises an optical delay device configured to apply a delay time to the light pulse after undergoing a first reflection from the deflection mirror and return the light pulse to the deflection mirror for a second reflection by the deflection mirror.

[0010] In a second aspect of the present invention, there is provided a method for spatially filtering optical pulses, the method comprising: spatially distributing a plurality of optical pulses on an optical surface according to a pulse energy of each optical pulse; and spatially filtering the optical pulses based on a position of each optical pulse on the optical surface obtained from the spatial distribution.

[0011] Preferably, spatially distributing the light pulses further comprises first reflecting the light pulses using a deflection mirror, applying a delay time to the light pulses first reflected from said deflection mirror and subsequently returning them to said deflection mirror, and secondly reflecting the delayed light pulses using a deflection mirror to spatially distribute said light pulses, wherein the deflection mirror is deflected upon each reflection of each light pulse, the deflection of the deflection mirror being dependent on the pulse energy of each light pulse.

[0012] In a third aspect of the present invention, there is provided a broadband light source device comprising the optical filter apparatus of the first aspect, the broadband light source device being configured to generate broadband output radiation comprising optical pulses upon receiving pump radiation.

[0013] Another aspect of the present invention is a measurement device including the broadband light source device of the third aspect. [Brief explanation of the drawings]

[0014] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: [Figure 1] 1 depicts a schematic diagram of a lithographic apparatus; [Figure 2] FIG. 1 is a schematic diagram of a lithography cell. [Figure 3] A schematic diagram of overall lithography, illustrating the collaboration between three key technologies to optimize semiconductor manufacturing. [Figure 4] 1 is a schematic diagram of a scatterometry device used as a measurement device, including a radiation source according to an embodiment of the present invention; FIG. [Figure 5] 1 is a schematic diagram of a level sensor device comprising a radiation source according to an embodiment of the present invention; [Figure 6] 1 is a schematic diagram of an alignment sensor apparatus comprising a radiation source according to an embodiment of the present invention; [Figure 7] 1 is a schematic cross-section, in a transverse plane (i.e. perpendicular to the axis of the optical fiber), of a hollow core optical fiber forming part of a radiation source according to an embodiment; [Figure 8] FIG. 1 is a schematic diagram of a radiation source for providing broadband output radiation in accordance with an embodiment; [Figure 9] 9(a) and 9(b) are schematic diagrams illustrating cross-sections of examples of hollow-core photonic crystal fiber (HC-PCF) designs for supercontinuum generation. [Figure 10] 1 is a plot showing measured APR as a function of laser repetition rate. [Figure 11] 11(a) and 11(b) are two example plots showing simulated pulse amplitudes before and after application of the optical filter device, respectively. [Figure 12] 1 is a diagram illustrating the operation principle of an optical filter device. [Figure 13] 1 is a plot showing the simulated time-dependent deflection of a deflecting mirror (e.g., cantilever) caused by single reflections of three light pulses, each having a different pulse energy. [Figure 14] 1 is a diagram illustrating an embodiment of an optical filter device. [Figure 15] 14 is a plot showing the time-dependent deflection of a simulated deflecting mirror (e.g., cantilever) induced by two reflections of two successive light pulses used in the simulation shown in FIG. 13. [Figure 16] FIG. 1 shows a block diagram of a computer system for controlling a broadband radiation source. DETAILED DESCRIPTION OF THE INVENTION

[0015] As used herein, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365, 248, 193, 157, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having wavelengths in the range of about 5 to 100 nm).

[0016] The terms "reticle," "mask," or "patterning device," as used herein, may be broadly interpreted to refer to a general patterning device that can be used to impart an incident radiation beam with a patterned cross-section that corresponds to the pattern to be created on a target portion of a substrate. The term "light valve" can also be used in this context. In addition to standard masks (transmissive or reflective; binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0017] 1 schematically depicts a lithographic apparatus LA comprising: an illumination system (also called an illuminator) IL configured to condition a radiation beam B (e.g. UV, DUV or EUV radiation), a mask support (e.g. mask table) MT constructed to support a patterning device (e.g. mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. wafer table) WT constructed to hold a substrate (e.g. resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0018] In operation, the illumination system IL receives a beam from the radiation source SO, for example via the beam delivery system BD. The illumination system IL may include various optical elements, or any combination of these, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical elements, for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0019] The term "projection system" PS as used herein should be interpreted broadly as covering any type of projection system including, for example, refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate to the exposure radiation used and / or other factors such as the use of an immersion liquid or a vacuum. Where the term "projection lens" is used herein, this may be considered as synonymous with the more general term "projection system" PS.

[0020] Lithographic apparatus LA may be of a type in which at least a portion of the substrate is covered by a liquid having a relatively high refractive index (e.g. water) so as to fill a gap between the projection system PS and the substrate W. This is also known as immersion lithography. Further details about immersion techniques are found in U.S. Patent No. 6,952,253, which is incorporated herein by reference.

[0021] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also called "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps in preparation for subsequent exposure of the substrate W may be performed on a substrate W located on one of the substrate supports WT, while a substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0022] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor can be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage can hold multiple sensors. The cleaning device can be configured to clean part of the lithographic apparatus, for example part of the projection system PS or part of a system for providing immersion liquid. When the substrate support WT is distant from the projection system PS, the measurement stage can be moved below the projection system PS.

[0023] In operation, a radiation beam B is incident on a patterning device, for example a mask MA, which is held on a mask support MT, and is patterned according to a pattern (design layout) present on the patterning device MA. After passing the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be precisely moved to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, a first positioner PM and possibly further position sensors (not explicitly shown in FIG. 1 ) can be used to precisely position the patterning device MA with respect to the path of the radiation beam B. The patterning device MA and substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may also be located in spaces between the target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are known as scribe-lane alignment marks.

[0024] As shown in FIG. 2, the lithography apparatus LA may form part of a lithography cell LC, sometimes referred to as a lithocell or (litho)cluster, which often also includes apparatus for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH, and a bake plate BK, e.g., for adjusting the temperature of the substrate W or for adjusting the solvent of the resist layer. A substrate handler or robot RO retrieves substrates W from input / output ports I / O1, I / O2, moves them between different process tools, and delivers them to a loading bay LB of the lithography apparatus LA. The apparatuses of the lithocell (often collectively referred to as tracks) are typically under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which may control the lithography apparatus LA, e.g., via a lithography control unit LACU.

[0025] In order to accurately and consistently expose substrates W exposed by lithography apparatus LA, it is desirable to inspect the substrates to measure characteristics of the pattern features, such as overlay error between subsequent layers, line width, critical dimension (CD), etc. For this purpose, an inspection tool (not shown) may be included in lithocell LC. If an error is detected, for example, the exposure of subsequent substrates or other processing steps performed on substrate W may be adjusted, particularly if inspection is done before other substrates W of the same batch or lot are yet to be exposed or processed.

[0026] Inspection apparatus, sometimes referred to as metrology apparatus, are used to determine the properties of substrates W, particularly how properties vary across different substrates W, or how properties associated with different layers of the same substrate W vary from layer to layer. Alternatively, the inspection apparatus may be configured to identify defects on the substrate W and may, for example, be part of a lithocell LC, integrated into a lithography apparatus LA, or may be a stand-alone apparatus. The inspection apparatus may measure properties of a latent image (an image of a resist layer after exposure), a semi-latent image (an image of a resist layer after a post-exposure bake step PEB), a developed resist image (an image in which exposed or unexposed portions of the resist have been removed), or an etched image (after a pattern transfer step such as etching).

[0027] Typically, the patterning process in a lithography tool (LA) is one of the most critical steps in processing, requiring high accuracy in the dimensioning and placement of structures on a substrate (W). To ensure this high accuracy, three systems can be combined in a so-called "holistic" controlled environment, as shown schematically in Figure 3. One of these systems is the lithography tool (LA), which is (virtually) connected to a metrology tool (MT) (second system) and a computer system (CL) (third system). The key to such a "holistic" environment is optimizing the interaction between these three systems to enforce the overall process window and provide a strict control loop that ensures that the patterning performed by the lithography tool (LA) stays within the process window. The process window defines the set of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device), typically the range within which the process parameters of the lithography or patterning process are allowed to vary.

[0028] The computer system CL can use (parts of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings will achieve the largest overall process window for the patterning process (shown in FIG. 3 by the double arrow at the first scale SC1). Typically, the resolution enhancement techniques are configured to match the patterning capabilities of the lithography apparatus LA. The computer system CL may also be used to detect where within the process window the lithography apparatus LA is currently operating (e.g., using input from the metrology tool MT) (shown in FIG. 3 by the arrow pointing to "0" at the second scale SC2), for example, to predict whether defects will be present due to suboptimal processing.

[0029] The metrology tool MT can provide input to the computer system CL to enable accurate simulations and predictions, and can provide feedback to the lithographic apparatus LA to identify possible drifts in, for example, the calibration state of the lithographic apparatus LA (indicated by multiple arrows at the third scale SC3 in Figure 3).

[0030] In lithography processes, it is frequently desirable to perform measurements of the structures created, e.g., for process control and verification. Tools for performing such measurements are typically referred to as metrology tools. Various types of metrology tools are known for performing such measurements, including scanning electron microscopes or various forms of scatterometers. Scatterometers are versatile instruments that enable the measurement of parameters of the lithography process by having a sensor in the pupil or at a plane conjugate to the pupil of the scatterometer objective (typically referred to as pupil-based measurements) or in the image plane or at a plane conjugate to the image plane (in which case the measurements are typically referred to as image- or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164A, which are incorporated herein by reference in their entireties. The scatterometers described above are capable of measuring gratings using light from soft x-rays and light in the visible to near-infrared wavelength range.

