Dual comb radiation source and heterodyne detection for alignment metrology

Abstract

Dual comb radiation sources and heterodyne detection for alignment metrology are described. A dual comb radiation source is configured to irradiate a metrology target with first radiation and second radiation. The first radiation and the second radiation have different repetition rates so one comb samples the repetitive pulse train from the other comb. A detector receives diffracted radiation and generates a detection signal by analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation to distinguish relevant wavelength ranges in an optical spectrum of diffracted radiation.

Description

DUAL COMB RADIATION SOURCE AND HETERODYNE DETECTION FOR ALIGNMENT METROLOGYCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 666,402 which was filed on July 1 , 2024 and which is incorporated herein in its entirety by reference.TECHNICAL FIELD

[0002] This description relates to dual comb radiation sources and heterodyne detection for alignment metrology systems and methods.BACKGROUND

[0003] A lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A patterning device (e.g., a mask) may include or provide a pattern corresponding to an individual layer of the IC (“design layout”), and this pattern can be transferred onto a target portion (e.g. comprising one or more dies) on a substrate (e.g., silicon wafer) that has been coated with a layer of radiation-sensitive material (“resist”), by methods such as irradiating the target portion through the pattern on the patterning device. In general, a single substrate includes a plurality of adjacent target portions to which the pattern is transferred successively by the lithographic projection apparatus, one target portion at a time. In one type of lithographic projection apparatus, the pattern on the entire patterning device is transferred onto one target portion in one operation. Such an apparatus is commonly referred to as a stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, a projection beam scans over the patterning device in a given reference direction (the “scanning” direction) while synchronously moving the substrate parallel or anti-parallel to this reference direction. Different portions of the pattern on the patterning device are transferred to one target portion progressively.

[0004] Prior to transferring the pattern from the patterning device to the substrate, the substrate may undergo various procedures, such as priming, resist coating, and a soft bake. After exposure, the substrate may be subjected to other procedures (“post-exposure procedures”), such as a post-exposure bake (PEB), development, a hard bake and measurement / inspection of the transferred pattern. This array of procedures is used as a basis to make an individual layer of a device, e.g., an IC. The substrate may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, deposition, chemo-mechanical polishing, etc., all intended to finish the individual layer of the device. If several layers are required in the device, then the whole procedure, or a variant thereof, is repeated for each layer. Eventually, a device will be present in each target portion on the substrate. These devices are then separated from one another by a technique such as dicing or sawing, such that the individual devices can be mounted on a carrier, connected to pins, etc.

[0005] This device manufacturing process may be considered a patterning process. A patterning process involves a patterning step, such as optical and / or nanoimprint lithography using a patterning device in a lithographic apparatus, to transfer a pattern on the patterning device to a substrate and typically, but optionally, involves one or more related pattern processing steps, such as resist development by a development apparatus, baking of the substrate using a bake tool, etching using the pattern using an etch apparatus, deposition, etc.

[0006] Lithography is a central step in the manufacturing of devices such as ICs, where patterns formed on substrates define functional elements of the devices, such as microprocessors, memory chips, etc. Similar lithographic techniques are also used in the formation of flat panel displays, micro-electro mechanical systems (MEMS) and other devices.

[0007] As semiconductor manufacturing processes continue to advance, the dimensions of functional elements have continually been reduced while the number of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as “Moore’s law.” At the current state of technology, layers of devices are manufactured using lithographic projection apparatuses that project a design layout onto a substrate using illumination from a deep-ultraviolet illumination source, creating individual functional elements having dimensions well below 100 nm, i.e. less than half the wavelength of the radiation from the illumination source (e.g., a 193 nm illumination source).

[0008] This process in which features with dimensions smaller than the classical resolution limit of a lithographic projection apparatus are printed, is commonly known as low-ki lithography, according to the resolution formula CD = kixL / NA, where X is the wavelength of radiation employed (currently in most cases 248nm or 193nm), NA is the numerical aperture of projection optics in the lithographic projection apparatus, CD is the “critical dimension’ -generally the smallest feature size printed-and ki is an empirical resolution factor. In general, the smaller ki the more difficult it becomes to reproduce a pattern on the substrate that resembles the shape and dimensions planned by a designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine-tuning steps are applied to the lithographic projection apparatus, the design layout, or the patterning device. These include, for example, but are not limited to, optimization of NA and optical coherence settings, customized illumination schemes, use of phase shifting patterning devices, optical proximity correction (OPC, sometimes also referred to as “optical and process correction”) in the design layout, or other methods generally defined as “resolution enhancement techniques” (RET).

[0009] Accurate and timely metrology is important for these resolution enhancement techniques.SUMMARY

[0010] Dual comb radiation sources and heterodyne detection for alignment metrology are described. A dual comb radiation source is configured to irradiate a metrology target with first radiation and second radiation. The first radiation and the second radiation have different repetition rates so one combsamples the repetitive pulse train from the other comb. A detector receives diffracted radiation and generates a radio-frequency (RF) detection signal by analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation to distinguish relevant wavelength ranges in an optical spectrum of diffracted radiation.

[0011] According to an embodiment, a metrology system is provided. The metrology system comprises a radiation source configured to irradiate an alignment metrology target in a patterned substrate with first radiation and second radiation. The first radiation and the second radiation have different repetition rates. The system comprises a detector configured to receive diffracted radiation from the alignment metrology target and generate a detection signal. The detector comprises one or more photodiodes and one or more processors operatively coupled to the one or more photodiodes configured to generate the detection signal by analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in an optical spectrum of the diffracted radiation. The detection signal is configured to be used for determining alignment of one layer of the patterned substrate relative to another.

[0012] In some embodiments, the radiation source comprises a dual comb radiation source, with a first comb of the dual comb radiation source configured to generate the first radiation, and a second comb of the radiation source configured to generate the second radiation. In some embodiments, the one or more processors form at least a portion of a radio frequency (RF) analyzer. The RF analyzer is configured to analyze heterodyne beating between radiation from pairs of comb teeth in the first comb and the second comb received at the one or more photodiodes to distinguish the relevant wavelength ranges of the optical spectrum of the diffracted radiation. The dual comb radiation source and the RF analyzer facilitate elimination of a radiation beam demultiplexer in the metrology system.

[0013] In some embodiments, the first comb and the second comb each comprise a mode locked comb configured to pass generated radiation through a nonlinear fiber to generate broadband radiation. The broadband radiation is the first radiation and / or the second radiation. The nonlinear fiber may be a photonic crystal fiber or a graded-index fiber, for example.

[0014] In some embodiments, the first comb and the second comb each comprise one or more ring resonators. Each of the one or more ring resonators may be associated with different colors of light, for example.

[0015] In some embodiments, the first comb and the second comb are configured to pass generated radiation through the same nonlinear fiber. In some embodiments, the first comb and the second comb are configured to pass generated radiation through different nonlinear fibers. In some embodiments, at least a portion of a fiber associated with each of the first comb and the second comb are arranged in proximity to each other to facilitate combination of the first radiation and the second radiation in each fiber. In some embodiments, combined first and second radiation in a first fiber is configured to be used as reference radiation, and combined first and second radiation in a second fiber is directed towardthe alignment metrology target.

[0016] In some embodiments, generated radiation from the first comb and the second comb passes through fibers and generates a supercontinuum, which broadens the comb bandwidth of the first comb and the second comb. In some embodiments, each supercontinuum generating fiber comprises a waveguide configured to broaden a spectrum of radiation that passes through.

[0017] In some embodiments, the relevant wavelength ranges in the optical spectrum of the diffracted radiation are associated with different colors. In some embodiments, there are twelve or more relevant wavelength ranges that correspond to twelve or more different colors.

[0018] In some embodiments, the first radiation and the second radiation comprise visible light.

[0019] In some embodiments, analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in the optical spectrum of the diffracted radiation comprises analyzing a beat frequency, which is a difference between two original frequencies of the first radiation and the second radiation.

[0020] In some embodiments, the one or more processors are configured to sample 100 sampling points or more as part of analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in an optical spectrum of the diffracted radiation.

[0021] In some embodiments, the system comprises one or more optical components including one or more lenses, mirrors, apertures, and / or beam splitters configured to direct the first radiation and the second radiation to the alignment metrology target, and direct the diffracted radiation from the alignment metrology target to the detector.

[0022] In some embodiments, the alignment metrology target comprises a grating. In some embodiments, the metrology system forms a portion of an alignment sensor. In some embodiments, the alignment sensor is configured for a patterned substrate comprising a semiconductor wafer. In some embodiments, the alignment sensor is configured to be used in a semiconductor manufacturing process.

