TW202441270A - Supercontinuum radiation source - Google Patents

Abstract

Disclosed is a broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each the supercontinuum generation stage comprising a respective nonlinear generation element. The plurality of supercontinuum generation stages comprises at least a first supercontinuum generation stage and a second supercontinuum generation stage, the second supercontinuum generation stage succeeding the first supercontinuum generation stage in the series. A damage tolerance of a first nonlinear generation element comprised within the first supercontinuum generation stage is greater than a damage tolerance of at least a second nonlinear generation element comprised within the second supercontinuum generation stage.

Description

Supercontinuum radiation source

The present invention relates to a hypercontinuum or broadband radiation source and, in particular, to such broadband radiation sources for metrological applications in integrated circuit manufacturing.

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

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

Low- _k1 lithography can be used to process features smaller than the classical resolution limit of the lithography equipment. In this procedure, the resolution formula 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 dimension" (usually the smallest feature size printed, but in this case half-pitch) and _k1 is an empirical resolution factor. In general, the smaller _k1 is, the more difficult it is to reproduce on the substrate a pattern that resembles 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 the design layout. These steps include, for example, but not limited to, optimization of NA, custom illumination schemes, use of phase-shift patterning devices, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement technology" (RET). Alternatively, a tight control loop for controlling the stability of the lithography equipment can be used to improve the reproduction of the pattern at low k1.

Metrology tools are used in many aspects of the IC manufacturing process, for example as alignment tools for properly positioning a substrate prior to exposure, as level metrology tools for measuring the surface topography of a substrate, as focus control and scatterometry based tools for inspecting/measuring exposed and/or etched products, for example in process control. In each case, a radiation source is required. Broadband or white light radiation sources are increasingly used in these metrology applications for various reasons, including measurement stability and accuracy. Modifications to current devices will be required for broadband radiation generation.

In a first aspect of the present invention, a broadband radiation source for generating output broadband radiation is provided, which comprises a plurality of supercontinuum generating stages arranged in series, each of the supercontinuum generating stages comprising a respective nonlinear generating element; wherein the plurality of supercontinuum generating stages comprises at least a first supercontinuum generating stage and a second supercontinuum generating stage. The second supercontinuum generating stage is a stage in the series of supercontinuum generating stages, wherein the second supercontinuum generating stage is after the first supercontinuum generating stage; and a damage tolerance of a first nonlinear generating element included in the first supercontinuum generating stage is greater than a damage tolerance of at least a second nonlinear generating element included in the second supercontinuum generating stage.

In a second aspect of the present invention, a broadband radiation source for generating output broadband radiation is provided, which comprises a plurality of supercontinuum generating stages arranged in series, each of the supercontinuum generating stages comprising a respective nonlinear generating element; wherein the plurality of supercontinuum generating stages comprises at least a first supercontinuum generating stage and a second supercontinuum generating stage, and in the series of supercontinuum generating stages, the second supercontinuum generating stage is after the first supercontinuum generating stage; and an optical nonlinearity of a first nonlinear generating element included in the first supercontinuum generating stage is lower than an optical nonlinearity of at least a second nonlinear generating element included in the second supercontinuum generating stage.

Other aspects of the present invention include a metrology device, which includes the broadband radiation source device of the first aspect or the second aspect.

In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including UV radiation (e.g., having a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 to 100 nm).

The terms "reduction mask", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a generic patterning device that can be used to impart a patterned cross-section to an incident radiation beam, which corresponds to the pattern to be produced in a target portion of a substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

FIG1 schematically depicts a lithography apparatus LA. The lithography apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask stage) MT constructed 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 constructed 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 is configured to project the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (eg, comprising one or more dies).

In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, 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 condition the radiation beam B to have a desired spatial and angular intensity distribution in a cross-section at the plane of the patterning device MA.

The term "projection system" PS as used herein should be interpreted broadly as covering various types of projection systems appropriate to the exposure radiation used and/or to other factors such as the use of an immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, synthetic, magnetic, electromagnetic and/or electro-optical systems or any combination thereof. Any use of the term "projection lens" herein should be considered synonymous with the more general term "projection system" PS.

The lithography apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid, such as water, having a relatively high refractive index, so as to fill the space between the projection system PS and the substrate W - this is also called immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference.

The lithography apparatus LA may also be of a type having two or more substrate supports WT (also known as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and/or a step of preparing the substrate W for subsequent exposure may be performed on a substrate W on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

In addition to the substrate support WT, the lithography apparatus LA may also comprise a measurement stage. The measurement stage is configured to hold sensors and/or cleaning devices. The sensors may be configured to measure characteristics of the projection system PS or of the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning device may be configured to clean parts of the lithography apparatus, for example parts of the projection system PS or parts of a system for providing an immersion liquid. The measurement stage may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

In operation, a radiation beam B is incident on a patterning device (e.g. a mask MA) held on a mask support MT and is 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. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, a first positioner PM and possibly a further position sensor (which is 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. The mask alignment marks M1, M2 and substrate alignment marks P1, P2 may be used to align the patterning device MA and the substrate W. Although the substrate alignment marks P1, P2 are shown occupying dedicated target portions, they may be located in the space between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe line alignment marks.

As shown in FIG. 2 , the lithography apparatus LA may form part of a lithography cell LC (sometimes also referred to as a lithocell or a (lithography) cluster), which often also comprises equipment for performing pre-exposure and post-exposure processes on a substrate W. As is known, such equipment comprises a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, cooling plates CH and baking plates BK, for example for regulating the temperature of the substrate W (for example for regulating the solvent in the resist layer). A substrate handler or robot RO picks up substrates W from input/output ports I/O1, I/O2, moves substrates W between different process equipment and delivers substrates W to a loading box LB of the lithography apparatus LA. The devices in the lithography manufacturing unit, which are often collectively referred to as automated photoresist coating and developing systems, are usually under the control of an automated photoresist coating and developing system control unit TCU. The automated photoresist coating and developing system control unit TCU itself can be controlled by a supervisory control system SCS, and the supervisory control system SCS can also control the lithography equipment LA, for example, via the lithography control unit LACU.

In order to correctly and consistently expose the substrate W exposed by the lithography apparatus LA, it is necessary to inspect the substrate to measure characteristics of the patterned structure, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithography fabrication cell LC. If an error is detected, then, for example, adjustments may be made to the exposure of subsequent substrates or to other processing steps to be performed on the substrate W, especially if the inspection is performed before other substrates W of the same batch or lot are still to be exposed or processed.

The inspection apparatus, which may also be referred to as metrology apparatus, is used to determine characteristics of a substrate W and, in particular, to determine how characteristics of different substrates W vary or how characteristics associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on the substrate W and may, for example, be part of a lithography fabrication cell LC or may be integrated into a lithography apparatus LA or may even be a stand-alone device. The inspection equipment can measure characteristics on a latent image (an image in a resist layer after exposure), or on a semi-latent image (an image in a resist layer after a post-exposure bake step (PEB), or on a developed resist image (where either the exposed or unexposed portions of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

Typically the patterning process in a lithography apparatus LA is one of the most critical steps in the process, which requires a high accuracy of the dimensioning and placement of the structures on the substrate W. In order to ensure this high accuracy, three systems can be combined in a so-called "holistic" control environment, as schematically depicted in FIG3 . One of these systems is the lithography apparatus LA, which is (actually) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and to provide a tight control loop, thereby ensuring that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines a set of process parameters (e.g., dose, focus, overlay) within which a particular fabrication process produces a defined result (e.g., a functional semiconductor device) - typically within which process parameters in a lithography process or patterning process are allowed to vary.

The computer system CL may use (a portion of) the design layout to be patterned to predict which resolution enhancement technique to use and perform computational lithography simulations and calculations to determine which mask layout and lithography equipment settings achieve the maximum overall process window for the patterning process (depicted in FIG. 3 by the double arrows in the first scale SC1). Typically, the resolution enhancement technique is configured to match the patterning possibilities of the lithography equipment LA. The computer system CL may also be used to detect where within the process window the lithography equipment LA is currently operating (e.g. using input from a metrology tool MT) to predict whether defects exist due to, for example, suboptimal processing (depicted in FIG. 3 by the arrow pointing to "0" in the second scale SC2).

The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithography apparatus LA to identify possible drifts in the calibration state of the lithography apparatus LA, for example (depicted in FIG. 3 by arrows in the third scale SC3).

In lithography processes it is frequently necessary to carry out measurements of the created structures, e.g. for process control and verification. The tool used to carry out such measurements is generally referred to as a metrology tool MT. Different types of metrology tools MT for carrying out such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments which allow measuring parameters of the lithography process either by having sensors in the pupil or in a plane conjugated to the pupil of the objective lens of the scatterometer (the measurement is generally referred to as pupil-based measurement), or by having sensors in the image plane or in a plane conjugated to the image plane, in which case the measurement is generally referred to as image- or field-based measurement. Such scatterometers and associated measurement techniques are further described in 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 visible to the near IR wavelength range.

