Dual comb radiation source and heterodyne detection for overlay metrology systems and methods

Abstract

Dual comb radiation sources and heterodyne detection for overlay 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 the radiation signals can pass through each other. A detector receives diffracted radiation and generates a detection signal. For overlay determination, the first radiation irradiates the metrology target and the second radiation is used as a reference. The detector performs heterodyne detection of interference between different corresponding portions of the first radiation diffracted by the metrology target and the second radiation to provide a spectral response of an amplitude and phase of the diffracted radiation.

Description

DUAL COMB RADIATION SOURCE AND HETERODYNE DETECTION FOR OVERLAY METROLOGY SYSTEMS AND METHODSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 653,431 which was filed on May 30, 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 overlay 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 individualdevices 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 device 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 = kjxk / NA, where I 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 overlay 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. This allows one radiation signal to pass through the other radiation signal. A detector receives diffracted radiation and generates a detection signal. For overlay determination, the first radiation irradiates the metrology target and the second radiation is used as a reference. The detector performs heterodyne detection of interference between different corresponding portions of the first radiation diffracted by the metrology target and the second radiation to provide a spectral response of an amplitude and phase of the diffracted radiation.

[0011] According to an embodiment, a metrology system is provided. The system comprises a radiation source configured to irradiate an overlay metrology target in a patterned substrate with first radiation, and generate second radiation for use as a reference. The first radiation and the second radiation have different repetition rates. The system comprises a detector configured to receive diffracted radiation from the overlay 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 performing heterodyne detection of interference between different corresponding portions of the first radiation diffracted by the metrology target and the second radiation to provide a spectral response of an amplitude and phase of the diffracted radiation. The detection signal is configured to be used for determining overlay 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.

[0013] In some embodiments, a difference frequency between the different corresponding portions of the first radiation diffracted by the overlay metrology target and the second radiation generates radiofrequency (RF) comb lines which represent beating of two optical frequency comb lines, which is a translation of an optical spectrum into an RF domain.

[0014] In some embodiments, the one or more processors are configured to perform a Fourier transform of a time-domain response of each photodiode of the detector such that each comb line represents distinguished beating of two combs in the optical spectrum, and a spacing between the comb lines is equal to a difference between the repetition rate between two frequency combs in an optical domain, where a total number of comb lines and the repetition rate of the combs are limited by optical bandwidth, acquisition speed, and photodetector bandwidth.

[0015] In some embodiments, the one or more processors comprise a vectorial spectrum analyzer configured to resolve the amplitude and phase of the diffracted radiation at each frequency corresponding to each comb line.

[0016] In some embodiments, the one or more processors are configured to generate a hyper-spectralimage of the amplitude and phase based on the amplitude and phase of the diffracted radiation at each frequency. The hyper-spectral image comprises the detection signal, configured to be used for determining overlay of one layer of the patterned substrate relative to another. In some embodiments, the one or more processors are configured to measure the hyper-spectral image within a narrowed bandwidth to extract information related to overlay metrology target asymmetry. The information related to overlay metrology mark asymmetry may comprise three dimensional variation of the overlay metrology mark, for example. In some embodiments, the hyper-spectral image comprises a broadband hyper-spectral image, and the broadband hyper-spectral image is configured to be used by the one or more processors to measure overlay for an entire bandwidth, eliminating a requirement for wavelength switching, which facilitates increased throughput.

[0017] In some embodiments, the first comb and the second comb each comprise a mode locked micro comb configured to pass generated radiation through a same nonlinear fiber, or two separate nonlinear fibers, to generate broadband radiation. The broadband radiation may be the first radiation and / or the second radiation. In some embodiments, the same nonlinear fiber, or the two separate nonlinear fibers, each comprise a photonic crystal fiber or a graded-index fiber.

[0018] 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 visible light.

[0019] In some embodiments, the same nonlinear fiber, or two separate nonlinear fibers each form a supercontinuum generator. Each supercontinuum generator may comprises a waveguide configured to broaden a spectrum of radiation passing through each fiber.

[0020] In some embodiments, the first radiation and the second radiation comprise visible light.

[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 overlay metrology target, and direct the diffracted radiation from the overlay metrology target to the detector.

[0022] In some embodiments, the overlay metrology target comprises gratings in four pads in each of two different layers of the patterned substrate. In some embodiments, the metrology system forms a portion of an overlay sensor, and the overlay sensor is configured for a patterned substrate comprising a semiconductor wafer. In some embodiments, the overlay sensor is configured to be used in a semiconductor manufacturing process.