[0031] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the diffraction grating. Such a reconstruction can result, for example, from simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0032] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed towards a target, and reflected or scattered radiation from the target is directed to a spectrometer detector that measures the spectrum of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile of the target giving rise to the detected spectrum can be reconstructed, for example by rigorous coupled-wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

[0033] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows for determining parameters of the lithography process by measuring scattered radiation of each polarization state. Such metrology tools emit polarized light (such as linearly, circularly, or elliptically), for example, by using appropriate polarizing filters in the illumination section of the metrology tool. A radiation source suitable for the metrology tool may also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. patent application Ser. Nos. 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated herein by reference in their entireties.

[0034] In one embodiment of the scatterometer MT, the scatterometer MT is configured to measure the overlay of two misaligned gratings or periodic structures by measuring the asymmetry of the reflectance spectra and / or the detection configuration. The asymmetry is related to the degree of overlay. The two (typically overlapping) grating structures may be applied to two different layers (not necessarily consecutive layers) and may be formed at substantially the same location on the wafer. The scatterometer can have a symmetric detection configuration to clearly distinguish asymmetries, as described, for example, in co-owned patent application EP 1,628,164 A. This allows for simple measurement of diffraction grating misalignment. Further examples for measuring overlay errors between two layers containing periodic structures as targets measured via the asymmetry of the periodic structures can be found in International Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application No. US 20160161863, which are incorporated herein by reference in their entireties.

[0035] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by a scatterometer (or alternatively by a scanning electron microscope), as described in U.S. Patent Application No. US2011-0249244, the entire contents of which are incorporated herein by reference. A single structure may be used, which has a unique combination of critical dimension and sidewall angle measurements for each point in a focus-energy matrix (FEM, also called a focus-exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​may be uniquely determined from these measurements.

[0036] Metrology targets may be collections of composite gratings, often formed in resist by lithography processes, but also formed after, for example, etching processes. Typically, the pitch and linewidth of structures in the gratings strongly depend on the measurement optics (particularly the NA of the optics) capable of capturing the diffraction orders coming from the metrology target. As previously indicated, the diffracted signal may be used to determine the shift between two layers (also referred to as "overlay") or to reconstruct at least a portion of the original grating produced by the lithography process. This reconstruction may be used to provide an indication of the quality of the lithography process and to control at least a portion of the lithography process. The target may have smaller subsegments configured to mimic the dimensions of the features of the design layout within the target. Due to this subsegmentation, the target will behave more similarly to the features of the design layout, such that measurements of the overall process parameters closely resemble the features of the design layout. The target may be measured in underfill or overfill mode. In underfill mode, the measurement beam creates a spot that is smaller than the entire target. In overfill mode, the measurement beam creates a spot that is larger than the entire target. In such overfill mode, it may be possible to simultaneously measure different targets, and thus simultaneously determine different process parameters.

[0037] The overall quality of a measurement of a lithography parameter using a particular target is determined at least in part by the measurement recipe used to measure that lithography parameter. The term "substrate measurement recipe" may include one or more parameters of the measurement itself, or may be used to measure one or more parameters of the measured lithography parameter. The measurement recipe may include one or more parameters of the pattern, or may include both. For example, if the measurement used in a substrate measurement recipe is a diffraction-based optical measurement, the one or more parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement recipe may be, for example, the sensitivity of one measurement parameter to process variations. More examples are described in U.S. Patent Application No. US2016-0161863 and published U.S. Patent Application No. US2016 / 0370717, which are incorporated by reference in their entireties.

[0038] A metrology device such as a scatterometer is shown in Figure 4. It comprises a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. Reflected or scattered radiation is passed to a spectrometer detector 4, which measures the spectrum 10 of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile giving rise to the detected spectrum can be reconstructed by a processing unit PU, for example, by rigorous coupled-wave analysis and nonlinear regression, or by comparison with a library of simulated spectra, as shown in the bottom of Figure 3. Typically, in reconstruction, the general form of the structure is known, and some parameters are inferred from knowledge of the process by which the structure was created, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0039] The overall measurement quality of a lithography parameter from a metrology target measurement is determined, at least in part, by the measurement recipe used to measure the lithography parameter. The term “substrate measurement recipe” can include one or more parameters of the measurement itself, one or more parameters of one or more measured patterns, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, the one or more parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the direction of the radiation relative to the patterns on the substrate, etc. One criterion for selecting a measurement recipe may be, for example, the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. Patent Application No. 2016 / 0161863 and published U.S. Patent Application No. 2016 / 0370717A1, which are incorporated by reference herein in their entireties.

[0040] Another type of metrology tool used in IC manufacturing is a topography measurement system, level sensor, or height sensor. Such tools can be integrated into lithography apparatus to measure the topography of the top surface of a substrate (or wafer). A map of the substrate's topography, also referred to as a height map, can be generated from these measurements, indicating the height of the substrate as a function of position on the substrate. This height map can then be used to correct the position of the substrate during transfer of a pattern to the substrate to provide an aerial image of the patterning device at the correct focus on the substrate. In this context, it will be understood that "height" generally refers to the out-of-plane dimension (also referred to as the Z-axis) relative to the substrate. Typically, a level or height sensor performs measurements at a fixed location (relative to its optics), and relative movement between the substrate and the optics of the level or height sensor results in height measurements at each location across the substrate.

[0041] An example of a level or height sensor LS known in the art is shown schematically in FIG. 5. This figure illustrates only the principle of operation. In this example, the level sensor comprises an optical system including a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO that provides a radiation beam LSB imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband light source such as a supercontinuum light source, polarized or unpolarized such as a polarized or unpolarized laser beam, pulsed or continuous. The radiation source LSO may also include multiple 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 alternatively include UV and / or IR radiation and any range of wavelengths suitable for reflection from the surface of the substrate.

[0042] The projection grating PGR is a periodic grating with a periodic structure that results in a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed towards a measurement location MLO on the substrate W at an angle of incidence ANG of between 0 and 90 degrees, typically 70 to 80 degrees, relative to an axis normal to the incident substrate surface (the Z-axis). At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0043] To determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grid DGR, a detector DET and a processing unit (not shown) for processing the output signal of the detector DET. The detection grid DGR may be identical to the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, e.g., indicative of the intensity of the received light, as in a photodetector, or representing the spatial distribution of the received intensity, as in a camera. The detector DET may comprise any combination of one or more detector types.

[0044] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends, inter alia, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0045] The projection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0046] In one embodiment, the detection grating DGR may be omitted and the detector DET may be placed at the location of the detection grating DGR, such a configuration providing a more direct detection of the image of the projection grating PGR.

[0047] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots that cover a larger measurement range.

[0048] Various height sensors of the general type are disclosed, for example, in U.S. Patent Nos. 7,265,364 and 7,646,471, which are incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in U.S. Patent Application Publication No. 2010233600A1, which is incorporated by reference. WO 2016102127A1, which is incorporated by reference, describes a compact height sensor that uses a multi-element detector to detect and recognize the position of a grating image without the need for a detection grating.

[0049] Another type of metrology tool used in IC manufacturing is an alignment sensor. Therefore, an important aspect of the performance of a lithographic apparatus is the ability to correctly and accurately position an applied pattern with respect to features defined in a previous layer (by the same or a different lithographic apparatus). To this end, one or more sets of marks or targets are provided on the substrate. Each mark is a structure whose position can be subsequently measured by a position sensor, typically an optical position sensor. The position sensor is sometimes called an "alignment sensor" and the marks are sometimes called "alignment marks."

[0050] A lithographic apparatus can include one or more (e.g., multiple) alignment sensors to accurately measure the position of alignment marks provided on a substrate. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatuses is based on a self-referencing interferometer, as described in U.S. Patent No. 6,961,115. Various extensions and modifications of position sensors have been developed, as disclosed, for example, in U.S. Patent Publication No. 2015261097A1. The contents of all these publications are incorporated herein by reference.

[0051] 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, for example as described in U.S. Patent No. 6,961,116, which is incorporated herein by reference. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is diverted by diverting optics as an illumination spot SP onto a mark, such as a mark AM, positioned on a substrate W. In this example, the diverting optics includes a spot mirror SM and an objective lens OL. The illumination spot SP illuminating the mark AM may have a diameter slightly smaller than the width of the mark itself.

[0052] Radiation diffracted by the alignment mark AM is collimated (in this example via an objective lens OL) into an information-bearing beam IB. The term "diffracted" is intended to include zero-order diffraction (which may be referred to as reflection) from the mark. A self-referencing interferometer SRI, for example of the type disclosed in the above-mentioned U.S. Patent No. 6,961,116, causes the beam IB to interfere with itself, which is then received by a photodetector PD. If more than one wavelength is produced by the radiation source RSO, additional optics (not shown) may be included to provide separate beams. The photodetector may be a single element, or may include many pixels, if desired. The photodetector may include a sensor array.

[0053] Guiding optics, which in this example include a spot mirror SM, may function to block zero-order radiation reflected from the mark, so that the information-bearing beam IB contains only higher-order diffracted radiation from the mark AM (this is not essential for the measurement, but improves the signal-to-noise ratio).

[0054] The intensity signal SI is fed to a processing unit PU. The combination of optical processing in block SRI and computational processing in unit PU outputs values ​​for the X and Y position on the substrate relative to a reference frame.

[0055] A single measurement of the type shown only fixes the position of the mark within a certain range corresponding to one pitch of the mark. to identify the period of the sine wave that contains this mark location. The same process can be repeated at different wavelengths and at coarse and / or fine levels for improved accuracy and / or robust detection of the mark, regardless of the material the mark is made of or the material provided below and / or above the mark. Wavelengths can be optically multiplexed and demultiplexed and / or multiplexed by time or frequency division to be processed simultaneously.

[0056] In this example, the alignment sensor and spot SP remain fixed, and it is the substrate W that moves. The alignment sensor can therefore scan the mark AM in a direction substantially opposite to the direction of movement of the substrate W, while being rigidly and precisely mounted to a reference frame. This movement of the substrate W is controlled by the substrate W being mounted on a substrate support and a substrate positioning system controlling 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 one embodiment, one or more (alignment) marks are provided on the substrate support. By measuring the positions of the marks provided on the substrate support, the position of the substrate support as determined by the position sensor can be calibrated (e.g., relative to a frame to which the alignment system is connected). By measuring the positions of the alignment marks provided on the substrate, the position of the substrate can be determined relative to the substrate support.