In a first embodiment, the scatterometer MT is an angle-resolving scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signal to reconstruct or calculate the properties of the grating. This reconstruction can, for example, result from simulating the interaction of the scattered radiation with a mathematical model of the target structure 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 that is similar to the diffraction pattern observed from a real target.

In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In this spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed onto a spectrometer detector which measures the spectrum of the spectroscopically reflected radiation (i.e. a measure of the intensity as a function of wavelength). From this data, the structure or contour of the target which gave rise to the detected spectrum can be reconstructed, for example by Rigorous Coupled Wave Analysis and nonlinear regression or by comparison with a library of simulated spectra.

In a third embodiment, the scatterometer MT is an elliptical metrology scatterometer. An elliptical metrology scatterometer allows to determine parameters of a lithography process by measuring the scattered radiation for each polarization state. The metrology apparatus emits polarized light (such as linear, annular or elliptical) by using, for example, appropriate polarization filters in the illumination section of the metrology apparatus. A source suitable for the metrology apparatus may also provide polarized radiation. Various embodiments of prior art elliptical measurement 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.

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 reflected spectrum and/or detecting asymmetries in the configuration, which asymmetries are related to the extent of the stacking. The two (usually overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers) and may be formed to be in substantially the same position on the wafer. The scatterometer may have a symmetric detection configuration such as described in the commonly owned patent application EP1,628,164A, so that any asymmetries can be clearly distinguished. This provides a direct way to measure misalignment in the gratings. Other examples of measuring overlay errors between two layers comprising a periodic structure when the target is measured through the asymmetry of the periodic structure can be found in PCT Patent Application Publication No. WO 2011/012624 or U.S. Patent Application US 20160161863, which are incorporated herein by reference in their entirety.

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

A metrology target may be an aggregate of a composite grating formed by a lithography process, primarily in a resist, and also formed, for example, after an etching process. Typically, the pitch and linewidth of the structures in the grating depend largely on the measurement optics (particularly the NA of the optics) to be able to capture the diffraction order from the metrology target. As indicated earlier, the diffraction signal may be used to determine the shift between two layers (also referred to as "overlap") or may be used to reconstruct at least a portion of the original grating produced by the lithography process. This reconstruction may be used to provide quality guidance for the lithography process, and may be used to control at least a portion of the lithography process. The target may have smaller sub-segments configured to mimic the size of a functional portion of a design layout in the target. Due to this sub-segmentation, the target will behave more like a functional part of the design layout, making the overall process parameter measurement better resemble the functional part of the design layout. The target can be measured in the underfill mode or in the overfill mode. In the underfill mode, the measurement beam produces a spot that is smaller than the overall target. In the overfill mode, the measurement beam produces a spot that is larger than the overall target. In this overfill mode, it is also possible to measure different targets at the same time and thus determine different process parameters at the same time.

The overall measurement quality of a lithographic parameter using a specific target is determined at least in part by the measurement recipe used to measure the lithographic parameter. The term "substrate measurement recipe" can include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, one or more of the measured parameters can 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 recipe can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US 2016/0370717A1, which are incorporated herein by reference in their entirety.

A metrological device, such as a scatterometer, is depicted in Figure 4. The metrological device comprises a broadband (white light) radiation projector 2 which 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 radiation reflected by the mirror (i.e. a measurement of the intensity as a function of wavelength). From this data, the structure or profile which gave rise to the detected spectrum can be reconstructed by the processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra as shown at the bottom of Figure 3. In general, for the reconstruction, the general form of the structure is known, and some parameters are assumed from knowledge of the procedure used to make the structure, leaving only a few parameters of the structure to be determined from the scattering measurement data. The scatterometer can be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

The overall measurement quality of a lithographic parameter of a metrology target is determined at least in part by the measurement recipe used to measure the lithographic parameter. The term "substrate measurement recipe" may include one or more parameters of the measurement itself, one or more parameters of one or more patterns measured, or both. For example, if the measurement used in the substrate measurement recipe 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 recipe may, for example, be the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016/0161863 and published U.S. patent application US 2016/0370717A1, which are incorporated herein by reference in their entirety.

Another type of metrology tool used in IC manufacturing is a topographic measurement system, a step sensor, or a height sensor. This tool can be integrated into a lithography apparatus and used to measure the topography of the top surface of a substrate (or wafer). A map of the substrate's topography, also called a height map, can be generated from these measurements that indicates the height of the substrate as a function of position on the substrate. This height map can then be used to correct the position of the substrate during transfer of the pattern onto the substrate in order to provide an aerial image of the patterned device in a properly focused position on the substrate. It should be understood that "height" in this context generally refers to the dimension from the plane to the substrate (also called the Z axis). Typically, a step or height sensor performs measurements at a fixed position (relative to its own optical system), and relative movement between the substrate and the optical system of the step or height sensor results in a height measurement at a position across the substrate.

An example of a level or height sensor LS known in the art is schematically shown in FIG5 , which merely illustrates the operating principle. In this example, the level sensor comprises an optical system, which comprises a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a radiation beam LSB, which is imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or wideband light source, such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO may comprise 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 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.

The projection grating PGR is a periodic grating comprising a periodic structure which generates a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed to a measuring position MLO on the substrate W with an incident angle ANG with respect to an axis (Z axis) perpendicular to the incident substrate surface, the incident angle ANG being between 0 and 90 degrees, typically between 70 and 80 degrees. At the measuring position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed to the detection unit LSD.

To determine the height level at the measuring position MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET generates a detector output signal, which is indicative of the received light, for example indicating the intensity of the received light, such as a light detector, or representing the spatial distribution of the received intensity, such as a camera. The detector DET may comprise any combination of one or more detector types.

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

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

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 located. This configuration provides a more direct detection of the image of the projection grating PGR.

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

For example, various height sensors of the general type are disclosed in US7265364 and US7646471, both of which are incorporated by reference. A height sensor that uses UV radiation rather than visible or infrared radiation is 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 the need to detect the grating.

Another type of metrology tool used in IC manufacturing is an alignment sensor. Therefore, a key aspect of the performance of a lithography apparatus is the ability to properly and accurately place an applied pattern relative to features placed in a previous layer (either by the same apparatus or a different lithography apparatus). For this purpose, a substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can later be measured using a position sensor (usually an optical position sensor). The position sensor may be referred to as an "alignment sensor" and the mark may be referred to as an "alignment mark."

A lithography apparatus may include one or more (e.g., multiple) alignment sensors by which the positions of alignment marks provided on a substrate can be accurately measured. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on a substrate. An example of an alignment sensor used in a current lithography apparatus is based on a self-referencing interferometer described in US6961116. Various enhancements and modifications of position sensors have been developed, such as disclosed in US2015261097A1. The contents of all such publications are incorporated herein by reference.

FIG6 is a schematic block diagram of an embodiment of a known alignment sensor AS as described, for example, in US6961116 and incorporated by reference. A radiation source RSO provides a radiation beam RB having one or more wavelengths which is redirected by a steering optic onto a mark, such as a mark AM on a substrate W, as an illumination spot SP. In this example, the steering optics comprises a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP with which the mark AM is illuminated may be slightly smaller than the width of the mark itself.

Radiation diffracted by the alignment mark AM (in this example via the objective lens OL) is collimated into an information-carrying beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI, for example of the type disclosed in US6961116 mentioned above, interferes with the beam IB with itself, whereupon the beam is received by the photodetector PD. Additional optics (not shown) may be included to provide separate beams in the event that more than one wavelength is generated by the radiation source RSO. The photodetector may be a single element, or it may comprise a plurality of pixels as required. The photodetector may comprise an array of sensors.

The steering optics comprising the spot mirror SM in this example may also be used to block the zero-order radiation reflected from the marker so that the information-carrying beam IB contains only the high-order diffraction radiation from the marker AM (which is not necessary for measurement but improves the signal-to-noise ratio).

The intensity signal SI is supplied to the processing unit PU. Through the combination of optical processing in block SRI and computational processing in unit PU, the values of the X and Y positions of the substrate relative to the reference frame are output.

A single measurement of the type described only fixes the position of the mark to a certain range corresponding to a spacing of the mark. A coarser measurement technique is used in conjunction with this measurement to identify which cycle of the sine wave is the cycle that contains the marked position. The same procedure at coarser and/or finer levels may be repeated at different wavelengths for increased accuracy and/or for robust detection of the mark independent of the material from which the mark is made and the material on which the mark is provided. The wavelengths may be optically multiplexed and demultiplexed so as to process them simultaneously and/or may be multiplexed by time or frequency division.