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

[0024] The above aspects and other aspects and features will become apparent to those ordinarilyskilled 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 is a schematic representation of a system for digital holography microscopy, and associated methodology for determining amplitudes and / or phases of a diffracted radiation signal, according to an embodiment.Fig. 7 illustrates dual comb hyper-spectral holography, according to an embodiment.Fig. 8 illustrates an example radiation source comprising two combs according to an embodiment.Fig. 9 illustrates a metrology system that uses digital holography microscopy techniques, with hyper-spectral holography techniques, and with a dual comb radiation source, for overlay determination, according to an embodiment.Fig. 10 shows the time domain and frequency domain response of output from each pixel / photodiode of a camera / detector array, according to an embodiment.Fig. 11 illustrates different views showing how, in some embodiments, a first comb and a second comb each comprise one or more ring resonators, according to an embodiment.Fig. 12 illustrates an overlay metrology method, according to an embodiment.Fig. 13 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 mark) 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 overlay and / or other parameters.

[0026] Optical overlay metrology based on digital holography microscopy (DHM) facilitates measurement of both the amplitude and phase of diffracted radiation. Knowing both the amplitude and phase facilitates determination and correction of lens aberrations, significantly reducing the complexity of the optical components in an overlay metrology system, and the cost of overlay metrology overall, as well as potentially increasing overlay metrology accuracy, among other advantages.

[0027] A dual comb radiation source may be used for hyper-spectral holography to reconstruct a three dimensional image of an object. Using a dual comb radiation source, radiation pulses from different combs have slightly different repetition rates, so one pulse train from a first comb (of the dual comb radiation source) “walks” through the other pulse train from the other comb, and the heterodyne detection of their interference provides the spectral response of the amplitude and the phase. Applying a dual comb radiation source in an overlay metrology system may decrease sensitivity to path length differences between the diffracted radiation signal are a reference radiation signal, increase overlay accuracy for high aspect ratio metrology targets and thick substrate layers, and / or may have other advantages.

[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 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 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 andthe 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 be arranged 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 theilluminator 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” 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 cross-section 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 radiationbeam 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 to correct 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 substratemay 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 system PS, 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 thesubstrate 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 be employed.

[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 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 chillplates 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 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), alignment, 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, overlay between successive layers formed in or on the patterned substrate, alignment, 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), asubstrate 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 substrate structure, 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 determine 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 similaror 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 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 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 theradiation, 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] In some embodiments, a metrology system may use reference radiation (e.g., radiation that has not been diffracted from a metrology target and is instead provided directly to a detector) to make metrology measurements. For example, reference radiation may be interfered with radiation diffracted from a metrology target to make various determinations. However, these metrology systems may be sensitive to the path length difference between the path length for a primary diffracted radiation signal and the path length of a reference beam of radiation (e.g., on the order of even just a few nano-meters).

[0063] Fig. 6 is a schematic representation of a system 600 for digital holography microscopy, and associated methodology for determining amplitudes 602 and / or phases 604 of a diffracted radiation signal 606. Amplitudes 602 and / or phases 604 of diffracted radiation 608 may be used for determining overlay and / or other metrics, for example. System 600 comprises a radiation source 610 (which may be similar to and / or the same as source 2 described above), a detector 612 (similar to and / or the same as detector 4 described above), one or more processors PRO, one or more lenses 614 and / or other elements configured to direct radiation to and / or from target 30 on substrate W, and / or other components. In this example, detector 612 may be a camera. In system 600, a first portion 620 of the radiation from source 610 is directed toward target 30, and a second portion 630 of the radiation is used as a reference and directed toward detector 612.

[0064] In Fig. 6, E generally indicates an electric field, Eorepresents a diffracted radiation beam, and Errepresents a reference beam of radiation. As illustrated, Idhm , represents a digital hologram signal, which is a measured power - based on the interference of Eoand Er, captured by detector 612. Processors PRO generate a digital hologram 650 based on Eoand Er, and determines the phase 604 and the amplitude 602 of the digital hologram signal using the mathematics shown in Fig. 6. In Fig. 6, the * indicates a complex conjugate of a complex number, and term 670 indicates a phase difference between the object beam (cp0) and the reference beam (krx). A fast Fourier transform (FFT) is performed for the spectrum 680, and then again to determine amplitudes 602 and / or phases 604.

[0065] Visually in Fig. 6, the mathematics shown on the left hand side of each equation (e.g., the IEO2I + IEr2l) corresponds to the large circle in the middle of spectrum 680, and can generally be ignored. Only one of the smaller circles (and the corresponding mathematics) is needed to extract the digital hologram signal. For example, an image in real space is a function of x and y, E(x,y). After a Fourier transform, this is a function of kx and ky. For one signal, the power is IEOI2, which is a real number, only the amplitude of the signal may be measured, but the digital hologram signal is determined, Er*Eo is determined, which is a complex number, and because Eris known (it is a reference), both the amplitude and the phase of Eocan be determined.

[0066] Optical metrology based on digital holography microscopy facilitates determination of both amplitude and phase (phase information was not available from some prior overlay metrology systems) information from diffracted signal 606 at the camera / detector location. A full knowledge of both amplitude and phase facilitates determinations based on signal 606 for any location on target 30, facilitates determination and / or correction of lens aberrations, decreases the complexity of system 600 compared to prior overlay metrology systems (e.g., two lenses instead of 15 or more lenses or lens like elements), decreases overlay metrology costs compared to prior systems, increases overlay accuracy compared to prior systems, and / or has other advantages.

