Digital holography system and method for diffraction based alignment

Abstract

A metrology system comprising: an illumination source, the illumination source configured to produce a beam of illumination; wherein at least a first portion of the beam is diffracted by a target and wherein at least a second portion of the beam is not diffracted by the target; a detector, the detector configured to detect a recombination of the first portion of the beam diffracted by the target and the second portion of the beam not diffracted by the target; and a processor operatively connected with the detector, the processor configured to determine a phase and / or amplitude of a waveform of the first portion of the beam after it has interacted with the target based on the recombination detected by the illumination detector.

Description

DIGITAL HOLOGRAPHY SYSTEM AND METHOD FOR DIFFRACTION BASED ALIGNMENTCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 640,511 which was filed on April 30, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates generally systems and methods for measurement of parameters of interest in semiconductor manufacturing and more specifically to system and methods for holographic evaluation and / or correction of diffraction-based alignment measurement.BACKGROUND

[0003] Manufacturing devices, such as semiconductor devices, typically involves processing a substrate (e.g., a semiconductor wafer) using a number of fabrication processes to form various features and multiple layers of the devices. Such layers and features are typically manufactured and processed using, e.g., deposition, lithography, etch, chemical-mechanical polishing, and ion implantation. Multiple devices may be fabricated on a plurality of dies on a substrate and then separated into individual devices. 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, etc. Patterning can occur in multiple layers, such that a multi-layer stack or device can be constructed from a set of patterned layers which are aligned with one another during patterning and other steps.

[0004] Lithography is a central step in the manufacturing of devices such as ICs, where patterns formed on substrates define functional elements of the devices, such as microprocessors, memory chips, etc. Similar lithographic techniques are also used in the formation of flat panel displays, microelectromechanical systems (MEMS) and other devices.

[0005] As semiconductor manufacturing processes continue to advance, the dimensions of functional elements have continually been reduced. At the same time, the number of functional elements, such as transistors, per device has been steadily increasing, 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 deepultraviolet illumination source, creating individual functional elements having dimensions well below100 nanometers (nm), i.e., less than half the wavelength of the radiation from the illumination source (e.g., a 193 nm illumination source).

[0006] 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-kl lithography, according to the resolution formula CD = klxl / NA, where X is the wavelength of radiation employed (currently in most cases 248nm or 193nm), NA is the numerical aperture of projection optics in the lithographic projection apparatus, CD is the “critical dimension”-generally the smallest feature size printed-and kl is an empirical resolution factor. In general, the smaller kl 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, which can include alignment tools, the design layout, or the patterning device.

[0007] Monitoring of device and material features, including CD, and of parameters of interest in a manufacturing process (e.g., fabrication parameters such as overlay offset, position, dose, symmetry, etc.), allows for process monitoring, control, and correction, including control of lithography and other fabrication steps. A metrology apparatus can be used to determine properties of devices and how properties of different devices vary or how properties associated with different layers of the same device vary from layer to layer. The metrology apparatus, which can be a diffraction-based apparatus, an optical apparatus, an electron microscopy apparatus, etc., may alternatively be constructed to identify defects on the device or to align the device and may, for example, be part of the lithographic apparatus or may be a stand-alone device. The metrology apparatus may measure the properties on a latent image (image in a resist layer after the exposure), or on a semi-latent image (image in a resist layer after a post-exposure bake step PEB), or on a developed resist image (in which the exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).SUMMARY

[0008] In an embodiment, a metrology system comprising an illumination source, the illumination source configured to produce a beam of illumination; wherein at least a first portion of the beam is diffracted by a target and wherein at least a second portion of the beam is not diffracted by the target; a detector, the detector configured to detect a recombination of the first portion of the beam diffracted by the target and the second portion of the beam not diffracted by the target; and a processor operatively connected with the detector, the processor configured to determine a phase and / or amplitude of a waveform of the first portion of the beam after it has interacted with the target based on the recombination detected by the detector.

[0009] In a further embodiment, a metrology system comprising: an illumination source, the illumination source configured to produce one or more wavelengths of illumination which are diffractedby a target; one or more detectors configured to detect locations of one or more diffraction orders of the illumination diffracted by the target; a beam splitter configured to select, for deflection to an additional detector, a subset of the one or more diffraction orders of the illumination; a beam conditioner to generate substantially temporally coherent illumination from the selected subset; the additional detector configured to detect a recombination of the selected subset of the substantially temporally coherent illumination and an additional substantially temporally coherent illumination; and a processor operatively connected with the one or more detectors and the additional detector, the processor configured to determine a location of the target based on the detected locations of the one or more diffraction orders as detected by the one or more detectors, the processor configured to determine a non-ideality in the determined location based on the recombination detected by the additional detector.

