Methods and apparatuses for spatially filtering optical pulses

Abstract

An optical filter apparatus comprising an optical divergence device, operable to receive optical pulses and spatially distribute the optical pulses over an optical plane in dependence with a pulse energy of each of the optical pulses; and a spatial filter, located at said optical plane, operable to apply spatial filtering to the optical pulses based on a location of each of the optical pulses at the optical plane resulting from the spatial distributing.

Description

[Technical Field]

[0001] The present invention relates to a method and apparatus for spatially filtering optical pulses, and more specifically, to such methods and apparatus in relation to noise reduction applications for pulsed radiation sources. [Previous Technology]

[0002] A lithography apparatus is a machine configured to apply a desired pattern onto a substrate. Lithography apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). A lithography apparatus can project a pattern (often referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0003] To project a pattern onto a substrate, lithography equipment can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Compared to lithography equipment using radiation with a wavelength of, for example, 193 nm, lithography equipment using extreme ultraviolet (EUV) radiation with wavelengths in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate.

[0004] Low-k1 lithography can be used to process features smaller than the typical resolution limit of lithography equipment. In this process, the resolution can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithography equipment, CD is the "critical size" (usually the smallest printed feature size, but in this case half-pitch), and k1 is an empirical resolution factor. Generally, the smaller k1 is, the more difficult it is to reproduce patterns on the substrate that resemble the shape and size planned by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithography projection equipment and / or design layout. These steps include, but are not limited to: NA optimization, illumination scheme development, use of phase-shift patterning devices, various optimizations of the design layout, such as optical proximity correction (OPC, sometimes also called "optical and procedural correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a strict control loop used to control the stability of lithography equipment can be used to improve pattern reproduction at low k1.

[0005] Measuring tools are used in many aspects of IC manufacturing processes, such as alignment tools for properly positioning substrates before exposure, leveling tools for measuring the surface topology of substrates, and tools based on focus control and scattering measurements for, for example, detecting / measuring exposed and / or etched products in process control. In each case, a radiation source is required. For various reasons, including measurement robustness and accuracy, broadband or white light radiation sources are increasingly used in such measuring applications. Existing devices will need to be modified to accommodate broadband radiation generation. [Summary of the Invention]

[0006] In a first aspect of the present invention, an optical filter device is provided, comprising: an optical diverging device operable to receive light pulses and spatially distributing the light pulses above an optical plane depending on the pulse energy of each of the light pulses; and a spatial filter located at the optical plane, operable to apply spatial filtering to the light pulses based on a position of each of the light pulses generated by the spatial distribution at the optical plane.

[0007] Preferably, the optical diverging device includes a deflectable mirror configured to receive and subsequently reflect the optical pulses; the deflectable mirror is further configured to deflect after each reflection of each of the optical pulses, the amount of deflection depending on the pulse energy of each of the optical pulses and the deflection resulting in the optical pulses being spatially distributed above the optical plane.

[0008] More preferably, the optical divergence device further includes an optical delay configuration configured to apply a delay time to the optical pulses after they have undergone a first reflection from one of the deflectable mirrors and guide them back to the deflectable mirrors to undergo a second reflection by the deflectable mirrors.

[0009] In a second aspect of the present invention, a method for spatially filtering optical pulses is provided, comprising: spatially distributing the optical pulses above an optical plane depending on the pulse energy of one of a plurality of optical pulses; and spatially filtering the optical pulses based on a position of each of the optical pulses generated by the spatial distribution at the optical plane.

[0010] Preferably, the spatial distribution of the light pulses further comprises: using a deflectable mirror to first reflect the light pulse; applying a delay time to the light pulses that are first reflected from the deflectable mirror and then guiding them back to the deflectable mirror; using the deflectable mirror to second reflect the delayed light pulse to spatially distribute the light pulses; wherein the deflectable mirror deflects after each reflection of each of the light pulses, the deflection of the deflectable mirror depending on a pulse energy of each of the light pulses.

[0011] In a third embodiment of the present invention, a broadband light source device is provided, which is configured to generate broadband output radiation containing such light pulses after receiving pump radiation, and includes an optical filter device of the first embodiment.

[0012] Other forms of the measuring device of the present invention include the broadband light source device of the third form.

Implementation Method

[0014] In this document, the terms “radiation” and “beam” are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365, 248, 193, 157 or 126 nm) and extreme ultraviolet radiation (EUV, e.g., having wavelengths in the range of about 5 to 100 nm).

[0015] As used herein, the terms “reducing mask,” “mask,” or “patterning device” can be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, the patterned cross-section corresponding to a pattern to be generated in a target portion of a substrate. In this context, the term “light valve” may also be used. Examples of such patterning devices, besides classic masks (transmission or reflection, binary, phase-shift, hybrid, etc.), include programmable mirror arrays and programmable LCD arrays.

[0016] Figure 1 schematically depicts a lithography apparatus LA. The lithography apparatus LA includes: an illumination system (also called an illuminator) IL configured to adjust a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask stage) MT configured to support a patterning device (e.g., a 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., a wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted by the patterning device MA to the radiation beam B onto a target portion C (e.g., containing one or more dies) of the substrate W.

[0017] In operation, the illumination system IL receives a radiated beam from the radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components for guiding, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components or any combination thereof. The illuminator IL may be used to adjust the radiated beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0018] The term "projection system" PS as used herein should be broadly interpreted to encompass all types of projection systems suitable for the exposure radiation used and / or other factors suitable for use with wetted liquids or in a vacuum, including refractive, reflective, reflective-refractive, composite, magnetic, electromagnetic, and / or electrostatic optical systems or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0019] A lithography apparatus LA may be a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, to fill the space between the projection system PS and the substrate W—this is also known as immersion lithography. Further information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0020] The lithography equipment LA may be of the type having two or more substrate supports WT (also known as "dual stages"). In this "multi-stage" machine, the substrate supports WT can be used in parallel, and / or a substrate W located on one of the substrate supports WT can be used to prepare the substrate W for subsequent exposure, while another substrate W on the other substrate support WT is used to expose a pattern on the other substrate W.

[0021] In addition to the substrate support WT, the lithography apparatus LA may also include a measurement stage. The measurement stage is configured to hold sensors and / or cleaning devices. The sensors may be configured to measure properties of the projection system PS and / or the properties of the radiated beam B. The measurement stage may hold multiple sensors. The cleaning devices may be configured to clean parts of the lithography apparatus, such as parts of the projection system PS or parts of the system providing the wetting liquid. When the substrate support WT moves away from the projection system PS, the measurement stage may move below the projection system PS.

[0022] In operation, the radiation beam B is incident on a patterning device (e.g., a mask) MA held on a mask support MT, and patterned by a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of the substrate W. The substrate support WT can be accurately moved by means of a second locator PW and a position measurement system IF, for example, to position different target portions C in the path of the radiation beam B at a focused but aligned position. Similarly, a first locator PM and possibly another position sensor (not explicitly depicted in FIG. 1) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device MA and the substrate W. Although the substrate alignment marks P1, P2, as described, occupy dedicated target portions, they can be positioned in the space between the target portions. When substrate alignment marks P1 and P2 are located between target portions C, these substrate alignment marks P1 and P2 are referred to as cut track alignment marks.

[0023] As shown in Figure 2, the lithography apparatus LA can form part of a lithography unit LC (sometimes also called a lithocell or lithocell cluster), which typically also includes equipment 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 cooling plate CH for adjusting the temperature of the substrate W (e.g., for adjusting the solvent in the resist layer), and a baking plate BK. A substrate handler or robot RO picks up the substrate W from input / output ports I / O1 and I / O2, moves the substrate W between different process devices, and delivers the substrate W to the loading area LB of the lithography apparatus LA. The devices in a lithography unit, often collectively referred to as the coating and developing system, are typically under the control of the coating and developing system control unit (TCU). The TCU itself can be controlled by a supervisory control system (SCS), which can also control the lithography equipment (LA) via, for example, the lithography control unit (LACU).

[0024] In order to correctly and consistently expose the substrate W exposed by the lithography equipment LA, it is necessary to inspect the substrate to measure the properties of the patterned structure, such as the overlap error between subsequent layers, line thickness, critical dimension (CD), etc. For this purpose, an inspection tool (not shown) can be included in the lithography unit LC. If an error is detected, adjustments can be made, for example, to the exposure of subsequent substrates or other processing steps to be performed on the substrate W, especially when inspection is performed before other substrates W in the same batch or group are still to be exposed or processed.

[0025] The inspection device, also known as a measuring device, is used to determine the properties of the substrate W, and in particular to determine how the properties of different substrates W vary or how the properties associated with different layers of the same substrate W vary between different layers. The inspection device may alternatively be constructed to identify defects on the substrate W, and may be, for example, part of a lithography unit LC, or integrated into a lithography device LA, or even a stand-alone device. The inspection device can measure properties on latent images (images in a resist layer after exposure), or on semi-latent images (images in a resist layer after a post-exposure baking step PEB), or on developed resist images (where the exposed or unexposed portions of the resist have been removed), or even on etched images (after a pattern transfer step such as etching).

[0026] Typically, the patterning process in the lithography equipment LA is one of the most critical steps in the process, requiring high accuracy in the dimensional calibration and placement of the structure on the substrate W. To ensure this high accuracy, three systems can be combined in a so-called "integrated" control environment, schematically depicted in Figure 3. One of these systems is the lithography equipment LA, which is (in fact) connected to the metrology tool MT (second system) and to the computer system CL (third system). The key to this "integrated" environment is to optimize the cooperation between these three systems to enhance the overall program window and provide a tight control loop to ensure that the patterning performed by the lithography equipment LA remains within the program window. The program window defines a series of program parameters (e.g., dosage, focus, overlap) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device) – typically within which program parameters in the lithography or patterning process are allowed to vary.

[0027] The computer system CL can use the design layout (part of it) to be patterned to predict which resolution enhancement technique to use and perform computational lithography simulation and calculations to determine which mask layout and lithography device settings achieve the maximum overall program window of the patterning procedure (described by the double arrows in the first scale SC1 in Figure 3). Typically, the resolution enhancement technique is configured to match the patterning possibilities of the lithography device LA. The computer system CL can also be used to detect where the lithography device LA is currently operating within the program window (e.g., using input from the metric tool MT) to predict whether there may be defects attributable to, for example, suboptimal processing (described by the arrow pointing to "0" in the second scale SC2 in Figure 3).

