System and method for overlay metrology with reduced coherence and speckle contrast

Abstract

An infinite impulse response optical filter includes at least one N x N multimode optical coupler and one or more mode-scrambling loops. The at least one N x N multimode optical coupler has N input ports and N output ports. One of the N input ports is coupled to a laser source that generates a light beam. The at least one N x N multimode optical coupler splits the light beam into a plurality of sub-beams divided in an amplitude domain across the N output ports. One of the N output ports is coupled to a detection system. The one or more mode-scrambling loops includes a mode scrambler and couples one of the N output ports to one of the N input ports. The one or more mode-scrambling loops produce a temporal incoherence and a spatial incoherence that reduce a peak power and a speckle contrast of the light beam.

Description

SYSTEM AND METHOD FOR OVERLAY METROLOGY WITH REDUCED COHERENCE AND SPECKLE CONTRASTCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 615,044 which was filed on 27 December 2023, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to incoherent illumination systems, for example, an infinite impulse response optical filter for overlay metrology in lithographic systems and methods.BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which can be a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiationsensitive material (photoresist or simply “resist”) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”- direction) while synchronously scanning the target portions parallel or anti-parallel to this scanning direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0004] During lithographic operation, different processing steps can entail different layers to be sequentially formed on the substrate. Accordingly, it can be necessary to position the substrate relative to prior patterns formed thereon with a high degree of accuracy. Generally, alignment marks are placed on the substrate to be aligned and are located with reference to a second object. A lithographic apparatus can use an alignment apparatus for detecting positions of the alignment marks and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask. Misalignment between the alignment marks at two different layers is measured as overlay error.

[0005] In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters can include, for example, an overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement can be performed on a product substrate and / or on a dedicated metrology target, such as a grating. There arevarious techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of a specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. By contrast, angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.

[0006] Such optical scatterometers can be used to measure parameters, such as critical dimensions of developed photosensitive resist or overlay error (OV) between two layers formed in or on the patterned substrate. Properties of the substrate can be determined by comparing the properties of an illumination beam before and after the beam has been reflected or scattered by the substrate.

[0007] Speckle is a laser phenomenon caused by self-interference of a coherent illumination beam, which can lead to random local illumination non-uniformity. Speckle contrast is a measurement of local illumination intensity variations across a speckle pattern. Non-uniformity in laser beam illumination can result in non-uniform patterning and can lead to errors in a lithographic process.SUMMARY

[0008] Accordingly, it is desirable to reduce speckle contrast in illumination systems of metrology apparatuses for improving accuracy in measurements performed in conjunction with lithographic processes.

[0009] In some aspects, an optical metrology incoherent illumination system can comprise a white light laser source, a detection system, and an infinite impulse response (HR) optical filter. In some aspects, the white light laser source can be configured to generate a light beam. In some aspects, the detection system can be configured to measure a characteristic of the light beam. In some aspects, the IIR optical filter can be disposed between the laser source and the detection system. In some aspects, the IIR optical filter can comprise at least one N x N multimode optical coupler and one or more modescrambling loops. In some aspects, the at least one N x N multimode optical coupler can include N input ports and N output ports. In some aspects, a first input port of the N input ports, forming an input of the IIR optical filter, can be coupled to the laser source by a first multimode optical fiber. In some aspects, the at least one N x N multimode optical coupler can be configured to split the light beam into a plurality of sub-beams divided in an amplitude domain across the N output ports. In some aspects, a first output port of the N output ports, forming an output of the IIR optical filter, can be coupled to the detectionsystem by a second multimode optical fiber. In some aspects, the one or more mode-scrambling loops can be formed by coupling at least one of the other N output ports to another one of the other N input ports with a multimode optical fiber loop. In some aspects, a length of the multimode optical fiber loop can be longer than a coherence length of the laser source to achieve a temporal incoherence between the plurality of sub-beams and the light beam. In some aspects, a mode scrambler can be disposed along the multimode optical fiber loop to achieve a spatial incoherence between the plurality of sub-beams and the light beam. In some aspects, the temporal incoherence provided by the length of the multimode optical fiber loop and the spatial incoherence provided by the mode scrambler can reduce a peak power of the light beam and can reduce a speckle contrast of the light beam measured by the detection system.

[0010] In some aspects, an IIR optical filter apparatus can comprise at least one N x N multimode optical coupler and one or more mode-scrambling loops. In some aspects, the at least one N x N multimode optical coupler can include N input ports and N output ports. In some aspects, a first input port of the N input ports, forming an input of the IIR optical filter, can be coupled to a laser source by a first multimode optical fiber. In some aspects, the laser source can be configured to generate a light beam. In some aspects, the at least one N x N multimode optical coupler can be configured to split the light beam into a plurality of sub-beams divided in an amplitude domain across the N output ports. In some aspects, a first output port of the N output ports, forming an output of the IIR optical filter, can be coupled to a detection system by a second multimode optical fiber. In some aspects, the detection system can be configured to measure a characteristic of the light beam. In some aspects, the one or more modescrambling loops can be formed by coupling at least one of the other N output ports to another one of the other N input ports with a multimode optical fiber loop. In some aspects, a length of the multimode optical fiber loop can be longer than a coherence length of the laser source to achieve a temporal incoherence between the plurality of sub-beams and the light beam. In some aspects, a mode scrambler can be disposed along the multimode optical fiber loop to achieve a spatial incoherence between the plurality of sub-beams and the light beam. In some aspects, the temporal incoherence provided by the length of the multimode optical fiber loop and the spatial incoherence provided by the mode scrambler can reduce a peak power of the light beam and can reduce a speckle contrast of the light beam measured by the detection system.

[0011] In some aspects, an incoherent illumination method for optical metrology can include generating a light beam with a white light laser source. In some aspects, the method can further include transmitting the light beam with a first multimode optical fiber to an input of an IIR optical filter. In some aspects, the IIR optical filter can comprise at least one N x N multimode optical coupler having N input ports and N output ports, wherein the first input port of the N input ports forms the input of the IIR optical filter. In some aspects, the method can further include splitting the light beam into a plurality of sub-beams divided in an amplitude domain across the N output ports. In some aspects, the method can further include delaying the plurality of sub-beams in a time domain with one or more modescrambling loops. In some aspects, the one or more mode-scrambling loops can be formed by couplingat least one of the other N output ports to another one of the other N input ports with a multimode optical fiber loop. In some aspects, the multimode optical fiber loop is longer than a coherence length of the laser source to achieve a temporal incoherence between the plurality of sub-beams and the light beam. In some aspects, the method can further include mixing a spatial mode of the plurality of sub-beams with a mode scrambler disposed along the multimode optical fiber loop to achieve a spatial incoherence between the plurality of sub-beams and the light beam. In some aspects, the method can further include reducing a peak power of the light beam and a speckle contrast of the light beam due to the temporal incoherence provided by the one or more mode-scrambling loops and the spatial incoherence provided by the mode scrambler. In some aspects, the method can further include recirculating the plurality of sub-beams through the one or more mode-scrambling loops. In some aspects, the method can further include transmitting an output light beam from an output of the IIR optical filter with a second multimode optical fiber to a detection system.

[0012] Further features of various aspects of the present disclosure are described in detail below with reference to the accompanying drawings. It is noted that the present disclosure is not limited to the specific aspects described herein. Such aspects are presented herein for illustrative purposes only. Additional aspects will be apparent to those skilled in the relevant art(s) based on the teachings contained herein.BRIEF DESCRIPTION OF THE DRAWINGS / FIGURES

[0013] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable those skilled in the relevant art(s) to make and use aspects described herein.

[0014] FIG. 1A shows a reflective lithographic apparatus, according to some aspects.

[0015] FIG. IB shows a transmissive lithographic apparatus, according to some aspects.

[0016] FIG. 2 shows more details of a reflective lithographic apparatus, according to some aspects.

[0017] FIG. 3 shows a lithographic cell, according to some aspects.

[0018] FIGS. 4 A and 4B show inspection apparatuses, according to some aspects.

[0019] FIG. 5 shows an optical metrology incoherent illumination system with a 2 x 2 multimode optical coupler, according to some aspects.

[0020] FIG. 6 shows an optical metrology incoherent illumination system with a 4 x 4 multimode optical coupler, according to some aspects.

[0021] FIG. 7 shows schematic plots indicating pulses and sub-pulses transmitted over a period, according to some aspects.

[0022] FIG. 8 shows an optical metrology incoherent illumination system with a series of 2 x 2 multimode optical couplers, according to some aspects.

[0023] FIG. 9 shows an optical metrology incoherent illumination system with a plurality of 2 x 2 multimode optical couplers arranged in nested mode-scrambling loops, according to some aspects.

[0024] FIG. 10 shows an incoherent illumination method for optical metrology, according to some aspects.

[0025] The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elements. Additionally, generally, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.DETAILED DESCRIPTION

[0026] The aspects described herein, and references in the specification to “one aspect,” “an aspect,” “an exemplary aspect,” “an example aspect,” etc., indicate that the aspects described can include a particular feature, structure, or characteristic, but every aspect may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same aspect. Further, when a particular feature, structure, or characteristic is described in connection with an aspect, it is understood that it is within the knowledge of those skilled in the art to effect such feature, structure, or characteristic in connection with other aspects whether or not explicitly described.

[0027] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, can be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein can likewise be interpreted accordingly.

[0028] The terms “about,” “approximately,” or the like can be used herein to indicate the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the terms “about,” “approximately,” or the like can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

[0029] Aspects of the present disclosure can be implemented in hardware, firmware, software, or any combination thereof. Aspects of the disclosure can also be implemented as instructions stored on a computer-readable medium, which can be read and executed by one or more processors. A machine- readable medium can include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium can include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g.,carrier waves, infrared signals, digital signals, etc.), and others. Furthermore, firmware, software, routines, and / or instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. The term “machine -readable medium” can be interchangeable with similar terms, for example, “computer program product,” “computer-readable medium,” “non-transitory computer- readable medium,” or the like. The term “non-transitory” can be used herein to characterize one or more forms of computer readable media except for a transitory, propagating signal.

[0030] Before describing such aspects in more detail, however, it is instructive to present an example environment in which aspects of the present disclosure can be implemented.

[0031] Example Lithographic Systems

[0032] FIGS. 1A and IB show a lithographic apparatus 100 and a lithographic apparatus 100’, respectively, in which aspects of the present disclosure can be implemented. Lithographic apparatus 100 and lithographic apparatus 100’ each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatus 100 and 100’ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100’, the patterning device MA and the projection system PS are transmissive.

[0033] The illumination system IL can include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B.

[0034] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatus 100 and 100’, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

[0035] The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

[0036] The patterning device MA can be transmissive (as in lithographic apparatus 100’ of FIG. IB) or reflective (as in lithographic apparatus 100 of FIG. 1A). Examples of patterning devices MA include reticles, masks, programmable mirror arrays, or programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase shift, or attenuated phase shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam B, which is reflected by a matrix of small mirrors.

[0037] The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0038] Lithographic apparatus 100 and / or lithographic apparatus 100’ can be of a type having two (dual stage) or more substrate tables WT (and / or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

[0039] The lithographic apparatus can also be of a type wherein at least a portion of the substrate can be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid can also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid. For example, a liquid can be located between the projection system and the substrate during exposure.