[0023] According to another embodiment, metrology method(s) are provided. The method comprises one or more of the operations described above performed by the metrology system(s).BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The above aspects and other aspects and features will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures.Fig. 1 schematically depicts a lithography apparatus, according to an embodiment.Fig. 2 schematically depicts an embodiment of a lithographic cell or cluster, according to an embodiment.Fig. 3 schematically depicts an example metrology system, according to an embodiment.Fig. 4 schematically depicts an example metrology technique, according to an embodiment.Fig. 5 illustrates the relationship between a radiation illumination spot of an inspection system and a metrology target, according to an embodiment.Fig. 6 illustrates a radiation beam demultiplexer typically included in prior alignment metrology systems.Fig. 7 illustrates the concept of dual comb spectroscopy, according to an embodiment.Fig. 8 further illustrates dual comb spectroscopy concepts, according to an embodiment.Fig. 9 illustrates an alignment metrology system configured to utilize a dual comb radiation source with heterodyne detection and corresponding spectroscopy to determine alignment, according to an embodiment.Fig. 10 illustrates details and examples of dual comb radiation generation related to fibers and ring resonators, according to an embodiment.Fig. 11 illustrates an alignment metrology method, according to an embodiment.Fig. 12 is a block diagram of an example computer system, according to an embodiment.DETAILED DESCRIPTION

[0025] In semiconductor device manufacturing, metrology operations typically include determining the position of a metrology target (or marks) and / or other target in a layer of a semiconductor device structure. This position is typically determined by irradiating a metrology target with radiation, and comparing characteristics of different diffraction orders of radiation diffracted from the metrology target. Such techniques are used to measure alignment, overlay, and / or other parameters.

[0026] Prior metrology systems use a bulky, multi-element demultiplexer to transmit sub-portions of diffracted radiation from a target such as a metrology mark to a detector. These demultiplexers increase the costs and size of a typical metrology system, among other disadvantages. For example, a demultiplexer is often used with an alignment sensor to separate multiple individual color bands from a single input into multiple outputs (channels). Prior alignment metrology system demultiplexers may use filters to separate colors, along with refractive and / or reflective optics to transmit light from input to output. Each alignment sensor needs one demultiplexer, and potentially additional filters, refractive and / or reflective optics, and / or other components for each output channel.

[0027] The optics used to transmit light from the input to the output comprise bulky lenses and / or other surfaces configured for bending light. These lenses and / or other surfaces can have substantial depth, taking up volume. In addition, as the number of colors () used by an alignment sensor has grown, the size, cost, complexity, and quantity of required demultiplexers has also grown (with more fibers, detectors, amplifiers, etc., needed). If industry trends continue as expected, future metrology systems may require 20-30x more demultiplexers compared to today’s systems. It is also likely that future systems will use multiple quantities of alignment sensors (lOx for instance), which each require one or more additional demultiplexers. Finding the physical space necessary for these components may beimpossible. New alignment metrology systems, which do not require the same large bulky demultiplexers at all (and primarily use computational demultiplexing or similar), are described herein.

[0028] By way of a brief introduction, the description below relates to semiconductor device manufacturing and patterning processes. The following paragraphs also describe several components of systems and / or methods for semiconductor device metrology. These systems and methods may be used for measuring alignment, overlay, etc., in a semiconductor device manufacturing process, for example, or for other operations.

[0029] Although specific reference may be made in this text to the measurement of alignment, overlay, or other parameters, and the manufacture of integrated circuits (ICs) for semiconductor devices, it should be understood that the description herein has many other applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle,” “wafer” or “die” in this text should be considered as interchangeable with the more general terms “mask,” “substrate” and “target portion,” respectively.

[0030] Fig. 1 schematically depicts an embodiment of a lithographic apparatus LA. The apparatus comprises an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation, or EUV radiation); a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters; a substrate table (e.g. a wafer table) WT (e.g., WTa, WTb or both) configured to hold a substrate (e.g. a resist-coated wafer) W and coupled to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies and often referred to as fields) of the substrate W. The projection system is supported on a reference frame RF. As depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array, or employing a reflective mask).

[0031] The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising for example suitable directing mirrors and / or a beam expander. In other cases, the source may be an integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0032] The illuminator IL may alter the intensity distribution of the beam. The illuminator may bearranged to limit the radial extent of the radiation beam such that the intensity distribution is non-zero within an annular region in a pupil plane of the illuminator IL. Additionally or alternatively, the illuminator IL may be operable to limit the distribution of the beam in the pupil plane such that the intensity distribution is non-zero in a plurality of equally spaced sectors in the pupil plane. The intensity distribution of the radiation beam in a pupil plane of the illuminator IL may be referred to as an illumination mode.

[0033] The illuminator IL may comprise adjuster AD configured to adjust the (angular / spatial) intensity distribution of the 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. The illuminator IL may be operable to vary the angular distribution of the beam. For example, the illuminator may be operable to alter the number, and angular extent, of sectors in the pupil plane wherein the intensity distribution is non-zero. By adjusting the intensity distribution of the beam in the pupil plane of the illuminator, different illumination modes may be achieved. For example, by limiting the radial and angular extent of the intensity distribution in the pupil plane of the illuminator IL, the intensity distribution may have a multi-pole distribution such as, for example, a dipole, quadrupole or hexapole distribution. A desired illumination mode may be obtained, e.g., by inserting an optic which provides that illumination mode into the illuminator IL or using a spatial light modulator.

[0034] The illuminator IL may be operable to alter the polarization of the beam and may be operable to adjust the polarization using adjuster AD. The polarization state of the radiation beam across a pupil plane of the illuminator IL may be referred to as a polarization mode. The use of different polarization modes may allow greater contrast to be achieved in the image formed on the substrate W. The radiation beam may be unpolarized. Alternatively, the illuminator may be arranged to linearly polarize the radiation beam. The polarization direction of the radiation beam may vary across a pupil plane of the illuminator IL. The polarization direction of radiation may be different in different regions in the pupil plane of the illuminator IL. The polarization state of the radiation may be chosen in dependence on the illumination mode. For multi-pole illumination modes, the polarization of each pole of the radiation beam may be generally perpendicular to the position vector of that pole in the pupil plane of the illuminator IL. For example, for a dipole illumination mode, the radiation may be linearly polarized in a direction that is substantially perpendicular to a line that bisects the two opposing sectors of the dipole. The radiation beam may be polarized in one of two different orthogonal directions, which may be referred to as X-polarized and Y-polarized states. For a quadrupole illumination mode, the radiation in the sector of each pole may be linearly polarized in a direction that is substantially perpendicular to a line that bisects that sector. This polarization mode may be referred to as XY polarization. Similarly, for a hexapole illumination mode the radiation in the sector of each pole may be linearly polarized in a direction that is substantially perpendicular to a line that bisects that sector. This polarization mode may be referred to as TE polarization.

[0035] In addition, the illuminator IL generally comprises various other components, such as an integrator IN and a condenser CO. The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, diffractive, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. Thus, the illuminator provides a conditioned beam of radiation B, having a desired uniformity and intensity distribution in its cross section.

[0036] The support structure MT supports the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” may be considered synonymous with the more general term “patterning device.”

[0037] The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a pattern in a target portion of the substrate. In an embodiment, a patterning device is any device that can be used to impart a radiation beam with a pattern in its crosssection to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in a target portion of the device, such as an integrated circuit.

[0038] A patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and 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 to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.

[0039] The term “projection system” should be broadly interpreted as encompassing 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 or the use of a vacuum. Any use of the term “projection lens” may be considered as synonymous with the more general term “projection system.”

[0040] The projection system PS may comprise a plurality of optical (e.g., lens) elements and may further comprise an adjustment mechanism configured to adjust one or more of the optical elements tocorrect for aberrations (phase variations across the pupil plane throughout the field). To achieve this, the adjustment mechanism may be operable to manipulate one or more optical (e.g., lens) elements within the projection system PS in one or more different ways. The projection system may have a coordinate system wherein its optical axis extends in the z direction. The adjustment mechanism may be operable to do any combination of the following: displace one or more optical elements; tilt one or more optical elements; and / or deform one or more optical elements. Displacement of an optical element may be in any direction (x, y, z, or a combination thereof). Tilting of an optical element is typically out of a plane perpendicular to the optical axis, by rotating about an axis in the x and / or y directions although a rotation about the z axis may be used for a non-rotationally symmetric aspherical optical element. Deformation of an optical element may include a low frequency shape (e.g. astigmatic) and / or a high frequency shape (e.g. free form aspheres). Deformation of an optical element may be performed for example by using one or more actuators to exert force on one or more sides of the optical element and / or by using one or more heating elements to heat one or more selected regions of the optical element. In general, it may not be possible to adjust the projection system PS to correct for apodization (transmission variation across the pupil plane). The transmission map of a projection system PS may be used when designing a patterning device (e.g., mask) MA for the lithography apparatus LA. Using a computational lithography technique, the patterning device MA may be designed to at least partially correct for apodization.

[0041] The lithographic apparatus may be of a type having two (dual stage) or more supports or tables (e.g., two or more wafer tables WTa, WTb, two or more patterning device tables, a substrate table WTa and a table WTb below the projection system without a substrate that is dedicated to, for example, facilitating measurement, and / or cleaning, etc.). In such “multiple stage” machines, the additional tables may be used in parallel, or preparatory steps may be conducted on one or more tables while one or more other tables are being used for exposure. For example, alignment measurements using an alignment sensor AS and / or level (height, tilt, etc.) measurements using a level sensor LS may be made.