[0028] The measurement tool MT can provide input to the computer system CL to achieve accurate simulation and prediction, and can provide feedback to the lithography device LA to identify possible drift, for example, in the calibration state of the lithography device LA (depicted by multiple arrows in the third scale SC3 in Figure 3).

[0029] In lithography, it is necessary to frequently measure the resulting structure, for example, for program control and verification. The tools used to perform such measurements are generally called metrology tools (MTs). Different types of metrology tools (MTs) used for such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools (MTs). Scatterometers are multifunctional instruments that allow the measurement of lithography parameters by means of a sensor in the pupil or a plane conjugate to the pupil of the scatterometer's objective lens (measurements are generally referred to as pupil-based measurements), or by means of a sensor in the image plane or a plane conjugate to the image plane, in which case the measurements are generally referred to as image or field-based measurements. Such scatterometers and related measurement techniques are further described in the patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure gratings using light from soft X-rays and light visible in the near-IR wavelength range.

[0030] In the first embodiment, the scatterometer MT is an angular resolving scatterometer. In this scatterometer, the reconstruction method can be applied to reconstruct or calculate the properties of the grating from the measured signal. This reconstruction can, for example, be caused by simulating the interaction between the scattered radiation and the target structure using a mathematical model and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0031] In the second embodiment, the scatterometer MT is a spectroradiometer MT. In this spectroradiometer MT, radiation emitted by a radiation source is guided to the target, and reflected or scattered radiation from the target is guided to a spectrometer detector, which measures the spectrum of the specular reflected radiation (i.e., the measurement of intensity varying with wavelength). From this data, the structure or profile of the target that generated the detected spectrum can be reconstructed, for example, by rigorous coupled wave analysis and nonlinear regression or by comparison with a simulated spectral library.

[0032] In a third embodiment, the scatterometer MT is an elliptic scatterometer. An elliptic scatterometer allows for the determination of parameters of a lithography process by measuring the scattered radiation for each polarization state. This metrological device emits polarized light (such as linear, circular, or elliptical) by using, for example, a suitable polarizing filter in the illumination section of the metrological device. The source suitable for the metrological device can also provide polarized radiation. Various embodiments of existing elliptic scatterometers are described in U.S. Patent Applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0033] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the stacking of two misaligned gratings or periodic structures by measuring the reflectance spectrum and / or detecting asymmetry in the configuration (which is related to the extent of the stacking). Two (typically overlapping) grating structures can be applied to two different layers (not necessarily consecutive layers), and the two grating structures can be formed to be substantially located at the same position on the wafer. The scatterometer can have a symmetrical detection configuration, such as described, for example, in the co-owned patent application EP1,628,164A, such that any asymmetry is clearly distinguishable. This provides a direct method for measuring misalignment in gratings. Further examples of measuring the overlap error between two layers containing a periodic structure as the target by means of the asymmetry of such periodic structure can be found in PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application No. US 20160161863, which are incorporated herein by reference in their entirety.

[0034] Other parameters of interest may be focus and dose. Focus and dose can be determined simultaneously by scattering techniques (or alternatively by scanning electron microscopy) as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure with a unique combination of critical dimensions and sidewall angle measurements for each point in the focus energy matrix (FEM, also known as the focus exposure matrix) can be used. If such a unique combination of critical dimensions and sidewall angles is available, the focus and dose values ​​can be uniquely determined based on these measurements.

[0035] The metrological target may be an assembly of composite gratings formed primarily in a resist by a lithography process and also formed, for example, after an etching process. Typically, the spacing and linewidth between structures in the grating are largely dependent on the measurement optics (especially the NA of the optics) to capture the diffraction order from the metrological target. As previously indicated, the diffraction signal can be used to determine the displacement between two layers (also known as 'overlap') or to reconstruct at least a portion of the original grating as produced by the lithography process. This reconstruction can be used to guide the quality of the lithography process and to control at least a portion of the lithography process. The target may have smaller sub-segments configured to mimic the size of functional portions of a design layout in the target. Due to these sub-segments, the target will behave more similarly to the functional portions of the design layout, making the overall process parameter measurements more similar to the functional portions of the design layout. The target can be measured in an underfill mode or an overfill mode. In the underfill mode, the measurement beam produces a spot smaller than the overall target. In the fill-over mode, the measurement beam produces a spot larger than the overall target. In this fill-over mode, it is also possible to measure different targets simultaneously, thereby determining different processing parameters at the same time.

[0036] The overall measurement quality of a lithography parameter for a specific target is determined at least in part by the measurement formulation used to measure that lithography parameter. The term "substrate measurement formulation" may include measuring one or more parameters of itself, measuring one or more parameters of one or more patterns, or both. For example, if the measurement used in the substrate measurement formulation is a diffraction-based optical measurement, one or more of the measured parameters may include the wavelength of radiation, the polarization of radiation, the angle of incidence of radiation relative to the substrate, the orientation of radiation relative to a pattern on the substrate, etc. One of the criteria used to select the measurement formulation may be, for example, the sensitivity of one of the measured parameters to processing variations. Further examples are described in U.S. Patent Application US2016-0161863, which is incorporated herein by reference in its entirety, and in U.S. Patent Application US2016 / 0370717A1, which has been published.

[0037] Figure 4 depicts a measuring device, such as a scatterometer. The scatterometer includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. The reflected or scattered radiation is transmitted to a spectrometer detector 4, which measures the spectrum 10 of the specular reflected radiation (i.e., the intensity of the intensity according to wavelength variations). From this data, the structure or profile of the detected spectrum can be reconstructed by the processing unit PU, for example, through rigorous coupled-wave analysis and nonlinear regression, or through comparison with the simulated spectral library shown at the bottom of Figure 3. Generally, for reconstruction, the general form of the structure is known, and some parameters are assumed based on knowledge of the procedures used to manufacture the structure, leaving only a few parameters of the structure to be determined based on the scattering measurement data. This scatterometer can be configured as a normal-incident scatterometer or an oblique-incident scatterometer.

[0038] The overall measurement quality of the lithography parameters of the target measured by means of measurement is determined at least in part by the measurement formulation used to measure these lithography parameters. The term "substrate measurement formulation" may include measuring one or more parameters of itself, measuring one or more parameters of one or more patterns, or both. For example, if the measurement used in the substrate measurement formulation is a diffraction-based optical measurement, one or more of the measured parameters may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria used to select the measurement formulation may be, for example, the sensitivity of one of the measured parameters to processing variations. Further examples are described in U.S. Patent Application US2016 / 0161863 and U.S. Patent Application US2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0039] Another type of measurement tool used in IC manufacturing is a configuration measurement system, a level sensor, or a height sensor. Such tools can be integrated into lithography equipment to measure the configuration of the top surface of a substrate (or wafer). A map of the substrate's configuration (also called a height map) can be generated from these measurements, indicating the height of the substrate as it varies depending on its position on the substrate. This height map can then be used to correct the substrate's position during pattern transfer onto the substrate, providing an aerial image of the patterned device at the appropriate focus position on the substrate. It should be understood that "height" in this context refers to a dimension relative to the substrate that is substantially outside the plane (also known as the Z-axis). Typically, a level or height sensor measures at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor causes height measurements across the substrate.

[0040] Figure 5 schematically illustrates an example of a level or height sensor LS known in the art, for illustrative purposes only. In this example, the level sensor includes an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiated beam LSB, which is imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrowband or broadband light source, such as a supercontinuum light source, polarized or unpolarized, pulsed or continuous, such as a polarized or unpolarized laser beam. The radiation source LSO may include a plurality of radiation sources with different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible light radiation, but may additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0041] The projection grating PGR is a periodic grating containing a periodic structure that causes a radiation beam BE1 with periodically varying intensity. The radiation beam BE1 with periodically varying intensity passes through a measurement position MLO on the guide substrate W, and the radiation beam has an incident angle ANG between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees, relative to an axis (Z-axis) perpendicular to the surface of the incident substrate. At the measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and guided toward the detection unit LSD.

[0042] To determine the height level at the measurement position MLO, the level sensor further includes a detection system comprising a detection grating DGR, a detector DET, and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR may be the same as the projection grating PGR. The detector DET generates a detector output signal that indicates the received light, such as indicating the intensity of the received light, like a light detector, or representing the spatial distribution of the received intensity, like a camera. The detector DET may comprise any combination of one or more detector types.

[0043] The height level at the measurement location MLO can be determined by using triangulation technology. The detected height level is usually related to the signal strength measured by the detector DET, which is particularly dependent on the design of the projection grating PGR and the periodicity of the (tilted) incident angle ANG.

[0044] The projection unit LSP and / or the detection unit LSD may include other optical elements, such as lenses and / or mirrors (not shown), along the path of the patterned radiating beam between the projection grating PGR and the detection grating DGR.

[0045] In one embodiment, the detection grating DGR may be omitted, and the detector DET may be placed at the location where the detection grating DGR is placed. This configuration provides more direct detection of the image of the projection grating PGR.

[0046] In order to effectively cover the surface of the substrate W, the level sensor LS can be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating a measurement area MLO or an array of light spots covering a larger measurement range.

[0047] Various height sensors of general types are disclosed, for example, in US7265364 and US7646471, both of which are incorporated by reference. Height sensors that use UV radiation instead of visible or infrared radiation are disclosed in US2010233600A1, which is incorporated by reference. In WO2016102127A1, which is incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without detecting the grating itself.

[0048] Another type of measurement tool used in IC manufacturing is an alignment sensor. Therefore, a key aspect of the performance of lithography equipment is the ability to properly and accurately place the applied pattern relative to features placed in a previous layer (by the same or different lithography equipment). For this purpose, the substrate has one or more sets of marks or targets. Each mark is a structure whose position can be measured later using a position sensor, typically an optical position sensor. The position sensor may be called an "alignment sensor," and the mark may be called an "alignment mark."

[0049] A lithography apparatus may include one or more (e.g., a plurality of) alignment sensors capable of accurately measuring the position of alignment marks provided on a substrate. The alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from the alignment marks formed on the substrate. Examples of alignment sensors used in current lithography apparatuses are based on self-reference interferometers as described in US6961116. Various enhancements and modifications of position sensors have been developed, for example, as disclosed in US2015261097A1. The contents of all such disclosures are incorporated herein by reference.

[0050] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as that described, for example, in US6961116 and incorporated herein by reference. A radiation source RSO provides a radiation beam RB having one or more wavelengths, which is directed by a steering optics onto a mark (such as a mark AM located on a substrate W) as an illumination spot SP. In this example, the steering optics include a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP used to illuminate the mark AM may be slightly smaller than the width of the mark itself.