[0040] Referring to FIGS. 1A and IB, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus 100, 100’ can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus 100 or 100’, and the radiation beam B passes from the sourceSO to the illuminator IL with the aid of a beam delivery system BD (in FIG. IB) including, for example, suitable directing mirrors and / or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus 100, 100’ , for example, when the source SO is a mercury lamp. A radiation system can comprise the source SO, the illuminator IL, and / or the beam delivery system BD.

[0041] The illuminator IL can include an adjuster AD (in FIG. IB) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent (commonly referred to as “o-outer” and “o-inner,” respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components (in FIG. IB), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

[0042] Referring to FIG. 1A, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask) MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0043] Referring to FIG. IB, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. The projection system has a pupil conjugate PPU to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at the mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

[0044] The projection system PS projects an image of the mask pattern MP, where the image is formed by diffracted beams produced from the mark pattern MP by radiation from the intensity distribution, onto a photoresist layer coated on the substrate W. For example, the mask pattern MP can include an array of lines and spaces. A diffraction of radiation at the array and different from zeroth order diffraction generates diverted diffracted beams with a change of direction in a direction perpendicular to the lines. Undiffracted beams (i.e., so-called zeroth order diffracted beams) traverse the pattern without any change in propagation direction. The zeroth order diffracted beams traverse an upper lensor upper lens group of the projection system PS, upstream of the pupil conjugate PPU of the projection system PS, to reach the pupil conjugate PPU. The portion of the intensity distribution in the plane of the pupil conjugate PPU and associated with the zeroth order diffracted beams is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The aperture device PD, for example, is disposed at or substantially at a plane that includes the pupil conjugate PPU of the projection system PS.

[0045] The projection system PS is arranged to capture (e.g., using a lens or lens group L) the zeroth order diffracted beams, first order diffracted beams, and / or higher order diffracted beams (not shown). In some aspects, dipole illumination for imaging line patterns extending in a direction perpendicular to a line can be used to utilize the resolution enhancement effect of dipole illumination. For example, first- order diffracted beams interfere with corresponding zeroth-order diffracted beams at the level of the wafer W to create an image of the line pattern MP at highest possible resolution and process window (i.e., usable depth of focus in combination with tolerable exposure dose deviations). In some aspects, astigmatism aberration can be reduced by providing radiation poles (not shown) in opposite quadrants of the illumination system pupil IPU. Further, in some aspects, astigmatism aberration can be reduced by blocking the zeroth order beams in the pupil conjugate PPU of the projection system associated with radiation poles in opposite quadrants. This is described in more detail in US 7,511,799 B2, issued Mar. 31, 2009, which is incorporated by reference herein in its entirety.

[0046] With the aid of the second positioner PW and position sensor IFD (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor (not shown in FIG. IB) can be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

[0047] In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks Pl, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks can be located between the dies.

[0048] Mask table MT and patterning device MA can be in a vacuum chamber V, where an in-vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum chamber. Alternatively, when mask table MT and patterning device MA are outside of the vacuum chamber, anout-of-vacuum robot can be used for various transportation operations, similar to the in-vacuum robot IVR. Both the in-vacuum and out-of-vacuum robots can be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

[0049] The lithographic apparatus 100 and 100’ can be used in at least one of the following modes:

[0050] 1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and / or Y direction so that a different target portion C can be exposed.

[0051] 2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT can be determined by the (de- jmagnification and image reversal characteristics of the projection system PS.

[0052] 3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be employed and the programmable patterning device is updated as needed after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

[0053] Combinations and / or variations on the described modes of use or entirely different modes of use can also be employed.

[0054] In some aspects, lithographic apparatus 100 includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0055] In some aspects, lithographic apparatus 100’ includes a deep ultraviolet (DUV) source, which is configured to generate a beam of DUV radiation for DUV lithography. In general, the DUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the DUV radiation beam of the DUV source.

[0056] FIG. 2 shows the lithographic apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 can be formed by a discharge produced plasma source. In some aspects, a plasma of excited tin (Sn) (e.g., excited via a laser) is provided to produce EUV radiation.

[0057] The radiation emitted by the EUV radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 can include a channel structure. Contamination trap 230 can also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure.

[0058] The collector chamber 212 can include a radiation collector CO, which can be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point INTF. The virtual source point INTF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the intermediate focus INTF is located at or near an opening 219 in the enclosing structure 220. The virtual source point INTF is an image of the EUV radiation emitting plasma 210. Grating spectral filter 240 is used in particular for suppressing infra-red (IR) radiation.

[0059] Subsequently the radiation traverses the illumination system IL, which can include a faceted field mirror device 222 and a faceted pupil mirror device 224 arranged to provide a desired angular distribution of the radiation beam 221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation 221 at the patterning device MA, held by the support structure MT, a patterned beam 226 is formed and the patterned beam 226 is imaged by the projection system PS via reflective elements 228, 229 onto a substrate W held by the wafer stage or substrate table WT.

[0060] More elements than shown can generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 can optionally be present, depending upon the type of lithographic apparatus. Further, there can be more mirrors present than those shown in the FIG. 2, for example there can be one to six additional reflective elements present in the projection system PS than shown in FIG. 2.

[0061] Collector optic CO, as illustrated in FIG. 2, is depicted as a nested collector with grazing incidence reflectors 253, 254, and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254, and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

[0062] Example Lithographic Cell

[0063] FIG. 3 shows a lithographic cell 300, also sometimes referred to a lithocell or cluster, according to some aspects. Lithographic apparatus 100 or 100’ can form part of lithographic cell 300. Lithographic cell 300 can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. Conventionally these include spin coaters SC to deposit resist layers, developers DE todevelop exposed resist, chill plates CH, and bake plates BK. A substrate handler, or robot, RO picks up substrates from input / output ports I / Ol, I / O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus 100 or 100’ . These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.

[0064] Example Inspection Apparatus

[0065] In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more inspection apparatuses for accurate positioning of marks on a substrate. These alignment apparatuses are effectively position measuring apparatuses. Different types of marks and different types of alignment apparatuses and / or systems are known from different times and different manufacturers. A type of system widely used in current lithographic apparatus is based on a self-referencing interferometer as described in U.S. Patent No. 6,961,116 (den Boef et al.). Generally marks are measured separately to obtain X- and Y-positions. A combined X- and Y-measurement can be performed using the techniques described in U.S. Publication No. 2009 / 195768 A (Bijnen et al.), however. The full contents of both of these disclosures are incorporated herein by reference.

[0066] FIG. 4A shows a cross-sectional view of an inspection apparatus 400 that can be implemented as a part of lithographic apparatus 100 or 100’, according to some aspects. In some aspects, inspection apparatus 400 can be configured to align a substrate (e.g., substrate W) with respect to a patterning device (e.g., patterning device MA). Inspection apparatus 400 can be further configured to detect positions of alignment marks on the substrate and to align the substrate with respect to the patterning device or other components of lithographic apparatus 100 or 100’ using the detected positions of the alignment marks. Such alignment of the substrate can ensure accurate exposure of one or more patterns on the substrate.

[0067] The terms “inspection apparatus,” “metrology system,” or the like can be used herein to refer to, e.g., a device used for measuring a property of a structure (e.g., overlay sensor, critical dimension sensor, or the like), a device or system used in a lithographic apparatus to inspect an alignment of a wafer (e.g., alignment sensor), or the like.

[0068] In some aspects, inspection apparatus 400 can include an illumination system 412, a beam splitter 414, an interferometer 426, a detector 428, a beam analyzer 430, and a processor 432. Illumination system 412 can be configured to provide an electromagnetic narrow band radiation beam 413 having one or more passbands. In an example, the one or more passbands can be within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, the one or more passbands can be discrete narrow passbands within a spectrum of wavelengths between about 500 nm to about 900 nm. Illumination system 412 can be further configured to provide one or more passbands havingsubstantially constant center wavelength (CWL) values over a long period of time (e.g., over a lifetime of illumination system 412). Such configuration of illumination system 412 can help to prevent the shift of the actual CWL values from the desired CWL values, as discussed above, in current alignment systems. And, as a result, the use of constant CWL values can improve long-term stability and accuracy of alignment systems (e.g., inspection apparatus 400) compared to the current alignment apparatuses.

[0069] In some aspects, beam splitter 414 can be configured to receive radiation beam 413 and split radiation beam 413 into at least two radiation sub-beams. For example, radiation beam 413 can be split into radiation sub-beams 415 and 417, as shown in FIG. 4A. Beam splitter 414 can be further configured to direct radiation sub-beam 415 onto a substrate 420 placed on a stage 422. In one example, the stage 422 is movable along direction 424. Radiation sub-beam 415 can be configured to illuminate an alignment mark or a target 418 located on substrate 420. Alignment mark or target 418 can be coated with a radiation sensitive film. In some aspects, alignment mark or target 418 can have one hundred and eighty degrees (i.e., 180°) symmetry. That is, when alignment mark or target 418 is rotated 180° about an axis of symmetry perpendicular to a plane of alignment mark or target 418, rotated alignment mark or target 418 can be substantially identical to an unrotated alignment mark or target 418. The target 418 on substrate 420 can be (a) a resist layer grating comprising bars that are formed of solid resist lines, or (b) a product layer grating, or (c) a composite grating stack in an overlay target structure comprising a resist grating overlaid or interleaved on a product layer grating. The bars can alternatively be etched into the substrate. This pattern is sensitive to chromatic aberrations in the lithographic projection apparatus, particularly the projection system PL, and illumination symmetry and the presence of such aberrations will manifest themselves in a variation in the printed grating. One in-line method used in device manufacturing for measurements of line width, pitch, and critical dimension makes use of a technique known as “scatterometry”. Methods of scatterometry are described in Raymond et al., “Multiparameter Grating Metrology Using Optical Scatterometry”, J. Vac. Sci. Tech. B, Vol. 15, no. 2, pp. 361-368 (1997) and Niu et al., “Specular Spectroscopic Scatterometry in DUV Lithography”, SPIE, Vol. 3677 (1999), which are both incorporated by reference herein in their entireties. In scatterometry, light is reflected by periodic structures in the target, and the resulting reflection spectrum at a given angle is detected. The structure giving rise to the reflection spectrum is reconstructed, e.g. using Rigorous Coupled- Wave Analysis (RCWA) or by comparison to a library of patterns derived by simulation. Accordingly, the scatterometry data of the printed gratings is used to reconstruct the gratings. The parameters of the grating, such as line widths and shapes, can be input to the reconstruction process, performed by processing unit PU, from knowledge of the printing step and / or other scatterometry processes.

[0070] In some aspects, beam splitter 414 can be further configured to receive diffraction radiation beam 419 and split diffraction radiation beam 419 into at least two radiation sub-beams, according to an aspect. Diffraction radiation beam 419 can be split into diffraction radiation sub-beams 429 and 439, as shown in FIG. 4A.