[0042] The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g. water, to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the patterning device 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, but rather only means that liquid is located between the projection system and the substrate during exposure.

[0043] In operation of the lithographic apparatus, a radiation beam is conditioned and provided by the illumination system IL. The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. Having traversed the patterning device MA, the radiation beam B passes through the projection systemPS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in Fig. 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the support structure MT may 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 may 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 support structure MT may be connected to a shortstroke actuator only, or may be fixed. Patterning device MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device MA, the patterning device alignment marks may be located between the dies.

[0044] The depicted apparatus may be used in at least one of the following modes. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while a pattern imparted to the radiation beam 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. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam 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 MT may be determined by the (de-) magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. In another mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed, and the programmable patterning device is updated as required 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 programmable patterning device, such as a programmable mirror array of a type as referred to above. Combinations and / or variations on the above-described modes of use or entirely different modes of use may also beemployed.

[0045] The substrate may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already includes multiple processed layers.

[0046] The terms “radiation” and “beam” used herein with respect to lithography encompass all types of electromagnetic radiation, including ultraviolet (UV) or deep ultraviolet (DUV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0047] Various patterns on or provided by a patterning device may have different process windows, i.e., a space of processing variables under which a pattern will be produced within specification. Examples of pattern specifications that relate to potential systematic defects include checks for necking, line pull back, line thinning, CD, edge placement, overlapping, resist top loss, resist undercut and / or bridging. The process window of the patterns on a patterning device or an area thereof may be obtained by merging (e.g., overlapping) process windows of each individual pattern. The boundary of the process window of a group of patterns comprises boundaries of process windows of some of the individual patterns. In other words, these individual patterns limit the process window of the group of patterns.

[0048] As shown in Fig. 2, the lithographic apparatus LA may form part of a lithographic cell LC, also sometimes referred to as a lithocell or cluster, which also includes apparatuses to perform pre- and post-exposure processes on a substrate. Conventionally these include one or more spin coaters SC to deposit one or more resist layers, one or more developers to develop exposed resist, one or more chill plates CH and / or one or more bake plates BK. A substrate handler, or robot, RO picks up one or more substrates from input / output port I / Ol , I / O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus. These apparatuses, which are often collectively referred to as the track, are under the control of a track control unit TCU which is itself controlled by the 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.

[0049] In order that a substrate that is exposed by the lithographic apparatus is exposed correctly and consistently and / or in order to monitor a part of the patterning process (e.g., a device manufacturing process) that includes at least one pattern transfer step (e.g., an optical lithography step), it is desirable to inspect a substrate or other object to measure or determine one or more properties such as alignment, overlay (which can be, for example, between structures in overlying layers or between structures in a same layer that have been provided separately to the layer by, for example, a double patterning process),line thickness, critical dimension (CD), focus offset, a material property, etc. Accordingly, a manufacturing facility in which lithocell LC is located also typically includes a metrology system that measures some or all of the substrates W (Fig. 1) that have been processed in the lithocell or other objects in the lithocell. The metrology system may be part of the lithocell LC, for example it may be part of the lithographic apparatus LA (such as alignment sensor AS (Fig. 1)).

[0050] The one or more measured parameters may include, for example, alignment, overlay between successive layers formed in or on the patterned substrate, critical dimension (CD) (e.g., critical linewidth) of, for example, features formed in or on the patterned substrate, focus or focus error of an optical lithography step, dose or dose error of an optical lithography step, optical aberrations of an optical lithography step, etc. This measurement is often performed on one or more dedicated metrology targets provided on the substrate. The measurement can be performed after-development of a resist but before etching, after-etching, after deposition, and / or at other times.

[0051] There are various techniques for making measurements of the structures formed in the patterning process, including the use of a scanning electron microscope, an image -based measurement tool and / or various specialized tools. A fast and non-invasive form of specialized metrology tool is one in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered (diffracted / reflected) beam are measured. By evaluating one or more properties of the radiation scattered by the substrate, one or more properties of the substrate can be determined. Traditionally, this may be termed diffraction-based metrology. Applications of this diffraction-based metrology include the measurement of overlay, alignment, etc. For example, overlay and / or alignment can be measured by comparing parts of the diffraction spectrum (for example, comparing different diffraction orders in the diffraction spectrum of a periodic grating).

[0052] Thus, in a device fabrication process (e.g., a patterning process or a lithography process), a substrate or other objects may be subjected to various types of measurement during or after the process. The measurement may determine whether a particular substrate is defective, may establish adjustments to the process and apparatuses used in the process (e.g., aligning two layers on the substrate or aligning the patterning device to the substrate), may measure the performance of the process and the apparatuses, or may be for other purposes.

[0053] Metrology results may be provided directly or indirectly to the supervisory control system SCS. If an error is detected, an adjustment may be made to exposure of a subsequent substrate (especially if the inspection can be done soon and fast enough that one or more other substrates of the batch are still to be exposed) and / or to subsequent exposure of the exposed substrate. Also, an already exposed substrate may be stripped and reworked to improve yield, or discarded, thereby avoiding performing further processing on a substrate known to be faulty. In a case where only some target portions of a substrate are faulty, further exposures may be performed only on those target portions which meet specifications. Other manufacturing process adjustments are contemplated.

[0054] A metrology system may be used to determine one or more properties of the substratestructure, and in particular, how one or more properties of different substrate structures vary, or different layers of the same substrate structure vary from layer to layer. The metrology system may be integrated into the lithographic apparatus LA or the lithocell LC, or may be a stand-alone device.

[0055] To enable the metrology, often one or more targets are specifically provided on the substrate. Typically, the target is specially designed and may comprise a periodic structure. For example, the target on a substrate may comprise one or more 1-D periodic structures (e.g., geometric features such as gratings), which are printed such that after development, the periodic structural features are formed of solid resist lines. As another example, the target may comprise one or more 2-D periodic structures (e.g., gratings), which are printed such that after development, the one or more periodic structures are formed of solid resist pillars or vias in the resist. The bars, pillars, or vias may alternatively be etched into the substrate (e.g., into one or more layers on the substrate).

[0056] Fig. 3 depicts an example metrology system 10 that may be used to detect alignment, overlay, and / or perform other metrology operations. It comprises a radiation or illumination source 2 which projects or otherwise irradiates radiation onto a substrate W (e.g., which may typically include a metrology target). The redirected radiation is passed to a sensor such as a spectrometer detector 4 and / or other sensors, which measures a spectrum (intensity as a function of wavelength) of the specular reflected and / or diffracted radiation, as shown, e.g., in the graph on the left of Fig. 4. The sensor may generate a metrology signal conveying metrology data indicative of properties of the reflected and / or diffracted radiation. From this data, the structure or profile giving rise to the detected spectrum may be reconstructed by one or more processors PRO, or by other operations.

[0057] As in the lithographic apparatus LA in Fig. 1, one or more substrate tables may be provided to hold the substrate W during measurement operations. The one or more substrate tables may be similar or identical in form to the substrate table WT (WTa or WTb or both) of Fig. 1. In an example where system 10 is integrated with the lithographic apparatus, they may even be the same substrate table. Coarse and fine positioners may be provided and configured to accurately position the substrate in relation to a measurement optical system. Various sensors and actuators are provided, for example, to acquire the position of a target portion of interest of a structure (e.g., a metrology mark), and to bring it into position under an objective lens. Typically, many measurements will be made on target portions of a structure at different locations across the substrate W. The substrate support can be moved in X and Y directions to acquire different targets, and in the Z direction to obtain a desired location of the target portion relative to the focus of the optical system. It is convenient to think and describe operations as if the objective lens is being brought to different locations relative to the substrate, when, for example, in practice the optical system may remain substantially stationary (typically in the X and Y directions, but perhaps also in the Z direction) and the substrate moves. Provided the relative position of the substrate and the optical system is correct, it does not matter in principle which one of those is moving, or if both are moving, or a combination of a part of the optical system is moving (e.g., in the Z and / or tilt direction) with the remainder of the optical system being stationary and the substrate is moving (e.g.,in the X and Y directions, but also optionally in the Z and / or tilt direction).

[0058] For typical metrology measurements, a target 30 on substrate W may be a 1-D grating, which is printed such that after development, the bars are formed of solid resist lines (e.g., which may be covered by a deposition layer), and / or other materials. Or the target 30 may be a 2-D grating, which is printed such that after development, the grating is formed of solid resist pillars, and / or other features in the resist.

[0059] The bars, pillars, vias, and / or other features may be etched into or on the substrate (e.g., into one or more layers on the substrate), deposited on a substrate, covered by a deposition layer, and / or have other properties. Target 30 (e.g., of bars, pillars, vias, etc.) is sensitive to changes in processing in the patterning process (e.g., optical aberration in the lithographic projection apparatus such as in the projection system, focus change, dose change, etc.) such that process variation manifests in variation in target 30. Accordingly, the measured data from target 30 may be used to determine an adjustment for one or more of the manufacturing processes, and / or used as a basis for making the actual adjustment.