[0010] In an embodiment, a metrology system comprising: an illumination source, the illumination source configured to produce one or more wavelengths of illumination which are diffracted by a target; one or more detectors configured to detect locations of one or more diffraction orders of the illumination diffracted by the target; an additional illumination source, the additional illumination source configured to generate substantially coherent light, wherein a first portion of the substantially coherent light is diffracted by the target and wherein a second portion of the substantially coherent light is undiffracted by the target; an additional detector configured to detect a recombination of the first portion of the substantially coherent light diffracted by the target and the second portion of the substantially coherent light undiffracted by the target; and a processor operatively connected with the one or more detectors and the additional detector, the processor configured to determine a location of the target based on the detected locations of the one or more diffraction orders as detected by the one or more detectors, the processor configured to determine a non-ideality in the determined location based on the recombination detected by the additional detector.

[0011] In a further embodiment, a method for metrology, comprising: illuminating a diffraction target with a first beam of illumination, the illumination generating at least one diffraction order; recombining the at least one diffraction order with a source of substantially coherent illumination; detecting the recombination by a two-dimensional detector; and determining, by a processor, a phase and / or amplitude of a waveform of the at least one diffraction order after it has interacted with the target based on the recombination detected by the two-dimensional detector.

[0012] In an embodiment, a method for metrology, comprising: illuminating a diffraction target with multiple wavelengths of illumination, the illumination generating multiple diffraction orders; detecting, by one or more detectors, locations of the multiple diffraction orders; determining, by a processor, a location of the diffraction target based on the detected location of the multiple diffraction orders; selecting, by a beam splitter, a portion of at least one diffraction order for deflection to an additional detector; recombining the selected portion of the at least one diffraction order with a substantially temporally coherent illumination source; detecting, by the additional detector, the recombination; determining, by the processor, a phase and / or amplitude of a waveform corresponding to the at leastone diffraction order after it has interacted with the target based on the recombination detected by the additional detector; and correcting, by the processor, the determined location of the diffraction target based on the phase and / or amplitude of the waveform corresponding to the at least one diffraction order.

[0013] In one embodiment, a machine -readable medium having instructions thereon, the instructions when executed by a processor being configured to perform the method of another embodiment.

[0014] In a further embodiment, a processor and a machine -readable medium as described in another embodiment.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0016] Figure 1 depicts a schematic overview of a lithographic apparatus, according to an embodiment.

[0017] Figure 2 depicts a schematic overview of a lithographic cell or cluster, according to an embodiment.

[0018] Figure 3 depicts a schematic representation of an example metrology system, according to an embodiment.

[0019] Figure 4 depicts a schematic representation of an example metrology technique, according to an embodiment.

[0020] Figure 5A depicts a schematic representation of an example aberration causing diffractionbased metrology errors, according to an embodiment.

[0021] Figure 5B depicts a schematic representation of an effect of an example aberration on detection of diffractions, according to an embodiment.

[0022] Figure 6 depicts a schematic representation of an example diffraction-based metrology system, according to an embodiment.

[0023] Figure 7A depicts a schematic representation of an example off-axis holography system working in concert with a diffraction-based metrology system, according to an embodiment.

[0024] Figure 7B depicts a schematic representation of off-axis illumination for diffraction-based holography, according to an embodiment.

[0025] Figure 8A depicts a schematic representation of an example zeroth order holography system working in concert with a diffraction-based metrology system, according to an embodiment.

[0026] Figure 8B depicts a schematic representation of on-axis illumination for diffraction-based holography, according to an embodiment.

[0027] Figure 9 depicts a schematic representation of an example technique for improvement of diffraction-based metrology with holography, according to an embodiment.

[0028] Figure 10 depicts a flowchart illustrating an exemplary method for diffraction-based holography, according to an embodiment.