[0071] It should be noted that even though beam splitter 414 is shown to direct radiation sub-beam 415 towards alignment mark or target 418 and to direct diffracted radiation sub-beam 429 towards interferometer 426, the disclosure is not so limiting. Other optical arrangements can be used to obtain the similar result of illuminating alignment mark or target 418 on substrate 420 and detecting an image of alignment mark or target 418.

[0072] As illustrated in FIG. 4A, interferometer 426 can be configured to receive radiation sub-beam 417 and diffracted radiation sub-beam 429 through beam splitter 414. In an example aspect, diffracted radiation sub-beam 429 can be at least a portion of radiation sub-beam 415 that can be reflected from alignment mark or target 418. In an example of this aspect, interferometer 426 comprises any appropriate set of optical-elements, for example, a combination of prisms that can be configured to form two images of alignment mark or target 418 based on the received diffracted radiation sub-beam 429. It should be appreciated that a good quality image need not be formed. It can be enough to have the features of alignment mark 418 resolved. Interferometer 426 can be further configured to rotate one of the two images with respect to the other of the two images 180° and recombine the rotated and unrotated images interferometrically.

[0073] In some aspects, detector 428 can be configured to receive the recombined image via interferometer signal 427 and detect interference as a result of the recombined image when alignment axis 421 of inspection apparatus 400 passes through a center of symmetry (not shown) of alignment mark or target 418. Such interference can be due to alignment mark or target 418 being 180° symmetrical, and the recombined image interfering constructively or destructively, according to an example aspect. Based on the detected interference, detector 428 can be further configured to determine a position of the center of symmetry of alignment mark or target 418 and consequently, detect a position of substrate 420. According to an example, alignment axis 421 can be aligned with an optical beam perpendicular to substrate 420 and passing through a center of image rotation interferometer 426. Detector 428 can be further configured to estimate the positions of alignment mark or target 418 by implementing sensor characteristics and interacting with wafer mark process variations.

[0074] In a further aspect, detector 428 determines the position of the center of symmetry of alignment mark or target 418 by performing one or more of the following measurements:1. measuring position variations for various wavelengths (position shift between colors);2. measuring position variations for various orders (position shift between diffraction orders);3. measuring position variations for various polarizations (position shift between polarizations); and4. measuring intensity difference between opposite orders of a diffraction order pair (e.g., to characterize and correct for asymmetry).

[0075] This data can be obtained using any type of alignment sensor, for example, a SMASH (SMart Alignment Sensor Hybrid) sensor, as described in U.S. Patent No. 6,961,116 that employs a selfreferencing interferometer with a single detector and four different wavelengths, and extracts the alignment signal in software, or Athena (Advanced Technology using High order ENhancement ofAlignment), as described in U.S. Patent No. 6,297,876, which directs each of seven diffraction orders to a dedicated detector, which are both incorporated by reference herein in their entireties.

[0076] In some aspects, beam analyzer 430 can be configured to receive and determine an optical state of diffracted radiation sub-beam 439. The optical state can be a measure of beam wavelength, polarization, or beam profile. Beam analyzer 430 can be further configured to determine a position of stage 422 and correlate the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420 can be accurately known with reference to stage 422. Alternatively, beam analyzer 430 can be configured to determine a position of inspection apparatus 400 or any other reference element such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400 or any other reference element. Beam analyzer 430 can be a point or an imaging polarimeter with some form of wavelength-band selectivity. In some aspects, beam analyzer 430 can be directly integrated into inspection apparatus 400, or connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other aspects.

[0077] In some aspects, beam analyzer 430 can be further configured to determine the overlay data between two patterns on substrate 420. One of these patterns can be a reference pattern on a reference layer. The other pattern can be an exposed pattern on an exposed layer. The reference layer can be an etched layer already present on substrate 420. The reference layer can be generated by a reference pattern exposed on the substrate by lithographic apparatus 100 and / or 100’. The exposed layer can be a resist layer exposed adjacent to the reference layer. The exposed layer can be generated by an exposure pattern exposed on substrate 420 by lithographic apparatus 100 or 100’. The exposed pattern on substrate 420 can correspond to a movement of substrate 420 by stage 422. In some aspects, the measured overlay data can also indicate an offset between the reference pattern and the exposure pattern. The measured overlay data can be used as calibration data to calibrate the exposure pattern exposed by lithographic apparatus 100 or 100’, such that after the calibration, the offset between the exposed layer and the reference layer can be minimized.

[0078] In some aspects, beam analyzer 430 can be further configured to determine a model of the product stack profile of substrate 420, and can be configured to measure overlay, critical dimension, and focus of target 418 in a single measurement. The product stack profile contains information on the stacked product such as alignment mark, target 418, or substrate 420, and can include mark process variation-induced optical signature metrology that is a function of illumination variation. The product stack profile can also include product grating profile, mark stack profile, and mark asymmetry information. An example of beam analyzer 430 is Yieldstar™, manufactured by ASML, Veldhoven, The Netherlands, as described in U.S. Patent No. 8,706,442, which is incorporated by reference herein in its entirety. Beam analyzer 430 can be further configured to process information related to a particular property of an exposed pattern in that layer. For example, beam analyzer 430 can process an overlay parameter (an indication of the positioning accuracy of the layer with respect to a previous layer on thesubstrate or the positioning accuracy of the first layer with respective to marks on the substrate), a focus parameter, and / or a critical dimension parameter (e.g., line width and its variations) of the depicted image in the layer. Other parameters are image parameters relating to the quality of the depicted image of the exposed pattern.

[0079] In some aspects, an array of detectors (not shown) can be connected to beam analyzer 430, and allows the possibility of accurate stack profile detection as discussed below. For example, detector 428 can be an array of detectors. For the detector array, a number of options are possible: a bundle of multimode fibers, discrete pin detectors per channel, or CCD or CMOS (linear) arrays. The use of a bundle of multimode fibers enables any dissipating elements to be remotely located for stability reasons. Discrete PIN detectors offer a large dynamic range but each need separate pre-amps. The number of elements is therefore limited. CCD linear arrays offer many elements that can be read-out at high speed and are especially of interest if phase-stepping detection is used.

[0080] In some aspects, a second beam analyzer 430’ can be configured to receive and determine an optical state of diffracted radiation sub-beam 429, as shown in FIG. 4B. The optical state can be a measure of beam wavelength, polarization, or beam profile. Second beam analyzer 430’ can be identical to beam analyzer 430. Alternatively, second beam analyzer 430’ can be configured to perform one or more of the functions of beam analyzer 430, such as determining a position of stage 422 and correlating the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420, can be accurately known with reference to stage 422. Second beam analyzer 430’ can also be configured to determine a position of inspection apparatus 400, or any other reference element, such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400, or any other reference element.

[0081] In some aspects, second beam analyzer 430’ can be directly integrated into inspection apparatus 400, or it can be connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other aspects. Alternatively, second beam analyzer 430’ and beam analyzer 430 can be combined to form a single analyzer (not shown) configured to receive and determine the optical states of both diffracted radiation sub-beams 429 and 439.

[0082] In some aspects, processor 432 receives information from detector 428 and beam analyzer 430. Processor 432 can create a basic correction algorithm based on the information received from detector 428 and beam analyzer 430, including but not limited to the optical state of the illumination beam, the alignment signals, associated position estimates, and the optical state in the pupil, image, and additional planes. The pupil plane is the plane in which the radial position of radiation defines the angle of incidence and the angular position defines the azimuth angle of the radiation. Processor 432 can utilize the basic correction algorithm to characterize the inspection apparatus 400 with reference to wafer marks and / or alignment marks 418.

[0083] Example Optical Metrology Incoherent Illumination System

[0084] As ICs continue to shrink, demand is increasing for lithographic tools capable of processing smaller and more densely packed metrology targets (e.g., alignment marks). A single wafer can include numerous targets for measurements (e.g., alignment, overlay, or the like). In turn, the large number of targets on the wafer can introduce delays in production due to the additional measurements, reducing production throughput. Therefore, it is desirable for metrology systems to measure targets faster for increasing wafer throughput.

[0085] A solution can be to reduce the time spent measuring each target. However, such an implementation can be challenging. For example, in an attempt to quickly move from target to target, a detection time (e.g., photon collection time or integration time) of a metrology system can be reduced. But in this scenario, the measurements can suffer signal-to-noise issues that degrade the reliability of the measurements. It is analogous to setting a very high shutter speed on a camera used in photography (e.g., a few milliseconds of exposure), resulting in the capture of a poorly resolved image due to the lack of sufficient illumination.

[0086] To address this issue, the amount of radiation detected by the metrology system can be increased (e.g., use a brighter source). Radiation sources may come in coherent and incoherent varieties. For metrology, a homogenous beam spot is desirable since inhomogeneity in the beam spot, such as a speckle pattern, can introduce errors in a measurement. The effect of a speckle pattern is analogous to a photograph image having uneven brightness — bright and dark areas.

[0087] Bright-source solutions are further constrained depending on the tool they are implemented in. For example, in typical microscopy applications, a light bulb can suffice for a number of reasons. Spatially incoherent radiation sources (e.g., lamps (light bulbs), LEDs, plasma sources (laser pumped plasma light source), or the like), can output bright and homogeneous light. The etendue of a light bulb (e.g., spread out in all directions) is not a problem for many microscopy applications. The term “etendue” can be used herein to refer to a property of light of an optical system that characterizes a spread of radiation intensity in area and angle. On the other hand, high-precision metrology systems, such as ones used in conjunction with lithographic processes, can have highly constraining etendue requirements, which can hinder light-bulb implementation. In the process of conditioning the etendue of a light bulb to conform to a high precision metrology system (e.g., via mirrors, lenses, apertures, and other optical hardware), much radiation can be lost. Lost radiation is counterproductive to the goal of providing a high-brightness source.

[0088] In some aspects, the terms “spatial coherence,” “spatially coherent,” “spatial incoherence,” or the like can be used to refer to coherence phenomena, or lack thereof, in which a portion of radiation (e.g., wavefronts) can interfere with a spatially shifted version of itself. Furthermore, the terms “temporal coherence,” “temporally coherent,” “temporal incoherence,” or the like, can be used to refer to coherence phenomena, or lack thereof, in which a portion of radiation can interfere with a time- delayed version of itself. It should be appreciated that, depending on the radiation source, radiation can be spatially coherent, temporally coherent, or both.

[0089] For high-precision metrology systems, spatially coherent radiation sources, such as a laser, are desirable for their high brightness and tight etendue (e.g., a highly directional beam). Spatially coherent radiation sources can also be more energy efficient than traditional incoherent sources. However, speckle phenomena — an interference effect — can reduce their desirability. In aspects described herein, the terms “interference pattern,” “speckle,” “speckling,” “speckling pattern,” or the like can be used to refer to coherent radiation having a cross section that exhibits inhomogeneous intensity.