[0060] For example, the measured data from target 30 may indicate alignment and / or overlay for a layer of a semiconductor device. The measured data from target 30 may be used (e.g., by the one or more processors PRO and / or other processors) for determining one or more semiconductor device manufacturing process parameters based the alignment and / or overlay, and determining an adjustment for a semiconductor device manufacturing apparatus based on the one or more determined semiconductor device manufacturing process parameters. In some embodiments, this may comprise a stage position adjustment, for example, or this may include determining an adjustment for a mask design, a metrology target design, a semiconductor device design, an intensity of the radiation, an incident angle of the radiation, a wavelength of the radiation, a pupil size and / or shape, a resist material, and / or other process parameters.

[0061] Fig. 5 illustrates a plan view of a typical target 30, and the extent of a typical radiation illumination spot. Typically, to obtain a diffraction spectrum that is free of interference from surrounding structures, the target 30, in an embodiment, is a periodic structure (e.g., grating) larger than the width (e.g., diameter) of the illumination spot S. The width of spot S may be smaller than the width and length of the target. The target, in other words, is ‘underfilled’ by the illumination, and the diffraction signal is essentially free from any signals from product features and the like outside the target itself. The illumination arrangement may be configured to provide illumination of a uniform intensity across a back focal plane of an objective, for example. Alternatively, by, for example, including an aperture in the illumination path, illumination may be restricted to on axis or off axis directions.

[0062] Fig. 6 illustrates a radiation beam demultiplexer 600 typically included in prior alignment metrology systems. The radiation beam demultiplexer 600 comprises different sub-portions 601 (e.g., a collimation lens) 602 (e.g., relay mirrors - see RM2 - RM12), 603 (e.g., a dichroic mirror), 604 (e.g., wedge filters), 606 (e.g., band pass filters), 608 (e.g., focusing lenses), and / or other sub-portions. Eachof the different sub-portions 601-608 is configured to direct and / or separate an associated diffracted radiation beam into narrower band sub-beams of radiation (e.g., channels CHI - CH2), each having its own wavelength range (e.g., color) and / or other characteristics for alignment determination. Demultiplexer 600 is a bulky, multi-element demultiplexer that increases the costs and size of a typical alignment metrology system, among other disadvantages. Prior alignment metrology system demultiplexers like demultiplexer 600 use filters and other components to separate colors, along with refractive and / or reflective optics to transmit light from input to output, as shown in Fig. 6. Each alignment metrology system needs multiple demultiplexers 600 (e.g., for parallel sensing, the number of demultiplexers 600 and any other associated components is proportional to a number of sensor), and potentially additional filters, refractive and / or reflective optics, and / or other components for each output channel (e.g., for 12 separate colors).

[0063] Current alignment metrology systems (such as system 10 shown in Fig. 3, and system 900 shown in Fig. 9 and Fig. 10 and described below) require smaller and smaller sensor components. However, decreasing the size of demultiplexer 600 (much less several demultiplexers 600 in one alignment metrology system) is difficult. Conventional approaches for decreasing the size of demultiplexer 600 are not feasible due to the large etendue of the multi-mode fiber used by such systems. In addition, to increase the accuracy of future alignment measurements, denser sampling of the spectrum of diffracted radiation (e.g., using more than 12 channels, which would make a single demultiplexer 600 even larger) is needed.

[0064] Dual comb spectroscopy (as described herein) is a fast and accurate technique that facilitates sufficient resolution of the diffracted radiation spectrum for a current alignment metrology system with increased alignment measurement accuracy requirements. Advantageously, dual comb spectroscopy requires no mechanical movement and facilitates resolution of the diffracted radiation spectrum in nanosecond or pico-second time frames (depending on sampling density, signal to noise ratio, and / or other factors).

[0065] Fig. 7 illustrates the concept of dual comb spectroscopy. Two frequency combs that generate radiation with slightly different repetition rates can cause a full spectral diffracted radiation response from an alignment metrology target, with only a single photodetector required (photodetectors are further described below). The spectral range of this diffracted radiation depends on the comb and photodetector bandwidths. The resolution depends on the difference (A) between the repetition rate (fr) of the two combsThe signal to noise ratio depends on the sampling resolution, comb power, scattering efficiency of the target, and / or other factors, but resolving an entire spectrum even with dense sampling and a high signal to noise ratio is achievable with a nano-second time scale. At (a), FIG. 7 illustrates an example of dual comb transmitted power 700 spectrums 702 in the optical frequency domain 704 and RF frequency spectrum detected by the photodetector 706 plots. The photodetector captures the beating between the two combs, so the optical spectrum is translated to the RF domain. At (b), Fig. 7 illustrates asymmetric (dispersive) 708 radiation 700 and symmetric (collinear) 710 radiation700, and corresponding frequency 712 versus amplitude 714, phase 716, and intensity 718 plots. For an alignment sensor, the symmetric structure is used, where the target is illuminated by both combs.

[0066] Fig. 8 further illustrates dual comb spectroscopy concepts relative to a frequency domain 800 (for parts (a) and (b)) and a time domain 802 (for parts (c) and (d)). At (a), two frequency combs 804 and 806 (see electric field 808 versus optical frequency 810 plot 812) are mixed to produce (b) an RF comb 814 shown on an RF voltage 816 versus RF frequency 818 plot 820. Lines 822 indicate filter functions applied in the RF and optical frequencies to avoid aliasing effects. Sometimes, the detector itself behaves as a filter because it has a limited response time. In Fig. 8, Av stands for optical bandwidth. At (c), an equivalent time-domain plot 850 (electric field 852 versus time 854) showing a pulse-to-pulse walk-off (e.g., differences) between the two comb pulse trains 856 and 858. At (d), a photodetector voltage 860 output is shown corresponding to the product of the two comb pulses, integrated over the detector bandwidth. This output can be viewed in normal time 862, where the samples are at time intervals of Mfr, or in effective time 864, where the samples are at time intervals of AT. In both time scales k is the sample number. The large “centerburst” 870 corresponds to the simultaneous arrival of the two pulses. The total time to resolve the spectrum is 1 / Afr. The optical spectrum bandwidth that needs to be measured within this time defines the required fr.

[0067] Fig. 9 illustrates an alignment metrology system 900. System 900 is configured to utilize a dual comb radiation source 902 with heterodyne detection and corresponding spectroscopy (e.g., see Fig. 7 and Fig. 8) to determine alignment. System 900 is the same as or similar to system 10 described above with respect to Fig. 3, with one or more components of system 900 being similar to and / or the same as one or more components of system 10 (and Fig. 9 illustrating additional possible components of the system). In some embodiments, one or more components of system 900 may replace, be used with, and / or otherwise augment one or more components of system 10. In Fig. 9, radiation 904 is generated by radiation source 902 and directed to (and from) an alignment metrology target (e.g., target 30) by one or more optical components 906. Optical components 906 may be similar to and / or the same as optical components in prior alignment metrology systems. Optical components 906 may include one or more lenses 908, mirrors 910, apertures 912, beam splitters 914, and / or other components 916 configured to direct radiation 904 to the alignment metrology target 30, and direct diffracted radiation 905 from the alignment metrology target 30 to a detector 920.

[0068] Alignment metrology target 30 (in a patterned substrate as described herein) is irradiated with first radiation 904a and second radiation 904b from radiation source 902 (e.g., a dual comb radiation source as described herein). First radiation 904a and second radiation 904b may comprise visible light and / or other radiation. First radiation 904a and second radiation 904b have different repetition rates and / or other characteristics. Radiation source 902 comprises a dual comb radiation source, with a first comb 902a of the dual comb radiation source 902 configured to generate first radiation 904a, and a second comb 902b of radiation source 902 configured to generate second radiation 904b.

[0069] Diffracted radiation 905 from the alignment metrology target 30 is received by detector 920,which generates a detection signal. Detector 920 comprises one or more photodiodes 950, 952, 954, 956, one or more processors PRO operatively coupled to the one or more photodiodes 950-956, and / or other components. In some embodiments, the one or more processors PRO form at least a portion of a radio frequency (RF) analyzer. The RF analyzer is configured to perform operations similar to and / or the same as those shown in Fig. 7 and / or Fig. 8, and as described below. The dual comb radiation source 902 and the RF analyzer facilitate elimination of a radiation beam demultiplexer in system 900, and allow (at least some portions of) system 900 to be much smaller in size compared to prior systems, among other advantages, for example. Thus, metrology system 900 (or multiple instances of metrology system 900) may be used for multiple parallel alignment measurements, so that alignment of a wafer and / or other substrate may be performed more quickly, and / or system 900 may have other advantageous uses.