In this example, the alignment sensor and the light spot SP remain stationary, while the substrate W moves. The alignment sensor can thus be stably 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 the substrate W on a substrate support and 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. The measurement of the position of the marks provided on the substrate support allows calibration of the position of the substrate support determined by the position sensor (e.g. relative to a frame to which the alignment system is connected). Measuring the position of alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

The metrology tools MT mentioned above (such as scatterometers, configuration measurement systems or position measurement systems) can use radiation originating from a radiation source to perform measurements. The characteristics of the radiation used by the metrology tool can affect the type and quality of 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 can be used. Multiple different frequencies may be able to propagate, irradiate and scatter the metrology target without interfering with other frequencies or with minimal interference with other frequencies. Therefore, for example, different frequencies can be used to obtain more metrology data simultaneously. Different radiation frequencies may also be able to query and discover different characteristics of the metrology target. Broadband radiation can be used in metrology systems such as position sensors, alignment marker measurement systems, scatterometry tools or detection tools. The broadband radiation source can be a hypercontinuum source.

High quality broadband radiation, such as supercontinuum radiation, may be difficult to produce. One method for producing broadband radiation may be, for example, to widen high power narrowband or single frequency input radiation or pump radiation using nonlinear higher order effects. The input radiation, which may be produced using a laser, may be referred to as pump radiation. Alternatively, the input radiation may be referred to as seed radiation. In order to obtain high power radiation for the widening effect, the radiation may be confined to a small area so that strongly localized high intensity radiation is achieved. In those areas, the radiation may interact with the widening structure and/or the material forming the nonlinear medium to produce 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.

In some implementations, broadband output radiation is generated in photonic crystal fibers (PCFs). In several embodiments, such PCFs have microstructures around their fiber cores that help confine radiation traveling through the fiber in the fiber core. The fiber core can be made of a solid material that has nonlinear properties and is capable of generating broadband radiation when high intensity pump radiation is transmitted through the fiber core. Although it is feasible to generate broadband radiation in solid core PCFs, there can be several disadvantages to using solid materials. For example, if UV radiation is generated in a solid core, it may not be present in the output spectrum of the fiber because it is absorbed by most solid materials and causes permanent damage.

In some implementations, as further discussed below with reference to FIG. 8 , methods and apparatus for broadening input radiation may use an optical fiber for limiting input radiation and for broadening the input radiation to output broadband radiation. The optical fiber may be a hollow core optical fiber and may include internal structures for achieving efficient guiding and confinement of radiation in the optical fiber. The optical fiber may be a hollow core photonic crystal fiber (HC-PCF), which is particularly suitable for achieving high radiation intensity by confining strong radiation primarily within the hollow core of the optical fiber. The hollow core of the optical fiber may be filled with a gas, which acts as a broadening medium for broadening the input radiation. This fiber and gas configuration can be used to produce a supercontinuum radiation source. The radiation input to the fiber can be electromagnetic radiation, such as radiation in one or more of the infrared, visible, UV, and extreme UV spectra. The output radiation can consist of or include broadband radiation, which can be referred to herein as white light.

Some embodiments relate to a new design of this broadband radiation source comprising an optical fiber. The optical fiber is a hollow core photonic crystal fiber (HC-PCF). In detail, the optical fiber can be a hollow core photonic crystal fiber of the type that includes an anti-resonance structure for limiting radiation. Such optical fibers including anti-resonance structures are known in the art as anti-resonance fibers, tubular fibers, single-loop fibers, negative curvature fibers, or suppressed coupling fibers. Various different designs of such optical fibers are known in the art. Alternatively, the optical fiber can be a photonic bandgap fiber (HC-PBF, such as Kagome fiber).

There are many types of HC-PCFs that can be engineered, each based on a different physical guidance mechanism. Two such HC-PCFs include: hollow core photonic bandgap fiber (HC-PBF) and hollow core 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 core anti-resonant reflective fiber), which are incorporated herein by reference. FIG. 9( a ) shows a Kagome fiber comprising a Kagome lattice structure.

An example of an optical fiber used in a radiation source is now described with reference to Figure 7, which is a schematic cross-sectional view of the optical fiber OF in a transverse plane. Other embodiments similar to the actual example of the optical fiber of Figure 7 are disclosed in WO2017/032454A1.

The optical fiber OF comprises an elongated body, the optical fiber OF being longer in one dimension compared to the other two dimensions of the optical fiber OF. This longer dimension may be referred to as the axial direction, and may define the axis of the optical fiber OF. The two other dimensions define a plane that may be referred to as a transverse plane. FIG. 7 shows a cross-section of the optical fiber OF in this transverse plane (i.e., perpendicular to the axis) labeled as the x-y plane. The transverse cross-section of the optical fiber OF may be substantially constant along the axis of the optical fiber.

It will be appreciated that the 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 a local optical axis, a local transverse cross section, etc. Furthermore, where components are described as being cylindrical or tubular, these terms will be understood to encompass such shapes that may have been deformed when the optical fiber OF is bent.

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

The optical fiber OF includes: 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 regarded as including a body (including the cladding portion and the support portion SP) having the hollow core HC. The cladding portion includes a plurality of anti-resonance elements for guiding radiation passing through the hollow core HC. In detail, the plurality of anti-resonance elements are configured to limit the radiation propagating through the optical fiber OF mainly inside the hollow core HC, and are configured to guide the 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, so that the axis of the optical fiber OF can also define the axis of the hollow core HC of the optical fiber OF.

The cladding portion includes a plurality of anti-resonance elements for guiding radiation propagation through the optical fiber OF. Specifically, in this embodiment, the cladding portion includes a single ring of six tubular capillaries CAP. Each of the tubular capillaries CAP acts as an anti-resonance element. It should be understood that these anti-resonance elements can be in different cross-sections, such as an elliptical cross-section or a nested circular cross-section, in which a smaller diameter circular tube is located in a larger diameter circular tube.

The capillaries CAP may also be referred to as tubes. The cross-section of the capillaries CAP may be circular, or may have another shape. Each capillary CAP comprises a substantially cylindrical wall portion WP which 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 appreciated that the wall portion WP may act as an anti-reflection Fabry-Perot resonator for radiation propagating through the hollow core HC (and which may be incident on the wall portion WP at a grazing angle of incidence). The thickness of the wall portion WP may be suitable so as to ensure that reflection returning to the hollow core HC is substantially enhanced while transmission into the capillary cavity CC is substantially suppressed. In some embodiments, the capillary wall portion WP may have a thickness between 0.01 μm and 10.0 μm.

It should be understood that as used herein, the term "cladding" is intended to mean the portion of the optical fiber OF that serves to guide radiation propagating through the optical fiber OF (i.e., the capillaries CAP that confine the radiation within the hollow core HC). The radiation may be confined in a transverse mode, thereby propagating along the fiber axis.

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

The capillaries CAP are configured so that each capillary does not contact any of the other capillaries CAP. Each of the capillaries CAP is in contact with the inner support portion SP and is spaced apart from adjacent capillaries CAP in the ring structure. This configuration may be beneficial because it may increase the transmission bandwidth of the optical fiber OF (relative to, for example, a configuration in which the capillaries are in contact with each other). Alternatively, in some embodiments, each of the capillaries CAP may be in contact with adjacent capillaries CAP in the ring structure.

The six capillaries CAP of the cladding are arranged in a ring structure around the hollow core HC. The inner surface of the ring structure of the capillaries CAP at least partially defines the hollow core HC of the optical fiber OF. The diameter d of the hollow core HC (which can be defined as the smallest dimension between the relative capillaries, indicated by the arrow d) can be between 10 µm and 1000 µm. The diameter d of the hollow core HC can affect the mode field diameter, impact loss, dispersion, modal multiplicity and nonlinear characteristics of the hollow core HC optical fiber OF.

In this embodiment, the cladding portion comprises a single ring arrangement of capillaries CAP (which act as anti-resonance elements). Thus, a line in any radial direction from the center of the hollow core HC to the outside of the optical fiber OF passes through no more than one capillary CAP.

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

FIG9( b) shows a modified embodiment of the HC-PCF with a single ring of tubular capillaries discussed above. In the example of FIG9( b), there are two coaxial rings of tubular capillaries 21. To hold the inner and outer rings of tubular capillaries 21, support tubes ST may be included in the HC-PCF. The support tubes may be made of silicon dioxide.

The tubular capillary of the examples of FIG. 7 and FIG. 9(a) and FIG. 9(b) may have a circular cross-sectional shape. Other shapes are also possible for the tubular capillary, such as an elliptical or polygonal cross-section. In addition, the solid material of the tubular capillary of the examples of FIG. 7 and FIG. 9(a) and FIG. 9(b) may include a plastic material such as PMA, glass such as silicon dioxide, or soft glass.