[0057] Metrology tools, such as the scatterometers, topography measurement systems, or position measurement systems described above, can use radiation generated by a radiation source to perform measurements. The characteristics of the radiation used in a metrology tool can affect the type and quality of measurements that can be performed. In some applications, it can be advantageous to use multiple radiation frequencies to measure a substrate. For example, broadband radiation can be used. Multiple different frequencies may be able to propagate, illuminate, and scatter the metrology target with no or minimal interference with other frequencies. Thus, for example, different frequencies can be used to simultaneously acquire more metrology data. Different radiation frequencies may also be able to probe and discover different characteristics of the metrology target. Broadband radiation can be useful in metrology systems, such as level sensors, alignment mark measurement systems, scatterometry tools, or inspection tools. The broadband radiation source can be a supercontinuum light source.

[0058] High-quality broadband radiation, e.g., supercontinuum radiation, can be difficult to generate. One method for generating broadband radiation is to broaden high-power, narrow-band or single-frequency input or pump radiation, for example, by utilizing nonlinear higher-order effects. The input radiation (which may be generated using a laser) may be referred to as pump radiation. Alternatively, the input radiation may be referred to as seed radiation. To obtain high-power radiation for broadening effects, the radiation can be confined to a small region to achieve highly localized, high-intensity radiation. In these regions, the radiation may interact with broadening structures and / or materials that form a nonlinear medium to generate broadband output radiation. In high-intensity radiation regions, different materials and / or structures can be used to enable and / or improve the broadening of the radiation by providing a suitable nonlinear medium.

[0059] In some embodiments, broadband output radiation is generated in a photonic crystal fiber (PCF). In some embodiments, such a photonic crystal fiber has a microstructure around the fiber core that helps confine radiation traveling through the fiber within the fiber core. The fiber core can be made of a solid material that has nonlinear properties and can generate broadband radiation when high-intensity pump radiation is transmitted through the fiber core. While it is possible to generate broadband radiation in a solid-core photonic crystal fiber, there can be some drawbacks to using solid-state materials. For example, if UV radiation is generated in the solid core, this radiation may be absorbed by most solid-state materials and therefore may not be present in the fiber's output spectrum.

[0060] In some implementations, as further described below with reference to FIG. 8 , methods and apparatus for broadening input radiation may use a fiber to confine the input radiation and to broaden the input radiation to output broadband radiation. The fiber may be a hollow-core fiber and may include an internal structure to achieve effective guiding and confinement of radiation within the fiber. The fiber may be a hollow-core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement, primarily inside the hollow core of the fiber, achieving high radiation intensity. The hollow core of the fiber may be filled with a gas, which serves as a broadening medium for broadening the input radiation. Such a fiber and gas arrangement can be used to create a supercontinuum radiation source. The radiation input to the fiber may be electromagnetic radiation, for example, radiation in one or more of the infrared, visible, UV, and extreme UV spectrums. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light.

[0061] Some embodiments relate to new designs for such broadband radiation, including optical fibers. The optical fiber is a hollow-core photonic crystal fiber (HC-PCF). In particular, the optical fiber can be a type of hollow-core photonic crystal fiber that includes an anti-resonant structure to confine the radiation. Such fibers that include an anti-resonant structure are known in the art as anti-resonant fibers, tubular fibers, single-ring fibers, negative curvature fibers, or inhibited coupling fibers. Various different designs of such fibers are known in the art. Alternatively, the optical fiber can be a photonic bandgap fiber (HC-PBF, e.g., Kagome fiber).

[0062] Many types of HC-PCFs can be engineered, each based on a different physical guiding mechanism. Two such HC-PCFs include hollow-core photonic bandgap fiber (HC-PBF) and hollow-core antiresonantly reflecting fiber (HC-ARF). Details of the design and fabrication of HC-PCFs are described in U.S. Patent US2004 / 015085A1 (for HC-PBF) and International Patent Application WO2017 / 032454A1 (for hollow-core antiresonantly reflecting fiber), which are incorporated herein by reference. Figure 9(a) shows a kagome fiber containing a kagome lattice structure.

[0063] An example of an optical fiber for use in a radiation source will now be described with reference to Figure 7, which shows a schematic cross-section of an optical fiber OF in a transverse plane. Further embodiments similar to the practical example of the fiber of Figure 7 are disclosed in WO 2017 / 032454 A1.

[0064] The optical fiber OF includes an elongated body having one dimension longer than the other two dimensions of the optical fiber OF. This longer dimension may be referred to as the axial direction and define the axis of the optical fiber OF. The other two dimensions define a plane called the transverse plane. Figure 7 shows a cross-section of the optical fiber OF at this transverse plane (i.e., perpendicular to the axis), which is labeled the xy plane. The transverse plane of the optical fiber OF may be substantially constant along the fiber axis.

[0065] It will be understood that the optical fiber OF has some flexibility, and therefore the axial orientation will generally not be uniform along the length of the optical fiber OF. Terms such as optical axis, cross section, etc. will be understood to mean the local optical axis, local cross section, etc. Furthermore, when a component is described as cylindrical or tubular, these terms will be understood to encompass the shape into which the optical fiber OF is distorted when bent.

[0066] It will be appreciated that the optical fiber OF can have any length, and the length of the optical fiber OF can depend on the application. The optical fiber OF can have a length between 1 cm and 10 m, for example, the optical fiber OF can have a length between 10 cm and 100 cm.

[0067] The optical fiber OF comprises a hollow core HC, a cladding portion surrounding the hollow core HC, and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be considered to comprise a body (including the cladding portion and the support portion SP) having the hollow core HC. The cladding portion comprises a plurality of anti-resonant elements for guiding radiation through the hollow core HC. In particular, the plurality of anti-resonant elements are arranged to confine radiation propagating through the optical fiber OF primarily inside the hollow core HC and guide the radiation along the optical fiber OF. The hollow core HC of the optical fiber OF can be located substantially in a central region of the optical fiber OF. The axis of the optical fiber OF can also define the axis of the hollow core HC of the optical fiber OF.

[0068] The cladding portion includes a plurality of anti-resonant elements for guiding radiation propagating through the optical fiber OF. In particular, in this embodiment, the cladding portion includes a single ring of six tubular capillary caps, each of which functions as an anti-resonant element.

[0069] The capillary CAP may also be referred to as a tube. The capillary CAP may be circular in cross section or may have another shape. Each capillary CAP includes a generally cylindrical wall portion WP that at least partially defines a hollow core HC of the optical fiber OF and separates the hollow core HC from a capillary cavity CC. It will be appreciated that the wall portion WP may function as an anti-reflection Fabry-Perot resonator for radiation propagating through the hollow core HC (and incident on the wall portion WP at a grazing incidence angle). The thickness of the wall portion WP may be appropriate to ensure that reflection into the hollow core HC is generally enhanced while transmission into the capillary cavity CC is generally suppressed. In some embodiments, the capillary wall portion WP may have a thickness between 0.01 and 10.0 μm.

[0070] It will be understood that the term cladding portion, as used herein, is intended to mean the part of the optical fiber OF intended to guide the radiation propagating therethrough (i.e. the capillary CAP that confines said radiation within the hollow core HC), where the radiation is confined in the form of transverse modes and propagates along the fiber axis.

[0071] The support portion is generally tubular and supports six capillary caps of the cladding portion, which, in the case of the inner support portion SP, are uniformly distributed around the inner surface, and the six capillary caps can be generally described as arranged in a hexagonal formation.

[0072] The capillary CAPs are arranged such that each capillary does not contact any of the other capillary CAPs. Each of the capillary CAPs contacts the inner support portion SP and is spaced apart from adjacent capillary CAPs in the ring structure. Such an arrangement can be beneficial because it can increase the transmission bandwidth of the optical fiber OF (e.g., compared to an arrangement in which the capillaries are in contact with each other). Alternatively, in some embodiments, each of the capillary CAPs may contact adjacent capillary CAPs in the ring structure.

[0073] The six capillary CAPs of the cladding portion are arranged in a ring structure around the hollow core HC. The inner surfaces of the ring structure of the capillary CAPs at least partially define the hollow core HC of the optical fiber OF. The diameter d of the hollow core HC (which may be defined as the smallest dimension between opposing capillaries and is indicated by arrow d) may be between 10 and 1000 μm. The diameter d of the hollow core HC may affect the mode field diameter, collisional loss, dispersion, modal plurality, and nonlinear characteristics of the hollow-core HC optical fiber OF.

[0074] In this embodiment, the cladding portion contains a single ring arrangement of capillary CAPs (which act as anti-resonant elements), so that any radial line from the center of the hollow core HC to the outside of the optical fiber OF passes through only one capillary CAP.

[0075] It will be appreciated that other embodiments may be provided with different arrangements of anti-resonant elements. These may include arrangements with multiple rings of anti-resonant elements and arrangements with nested anti-resonant elements. FIG. 9(a) shows an embodiment of an HC-PCF with three rings of capillary caps stacked radially on top of each other. In this embodiment, each capillary cap contacts other capillaries, both within the same ring and within different rings. Furthermore, while the embodiment shown in FIG. 7 includes six rings of capillaries, other embodiments may provide one or more rings containing any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) in the cladding portion.

[0076] Figure 9(b) shows a modified embodiment of the above HC-PCF with a single ring tubular capillary. In the example of Figure 9(b), there are two concentric rings of tubular capillary 21. A support tube ST may be included in the HC-PCF to hold the inner and outer rings of tubular capillary 21. The support tube may be made of silica.

[0077] The tubular capillaries of the examples of Figures 7 and 9(a) and (b) may have a circular cross-sectional shape. Other shapes for the tubular capillaries are also possible, such as oval or polygonal cross-sections. Furthermore, the solid material of the tubular capillaries of the examples of Figures 7 and 9(a) and (b) may include a plastic material such as PMA, a glass such as silica, or a soft glass.