[0090] In some aspects, radiation from a spatially coherent source can be adjusted so as to reduce undesirable effects of speckles. The terms “scrambled,” “coherence scrambling,” “mode scrambling,” or the like, can be used herein to refer to the phenomena where coherent radiation is converted into incoherent or partially-coherent radiation (either in part or in full) by, e.g., increasing incoherence of the radiation or changing a spatial intensity distribution of wavefronts of the radiation over time (e.g., varying the speckle pattern over time). The term “coherence adjustment,” “mode adjustment,” or the like can be used herein to refer to the process of adjusting a state of coherence of radiation — for example, adjusting a speckle pattern. It should be appreciated that, in the process of varying or scrambling a speckle pattern of coherent radiation, the instantaneous speckle pattern is an interference pattern of spatially coherent radiation, but the coherent radiation can appear to be scrambled or incoherent from the perspective of a detector that integrates the variation of the speckle pattern over a given time frame (e.g., averaging). The terms “mode scrambler,” “mode adjuster,” “coherence scrambler,” “coherence adjuster,” or the like can be used herein to refer to devices that implement adjustment of coherence to achieve mode scrambling.

[0091] One method includes using a mechanical coherence adjuster, such as a diffuser plate. In some aspects, a mechanical coherence adjuster can comprise a source of spatially coherent radiation and an actuatable diffuser plate. Coherent radiation can be incident on the actuatable diffuser plate. The radiation can scatter with unpredictable phases (e.g., randomized). The phases can be varied by moving the diffuser plate such that the incidence of the radiation on a rough surface changes over time. The radiation scattered (and coherence scrambled) by the diffuser plate can be collected to be sent to a target. The beam of scrambled radiation can have a speckle pattern that continually changes at a rate based on the roughness profile of the diffuser plate and rate of actuation.

[0092] In some aspects, a metrology system can comprise the mechanical coherence adjuster to generate a beam of scrambled radiation to send to a target. As the radiation scattered from the target is received at a detector, the varying speckle pattern is integrated over a finite detection period (e.g., averaged out).

[0093] Certain mechanical coherence adjusters, such as a diffuser plate, can suffer a variety of issues. In some aspects, diffuser plates can cause inefficient use of photons (e.g., some stray photons can be lost). Regarding speed of measurement, demands of the industry are gravitating toward metrology systems that can measure a target in milliseconds or less. It is desirable that a coherence adjuster be able to vary a speckle pattern over numerous distinct patterns in a detection period (e.g., 1000 variations ina millisecond) to attain a suitable averaging (a non-limiting example). However, a typical diffuser plate can be designed for pattern adjustment speeds in the range of kHz. Thus, some diffuser plate implementations can fall short of providing a satisfactory variation speed of a speckle pattern. Another problem of a diffuser plate is that it can produce random speckle patterns, and therefore might not produce enough variants of the pattern to scramble coherence within a given integration time — difficult to achieve optimal coherence scrambling. Additionally, having fast moving mechanical components in a clean environment (e.g., in a lithographic apparatus) can present problems, such as vibrations, release of contaminants, and catastrophic failures. Furthermore, some diffuser plates can be undesirably large and bulky for certain applications.

[0094] Aspects of the present disclosure provide structures and functions to more quickly and efficiently perform inspection of structures on a substrate, for example, using a coherence adjuster (e.g., an infinite impulse response optical filter with coherence scrambling capabilities) to convert a powerful coherent source into a powerful coherence scrambled source for irradiating a target. In some aspects, structures and functions of coherence adjusters can be implemented using, e.g., a multi-mode fiber, a diffuser device, an actuatable fiber, or the like.

[0095] FIG. 5 shows an optical metrology incoherent illumination system (e.g., illumination system 500), according to some aspects. In some aspects, illumination system 500 can include a laser source 502, an infinite impulse response (HR) optical filter 504, and a detection system 506.

[0096] In some aspects, laser source 502 can comprise or be part of radiation source SO of FIGS. 1 A and IB. In some aspects, laser source 502 can be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or non-polarized, and / or pulsed or continuous. In some aspects, laser source 502 can include a plurality of radiation sources having different colors, or wavelength ranges. In some aspects, laser source 502 can include additional optics that can prevent undesirable reflection and can provide for a suitable selection of one or more wavelength ranges or wavelengths. As a non-limiting example, laser source 502 can include a white light laser. The term “white light laser” can be interpreted as meaning that radiation is emitted over a range of wavelengths, and is not intended to imply that the emitted radiation is limited to visible radiation. Therefore, in some aspects, laser source 502 can additionally or alternatively encompass UV and / or IR radiation and any range of wavelengths suitable to reflect from a surface of a substrate W or from a layer at the substrate W.

[0097] In some aspects, laser source 502 can comprise a pump laser. In some aspects, laser source 502 can be configured to provide a pulsed laser beam. In some aspects, laser source 502 can emit pulses with a repetition rate of about 5MHz and with pulse width of less than about 1 ps. In some aspects, laser source 502 can pump high peak power pulses due to ultrashort pulse durations. For example, a peak intensity of laser source 502 can range from about 100 W / cm2to about 900 W / cm2. In some aspects, laser source 502 can include a pulse compression system, which can be arranged outside of an oscillatorof the laser source 502, and which is used to reduce the duration of laser pulses output by laser source 502.

[0098] In some aspects, laser source 502 can have a coherence length (Lc) ranging from about 10s of microns to about 10s of meters. For example, laser source 502 can have a coherence length of about 70 pm (corresponding to a 5 nm bandwidth at a 600 nm central wavelength).

[0099] In some aspects, laser source 502 can generate a light beam 508. In some aspects, light beam 508 can be coherent radiation. For example, light beam 508 can be generated by laser source 502 as spatially coherent radiation, temporally coherent radiation, or both. In some aspects, light beam 508 can have a wavelength ranging from about 400 nm to about 2000 nm. In some aspects, light beam 508 can have a wavelength ranging from about 400 nm to about 1000 nm.

[0100] In some aspects, laser source 502 can be coupled to a single-mode optical fiber connected to a first multimode optical fiber (e.g., input optical fiber 510) polarization-maintaining (PM) splice. The schematic diagram of FIG. 5 simplifies this connection for illustration purposes and merely shows laser source 502 coupled directly to input optical fiber 510. In some aspects, input optical fiber 510 can propagate radiation (e.g., light beam 508) by directing the radiation along a guide and / or medium. In some aspects, input optical fiber 510 can be configured to receive light beam 508 from the single-mode optical fiber and transmit light beam 508 to IIR optical filter 504. It is noted that, in FIGS. 5, 6, 8, and 9, each solid line represents an optical fiber, and an arrow represents a propagation direction of the light within the optical fiber.

[0101] In some aspects, IIR optical filter 504 can be configured to reduce coherence effects of at least one of spatial coherence, temporal coherence, or both. In some aspects, IIR optical filter 504 can comprise a single optical coupler (also referred as “fiber optic splitters”), such as N x N multimode optical coupler 512, and at least one mode-scrambling loop (e.g., mode-scrambling loop 526).

[0102] In some aspects, N x N multimode optical coupler 512 can have N input ports and N output ports. In some aspects, N x N multimode optical coupler 512 can be a commercially available optical coupler. In the exemplary aspect shown in FIG. 5, N x N multimode optical coupler 512 can be a 2 x 2 multimode optical coupler. In this configuration, N x N multimode optical coupler 512 can have input ports 514a, 514b and output ports 516a, 516b. In some aspects, one of input ports 514a, 514b can serve as an IIR filter input 518 configured to receive light beam 508 from input optical fiber 510. For example, first input port 514a can serve as IIR filter input 518. In some aspects, one of output ports 516a, 516b can serve as an IIR filter output 520 configured to transmit output light beam 532 to detection system 506 by way of a second multimode optical fiber (e.g., output optical fiber 524). For example, first output port 516a can serve as IIR filter output 520.

[0103] In some aspects, N x N multimode optical coupler 512 can be configured to split light beam 508 into sub-beams. In the exemplary aspect shown in FIG. 5, N x N multimode optical coupler 512 can be configured to split light beam 508 into sub-beams 522a-522d (depicted as dashed arrows). In some aspects, N x N multimode optical coupler 512 can be classified by a transmission (T) characteristicindicating a signal power percentage that is transmitted to an output port according to a coupling ratio. For example, N x N multimode optical coupler 512 can split light beam 508 according to a coupling ratio of T:(l-T). In some aspects, N x N multimode optical coupler 512 can transmit T% power of light beam 508 through first output port 516a, and therefore IIR filter output 520, and can reflect (1-T)% power of light beam 508 to second output port 516b. In some aspects, N x N multimode optical coupler 512 can have values for T ranging from about 0% to about 100%. Therefore, a skilled artisan will recognize that N x N multimode optical coupler 512 can have a coupling ratio of any value.

[0104] In one exemplary aspect, N x N multimode optical coupler 512 can be a 2 x 2 multimode optical coupler classified by a coupling ratio of 50:50, such that T is 50% power of light beam 508 and (1-T) is 50% power of light beam 508. In this configuration, N x N multimode optical coupler 512 can split light beam 508 at first input port 514a into first sub-beam 522a and second sub-beam 522b. In this example, first sub-beam 522a can be transmitted to first output port 516a at 50% power of light beam 508. In this example, second sub-beam 522b can be transmitted to second output port 516b at 50% power of light beam 508. For example, if the power of light of beam 508 were 100 mW, then the power of first sub-beam 522a can be 50 mW and the power of second sub-beam 522b can be 50 mW.

[0105] In some aspects, mode-scrambling loop 526 can be configured to perform mode scrambling on the signal of second sub-beam 522b. In some aspects, mode-scrambling loop 526 can include a multimode optical fiber loop 528 and a mode scrambler 530. In some aspects, mode-scrambling loop 526 can be formed by connecting an output port of N x N multimode optical coupler 512 to an input port of N x N multimode optical coupler 512. For example, in the configuration shown in FIG. 5, second output port 516a can be connected to second input port 514b. In some aspects, mode-scrambling loop 526 can be configured to transmit second sub-beam 522b from second output port 516b to second input port 514b. In some aspects, second input port 514b can split the received sub-beam into sub-beams 522c, 522d. In some aspects, sub-beam 522c can be transmitted to first output port 516a. In some aspects, sub-beam 522d can be transmitted to second output port 516b for recirculation through modescrambling loop 526.

[0106] In some aspects, with each sub-beam signal transmitted through mode-scrambling loop 526 M times (where M is an integer equal to or greater than 1), each successive divided copy of the input signal (e.g., light beam 508) can be equivalent to a power level T(1-T)M. In the exemplary aspect described in paragraph

[0107] with T equal to 50% power, second input port 514b can produce sub-beams 522c, 522d at 50% power of sub-beam 522b. Therefore, each successive copy of the input signal (e.g., light beam 508) can be equivalent to a power level 0.5 * (0.5)M. As a result, mode-scrambling loop 526 can produce copies of the input signal with decreasing peak power levels.

[0107] In some aspects, multimode optical fiber loop 528 can have a length Li longer than the temporal coherence length Lc of laser source 502. In some aspects, multimode optical fiber loop 528 can be configured to delay a received sub-beam signal in a time domain by a predetermined length relative to a previous sub-beam signal. Therefore, multimode optical fiber loop 528 can create such a delay thatmakes a successive sub-beam signal temporally incoherent with a previous sub-beam signal. For example, one meter of fiber can produce about 5 ns of delay, so each successive sub-beam signal can be delayed by 5 ns from a previous sub-beam signal. In some aspects, multimode optical fiber loop 528 can have a length selected such that no sub-beam signals interfere with themselves or each other. Accordingly, with each sub-beam signal transmitted through multimode optical fiber loop 528 M times (where M is an integer equal to or greater than 1), each successive divided copy of the input signal (e.g., light beam 508) can be temporally incoherent with every previous copy of the input signal.