[0067] In some embodiments, hyper-spectral holography techniques with dual comb radiation sources may be used in combination with digital holography microscopy techniques (e.g., as shown in Fig. 6). For example, Fig. 7 illustrates a dual comb hyper-spectral holography system 700. In dual comb hyper-spectral holography system 700, first radiation 702 from a first comb radiation source 704 has a slightly different repetition rate and / or other characteristics compared to second radiation 706 from a second comb radiation source 708, so that one pulse train of radiation from one of the two combs “walks” 710 through the other pulse train of radiation from the other comb. The heterodyne detection 712 of their interference creates a radio frequency (RF) signal which provides the optical spectral response of the amplitude and the phase. For example, time domain detections 714 may be Fourier transformed 715 (the top plot in 715 is in the time domain, the bottom one is in the frequency domain) into amplitude and phase information in the frequency domain 716. Heterodyne detection refers to comparing detections from the two combs (heterodyne means more than one frequency). The spectral response of the diffracted light gives more information about the depth of the overlay target. This helps to calibrate the errors related to the mark asymmetry due to the fabrication imperfections.

[0068] Fig. 8 illustrates an example spectral response of radiation source 800 (e.g., generating radiation similar to and / or the same as source 2 shown in Fig. 3) comprising a first comb 802 and a second comb 804. First radiation 806 from first comb 802 radiation has a slightly different repetition rate and / or other characteristics compared to second radiation 808 from second comb 804, which means the spacing between comb lines (narrow spectral lines) in different sources are slightly different in the spectral domain. Advantageously, first comb 802 and / or second comb 804 require no mechanicalmovement, and facilitate resolution of the diffracted radiation spectrum in nano-second or pico-second time frames (depending on sampling density, signal to noise ratio, and / or other factors). Combs (e.g., first comb 802 and second comb 804) that generate radiation 806 and 808 with slightly different repetition rates can cause a full spectral diffracted radiation response from an overlay metrology target. First radiation 806 and second radiation 808 may comprise visible, infrared, or ultraviolet light and / or other radiation.

[0069] Fig. 9 illustrates a metrology system 900 that uses digital holography microscopy techniques, with hyper-spectral holography techniques, and with a dual comb radiation source, for overlay determination. System 900 is the same as or similar to system 10 described above with respect to Fig. 3, and / or aspects of the systems shown in Fig. 6, Fig. 7, and / or Fig. 8, with one or more components of system 900 being similar to and / or the same as one or more components of system 10, system 600, system 700, etc., (and Fig. 9 illustrating additional and / or different 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, system 600, system 700, etc.. In system 900, radiation is generated by radiation source 902 and directed to (and from) a 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 overlay metrology systems. Optical components 906 may include one or more lenses, fibers, mirrors, apertures, beam splitters, and / or other components configured to direct radiation 904 to the overlay metrology target 30, direct diffracted radiation 905 from the overlay metrology target 30, and / or reference radiation 907 to a detector 920. In this example, target 30 may comprise gratings in four pads in each of two different layers of the patterned substrate W, as one of many possibilities.

[0070] Radiation source 902 is configured to irradiate overlay metrology target 30 in patterned substrate W with first radiation 904, and generate second radiation 907 for use as a reference. In some embodiments, radiation source 902 comprises a dual comb radiation source, with a first comb 910 of the dual comb radiation source 902 configured to generate (first) radiation 904, and a second comb 912 of radiation source 902 configured to generate (second or reference) radiation 907. First radiation 904 and second radiation 907 have different repetition rates (fT) (where A fTis the difference in repetition rate between the two combs) and / or other characteristics so the radiation signals may pass through each other. Diffracted radiation 905 from overlay metrology target 30, and reference radiation 907, is received by detector 920, which generates a detection signal 922.

[0071] Detector 920 comprises a camera, one or more photodiodes, one or more processors PRO operatively coupled to the camera and / or one or more photodiodes, and / or other components. In some embodiments, the one or more processors PRO form at least a portion of a radio frequency (RF) spectrum analyzer. In Fig. 9, Eoagain represents a diffracted radiation beam, and Errepresents a reference beam of radiation. As indicated by Eoeltp(f), the phase is a function of frequency. This couldbe re-written as Eo= IEoleltp(f), for example. Detector 920 and / or processors PRO generate a digital hologram 951 based on Eoand Er, and / or other information. With a hologram image and a known lens aberration, back propagation can be used to determine what the corresponding field looks like without the aberration, so the aberration and other imperfections can be corrected (Eris a reference, so it is assumed that it is already determined).