FIG8 depicts a known radiation source RDS for providing broadband output radiation. The radiation source RDS may include a pulsed pump radiation source PRS, a continuous wave source, or any type of source capable of producing short pulses of the desired length and energy level; an optical fiber OF with a hollow core HC (e.g., of the type shown in FIG7 ); and a working medium WM (e.g., a gas) disposed within the hollow core HC. Although in FIG8 the radiation source RDS includes the optical fiber OF shown in FIG7 , in alternative embodiments other types of hollow core HC optical fibers OF may be used.

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

The radiation source RDS further comprises a reservoir RSV. The optical fiber OF is placed inside the reservoir RSV. The reservoir RSV may also be referred to as a shell, a container or an air cell. The reservoir RSV is configured to contain a working medium WM. The reservoir RSV may include one or more features known in the art for controlling, regulating and/or monitoring the composition of the working medium WM (which may be a gas or a liquid) inside the reservoir RSV. The reservoir RSV may include a first transparent window TW1. In use, the optical fiber OF is placed inside the reservoir RSV so 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 reservoir RSV. The first transparent window TW1 may be transparent at least to the received input radiation frequency so that the received input radiation IRD (or at least a large part thereof) may be coupled into the optical fiber OF located inside the reservoir RSV. It will be appreciated that optics (not shown) may be provided for coupling the input radiation IRD into the optical fiber OF.

The reservoir RSV comprises a second transparent window TW2 forming part of a wall of the reservoir RSV. In use, when the optical fiber OF is placed inside the reservoir RSV, the second transparent window TW2 is located close to the output end OE of the optical fiber OF. The second transparent window TW2 may be transparent at least for the frequency of the broadband output radiation ORD of the device 120.

Alternatively, in another embodiment, the two opposite ends of the optical fiber OF may be placed inside different collectors. The optical fiber OF may include a first end section configured to receive input radiation IRD, and a second end section for outputting broadband output radiation ORD. The first end section may be placed inside a first collector including a working medium WM. The second end section may be placed inside a second collector, wherein the second collector may also include a working medium WM. The operation of the collector may be as described above with respect to Figure 8. The first collector may include a first transparent window, which is configured to be transparent to the input radiation IRD. The second collector may include a second transparent window, which is 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 the reservoir and partially placed outside the reservoir so that the gas can be sealed inside the reservoir. The optical fiber OF may further include a middle section that is not contained within the reservoir. This configuration using two separate gas reservoirs may be particularly convenient for embodiments where the optical fiber OF is relatively long (e.g., when the length exceeds 1 m). It should 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 gas within the two reservoirs) can be considered as an apparatus for providing a working medium WM within the hollow core HC of the optical fiber OF.

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

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

The working medium WM may include: rare gases, such as argon, krypton and xenon; Raman active gases, such as hydrogen, deuterium and nitrogen; gas mixtures, such as argon/hydrogen mixtures, xenon/deuterium mixtures, krypton/nitrogen mixtures; molecular gas mixtures, such as nitrogen/hydrogen mixtures; atomic gas mixtures, such as argon/helium, krypton/helium, xenon/helium; or ternary gas mixtures, such as argon/helium/hydrogen, or krypton/helium/hydrogen. Depending on the thermodynamic conditions of the RSV, such as the type of filling gas, its pressure and temperature, and on the laser conditions, such as pulse duration, energy and wavelength, nonlinear optical processes may include modulation instability (MI), soliton self-compression, soliton splitting, Kerr effect, Raman effect and dispersive wave generation (DWG), the details of which are described in WO2018/127266A1 and US9160137B1 (both of which are hereby incorporated by reference). Since the aforementioned parameters can be tuned, the generated broadband pulse dynamics and the associated spectral broadening characteristics can be adjusted in order to optimize the frequency conversion. For example, the dispersion of an air-filled hollow-core fiber, which is a key element in defining the range of interaction between the laser pulse and the WM, can be tuned by varying the working medium WM pressure (i.e., the air cell pressure) in the reservoir RSV.

In one implementation, the working medium WM may be disposed in the hollow core HC at least during the reception of input radiation IRD for generating broadband output radiation ORD. It should be understood that when the optical fiber OF does not receive input radiation IRD for generating broadband output radiation, the gas WM may not be present in the hollow core HC in whole or in part.

In order to achieve frequency broadening, high intensity radiation may be required. An advantage of having a hollow core HC fiber OF is that it can achieve high intensity radiation via strong spatial confinement of the radiation propagating through the fiber OF, thereby achieving a high localized radiation intensity. The radiation intensity inside the fiber OF may be higher, for example, due to a high received input radiation intensity and/or due to the strong spatial confinement of the radiation inside the fiber OF. The advantage of hollow core optical fibers is that they can guide radiation having a wider range of wavelengths than solid core optical fibers, and in particular, hollow core optical fibers can guide radiation in the ultraviolet and infrared ranges in the region of the spectrum that fused silica (the typical glass used in optical fibers) absorbs.

The advantage of using a hollow core HC optical fiber OF may be that most of the radiation guided inside the optical fiber OF is confined in the hollow core HC. Therefore, most of the interaction of the radiation inside the optical fiber OF is with the working medium WM, which is disposed inside the hollow core HC of the optical fiber OF. As a result, the broadening effect of the working medium WM on the radiation may be increased.

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 include ultrafast pulses such as those generated by a laser.

The input radiation IRD may be radiation that is temporally and/or spatially coherent. The input radiation IRD may be collimated radiation, which may have the advantage of facilitating and increasing the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD may comprise a single frequency or a narrow frequency range. The input radiation IRD may be generated by a laser. Similarly, the output radiation ORD may be collimated and/or may be temporally and/or spatially coherent.

The broadband range of output radiation ORD may be a continuous range, including a continuous range of radiation frequencies. The output radiation ORD may include hypercontinuum radiation. Continuous radiation may be beneficial for use in several applications, such as in metrology applications. For example, a continuous frequency range may be used to interrogate a large number of characteristics. The continuous frequency range may, for example, be used to determine and/or eliminate frequency dependencies of measured characteristics and/or enable the use of spectral filtering devices to quickly select and/or switch between different frequencies. The hypercontinuum output radiation ORD may include, for example, electromagnetic radiation in the wavelength range of 100 nm to 4000 nm. The broadband output radiation ORD frequency range may be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD may include white light.

The input radiation IRD provided by a pulsed pump radiation source PRS (or other input radiation source) may be pulsed. The input radiation IRD may include electromagnetic radiation of one or more frequencies between 200 nm and 3 µm. The input radiation IRD may, for example, include electromagnetic radiation having a wavelength of 1.03 µm. The repetition rate of the pulsed radiation IRD may be of the order of 1 kHz to 100 MHz. The pulse energy may be of the order of 1 nJ to 100 µJ (e.g. 1 µJ to 10 µJ). The pulse duration of the input radiation IRD may be between 1 fs and 10 ps, e.g. 300 fs. The average power input to the radiation IRD may be between 100 mW and several 100 W. The average power input to the radiation IRD may be, for example, 20 to 50 W.

The pulsed pump radiation source PRS may be a laser. The spatiotemporal transmission properties of this laser pulse transmitted along the optical fiber OF, such as its spectral amplitude and phase, may be varied and tuned by adjusting the (pump) laser parameters, the tuned thermodynamic properties of the working medium WM and/or the optical fiber OF parameters, such as its core diameter and/or length. These spatiotemporal transmission properties may include one or more of the following: output power, output mode profile, output temporal profile, width of the output temporal profile (or output pulse width), output spectral profile and bandwidth of the output spectral profile (or output spectral bandwidth). The input radiation source IRS parameters may include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate. The optical fiber OF parameters may include one or more of the following: optical fiber length, size and shape of hollow core HC, size and shape of capillary, thickness of the wall of capillary surrounding hollow core HC. The working component WM (e.g. filling gas) parameters may include one or more of the following: gas type, gas pressure and gas temperature, or gas composition and/or partial pressure in the case where WM is a mixture of different gases.

The broadband output radiation ORD provided by the radiation source RDS may have an average output power of at least 100 mW. The average output power may be 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 be a continuous wave CW broadband output radiation ORD. The broadband output radiation ORD may have a power spectral density in the entire wavelength band of the output radiation of at least 0.01 mW/nm. The power spectral density in the entire wavelength band of the broadband output radiation may be at least 3 mW/nm.