[0108] In some aspects, mode scrambler 530 can perform mode mixing to alter a spatial mode distribution of each successive sub-beam signal traveling through multimode optical fiber loop 528 in mode-scrambling loop 526. In some aspects, mode scrambler 530 can add local stress and modify a refractive index of the multimode optical fiber loop 528, thereby facilitating mode mixing and creating a different speckle pattern from the initial light beam 508. For example, mode scrambler 530 can be configured to physically bend optical fibers. Accordingly, with each sub-beam signal transmitted through multimode optical fiber loop 528 M times (where M is an integer equal to or greater than 1), each successive divided copy of the input signal (e.g., light beam 508) can be spatially incoherent with every previous copy. As a result, mode scrambler 530 can alter the power among mode balances and can create a new mode ensemble at IIR filter output 520.

[0109] In some aspects, mode scrambler 530 can perform at least one of active mode scrambling or passive mode scrambling by way of a variety of different physical configurations including, for example, at least one of a clamping device, a winding device, a piezoelectric active mode scrambler, a coil in multimode optical fiber loop 528, a twist in multimode optical fiber loop 528, an off-axis splice in multimode optical fiber loop 528, or a mirror-based ring resonator, or a combination. In some aspects, mode scrambler 530 can be a commercially available mode-scrambling device. Various aspects of mode scrambler 530 are described in more detail in US 10,845,523 B2, issued November 24, 2020, which is incorporated by reference herein in its entirety.

[0110] In some aspects, each output light beam 532 in time can be a light signal output from IIR filter output 520. In some aspects, each output light beam 532 can have a reduced peak power and a reduced coherence compared to initial light beam 508, as a result of the signal divisions performed by IIR optical filter 504. In some aspects, each output light beam 532 can be transmitted through output optical fiber 524 to detection system 506 for measuring the speckle pattern.

[0111] In some aspects, detection system 506 can be a device used measure a property of a structure (e.g., overlay sensor, critical dimension sensor, or the like), a device or system used in a lithographic apparatus to inspect an alignment of a wafer (e.g., alignment sensor), or the like. In some aspects, detection system 506 can be detector 428 described in FIGS. 4A and 4B. In some aspects, detection system 506 can be configured to average an image of all output signals (e.g., each output light beam 532) received from IIR optical filter 504 as a function of time. Such averaging operations performed bydetection system 506 can thereby average the speckle pattern originating from laser source 502. Accordingly, an average of the plurality of sub-beams can result in spatial coherence suppression.

[0112] FIG. 6 shows an optical metrology incoherent illumination system (e.g., illumination system 600), according to some aspects. In some aspects, illumination system 600 can be an alternative aspect of illumination system 500. It is noted that, the same components that have been described above in connection with FIG. 5, such as laser source 502, light beam 508, detection system 506, etc., are not repeated herein. In some aspects, illumination system 600 can include IIR optical filter 605.

[0113] In some aspects, IIR optical filter 605 can be an alternative aspect of IIR optical filter 504. In some aspects, IIR optical filter 605 can comprise a single optical coupler, such as N x N multimode optical coupler 613, and a plurality of mode-scrambling loops (e.g., mode-scrambling loops 526a-526c). In some aspects, IIR optical filter 605 can reduce a peak power of light beam 508 by 75% while maintaining consistent average power, thereby improving a lifespan of detection system 506. Additionally, in some aspects, IIR optical filter 605 can reduce speckle contrast of light beam 508.

[0114] In some aspects, N x N multimode optical coupler 613 can have N input ports and N output ports. In some aspects, N x N multimode optical coupler 613 can be a commercially available optical coupler. In the exemplary aspect shown in FIG. 6, N x N multimode optical coupler 613 can be a 4 x 4 multimode optical coupler. In this configuration, N x N multimode optical coupler 613 can have input ports 514a-514d and output ports 516a-516d. In some aspects, one of input ports 514a-514d can serve as an IIR filter input 518 configured to receive light beam 508 from input optical fiber 510. For example, first input port 514a can serve as IIR filter input 518. In some aspects, one of output ports 516a-516d can serve as an IIR filter output 520 configured to transmit output light beam 532 to detection system 506 by way of a second multimode optical fiber (e.g., output optical fiber 524). For example, first output port 516a can serve as an IIR filter output 520.

[0115] In some aspects, N x N multimode optical coupler 613 can be configured to split light beam 508 into sub-beams. In the exemplary aspect shown in FIG. 6, N x N multimode optical coupler 613 can be configured to split light beam 508 into sub-beams 622 (depicted as dashed arrows). In one exemplary aspect, N x N multimode optical coupler 613 can be a 4 x 4 multimode optical coupler that splits each input signal at each of input ports 514a-514d into four sub-beams 622 with 25% power of the input signal for transmission to each of output ports 516a-516d. For example, first input port 514a can transmit four sub-beams 622 to output ports 516a-516d, wherein each sub-beam 622 with 25% power of an input signal is transmitted to a corresponding one of output ports 516a-516d. A skilled artisan will recognize that N x N multimode optical coupler 613 can have a coupling ratio of any value.

[0116] In some aspects, IIR optical filter 605 can include a plurality of mode-scrambling loops. In the exemplary aspect shown in FIG. 6 where N x N multimode optical coupler 613 is a 4 x 4 multimode optical coupler, IIR optical filter 605 can include three mode-scrambling loops 526a-526c. In this configuration, light input into IIR optical filter 605 can be split into three mode-scrambling loops with three different fiber loop lengths, namely Li, L2, and L3, respectively. In this configuration, first mode-scrambling loop 526a can be formed by coupling fourth input port 514d to fourth output port 516d with multimode optical fiber loop 528a of length Li. In this configuration, second mode-scrambling loop 526b can be formed by coupling third input port 514c to third output port 516c with multimode optical fiber loop 528b of length L2. In this configuration, third mode-scrambling loop 526c can be formed by coupling second input port 514b to second output port 516b with multimode optical fiber loop 528c of length L3.

[0117] In some aspects, the lengths Li, L2, and L3 of multimode optical fiber loops 528a-528c, respectively, can be predetermined values such that sub-beams 622 do not have a constant relative phase difference with each other, thereby reducing coherence effects. In some aspects, the lengths Li, L2, and L3 of multimode optical fiber loops 528a-528c, respectively, can be predetermined according to the following rule: Li » Ni*Lc, L2 » N2*Lc and L3 » N3*Lc, whereby Li, L2, and L3 are individually much larger than the coherence length Lc of laser source 502, Li L2L3, and Ni, N2, and N3 are real numbers. Additionally, lengths for Li, L2, and L3 and values for Ni, N2, and N3 can be predetermined according to the following rule: | N-L * 1^ - N2* L2| « Lc, | N-L * L-L — N3* L31 « Lc, and |N2* L2— N3* L3| « Lc, whereby Li, L2, and L3 are individually much larger than the coherence length Lc of laser source 502, Li L2L3, and Ni, N2, and N3 are real numbers. The above-mentioned rules can ensure that any two sub-beams 622 are outside of a coherence gate.

[0118] In some aspects, Ni, N2, and N3 can be predetermined to reduce application-specific dose noise in a lithographic apparatus. In some aspects, Ni, N2, and N3 can be predetermined such that any length of any combination of transmissions through multimode optical fiber loops 528a-528c are unique lengths separated by more than the coherence length Lc. In some aspects, Ni, N2, and N3 can be predetermined such that any sub-beam signals typically do not have a constant relative phase difference. For example, Ni, N2, and N3 can be prime numbers such that any sub-beam signals typically do not have a constant relative phase difference. In some aspects, Ni, N2, and N3 can be predetermined such that any sub-beam signals have a constant relative phase difference after a certain predetermined light amplitude threshold. For example, the amplitude of each output light beam 532 can be imperceptibly low according to detection system 506 after a 10-transmission threshold through mode-scrambling loops 526a-526c. In this example, Ni can be set at 1, N2 can be set at 10, and N3 can be set at 100. Therefore, in this example, each output light beam 532 can begin to add together coherently after the 10-transmission threshold, but the intensity of the signal received at detection system 506 would be low enough for any coherence at that level to be considered acceptable.

[0119] FIG. 7 shows schematic plots 734, 736 indicating amplitude 740 (arbitrary units) of pulses and sub-pulses transmitted over a period of time 742 (ns), according to some aspects. In other aspects, period of time 742 can be any unit of time. It is noted that FIG. 7 should not be interpreted as a to-scale drawing.

[0120] In some aspects, plot 734 can indicate the use of a laser source without an IIR optical filter (e.g., IIR optical filter 605). In some aspects, plot 734 can indicate an initial unfiltered pulse 744 transmitted by a laser source at an initial time 746. In some aspects, plot 734 can indicate a subsequent unfilteredpulse 748 transmitted by a laser source at a subsequent time 750. In some aspects, subsequent time 750 can be delayed from initial time 746 by the reciprocal of a predetermined pulse repetition frequency (1 / PRF). The amplitude 740 of initial unfiltered pulse 744 and the amplitude 740 of subsequent unfiltered pulse 748 can be considered to be undesirably high.

[0121] In some aspects, plot 736 can indicate the use of a laser source with an IIR optical filter (e.g., IIR optical filter 605) and the associated benefits. In some aspects, plot 736 can indicate an initial subpulse 752 output from an IIR optical filter (e.g., IIR optical filter 605) at an initial time 746. In some aspects, initial sub-pulse 752 can be representative of, according to the exemplary aspect shown in FIG. 6, a sub-beam 622 transmitted from first input port 514a to first output port 516a. In this example, the amplitude 740 of initial sub-pulse 752 can be 25% of the amplitude 740 of initial unfiltered pulse 744.

[0122] In some aspects, plot 736 can indicate first-transit sub-pulses 756a-756c that have traveled through mode-scrambling loops one time. In some aspects, first-transit sub-pulse 756a can be representative of, according to the exemplary aspect shown in FIG. 6, a sub-beam 622 transmitted from first input port 514a to fourth output port 516d, then through mode-scrambling loop 526a, and then from fourth input port 514d to first output port 516a. In this example, the amplitude 740 of first-transit subpulse 756a can be 6.25% of the amplitude 740 of initial unfiltered pulse 744.

[0123] In some aspects, first-transit sub-pulse 756b can be representative of, according to the exemplary aspect shown in FIG. 6, a sub-beam 622 transmitted from first input port 514a to third output port 516c, then through mode-scrambling loop 526b, and then from third input port 514c to first output port 516a. In this example, the amplitude 740 of first-transit sub-pulse 756b can be 6.25% of the amplitude 740 of initial unfiltered pulse 744.

[0124] In some aspects, first-transit sub-pulse 756c can be representative of, according to the exemplary aspect shown in FIG. 6, a sub-beam 622 transmitted from first input port 514a to second output port 516b, then through mode-scrambling loop 526c, and then from second input port 514b to first output port 516a. In this example, the amplitude 740 of first-transit sub-pulse 756c can be 6.25% of the amplitude 740 of initial unfiltered pulse 744.