[0072] Detector 920 is configured to generate detection signal 922 by performing heterodyne detection of interference between different corresponding portions of first radiation 904 diffracted by metrology target 30 (so that it becomes diffracted radiation 905) and second radiation 907 to provide a spectral response of an amplitude 950 and phase 955 (based on hyper-spectral images 960 in system 900) of diffracted radiation 905. Detection signal 922 is configured to be used for determining overlay of one layer of the patterned substrate W relative to another.

[0073] For example, one or more processors PRO are configured to generate hyper-spectral images 960 of the amplitude and phase based on the amplitude and phase of the diffracted radiation 905 at each frequency. The hyper-spectral images 960 may comprise (or be determined based on) detection signal 922, and be used for determining overlay of one layer of the patterned substrate relative to another. In some embodiments, one or more processors PRO are configured to measure a hyper-spectral image 960 within a narrowed bandwidth to extract information related to overlay metrology target asymmetry. The information related to overlay metrology mark asymmetry may comprise three dimensional variation of the overlay metrology target, for example. In some embodiment, the hyper-spectral images comprises broadband hyper-spectral images, with the broadband hyper-spectral images configured to be used by the one or more processors PRO to measure overlay for an entire bandwidth, eliminating a requirement for wavelength switching, which facilitates increased throughput and / or has other advantages.

[0074] In some embodiments, a difference frequency between different corresponding portions of (first) radiation 904 diffracted by overlay metrology target 30 (to become diffracted radiation 905) and (second or reference) radiation 907 generates radiofrequency (RF) comb lines which represent beating of two optical frequency comb lines, and is a translation of an optical spectrum into an RF domain. The one or more processors PRO are configured to perform a Fourier transform of a time-domain response of each photodiode (for example) of detector 920 such that each comb line represents distinguished beating of two combs (e.g., first comb 910 and second comb 912) in the optical spectrum, and a spacing between the comb lines is equal to a difference between the repetition rate between the two (frequency) combs 910 and 912 in an optical domain. A total number of comb lines and the repetition rate of combs 910 and 912 may be limited by optical bandwidth, acquisition speed, photodetector bandwidth, and / or other factors. For example, the one or more processors PRO may comprise a vectorial spectrum analyzer configured to resolve the amplitude and phase of the diffracted radiation at each frequency corresponding to each comb line.

[0075] If the repetition rate of the first comb 910 and the second comb 912 are different, their interference at detector 920, 1 oc IEO+ ErI2, has two parts. As shown in Fig. 9, the first part 981 is a DC signal, which represents the total power of the reference radiation 907 and the diffracted radiation 905. However, the second term 991 comprises a sum frequency and difference frequency of the radiation from the two combs. The sum frequency generation occurs at relatively high frequencies, so it is not detected by detector 920, but the difference frequency generation process, generates RF comb lines which represent the beating of two optical frequency comb lines. Hence, the optical spectrum is translated to RF domain. Fig. 9 illustrates an RF comb amplitude 992 and an RF comb phase 994. In Fig. 9, * indicates a complex conjugate, fo indicates a central frequency of the combs, n indicates the comb line number which is an integer number, i =c.c. indicates a complex conjugate, t indicates time.

[0076] The electric field for a comb line in the time-domain can be written as E(t)=Ee12rf0t, and the Fourier transform of this signal in the frequency domain is a delta function E(f)= E5(f-f0). This equation shows that the detected signal (for each pixel at a photodetector, for example) is a train of comb lines (delta functions) in the frequency domain, which are spaced equally with a spacing of A / r. (which is at low-frequency domain), but the amplitude and phase of each of these RF comb lines represents the optical comb line at fo+nfr(fo+nfr» nA / r).

[0077] For example, Fig. 10 shows the time domain 1000 and frequency domain 1002 response of output 1001 from each pixel / photodiode of a camera / detector 920 array. An example of a signal composed of DC signal 1004 and a heterodyned signal 1006 are illustrated in time domain 1000. An nth RF comb line representing an nth optical comb line 1008, and detector 920 RF bandwidth 1010, are illustrated in frequency domain 1002. Using a vectorial spectrum analyzer (formed by the one or more processors PRO), the amplitude and phase can be resolved at each frequency, which means we a hyper- spectral image representing the amplitude and phase of an overlay signal can be generated. Fig. 10 shows the time-domain response of each pixel / photodiode (top) and its Fourier transform (bottom). Each comb line represents distinguished beating of two combs in the optical regime. The spacing between the comb lines is equal to the difference between the repetition rate between the two frequency combs and the total number of comb lines are limited by the photodetector bandwidth.

[0078] Returning to Fig. 9, in some embodiments, generated radiation 904 and 907 from first comb 910 and second comb 912, respectively, passes through one or more fibers 980 and generates a supercontinuum, which broadens a comb bandwidth of first comb 910 and / or second comb 912. Each supercontinuum generating fiber 980 may comprise a waveguide configured to broaden a spectrum of radiation 904 and 907 that passes through, and / or other components.