[0070] For example, using a dual comb radiation source and photodetectors makes these component of system 900 much less massive (e.g., lOOx less massive) than corresponding components associated with prior demultiplexers, which significantly reduces the weight and volume of system 900, provides for less complex mounts for the components of system 900, and less optical yielding compared to prior systems. In addition, without a demultiplexer, system 900 is more robust against environmental changes and possible cost reduction requirements for metrology system 900. Also instead of 8x12=96 photodetectors and amplifiers for each sensor with 8 channels (950,952,954,956 and 4 intensity channels), system 900 comprises one detector and one amplifier for each channel (8 total: 4 phase channels, 4 intensity channels). The components of system 900 facilitate cost reduction compared to prior systems because multiple channels are processed using a dual comb radiation source and photodiodes rather than a big bulky demultiplexer, reducing the number of individual elements to manufacture and mount compared to a typical metrology system. Costs may also be reduced because a quantity of high precession optical surfaces is less when the dual comb radiation source and photodiodes are used in metrology system 900.

[0071] Detector 920 is configured to generate the detection signal by analyzing heterodyne beating between different corresponding portions (e.g., corresponding wavelength ranges or colors of radiation) of first radiation 904a and second radiation 904b diffracted by metrology target 30 to distinguish relevant wavelength ranges in an optical spectrum of the diffracted radiation. The relevant wavelength ranges in the optical spectrum of the diffracted radiation may be associated with different colors, for example. This may be similar to a demultiplexer dividing diffracted radiation into different color channels. However, with system 900, there may be twelve or more relevant wavelength ranges that correspond to twelve or more different colors (in this example - 10’s, 100’s, 1000’s or more channels are possible using the principles described herein).

[0072] The detection signal is generated by the one or more photodiodes 950-956 and the one or more processors PRO. The detection signal comprises measurement information pertaining to a target 30 such as a grating on a semiconductor wafer. For example, the detection signal may be an alignmentsignal comprising alignment measurement information, and / or other metrology signals. The detection signal comprises an electronic signal that represents and / or otherwise corresponds to radiation reflected from target 30. Diffracted radiation 905 from target 30 may comprise + and - order diffracted radiation. The detection signal may indicate a metrology value associated with an alignment signal, a diffraction intensity of grating target 30, for example, and / or other information. Generating the detection signal comprises sensing the diffracted radiation 905 and converting the sensed diffracted radiation into the electronic signal. In some embodiments, generating the detection signal comprises sensing different portions of diffracted radiation 905 from different areas and / or different geometries of target 30, and / or multiple targets 30, and combining the different portions of the diffracted radiation 905 to form the detection signal. This sensing and converting may be performed by components similar to and / or the same as detector 4 and / or processors PRO shown in Fig. 3, detector 920 shown in Fig. 9, and / or other components.

[0073] Alignment of one layer of a patterned substrate (e.g., a layer that includes target 30) relative to another layer may be determined by one or more processors PRO and / or other components of system 900. The alignment may be determined based on the information in the detection signal and / or other information. For example, the RF analyzer (described above) formed by processors PRO is configured to analyze heterodyne beating between radiation 904 from pairs of comb teeth in first comb 902a and second comb 902b received at the one or more photodiodes 950-956 to distinguish the relevant wavelength ranges of the optical spectrum of diffracted radiation 905. In some embodiments, analyzing heterodyne beating between different corresponding portions of first radiation 904a and second radiation 904b diffracted by the metrology target to distinguish relevant wavelength ranges in the optical spectrum of diffracted radiation 905 comprises analyzing a beat frequency, which is a difference between two original frequencies of first radiation 904a and second radiation 904b. The one or more processors PRO may be configured to sample 100 sampling points or more as part of analyzing heterodyne beating between different corresponding portions of first radiation 904a and second radiation 904b diffracted by metrology target 30. Alignment is determined based on the heterodyne beating beat frequency and / or other information.

[0074] In some embodiments, system 900 is configured to use a symmetric (collinear) 710 approach as shown on the bottom right of (b) of Fig. 7. With this approach, radiation 904 from both combs (e.g., first radiation 904a from first comb 902a, and second radiation 904b from second comb 902b) illuminates metrology target 30 and the radiation from each comb is used to generate an alignment signal separately. Since the radiation from each comb has slightly different repetition rates, they do not interfere with each other. However, heterodyne beating between radiation from pairs of optical comb teeth at photodetectors 950-956 generates distinguished RF comb teeth which represent the optical spectrum as shown at (a) in Fig. 8. As the target mark is scanned, the alignment signal at each location of the mark is collected by each photodetector at the phase channels (950, 952, 954, 956). This signal includes the alignment position information for each color. The dual-comb spectroscopy technique,extracts the alignment signal for all colors (alignment signal spectrum) before the detector collects the next alignment signal of the shifted mark.

[0075] In some embodiments, generated radiation 904a and 904b from first comb 902a and second comb 902b, respectively, passes through one or more fibers 980 and generates a supercontinuum, which broadens a comb bandwidth of first comb 902a and / or second comb 902b. Each supercontinuum generating fiber 980 may comprise a waveguide configured to broaden a spectrum of radiation 904a and 904b that passes through, and / or other components.

[0076] In some embodiments, first comb 902a and second comb 902b may each comprise a mode locked comb configured to pass generated radiation 904a and / or 904b through a nonlinear fiber 980 to generate broadband radiation, with the broadband radiation being first radiation 904a and / or second radiation 904b. The nonlinear fiber 980 may be a photonic crystal fiber, a graded-index fiber, and / or other nonlinear fibers. In some embodiments, first comb 902a and second comb 902b are configured to pass generated radiation 904a and 904b through a same fiber 980. In some embodiments, first comb 902a and second comb 902b are configured to pass generated first radiation 904a and second radiation 904b through different nonlinear fibers. In some embodiments, at least a portion of a fiber associated with each of first comb 902a and second comb 902b are arranged in proximity to each other to facilitate combination of first radiation 904a and second radiation 904b in each fiber. In some embodiments, combined first radiation 904a and second radiation 904b in a first fiber is configured to be used as reference radiation, and combined first radiation 904a and second radiation 904b in a second fiber is directed toward alignment metrology target 30.

[0077] In some embodiments, first comb 902a and second comb 902b each comprise one or more ring resonators. Each of the one or more ring resonators may be associated with different colors of light, for example. Different colors of radiation from each of the ring resonators may be combined to form the broadband radiation. Advantageously, frequency combs (e.g., such as first comb 902a and second comb 902b) based on ring resonators may be configured with repetition rates scaled up to 10s of THz. In addition, to form a broadband radiation source, large bandwidth nonlinear phase matching can be achieved by dispersion engineering in ring resonators. Octave spanning frequency combs which can cover the entire visible radiation wavelength range can be achieved in with a dispersion engineered thin-film lithium niobate platform, for example. This can also be achieved by second harmonic generation of a broadband mid-infrared frequency comb.

[0078] Additional details and examples related to the fibers and ring resonators is described below with respect to Fig. 10. Fig. 10 illustrates different views 1000, 1002, and 1004 showing how, in some embodiments, first comb 902a and second comb 902b each comprise one or more ring resonators 1010. In this example, first comb 902a and second comb 902b also each comprise a laser radiation source 1012. To generate a broadband comb, a passive or active microresonator may be pumped by a continuous wave (CW) or pulsed laser near a cavity, and for sufficiently strong nonlinearity and with appropriate dispersion, a comb is generated through three-wave mixing, four-wave mixing, electro-optics modulation, etc. The spacing between the comb lines is specified by the free spectral range (FSR) of the microresonator. Each of the one or more ring resonators 1010 may be associated with different colors of light (radiation) or combination of colors, for example. Different colors of light from each of the ring resonators 1010 may be combined 1020 to form the broadband radiation (radiation 904). Ring resonators 1010 may be composed of silica, SiN, LN, GaN, LTa, or TaN on a chip, for example. Radiation 904 pulse trains may be combined on chip or using evanescent coupling through taper fibers 980, for example. A reference (e.g., an atomic clock) is used to calibrate the spectrum (to mode lock all the combs together). The two ring resonators 1010 shown in each view 1000, 1002, and 1004 are locked to the same reference (e.g., the atomic clock) to ensure they are mutually coherent with the desired career envelop offset (fceo,i ,

[0079] In some embodiments, first comb 902a and second comb 902b may each comprise a mode locked comb configured to pass generated radiation 904a and / or 904b through a nonlinear fiber 980 to generate broadband radiation, with the broadband radiation being first radiation 904a and / or second radiation 904b (or more generally radiation 904). The nonlinear fiber 980 may be a photonic crystal fiber, a graded-index fiber, and / or other nonlinear fibers. In some embodiments, generated radiation 904a and 904b from first comb 902a and second comb 902b, respectively, passes through one or more fibers 980 and generates 1050 a supercontinuum (see view 1002 specifically), which broadens a comb bandwidth of first comb 902a and / or second comb 902b. Each supercontinuum generating fiber 980 may comprise a waveguide configured to broaden a spectrum of radiation 904a and 904b that passes through, and / or other components. To increase the repetition rate of supercontinuum generation and form a frequency comb (e.g., first comb 902a and / or second comb 902b), a driver of the supercontinuum process may be a mode-locked high repetition rate narrow-band laser (e.g., laser radiation source 1012), for example, or frequency comb as illustrated in Fig. 6.