[0078] Figure 8 shows a radiation source RDS for providing broadband output radiation. The source RDS comprises a pulsed pump radiation source PRS or other type of radiation source capable of generating short pulses of a desired length and energy level, an optical fiber OF with a hollow core HC (for example of the type shown in Figure 7), and a working medium WM (e.g. a gas) disposed within the hollow core HC. In Figure 8, the source RDS includes the optical fiber OF shown in Figure 7, although in alternative embodiments other types of hollow core HC optical fiber could be used.

[0079] The pulsed pump radiation source PRS is configured to provide input radiation IRD. The hollow core HC of the optical fiber OF is configured to receive the input radiation IRD from the pulsed pump radiation source PRS and broaden it to provide output radiation ORD. The working medium WM enables broadening the frequency range of the received input radiation IRD to provide broadband output radiation ORD.

[0080] The radiation source RDS further includes a reservoir RSV. The optical fiber OF is disposed inside the reservoir RSV. The reservoir RSV may also be referred to as a housing, container, or gas cell. The reservoir RSV is configured to contain a working medium WM. The reservoir RSV may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the working medium WM (which may be a gas) inside the reservoir RSV. The reservoir RSV may include a first transparent window TW1. During use, the optical fiber OF is disposed inside the reservoir RSV such that the first transparent window TW1 is disposed proximate to the input end IE of the optical fiber OF. The first transparent window TW1 may form a portion of a wall of the reservoir RSV. The first transparent window TW1 may be transparent to at least the received input radiation frequency such that the received input radiation IRD (or at least a majority thereof) may be coupled to the optical fiber OF disposed inside the reservoir RSV. It will be appreciated that optics (not shown) may be provided to couple the input radiation IRD into the optical fiber OF.

[0081] The reservoir RSV includes a second transparent window TW2 that forms part of a wall of the reservoir RSV. In use, when the optical fiber OF is disposed inside the reservoir RSV, the second transparent window TW2 is disposed adjacent to the output end OE. The second transparent window TW2 may be transparent to at least the frequencies of the broadband output radiation ORD of the device 120.

[0082] Alternatively, in another embodiment, the two opposing ends of the optical fiber OF may be disposed in different reservoirs. The optical fiber OF may include a first end section configured to receive input radiation IRD and a second end section for outputting broadband output radiation ORD. The first end section may be disposed in a first reservoir containing a working medium WM. The second end section may be disposed inside a second reservoir, which may also contain the working medium WM. The reservoirs may function as described above in connection with FIG. 8. The first reservoir may include a first transparent window configured to be transparent to the input radiation IRD. The second reservoir may include a second transparent window configured to be transparent to the broadband output broadband radiation ORD. The first and second reservoirs may also include sealable openings that allow the optical fiber OF to be disposed partially inside and partially outside the reservoirs, thereby sealing a gas inside the reservoirs. The optical fiber OF can further include an intermediate section that is not contained within a reservoir. Such an arrangement using two separate gas reservoirs is particularly convenient for embodiments in which the optical fiber OF is relatively long (e.g., greater than 1 meter in length). In such a configuration using two separate gas reservoirs, the two reservoirs (which may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the gases in the two reservoirs) can be considered to provide an apparatus for providing the working medium WM within the hollow core HC of the optical fiber OF.

[0083] In this context, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of radiation incident on the window at that frequency is transmitted through the window.

[0084] Both the first transparent window TW1 and the second transparent window TW2 can form an airtight seal within the wall of the reservoir RSV, such that the working medium WM (which can be a gas) can be contained within the reservoir RSV. The gas WM can be contained within the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0085] The working medium (WM) may contain noble gases such as argon, krypton, or xenon; Raman-active gases such as hydrogen, deuterium, or nitrogen; or gas mixtures such as argon / hydrogen, xenon / deuterium, krypton / nitrogen, or nitrogen / hydrogen. Depending on the type of filling gas, nonlinear optical processes may include modulation instability (MI), soliton self-compression, soliton splitting, the Kerr effect, the Raman effect, and dispersive wave generation (DWG), details of which are described in WO 2018 / 127266 A1 and US 9,160,137 B1 (both incorporated herein by reference). The dispersion of the filling gas can be tuned by changing the working medium (WM) pressure (i.e., gas cell pressure) in the reservoir (RSR), thereby adjusting the dynamics and associated spectral broadening characteristics of the generated broadband pulses to optimize frequency conversion.

[0086] In one embodiment, the working medium WM may be disposed within the hollow core HC at least during reception of the input radiation IRD to generate the broadband output radiation ORD. It will be appreciated that the gas WM may be completely or partially absent from the hollow core HC while the optical fiber OF is not receiving the input radiation IRD to generate the broadband output radiation.

[0087] In order to broaden the frequency, high-intensity radiation may be desired. An advantage of having a hollow-core HC optical fiber OF is that high-intensity radiation can be achieved due to the strong spatial confinement of radiation propagating through the optical fiber OF, and high local radiation intensities can be achieved. The radiation intensity within the optical fiber OF may be high, for example, due to high received input radiation intensity and / or due to the strong spatial confinement of radiation within the optical fiber OF. An advantage of hollow-core optical fibers is that they can guide radiation over a wider wavelength range than solid-core optical fibers, and in particular, hollow-core optical fibers can guide radiation in both the ultraviolet and infrared ranges.

[0088] The advantage of using a hollow-core HC optical fiber OF is that most of the radiation guided into the optical fiber OF is confined to the hollow core HC. Therefore, most of the interaction of the radiation within the optical fiber OF is with the working medium WM provided inside the hollow core HC of the optical fiber OF. As a result, the spreading effect of the working medium WM on the radiation can be increased.

[0089] The received input radiation IRD may be electromagnetic radiation. The input radiation IRD may be received as pulsed radiation. For example, the input radiation IRD may comprise ultrafast pulses, for example generated by a laser.

[0090] The input radiation IRD may be coherent radiation. The input radiation IRD may be collimated radiation. The advantage of this may be to facilitate and improve the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD may include a single frequency or a narrow range of frequencies. The input radiation IRD may be generated by a laser. Similarly, the output radiation ORD may be collimated and / or coherent.

[0091] The broadband range of the output radiation ORD may be a continuous range, including a continuous range of radiation frequencies. The output radiation ORD may include supercontinuum radiation. Continuum radiation may be useful in many applications, such as metrology applications. For example, a continuous range of frequencies may be used to probe numerous properties. A continuous range of frequencies may be used, for example, to determine and / or eliminate frequency dependence of a measured property. The supercontinuum output radiation ORD may include electromagnetic radiation spanning a wavelength range of, for example, 100 nm to 4000 nm. The frequency range of the broadband output radiation ORD may be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD may include white light.

[0092] The input radiation IRD provided by the pulsed pump radiation source PRS may be pulsed. The input radiation IRD may include electromagnetic radiation at one or more frequencies between 200 nm and 2 μm. The input radiation IRD may include, for example, electromagnetic radiation having a wavelength of 1.03 μm. The repetition rate of the pulsed radiation 122 may be on the order of 1 kHz to 100 μM. The pulse energy may have a magnitude of 0.1 μJ to 100 μJ, for example, on the order of 1-10 μJ. The pulse duration of the input radiation IRD may be between 10 fs and 10 ps, ​​for example, 300 fs. The average power of the input radiation IRD may be between 100 mW and several hundred W. The average power of the input radiation IRD may be, for example, 20-50 W.

[0093] The pulsed pump radiation source PRS may be a laser. The spatiotemporal transmission characteristics of such laser pulses, such as their spectral amplitude and phase transmitted along the optical fiber OF, can be changed and tuned through adjustment of (pump) laser parameters, working medium WM variations, and optical fiber OF parameters. The spatiotemporal transmission characteristics may include one or more of output power, output mode profile, output temporal profile, width of the output temporal profile (or output pulse width), output spectral profile, and bandwidth of the output spectral profile (or output spectral bandwidth). The pulsed pump radiation source PRS parameters may include one or more of pump wavelength, pump pulse energy, pump pulse width, and pump pulse repetition rate. The optical fiber OF parameters may include one or more of optical fiber length, size and shape of the hollow core HC, size and shape of the capillary, and thickness of the capillary wall surrounding the hollow core HC. The parameters of the working medium WM, such as a fill gas, may include one or more of gas type, gas pressure, and gas temperature.

[0094] The broadband output radiation ORD provided by the radiation source RDS may have an average output power of at least 1 W. The average output power may be at least 5 W. The average output power may be at least 10 W. The broadband output radiation ORD may be a pulsed broadband output radiation ORD. The broadband output radiation ORD may have a power spectral density over the entire wavelength band of the output radiation of at least 0.01 mW / nm. The power spectral density over the entire wavelength band of the broadband output radiation may be at least 3 mW / nm.

[0095] In many applications requiring broadband output radiation ORDs, such as the metrology applications mentioned above, there is growing interest in further reducing the noise of broadband output radiation ORDs. For pulsed laser sources, such as the hollow-core (HC) optical fiber (OF)-based broadband radiation sources mentioned above, the primary source of laser noise is pulse-to-pulse fluctuations in the output radiation. When such pulsed laser sources are used, for example, in wafer alignment applications, laser noise directly impacts the alignment position repeatability (APR) and, in turn, overlay. The higher the laser noise, the higher the APR (and therefore overlay). Because relatively high pulse-to-pulse fluctuations are fundamental to the broadband radiation generation process, there is currently no direct solution to this inherent problem. Therefore, it is necessary to indirectly reduce laser noise downstream of the broadband radiation source.

[0096] For example, in existing methods, a beam of broadband radiation is used to illuminate an alignment mark. The signal beam diffracted from the alignment mark is then split into two sub-signals, each carrying the same laser noise. By controlling the relative phase delay between the two sub-signals so that their phases are completely different (or the phase delay is π), the two sub-signals are added together to form a final signal. Because two sub-signals carrying the same laser noise are out of phase, adding these two sub-signals together cancels them out, leaving only the laser noise. Once determined, the laser noise can be removed from the laser signal. However, these existing methods alone are not sufficient to lower the APR or completely remove the laser noise. This is because imperfections in the measurement (e.g., the two sub-signals are partially out of phase) can result in residual laser noise, which can lead to a high APR.