[0079] In some embodiments, first comb 910 and second comb 912 may each comprise a mode locked comb configured to pass generated radiation 904 and / or 907 through a nonlinear fiber 980 to generate broadband radiation, with the broadband radiation being first radiation 904 and / or second orreference radiation 907. The nonlinear fiber 980 may be a single-mode fiber, multi-mode fiber, photonic crystal fiber, a graded-index fiber, and / or other nonlinear fibers. In some embodiments, first comb 910 and second comb 912 are configured to pass generated radiation through a same nonlinear fiber 980 (not illustrated in Fig. 9). In some embodiments, first comb 910 and second comb 912 are configured to pass generated radiation through different nonlinear fibers 980 (e.g., as shown in Fig. 9). In some embodiments, at least a portion of a fiber associated with each of first comb 910 and second comb 912 are arranged in proximity to each other to facilitate combination of generated radiation in each fiber. In some embodiments, combined radiation in a first fiber is configured to be used as reference radiation, and combined radiation in a second fiber is directed toward metrology target 30.

[0080] In some embodiments, first comb 910 and second comb 912 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 910 and second comb 912) 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.

[0081] Additional details and examples related to the fibers and ring resonators is described below with respect to Fig. 11. Fig. 11 illustrates different views 1100, 1102, and 1104 showing how, in some embodiments, first comb 910 and second comb 912 each comprise one or more ring resonators 1110. In this example, first comb 910 and second comb 912 also each comprise a laser radiation source 1112. Due to the field enhancement in a micro-ring resonator nonlinear light-matter interaction is enhanced in a micro-ring resonator. If the linear and nonlinear dispersion of the resonator is designed properly, the spectrum of the generated signal within the cavity is broadened leading to generation of a short pulse. The repetition rate of the pulse bain depends on the length and the dispersion of the cavity. Each of the one or more ring resonators 1110 may be associated with different but narrower spectral bands of light (radiation), for example, but their combination covers a broad spectral band. In some embodiments, different colors of light from each of the ring resonators 1110 may be combined 1120 to form the broadband radiation (first radiation 904 and / or second or reference radiation 907).

[0082] In some embodiments, first comb 910 and second comb 912 may each comprise a mode locked comb configured to pass generated radiation 1105 and / or 1108 through a nonlinear fiber 980 to generate broadband radiation, with the broadband radiation being first radiation 904 and / or second or reference radiation 907. The nonlinear fiber 980 may be a photonic crystal fiber, a graded-index fiber, and / or other nonlinear fibers. In some embodiments, generated radiation 1105 and 1108 from firstcomb 910 and second comb 912, respectively, passes through one or more fibers 980 and generates 1150 a supercontinuum (see view 1102 specifically), which broadens a comb bandwidth of first comb 910 and / or second comb 912. Each supercontinuum generating fiber 980 may comprise a waveguide configured to broaden a spectrum of radiation 1105 and 1108 that passes through, and / or other components. To increase the repetition rate of supercontinuum generation and form a frequency comb (e.g., first comb 910 and / or second comb 912), a driver of the supercontinuum process may be a mode- locked high repetition rate narrow-band laser (e.g., laser radiation source 1112) and / or other drivers.

[0083] In some embodiments, first comb 910 and second comb 912 are configured to pass generated radiation 1105 and 1108 through a same nonlinear fiber 980. In some embodiments, first comb 910 and second comb 912 are configured to pass generated radiation 1105 and radiation 1108 through different nonlinear fibers 980 (e.g., as shown in views 1100, 1102, and 1104 of Fig. 11). In some embodiments, at least a portion of a fiber associated with each of first comb 910 and second comb 912 are arranged in proximity to each other (e.g., where different colors of light from each of the ring resonators 1110 may be combined 1120) to facilitate combination of radiation 1105 and radiation 1108 in each fiber 980. In some embodiments, combined first radiation 904 and second radiation 907 in a first fiber 980 is configured to be used as reference radiation 907 (e.g., for eventual metrology measurement determination), and combined first radiation 904 and second radiation 907 in a second fiber 980 is directed toward alignment metrology target 30 (Fig. 9) via one or more optical components 906.

[0084] Fig. 12 illustrates a metrology method 1201. In some embodiments, method 1201 is performed as part of an overlay sensing operation in a semiconductor device manufacturing process, for example. In some embodiments, one or more operations of method 1201 may be implemented in or by a metrology system such as system 10 illustrated in Fig. 3, system 900 shown in Fig. 9, a computer system (e.g., as illustrated in Fig. 13 and described below), and / or in or by other systems, for example. In some embodiments, method 1201 comprises irradiating (operation 1202) an overlay metrology target in a patterned substrate, receiving diffracted radiation and generating a detection signal (operation 1204), determining (operation 1206) overlay, and / or other operations.