[0051] Radiation diffracted by the alignment mark AM (in this example via the objective lens OL) is collimated to carry the beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). For example, a self-reference interferometer SRI of the type disclosed in US6961116 mentioned above interferes with itself with the beam IB, which is then received by a photodetector PD. Additional optics (not shown) may be included to provide a separate beam in cases where more than one wavelength is generated by the radiation source RSO. The photodetector may be a single element, or may include multiple pixels as needed. The photodetector may include a sensor array.

[0052] In this example, the steering optics of the spot mirror SM can also be used to block the zero-order radiation reflected by the self-marker, so that the information-carrying beam IB contains only the higher-order diffraction radiation from the marker AM (this is not necessary for measurement, but improves the signal-to-noise ratio).

[0053] The intensity signal SI is supplied to the processing unit PU. The X and Y positions of the substrate relative to the reference frame are output by combining the optical processing performed in the block SRI with the calculation processing performed in the unit PU.

[0054] The single measurement of the type described fixes the position of the mark within a certain range corresponding to one pitch of the mark. This measurement is combined with a coarser measurement technique to identify which period of the sine wave contains the period passing the mark position. The same procedure at coarser and / or finer levels can be repeated at different wavelengths to improve accuracy and / or to ensure robust mark detection, regardless of the material used to make the mark or the material provided above and / or below it. Wavelengths can be optically multiplexed and demultiplexed to process them simultaneously, and / or multiplexed by time-division or frequency-division.

[0055] In this example, the alignment sensor and the light spot SP remain stationary, while the substrate W moves. Therefore, the alignment sensor can be rigidly and accurately mounted to the reference frame, while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. During this movement, the substrate W is controlled by mounting it onto a substrate support and by the 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. Measurement of the position of the marks provided on the substrate support allows calibration of the position of the substrate support (e.g., relative to the frame to which the alignment system is connected), as determined by the position sensor. Measurement of the position of the alignment marks provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0056] The metrological instruments (MTs) mentioned above (such as scatterometers, configuration measurement systems, or position measurement systems) can use radiation from a radiation source for measurement. The properties of the radiation used by the metrological instrument can affect the type and quality of the measurements that can be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation. Multiple different frequencies can propagate, irradiate, and scatter the metrological target without interfering with other frequencies or with minimal interference. Therefore, different frequencies can be used, for example, to obtain more metrological data simultaneously. Different radiation frequencies can also be used to query and discover different properties of the metrological target. Broadband radiation can be used in metrological systems (MTs) such as level sensors, alignment mark measurement systems, scattering measurement instruments, or detection instruments. The broadband radiation source can be a supercontinuum spectral source.

[0057] For example, high-quality broadband radiation in the supercontinuum spectrum may be difficult to generate. One method for generating broadband radiation is, for example, to broaden high-power narrow-band or single-frequency input radiation using nonlinear higher-order effects. The input radiation (which can be generated using a laser) may be called pump radiation. Alternatively, the input radiation may be called seed radiation. To obtain high-power radiation for the broadening effect, the radiation can be confined to a small region to achieve highly localized high-intensity radiation. In such regions, the radiation can interact with the broadening structure and / or the material forming the nonlinear medium to form broadband output radiation. In the high-intensity radiation region, different materials and / or structures can be used to achieve and / or improve radiation broadening by providing a suitable nonlinear medium.

[0058] In some embodiments, broadband output radiation is generated in a photonic crystal fiber (PCF). In several embodiments, this photonic crystal fiber has microstructures around its core that help confine radiation passing through the fiber in the core. The core may be made of a solid material having nonlinear properties and capable of generating broadband radiation when high-intensity pump radiation is transmitted through the core. Although generating broadband radiation in a solid-core photonic crystal fiber is feasible, there are several disadvantages to using a solid material. For example, if UV radiation is generated in a solid core, this radiation may not be present in the output spectrum of the fiber because the radiation is absorbed by most solid materials.

[0059] In some embodiments, as further discussed below with reference to Figure 8, the method and apparatus for broadening input radiation may use optical fiber to confine the input radiation and to broaden the input radiation to output broadband radiation. The fiber may be a hollow fiber and may contain internal structures for effective guidance 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 within the hollow core of the fiber, thereby achieving high radiation intensity. The hollow core of the fiber may be filled with a gas, which acts as a broadening medium for broadening the input radiation. Such fiber and gas configurations can be used to generate supercontinuum spectral radiation sources. The radiation input to the fiber may be electromagnetic radiation, such as radiation in one or more of the infrared, visible, UV, and extreme UV spectra. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light.

[0060] Some embodiments relate to novel designs of this broadband radiation source comprising an optical fiber. The optical fiber is a hollow-core photonic crystal fiber (HC-PCF). Specifically, the optical fiber may be a hollow-core photonic crystal fiber containing an anti-resonant structure for confining radiation. Such fibers containing an anti-resonant structure are known in the art as anti-resonant fibers, tubular fibers, single-ring fibers, negative curvature fibers, or suppression coupling fibers. Various designs of such fibers are known in the art. Alternatively, the optical fiber may be a photonic bandgap fiber (HC-PBF, such as Kagome fiber).

[0061] Various types of HC-PCFs can be engineered, each based on a different physical guidance mechanism. Two such HC-PCFs include: hollow photonic bandgap fiber (HC-PBF) and hollow anti-resonant reflective fiber (HC-ARF). Details of the design and fabrication of HC-PCFs can be found in U.S. Patent US2004 / 015085A1 (for HC-PBF) and International PCT Patent Application WO2017 / 032454A1 (for hollow anti-resonant reflective fiber), which are incorporated herein by reference. Figure 9(a) shows a Kagome fiber containing a Kagome lattice structure.

[0062] An example of an optical fiber used in a radiation source will now be described with reference to FIG7, which is a schematic cross-sectional view of an optical fiber OF in a transverse plane. Other embodiments of actual examples of fibers similar to those in FIG7 are disclosed in WO2017 / 032454A1.

[0063] An optical fiber (OF) may comprise an elongated body, longer in one dimension than the other two dimensions of the optical fiber. This longer dimension may be called the axial direction and may define the axis of the optical fiber. The two other dimensions define a plane that may be called the transverse plane. Figure 7 shows a cross-section of the optical fiber in this transverse plane (i.e., perpendicular to the axis), labeled the xy plane. The transverse cross-section of the optical fiber may be substantially constant along the fiber axis.

[0064] It should be understood that optical fiber (OF) has a certain degree of flexibility, and therefore, in general, the direction of the axis will not be uniform along the length of the optical fiber (OF). Terms such as optical axis, transverse cross section, and the like should be understood to mean local optical axis, local transverse cross section, etc. Furthermore, when the component is described as cylindrical or tubular, these terms should be understood to cover such shapes that may have been deformed when the optical fiber (OF) is bent.

[0065] Fiber optic OF can have any length, and it will be understood that the length of fiber optic OF can depend on the application. Fiber optic OF can have lengths between 1 cm and 10 m, for example, fiber optic OF can have lengths between 10 cm and 100 cm.

[0066] An 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 as comprising a body having a hollow core HC (including the cladding portion and the support portion SP). The cladding portion includes a plurality of anti-resonant elements for guiding radiation through the hollow core HC. Specifically, the plurality of anti-resonant elements are configured to constrain radiation propagating primarily within the hollow core HC through the optical fiber OF, and are configured to guide radiation along the optical fiber OF. The hollow core HC of the optical fiber OF can be substantially disposed in the central region of the optical fiber OF such that the axis of the optical fiber OF can also define the axis of the hollow core HC.

[0067] The cladding portion includes a plurality of anti-resonance elements for guiding radiation propagating through the optical fiber (OF). Specifically, in this embodiment, the cladding portion includes a single ring of six tubular capillary CAPs. Each of the tubular capillary CAPs acts as an anti-resonance element.

[0068] The capillary CAP can also be referred to as a tube. In cross-section, the capillary CAP can be circular or may have another shape. Each capillary CAP includes a generally cylindrical wall portion WP that at least partially defines the hollow core HC of the optical fiber OF and separates the hollow core HC from the capillary cavity CC. It will be understood that the wall portion WP can act as an anti-reflection Fabry-Perot resonator for radiation propagating through the hollow core HC (and the radiation can be incident on the wall portion WP at a grazing angle of incidence). The thickness of the wall portion WP can be suitable to ensure that reflections returning to the hollow core HC are 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 µm and 10.0 µm.

[0069] It should be understood that, as used herein, the term cladding portion is intended to mean the portion of an optical fiber OF used to guide radiation propagating through the optical fiber OF (i.e., confining the radiation within the capillary CAP inside the hollow core HC). The radiation can be confined in a transverse mode, thereby propagating along the fiber axis.

[0070] The support portion is generally tubular and supports six capillary CAPs of the cladding portion. The six capillary CAPs are evenly distributed around the inner surface of the inner support portion SP. The six capillary CAPs can be described as being arranged in a generally hexagonal form.

[0071] The capillary CAPs are configured 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. This configuration is advantageous because it can increase the transmit bandwidth of the optical fiber OF (relative to, for example, a configuration where 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.

[0072] The six capillary capillaries (CAPs) of the cladding portion are arranged in a ring structure around the hollow HC. The inner surface of the ring structure of the capillary CAPs at least partially defines the hollow HC of the fiber OF. The diameter d of the hollow HC (which can be defined as the minimum dimension between opposing capillaries, indicated by arrow d) can be between 10 µm and 1000 µm. The diameter d of the hollow HC can affect the mode field diameter, impact loss, dispersion, modal multivariability, and nonlinear properties of the hollow HC fiber OF.

[0073] In this embodiment, the cladding portion comprises a single-ring configuration of capillary CAPs (which act as anti-resonance elements). Therefore, a line in any radial direction from the center of the hollow HC to the outside of the fiber OF passes through no more than one capillary CAP.

[0074] It should be understood that other embodiments may have different configurations of anti-resonance elements. Such configurations may include configurations of multiple rings with anti-resonance elements and configurations of nested anti-resonance elements. Figure 9(a) shows an embodiment of an HC-PCF with three rings of capillary CAPs stacked on top of each other in the radial direction. In this embodiment, each capillary CAP contacts other capillaries in the same ring and in different rings. Furthermore, although the embodiment shown in Figure 7 includes six capillary rings, in other embodiments, one or more rings containing any number of anti-resonance elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) may be disposed in the cladding portion.