As described above, there are many nonlinear optical processes involved in generating broadband output radiation ORD (e.g., supercontinuum or white light). Which nonlinear optical processes have a more pronounced spectral broadening effect than others will depend on how the operating parameters are set. For example, by choosing the pump wavelength and/or the fiber OF so that the pump pulse propagates through the fiber in the normal dispersion region (positive group velocity dispersion (GVD)), self-phase modulation becomes the dominant nonlinear optical process and is responsible for the spectral broadening of the pump pulse. However, in most cases, the spectral broadening of the input radiation IRD provided by the pulsed pump radiation source PRS is driven by the soliton dynamics that require the pump pulse to propagate in the optical fiber OF in the anomalous dispersion region (negative GVD). This is because in the anomalous dispersion region, the effects of Kerr nonlinearity and dispersion act in opposition to each other, causing the pulse to maintain and/or enhance its peak intensity. When the pulse parameters of the pump pulse launched into the optical fiber OF with anomalous dispersion (e.g., HC-PCF) do not exactly match the pulse parameters of the soliton, the pump pulse will evolve into a soliton pulse with a certain soliton order and a dispersion wave.

It is well known that soliton self-compression and modulation instability are the two main mechanisms for spectral broadening in the generation of soliton driven broadband radiation. The distinction between the two mechanisms is that the soliton self-compression process is associated with low soliton orders, while the modulation instability process is associated with high soliton orders. The soliton order N of a pulsed input radiation IRD is a convenient parameter that can be used to distinguish between conditions where the spectral broadening is dominated by modulation instability and conditions where the spectral broadening is dominated by soliton self-compression. The soliton order N of a pulsed input radiation IRD is given as follows: (1) Where γ is the nonlinear phase (or nonlinear parameter); _Pp is the pump peak power of the pulsed input radiation IRD; τ is the pump pulse duration of the pulsed input radiation IRD; and _β2 is the group velocity dispersion of the nonlinear elements (e.g., optical fiber and working medium WM).

when , spectral broadening is usually dominated by modulation instability, while , the spectral broadening is usually dominated by soliton self-compression.

Some known broadband radiation sources use a configuration that produces spectral broadening of the pulsed pump radiation but in which the parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to allow modulation instabilities to produce spectral broadening. There are a number of reasons why modulation instabilities are used to produce spectral broadening. First, modulation instabilities are known to produce broadband radiation with a relatively flat intensity wavelength distribution, with the limitation that a sufficient number of pulses are averaged. Such broadband radiation sources may be referred to as white light radiation sources (due to the relatively flat spectral intensity distribution). Second, modulation instability can be achieved using relatively low-cost laser sources as pump radiation sources.

On the other hand, in the soliton self-compression regime, the input pump pulse undergoes compression in the time domain, which is accompanied by an increase in the spectral width. After soliton self-compression, the compressed pulse undergoes soliton fissioning, in which the pulse splits into multiple solitons. This soliton fissioning causes a temporal broadening and spectral shift of the radiation pulse.

To summarize the above, for example, the principle behind the radiation source of Figure 8, this technology for supercontinuum (SC) generation relies on driving the interaction of laser radiation with nonlinear elements in order to increase the spectral bandwidth. In this case, the peak intensity of the laser radiation is high enough that it undergoes a number of optical nonlinear effects, thereby broadening its spectrum. Typically, the wavelength of the laser pulse is chosen so that it falls within the normal or anomalous dispersion range of the nonlinear element. In the former case, the broadening is governed by self-phase modulation (SPM), while in the latter case, the spectral broadening is governed by soliton dynamics. Alternatively, nonlinear elements can be designed to provide pure normal dispersion, such as all-normal dispersion (ANDi) fiber. In this case, the widening is controlled by SPM and optical wave breaking.

The method can produce very non-uniform power spectral density (PSD). For example, the PSD of the pump laser may drop by two to three orders of magnitude. This is an indication of low conversion efficiency from the driving laser pulse to the supercontinuum region of interest (which may be, for example, 100 nm to 1200 nm, 300 nm to 1000 nm, or 400 nm to 900 nm). The same is true for modulation instability (MI) based systems, which are a result of the stochastic nature of the high-order soliton based supercontinuum generation dynamics and the broadening process starting from the noise.

It is known to use polychromatic pumping to expand the supercontinuum spectrum and/or increase its PSD at the short-wavelength long edge of the supercontinuum spectrum. A typical configuration may include using a pump laser fundamental harmonic and its (e.g., second) harmonic to pump nonlinear elements. However, this approach has disadvantages. For example, there is a large gap in the spectrum between the fundamental frequency of the pump laser and its second-order harmonic, where no substantially strong radiation is provided to the nonlinear elements. In addition, the fundamental harmonic and its harmonics are in different dispersion ranges of the nonlinear medium. This causes the laser radiation in normal dispersion to broaden rapidly over time, thereby losing its peak intensity, which in turn leads to a reduction in spectral broadening. In addition, several laser systems or harmonic generation stages may be required to implement this method. Therefore, for high-power applications, harmonic generation in high-power laser systems is extremely challenging and technically expensive.

In single-frequency or multicolor generation, the supercontinuum is the result of the interaction of a laser pulse (or multiple laser pulses) with only a single nonlinear element level. The spectral expansion is therefore limited to the interaction range of only one nonlinear element and is limited by its nonlinear properties and propagation dynamics. For example, the spectral shape in soliton self-compression (SSC) is determined by the phase matching conditions of the dispersive waves via gas pressure, core diameter, etc. After this broadening phase, the PSD will remain in the state provided by the nonlinear interaction.

Nonlinear media are pumped with laser pulses to achieve sufficiently high peak intensities to initiate nonlinear optical processes such as SSC or MI. However, recent advances in supercontinuum generation and nonlinear optical media have enabled the generation of supercontinuum with high peak intensities that are also sufficient to initiate nonlinear optical processes in subsequent nonlinear elements/media. For example, high energy, high peak intensity supercontinuum can be generated in air-filled hollow core fibers (e.g., HC-PCF) via MI or SSC processes.

Thus, it is proposed to generate broadband radiation by pumping nonlinear media/components with supercontinuum instead of laser pulses. The supercontinuum should be strong enough and/or wide enough to drive further nonlinear optical phenomena in the optically nonlinear medium. In the context of the present invention, a supercontinuum comprises radiation (e.g. originating from a laser) which subsequently undergoes at least one spectral widening step through (or in) a nonlinear medium. The required intensity level may be related to the optical nonlinearity and optical impairments of subsequent nonlinear component stages.

The proposed radiation source comprises two or more consecutive supercontinuum generating stages, each of which comprises a respective nonlinear generating element, and wherein the damage tolerance of a first nonlinear generating element included in a first supercontinuum generating stage is greater than the damage tolerance of at least one second nonlinear generating element included in a second supercontinuum generating stage, and wherein the second supercontinuum generating stage is immediately after the first supercontinuum generating stage.

The first supercontinuum generation stage may be configured to generate a first supercontinuum spectrum and provide the first supercontinuum spectrum to the second supercontinuum generation stage to generate a second supercontinuum spectrum. The second supercontinuum generation stage may be a final supercontinuum generation stage, and the second supercontinuum spectrum is used as an output broadband radiation. Alternatively, in the case where the second supercontinuum generation stage is not the final supercontinuum generation stage, the second supercontinuum spectrum may be provided to a subsequent supercontinuum generation stage. Thus, each successive supercontinuum generation stage before the final supercontinuum generation stage may generate a supercontinuum for the next supercontinuum generation stage.

The radiation source may include a radiation source configured to provide laser radiation to the first supercontinuum generation stage so as to generate the first supercontinuum.

In one embodiment, where there are more than two consecutive supercontinuum generation stages, each consecutive supercontinuum generation stage may have a lower corruption tolerance than its preceding supercontinuum generation stage (i.e., the stages may be arranged in order of their corruption tolerance: from higher tolerance to lower tolerance). However, in other embodiments, the second supercontinuum generation stage may have a lower corruption tolerance than the first supercontinuum generation stage, and successive supercontinuum generation stages after the second stage may have any corruption tolerance; for example, the corruption tolerance may remain constant or become larger for successive supercontinuum generation stages after the second stage.

The damage tolerance can be evaluated for each individual nonlinear generation element included in each hypercontinuum generation stage based on the damage threshold.

Also disclosed is a radiation source in which the optical nonlinearity of a first nonlinear generating element included in a first supercontinuum generating stage is lower than the optical nonlinearity of at least one second nonlinear generating element included in a second supercontinuum generating stage.

In one embodiment, there may be free space coupling between at least the first supercontinuum generation stage and the second supercontinuum generation stage. In one embodiment, there may be free space coupling between each pair of the two or more consecutive supercontinuum generation stages.

In one embodiment, the first nonlinearity generating element may include a hollow core optical fiber (e.g., HC-PCF). Alternatively or additionally, the second nonlinearity generating element may include a solid nonlinearity generating medium. The solid nonlinearity generating medium may include a solid core optical fiber (e.g., SC-PCF), or a (synthetic) crystal (e.g., a BBO crystal or a PPLN crystal).