[0097] Figure 10 is an example plot showing the measured APR as a function of laser repetition rate. As shown in Figure 10, each data point represents the APR measured using a broadband output radiation ORD at a selected repetition rate. More specifically, at each selected repetition rate, a first and second portion of the broadband output radiation ORD is used to illuminate the alignment mark, generating a first alignment signal and a second alignment signal, respectively. The first and second alignment signals are then used to generate a signal difference. The measurement is repeated multiple times to obtain a statistically meaningful set of data. The data set is then used to generate an APR data point for the selected repetition rate. Subtracting one signal from another cancels out the dynamic vibration noise, so the resulting APR is primarily caused by laser noise.

[0098] This figure clearly shows that the APR measured at the lowest repetition rate (i.e., 2.5 MHz) is more than 0.12 nm higher than that measured at the highest repetition rate (i.e., 40 MHz). Therefore, increasing the repetition rate of broadband radiation sources is considered a potential solution for reducing the APR caused by laser noise. However, for many broadband radiation sources, especially those based on hollow-core HC optical fibers (e.g., HC-PCFs), the laser lifetime is inversely proportional to the laser repetition rate. That is, the higher the laser repetition rate, the shorter the laser lifetime. For this reason, HC-PCF-based radiation sources typically operate at relatively low repetition rates, e.g., in the range of 1 MHz to 5 MHz, which causes a relatively high APR (e.g., 0.1 to 0.2 nm) and therefore a high overlay. Therefore, a method that can effectively reduce the laser noise of broadband radiation sources without compromising the laser lifetime is highly desirable.

[0099] Relative intensity noise (RIN) is typically used to describe the degree of pulse-to-pulse variation. RIN is the power noise normalized to the average power level and can be expressed as:

number

[0100] This disclosure proposes methods and apparatus to overcome the aforementioned problems associated with existing methods. The proposed method and apparatus, embodied in the following examples, provide a flexible and effective way to reduce laser noise in broadband radiation sources without affecting the laser's lifetime. This is achieved by applying an optical filter device to pulses emitted from the broadband radiation source, stabilizing the pulse amplitude and thus reducing the degree of pulse-to-pulse amplitude or energy fluctuations. Figures 11(a) and 11(b) are two example plots showing simulated pulse amplitudes before and after application of the optical filter device, respectively. For the simulated pulse amplitude shown in Figure 11(a), the pulse amplitude statistics may follow a particular distribution, e.g., a Poisson distribution. The purpose of applying the optical filter device to the laser pulse is to selectively filter the amplitude distribution so that only a desired portion of the distribution (e.g., a central portion corresponding to pulse amplitudes with a higher occurrence probability) is retained. As is evident from Figure 11(b), after removing the undesired portion, the pulse-to-pulse amplitude or energy fluctuations can be significantly reduced, e.g., by 30% or more.

[0101] It should be noted that the proposed method does not necessarily require that the entire spectrum of the broadband output radiation ORD follows the same statistical distribution. The proposed method is applicable as long as a specific optical wavelength or wavelength range follows a specific statistical distribution, and that distribution has a finite width.

[0102] FIG. 12 schematically illustrates the operating principle of an optical filter device. The optical filter device may include a light diverging device operable to receive light pulses and spatially distribute the light pulses on an optical surface according to the pulse energy of each light pulse; and a spatial filter disposed on the optical surface and operable to apply spatial filtering to the light pulses based on the position of each light pulse on the optical surface resulting from the spatial distribution. Note that the operating principle illustrated schematically in FIG. 12 is based on an embodiment of the optical filter device in which the light diverging device includes a deflecting mirror DM. Such a deflecting mirror DM may be used to reflect a beam of broadband output radiation ORD, for example, toward a spatial filter SF. The beam of broadband output radiation ORD may originate from, for example, an HC-PCF-based broadband radiation source and include a train of light pulses at a specific repetition rate. One end (or fixed end) of the deflecting mirror may be fixed to a fixed point, such as a mirror support, while the other end (or free end) may be freely movable or displaceable within a reflective surface from which the beam of broadband output radiation ORD is reflected. The deflection mirror may be configured to be deflectable or tiltable about a fixed point FP, the angle of deflection or tilt being dependent on the radiation pressure exerted by the pulse of broadband output radiation ORD.

[0103] For example, when no pulse is incident on the deflection mirror DM, the deflection mirror DM may remain at the default deflection position P0. In contrast, when a pulse having a specific pulse energy or amplitude is incident on the deflection mirror DM, it may be deflected by radiation pressure to specific deflection positions, such as the first, second, and third deflection positions P1, P2, and P3 shown in FIG. 12. The three deflection positions P1, P2, and P3 correspond to the first, second, and third deflection angles DA1, DA2, and DA3 formed with respect to the default deflection position P0, respectively, and the first, second, and third pulse energies PE1, PE2, and PE3. The third pulse energy or amplitude PE3 may be higher than the second pulse energy or amplitude PE2, and the second pulse energy or amplitude PE2 may be higher than the first pulse energy or amplitude PE1. Because the deflection angle is proportional to the applied radiation pressure and thus the incident pulse energy, the third deflection angle DA3 can be greater than the second deflection angle DA2, which in turn can be greater than the first deflection angle DA1. Each deflection angle (e.g., the second deflection angle DA2) can be approximated by the corresponding vertical displacement (e.g., d2) of the free edge of the deflection mirror divided by the length L of the deflection mirror DM, i.e., DA2 = d2 / L. Note that in the following description, the deflection of a deflection mirror is evaluated by the vertical displacement of the free edge of the mirror.

[0104] Note that for simplicity, Figure 12 shows only three different scenarios, each corresponding to pulses with different pulse energies. In reality, the pulse train of the broadband output radiation contains multiple pulses with multiple pulse energies. Upon reflection, while still separated in time, some of the reflected pulses (e.g., pulses with pulse energies or amplitudes near the center of the Poisson distribution) at least partially overlap spatially, thus forming a spatially divergent reflected beam DRB. Assuming that the broadband output radiation ORD is sufficiently collimated before interacting with the deflecting mirror DM, the spatial divergence of the reflected beam DRB is primarily caused by mirror reflections at various different reflection angles, and therefore the reflected beam DRB diverges primarily within the reflection plane. As a result, the reflected beam DRB contains an elliptical beam profile whose major axis LA lies within the reflection plane. Other reflected pulses with pulse amplitudes or energies outside the desired range (e.g., pulse amplitudes or energies that are too low or too high) are located around the two edges along the major axis of the spatially divergent reflected beam DRB and, in some cases, are spatially separated from the spatially divergent reflected beam DRB. By spatially filtering out the undesired portions of the spatially divergent reflected beam DRB using the spatial filter SF, the remaining portion of the reflected beam DRB will be composed of pulses with more consistent pulse amplitude or energy, resulting in a lower pulse-to-pulse variation or RIN.

[0105] According to the reference H.-J. Butt et al., Surface Science Reports 59 (2005) 1-152 (incorporated herein by reference), the force exerted by an incident radiation beam is given by:

number

number

[0106] In static analysis, the deflection of the deflecting mirror DM for a particular force is given by:

number

number

[0107] However, here each pulse is processed independently, which requires dynamic processing. The deflection mirrors act as a mass-spring system and therefore follow simple harmonic motion. Therefore, each deflection mirror follows a second-order equation of motion. According to the reference book "Fundamentals of Vibrations" by Leonard Meirovitch (McGraw-Hill, 2001), the second-order equation of motion is given as follows:

number

number

number

number

number

number

[0108] Note that the above equation of motion, i.e., equation [6], and its solution, i.e., equation [7], assume that the applied force is zero. In our case, the force is not zero, and there is a "shock" caused by each incident pulse. The impulse force is known to generate an initial velocity given by:

number

number

number

number

[0109] Therefore, the response of the deflecting mirror DM to a pulse of energy ε is completely determined. Note that the pulse width does not actually affect the response of the deflecting mirror DM (within the approximations used above).

[0110] The above equations, i.e., equations [4] through

[15] , along with the relevant parameters, can be used to simulate a deflection mirror-based mass-spring system. To facilitate the simulation, we can use the parameters of an example cantilever from a commercially available atomic force microscope (AFM). AFM cantilevers have many advantages when used as deflection mirrors. First, AFM cantilevers are a known technology and are commercially available. Second, many different cantilevers with various different parameters (e.g., different stiffness) are already available on the market, enabling a variety of use cases. Third, most, if not all, commercially available AFM cantilevers are already coated with one or more optical coatings to reflect laser light in AFMs, potentially allowing for their direct use. If the existing coating on the AFM cantilever is not suitable for the incident broadband output radiation, a different reflective coating can be applied to the cantilever, and the reflective coating is optimized according to the spectral profile of the incident broadband output radiation.