[0080] In some embodiments, first comb 902a and second comb 902b are configured to pass generated radiation 904a and 904b through a same nonlinear fiber 980 (e.g., as shown in Fig. 9). In some embodiments, first comb 902a and second comb 902b are configured to pass generated first radiation 904a and second radiation 904b through different nonlinear fibers 980 (e.g., as shown in views 1000, 1002, and 1004 of Fig. 10). In some embodiments, at least a portion of a fiber associated with each of first comb 902a and second comb 902b are arranged in proximity to each other (e.g., where different colors of light from each of the ring resonators 1010 may be combined 1020) to facilitate combination of first radiation 904a and second radiation 904b in each fiber 980. In some embodiments, combined first radiation 904a and second radiation 904b in a first fiber 980 is configured to be used as reference radiation 1060 (e.g., for eventual alignment measurement determination), and combined first radiation 904a and second radiation 904b in a second fiber 980 is directed toward alignment metrology target 30 (Fig. 9) via one or more optical components 906.

[0081] View 1004 illustrates splitting a radiation 904 spectrum into narrower bands and performing dual comb spectroscopy with a pair of combs (e.g., first comb 902a and second comb 902b). In orderto measure the spectrum of all bands simultaneously, the embodiment shown in view 1004 is configured to ensure the RF beatings of first radiation 904a and second radiation 904b are distinguishable. To achieve this, Afr for all bands is the same (or the difference is small enough that aliasing effects does not occur). In addition, a carrier envelop offset (CEO) frequency ,fceo, for the combs are different. The difference between fceo of different pairs is larger than a linewidth of comb lines, but less than Afr, so the RF comb lines are distinguishable in the RF spectrum as shown. Parallel sources facilitates power scale up if needed, or increasing the overall bandwidth with narrow-band combs, for example.

[0082] Radiation 904 can be split into narrower bands, and using specific comb parameters (shown on the top right of view 1004), the entire spectrum may be translated into distinguishable RF comb lines, and the entire spectrum may be resolved with single measurement. Each comb (e.g., first comb 902a and / or second comb 902b) may be locked into a different carrier envelope offset (CEO) frequency as shown in view 1004, or the repetition rate of each dual comb (first comb 902a and second comb 902b in combination) may be engineered such that each pair of combs (first comb 902a and second comb 902b) are translated into a different RF band, for example. As another option, instead of using a pair of ring resonators 1010, a single ring resonator 1010 may be used with counter-propagating modes to generate dual combs.

[0083] Fig. 11 illustrates an alignment metrology method 1101. In some embodiments, method 1101 is performed as part of an alignment sensing operation in a semiconductor device manufacturing process, for example. In some embodiments, one or more operations of method 1101 may be implemented in or by a metrology system such as system 10 illustrated in Fig. 3 and / or system 900 illustrated in Fig. 9- 10, a computer system (e.g., as illustrated in Fig. 12 and described below), and / or in or by other systems, for example. In some embodiments, method 1101 comprises irradiating (operation 1102) an alignment metrology target in a patterned substrate, receiving diffracted radiation and generating a detection signal (operation 1104), determining (operation 1106) alignment of one layer of the patterned substrate relative to another, and / or other operations.

[0084] The operations of method 1101 are intended to be illustrative. In some embodiments, method 1101 may be accomplished with one or more additional operations not described, and / or without one or more of the operations discussed. For example, in some embodiments, method 1101 may include an additional operation comprising determining overlay and / or alignment for a semiconductor wafer, and determining an adjustment for a semiconductor device manufacturing process. Additionally, the order in which the operations of method 1101 are illustrated in Fig. 11 and described herein is not intended to be limiting.

[0085] In some embodiments, one or more portions of method 1101 may be implemented in and / or controlled by one or more processing devices (e.g., a digital processor, an analog processor, a digital circuit designed to process information, an analog circuit designed to process information, a state machine, and / or other mechanisms for electronically processing information). The one or more processing devices may include one or more devices executing some or all of the operations of method1101 in response to instructions stored electronically on an electronic storage medium. The one or more processing devices may include one or more devices configured through hardware, firmware, and / or software to be specifically designed for execution of one or more of the operations of method 1101 (e.g., see discussion related to Fig. 12 below).

[0086] At operation 1102, an alignment metrology target in a patterned substrate is irradiated with first radiation and second radiation from a radiation source (e.g., a dual comb radiation source as described herein). The first radiation and the second radiation may comprise visible light and / or other radiation. The first radiation and the second radiation have different repetition rates and / or other characteristics. The radiation source may irradiate an alignment target on a substrate such as a grating on a semiconductor wafer, and / or other components.

[0087] As described herein, the radiation source comprises a dual comb radiation source, with a first comb of the dual comb radiation source configured to generate the first radiation, and a second comb of the radiation source configured to generate the second radiation. In some embodiments, generated radiation from the first comb and the second comb passes through one or more fibers and generates a supercontinuum, which broadens the bandwidth of the first comb and / or the second comb. Each supercontinuum generating fiber may comprise a waveguide configured to broaden a spectrum of radiation that passes through and / or other components.

[0088] In some embodiments, the first comb and the second comb may each comprise a mode locked comb configured to pass generated radiation through a nonlinear fiber to generate broadband radiation, with the broadband radiation being the first radiation and / or the second radiation. The nonlinear fiber may be a photonic crystal fiber, a graded-index fiber, and / or other nonlinear fibers. In some embodiments, the first comb and the second comb are configured to pass generated radiation through the same nonlinear fiber. In some embodiments, the first comb and the second comb are configured to pass generated radiation through different nonlinear fibers. In some embodiments, at least a portion of a fiber associated with each of the first comb and the second comb are arranged in proximity to each other to facilitate combination of the first radiation and the second radiation in each fiber. In some embodiments, combined first and second radiation in a first fiber is configured to be used as reference radiation, and combined first and second radiation in a second fiber is directed toward the alignment metrology target.

[0089] In some embodiments, the first comb and the second comb each comprise one or more ring resonators. Each of the one or more ring resonators may be associated with different colors of light, for example. Different colors of radiation from each of the ring resonators may be combined to form the broadband radiation.

[0090] At operation 1104, diffracted radiation from the alignment metrology target is received by a detector, which generates a detection signal. The detector comprises one or more photodiodes and one or more processors operatively coupled to the one or more photodiodes. In some embodiments, the one or more processors form at least a portion of a radio frequency (RF) analyzer. The dual comb radiationsource and the RF analyzer facilitate elimination of a radiation beam demultiplexer in a metrology system configured to perform method 1101.

[0091] The detector is configured to generate the detection signal by analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in an optical spectrum of the diffracted radiation. The relevant wavelength ranges in the optical spectrum of the diffracted radiation may be associated with different colors. In some embodiments, there are twelve or more relevant wavelength ranges that correspond to twelve or more different colors.

[0092] The detection signal is generated by the one or more photodiodes and the one or more processors. The detection signal comprises measurement information pertaining to a target such as a grating on a semiconductor wafer. For example, the detection signal may be an alignment signal comprising alignment measurement information, and / or other metrology signals. The detection signal comprises an electronic signal that represents and / or otherwise corresponds to radiation reflected from the target. Diffracted radiation from a target may comprise + and - order diffracted radiation. The detection signal may indicate a metrology value associated with a diffraction grating target, for example, and / or other information. Generating the detection signal comprises sensing the diffracted radiation and converting the sensed diffracted radiation into the electronic signal. In some embodiments, generating the detection signal comprises sensing different portions of the diffracted radiation from different areas and / or different geometries of the target, and / or multiple targets, and combining the different portions of the diffracted radiation to form the detection signal. This sensing and converting may be performed by components similar to and / or the same as detector 4 and / or processors PRO shown in Fig. 3, the detector shown in Fig. 9, and / or other components.

[0093] In some embodiments, operations 1102 and / or 1104 comprise directing, with one or more optical components including one or more lenses, mirrors, apertures, and / or beam splitters, the first radiation and the second radiation to the alignment metrology target, and directing the diffracted radiation from the alignment metrology target to the detector.

[0094] At operation 1106, alignment of one layer of the patterned substrate relative to another layer is determined. The alignment may be determined based on the information in the detection signal and / or other information. For example, the RF analyzer (described above) is configured to analyze heterodyne beating between radiation from pairs of comb teeth in the first comb and the second comb received at the one or more photodiodes to distinguish the relevant wavelength ranges of the optical spectrum of the diffracted radiation. In some embodiments, analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in the optical spectrum of the diffracted radiation comprises analyzing a beat frequency, which is a difference between two original frequencies of the first radiation and the second radiation. The one or more processors may be configured to sample 100 sampling points or more as part of analyzing heterodyne beating between different corresponding portions of the firstradiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in an optical spectrum of the diffracted radiation. Alignment is determined based on the heterodyne beating beat frequency and / or other information, as described herein.