[0085] The operations of method 1201 are intended to be illustrative. In some embodiments, method 1201 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 1201 may include an additional operation comprising determining overlay for a semiconductor wafer, and determining an adjustment for a semiconductor device manufacturing process. Additionally, the order in which the operations of method 1201 are illustrated in Fig. 12 and described herein is not intended to be limiting.

[0086] In some embodiments, one or more portions of method 1201 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 statemachine, 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 method 1201 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 1201 (e.g., see discussion related to Fig. 13 below).

[0087] At operation 1202, an overlay metrology target in a patterned substrate is irradiated with first radiation. Second radiation is generated for use as a reference. The first and second radiation are generated by a radiation source such as 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 overlay target on a patterned substrate such as a grating on a semiconductor wafer, and / or other components. In some embodiments, the overlay metrology target comprises gratings in four pads in each of two different layers of the patterned substrate.

[0088] 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.

[0089] 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 - which may eventually split into metrology target directed first radiation and reference second radiation fibers. 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 second fiber is configured to be used as reference radiation, and combined first and second radiation in a first fiber is directed toward the overlay metrology target.

[0090] 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, forexample. Different colors of radiation from each of the ring resonators may be combined to form the broadband radiation.

[0091] At operation 1204, diffracted radiation from the overlay metrology target and the reference radiation 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. The detector is configured to generate the detection signal by performing heterodyne detection of interference between different corresponding portions of the first radiation diffracted by the metrology target and the second radiation to provide a spectral response of an amplitude and phase of the diffracted radiation. The detection signal is configured to be used for determining overlay of one layer of the patterned substrate relative to another.

[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 overlay signal comprising overlay measurement information, and / or other metrology signals. The detection signal comprises an electronic signal that represents and / or otherwise corresponds to diffracted radiation 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 1202 and / or 1204 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 overlay metrology target, and directing the diffracted radiation from the overlay metrology target to the detector.

[0094] At operation 1206, overlay of one layer of the patterned substrate relative to another layer is determined. The overlay may be determined based on the information in the detection signal and / or other information. For example, a difference frequency between the different corresponding portions of the first radiation diffracted by the overlay metrology target and the second radiation generates radiofrequency (RF) comb lines which represent beating of two optical frequency comb lines, which is a translation of an optical spectrum into an RF domain. The one or more processors are configured to perform a Fourier transform of a time-domain response of each photodiode of the detector such that each comb line represents distinguished beating of two combs in the optical spectrum. A spacingbetween the comb lines is equal to a difference between the repetition rate between two frequency combs in an optical domain. A total number of comb lines and the repetition rate of the combs are limited by optical bandwidth, acquisition speed, and photodetector bandwidth.

[0095] In some embodiments, the one or more processors comprise a vectorial spectrum analyzer configured to resolve the amplitude and phase of the diffracted radiation at each frequency corresponding to each comb line. In addition, the one or more processors are configured to generate a hyper-spectral image of the amplitude and phase based on the amplitude and phase of the diffracted radiation at each frequency. (The hyper-spectral image comprises the detection signal.) The one or more processors are configured to measure the hyper-spectral image within a narrowed bandwidth to extract information related to overlay metrology target asymmetry. The information related to overlay metrology mark asymmetry comprises three dimensional variation of the overlay metrology mark and / or other information. The hyper-spectral image comprises a broadband hyper-spectral image. The broadband hyper-spectral image is configured to be used by the one or more processors to measure overlay for an entire bandwidth, eliminating a requirement for wavelength switching, which facilitates increased throughput.

[0096] In some embodiments, method 1201 comprises determining an adjustment for a semiconductor device manufacturing process. In some embodiments, method 1201 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 overlay 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 overlay 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.

[0097] In some embodiments, method 1201 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 acceptabledevices.

[0098] 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 1201), for example. In some embodiments, method 1201 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.

[0099] Fig. 13 is a diagram of an example computer system CS that may be used for one or more of the 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) 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.

[0100] 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.

[0101] 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 ofinstructions 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.

[0102] 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 during radio 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.

[0103] 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.

[0104] 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 interfaceCI 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.

[0105] 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.

[0106] Computer system CS can send messages and receive data, including program code, through the network(s), network data link NDL, and communication interface CL In the Internet example, host computer 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.