[0111] By applying the following parameters to the above equation, the response of a deflection mirror (e.g., an AFM cantilever) to an incident pulse of a given energy can be determined. Note that the parameter values ​​shown below are based on an example implementation. Other different parameter values ​​based on different implementations may apply as well. dt = 1e-9; % time step t = (0 : dt : 0.6e-6); % time c = 80e-7; % viscous damping coefficient w = 20e-6; % cantilever width L = 25e-6; % cantilever length thick = 1.25e-6; % cantilever thickness E = 100e9; % Young's modulus of silicon rh = 2329; % silicon density m = 0.2427*w*thick*L*rho; % effective mass of the cantilever z0 = 0e-6; % initial displacement e = 0.5*5e-6; % incident pulse energy

[0112] Figure 13 is a plot showing the simulated time-dependent deflection of a cantilever caused by a single reflection of three optical pulses with different pulse energies. In the simulation, the cantilever material is silicon, and the width, length, and thickness of the cantilever are 20 μm, 25 μm, and 1.25 μm, respectively. The incident angles of all pulses are set to 0 degrees or close to 0 degrees. The deflection of the deflecting mirror is in nanometers, and is the angle of the free end of the deflecting mirror DM relative to the fixed end after each optical pulse. circumference This results in a shift in the direction of the deflection. As shown in Figure 13, the maximum deflection increases with pulse energy. For all three deflection curves, the maximum deflection points M1, M2, and M3 occur approximately 80 ns after the cantilever is irradiated with the pulse of broadband output radiation ORD. How quickly the response decays depends on the cantilever's geometry. In this particular simulation shown in Figure 13, the cantilever requires at least 400 ns to decay the response caused by the incident pulse. In other words, the cantilever deflection returns to zero approximately 400 ns after the pulse of broadband output radiation ORD irradiates the cantilever. A response time of 400 ns corresponds to a pulse repetition rate of 2.5 MHz. Therefore, to ensure that the cantilever deflection caused by the previous pulse has at least substantially returned to zero by the time the next pulse arrives at the cantilever, the pulse repetition rate of the broadband output radiation can be kept below 2.5 MHz. As mentioned previously, this is not a limitation, as higher repetition rates generally result in lower laser noise, making it less necessary for the present invention. Furthermore, the 400 ns value can be altered by changing the environment around the cantilever, causing the decay to occur more quickly.

[0113] It should be understood that pulses of broadband output radiation ORD undergo reflection before significant deflection of the deflecting mirror. This is evident in the simulation data shown in Figure 13, where the maximum cantilever deflection is recorded with a delay time of approximately 80 ns relative to the time at which the pulse is reflected by the cantilever. This delayed response is primarily due to the finite inert mass of the deflecting mirror (e.g., the cantilever)-based mass-spring system, which exhibits an acceleration time much longer than the duration of the incident optical pulse (e.g., the pulse of output radiation generated from an HC-PCF-based broadband radiation source). While transferring momentum to the deflecting mirror, the single pulse essentially reflects off the mirror undeflected. Since a larger mirror deflection allows for better spatial separation of pulses with different pulse energies and, consequently, more effective removal of pulses with unwanted amplitudes or energies, it is desirable to configure the optical filter device OFA so that the reflected pulse, which has already deflected the deflecting mirror DM on the first reflection, is returned to the deflecting mirror DM for a second reflection once the deflecting mirror's deflection reaches its maximum. The redirection of each light pulse for a second reflection on the deflection mirror DM can be achieved, for example, by an optical delay line.

[0114] FIG. 14 schematically illustrates one embodiment of the optical filter device OFA. In this embodiment, the optical filter device OFA may include a deflection mirror DM, an optical delay device ODA, and a spatial filter. A beam of broadband output radiation ORD containing a train of optical pulses may be obliquely incident on the deflection mirror DM. The broadband output radiation ORD may be generated, for example, by a hollow-core optical fiber-based broadband radiation source. Therefore, the values ​​of the relevant laser parameters may fall within the typical parameter ranges described in the above paragraph. It should be noted that the proposed method and device are not limited to reducing laser noise in broadband radiation; they may be equally applicable to reducing laser noise in radiation with a narrow spectral bandwidth.

[0115] The deflecting mirror DM may be any mirror that can be deflected or tilted around a fixed point FP of the deflecting mirror DM upon reflection of an incident laser pulse. The deflection or tilt of the deflecting mirror DM may occur at a reflective surface where the broadband output radiation ORD is reflected by the deflecting mirror DM. In the embodiment of FIG. 14, the deflecting mirror may comprise a cantilever, one end of which is fixed at a fixed point FP, e.g., to a mirror support (not shown), and the other end of which is movable or displaceable. An exemplary cantilever may be made of silicon. The dimensions of the cantilever are the same as those used in the simulation shown in FIG. 13.

[0116] In different embodiments, other types of deflection mirrors DM made of different materials other than silicon may be used. Furthermore, the dimensions of the deflection mirror DM can be flexibly selected to meet the needs of various applications. In some embodiments, the deflection mirror may include one or both of a width and a length in the range of 1 μm to 1000 μm, 1 μm to 500 μm, 1 μm to 100 μm, or 1 μm to 10 μm. And, a thickness in the range of 1 μm to 5 μm, 1 μm to 10 μm, or 1 μm to 100 μm. In some embodiments, the deflection mirror DM may include one or more reflective coatings configured to provide high reflectivity in a desired spectral range, e.g., the spectral range of the incident light pulse. The high reflectivity in the desired spectral range may be at least 80%, or at least 85%, or at least 90%, or at least 95%, or at least 99%. The spectral range covered by the reflective coating may be 100 nm to 4000 nm, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. In some embodiments, the deflecting mirror DM is capable of providing sufficient deflection after receiving an optical pulse having a pulse energy in the range between 0.1 μJ and 100 μJ.

[0117] Upon first reflection from a deflection mirror DM (e.g., a cantilever), the incident light pulse may be reflected to an optical delay device ODA. Note that for simplicity, it is assumed that the light pulses of the broadband output radiation ORD contain the same pulse energy and therefore follow the same trajectory upon each reflection from the deflection mirror DM (either deflected or undeflected). This is why the reflected light pulses are represented by a single solid line in Figure 14. However, in reality, the light pulses will contain different pulse energies and therefore exert different radiation pressures on the deflection mirror DM. As a result, the light pulses will follow different trajectories upon reflection from the deflection mirror, resulting in the formation of a spatially divergent beam, such as the spatially divergent reflected beam DRB shown in Figure 12. Driven by the radiation pressure exerted by the incident light pulse during the first reflection, the deflection mirror DM may gradually move from its default position P0 toward a first new position P1', where the deflection caused by the first reflection reaches a maximum.

[0118] The optical delay device ODA may be configured to apply a delay time to the reflected light pulse RP and then return the reflected light pulse RP to the deflection mirror DM. The delay time applied to the reflected light pulse RP by the optical delay device ODA may be adjustable. In one embodiment, the optical delay device ODA may include two mirrors R1 and R2 arranged on a translation stage TS. By moving the translation stage TS, the travel distance between the two mirrors R1 and R2 of the optical delay device ODA and the deflection mirror DM can be changed, resulting in a change in the delay time applied to the light pulse. To maximize the maximum deflection of the deflection mirror DM, the optical delay time can be optimized so that the second reflection of the reflected light pulse RP on the deflection mirror DM occurs substantially simultaneously with the maximum deflection of the deflection mirror DM, e.g., a first new position P1′. Substantially simultaneously with the maximum deflection can be defined, for example, as arriving at the deflection mirror when the deflection mirror is within 1%, 3%, 5%, 10%, 15%, or 20% of its maximum deflection.

[0119] As mentioned above, the maximum deflection depends on the intensity of the radiation pressure exerted by the incident optical pulse, which in turn depends on the pulse energy of the incident optical pulse. Note that while the maximum deflection depends on the incident pulse energy, the point in time at which this occurs is independent of the pulse energy and is determined by the properties (shape, material, etc.) of the deflecting mirror and the environment (attenuation coefficient, etc.). In some embodiments, the delay time applied to the reflected pulse RP by the optical delay device ODA may be in the range of 1 ns to 100 ns, 1 ns to 200 ns, 1 ns to 500 ns, or 1 ns to 1000 ns.

[0120] Upon a second reflection from the deflection mirror DM, the reflected light pulse RP may follow a new propagation direction determined by the pulse energy, which depends on the deflection of the deflection mirror DM. For example, in the embodiment of FIG. 14, the second reflection of the reflected light pulse RP may occur when the deflection mirror DM moves to a first new position P1′. The second reflection of the reflected light pulse RP may cause further mirror deflection in addition to the existing deflection caused by the first reflection of the same pulse. Driven by the radiation pressure exerted by the reflected light pulse RP during the second reflection, the deflection mirror DM may continue to deflect and move from the first new position P1′ to a second new position P2′, where the deflection due to the second reflection reaches a maximum. Therefore, the total cumulative deflection of the deflection mirror DM, i.e., from P0 to P2′, may be the sum of the maximum deflection caused by the first reflection and the maximum deflection caused by the second reflection.

[0121] Figure 15 is a plot showing the time-dependent deflection of a simulated deflecting mirror (e.g., a cantilever) caused by two reflections of two consecutive incident light pulses, as used in the simulation shown in Figure 13. The simulation shown in Figure 15 is an extension of the simulation shown in Figure 13. This means that most of the parameter values ​​used in the simulation shown in Figure 13 (such as those related to the deflecting mirror DM and the light pulses) are also used in the simulation shown in Figure 15. The main difference between the two simulations is that the simulation shown in Figure 15 allows for a delay time to be applied to the second reflection of the light pulse, thereby demonstrating the additional mirror deflection caused by the second reflection of the same light pulse.

[0122] As shown in FIG. 15, the first reflection of the first pulse moves the deflection mirror DM from a default position with zero deflection (e.g., the default position P0 shown in FIG. 14) to a first new position where the mirror deflection reaches a first deflection point D1 at the first instance T1 of 80 ns (e.g., the first new position P1' shown in FIG. 14). In this simulation, the first deflection point D1 is selected to be the maximum deflection achievable by the radiation pressure exerted by the light pulse during the first reflection. By applying an 80 ns delay time (same as in the simulation shown in FIG. 13) to the reflected light pulse RP, the second reflection of the same pulse occurs exactly or nearly simultaneously with the deflection of the deflection mirror DM reaching the first deflection point D1. Upon the second reflection, the deflection mirror DM continues deflecting and moves from the first new position to a second new position where the mirror deflection reaches a second deflection point D2 at the second instance T2 of 130 ns (e.g., the second new position P2' shown in FIG. 14). In this simulation, the second deflection point D2 is chosen to be the maximum deflection achievable by the radiation pressure exerted by the light pulse during the second reflection. Therefore, the second deflection point D2 is the maximum deflection point of the first peak of the deflection curve and has a value of 3.05 nm, which is equal to the sum of the maximum deflection caused by the first reflection (1.75 nm) and the maximum deflection caused by the second reflection (1.3 nm).