[0125] In some aspects, plot 736 can indicate second- transit sub-pulses 758a-758c that have traveled through mode-scrambling loops two times. In some aspects, second-transit sub-pulses 758a-758c can be representative of, according to the exemplary aspect shown in FIG. 6, sub-beams 622 that have been divided in an amplitude domain by passing through mode-scrambling loops 526a-526c two times. As a result, second-transit sub-pulses 758a-758c can be 1.56% of the amplitude 740 of initial unfiltered pulse 744.

[0126] In some aspects, after an arbitrary period 760, plot 736 can indicate Mlll-transit sub-pulses 762a- 762c that have traveled through mode-scrambling loops M times (where M is an integer greater than 2 in this example). In some aspects, Mlll-transit sub-pulses 762a-762c can be representative of, according to the exemplary aspect shown in FIG. 6, sub-beams 622 that have been divided in an amplitude domain by passing through mode-scrambling loops 526a-526c M times. With every completed loop, amplitude740 of each recirculated sub-pulse can be divided into a quarter of its input signal. As a result, in this example, Mth-transit sub-pulses 762a-762c can be (0.25*(0.25)M*100)% of the amplitude 740 of initial unfiltered pulse 744.

[0127] In some aspects, plot 736 can indicate a subsequent sub-pulse 754 (which has not traveled through any mode-scrambling loops) output from an IIR optical filter (e.g., IIR optical filter 605) at a subsequent time 750. In some aspects, subsequent sub-pulse 754 can be representative of, according to the exemplary aspect shown in FIG. 6, a sub-beam 622 transmitted from first input port 514a to first output port 516a. In this example, the amplitude 740 of subsequent sub-pulse 754 can be 25% of the amplitude 740 of initial unfiltered pulse 744.

[0128] In some aspects, any number of sub-pulses can occur between the transmission of initial subpulse 752 at initial time 746 and subsequent sub-pulse 754 at subsequent time 750. As a result, the IIR optical filter (e.g., IIR optical filter 605) can reduce a peak power of input signals because the IIR optical filter is configured to divided the power across a plurality of sub-pulses occurring as a function of time. In some aspects, a detection system (e.g., detection system 506 shown in FIG. 6) can average the plurality of sub-pulses to determine a reduced peak power and a reduced speckle contrast.

[0129] FIG. 8 shows an optical metrology incoherent illumination system (e.g., illumination system 800), according to some aspects. In some aspects, illumination system 800 can be an alternative aspect of illumination systems 500 or 600. It is noted that, the same components that have been described above in connection with FIG. 5, such as laser source 502, light beam 508, detection system 506, etc., are not repeated herein. In some aspects, illumination system 800 can include an IIR optical filter 865 that is equipped with a series of N x N multimode optical couplers.

[0130] In some aspects, IIR optical filter 865 can be an alternative aspect of IIR optical filters 504 or 605. In some aspects, IIR optical filter 865 can comprise a series of optical couplers, such as N x N multimode optical couplers 512a-512c, and a plurality of mode-scrambling loops (e.g., modescrambling loops 526a-526c). In some aspects, IIR optical filter 865 can reduce a peak power of light beam 508 while maintaining consistent average power, thereby improving a lifespan of detection system 506. Additionally, in some aspects, IIR optical filter 865 can reduce speckle contrast of light beam 508.

[0131] In some aspects, N x N multimode optical couplers 512a-512c can be a series of optical couplers comprising a plurality of the N x N multimode optical coupler 512 shown in FIG. 5. In the exemplary aspect shown in FIG. 8, N x N multimode optical couplers 512a-512c can be a series of 2 x 2 multimode optical couplers. In some aspects, first input port 514a of N x N multimode optical coupler 512a can serve as an IIR filter input 518 configured to receive light beam 508 from input optical fiber 510. In some aspects, first output port 516a of N x N multimode optical coupler 512a can be coupled to first input port 514a of N x N multimode optical coupler 512b with a connector optical fiber 866a. In some aspects, first output port 516a of N x N multimode optical coupler 512b can be coupled to first input port 514a of N x N multimode optical coupler 512c with a connector optical fiber 866b. In some aspects, first output port 516a of N x N multimode optical coupler 512c can serve as an IIR filter output 520configured to transmit output light beam 532 to detection system 506 by way of a second multimode optical fiber (e.g., output optical fiber 524).

[0132] In some aspects, the lengths of mode-scrambling loops 526a-526c can be tuned such that any transmitted sub-beams do not overlap in order to maintain the mode scrambling. For example, the lengths of mode-scrambling loops 526a-526c can be calibrated to maintain a temporal delay between any combination of signals traveling across mode-scrambling loops 526a-526c.

[0133] In some aspects, IIR optical filter 865 can be configured to use any amount and any size of N x N multimode optical couplers in series based on the specifications of the detection system. For example, a skilled artisan will recognize that the series of N x N multimode optical couplers can comprise a plurality of the N x N multimode optical coupler 613 shown in FIG. 6 (e.g., a 4 x 4 multimode optical coupler).

[0134] FIG. 9 shows an optical metrology incoherent illumination system (e.g., illumination system 900), according to some aspects. In some aspects, illumination system 900 can be an alternative aspect of illumination systems 500, 600, or 800. It is noted that, the same components that have been described above in connection with FIG. 5, such as laser source 502, light beam 508, detection system 506, etc., are not repeated herein. In some aspects, illumination system 900 can include an IIR optical filter 968 that is equipped with a plurality of N x N multimode optical couplers arranged in nested modescrambling loops.

[0135] In some aspects, IIR optical filter 968 can be an alternative aspect of IIR optical filters 504, 605, or 865. In some aspects, IIR optical filter 968 can comprise a plurality of N x N multimode optical couplers, such as N x N multimode optical couplers 512a-512e, arranged in a plurality of nested modescrambling loops (e.g., nested mode-scrambling loops 870a, 870b). In some aspects, IIR optical filter 968 can reduce a peak power of light beam 508 while maintaining consistent average power, thereby improving a lifespan of detection system 506. Additionally, in some aspects, IIR optical filter 865 can reduce speckle contrast of light beam 508.

[0136] In some aspects, N x N multimode optical couplers 512a-512c can be a series of optical couplers comprising a plurality of the N x N multimode optical coupler 512 shown in FIG. 5. In the exemplary aspect shown in FIG. 9, N x N multimode optical couplers 512a-512c can be a series of 2 x 2 multimode optical couplers. In some aspects, first input port 514a of N x N multimode optical coupler 512a can serve as an IIR filter input 518 configured to receive light beam 508 from input optical fiber 510. In some aspects, first output port 516a of N x N multimode optical coupler 512a can be coupled to first input port 514a of N x N multimode optical coupler 512b with a connector optical fiber 866a. In some aspects, first output port 516a of N x N multimode optical coupler 512b can be coupled to first input port 514a of N x N multimode optical coupler 512c with a connector optical fiber 866b. In some aspects, first output port 516a of N x N multimode optical coupler 512c can serve as an IIR filter output 520 configured to transmit output light beam 532 to detection system 506 by way of a second multimodeoptical fiber (e.g., output optical fiber 524). In some aspects, output optical fiber 524 can be can be spliced to a patchcord of a square core multimode fiber to support a top hat beam profile.

[0137] In some aspects, N x N multimode optical couplers 512a-512e can each have a predetermined coupling ratio of any value. In some aspects, N x N multimode optical coupler 512a can have a coupling ratio of 10:90. In this configuration, the output light of the first output port 516a of optical coupler 512a contains 10% power of the input light from the input ports 514a, 514b of optical coupler 512a. In this configuration, the output light of the first output port 516a of optical coupler 512a contains 10% power of the input light from the input ports 514a, 514b of optical coupler 512a.

[0138] In some aspects, N x N multimode optical coupler 512b can be a 3dB coupler with a coupling ratio of 50:50. In this configuration, the output light of the first output port 516a of optical coupler 512b contains 50% power of the input light from the input ports 514a, 514b of optical coupler 512b. In this configuration, the output light of the first output port 516a of optical coupler 512b contains 50% power of the input light from the input ports 514a, 514b of optical coupler 512b.

[0139] In some aspects, N x N multimode optical coupler 512c can have a coupling ratio of 99:1. In this configuration, the output light of the first output port 516a of optical coupler 512c contains 99% power of the input light from the input ports 514a, 514b of optical coupler 512c. In this configuration, the output light of the first output port 516a of optical coupler 512c contains 1% power of the input light from the input ports 514a, 514b of optical coupler 512c.

[0140] In some aspects, N x N multimode optical coupler 512d can be a 3dB coupler with a coupling ratio of 50:50. In this configuration, the output light of the first output port 516a of optical coupler 512d contains 50% power of the input light from the input ports 514a, 514b of optical coupler 512d. In this configuration, the output light of the first output port 516a of optical coupler 512d contains 50% power of the input light from the input ports 514a, 514b of optical coupler 512d.

[0141] In some aspects, N x N multimode optical coupler 512e can be a 3dB coupler with a coupling ratio of 50:50. In this configuration, the output light of the first output port 516a of optical coupler 512e contains 50% power of the input light from the input ports 514a, 514b of optical coupler 512e. In this configuration, the output light of the first output port 516a of optical coupler 512e contains 50% power of the input light from the input ports 514a, 514b of optical coupler 512e.

[0142] In some aspects, a first nested mode-scrambling loop 870a can begin at the second output port 516b of optical coupler 512a. In some aspects, second output port 516b of optical coupler 512a can be coupled to a mode scrambler 530a with a connector optical fiber 866c. The function of mode scrambler 530a is described in detail above with reference to mode scrambler 530 shown in FIG. 5. In some aspects, mode scrambler 530a can be coupled to first input port 514a of optical coupler 512d. In some aspects, first output port 516a of optical coupler 512d can be coupled to first input port 514a of optical coupler 512e with a connector fiber 866e with a length of Li. In some aspects, second output port 516b of optical coupler 512d can be coupled to second input port 514b of optical coupler 512e with a connector fiber 866i with a length of L2. In some aspects, first output port 516a of optical coupler 512ecan be coupled to second input port 514b of optical coupler 512b with a connector fiber 866f. In some aspects, second output port 516b of optical coupler 512e can be coupled to second input port 514b of optical coupler 512a with a connector fiber 866j with a length of L3. In some aspects, after passing through mode scrambler 530a, a light signal can be split across optical couplers 512d, 512e and connector optical fibers of different lengths for recirculation back into at least one of optical couplers 512a, 512b.