[0075] Figure 9(b) shows a modified embodiment of the HC-PCF with a single ring having a tubular capillary discussed above. In the example of Figure 9(b), there are two coaxial rings of the tubular capillary 21. To hold the inner and outer rings of the tubular capillary 21, a support tube ST may be included in the HC-PCF. The support tube may be made of silicon dioxide.

[0076] The tubular capillaries in the examples of Figures 7 and 9(a) and (b) may have a circular cross-sectional shape. Other shapes are also possible for tubular capillaries, such as elliptical or polygonal cross-sections. In addition, the solid material of the tubular capillaries in the examples of Figures 7 and 9(a) and (b) may include plastic materials such as PMA, glass such as silica, or soft glass.

[0077] Figure 8 depicts a radiation source RDS for providing broadband output radiation. The radiation source RDS includes: a pulse-pumped radiation source PRS or any other type of source capable of generating short pulses of desired length and energy level; an optical fiber OF with a hollow core HC (e.g., the type shown in Figure 7); and a working medium WM (e.g., gas) disposed within the hollow core HC. Although the radiation source RDS in Figure 8 includes the optical fiber OF shown in Figure 7, other types of hollow core HC optical fiber OFs may be used in alternative embodiments.

[0078] The pulsed-pumped radiation source (PRS) is configured to provide the input radiation IRD. The hollow core HC of the optical fiber OF is configured to accommodate the input radiation IRD from the pulsed-pumped radiation source (PRS) and to widen the input radiation IRD to provide the output radiation ORD. The working medium WM can widen the frequency range of the received input radiation IRD to provide a wideband output radiation ORD.

[0079] The radiation source RDS further includes a storage unit RSV. An optical fiber OF is disposed within the storage unit RSV. The storage unit RSV may also be referred to as a housing, container, or gas cell. The storage unit RSV is configured to contain a working medium WM. The storage unit 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 storage unit RSV. The storage unit RSV may include a first transparent window TW1. In use, the optical fiber OF is disposed within the storage unit RSV such that the first transparent window TW1 is positioned close to the input end IE of the optical fiber OF. The first transparent window TW1 may form part of the wall of the storage unit RSV. The first transparent window TW1 may be transparent at least to the received input radiation frequency, such that the received input radiation IRD (or at least a large portion thereof) can be coupled to the optical fiber OF located inside the storage unit RSV. It will be understood that optical devices (not shown) may be available for coupling the input radiation IRD to the fiber OF.

[0080] The storage unit RSV includes a second transparent window TW2 forming part of the wall of the storage unit RSV. In use, when the optical fiber OF is placed inside the storage unit RSV, the second transparent window TW2 is located near the output end OE of the optical fiber OF. The second transparent window TW2 is transparent at least to the frequency of the broadband output radiation ORD of the device 120.

[0081] Alternatively, in another embodiment, the two opposing ends of the optical fiber OF may be placed inside different reservoirs. The optical fiber OF may include a first end segment configured to receive input radiation IRD and a second end segment for outputting broadband output radiation ORD. The first end segment may be placed inside a first reservoir containing a working medium WM. The second end segment may be placed inside a second reservoir, which may also contain the working medium WM. The operation of the reservoirs may be performed as described above with respect to FIG8. 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 to allow the optical fiber OF to be partially placed inside and partially placed outside the reservoir, such that gas can be sealed inside the reservoir. The optical fiber OF may further include an intermediate section not contained within the reservoir. This configuration using two separate gas reservoirs is particularly convenient for embodiments in which the optical fiber OF is relatively long (e.g., when the length exceeds 1 m). It will be understood that for 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 within the two reservoirs) can be considered as devices providing the working medium WM within the hollow core HC of the optical fiber OF.

[0082] In this context, if at least 50%, 75%, 85%, 90%, 95% or 99% of incident radiation of a certain frequency is transmitted through the window, then the window may be transparent to that frequency.

[0083] The first TW1 and the second TW2 transparent windows can form an airtight seal within the wall of the reservoir RSV, so that the working medium WM (which may be a gas) can be contained within the reservoir RSV. It should be understood that the gas WM can be contained within the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0084] The working medium WM may contain: inert gases such as argon, krypton, and xenon; Raman-active gases such as hydrogen, deuterium, and nitrogen; or gas mixtures such as argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures, or nitrogen / hydrogen mixtures. Depending on the type of fill gas, nonlinear optical procedures may include modulation instability (MI), soliton self-compression, soliton splitting, Kerr effect, Raman effect, and dispersive wave generation (DWG), detailed in WO2018 / 127266A1 and US9160137B1 (both of which are hereby incorporated by reference). Since the dispersion of the fill gas can be tuned by changing the pressure of the working medium WM (i.e., the gas cell pressure) in the reservoir RSR, the resulting broadband pulse dynamics and associated spectral broadening characteristics can be adjusted to optimize frequency conversion.

[0085] In one embodiment, the working medium WM may be housed within the hollow core HC at least during the period when receiving the input radiation IRD for generating the broadband output radiation ORD. It should be understood that when the optical fiber OF does not receive the input radiation IRD for generating the broadband output radiation, the gas WM may be wholly or partially absent from the hollow core HC.

[0086] To achieve frequency broadening, high-intensity radiation may be required. The advantage of hollow-core HC fiber OF is that it can achieve high-intensity radiation through strong spatial confinement of radiation propagating through the fiber OF, thereby achieving high localized radiation intensity. The radiation intensity inside the fiber OF can be high, for example, due to high received input radiation intensity and / or due to the strong spatial confinement of radiation within the fiber OF. The advantage of hollow-core fiber is that it can guide radiation with a wider wavelength range than solid core fibers, and specifically, hollow-core fiber can guide radiation in both the ultraviolet and infrared ranges.

[0087] The advantage of using hollow-core HC fiber OF is that most of the radiation guided inside the fiber OF is confined within the hollow HC. Therefore, most of the radiation interaction inside the fiber OF occurs with the working medium WM, which is disposed inside the hollow HC of the fiber OF. Thus, the broadening effect of the working medium WM on radiation can be increased.

[0088] 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 contain, for example, an ultrafast pulse generated by a laser.

[0089] The input radiation IRD can be coherent radiation. The input radiation IRD can also be collimated radiation, which has the advantage of promoting and improving the efficiency of coupling the input radiation IRD to the optical fiber OF. The input radiation IRD can contain a single frequency or a narrow frequency range. The input radiation IRD can be generated by a laser. Similarly, the output radiation ORD can be collimated and / or coherent.

[0090] The bandwidth of the output radiation ORD can be a continuous range, encompassing a continuous range of radiation frequencies. The output radiation ORD may include supercontinuum spectral radiation. Continuous radiation is advantageous for use in numerous applications, such as in metrology applications. For example, a continuous frequency range can be used to look up a large number of attributes. A continuous frequency range can, for example, be used to determine and / or eliminate the frequency dependence of the measured attribute. A supercontinuum spectral output radiation ORD may include, for example, electromagnetic radiation in the wavelength range of 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. A supercontinuum spectral output radiation ORD may include white light.

[0091] The input radiation IRD provided by the pulsed pumped radiation source (PRS) can be pulsed. The input radiation IRD can contain electromagnetic radiation at one or more frequencies between 200 nm and 2 µm. The input radiation IRD can, for example, contain electromagnetic radiation with a wavelength of 1.03 µm. The repetition rate of the pulsed radiation IRD can be on the order of 1 kHz to 100 MHz. The pulse energy can be on the order of 0.1 µJ to 100 µJ, for example, 1 to 10 µJ. The pulse duration of the input radiation IRD can be between 10 fs and 10 ps, ​​for example, 300 fs. The average power of the input radiation IRD can be between 100 mW and several hundred W. The average power of the input radiation IRD can, for example, be between 20 W and 50 W.

[0092] The pulsed pumped radiation source (PRS) can be a laser. The spatiotemporal transmission characteristics (e.g., spectral amplitude and phase) of such laser pulses transmitted along the fiber OF can be altered and tuned by adjusting the (pumping) laser parameters, the working component WM, and the fiber OF parameters. These spatiotemporal transmission characteristics may include one or more of the following: output power, output mode profile, output time profile, width of the output time profile (or output pulse width), output spectral profile, and bandwidth of the output spectral profile (or output spectral bandwidth). The PRS parameters may include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, and pump pulse repetition rate. The OF parameters may include one or more of the following: the length, size, and shape of the hollow HC fiber; the size and shape of the capillary; and the thickness of the wall surrounding the hollow HC capillary. The working component WM (e.g., the filling gas) parameters may include one or more of the following: gas type, gas pressure, and gas temperature.

[0093] 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 of at least 0.01 mW / nm across the entire wavelength band of the output radiation. The power spectral density of the broadband output radiation across the entire wavelength band may be at least 3 mW / nm.

[0094] In many applications requiring wideband output radiation ORD (such as the aforementioned metrology applications), there is a growing concern about further reducing noise in wideband output radiation ORD. For pulsed laser sources, such as wideband radiation sources based on the aforementioned hollow-core (HC) fiber (OF), the primary source of laser noise is the inter-pulse variation of the output radiation. When such pulsed laser sources are used, for example, in wafer alignment applications, laser noise has a direct impact on alignment position reproducibility (APR), which in turn directly affects stacking. The higher the laser noise, the higher the APR (and therefore the stacking). Since relatively high inter-pulse variation is fundamental to the wideband radiation generation process, there is currently no direct solution to this inherent problem of the light source. Therefore, laser noise needs to be reduced indirectly downstream of the wideband radiation source.

[0095] For example, in existing methods, a broadband radiated beam 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, the two sub-signals are made completely out of phase (or π phase delay), and then the two sub-signals are added together to form the final signal. Because the two sub-signals carrying the same laser noise are out of phase, the addition of these two sub-signals results in the elimination of the two sub-signals while retaining the laser noise. Once determined, the laser noise can then be removed from the laser signal. However, this existing method alone is insufficient to reduce the APR or remove the entire laser noise. This is because any defects in the measurement (e.g., partial out-of-phase of the two sub-signals) can lead to residual laser noise and therefore a high APR.

[0096] Figure 10 is an example graph showing the measured APR varying according to the 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, the first and second portions of the broadband output radiation ORD are used to illuminate the alignment mark and generate a first alignment signal and a second alignment signal, respectively. Subsequently, the first and second alignment signals are used to generate a signal difference. The measurement is repeated multiple times to obtain a statistically meaningful set of data. The dataset is then used to generate APR data points for the selected repetition rate. Dynamic vibration noise is eliminated by subtracting one signal from another, and therefore the resulting APR is mainly caused by laser noise.