[0123] By the time the deflection of the deflection mirror DM reaches the second deflection point D2 at the second instance of 130 ns, it has begun to return to its default position, where the deflection is zero. To reduce or avoid the effects of residual mirror deflection, it is desirable for subsequent optical pulses to arrive at the deflection mirror DM when the deflection of the deflection mirror DM is zero or substantially close to zero. In the simulation shown in Figure 15, the second pulse strikes the deflection mirror DM at the third instance T3 of 400 ns, when the mirror deflection decreases to the residual deflection point D0, which has a residual deflection of 0.2 nm. Because this residual deflection is more than an order of magnitude smaller than the maximum deflection at the second deflection point D2, i.e., 3.05 nm, its effect on subsequent optical pulses is negligible.

[0124] To ensure a pulse separation time of at least 400 ns, the repetition rate of the incident optical pulses can be kept below 2.5 MHz. Note that the above values ​​are based on the example configuration of the optical filter device OFA employed in the simulation shown in Figure 15. Different values ​​may be obtained if the optical filter device OFA is configured in a different way.

[0125] As shown in Figure 15, the second peak of the deflection curve resulting from the second reflection of the second pulse is similar to the first peak of the deflection curve resulting from the second reflection of the first pulse in that the deflection of the deflecting mirror DM increases from the residual deflection point D0 to the first deflection point D1' (after a delay of 80 ns) and then reaches the second deflection point D2'. Here, the second deflection point D2' is also the maximum deflection point of the second peak of the deflection curve. The deflection at the first deflection point D1 is the maximum deflection caused by the first reflection of the second pulse, and the deflection at the second deflection point D2' is the sum of the maximum deflections caused by each of the two reflections of the second pulse. Once the deflection of the deflecting mirror DM reaches its maximum value, i.e., the second deflection point D2', it begins to decrease again to zero.

[0126] Referring back to FIG. 14, after two reflections from the deflection mirror DM, each reflected light pulse can be directed to the spatial filter SF by following a trajectory that depends on the pulse energy. The spatial filter SF can be used to at least partially block portions (e.g., peripheral regions) of a spatially divergent light beam that contain undesired pulse energies, e.g., light pulses with pulse energies outside a certain range. Thus, the energy distribution of the light pulses passing through the spatial filter SF becomes more uniform; in other words, the amplitude or energy variation between pulses is reduced. The pulse energy range can be within ±5%, ±10%, or ±15% of the average energy of the light pulses.

[0127] In some embodiments, the spatial filter SF may comprise an aperture, including an absorptive, scattering, and / or reflective substrate containing holes. In a preferred embodiment, the spatial filter SF may comprise a soft aperture, the substrate of which comprises a material (e.g., suitable glass) with a gradually varying transmittance for the light pulses. In different embodiments, the spatial filter SF may be configured to be dynamically adjustable. For example, the position and / or size of the aperture of the spatial filter SF may be dynamically adjustable so that the spatial filter SF is always well aligned with the deflection mirror DM and the degree of laser noise reduction can be actively controlled by changing the aperture size of the spatial filter SF. In different embodiments, the spatial filter SF may operate in a reflective configuration. For example, the spatial filter may comprise a reflective mirror that selectively reflects a portion (e.g., a central region) of a spatially diverging light beam while absorbing or transmitting an unselected portion (e.g., a peripheral region) of the light beam.

[0128] It should be noted that the above embodiments are provided for illustrative purposes only and are not limiting, and that other embodiments with different deflecting mirrors and / or different spatial filters are possible. For example, in some embodiments, the deflecting mirror DM can operate in a different medium other than air (e.g., water, gas, oil), which may result in a different response of the deflecting mirror DM to an optical pulse. This may be due to the fact that a deflecting mirror DM operating in a different medium may have a different viscous damping coefficient, i.e., a different value of c in Equation

[11] , than in air. In different embodiments, one or more optical lenses may be used to control (e.g., increase or decrease) the beam diameter of the incident radiation, e.g., broadband output radiation ORD, so that the physical dimension requirements of the deflecting mirror DM can be relaxed. It should also be noted that, although the above embodiments are described in the context of reducing laser noise in broadband pulsed radiation, the optical filter device OFA should be understood as a tool that can improve the noise performance of pulsed radiation having different temporal and spectral characteristics (e.g., pulse width, spectral bandwidth, center wavelength).

[0129] 16 is a block diagram illustrating a computer system 1600 that can assist in implementing the methods and flows disclosed herein. The computer system 1600 includes a bus 1602 or other communication mechanism for communicating information and a processor 1604 (or multiple processors 1604 and 1605) coupled to the bus 1602 for processing information. The computer system 1600 also includes a main memory 1606, such as a random access memory (RAM) or other dynamic storage device, coupled to the bus 1602 for storing information and instructions executed by the processor 1604. The main memory 1606 may also be used for storing temporary variables or other intermediate information during execution of instructions executed by the processor 1604. The computer system 1600 further includes a read-only memory (ROM) 1608 or other static storage device coupled to the bus 1602 for storing static information and instructions for the processor 1604. A storage device 1610, such as a magnetic or optical disk, is provided and coupled to the bus 1602 for storing information and instructions.

[0130] The computer system 1600 may be coupled via bus 1602 to a display 1612, such as a cathode ray tube (CRT) or flat-panel or touch-panel display, for displaying information to a computer user. An input device 1614, including alphanumeric and other keys, is coupled to bus 1602 for communicating information and command selections to the processor 1604. Another type of user input device is a cursor control 1616, such as a mouse, trackball, or cursor direction keys, for communicating directional information and command selections to the processor 1604 and for controlling cursor movement on the display 1612. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), allowing the device to specify a position in a plane. A touch-panel (screen) display may also be used as an input device.

[0131] One or more of the methods described herein may be performed by computer system 1600 in response to processor 1604 executing one or more sequences of one or more instructions contained in main memory 1606. Such instructions may be read into main memory 1606 from another computer-readable medium, such as storage device 1610. Execution of the sequence of instructions contained in main memory 1606 causes processor 1604 to perform the process steps described herein. One or more processors in a multiprocessing configuration may also be used to execute the sequence of instructions contained in main memory 1606. In alternative embodiments, hardwired circuitry may be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0132] The term "computer-readable medium," as used herein, refers to any medium that participates in providing instructions to processor 1604 for execution. Such media may take many forms, including, but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1610. Volatile media include dynamic memory, such as main memory 1606. Transmission media include coaxial cables, copper wire, and fiber optics, including the wires that comprise bus 1602. Transmission media may also take the form of acoustic or light waves such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include floppy disks, flexible disks, hard disks, magnetic tape, other magnetic media, CD-ROMs, DVDs, other optical media, punch cards, paper tape, other physical media with patterns of holes, RAM, PROM, EPROM, Flash EPROM, other memory chips or cartridges, carrier waves, or any other medium from which a computer can read.

[0133] Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to processor 1604 for execution. For example, the instructions may initially be stored on a magnetic disk of a remote computer. The remote computer may load the instructions into a dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 1600 may receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus 1602 may receive the data carried in the infrared signal and place the data on bus 1602. Bus 1602 carries the data to main memory 1606, from which processor 1604 retrieves and executes the instructions. The instructions received by main memory 1606 may optionally be stored on storage device 1610 either before or after execution by processor 1604.

[0134] Computer system 1600 also preferably includes a communication interface 1618 coupled to bus 1602. The communication interface 1618 provides a two-way data communication coupling to a network link 1620 that is connected to a local network 1622. For example, communication interface 1618 may be an Integrated Services Digital Network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 1618 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In such implementations, communication interface 1618 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0135] Network link 1620 typically provides data communication through one or more networks to other data devices. For example, network link 1620 may provide a connection through local network 1622 to a host computer 1624 or to data equipment operated by an Internet Service Provider (ISP) 1626. ISP 1626, in turn, provides data communication services through a worldwide packet data communication network. Local network 1622 and Internet 1628 both use electrical, electromagnetic, or optical signals that carry digital data streams. The signals through the various networks, network link 1620, and communication interface 1618, which carry the digital data to and from computer system 1600, are exemplary forms of carrier waves transporting the information.

[0136] Computer system 1600 can send messages and receive data, including program code, through the network(s), network link 1620, and communication interface 1618. In the Internet example, a server 1630 might transmit a requested code for an application program via Internet 1628, ISP 1626, local network 1622, and communication interface 1618. One such downloaded application may, for example, provide one or more of the techniques described herein. The received code may be executed by processor 1604 as received, and / or stored in storage device 1610 or other non-volatile storage for later execution. In this manner, computer system 1600 can obtain application code in the form of a carrier wave.