[0107] 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 overlay metrology target in a patterned substrate with first radiation, and generate second radiation for use as a reference, the first radiation and the second radiation having different repetition rates; and a detector configured to receive diffracted radiation from the overlay 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 performing heterodyne detection of interference between different corresponding portions of the first radiation diffracted by the metrology target and the second radiation to provide a spectral response of an amplitude and phase of the diffracted radiation, the detection signal configured to be used for determining overlay 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 secondcomb of the radiation source configured to generate the second radiation.3. The system of any of the previous clauses, wherein a difference frequency between the different corresponding portions of the first radiation diffracted by the overlay metrology target and the second radiation generates radiofrequency (RF) comb lines which represent beating of two optical frequency comb lines, which is a translation of an optical spectrum into an RF domain.4. The system of any of the previous clauses, wherein the one or more processors are configured to perform a Fourier transform of a time-domain response of each photodiode of the detector such that each comb line represents distinguished beating of two combs in the optical spectrum, and a spacing between the comb lines is equal to a difference between the repetition rate between two frequency combs in an optical domain, wherein a total number of comb lines and the repetition rate of the combs are limited by optical bandwidth, acquisition speed, and photodetector bandwidth.5. The system of any of the previous clauses, wherein the one or more processors comprise a vectorial spectrum analyzer configured to resolve the amplitude and phase of the diffracted radiation at each frequency corresponding to each comb line.6. The system of any of the previous clauses, wherein the one or more processors are configured to generate a hyper-spectral image of the amplitude and phase based on the amplitude and phase of the diffracted radiation at each frequency, the hyper-spectral image comprising the detection signal configured to be used for determining overlay of one layer of the patterned substrate relative to another.7. The system of any of the previous clauses, wherein the one or more processors are configured to measure the hyper-spectral image within a narrowed bandwidth to extract information related to overlay metrology target asymmetry.8. The system of any of the previous clauses, wherein the information related to overlay metrology mark asymmetry comprises three dimensional variation of the overlay metrology mark.9. The system of any of the previous clauses, wherein the hyper-spectral image comprises a broadband hyper-spectral image, and the broadband hyper-spectral image is configured to be used by the one or more processors to measure overlay for an entire bandwidth, eliminating a requirement for wavelength switching, which facilitates increased throughput.10. The system of any of the previous clauses, wherein the first comb and the second comb each comprise a mode locked micro comb configured to pass generated radiation through a same nonlinear fiber, or two separate nonlinear fibers, to generate broadband radiation, the broadband radiation being the first radiation and / or the second radiation.11. The system of any of the previous clauses, wherein the same nonlinear fiber, or the two separate nonlinear fibers, each comprise a photonic crystal fiber or a graded-index fiber.12. The system of any of the previous clauses, wherein the first comb and the second comb each comprise one or more ring resonators.13. The system of any of the previous clauses, wherein each of the one or more ring resonators isassociated with different colors of visible light.14. The system of any of the previous clauses, wherein the same nonlinear fiber, or two separate nonlinear fibers each form a supercontinuum generator.15. The system of any of the previous clauses, wherein each supercontinuum generator comprises a waveguide configured to broaden a spectrum of radiation passing through each fiber.16. The system of any of the previous clauses, wherein the first radiation and the second radiation comprise visible light.17. 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 overlay metrology target, and direct the diffracted radiation from the overlay metrology target to the detector.18. The system of any of the previous clauses, wherein the overlay metrology target comprises gratings in four pads in each of two different layers of the patterned substrate.19. The system of any of the previous clauses, wherein the metrology system forms a portion of an overlay sensor, and the overlay sensor is configured for a patterned substrate comprising a semiconductor wafer.20. The system of any of the previous clauses, wherein the overlay sensor is configured to be used in a semiconductor manufacturing process.21. A metrology method, comprising: irradiating, with a radiation source, an overlay metrology target in a patterned substrate with first radiation, and generating second radiation for use as a reference, the first radiation and the second radiation having different repetition rates; and receiving, with a detector, diffracted radiation from the overlay 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 performing heterodyne detection of interference between different corresponding portions of the first radiation diffracted by the metrology target and the second radiation to provide a spectral response of an amplitude and phase of the diffracted radiation, the detection signal configured to be used for determining overlay of one layer of the patterned substrate relative to another.22. The method of clause 21, 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.23. The method of any of the previous clauses, wherein a difference frequency between the different corresponding portions of the first radiation diffracted by the overlay metrology target and the second radiation generates radiofrequency (RF) comb lines which represent beating of two optical frequency comb lines, which is a translation of an optical spectrum into an RF domain.24. The method of any of the previous clauses, wherein the one or more processors are configured toperform a Fourier transform of a time-domain response of each photodiode of the detector such that each comb line represents distinguished beating of two combs in the optical spectrum, and a spacing between the comb lines is equal to a difference between the repetition rate between two frequency combs in an optical domain, wherein a total number of comb lines and the repetition rate of the combs are limited by optical bandwidth, acquisition speed, and photodetector bandwidth.25. The method of any of the previous clauses, wherein the one or more processors comprise a vectorial spectrum analyzer configured to resolve the amplitude and phase of the diffracted radiation at each frequency corresponding to each comb line.26. The method of any of the previous clauses, wherein the one or more processors are configured to generate a hyper-spectral image of the amplitude and phase based on the amplitude and phase of the diffracted radiation at each frequency, the hyper-spectral image comprising the detection signal configured to be used for determining overlay of one layer of the patterned substrate relative to another.27. The method of any of the previous clauses, wherein the one or more processors are configured to measure the hyper-spectral image within a narrowed bandwidth to extract information related to overlay metrology target asymmetry.28. The method of any of the previous clauses, wherein the information related to overlay metrology mark asymmetry comprises three dimensional variation of the overlay metrology mark.29. The method of any of the previous clauses, wherein the hyper-spectral image comprises a broadband hyper-spectral image, and the broadband hyper-spectral image is configured to be used by the one or more processors to measure overlay for an entire bandwidth, eliminating a requirement for wavelength switching, which facilitates increased throughput.30. The method of any of the previous clauses, wherein the first comb and the second comb each comprise a mode locked micro comb configured to pass generated radiation through a same nonlinear fiber, or two separate nonlinear fibers, to generate broadband radiation, the broadband radiation being the first radiation and / or the second radiation.31. The method of any of the previous clauses, wherein the same nonlinear fiber, or the two separate nonlinear fibers, each comprise a photonic crystal fiber or a graded-index fiber.32. The method of any of the previous clauses, wherein the first comb and the second comb each comprise one or more ring resonators.33. The method of any of the previous clauses, wherein each of the one or more ring resonators is associated with different colors of visible light.34. The method of any of the previous clauses, wherein the same nonlinear fiber, or two separate nonlinear fibers each form a supercontinuum generator.35. The method of any of the previous clauses, wherein each supercontinuum generator comprises a waveguide configured to broaden a spectrum of radiation passing through each fiber.36. The method of any of the previous clauses, wherein the first radiation and the second radiationcomprise visible light.37. 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 overlay metrology target, and directing the diffracted radiation from the overlay metrology target to the detector.38. The method of any of the previous clauses, wherein the overlay metrology target comprises gratings in four pads in each of two different layers of the patterned substrate.39. The method of any of the previous clauses, wherein the method is performed by a metrology system that forms a portion of an overlay sensor, and the overlay sensor is configured for a patterned substrate comprising a semiconductor wafer.40. The method of any of the previous clauses, wherein the overlay sensor is configured to be used in a semiconductor manufacturing process.