In the context of the present invention, a nonlinear generating element may include a nonlinear generating medium (e.g., where the nonlinear generating medium is a solid element), or a nonlinear generating element may include an element for confining the nonlinear generating medium (e.g., where the nonlinear generating medium is a gas, for example, the nonlinear generating element may be a hollow core optical fiber). It is understood that in either case, any nonlinear generating medium may also be a liquid.

Fig. 10 schematically illustrates the concept according to a general embodiment. A driving laser DL generates a driving radiation DR, which is received by a first supercontinuum generation stage SCGS1. The first supercontinuum generation stage comprises a first nonlinear generation element having a first damage threshold. The first supercontinuum generation stage generates a first supercontinuum _SC1 , which is received by a next supercontinuum generation stage or a second supercontinuum generation stage, the second supercontinuum generation stage comprising a second nonlinear generation element having a second damage threshold less than the first damage threshold. The second supercontinuum generation stage may be a final supercontinuum generation stage SCGSn (ie, n=2), such that the supercontinuum spectrum output from this supercontinuum generation stage comprises broadband output radiation _SCout . Alternatively, there may be any number of intermediate supercontinuum generation stages (i.e., n>2) between the first supercontinuum generation stage SCG1 and the final supercontinuum generation stage SCGSn, each intermediate supercontinuum generation stage receiving a supercontinuum spectrum from the immediately preceding supercontinuum generation stage and generating a supercontinuum spectrum for the immediately following supercontinuum generation stage, the penultimate stage generating a supercontinuum spectrum SCn _-1 for pumping the final supercontinuum generation stage. For each successive supercontinuum generation stage after the second supercontinuum generation stage (ie, from the input to the output of the radiation source), the damage threshold of the nonlinear generation element of each supercontinuum generation stage can also be reduced (or at least not increased).

Thus, each supercontinuum generated is sufficiently strong relative to the optical nonlinearity of the associated stage (at least before the final stage) to drive further nonlinear optical phenomena in the next supercontinuum generation stage, such as mixing and spectral broadening, thereby producing a new enhanced supercontinuum radiation at the output of each successive supercontinuum generation stage. In this context, enhancement can be described as having improved PSD flatness (e.g., flatter PSD) and increased spectral coverage, especially in the supercontinuum region of interest, and/or improved PSD values.

Several specific examples of proposed broadband radiation sources will now be described, each being an embodiment of the broadband radiation source depicted in FIG. 10 .

FIG11 shows a configuration in which (at least) the first supercontinuum generation stage comprises an air-filled hollow core fiber HCF (e.g., HC-PCF). This hollow core fiber HCF can generate a supercontinuum via MI and/or SSC self-driven radiation DR (e.g., emitted by a drive laser DL). In the case of MI, the generated first supercontinuum radiation SC ₁ has a fine time structure consisting of a sharp (e.g., about 10 femtoseconds) time structure. The supercontinuum radiation SC ₁ can be received by the next supercontinuum generation stage comprising a solid core fiber SCF (or other solid nonlinear medium) (such as SC-PCF). The solid core fiber SCF may generate an output supercontinuum _SCout (as depicted) or another supercontinuum for another supercontinuum generation stage (as described above, there may be any number of such stages). Since the damage threshold of the air-filled hollow core fiber is higher than that of the solid core PCF, the peak intensity of the peak of the supercontinuum _SC1 is high enough to drive nonlinear broadening in a second nonlinear element (such as a solid core PCF).

In one embodiment, the first supercontinuum radiation _SC1 can be coupled into a solid core fiber SCF via free space optics. Alternatively, a hollow core fiber HCF can be spliced to the solid core fiber SCF, making the free space optics redundant.

In another embodiment, the MI- or SSC-based supercontinuum can drive further nonlinear effects in another gas. The following stage benefits from the fine time structure of the MI-based supercontinuum, or the short pulses of the SSC supercontinuum, to drive further nonlinear processes.

A large PSD drop has been observed between the dispersive waves and the SPM-broadened spectrum of the first supercontinuum radiation SC _1. This can make the output radiation SC _out unsuitable for some metrological applications. To solve this problem, it is proposed to reduce the pressure of the gas (nonlinear medium) inside the hollow core fiber HCF to less than 100 bar. This step is optional and converts the dispersive waves to shorter wavelengths. The first supercontinuum radiation is then coupled into the solid core fiber SCF. In this case, the spectrum of the supercontinuum SC ₁ contains the normal and anomalous dispersion range of the solid core fiber, but this is not a necessary condition. As the pulse propagates along the solid core fiber, the dispersion wave is red-shifted, thereby reducing the filling PSD. In addition, the remaining part of the pump widened by the SPM undergoes four-wave mixing (FWM) and converts its energy to other wavelengths on both the short-wave side and the long-wave side, further smoothing the spectrum. These combined effects improve the PSD of the output radiation SC _out .

Fig. 12 and Fig. 13 each show a configuration in which a first supercontinuum SC ₁ is generated in a transparent solid or a fibrillated element FE by fibrillation. Generating a supercontinuum in a solid by fibrillation is a good way to manufacture compact, alignment-free, stable solid-state broadband light sources at low cost. As already described, there is a need to improve the spectral flatness and increase the spectral coverage of the output supercontinuum SC _out . The damage threshold of the proposed bulk medium (e.g., a first nonlinear medium) (e.g., sapphire, fused silica, or a laser crystal like KGW or YAG) used for the filamentation element FE is quite high, e.g., up to a fluence level of J/cm ² for femtosecond pulses. Thus, such media can be used for (at least) a first supercontinuum generation stage and then for one or more additional supercontinuum generation stages, which include respective nonlinear elements with lower damage thresholds. For example, these nonlinear elements may include one or more of a solid core fiber SCF (as depicted in FIG. 12 ) or a mixing stage FMS (as depicted in FIG. 13 ) in order to increase the bandwidth of the hypercontinuum and smooth its PSD.

The mixing stage FMS may comprise a synthetic crystal, such as periodically polarized lithium niobate (PPLN). The mixing stage may support wideband phase matching. This may be achieved, for example, by quasi-phase-matched wideband second-order harmonic generation. Other techniques for wideband phase matching may also be used, such as angular dispersive supercontinuum spectroscopy.

FIG. 14 illustrates another embodiment in which supercontinuum spectrum generation is performed via progressive widening in a series of two or more (e.g., solid core) optical fibers having core diameters that decrease for each successive stage. In the particular example shown, three such stages are provided, including a first solid core optical fiber SCF1 having a first diameter, a second solid core optical fiber SCF2 having a second diameter that is smaller than the first diameter, and a third solid core optical fiber SCF3 having a third diameter that is smaller than the second diameter. The laser induced damage threshold (LIDT) in optical fibers increases with And adjust proportionally, that is, , so that the damage threshold is reduced for each consecutive nonlinear element or solid core fiber SCF1, SCF2, SCF3. This configuration makes it possible to exploit various dispersion profiles provided by different fibers with different core diameters. Optionally, the material constituting the solid core fiber can be different for one or more of the fibers, so that also various dispersion profiles provided by different core materials are exploited.

The coupling between the different fibers can be done both via free space optics, both via splicing, or a combination of both (i.e., the first pair SCF1, SCF2 is free space coupled and the second pair SCF2, SCF3 is spliced or vice versa). When spliced, an adapter can be placed between the fibers to mode fill the diameter of each consecutive fiber.

FIG. 15 shows another embodiment in which at least one supercontinuum generation stage after the first supercontinuum stage SCGS1 includes a crystal such as a barium borate (BBO) crystal BBO as a nonlinear medium. For example, the BBO crystal may include a cut angle θ _c = 22° under type I phase matching. This BBO crystal enhances the output broadband radiation characteristics in the visible light frequency range. It is noteworthy that by using a single BBO crystal in this way, a larger portion of the IR region of the output broadband radiation can be converted into visible light. In this embodiment, the first supercontinuum stage SCGS1 may include, for example, a hollow core optical fiber (e.g., HC-PCF).

It will be appreciated that Figures 11-15 show a non-exhaustive number of different configurations that are possible within the context of the disclosure herein and the general configuration of Figure 10. Other configurations are possible within the scope of the present invention.

In one embodiment, the feedback loop may be configured to control (e.g., adjust) input radiation (e.g., in terms of power) based on a performance metric of each hypercontinuum level, such as by measuring PSD; for example, this may be done to prevent optical corruption.

FIG16 is a block diagram of 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 communicating information and a processor 1604 (or multiple processors 1604 and 1605) coupled to the bus 1602 for processing information. The computer system 1600 also includes a main memory 1606, such as a random access memory (RAM) or other dynamic storage device, coupled to the bus 1602 for storing information and instructions to be executed by the processor 1604. Main memory 1606 may also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by processor 1604. Computer system 1600 further includes a read-only memory (ROM) 1608 or other static storage device coupled to bus 1602 for storing static information and instructions for processor 1604. Storage device 1610, such as a magnetic or optical disk, is provided and coupled to bus 1602 for storing information and instructions.