[0137] Further embodiments are disclosed in the following numbered paragraphs: 1. A light diverging device operable to receive light pulses and spatially distribute the light pulses on an optical surface according to the pulse energy of each said light pulse; a spatial filter disposed at the optical surface and operable to apply spatial filtering to the light pulses based on a position of each of the light pulses at the optical surface resulting from the spatial distribution; An optical filter device comprising: 2. The optical filter device described in item 1, wherein the light diverging device comprises a deflection mirror configured to receive and subsequently reflect the light pulses, the deflection mirror being further configured to deflect each reflection of the light pulses, the magnitude of the deflection being dependent on the pulse energy of each of the light pulses, and the deflection spatially distributing the light pulses on the optical surface. 3. The optical filter device described in item 2, wherein the deflection mirror comprises a cantilever having a fixed end fixed at a predetermined position and a free end movable within the reflecting surface. 4. The deflection of the deflection mirror is the ratio of the free end to the fixed end upon each impact by the light pulse. circumference Item 4. The optical filter device according to item 3, including movement in a direction. 5. The optical filter device according to any one of items 2 to 4, wherein the deflection mirror is made of silicon. 6. The optical filter device according to any one of items 2 to 5, wherein the deflection mirror has a width in the range of 1 μm to 1000 μm. 7. The optical filter device according to any one of items 2 to 6, wherein the deflection mirror has a length in the range of 1 μm to 1000 μm. 8. The optical filter device according to any one of items 2 to 7, wherein the deflection mirror has a thickness in the range of 0.1 μm to 100 μm. 9. An optical filter device according to any one of items 2 to 8, wherein the deflection mirror is configured so that, upon second reflection, each of the optical pulses is reflected in a propagation direction according to its pulse energy. 10. An optical filter device according to any one of paragraphs 2 to 9, wherein the deflecting mirror comprises at least one reflective coating configured to reflect the light pulses. 11. The optical filter device of claim 10, wherein the at least one reflective coating provides a reflectivity of at least 80% in the spectral range defined by the light pulse. 12. The optical filter device according to item 11, wherein the spectral range covered by the at least one reflective coating is between 100 nm and 4000 nm. 13. An optical filter device described in any one of items 2 to 12, wherein the deflection mirror is configured to provide sufficient deflection to spatially distribute the light pulse when the pulse energy of the light pulse is in the range of 0.1 μJ to 100 μJ. 14. An optical filter device described in any of items 2 to 13, wherein the light diverging device further comprises an optical delay device configured to apply a delay time to the light pulse after receiving a first reflection from the deflection mirror and return the light pulse to the deflection mirror for a second reflection by the deflection mirror. 15. An optical filter device as described in paragraph 14, wherein the delay time is applied by the optical delay device such that each of the light pulses reaches the deflection mirror for a second reflection when the deflection of the deflection mirror is substantially maximum. 16. The optical filter device according to claim 14 or 15, wherein the optical delay device has a configurable delay time. 17. An optical filter device described in any of items 14 to 16, wherein the optical delay device is configured to include at least two optical mirrors arranged on a translation stage, and the translation stage is movable to change the distance between the optical delay device and the deflection mirror. 18. An optical filter device described in any of items 1 to 17, wherein the spatial filter comprises a first region configured to select a first portion of the light pulse and a second region configured to at least partially block a second portion of the light pulse, the position of the first portion being included in the first region and the position of the second portion being included in the second region. 19. The optical filter device according to item 18, wherein the spatial filter comprises a substrate having an aperture. 20. The optical filter device of claim 18, wherein the spatial filter is reflective such that the first region comprises a reflective region and the second region comprises an absorptive region. 21. An optical filter device according to any of paragraphs 18 to 20, wherein the at least partially blocked light pulses include light pulses having pulse energies that deviate by more than 5% from the average energy of the light pulses. 22. An optical filter device according to any of paragraphs 18 to 20, wherein the at least partially blocked light pulses include light pulses having pulse energies that deviate by more than 10% from the average energy of the light pulses. 23. An optical filter device according to any of paragraphs 18 to 20, wherein the at least partially blocked light pulses include light pulses having pulse energies that deviate by more than 15% from the average energy of the light pulses. 24. The spatial filter is configured such that the size and / or position of the first region is adjustable. from 24. An optical filter device according to any one of 23. 25. An optical filter device according to any one of items 1 to 24, wherein the distance between the spatial filter and the light diverging device is adjustable. 26. The optical system of claim 1, further comprising one or more optical lenses configured to control the beam diameter of the light pulses prior to receipt of the light pulses by the light diverging device. from 26. An optical filter device according to any one of 25. 27. A broadband light source device comprising an optical filter device according to any one of paragraphs 1 to 26, configured to receive pump radiation and generate broadband output radiation comprising said optical pulses. 28. A broadband light source device configured to generate broadband output radiation comprising the optical pulses upon receiving pump radiation, the broadband light source device comprising an optical filter device according to any one of paragraphs 2 to 17, and configured such that the pulse separation time of the optical pulses is equal to or greater than the time required for the deflection of the deflection mirror to decrease to zero or nearly zero. 29. The broadband light source device according to paragraph 27 or 28, wherein the broadband output radiation is generated in a hollow-core photonic crystal fiber (HC-PCF). 30. A measurement device comprising a broadband light source device according to any one of paragraphs 26 to 28. 31. The metrology device according to clause 30, comprising a scatterometer, a level sensor, or an alignment sensor. 32. A method for spatially filtering optical pulses, comprising: spatially distributing a plurality of light pulses on an optical surface according to a pulse energy of each said light pulse; spatially filtering the light pulses based on a position of each light pulse at the optical surface obtained from a spatial distribution; A method for providing the above. 33. The spatial distribution of the light pulses further comprises: first reflecting the light pulse using a deflecting mirror; applying a delay time to the light pulses first reflected from the deflection mirror and then returning them to the deflection mirror; and performing a second reflection of the delayed light pulses using the deflecting mirror to spatially distribute the light pulses; 33. The method of claim 32, wherein the deflection mirror is deflected upon reflection of each of the light pulses, the deflection of the deflection mirror depending on the pulse energy of each of the light pulses. 34. The method of claim 33, wherein the delay time is applied such that each of the light pulses reaches the deflection mirror for a second reflection when the deflection of the deflection mirror is substantially maximum. 35. The method of claim 33 or 34, wherein the pulse separation time of at least one of the light pulses is substantially equal to or longer than the time required for the deflection of the deflection mirror to return to zero or nearly zero. 36. The method of any of paragraphs 32 to 35, wherein the spatial filtering comprises at least partially blocking a portion of the light pulses spatially distributed at the optical surface. 37. The method of claim 36, wherein the at least partially blocked light pulses include light pulses having pulse energies that deviate by more than 5% from the average energy of the light pulses. 38. The method of claim 37, wherein the at least partially blocked light pulses include light pulses having pulse energies that deviate by more than 10% from the average energy of the light pulses. 39. The method of claim 38, wherein the at least partially blocked light pulses include light pulses having pulse energies that deviate by more than 15% from the average energy of the light pulses. 40. The method of any of paragraphs 32 to 39, further comprising controlling the beam diameter of the light pulses prior to spatial distribution of the light pulses on the optical surface.

[0138] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, including the production of guidance and detection patterns for integrated optical systems, magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0139] Although specific reference may be made herein to embodiments of the invention in connection with lithography apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatuses may be generally referred to as lithography tools. Such lithography tools may use vacuum or ambient (non-vacuum) conditions.

[0140] Although specific reference has been made above to the use of embodiments of the present invention in the context of optical lithography, the present invention is not limited to optical lithography and may also be used in other applications, such as imprint lithography, where the context permits.

[0141] While specific embodiments of the present invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The above description is intended to be illustrative and not limiting. Thus, it will be apparent to those skilled in the art that modifications can be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. a light diverging device operable to receive light pulses and spatially distribute the light pulses on an optical surface according to the pulse energy of each said light pulse; a spatial filter disposed at the optical surface and operable to apply spatial filtering to the light pulses based on a position of each of the light pulses at the optical surface resulting from the spatial distribution; An optical filter device comprising:

2. 2. The optical filter device of claim 1, wherein the light diverging device comprises a deflecting mirror configured to receive and subsequently reflect the light pulses, the deflecting mirror further configured to deflect upon each reflection of the light pulses, the magnitude of the deflection being dependent on the pulse energy of each of the light pulses, and the deflection spatially distributing the light pulses on the optical surface.

3. 3. The optical filter device of claim 2, wherein the deflection mirror comprises a cantilever having a fixed end fixed at a predetermined position and a free end movable within the reflecting surface.

4. 4. The optical filter device of claim 3, wherein the deflection of the deflecting mirror comprises circumferential movement of the free end relative to the fixed end upon each impact by the light pulse.

5. The width of the deflection mirror is in the range of 1 μm to 1000 μm. The length of the deflection mirror is in the range of 1 μm to 1000 μm. The thickness of the deflection mirror is in the range of 0.1 μm to 100 μm.

5. The optical filter device according to claim 2, wherein at least one of the following is satisfied.

6. 6. The optical filter device according to claim 2, wherein the deflection mirror is configured such that, upon a second reflection, each of the optical pulses is reflected in a propagation direction according to its pulse energy.

7. the deflecting mirror comprises at least one reflective coating configured to reflect the light pulses; 7. An optical filter device according to claim 2, wherein the at least one reflective coating provides a reflectivity of at least 80% in a spectral range defined by the light pulse.

8. 8. The optical filter device of claim 7, wherein the at least one reflective coating covers a spectral range between 100 nm and 4000 nm.

9. 9. The optical filter device of claim 2, wherein the deflection mirror is configured to provide sufficient deflection to spatially distribute the light pulses when the pulse energy of the light pulses is in the range of 0.1 μJ to 100 μJ.

10. 10. The optical filter device of claim 2, wherein the light diverging device further comprises an optical delay device configured to apply a delay time to the light pulses after a first reflection from the deflection mirror and return the light pulses to the deflection mirror for a second reflection by the deflection mirror, the delay time being applied by the optical delay device such that each of the light pulses arrives at the deflection mirror for a second reflection when the deflection of the deflection mirror is substantially maximum.

11. The optical filter device of claim 10 , wherein the optical delay device comprises a configurable delay time.

12. 12. A broadband light source device configured to generate broadband output radiation comprising said optical pulses upon receiving pump radiation, said broadband light source device comprising an optical filter device according to any preceding claim.

13. 12. A broadband light source device configured to generate broadband output radiation comprising the optical pulses upon receiving pump radiation, the broadband light source device comprising an optical filter device according to any one of claims 2 to 11, wherein the pulse separation time of the optical pulses is configured to be substantially equal to or longer than the time required for the deflection of the deflection mirror to reduce to zero or substantially zero.

14. A measurement device comprising the optical filter device according to any one of claims 1 to 11 or the broadband light source device according to claim 12 or 13, the measurement device comprising a scatterometer measurement device, a level sensor or an alignment sensor.

15. 1. A method for spatially filtering optical pulses, comprising: spatially distributing a plurality of light pulses on an optical surface according to a pulse energy of each said light pulse; spatially filtering the light pulses based on a position of each light pulse at the optical surface obtained from a spatial distribution; A method for providing the above.

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