[0097] This figure clearly shows that the APR measured at the lowest repetition rate (i.e., 2.5 MHz) is 0.12 nm higher than the APR measured at the highest repetition rate (i.e., 40 MHz). Therefore, increasing the repetition rate of a broadband radiation source appears to be a potential solution for reducing laser noise-induced APR. However, for many broadband radiation sources, especially those based on hollow-core HC fiber OF (e.g., HC-PCF), laser lifetime is inversely proportional to laser repetition rate; that is, the higher the repetition rate, the shorter the laser lifetime. For this reason, HC-PCF-based radiation sources typically operate at relatively low repetition rates, for example, in the range of 1 MHz to 5 MHz, resulting in a relatively high APR (e.g., 0.1 nm to 0.2 nm) and thus high overlap. Therefore, there is a particular need for a method that can effectively reduce laser noise in broadband radiation sources without compromising laser lifetime.

[0098] Relative intensity noise (RIN) is generally used to describe the range of inter-pulse variation. RIN is power noise normalized to the average power level and can be expressed as follows: [1] where and are the standard deviation and average value of the pulse amplitude distribution, respectively. The RIN of a typical broadband radiation source based on HC-PCF can be in the range of, for example, 0.4 to 0.6, depending on the wavelength or wavelength range being measured.

[0099] In this invention, a method and apparatus are proposed 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 from a broadband radiation source without affecting laser lifetime. This can be achieved by applying an optical filter configuration to a pulse emitted from a broadband radiation source to stabilize the pulse amplitude and thus reduce the range of amplitude or energy variations between pulses. Figures 11(a) and 11(b) are two example graphs showing simulated pulse amplitudes before and after the application of the optical filter configuration, respectively. In the case of the simulated pulse amplitude shown in Figure 11(a), the pulse amplitude statistics may follow a distribution, such as a Passon distribution. The purpose of applying the optical filter configuration to the laser pulse is to then selectively filter out the amplitude distribution so that only the desired portion of the distribution is retained (e.g., the central portion corresponding to the pulse amplitude with a higher probability of occurrence). After filtering out the unwanted portions, the amplitude or energy variation between pulses can be significantly reduced, for example, by more than 30%, as is evident in Figure 11(b).

[0100] It should be noted that the proposed method does not necessarily require the entire spectrum of the broadband output radiation ORD to follow the same statistical distribution. The proposed method is applicable as long as certain optical wavelengths or wavelength ranges follow a certain statistical distribution and that distribution has a finite width.

[0101] Figure 12 schematically illustrates the operating principle of an optical filter configuration. The optical filter configuration may include: an optical divergence device operable to receive optical pulses and spatially distribute the optical pulses above an optical plane depending on the pulse energy of each of the optical pulses; and a spatial filter located at the optical plane, operable to apply spatial filtering to the optical pulses based on the position of each of the optical pulses generated by the spatial distribution at the optical plane. It should be noted that the operating principle schematically illustrated in Figure 12 is based on an embodiment of the optical filter configuration, wherein the optical divergence device includes a deflectable mirror DM. This deflectable mirror DM can be used to reflect a beam of broadband output radiation ORD toward, for example, a spatial filter SF. The beam of broadband output radiation ORD can be emitted from, for example, a broadband radiation source based on HC-PCF, and can consist of a series of optical pulses at a certain repetition rate. One end (or fixed end) of the deflectable mirror can be fixed at a fixed point, such as to a mirror support, while the other end (or free end) can move or displace freely in the reflection plane of the beam reflecting the broadband output radiation ORD. The deflectable mirror can be configured to deflect or tilt about a fixed point FP, and the deflection or tilt angle depends on the radiative force applied by the pulse of the broadband output radiation ORD.

[0102] For example, when no pulse is incident on the deflectable mirror DM, the deflectable mirror DM can remain at a preset deflection position P0. In contrast, when a pulse with a certain pulse energy or amplitude is incident on the deflectable mirror DM, the deflectable mirror DM can be deflected to a certain deflection position by radiation force, such as the first deflection position P1, the second deflection position P2, and the third deflection position P3 shown in FIG12. The three deflection positions P1, P2, and P3 can respectively correspond to the first deflection angle DA1, the second deflection angle DA2, and the third deflection angle DA3 formed relative to the preset deflection position P0, as well as the pulses with the first pulse energy PE1, the second pulse energy PE2, and the third pulse energy PE3. The third pulse energy or amplitude PE3 can be higher than the second pulse energy or amplitude PE2, which in turn can be higher than the first pulse energy or amplitude PE1. Since the deflection angle is proportional to the applied radiating force and therefore the incident pulse energy, the third deflection angle DA3 can therefore 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 estimated by dividing the corresponding vertical displacement (e.g., d2) of the edge of the free end of the deflectable mirror by the length L of the deflectable mirror DM. It should be noted that in the following description, the deflection of the deflectable mirror is evaluated by the vertical displacement obtained from the edge of the free end of the mirror.

[0103] It should be noted that, for simplicity, Figure 12 illustrates only three different scenarios, each corresponding to pulses with different pulse energies. In reality, the pulse train of broadband output radiation contains a plurality of pulses with a plurality of pulse energies. After reflection, although still temporally separated, some of the reflected pulses (e.g., those with pulse energies or amplitudes at the very center of the Passon distribution) will at least partially overlap spatially and thus form a spatially divergent reflected beam DRB. Assuming that the broadband output radiation ORD is well collimated before interacting with the deflectable mirror DM, the spatial divergence of the reflected beam DRB is therefore mainly caused by specular reflection at various different reflection angles, and thus the reflected beam DRB diverges primarily in the reflecting plane. Therefore, the reflected beam DRB contains an elliptical beam profile, with the major axis LA in the reflecting plane. Additional reflected pulses with pulse amplitudes or energies falling outside the desired range (e.g., excessively low or high pulse amplitudes or energies) will be positioned around the two edges along the long axis of the spatially divergent reflected beam (DRB), and in some cases may even be spatially separated from the DRB. By using a spatial filter (SF) to spatially filter out the unwanted portion of the DRB, the remaining portion of the reflected beam will contain pulses with more consistent pulse amplitudes or energies, resulting in low inter-pulse variation or RIN.

[0104] According to reference, H.-J. Butt et al., Surface Science Reports 59 (2005) 1-152 (incorporated hereby), the applied force due to the incident radiation beam is given by the following equation:, [2] where is the reflected power, is the absorbed power, is the angle of incidence, and is the speed of light. Assuming that the deflectable mirror DM is coated with a reflective coating, then. The (instantaneous) power due to reflection is related to the pulse energy and pulse width by the following equation:. [3]

[0105] In static analysis, the deflection system of the deflectable mirror DM for a given force is given by the following formula: , [4] where is the spring constant, and is the deflection. The spring constant of the deflectable mirror DM is given by the following formula: . [5] where is Young's modulus, and and are the width, thickness and length of the deflectable mirror, respectively.

[0106] However, each pulse here is processed independently, and this requires dynamic processing. The deflectable mirror acts as a mass-spring system and therefore follows simple harmonic motion. Thus, each deflectable mirror will follow the second-order equation of motion. According to the reference, Fundamentals of Vibrations, Leonard Meirovitch, McGraw-Hill, 2001, the second-order equation of motion is given by: , [6] where is the (effective) mass of the deflectable mirror DM and is the coefficient of viscous damping. The operating state of the operation results in an underdamped second-order system. The solution to this system is known. Specifically, the response of the underdamped second-order system subjected to the initial conditions is: , [7], [8]; [9];

[10] .

[11]

[0107] It should be noted that the equations of motion (i.e., equation [6]) and their solutions (i.e., equation [7]) provided above assume the application of zero force. In our case, the force is not zero, but rather there is an 'effect' caused by each incident pulse. It is known that the impulse produces an initial velocity given by the following equation: ,

[12] where, for a pulse with a unit area under the force-time curve, kg·m / s. However, since the pulse used is a Gaussian pulse with a finite pulse width characterized by, for example, full width half maximum (FWHM), it is not 1 and should be derived. To do this, it can be assumed that the incoming pulse is rectangular. This is a good assumption given the Gaussian nature of the pulse. Furthermore, it is assumed that the angle of incidence is zero. Then: ,

[13] ,

[14] .

[15]

[0108] Therefore, the response of the deflectable mirror DM to the energy pulse has now been fully determined. It should be noted that the pulse width does not actually affect the response of the deflectable mirror DM (within the approximation used above).

[0109] The above equations (i.e., equations [4] to

[15] ) along with the relevant parameters can be used to simulate a mass spring system based on a deflectable mirror. To facilitate the simulation, the parameters of example cantilevers of commercial atomic force microscopes (AFM) can be used. AFM cantilevers have many advantages for use as deflectable mirror surfaces. First, AFM cantilevers are known technology and commercially available. Second, there are many different cantilevers available on the market with various parameters (e.g., different hardness), thereby allowing for different applications. Third, most (if not all) commercial AFM cantilevers are coated with one or more optical coatings to reflect laser light in the AFM, so it may be possible to use such cantilevers directly. If the existing coating of the AFM cantilevers is not suitable for the incident broadband output radiation ORD, different reflective coatings can be applied to the cantilevers, which are optimized according to the spectral profile of the incident broadband output radiation ORD.

[0110] By applying the following parameters to the above equations, the response of a deflectable mirror (e.g., an AFM cantilever) to an incident pulse of a given energy can be determined. It should be noted that the parameter values ​​shown below are based on an example implementation; other parameter values ​​based on different implementations are also applicable. 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 / thickness = 1.25e-6; %Cantilever thickness E = 100e9; %Young's modulus of silicon rho = 2329; %Density of silicon m = 0.2427*w*thickness*L*rho; %Effective mass of cantilever z0 = 0e-6; %Initial displacement e = 0.5*5e-6; %Incident pulse energy

[0111] Figure 13 is a graph showing the simulated time-dependent deflection of a cantilever induced by a single reflection of three light pulses, each with different pulse energies. In the simulation, the cantilever is made of silicon, and its width, length, and thickness are 20 µm, 25 µm, and 1.25 µm, respectively. The incident angles of all pulses are set to or substantially close to 0 degrees. The deflection of the deflectable mirror is in nanometers and is the result of the radial movement of the free end of the deflectable mirror DM relative to its fixed end after being struck by each light pulse. As shown in Figure 13, the maximum deflection increases with pulse energy, and for all three deflection curves, the maximum deflection points M1, M2, and M3 are located at approximately 80 ns after the cantilever is struck by a pulse from the broadband output radiation ORD. The damped response velocity is dependent on the geometry of the cantilever. In this particular simulation shown in Figure 13, the cantilever requires at least 400 ns to dampen the response induced by the incident pulse. In other words, after the cantilever is struck by a pulse from the broadband output radiation ORD, the cantilever deflection returns to zero at approximately 400 ns. This 400 ns response time corresponds to a pulse repetition rate of 2.5 MHz. Therefore, the pulse repetition rate of the broadband output radiation can be maintained below 2.5 MHz to ensure that when the next pulse arrives at the cantilever, the deflection of the cantilever caused by the previous pulse has at least substantially returned to zero. This is not a limitation, as, as previously mentioned, higher repetition rates generally have lower laser noise and would be less necessary for this invention. Furthermore, the 400 ns value can be altered by changing the environment around the cantilever to cause damping to occur more quickly.