[0143] In some aspects, a second nested mode-scrambling loop 870b can begin at the second output port 516b of optical coupler 512b. In some aspects, second output port 516b of optical coupler 512b can be coupled to a mode scrambler 530b with a connector optical fiber 866g. The function of mode scrambler 530b is described in detail above with reference to mode scrambler 530 shown in FIG. 5. In some aspects, mode scrambler 530b can be coupled to second input port 514b of optical coupler 512d. In some aspects, first output port 516a of optical coupler 512d can be coupled to first input port 514a of optical coupler 512e with a connector fiber 866e with a length of Li. In some aspects, second output port 516b of optical coupler 512d can be coupled to second input port 514b of optical coupler 512e with a connector fiber 866i with a length of L2. In some aspects, first output port 516a of optical coupler 512e can be coupled to second input port 514b of optical coupler 512b with a connector fiber 866f. In some aspects, second output port 516b of optical coupler 512e can be coupled to second input port 514b of optical coupler 512a with a connector fiber 866j with a length of L3. In some aspects, after passing through mode scrambler 530b, a light signal can be split across optical couplers 512d, 512e and connector optical fibers of different lengths for recirculation back into at least one of optical couplers 512a, 512b.

[0144] In some aspects, connector optical fibers 866e, 866i, and 866j can be low bandwidth 50 pm step index fibers configured to spread each additional copy of light beam 508 in a time domain. In some aspects, the lengths Li, L2, and L3 of connector optical fibers 866e, 866i, and 866j, respectively, can be predetermined values such that any light signals do not have a constant relative phase difference with each other, thereby reducing coherence effects. In some aspects, the lengths Li, L2, and L3 of connector optical fibers 866e, 866i, and 866j, respectively, can be predetermined according to the following rule: Li » Ni*Lc, L2 » N2*LC and L3 » N3*Lc, whereby Li, L2, and L3 are individually much larger than the coherence length Lc of laser source 502, Li L2L3, and Ni, N2, and N3 are real numbers. Additionally, lengths for Li, L2, and L3 and values for Ni, N2, and N3 can be predetermined according to the following rule:L31 « Lc, whereby Li, L2, and L3 are individually much larger than the coherence length Lc of laser source 502, Li L2L3, and Ni, N2, and N3 are real numbers. The above-mentioned rules can ensure that any two light signals are outside of a coherence gate.

[0145] In some aspects, Ni, N2, and N3 can be predetermined to reduce application-specific dose noise in a lithographic apparatus. In some aspects, Ni, N2, and N3 can be predetermined such that any length of any combination of transmissions through nested mode-scrambling loops 870a, 870b are uniquelengths separated by more than the coherence length Lc. In some aspects, Ni, N2, and N3 can be predetermined such that any light signals typically do not have a constant relative phase difference. For example, Ni, N2, and N3 can be prime numbers such that any light signals typically do not have a constant relative phase difference. In some aspects, Ni, N2, and N3 can be predetermined such that any sub-beam signals have a constant relative phase difference after a certain predetermined light amplitude threshold. For example, the amplitude of each output light beam 532 can be imperceptibly low according to detection system 506 after a 10-transmission threshold through nested mode-scrambling loops 870a, 870b. In this example, Ni can be set at 1, N2 can be set at 10, and N3 can be set at 100. Therefore, in this example, each output light beam 532 can begin to add together coherently after the 10-transmission threshold, but the intensity of the signal received at detection system 506 would be low enough for any coherence at that level to be considered acceptable.

[0146] FIG. 10 shows an incoherent illumination method 1000 for optical metrology, according to some aspects. In some aspects, at step S1002, a light beam (e.g., light beam 508 shown in FIG. 5) can be generated with a white light laser source (e.g., laser source 502 shown in FIG. 5). In some aspects, the light beam can comprise at least one of a continuous light wave or a plurality of light pulses.

[0147] In some aspects, at step S1004, the light beam can be transmitted with a first multimode optical fiber (e.g., input optical fiber 510 shown in FIG. 5) to an input (e.g., IIR filter input 518 shown in FIG. 5) of an IIR optical filter (e.g., IIR optical filter 504 shown in FIG. 5). In some aspects, the IIR optical filter can comprise at least one N x N multimode optical coupler having N input ports (e.g., input ports 514 shown in FIG. 5) and N output ports (e.g., output ports 516 shown in FIG. 5), wherein the first input port of the N input ports forms the input of the IIR optical filter.

[0148] In some aspects, the at least one N x N multimode optical coupler can comprise at least one of a 2 x 2 multimode optical coupler (e.g., N x N multimode optical coupler 512 shown in FIG. 5), a 3 x 3 multimode optical coupler, or a 4 x 4 multimode optical coupler (e.g., N x N multimode optical coupler 613 shown in FIG. 6). In some aspects, the at least one N x N multimode optical coupler can comprise a series of 2 x 2 multimode optical couplers (e.g., N x N multimode optical couplers 512a- 512c shown in FIG. 8) such that one of the N output ports of each 2 x 2 multimode optical coupler is coupled to one of the N input ports of each successive 2 x 2 multimode optical coupler. In some aspects, the at least one N x N multimode optical coupler can comprise a plurality of 2 x 2 multimode optical couplers (e.g., N x N multimode optical couplers 512a-512e shown in FIG. 9) arranged in nested modescrambling loops (e.g., nested mode-scrambling loops 870a, 870b) that produce a plurality of sub-beams divided in an amplitude domain by a predetermined coupling ratio at each 2 x 2 multimode optical coupler.

[0149] In some aspects, at step S1006, the light beam can be split into a plurality of sub-beams divided in an amplitude domain across the N output ports.

[0150] In some aspects, at step S 1008, the plurality of sub-beams can be delayed in a time domain with one or more mode-scrambling loops (e.g., mode-scrambling loop 526 shown in FIG. 5) formed bycoupling at least one of the other N output ports to another one of the other N input ports with a multimode optical fiber loop (e.g., multimode optical fiber loop 528 shown in FIG. 5). In some aspects, the multimode optical fiber loop can be longer than a coherence length of the laser source to achieve a temporal incoherence between the plurality of sub-beams and the light beam.

[0151] In some aspects, at step S1010, a spatial mode of the plurality of sub-beams can be mixed with a mode scrambler (e.g., mode scrambler 530) disposed along the multimode optical fiber loop to achieve a spatial incoherence between the plurality of sub-beams and the light beam. In some aspects, the mode scrambler can comprise at least one of a piezoelectric active mode scrambler, a coil in the multimode optical fiber loop, a twist in the multimode optical fiber loop, an off-axis splice in the multimode optical fiber loop, or a mirror-based ring resonator, or a combination.

[0152] In some aspects, at step S1012, a peak power of the light beam can be reduced and a speckle contrast of the light beam can be reduced due to the temporal incoherence provided by the one or more mode-scrambling loops and the spatial incoherence provided by the mode scrambler.

[0153] In some aspects, at step S1014, the plurality of sub-beams can be recirculated through the one or more mode-scrambling loops.

[0154] In some aspects, at step S1016, an output light beam (e.g., output light beam 532 shown in FIG. 5) can be transmitted from an output (e.g., IIR filter output 520 shown in FIG. 5) of the IIR optical filter with a second multimode optical fiber (e.g., output optical fiber 524 shown in FIG. 5) to a detection system (e.g., detection system 506 shown in FIG. 5).

[0155] The method steps of FIG. 10 can be performed in any conceivable order and it is not required that all steps be performed. Moreover, the method steps of FIG. 10 described above merely reflect an example of steps and are not limiting. That is, further method steps and functions are envisaged based aspects described in reference to FIGS. 1A-9.