[0108] 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.

[0109] 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.

[0110] 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 overlay metrology target in a patterned substrate with first radiation, and generate second radiation for use as a reference, the first radiation and the second radiation having different repetition rates; and a detector configured to receive diffracted radiation from the overlay 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 performing heterodyne detection of interference between different corresponding portions of the first radiation diffracted by the metrology target and the second radiation to provide a spectral response of an amplitude and phase of the diffracted radiation, the detection signal configured to be used for determining overlay 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 a difference frequency between the different corresponding portions of the first radiation diffracted by the overlay metrology target and the second radiation generates radiofrequency (RF) comb lines which represent beating of two optical frequency comb lines, which is a translation of an optical spectrum into an RF domain.

4. The system of claim 3, wherein the one or more processors are configured to perform a Fourier transform of a time-domain response of each photodiode of the detector such that each comb line represents distinguished beating of two combs in the optical spectrum, and a spacing between the comb lines is equal to a difference between the repetition rate between two frequency combs in an optical domain, wherein a total number of comb lines and the repetition rate of the combs are limited by optical bandwidth, acquisition speed, and photodetector bandwidth.

5. The system of claims 3 or 4, wherein the one or more processors comprise a vectorial spectrum analyzer configured to resolve the amplitude and phase of the diffracted radiation at each frequency corresponding to each comb line.

6. The system of claim 5, wherein the one or more processors are configured to generate a hyper- spectral image of the amplitude and phase based on the amplitude and phase of the diffractedradiation at each frequency, the hyper-spectral image comprising the detection signal configured to be used for determining overlay of one layer of the patterned substrate relative to another.

7. The system of claim 6, wherein the one or more processors are configured to measure the hyper- spectral image within a narrowed bandwidth to extract information related to overlay metrology target asymmetry.

8. The system of claim 7, wherein the information related to overlay metrology mark asymmetry comprises three dimensional variation of the overlay metrology mark.

9. The system of any of claims 6-8, wherein the hyper-spectral image comprises a broadband hyper- spectral image, and the broadband hyper-spectral image is configured to be used by the one or more processors to measure overlay for an entire bandwidth, eliminating a requirement for wavelength switching, which facilitates increased throughput.

10. The system of any of claims 2-9, wherein the first comb and the second comb each comprise a mode locked micro comb configured to pass generated radiation through a same nonlinear fiber, or two separate nonlinear fibers, to generate broadband radiation, the broadband radiation being the first radiation and / or the second radiation.

11. The system of claim 10, wherein the same nonlinear fiber, or the two separate nonlinear fibers, each comprise a photonic crystal fiber or a graded-index fiber.

12. The system of any of claims 2-11, wherein the first comb and the second comb each comprise one or more ring resonators.

13. The system of claim 12, wherein each of the one or more ring resonators is associated with different colors of visible light.

14. The system of any of claims 10-13, wherein the same nonlinear fiber, or two separate nonlinear fibers each form a supercontinuum generator.

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

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