[0112] It should be understood that the pulse of broadband output radiation ORD is reflected before any significant deflection by the deflectable mirror. This is evident in the simulation data shown in Figure 13, where the maximum deflection of the cantilever is recorded at a delay of approximately 80 ns relative to the time when the pulse is reflected by the cantilever. This delayed response is primarily due to the fact that the mass-spring system based on the deflectable mirror (e.g., the cantilever) has a finite moment of inertia, exhibiting an acceleration time much longer than the duration of the incident light pulse (e.g., the pulse of output radiation generated from a broadband radiation source based on HC-PCF). Although momentum has been transferred to the deflectable mirror, a single pulse will be reflected from a substantially undeflected mirror. Because a larger mirror deflection enables better spatial separation of pulses with different pulse energies, and allows for more efficient removal of pulses with undesirable amplitudes or energies, it is desirable to configure the optical filter (OFA) in a way that allows reflected pulses, after being deflected by the deflectable mirror DM following the first reflection, to be guided back to the deflectable mirror DM for a second reflection when the deflection of the deflectable mirror reaches its maximum value. For example, the redirection of each optical pulse for the second reflection on the deflectable mirror DM can be achieved using an optical delay line.

[0113] Figure 14 schematically illustrates an embodiment of an optical filter configuration (OFA). In this embodiment, the optical filter configuration (OFA) may include a deflectable mirror (DM), an optical delay configuration (ODA), and a spatial filter. A beam containing a broadband output radiation (ORD) of a series of optical pulses may be incident obliquely on the deflectable mirror (DM). The broadband output radiation (ORD) may be generated, for example, by a broadband radiation source based on hollow fiber. Therefore, the values ​​of the relevant laser parameters may fall within the typical parameter range described in the preceding paragraphs. It should be noted that the proposed methods and apparatus are not limited to reducing laser noise from broadband radiation, but are equally applicable to reducing laser noise from radiation using narrow spectral bandwidths.

[0114] The deflectable mirror DM can be any mirror capable of deflecting or tilting around a fixed point FP of the deflectable mirror DM after reflecting an incident laser pulse. The deflection or tilting of the deflectable mirror DM can occur in a reflecting plane in which the broadband output radiation ORD is reflected by the deflectable mirror DM. In the embodiment of FIG14, the deflectable mirror may include a cantilever, one end of which is fixed at the fixed point FP, for example, to a mirror support (not shown), and the other end is movable or displaceable. Example cantilever can be made of silicon. The dimensions of the cantilever are the same as those used in the simulation shown in FIG13.

[0115] In different embodiments, other types of deflectable mirrors DM made of different materials other than silicon can be used. Additionally, the size of the deflectable mirror DM can be flexibly selected to meet different application needs. In some embodiments, the deflectable mirror may include one or both of the following width and length ranges: between 1 µm and 1000 µm, between 1 µm and 500 µm, between 1 µm and 100 µm, or between 1 µm and 10 µm; and may include a thickness range between 1 µm and 5 µm, 1 µm and 10 µm, or 1 µm and 100 µm. In some embodiments, the deflectable mirror DM may include one or more reflective coatings configured to provide high reflectivity within a desired spectral range (e.g., the spectral range of the incident light pulse). The high reflectivity within the desired spectral range may be at least 80%, or at least 85%, at least 90%, at least 95%, or at least 99%. The reflective coating can cover a spectral range between 100 nm and 4000 nm, 400 nm and 900 nm, 500 nm and 900 nm, or 200 nm and 2000 nm. In some embodiments, the deflectable mirror DM can provide sufficient deflection after being subjected to light pulses with pulse energies in the range of 0.1 µJ to 100 µJ.

[0116] After the first reflection from the deflectable mirror DM (e.g., a cantilever), the incident light pulse can be reflected to the optical delay configuration ODA. It should be noted that, for simplicity, it is assumed that the light pulses of the broadband output radiation ORD contain the same pulse energy, and therefore, after each reflection from the deflectable mirror DM (deflected or undeflected), all pulses follow the same trajectory. 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 apply different radiative forces to the deflectable mirror DM. Thus, the light pulses will follow different trajectories after reflection from the deflectable 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 radiative force applied by the incident light pulse during the first reflection, the deflectable mirror DM can gradually move away from the preset position P0 toward a first new position P1' where the deflection induced by the first reflection reaches its maximum value.

[0117] An optical delay configuration (ODA) can be configured to apply a delay time to a reflected light pulse RP and subsequently guide the reflected light pulse RP back to a deflectable mirror DM. This delay time applied to the reflected light pulse RP by the optical delay configuration (ODA) can be adjustable. In one embodiment, the optical delay configuration (ODA) may include two mirrors R1, R2 disposed on a translation stage TS. Moving the translation stage TS allows for changes in the travel distance between the two mirrors R1, R2 of the optical delay configuration (ODA) and the deflectable mirror DM, thereby causing a change in the delay time applied to the light pulse. To best utilize the maximum deflection of the deflectable mirror DM, the optical delay time can be optimized such that the second reflection of the reflected light pulse RP on the deflectable mirror DM occurs substantially simultaneously with the maximum deflection of the deflectable mirror DM (e.g., a first new position P1'). Essentially, the occurrence of the maximum deflection can be defined, for example, when the deflectable mirror is within 1%, 3%, 5%, 10%, 15%, or 20% of its maximum deflection, reaching the deflectable mirror.

[0118] As described above, the maximum deflection may depend on the intensity of the radiative force applied by the incident light pulse, which in turn depends on the pulse energy of the incident light pulse. It should be noted that although the maximum deflection depends on the incident pulse energy, the timing of this occurrence is independent of the pulse energy and is influenced by the properties of the deflectable mirror (e.g., geometry, material) and the environment (e.g., damping coefficient). In some embodiments, the delay time applied to the reflected pulse RP by the optical delay configuration ODA may be within the range of 1 ns to 100 ns, 1 ns to 200 ns, 1 ns to 500 ns, or 1 ns to 1000 ns.

[0119] After the second reflection from the deflectable mirror DM, the reflected light pulse RP can follow a new propagation direction determined by the pulse energy-dependent deflection of the deflectable mirror DM. For example, in the embodiment of FIG14, the second reflection of the reflected light pulse RP can occur when the deflectable mirror DM has moved to a first new position P1'. The second reflection of the reflected light pulse RP can induce another mirror deflection besides the existing deflection induced by the first reflection of the same pulse. Driven by the radiative force applied by the reflected light pulse RP during the second reflection, the deflectable mirror DM can continue to deflect and move from the first new position P1' to a second new position P2', where the deflection induced by the second reflection reaches its maximum value. The total cumulative deflection of the deflectable mirror DM (i.e., from P0 to P2') can therefore be the sum of the maximum deflection induced by the first reflection and the maximum deflection induced by the second reflection.

[0120] Figure 15 is a graph showing the simulated time-dependent deflection of a deflectable mirror (e.g., a cantilever) induced by two reflections of two consecutive 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 (e.g., the parameter values ​​associated with the deflectable 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 can apply a delay time for the second reflection of the light pulse, and therefore can show the additional mirror deflection induced by the second reflection of the same light pulse.

[0121] As shown in Figure 15, the first reflection of the first pulse causes the deflectable mirror DM to move from a preset position with zero deflection (e.g., the preset position P0 shown in Figure 14) to a first new position, where the mirror deflection reaches a first deflection point D1 (e.g., the first new position P1' shown in Figure 14) at a first time term T1 of 80 ns. In this simulation, the first deflection point D1 is selected as the maximum deflection achievable by the radiative force applied by the light pulse during the first reflection. By applying a delay time of 80 ns (the same as in the simulation shown in Figure 13) to the reflected light pulse RP, the second reflection of the same pulse occurs at a time precisely or substantially the same as the time when the deflection of the deflectable mirror DM reaches the first deflection point D1. Following the second reflection, the deflectable mirror DM continues to deflect and moves from the first new position to a second new position (e.g., the second new position P2' shown in Figure 14), where the deflection reaches a second deflection point D2 at a second time term T2 of 130 ns. In this simulation, the second deflection point D2 is chosen as the maximum deflection achievable by the radiative force applied by the light pulse during the second reflection. The second deflection point D2 is therefore the maximum deflection point of the first peak of the deflection curve and has a value of 3.05 nm, which is equivalent to the sum of the maximum deflection of 1.75 nm induced by the first reflection and the maximum deflection of 1.3 nm induced by the second reflection.

[0122] When the deflection of the deflectable mirror DM reaches the second deflection D2 at the second time term of 130 ns, the deflectable mirror DM begins to shift back to the preset position where the deflection is zero. To reduce or avoid the influence of any residual mirror deflection, subsequent light pulses need to reach the deflectable mirror DM when the deflection of the deflectable mirror DM is zero or substantially close to zero. In the simulation shown in Figure 15, the second pulse strikes the deflectable mirror DM at the third time term T3 of 400 ns, where the mirror deflection has been reduced to a residual deflection point D0 with a residual deflection of 0.2 nm. Since this residual deflection is more than an order of magnitude lower than the maximum deflection at the second deflection point D2 (i.e., 3.05 nm), its influence on subsequent light pulses is negligible.

[0123] To ensure a pulse interval of at least 400 ns, the repetition rate of the incident light pulses can be maintained at no more than 2.5 MHz. It should be noted that the aforementioned values ​​are based on an example configuration of the optical filter configuration OFA used in the simulation shown in Figure 15; different values ​​can be obtained if the optical filter configuration OFA is configured in different ways.