[0156] The embodiments may further be described using the following clauses:1. An optical metrology incoherent illumination system comprising: a white light laser source configured to generate a light beam; a detection system configured to measure a characteristic of the light beam; and an infinite impulse response (IIR) optical filter disposed between the laser source and the detection system, the IIR optical filter comprising: at least one N x N multimode optical coupler having N input ports and N output ports, wherein a first input port of the N input ports, forming an input of the IIR optical filter, is coupled to the laser source by a first multimode optical fiber, and the at least one N x N multimode optical coupler is configured to split the light beam into a plurality of sub-beams divided in an amplitude domain across the N output ports; a first output port of the N output ports, forming an output of the IIR optical filter, is coupled to the detection system by a second multimode optical fiber; andone or more mode-scrambling loops formed by coupling at least one of the other N output ports to another one of the other N input ports with a multimode optical fiber loop, wherein a length of the multimode optical fiber loop is longer than a coherence length of the laser source to achieve a temporal incoherence between the plurality of sub-beams and the light beam, a mode scrambler is disposed along the multimode optical fiber loop to achieve a spatial incoherence between the plurality of sub-beams and the light beam, and the temporal incoherence provided by the length of the multimode optical fiber loop and the spatial incoherence provided by the mode scrambler reduce a peak power of the light beam and reduce a speckle contrast of the light beam measured by the detection system.2. The optical metrology incoherent illumination system of clause 1, wherein the light beam comprises at least one of a continuous light wave or a plurality of light pulses.3. The optical metrology incoherent illumination system of clause 1, wherein the at least one N x N multimode optical coupler comprises at least one of a 2 x 2 multimode optical coupler, a 3 x 3 multimode optical coupler, or a 4 x 4 multimode optical coupler.4. The optical metrology incoherent illumination system of clause 1 , wherein the at least one N x N multimode optical coupler comprises a series of 2 x 2 multimode optical couplers such that one of the N output ports of each 2 x 2 multimode optical coupler is coupled to one of the N input ports of each successive 2 x 2 multimode optical coupler.5. The optical metrology incoherent illumination system of clause 1, wherein the at least one N x N multimode optical coupler comprises a plurality of 2 x 2 multimode optical couplers arranged in nested mode-scrambling loops that produce a plurality of sub-beams divided in an amplitude domain by a predetermined coupling ratio at each 2 x 2 multimode optical coupler.6. The optical metrology incoherent illumination system of clause 1, wherein the length of the multimode optical fiber loop in each of the one or more mode-scrambling loops comprises a predetermined length unique from any other multimode optical fiber loop such that the plurality of subbeams remain temporally incoherent and spatially incoherent relative to each other such that an average of the plurality of sub-beams results in a spatial coherence suppression.7. The optical metrology incoherent illumination system of clause 1, wherein the mode scrambler comprises at least one of a piezoelectric active mode scrambler, a coil in the multimode optical fiber loop, a twist in the multimode optical fiber loop, an off-axis splice in the multimode optical fiber loop, or a mirror-based ring resonator, or a combination.8. An infinite impulse response (IIR) optical filter apparatus comprising: at least one N x N multimode optical coupler having N input ports and N output ports, wherein a first input port of the N input ports, forming an input of the IIR optical filter, is coupled to a white light laser source by a first multimode optical fiber, the laser source configured to generate a light beam, andthe at least one N x N multimode optical coupler is configured to split the light beam into a plurality of sub-beams divided in an amplitude domain across the N output ports; a first output port of the N output ports, forming an output of the IIR optical filter, is coupled to a detection system by a second multimode optical fiber, the detection system configured to measure a characteristic of the light beam; and one or more mode-scrambling loops formed by coupling at least one of the other N output ports to another one of the other N input ports with a multimode optical fiber loop, wherein a length of the multimode optical fiber loop is longer than a coherence length of the laser source to achieve a temporal incoherence between the plurality of sub-beams and the light beam, a mode scrambler is disposed along the multimode optical fiber loop to achieve a spatial incoherence between the plurality of sub-beams and the light beam, and the temporal incoherence provided by the length of the multimode optical fiber loop and the spatial incoherence provided by the mode scrambler reduce a peak power of the light beam and reduce a speckle contrast of the light beam measured by the detection system.9. The IIR optical filter apparatus of clause 8, wherein the light beam comprises at least one of a continuous light wave or a plurality of light pulses.10. The IIR optical filter apparatus of clause 8, wherein the at least one N x N multimode optical coupler comprises at least one of a 2 x 2 multimode optical coupler, a 3 x 3 multimode optical coupler, or a 4 x 4 multimode optical coupler.11. The IIR optical filter apparatus of clause 8, wherein the at least one N x N multimode optical coupler comprises a series of 2 x 2 multimode optical couplers such that one of the N output ports of each 2 x 2 multimode optical coupler is coupled to one of the N input ports of each successive 2 x 2 multimode optical coupler.12. The IIR optical filter apparatus of clause 8, wherein the at least one N x N multimode optical coupler comprises a plurality of 2 x 2 multimode optical couplers arranged in nested mode-scrambling loops that produce a plurality of sub-beams divided in an amplitude domain by a predetermined coupling ratio at each 2 x 2 multimode optical coupler.13. The IIR optical filter apparatus of clause 8, wherein the length of the multimode optical fiber loop in each of the one or more mode-scrambling loops comprises a predetermined length unique from any other multimode optical fiber loop such that the plurality of sub-beams remain temporally incoherent and spatially incoherent relative to each other such that an average of the plurality of subbeams results in a spatial coherence suppression.14. The IIR optical filter apparatus of clause 8, wherein the mode scrambler comprises at least one of a piezoelectric active mode scrambler, a coil in the multimode optical fiber loop, a twist in the multimode optical fiber loop, an off-axis splice in the multimode optical fiber loop, or a mirror-based ring resonator, or a combination.15. An incoherent illumination method for optical metrology comprising:generating a light beam with a white light laser source; transmitting the light beam with a first multimode optical fiber to an input of an infinite impulse response (IIR) optical filter, the IIR optical filter comprising at least one N x N multimode optical coupler having N input ports and N output ports, wherein a first input port of the N input ports forms the input of the IIR optical filter; splitting the light beam into a plurality of sub-beams divided in an amplitude domain across the N output ports; delaying the plurality of sub-beams in a time domain with one or more mode-scrambling loops formed by coupling at least one of the other N output ports to another one of the other N input ports with a multimode optical fiber loop, wherein the multimode optical fiber loop is longer than a coherence length of the laser source to achieve a temporal incoherence between the plurality of sub-beams and the light beam; mixing a spatial mode of the plurality of sub-beams with a mode scrambler disposed along the multimode optical fiber loop to achieve a spatial incoherence between the plurality of sub-beams and the light beam; reducing a peak power of the light beam and a speckle contrast of the light beam due to the temporal incoherence provided by the one or more mode-scrambling loops and the spatial incoherence provided by the mode scrambler; recirculating the plurality of sub-beams through the one or more mode-scrambling loops; and transmitting an output light beam from an output of the IIR optical filter with a second multimode optical fiber to a detection system.16. The incoherent illumination method of clause 15, wherein the light beam comprises at least one of a continuous light wave or a plurality of light pulses.17. The incoherent illumination method of clause 15, wherein the at least one N x N multimode optical coupler comprises at least one of a 2 x 2 multimode optical coupler, a 3 x 3 multimode optical coupler, or a 4 x 4 multimode optical coupler.18. The incoherent illumination method of clause 15, wherein the at least one N x N multimode optical coupler comprises a series of 2 x 2 multimode optical couplers such that one of the N output ports of each 2 x 2 multimode optical coupler is coupled to one of the N input ports of each successive 2 x 2 multimode optical coupler.19. The incoherent illumination method of clause 15, wherein the at least one N x N multimode optical coupler comprises a plurality of 2 x 2 multimode optical couplers arranged in nested modescrambling loops that produce a plurality of sub-beams divided in an amplitude domain by a predetermined coupling ratio at each 2 x 2 multimode optical coupler.20. The incoherent illumination method of clause 15, wherein the mode scrambler comprises at least one of a piezoelectric active mode scrambler, a coil in the multimode optical fiber loop, a twist inthe multimode optical fiber loop, an off-axis splice in the multimode optical fiber loop, or a mirrorbased ring resonator, or a combination.

[0157] The terms “radiation,” “beam,” “light,” “illumination,” or the like can be used herein to refer to one or more types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength I of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-100 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G- line 436 nm; H-line 405 nm; and / or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in some aspects, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of about 5 to about 20 nm.

[0158] Although some aspects of the present disclosure are described in the context of lithographic apparatuses in the manufacture of ICs, it should be understood that lithographic apparatuses described herein can be used in other applications, for example, in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as specific examples of the more general terms “substrate” or “target portion”, respecively. A substrate can be processed before or after exposure in, for example, a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) and / or a metrology unit. Where applicable, aspects disclosed herein can be applied to such and other substrate processing tools. Furthermore, a substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains multiple processed layers.

[0159] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0160] The present disclosure has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. The foregoing description of specific aspects will so fully reveal the generalnature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and / or adapt for various applications such specific aspects, without undue experimentation and without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed aspects, based on the teaching and guidance presented herein.

[0161] It is to be understood that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections can set forth one or more, but not necessarily all, aspects of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the present disclosure and the appended claims in any way. The breadth and scope of the protected subject matter should not be limited by any of the above-described aspects, but should be defined in accordance with the following claims and their equivalents.

Claims

CLAIMS:

1. An optical metrology incoherent illumination system comprising: a white light laser source configured to generate a light beam; a detection system configured to measure a characteristic of the light beam; and an infinite impulse response (HR) optical filter disposed between the laser source and the detection system, the IIR optical filter comprising: at least one N x N multimode optical coupler having N input ports and N output ports, wherein a first input port of the N input ports, forming an input of the IIR optical filter, is coupled to the laser source by a first multimode optical fiber, and the at least one N x N multimode optical coupler is configured to split the light beam into a plurality of sub-beams divided in an amplitude domain across the N output ports; a first output port of the N output ports, forming an output of the IIR optical filter, is coupled to the detection system by a second multimode optical fiber; and one or more mode-scrambling loops formed by coupling at least one of the other N output ports to another one of the other N input ports with a multimode optical fiber loop, wherein a length of the multimode optical fiber loop is longer than a coherence length of the laser source to achieve a temporal incoherence between the plurality of sub-beams and the light beam, a mode scrambler is disposed along the multimode optical fiber loop to achieve a spatial incoherence between the plurality of sub-beams and the light beam, and the temporal incoherence provided by the length of the multimode optical fiber loop and the spatial incoherence provided by the mode scrambler reduce a peak power of the light beam and reduce a speckle contrast of the light beam measured by the detection system.

2. The optical metrology incoherent illumination system of claim 1, wherein the light beam comprises at least one of a continuous light wave or a plurality of light pulses.

3. The optical metrology incoherent illumination system of claim 1, wherein the at least one N x N multimode optical coupler comprises at least one of a 2 x 2 multimode optical coupler, a 3 x 3 multimode optical coupler, or a 4 x 4 multimode optical coupler.

4. The optical metrology incoherent illumination system of claim 1 , wherein the at least one N x N multimode optical coupler comprises a series of 2 x 2 multimode optical couplers such that one of the N output ports of each 2 x 2 multimode optical coupler is coupled to one of the N input ports of each successive 2 x 2 multimode optical coupler.

5. The optical metrology incoherent illumination system of claim 1, wherein the at least one N x N multimode optical coupler comprises a plurality of 2 x 2 multimode optical couplers arranged in nested mode-scrambling loops that produce a plurality of sub-beams divided in an amplitude domain by a predetermined coupling ratio at each 2 x 2 multimode optical coupler.

6. The optical metrology incoherent illumination system of claim 1, wherein the length of the multimode optical fiber loop in each of the one or more mode-scrambling loops comprises a predetermined length unique from any other multimode optical fiber loop such that the plurality of subbeams remain temporally incoherent and spatially incoherent relative to each other such that an average of the plurality of sub-beams results in a spatial coherence suppression.

7. The optical metrology incoherent illumination system of claim 1, wherein the mode scrambler comprises at least one of a piezoelectric active mode scrambler, a coil in the multimode optical fiber loop, a twist in the multimode optical fiber loop, an off-axis splice in the multimode optical fiber loop, or a mirror-based ring resonator, or a combination.

8. An infinite impulse response (IIR) optical filter apparatus comprising: at least one N x N multimode optical coupler having N input ports and N output ports, wherein a first input port of the N input ports, forming an input of the IIR optical filter, is coupled to a white light laser source by a first multimode optical fiber, the laser source configured to generate a light beam, and the at least one N x N multimode optical coupler is configured to split the light beam into a plurality of sub-beams divided in an amplitude domain across the N output ports; a first output port of the N output ports, forming an output of the IIR optical filter, is coupled to a detection system by a second multimode optical fiber, the detection system configured to measure a characteristic of the light beam; and one or more mode-scrambling loops formed by coupling at least one of the other N output ports to another one of the other N input ports with a multimode optical fiber loop, wherein a length of the multimode optical fiber loop is longer than a coherence length of the laser source to achieve a temporal incoherence between the plurality of sub-beams and the light beam, a mode scrambler is disposed along the multimode optical fiber loop to achieve a spatial incoherence between the plurality of sub-beams and the light beam, and the temporal incoherence provided by the length of the multimode optical fiber loop and the spatial incoherence provided by the mode scrambler reduce a peak power of the light beam and reduce a speckle contrast of the light beam measured by the detection system.

9. The IIR optical filter apparatus of claim 8, wherein the light beam comprises at least one of a continuous light wave or a plurality of light pulses.

10. The IIR optical filter apparatus of claim 8, wherein the at least one N x N multimode optical coupler comprises at least one of a 2 x 2 multimode optical coupler, a 3 x 3 multimode optical coupler, or a 4 x 4 multimode optical coupler.

11. The IIR optical filter apparatus of claim 8, wherein the at least one N x N multimode optical coupler comprises a series of 2 x 2 multimode optical couplers such that one of the N output ports of each 2 x 2 multimode optical coupler is coupled to one of the N input ports of each successive 2 x 2 multimode optical coupler.

12. The IIR optical filter apparatus of claim 8, wherein the at least one N x N multimode optical coupler comprises a plurality of 2 x 2 multimode optical couplers arranged in nested mode-scrambling loops that produce a plurality of sub-beams divided in an amplitude domain by a predetermined coupling ratio at each 2 x 2 multimode optical coupler.

13. The IIR optical filter apparatus of claim 8, wherein the length of the multimode optical fiber loop in each of the one or more mode-scrambling loops comprises a predetermined length unique from any other multimode optical fiber loop such that the plurality of sub-beams remain temporally incoherent and spatially incoherent relative to each other such that an average of the plurality of subbeams results in a spatial coherence suppression.

14. The IIR optical filter apparatus of claim 8, wherein the mode scrambler comprises at least one of a piezoelectric active mode scrambler, a coil in the multimode optical fiber loop, a twist in the multimode optical fiber loop, an off-axis splice in the multimode optical fiber loop, or a mirror-based ring resonator, or a combination.

Images