[0095] In some embodiments, method 1101 comprises determining an adjustment for a semiconductor device manufacturing process. In some embodiments, method 1101 includes determining one or more semiconductor device manufacturing process parameters. The one or more semiconductor device manufacturing process parameters may be determined based on an alignment value indicated by the detection signal, and / or other similar systems, and / or other information. The one or more parameters may include a parameter of the radiation (the radiation used for metrology), an alignment value, a metrology inspection location on a layer of a semiconductor device structure, a radiation beam trajectory across a target, and / or other parameters. In some embodiments, process parameters can be interpreted broadly to include a stage position, a mask design, a metrology target design, a semiconductor device design, an intensity of the radiation (used for exposing resist, etc.), an incident angle of the radiation (used for exposing resist, etc.), a wavelength of the radiation (used for exposing resist, etc.), a pupil size and / or shape, a resist material, and / or other parameters.

[0096] In some embodiments, method 1101 includes determining a process adjustment based on the one or more determined semiconductor device manufacturing process parameters, adjusting a semiconductor device manufacturing apparatus based on the determined adjustment, and / or other operations. For example, if a determined metrology measurement is not within process tolerances, the out of tolerance measurement may be caused by one or more manufacturing processes whose process parameters have drifted and / or otherwise changed so that the process is no longer producing acceptable devices (e.g., measurements may breach a threshold for acceptability). One or more new or adjusted process parameters may be determined based on the measurement determination. The new or adjusted process parameters may be configured to cause a manufacturing process to again produce acceptable devices.

[0097] For example, a new or adjusted process parameter may cause a previously unacceptable measurement value to be adjusted back into an acceptable range. The new or adjusted process parameters may be compared to existing parameters for a given process. If there is a difference, that difference may be used to determine an adjustment for an apparatus that is used to produce the devices (e.g., parameter “x” should be increased / decreased / changed so that it matches the new or adjusted version of parameter “x” determined as part of method 1101), for example. In some embodiments, method 1101 may include electronically adjusting an apparatus (e.g., based on the determined process parameters). Electronically adjusting an apparatus may include sending an electronic signal, and / or other communications to the apparatus, for example, which causes a change in the apparatus. The electronic adjustment may include changing a setting on the apparatus, for example, and / or other adjustments.

[0098] Fig. 12 is a diagram of an example computer system CS that may be used for one or more ofthe operations described herein. Computer system CS includes a bus BS or other communication mechanism for communicating information, and a processor PRO (or multiple processors similar to and / or the same as processor PRO shown in Fig. 3 and / or other figures) coupled with bus BS for processing information. Computer system CS also includes a main memory MM, such as a random access memory (RAM) or other dynamic storage device, coupled to bus BS for storing information and instructions to be executed by processor PRO. Main memory MM also may be used for storing temporary variables or other intermediate information during execution of instructions by processor PRO. Computer system CS further includes a read only memory (ROM) ROM or other static storage device coupled to bus BS for storing static information and instructions for processor PRO. A storage device SD, such as a magnetic disk or optical disk, is provided and coupled to bus BS for storing information and instructions.

[0099] Computer system CS may be coupled via bus BS to a display DS, such as a flat panel or touch panel display or a cathode ray tube (CRT) for displaying information to a computer user. An input device ID, including alphanumeric and other keys, is coupled to bus BS for communicating information and command selections to processor PRO. Another type of user input device is cursor control CC, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor PRO and for controlling cursor movement on display DS. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. A touch panel (screen) display may also be used as an input device.

[0100] In some embodiments, one or more operations described herein may be performed by computer system CS in response to processor PRO executing one or more sequences of one or more instructions contained in main memory MM. Such instructions may be read into main memory MM from another computer-readable medium, such as storage device SD. Execution of the sequences of instructions included in main memory MM causes processor PRO to perform the process steps (operations) described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory MM. In some embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0101] The term “computer-readable medium” or “machine -readable medium” as used herein refers to any medium that participates in providing instructions to processor PRO for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device SD. Volatile media include dynamic memory, such as main memory MM. Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise bus BS. Transmission media can also take the form of acoustic or light waves, such as those generated duringradio frequency (RF) and infrared (IR) data communications. Computer-readable media can be non- transitory, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge. Non-transitory computer readable media can have instructions recorded thereon. The instructions, when executed by a computer, can implement any of the operations described herein. Transitory computer-readable media can include a carrier wave or other propagating electromagnetic signal, for example.

[0102] Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor PRO for execution. For example, the instructions may initially be borne on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system CS can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus BS can receive the data carried in the infrared signal and place the data on bus BS. Bus BS carries the data to main memory MM, from which processor PRO retrieves and executes the instructions. The instructions received by main memory MM may optionally be stored on storage device SD either before or after execution by processor PRO.

[0103] Computer system CS may also include a communication interface CI coupled to bus BS. Communication interface CI provides a two-way data communication coupling to a network link NDL that is connected to a local network LAN. For example, communication interface CI may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface CI may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface CI sends and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information.

[0104] Network link NDL typically provides data communication through one or more networks to other data devices. For example, network link NDL may provide a connection through local network LAN to a host computer HC. This can include data communication services provided through the worldwide packet data communication network, now commonly referred to as the “Internet” INT. Local network LAN (Internet) may use electrical, electromagnetic, or optical signals that carry digital data streams. The signals through the various networks and the signals on network data link NDL and through communication interface CI, which carry the digital data to and from computer system CS, are exemplary forms of carrier waves transporting the information.

[0105] Computer system CS can send messages and receive data, including program code, through the network(s), network data link NDL, and communication interface CI. In the Internet example, hostcomputer HC might transmit a requested code for an application program through Internet INT, network data link NDL, local network LAN, and communication interface CL One such downloaded application may provide all or part of a method described herein, for example. The received code may be executed by processor PRO as it is received, and / or stored in storage device SD, or other non-volatile storage for later execution. In this manner, computer system CS may obtain application code in the form of a carrier wave.