[0124] As shown in Figure 15, the second peak of the deflection curve generated by the two reflections of the second pulse is similar to the first peak generated by the two reflections of the first pulse. This is because the deflection of the deflectable mirror DM increases from the residual deflection point D0 to the first deflection point D1' (after a delay of 80 ns) before reaching 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 induced by the first reflection of the second pulse, while the deflection at the second deflection point D2' is the sum of the maximum deflections induced by the two reflections of the second pulse, respectively. Once the deflection of the deflectable mirror DM reaches its maximum value, i.e., the second deflection point D2', the deflection begins to decrease to zero again.

[0125] Referring back to Figure 14, after two reflections from the deflectable mirror DM, each reflected light pulse can be guided to the spatial filter SF by following a pulse energy-dependent trajectory. The spatial filter SF can be used to at least partially block a portion (e.g., in the peripheral region) of a spatially divergent beam containing light pulses with undesired pulse energies (e.g., pulse energies exceeding a certain range). Consequently, the energy distribution of the light pulses transmitted through the spatial filter SF becomes more uniform, or 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.

[0126] In some embodiments, the spatial filter SF may include an aperture comprising an absorptive, scattering, and / or reflective substrate, the substrate including a hole. In a preferred embodiment, the spatial filter SF may include a soft aperture, the substrate of which comprises a material (e.g., suitable glass) having a gradually varying transmittance to light pulses. In various 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 such that the spatial filter SF is always well aligned with the deflectable mirror DM, and the degree of laser noise reduction can be actively controlled by changing the size of the aperture of the spatial filter SF. In various embodiments, the spatial filter SF may be configured to operate reflectively. For example, the spatial filter may include a reflective mirror that selectively reflects a portion of the spatially diverging beam (e.g., in the central region) while absorbing or transmitting an unselected portion of the beam (e.g., in the peripheral region).

[0127] It should be noted that the above embodiments are provided herein for illustrative purposes and are therefore not limiting, and other embodiments including different deflectable mirrors and / or different spatial filters are also possible. For example, in some embodiments, the deflectable mirror DM can operate in different media other than air (e.g., water, gas, oil), and therefore the response of the deflectable mirror DM to light pulses can be different. This can be attributed to the fact that a deflectable mirror DM operating in a different medium can have a different viscous damping coefficient value compared to a deflectable mirror DM operating in air, i.e., in Equation

[11] . In different embodiments, one or more optical lenses can be used to control (e.g., increase or decrease) the beam diameter of the incident radiation (e.g., broadband output radiation ORD), thereby relaxing the requirements on the physical dimensions of the deflectable mirror DM. It should also be noted that although the above embodiments are described in the context of reducing laser noise from pulsed broadband radiation, the optical filter configuration OFA should be understood as a tool capable of improving the noise performance of pulsed radiation with different temporal and spectral characteristics (e.g., pulse width, spectral bandwidth, and center wavelength).

[0128] Figure 16 is a block diagram illustrating a computer system 1600 that can assist in implementing the methods and processes disclosed herein. The computer system 1600 includes a bus 1602 or other communication mechanism for transmitting 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 main memory 1606, such as random access memory (RAM) or other dynamic storage device, coupled to the bus 1602 for storing information and instructions to be executed by the processor 1604. The main memory 1606 can also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by the processor 1604. The computer system 1600 further includes 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 disk or optical disk, is provided, and the storage device 1610 is coupled to a bus 1602 for storing information and instructions.

[0129] The computer system 1600 may be coupled via bus 1602 to a display 1612 for displaying information to a computer user, such as a cathode ray tube (CRT), flat panel display, or touch panel display. An input device 1614, including alphanumeric keypads and other keys, is coupled to bus 1602 to transmit information and command selections to processor 1604. Another type of user input device is a cursor control 1616, such as a mouse, trackball, or cursor direction keys, for transmitting directional information and command selections to processor 1604 and for controlling cursor movement on display 1612. This input device typically has two degrees of freedom on 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. Touch panel (screen) displays can also be used as input devices.

[0130] One or more of the methods described herein may be executed by computer system 1600 in response to processor 1604 executing one or more sequences of 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 execute the program steps described herein. One or more processors in a multiprocessor configuration may also be used to execute the sequence of instructions contained in main memory 1606. In an alternative embodiment, a hard-wired circuit system may be used instead of or in combination with software instructions. Therefore, the description herein is not limited to any particular combination of hardware circuit systems and software.

[0131] As used herein, the term "computer-readable media" refers to any media that participates in providing instructions to the 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 discs or magnetic disks, such as storage devices 1610. Volatile media include dynamic memory, such as main memory 1606. Transmission media include coaxial cables, copper wires, and optical fibers, including wires containing bus 1602. Transmission media may also take the form of sound waves or light waves, such as sound waves or light waves generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, floppy disks, hard disks, magnetic tapes, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tapes, any other physical media with a perforated pattern, RAM, PROMs and EPROMs, FLASH-EPROMs, any other memory chips or cartridges, carrier waves as described below, or any other media that can be read by a computer.

[0132] Various forms of computer-readable media may involve carrying one or more sequences of instructions to processor 1604 for execution. For example, such instructions may initially be carried on a disk of a remote computer. The remote computer may load the instructions into its dynamic memory and transmit the instructions via a telephone line using a modem. The modem at the local end of computer system 1600 may receive data on the telephone line and convert the data into an infrared signal using an infrared transmitter. 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 fetches and executes the instructions. Instructions received from main memory 1606 may be stored on storage device 1610, as appropriate, before or after execution by processor 1604.

[0133] The computer system 1600 also preferably includes a communication interface 1618 coupled to a bus 1602. The communication interface 1618 provides bidirectional data communication coupling to a network link 1620 connected to a local area network 1622. For example, the communication interface 1618 may be an integrated Services Digital Network (ISDN) card or modem to provide data communication connectivity to a corresponding type of telephone line. As another example, the communication interface 1618 may be a local area network (LAN) card that provides data communication connectivity to a LAN-compatible network. A wireless link may also be implemented. In any such implementation, the communication interface 1618 transmits and receives electrical, electromagnetic, or optical signals carrying digital data streams representing various types of information.

[0134] Network link 1620 typically provides data communication to other data devices via one or more networks. For example, network link 1620 may provide connectivity to host computer 1624 or to data equipment operated by Internet Service Provider (ISP) 1626 via local area network 1622. ISP 1626 provides data communication services via a global packet data communication network (now commonly referred to as the "Internet" 1628). Both local area network 1622 and Internet 1628 use electrical, electromagnetic, or optical signals carrying digital data streams. Signals via various networks and signals on network link 1620 and via communication interface 1618 are carrier waves, an exemplary form of information transmission, which carry digital data to and from computer system 1600.

[0135] Computer system 1600 can send messages and receive data, including program code, via a network, network link 1620, and communication interface 1618. In an Internet example, server 1630 can transmit requested program code for an application via Internet 1628, ISP 1626, local area network 1622, and communication interface 1618. For example, such a downloaded application may provide one or more of the techniques described herein. The received program code may be executed by processor 1604 upon receipt and / or stored in storage device 1610 or other non-volatile storage for later execution. In this way, computer system 1600 can obtain application code in carrier form.