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

One or more of the methods described herein may be performed by the computer system 1600 in response to the processor 1604 executing one or more sequences of one or more instructions contained in the main memory 1606. Such instructions may be read into the main memory 1606 from another computer-readable medium, such as the storage device 1610. Execution of the sequence of instructions contained in the main memory 1606 causes the processor 1604 to perform the program steps described herein. One or more processors in a multi-processing configuration may also be used to execute the sequence of instructions contained in the main memory 1606. In an alternative embodiment, hard-wired circuitry may be used instead of or in conjunction with software instructions. Therefore, the description herein is not limited to any particular combination of hardware circuitry and software.

As used herein, the term "computer-readable media" refers to any media that participates in providing instructions to processor 1604 for execution. This media can take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1610. Volatile media include dynamic memory, such as main memory 1606. Transmission media include coaxial cables, copper wire, and optical fibers, including the wires that comprise bus 1602. Transmission media can also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, floppy disks, diskettes, hard disks, magnetic tapes, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tape, any other physical media with a pattern of holes, RAM, PROM and EPROM, FLASH-EPROM, any other memory chip or cartridge, the carriers described below, or any other medium that can be read by a computer.

Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to processor 1604 for execution. For example, the instructions may initially be carried on a disk of a remote computer. The remote computer may load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A 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. The bus 1602 carries the data to the main memory 1606, and the processor 1604 retrieves and executes the instructions from the main memory 1606. The instructions received by the main memory 1606 may be stored on the storage device 1610 before or after execution by the processor 1604, as appropriate.

The computer system 1600 also preferably includes a communication interface 1618 coupled to the bus 1602. The communication interface 1618 provides a two-way data communication coupling with a network link 1620, which is connected to a local area network 1622. For example, the communication interface 1618 can be an integrated services digital network (ISDN) card or a modem to provide a data communication connection with a corresponding type of telephone line. As another example, the communication interface 1618 can be a local area network (LAN) card that provides a data communication connection with a compatible LAN. Wireless links can also be implemented. In any such implementation, the communication interface 1618 sends and receives electrical signals, electromagnetic signals, or optical signals that carry digital data streams representing various types of information.

Network link 1620 typically provides data communications to other data devices through one or more networks. For example, network link 1620 may provide a connection to a host computer 1624 or to data equipment operated by an Internet Service Provider (ISP) 1626 through a local area network 1622. ISP 1626, in turn, provides data communications services through the global packet data communications network, now commonly referred to as the "Internet" 1628. Both local area network 1622 and Internet 1628 use electrical, electromagnetic, or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 1620 and through communication interface 1618 are carrier waves that are exemplary forms of transporting information, where the signals carry digital data to and from computer system 1600.

Computer system 1600 can send messages and receive data, including program code, via the network, network link 1620, and communication interface 1618. In the Internet example, server 1630 can transmit the requested program code for an application via Internet 1628, ISP 1626, local area network 1622, and communication interface 1618. For example, one such downloaded application can provide one or more of the techniques described herein. The received program code can be executed by processor 1604 as it is received, 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.

Additional embodiments of the present invention are listed in the following list of numbered clauses: 1. A broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generating stages arranged in series, each of which comprises a respective nonlinear generating element; wherein the plurality of supercontinuum generating stages comprises at least a first supercontinuum generating stage and a second supercontinuum generating stage, wherein the second supercontinuum generating stage is after the first supercontinuum generating stage in the series of supercontinuum generating stages; and wherein a damage tolerance of a first nonlinear generating element contained in the first supercontinuum generating stage is greater than a damage tolerance of at least a second nonlinear generating element contained in the second supercontinuum generating stage. 2. A broadband radiation source as in clause 1, wherein the first supercontinuum generation stage is configured to generate a first supercontinuum spectrum and provide the first supercontinuum spectrum to the second supercontinuum generation stage to generate a second supercontinuum spectrum. 3. A broadband radiation source as in clause 2, further comprising a laser radiation source, the laser radiation source being configured to provide laser radiation to the first supercontinuum generation stage to generate the first supercontinuum spectrum. 4. A broadband radiation source as in any of the preceding clauses, wherein the second supercontinuum generation stage is a final supercontinuum generation stage, so that the output broadband radiation comprises the second supercontinuum spectrum. 5. A broadband radiation source as claimed in clause 1, 2 or 3, comprising one or more other supercontinuum generation stages, wherein the second supercontinuum is provided to the next sequential supercontinuum generation stage in the one or more other supercontinuum generation stages, so that each sequential supercontinuum generation stage before a final supercontinuum generation stage generates a supercontinuum for the next supercontinuum generation stage in the series of supercontinuum generation stages, and the output broadband radiation comprises a supercontinuum from the final supercontinuum generation stage. 6. A broadband radiation source as in clause 5, wherein each successive supercontinuum generation stage after the second supercontinuum generation stage comprises a respective nonlinear generation element, a damage tolerance of the respective nonlinear generation element being lower than or equal to the damage tolerance of the nonlinear generation element of the immediately preceding supercontinuum generation stage. 7. A broadband radiation source as in clause 5, wherein each successive supercontinuum generation stage comprises a respective nonlinear generation element, a damage tolerance of the respective nonlinear generation element being lower than the damage tolerance of the nonlinear generation element of the immediately preceding supercontinuum generation stage. 8. A broadband radiation source as in any preceding clause, comprising free-space coupling between at least the first supercontinuum generating stage and the second supercontinuum generating stage. 9. A broadband radiation source as in any preceding clause, comprising free-space coupling between all of the plurality of supercontinuum generating stages. 10.   A broadband radiation source as in any of clauses 1 to 8, comprising fiber-based coupling between all of the plurality of supercontinuum generating stages. 11.   A broadband radiation source as in any of clauses 1 to 8, wherein the first nonlinear element and the second nonlinear element are bonded together. 12.   A broadband radiation source as in clause 11, wherein all of the respective nonlinear generating elements are bonded together. 13.   A broadband radiation source as in any of the preceding clauses, wherein at least the first nonlinear generating element comprises a hollow core optical fiber. 14.   A broadband radiation source as in clause 13, wherein the hollow core optical fiber comprises a hollow core photonic crystal optical fiber. 15.   A broadband radiation source as in any of clauses 1 to 12, wherein at least the first nonlinear generating element comprises a solid core optical fiber. 16.   A broadband radiation source as in any of clauses 1 to 12, wherein at least the first nonlinear generating element comprises a fibrillation element, wherein the fibrillation element can be used to generate a supercontinuum spectrum by fibrillation. 17.   A broadband radiation source as in any preceding clause, wherein at least the second nonlinear generating element comprises a solid nonlinear generating element. 18    A broadband radiation source as in clause 17, wherein the solid nonlinear generating element comprises a solid core optical fiber. 19.   A broadband radiation source as in clause 17, wherein the solid nonlinear generating element comprises a crystal. 20.   A broadband radiation source as in clause 19, wherein the solid nonlinear generating element comprises a BBO crystal. 21.   A broadband radiation source as in any of clauses 1 to 17, wherein at least the second supercontinuum generating stage comprises a mixing stage. 22.   The broadband radiation source of clause 21, wherein the second supercontinuum generation stage supports broadband phase matching. 23.   The broadband radiation source of clause 21 or 22, wherein the second nonlinear generation element comprises a periodically polarized lithium niobate. 24.   The broadband radiation source of any of clauses 1 to 12, wherein the plurality of supercontinuum generation stages each comprises a respective optical fiber, each optical fiber having a diameter that decreases for each successive supercontinuum generation stage. 25.   The broadband radiation source of any preceding clause, wherein the output broadband radiation comprises wavelengths at least between 100 nm and 1200 nm. 26.   A broadband radiation source as in any preceding clause, wherein the output broadband radiation comprises wavelengths at least in a range between 400 nm and 900 nm. 27.   A broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generating stages arranged in series, each of the supercontinuum generating stages comprising a respective nonlinear generating element; wherein the plurality of supercontinuum generating stages comprises at least a first supercontinuum generating stage and a second supercontinuum generating stage, wherein the second supercontinuum generating stage is after the first supercontinuum generating stage in the series of supercontinuum generating stages; and wherein an optical nonlinearity of a first nonlinear generating element contained in the first supercontinuum generating stage is lower than an optical nonlinearity of at least a second nonlinear generating element contained in the second supercontinuum generating stage. 28.   A metrological device comprising a radiation source as in any preceding clause. 29.   A metrological device as in clause 28 comprising a scatterometer metrological device, a first order sensor or an alignment sensor.