[0106] Various embodiments of the present systems and methods are disclosed in the subsequent list of numbered clauses. In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of clauses that may be optionally claimed in any combination:1. A metrology system, comprising: a radiation source configured to irradiate an alignment metrology target in a patterned substrate with first radiation and second radiation, the first radiation and the second radiation having different repetition rates; and a detector configured to receive diffracted radiation from the alignment metrology target and generate a detection signal, the detector comprising one or more photodiodes and one or more processors operatively coupled to the one or more photodiodes configured to generate the detection signal by analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in an optical spectrum of the diffracted radiation, the detection signal configured to be used for determining alignment of one layer of the patterned substrate relative to another.2. The system of clause 1, wherein the radiation source comprises a dual comb radiation source, with a first comb of the dual comb radiation source configured to generate the first radiation, and a second comb of the radiation source configured to generate the second radiation.3. The system of any of the previous clauses, wherein the one or more processors form at least a portion of a radio frequency (RF) analyzer.4. The system of any of the previous clauses, wherein the RF analyzer is configured to analyze heterodyne beating between radiation from pairs of comb teeth in the first comb and the second comb received at the one or more photodiodes to distinguish the relevant wavelength ranges of the optical spectrum of the diffracted radiation.5. The system of any of the previous clauses, wherein the dual comb radiation source and the RF analyzer facilitate elimination of a radiation beam demultiplexer in the metrology system.6. The system of any of any of the previous clauses, wherein the first comb and the second comb each comprise a mode locked comb configured to pass generated radiation through a nonlinear fiber to generate broadband radiation, the broadband radiation being the first radiation and / or the second radiation.7. The system of any of the previous clauses, wherein the nonlinear fiber is a photonic crystal fiber or a graded-index fiber.8. The system of any of the previous clauses, wherein the first comb and the second comb each comprise one or more ring resonators.9. The system of any of the previous clauses, wherein each of the one or more ring resonators is associated with different colors of light.10. The system of any of the previous clauses, wherein the first comb and the second comb are configured to pass generated radiation through a same nonlinear fiber.11. The system of any of the previous clauses, wherein the first comb and the second comb are configured to pass generated radiation through different nonlinear fibers.12. The system of any of the previous clauses, wherein at least a portion of a fiber associated with each of the first comb and the second comb are arranged in proximity to each other to facilitate combination of the first radiation and the second radiation in each fiber.13. The system of any of the previous clauses, wherein combined first and second radiation in a first fiber is configured to be used as reference radiation, and wherein combined first and second radiation in a second fiber is directed toward the alignment metrology target.14. The system of any of the previous clauses, wherein generated radiation from the first comb and the second comb passes through fibers and generates a supercontinuum, which broadens a comb bandwidth of the first comb and the second comb.15. The system of any of the previous clauses, wherein each supercontinuum generating fiber comprises a waveguide configured to broaden a spectrum of radiation that passes through.16. The system of any of the previous clauses, wherein the relevant wavelength ranges in the optical spectrum of the diffracted radiation are associated with different colors.17. The system of any of the previous clauses, wherein there are twelve or more relevant wavelength ranges that correspond to twelve or more different colors.18. The system of any of the previous clauses, wherein the first radiation and the second radiation comprise visible light.19. The system of any of the previous clauses, wherein analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in the optical spectrum of the diffracted radiation comprises analyzing a beat frequency, which is a difference between two original frequencies of the first radiation and the second radiation.20. They system of any of the previous clauses, wherein the one or more processors are configured to sample 100 sampling points or more as part of analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in an optical spectrum of the diffracted radiation.21. The system of any of the previous clauses, further comprising one or more optical components including one or more lenses, mirrors, apertures, and / or beam splitters configured to direct the first radiation and the second radiation to the alignment metrology target, and direct the diffracted radiationfrom the alignment metrology target to the detector.22. The system of any of the previous clauses, wherein the alignment metrology target comprises a grating.23. The system of any of the previous clauses, wherein the metrology system forms a portion of an alignment sensor.24. The system of any of the previous clauses, wherein the alignment sensor is configured for a patterned substrate comprising a semiconductor wafer.25. The system of any of the previous clauses, wherein the alignment sensor is configured to be used in a semiconductor manufacturing process.26. A metrology method, comprising: irradiating, with a radiation source, an alignment metrology target in a patterned substrate with first radiation and second radiation, the first radiation and the second radiation having different repetition rates; and receiving, with a detector, diffracted radiation from the alignment metrology target and generating a detection signal, the detector comprising one or more photodiodes and one or more processors operatively coupled to the one or more photodiodes configured to generate the detection signal by analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in an optical spectrum of the diffracted radiation, the detection signal configured to be used for determining alignment of one layer of the patterned substrate relative to another.27. The method of clause 26, wherein the radiation source comprises a dual comb radiation source, with a first comb of the dual comb radiation source configured to generate the first radiation, and a second comb of the radiation source configured to generate the second radiation.28. The method of any of the previous clauses, wherein the one or more processors form at least a portion of a radio frequency (RF) analyzer.29. The method of any of the previous clauses, wherein the RF analyzer is configured to analyze heterodyne beating between radiation from pairs of comb teeth in the first comb and the second comb received at the one or more photodiodes to distinguish the relevant wavelength ranges of the optical spectrum of the diffracted radiation.30. The method of any of the previous clauses, wherein the dual comb radiation source and the RF analyzer facilitate elimination of a radiation beam demultiplexer in a metrology system configured to perform the method.31. The method of any of the previous clauses, wherein the first comb and the second comb each comprise a mode locked comb configured to pass generated radiation through a nonlinear fiber to generate broadband radiation, the broadband radiation being the first radiation and / or the second radiation.32. The method of any of the previous clauses, wherein the nonlinear fiber is a photonic crystal fiber or a graded-index fiber.33. The method of any of the previous clauses, wherein the first comb and the second comb each comprise one or more ring resonators.34. The method of any of the previous clauses, wherein each of the one or more ring resonators is associated with different colors of light.35. The method of any of the previous clauses, wherein the first comb and the second comb are configured to pass generated radiation through a same nonlinear fiber.36. The method of any of the previous clauses, wherein the first comb and the second comb are configured to pass generated radiation through different nonlinear fibers.37. The method of any of the previous clauses, wherein at least a portion of a fiber associated with each of the first comb and the second comb are arranged in proximity to each other to facilitate combination of the first radiation and the second radiation in each fiber.38. The method of any of the previous clauses, wherein combined first and second radiation in a first fiber is configured to be used as reference radiation, and wherein combined first and second radiation in a second fiber is directed toward the alignment metrology target.39. The method of any of the previous clauses, wherein generated radiation from the first comb and the second comb passes through fibers and generates a supercontinuum, which broadens a comb bandwidth of the first comb and the second comb.40. The method of any of the previous clauses, wherein each supercontinuum generating fiber comprises a waveguide configured to broaden a spectrum of radiation that passes through.41. The method of any of the previous clauses, wherein the relevant wavelength ranges in the optical spectrum of the diffracted radiation are associated with different colors.42. The method of any of the previous clauses, wherein there are twelve or more relevant wavelength ranges that correspond to twelve or more different colors.43. The method of any of the previous clauses, wherein the first radiation and the second radiation comprise visible light.44. The method of any of the previous clauses, wherein analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in the optical spectrum of the diffracted radiation comprises analyzing a beat frequency, which is a difference between two original frequencies of the first radiation and the second radiation.45. They method of any of the previous clauses, wherein the one or more processors are configured to sample 100 sampling points or more as part of analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in an optical spectrum of the diffracted radiation.46. The method of any of the previous clauses, further comprising directing, with one or more optical components including one or more lenses, mirrors, apertures, and / or beam splitters, the first radiation and the second radiation to the alignment metrology target, and directing the diffracted radiation fromthe alignment metrology target to the detector.47. The method of any of the previous clauses, wherein the alignment metrology target comprises a grating.48. The method of any of the previous clauses, wherein the method is performed by a metrology system that forms a portion of an alignment sensor.49. The method of any of the previous clauses, wherein the alignment sensor is configured for a patterned substrate comprising a semiconductor wafer.50. The method of any of the previous clauses, wherein the alignment sensor is configured to be used in a semiconductor manufacturing process.

[0107] The concepts disclosed herein may be associated with any generic imaging system for imaging sub wavelength features, and may be especially useful with emerging imaging technologies capable of producing increasingly shorter wavelengths. Emerging technologies already in use include EUV (extreme ultra violet), DUV lithography that is capable of producing a 193nm wavelength with the use of an ArF laser, and even a 157nm wavelength with the use of a Fluorine laser. Moreover, EUV lithography is capable of producing wavelengths within a range of 20-5nm by using a synchrotron or by hitting a material (either solid or a plasma) with high energy electrons in order to produce photons within this range.

[0108] While the concepts disclosed herein may be used for imaging on a substrate such as a silicon wafer, it shall be understood that the disclosed concepts may be used with any type of lithographic imaging systems, e.g., those used for imaging on substrates other than silicon wafers. In addition, the combination and sub-combinations of disclosed elements may comprise separate embodiments.

[0109] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A metrology system, comprising: a radiation source configured to irradiate an alignment metrology target in a patterned substrate with first radiation and second radiation, the first radiation and the second radiation having different repetition rates; and a detector configured to receive diffracted radiation from the alignment metrology target and generate a detection signal, the detector comprising one or more photodiodes and one or more processors operatively coupled to the one or more photodiodes configured to generate the detection signal by analyzing heterodyne beating between different corresponding portions of the first radiation and the second radiation diffracted by the metrology target to distinguish relevant wavelength ranges in an optical spectrum of the diffracted radiation, the detection signal configured to be used for determining alignment of one layer of the patterned substrate relative to another.

2. The system of claim 1 , wherein the radiation source comprises a dual comb radiation source, with a first comb of the dual comb radiation source configured to generate the first radiation, and a second comb of the radiation source configured to generate the second radiation.

3. The system of claim 2, wherein the one or more processors form at least a portion of a radio frequency (RF) analyzer.

4. The system of claim 3, wherein the RF analyzer is configured to analyze heterodyne beating between radiation from pairs of comb teeth in the first comb and the second comb received at the one or more photodiodes to distinguish the relevant wavelength ranges of the optical spectrum of the diffracted radiation.

5. The system of claim 4, wherein the dual comb radiation source and the RF analyzer facilitate elimination of a radiation beam demultiplexer in the metrology system.

6. The system of any of claims 2-5, wherein the first comb and the second comb each comprise a mode locked comb configured to pass generated radiation through a nonlinear fiber to generate broadband radiation, the broadband radiation being the first radiation and / or the second radiation.

7. The system of claim 6, wherein the nonlinear fiber is a photonic crystal fiber or a graded-index fiber.

8. The system of claims 6 or 7, wherein the first comb and the second comb each comprise one or more ring resonators.

9. The system of claim 8, wherein each of the one or more ring resonators is associated with different colors of light.

10. The system of any of claims 6-8, wherein the first comb and the second comb are configured to pass generated radiation through a same nonlinear fiber.

11. The system of any of claims 6-8, wherein the first comb and the second comb are configured to pass generated radiation through different nonlinear fibers.

12. The system of claim 11, wherein at least a portion of a fiber associated with each of the first comb and the second comb are arranged in proximity to each other to facilitate combination of the first radiation and the second radiation in each fiber.

13. The system of claim 12, wherein combined first and second radiation in a first fiber is configured to be used as reference radiation, and wherein combined first and second radiation in a second fiber is directed toward the alignment metrology target.

14. The system of any of claims 6-13, wherein generated radiation from the first comb and the second comb passes through fibers and generates a supercontinuum, which broadens a comb bandwidth of the first comb and the second comb.

15. The system of claim 14, wherein each supercontinuum generating fiber comprises a waveguide configured to broaden a spectrum of radiation that passes through.

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