[0136] Further embodiments are disclosed in the following list of numbered items: 1. An optical filter device comprising: an optical diverging device operable to receive light pulses and spatially distributing the light pulses above an optical plane depending on the pulse energy of each of the light pulses; and a spatial filter located at the optical plane, operable to apply spatial filtering to the light pulses based on the position of each of the light pulses generated by the spatial distribution at the optical plane. 2. The optical filter device of item 1, wherein the optical diverging device comprises a deflectable mirror configured to receive and subsequently reflect the light pulses; the deflectable mirror is further configured to deflect after each reflection of each of the light pulses, the amount of deflection depending on the pulse energy of each of the light pulses and the deflection resulting in the spatial distribution of the light pulses above the optical plane. 3. An optical filter device as described in clause 2, wherein the deflectable mirror comprises a cantilever having a fixed end fixed in place and a free end movable in the reflective plane. 4. An optical filter device as described in clause 3, wherein the deflection of the deflectable mirror comprises radial movement of the free end relative to the fixed end at each effect of a light pulse. 5. An optical filter device as defined in any one of clauses 2 to 4, wherein the deflectable mirror comprises silicon. 6. An optical filter device as defined in any one of clauses 2 to 5, wherein the deflectable mirror comprises a width in the range of 1 µm to 1000 µm. 7. An optical filter device as defined in any one of clauses 2 to 6, wherein the deflectable mirror comprises a length in the range of 1 µm to 1000 µm. 8. An optical filter device as defined in any one of clauses 2 to 7, wherein the deflectable mirror has a thickness in the range of 0.1 µm to 100 µm. 9. An optical filter device as defined in any one of clauses 2 to 8, wherein the deflectable mirror is configured such that, after a second reflection, each of the light pulses is reflected in the propagation direction according to its pulse energy. 10. An optical filter device as defined in any one of clauses 2 to 9, wherein the deflectable mirror includes at least one reflective coating configured to reflect the light pulse. 11. An optical filter device as defined in clause 10, wherein the at least one reflective coating provides at least 80% reflectivity in the spectral range defined by the light pulse. 12. An optical filter device as defined in clause 11, wherein the spectral range covering the at least one reflective coating is between 100 nm and 4000 nm. 13. An optical filter device as defined in any one of clauses 2 to 12, wherein the deflectable mirror is configured to provide sufficient deflection to spatially distribute the optical pulse when the pulse energy of the optical pulse is in the range of 0.1 µJ to 100 µJ.14. An optical filter device as defined in any one of clauses 2 to 13, wherein the optical divergence device further includes an optical delay configuration configured to apply a delay time to the optical pulses after they have undergone a first reflection from the deflectable mirror and guide them back to the deflectable mirror for a second reflection. 15. An optical filter device as described in clause 14, wherein the delay time applied by the optical delay configuration is such that each of the optical pulses reaches the deflectable mirror for a second reflection when the deflection of the deflectable mirror is substantially at its maximum. 16. An optical filter device as described in clauses 14 or 15, wherein the optical delay configuration includes a configurable delay time. 17. An optical filter device as defined in any one of clauses 14 to 16, wherein the optical retardation configuration is configured to include at least two optical mirrors placed on a translation stage, the translation stage being movable to change the distance between the optical retardation configuration and the deflectable mirrors. 18. An optical filter device as defined in any of the preceding clauses, wherein the spatial filter includes: a first region configured to select a first portion of the optical pulses, the first portion being located within the first region; and a second region configured to at least partially block a second portion of the optical pulses, the second portion being located within the second region. 19. An optical filter device as defined in clause 18, wherein the spatial filter includes a substrate having an aperture. 20. An optical filter device as defined in clause 18, wherein the spatial filter is reflective, such that the first region includes a reflective region and the second region includes an absorptive region. 21. An optical filter device as defined in any one of clauses 18 to 20, wherein at least partially blocked optical pulses comprise optical pulses whose pulse energy deviates from the average energy of the optical pulses by more than 5%. 22. An optical filter device as defined in any one of clauses 18 to 20, wherein at least partially blocked optical pulses comprise optical pulses whose pulse energy deviates from the average energy of the optical pulses by more than 10%. 23. An optical filter device as defined in any one of clauses 18 to 20, wherein at least partially blocked optical pulses comprise optical pulses whose pulse energy deviates from the average energy of the optical pulses by more than 15%. 24. An optical filter device as defined in any one of clauses 18 to 23, wherein the spatial filter is configured such that the size and / or position of the first region is adjustable. 25. An optical filter device as defined in any of the preceding clauses, wherein the distance between the spatial filter and the optical divergence device is adjustable. 26. An optical fiber device as defined in any of the preceding clauses, further comprising one or more optical lenses configured to control the beam diameter of the optical pulses before they are received by an optical diverging device.27. A broadband light source device configured to generate broadband output radiation comprising such optical pulses after receiving pump radiation, the broadband light source device comprising an optical filter device as defined in any of the preceding clauses. 28. A broadband light source device configured to generate broadband output radiation comprising such optical pulses after receiving pump radiation, the broadband light source device comprising an optical filter device as defined in any of clauses 2 to 17, and configured such that the pulse interval time of the optical pulses is configured to be substantially equal to or longer than the time period required for the deflection of a deflectable mirror to be reduced to zero or substantially close to zero. 29. A broadband light source device as described in clause 27 or 28, wherein the broadband output radiation is generated in a hollow photonic crystal fiber (HC-PCF). 30. A measuring device comprising a broadband light source device as defined in any of clauses 26 to 28. 31. The measuring device of claim 30, comprising a scatterometer measuring device, a level sensor, or an alignment sensor. 32. A method for spatially filtering optical pulses, comprising: spatially distributing the optical pulses above an optical plane based on the pulse energy of each of a plurality of optical pulses; and spatially filtering the optical pulses based on the position of each of the optical pulses generated by the spatial distribution at the optical plane. 33. The method of claim 32, wherein the spatial distribution of the optical pulses further comprises: using a deflectable mirror to first reflect the optical pulses; applying a delay time to the optical pulses initially reflected from the deflectable mirror and subsequently guiding them back to the deflectable mirror; using the deflectable mirror to second reflect the delayed optical pulses to spatially distribute the optical pulses; wherein the deflectable mirror deflects after each reflection of each of the optical pulses, the deflection of the deflectable mirror depending on the pulse energy of each of the optical pulses. 34. The method of clause 33, wherein the applied delay time is such that each of the optical pulses reaches the deflectable mirror for a second reflection when the deflection of the deflectable mirror is substantially at its maximum. 35. The method of clauses 33 or 34, wherein the pulse interval of at least one optical pulse is substantially equal to or longer than the time period required for the deflection of the deflectable mirror to return to zero or substantially approach zero. 36. The method as defined in any one of clauses 32 to 35, wherein spatial filtering comprises at least partially blocking some of the optical pulses spatially distributed at the optical plane. 37. The method of clause 36, wherein the at least partially blocked optical pulses comprise optical pulses whose pulse energy deviates from the average energy of the optical pulses by more than 5%. 38. The method of clause 37, wherein the at least partially locked optical pulses comprise optical pulses whose pulse energy deviates from the average energy of the optical pulses by more than 10%. 39. The method of clause 38, wherein the light pulse that is at least partially blocked comprises a light pulse whose pulse energy deviates from the average energy of the light pulse by more than 15%.40. The method as defined in any one of clauses 32 to 39 further comprises controlling the beam diameter of the optical pulses before the optical pulses are spatially distributed above the optical plane.

[0137] While reference may be specifically made herein to the use of lithography equipment in IC manufacturing, it should be understood that the lithography equipment described herein may have other applications. Possible other applications include manufacturing integrated optical systems, guiding and detecting magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0138] Although embodiments of the invention may be specifically referenced herein in the context of lithography equipment, embodiments of the invention may be used in other equipment. Embodiments of the invention may form part of a mask inspection device, a metrology device, or any device for measuring or processing objects such as wafers (or other substrates) or masks (or other patterning devices). Such devices may generally be referred to as lithography tools. These lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0139] Although the foregoing may have specifically referenced the use of embodiments of the invention in the context of optical lithography, it should be understood that, where the context permits, the invention is not limited to optical lithography and can be used in other applications (e.g., embossing).

[0140] Although specific embodiments of the invention have been described above, it will be understood that the invention may be practiced in other ways different from those described. The above description is intended to be illustrative and not restrictive. Therefore, it will be apparent to those skilled in the art that modifications can be made to the described invention without departing from the scope of the claims set forth below. [Simplified Explanation of the Diagram]

[0013] Embodiments of the invention will now be described by way of example only with reference to the accompanying illustrative drawings, in which: - Figure 1 depicts a schematic overview of a lithography apparatus; - Figure 2 depicts a schematic overview of a lithography unit; - Figure 3 depicts a schematic illustration of a complete lithography, representing the collaboration between three key technologies for optimizing semiconductor manufacturing; - Figure 4 depicts a schematic overview of a scattering measurement device that may include a radiation source as a measuring device according to an embodiment of the invention; - Figure 5 depicts a schematic overview of a level sensor device that may include a radiation source according to an embodiment of the invention; - Figure 6 depicts a schematic overview of an alignment sensor device that may include a radiation source according to an embodiment of the invention; - Figure 7 is a schematic cross-sectional view of a hollow optical fiber in which a portion of a radiation source according to an embodiment can be formed in a transverse plane (i.e., perpendicular to the axis of the optical fiber); - Figure 8 depicts a schematic representation of a radiation source according to an embodiment for providing broadband output radiation; Figures 9(a) and 9(b) schematically depict cross-sections of examples of hollow-core photonic crystal fiber (HC-PCF) designs for supercontinuum spectrum generation; - Figure 10 is a graph showing the measured APR varying according to the laser repetition rate; - Figures 11(a) and 11(b) are two examples of simulated pulse amplitudes before and after the application of an optical filter configuration, respectively; - Figure 12 schematically depicts the operating principle of the optical filter configuration; - Figure 13 is a graph showing the simulated time-dependent deflection of a deflectable mirror (e.g., a cantilever) induced by a single reflection of three optical pulses, each with different pulse energies; - Figure 14 schematically depicts an embodiment of the optical filter configuration; - Figure 15 is a graph showing the simulated time-dependent deflection of a deflectable mirror (e.g., a cantilever) induced by two reflections of two consecutive optical pulses, as shown in the simulation in Figure 13; and - Figure 16 depicts a block diagram of a computer system used to control broadband radiation sources.

Claims

1. An optical filter device comprising: an optical divergence device operable to receive light pulses and spatially distributing the light pulses above an optical plane depending on the pulse energy of one of the light pulses; and a spatial filter located at the optical plane, operable to apply spatial filtering to the light pulses based on a position of each of the light pulses generated by the spatial distribution at the optical plane.

2. The optical filter device of claim 1, wherein the optical diverging device includes a deflectable mirror configured to receive and subsequently reflect the optical pulses; the deflectable mirror is further configured to deflect after each reflection of each of the optical pulses, the amount of deflection depending on a pulse energy of each of the optical pulses and the deflection resulting in the optical pulses being spatially distributed above the optical plane.

3. The optical filter device of claim 2, wherein the deflectable mirror includes a cantilever having a fixed end fixed in place and a free end movable in the reflective plane.

4. The optical filter device of claim 3, wherein the deflection of the deflectable mirror includes radial movement of the free end relative to one of the fixed ends attribution to each effect of the light pulses.

5. An optical filter device as claimed in any one of claims 2 to 4, wherein at least one of the following is true: the deflectable mirror has a width in the range of 1 µm to 1000 µm, the deflectable mirror has a length in the range of 1 µm to 1000 µm, and the deflectable mirror has a thickness in the range of 0.1 µm to 100 µm.

6. An optical filter device as claimed in any of claims 2 to 4, wherein the deflectable mirror is configured such that, after a second reflection, each of the optical pulses is reflected in a propagation direction according to its pulse energy.

7. An optical filter device as claimed in any one of claims 2 to 4, wherein the deflectable mirror includes at least one reflective coating configured to reflect the light pulses, and wherein, where appropriate, the at least one reflective coating provides at least 80% reflectivity in a spectral range defined by the light pulses.

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

9. An optical filter device as claimed in any one of claims 2 to 4, wherein the deflectable mirror is configured to provide sufficient deflection to spatially distribute the optical pulses when the pulse energy of the optical pulses is in the range of 0.1 µJ to 100 µJ.

10. An optical filter device as claimed in any one of claims 2 to 4, wherein the optical divergence device further includes an optical delay configuration configured to apply a delay time to the optical pulses after they have undergone a first reflection from one of the deflectable mirrors and guide them back to the deflectable mirrors to undergo a second reflection by the deflectable mirrors, wherein, where appropriate, the delay time applied by the optical delay configuration causes each of the optical pulses to reach the deflectable mirrors when the deflection of the deflectable mirrors is substantially at its maximum for the second reflection.

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

12. A broadband light source device configured to generate broadband output radiation comprising light pulses after receiving pump radiation, the broadband light source device comprising an optical filter device as claimed in any one of claims 1 to 11.

13. A broadband light source device configured to generate broadband output radiation comprising light pulses after receiving pump radiation, the broadband light source device comprising an optical filter device as claimed in any one of claims 2 to 11, and configured such that the pulse interval of one of the light pulses is configured to be substantially equal to or longer than a period of time required for the deflection of the deflectable mirror to be reduced to zero or substantially close to zero.

14. A measuring device comprising an optical filter device as claimed in any one of claims 1 to 11 or a broadband light source device as claimed in any one of claims 12 or 13, and, where appropriate, the measuring device comprising a scatterometer measuring device, a first-order sensor or an alignment sensor.

15. A method for spatially filtering optical pulses, comprising: spatially distributing the optical pulses above an optical plane based on the pulse energy of one of a plurality of optical pulses; and spatially filtering the optical pulses based on a position of each of the optical pulses generated by the spatial distribution at the optical plane.

Images