Although specific reference may be 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 the manufacture of integrated optical systems, guide and detection patterns for magnetic memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

Although specific reference may be made herein to embodiments of the invention 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 equipment, a metrology equipment, or any equipment that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterned device). Such equipment may generally be referred to as a lithography tool. The lithography tool may use vacuum conditions or ambient (non-vacuum) conditions.

Although the above may specifically refer to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present invention is not limited to optical lithography and may be used in other applications (such as imprint lithography) where the context permits.

Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative rather than restrictive. Thus, it will be apparent to those skilled in the art that modifications may be made to the present invention as described without departing from the scope of the claims set forth below.

2: Broadband (white light) radiation projector 4: Spectrometer detector 6: Substrate 1600: Computer system 1602: Bus 1604: Processor 1605: Processor 1606: Main memory 1608: Read-only memory (ROM) 1610: Storage device 1612: Display 1614: Input device 1616: Cursor control 1618: Communication interface 1620: Network link 1622: Local area network 1624: Host computer 1626: Internet service provider (ISP) 1628: Internet 1630: Server AM: Marker ANG: Incident angle AS: Known alignment sensor B: Radiation beam BBO: Barium borate (BBO) crystal BD: Beam delivery system BE1: Radiation beam BE2: Arrow BK: Bake plate C: Target part CAP: Tubular capillary CC: Capillary cavity CH: Cooling plate CL: Computer system d: Diameter DE: Display DET: Detector DGR: Detection grating DL: Drive laser DR: Drive radiation FE: Transparent Solid or filamentary components FMS: Mixing stage HC: Hollow core HCF: Hollow core fiber IB: Information carrying beam IE: Input end IF: Position measurement system IL: Illumination system IRD: Received input radiation I/O1: Input/output port I/O2: Input/output port LA: Lithography equipment LACU: Lithography control unit LB: Loading box LC: Lithography production unit LS: Level or height sensor LSB: Radiation beam LSD: Detection unit LSO: Radiation source LSP: Projection unit M ₁ : Mask alignment mark M ₂ : Mask alignment mark MA: Patterning device MLO: Measurement position MT: Mask support/Metrology tool/Scatterometer OE: Output end OF: Optical fiber OL: Objective lens ORD: Broadband output radiation P ₁ : Substrate alignment mark P ₂ : Substrate alignment mark PD: Photodetector PEB: Post-exposure bake step PGR: Projection grating PM: First positioner PRS: Pulsed pump radiation source PS: Projection system PU: Processing unit PW: Second positioner RB: Radiation beam RDS: Known radiation source RO: Substrate handler or robot RSO: Radiation source RSV: Reservoir SC: Spin coater SC ₁ : First hypercontinuum SC _n-1 : Hypercontinuum SC _out : broadband output radiation SC1: first scale SC2: second scale SC3: third scale SCF: solid core fiber SCF1: first solid core fiber SCF2: second solid core fiber SCF3: third solid core fiber SCGS1: first supercontinuum generation stage SCGSn: final supercontinuum generation stage SCS: supervisory control system SI: intensity signal SM: light spot mirror SO: radiation source SP: illumination light spot/internal support part SRI: self-reference interferometer ST: support tube TCU: automated photoresist coating and development system control unit TW1: first transparent window TW2: second transparent window W: substrate WM: working medium WP: cylindrical wall part WT: substrate support

Embodiments of the invention will now be described by way of example only with reference to the accompanying schematic drawings, in which: - FIG. 1 depicts a schematic overview of a lithography apparatus; - FIG. 2 depicts a schematic overview of a lithography manufacturing unit; - FIG. 3 depicts a schematic representation of overall lithography showing the cooperation between three key technologies for optimizing semiconductor manufacturing; - FIG. 4 depicts a schematic overview of a scattering measurement apparatus used as a metrology device that may include a radiation source according to an embodiment of the invention; - FIG. 5 depicts a schematic overview of a level sensor apparatus that may include a radiation source according to an embodiment of the invention; - FIG. 6 depicts a schematic overview of an alignment sensor apparatus that may include a radiation source according to an embodiment of the invention; - FIG. 7 is a schematic cross-sectional view of a hollow core optical fiber that may form part of a radiation source according to one embodiment in a transverse plane (i.e., perpendicular to the axis of the optical fiber); - FIG. 8 depicts a schematic representation of a known radiation source for providing broadband output radiation; - FIG. 9(a) and FIG. 9(b) schematically depict transverse cross-sections of an example of a hollow core photonic crystal fiber (HC-PCF) design for supercontinuum generation; - FIG. 10 schematically depicts a broadband radiation source according to a first embodiment; - FIG. 11 schematically depicts a broadband radiation source according to a first embodiment; - FIG. 12 schematically depicts a broadband radiation source according to a second embodiment; - Figure 13 schematically illustrates a broadband radiation source according to a third embodiment; - Figure 14 schematically illustrates a broadband radiation source according to a fourth embodiment; - Figure 15 schematically illustrates a broadband radiation source according to a fifth embodiment; and - Figure 16 depicts a block diagram of a computer system for controlling a broadband radiation source.

DL: Drive Laser

DR: Driving Radiation

SC ₁ : First Supercontinuum

SC _n-1 :Supercontinuum

SC _out : Broadband output radiation

SCGS1: First supercontinuum spectrum generation level

SCGSn: Final Supercontinuum Spectrum Generator

Claims (20)

A broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generating stages arranged in series, each of which comprises a respective nonlinear generating element; wherein the plurality of supercontinuum generating stages comprises at least a first supercontinuum generating stage and a second supercontinuum generating stage, and in the series of supercontinuum generating stages, the second supercontinuum generating stage follows the first supercontinuum generating stage; and wherein a damage tolerance of a first nonlinear generating element contained in the first supercontinuum generating stage is greater than a damage tolerance of at least a second nonlinear generating element contained in the second supercontinuum generating stage. A broadband radiation source as claimed in claim 1, wherein the first supercontinuum generation stage is configured to generate a first supercontinuum spectrum and provide the first supercontinuum spectrum to the second supercontinuum generation stage to generate a second supercontinuum spectrum. The broadband radiation source of claim 2 further comprises a laser radiation source configured to provide laser radiation to the first supercontinuum generation stage so as to generate the first supercontinuum. The broadband radiation source of claim 1, wherein the second supercontinuum generation stage is a final supercontinuum generation stage, so that the output broadband radiation includes the second supercontinuum. A broadband radiation source as claimed in claim 1, further comprising one or more other supercontinuum generation stages, wherein the second supercontinuum is provided to a next sequential supercontinuum generation stage in the one or more other supercontinuum generation stages, so that each sequential supercontinuum generation stage before a final supercontinuum generation stage generates a supercontinuum for a next supercontinuum generation stage in the series of supercontinuum generation stages, and the output broadband radiation includes a supercontinuum from the final supercontinuum generation stage. A broadband radiation source as claimed in claim 5, wherein each successive supercontinuum generating stage after the second supercontinuum generating stage comprises a respective nonlinear generating element, a damage tolerance of the respective nonlinear generating element being lower than or equal to the damage tolerance of the nonlinear generating element of the immediately preceding supercontinuum generating stage. A broadband radiation source as claimed in claim 1, comprising a free-space coupling between at least the first supercontinuum generation stage and the second supercontinuum generation stage. A broadband radiation source as claimed in claim 1, comprising a fiber-based coupling between the first supercontinuum generation stage and the second supercontinuum generation stage. A broadband radiation source as claimed in claim 1, wherein at least the first nonlinear generating element comprises a hollow core optical fiber. A broadband radiation source as claimed in claim 9, wherein the hollow core optical fiber is a hollow core photonic crystal optical fiber. A broadband radiation source as claimed in claim 1, wherein at least the first nonlinear generating element comprises a fissioning element, and the fissioning element can be used to generate a supercontinuum spectrum through fissioning. A broadband radiation source as claimed in claim 1, wherein at least the second nonlinear generating element comprises a solid nonlinear generating element. A broadband radiation source as claimed in claim 12, wherein the solid nonlinear generating element comprises a solid core optical fiber. A broadband radiation source as claimed in claim 12, wherein the solid nonlinear generating element comprises a crystal. A broadband radiation source as claimed in claim 1, wherein at least the second supercontinuum generation stage comprises a mixing stage. A broadband radiation source as claimed in claim 1, wherein the plurality of supercontinuum generating stages each comprises a respective optical fiber, each optical fiber having a diameter that decreases for each successive supercontinuum generating stage. A metrological device comprising a radiation source as claimed in claim 1. A metrological device as claimed in claim 17, comprising a scatterometer metrological device, a first order sensor or an alignment sensor. A detection device comprising a radiation source as claimed in claim 1. The inspection apparatus of claim 19, wherein the inspection apparatus is configured to identify defects on a substrate.