[0029] Figure 11 is a block diagram of an example computer system, according to an embodiment of the present disclosure.DETAILED DESCRIPTION

[0030] Embodiments of the present disclosure are described in detail with reference to the drawings, which are provided as illustrative examples of the disclosure so as to enable those skilled in the art to practice the disclosure. Notably, the figures and examples below are not meant to limit the scope of the present disclosure to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present disclosure can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present disclosure will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the disclosure. Embodiments described as being implemented in software should not be limited thereto, but can include embodiments implemented in hardware, or combinations of software and hardware, and vice-versa, as will be apparent to those skilled in the art, unless otherwise specified herein. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the disclosure is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present disclosure encompasses present and future known equivalents to the known components referred to herein by way of illustration.

[0031] In semiconductor device manufacturing, metrology operations typically include determining the position of a metrology target (or targets) and / or other target in a layer of a semiconductor device structure. This position is typically determined by irradiating a metrology target with radiation, and comparing characteristics of different diffraction orders of radiation reflected from the metrology target. Such techniques are used to measure overlay, alignment, and / or other parameters.

[0032] 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, alignment, etc., in a semiconductor device manufacturing process, for example, or for other operations.

[0033] Although specific reference may be made in this text to the manufacture of ICs, it should be explicitly understood that the description herein has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilledartisan 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.

[0034] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g., having a wavelength in the range of about 5- 100 nm).

[0035] In the present document, the term “wavelength” is used to encompass a wavelength range. For example, an emitted (or diffracted, transmitted, detected, etc.) wavelength may encompass a range of wavelengths, such as with a non-zero bandwidth (or full width half max (FWHM)). A wavelength may have a narrow wavelength range, such as 1 nm, 0.5 nm, 30 femtometers (fm), etc., and may be produced by a coherent source. A wavelength range may be centered on or otherwise include a representative wavelength. A wavelength range may be a closed or open interval.

[0036] A (e.g., semiconductor) patterning device can comprise, or can form, one or more patterns. The pattern can be generated utilizing CAD (computer-aided design) programs, based on a pattern or design layout, this process often being referred to as EDA (electronic design automation). Most CAD programs follow a set of predetermined design rules in order to create functional design layouts / patterning devices. These rules are set by processing and design limitations. For example, design rules define the space tolerance between devices (such as gates, capacitors, etc.) or interconnect lines, so as to ensure that the devices or lines do not interact with one another in an undesirable way. The design rules may include and / or specify specific parameters, limits on and / or ranges for parameters, and / or other information. One or more of the design rule limitations and / or parameters may be referred to as a “critical dimension” (CD). A critical dimension of a device can be defined as the smallest width of a line or hole or the smallest space between two lines or two holes, or other features. Thus, the CD determines the overall size and density of the designed device. One of the goals in device fabrication is to faithfully reproduce the original design intent on the substrate (via the patterning device).

[0037] The term “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic semiconductor patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective; binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0038] As used herein, the term “patterning process” generally means a process that creates an etched substrate by the application of specified patterns of light as part of a lithography process. However, “patterning process” can also include (e.g., plasma) etching, as many of the features described herein can provide benefits to forming printed patterns using etch (e.g., plasma) processing.

[0039] As used herein, the term “pattern” means an idealized pattern that is to be etched on a substrate (e.g., wafer) - e.g., based on the design layout described above. A pattern may comprise, for example, various shape(s), arrangement(s) of features, contour(s), etc.

[0040] As used herein, a “printed pattern” means the physical pattern on a substrate that was etched based on a target pattern. The printed pattern can include, for example, troughs, channels, depressions, edges, or other two- and three-dimensional features resulting from a lithography process.

[0041] A patterning system may be a system comprising any or all of the components described above, plus other components configured to performing any or all of the operations associated with these components. A patterning system may include a lithographic projection apparatus, a scanner, systems configured to apply and / or remove resist, etching systems, and / or other systems, for example.

[0042] As used herein, the term “diffraction” refers to the behavior of a beam of light or other electromagnetic radiation when encountering an aperture or series of apertures, including a periodic structure or grating. “Diffraction” can include both constructive and destructive interference, including scattering effects and interferometry. As used herein, a “grating” is a periodic structure, which can be one-dimensional (i.e., comprised of posts of dots), two-dimensional, or three-dimensional, and which causes optical interference, scattering, or diffraction. A “grating” can be a diffraction grating.

[0043] As used throughout this application “or”, unless indicated otherwise, takes a non-exclusive meaning, e.g., encompassing both “and” and “or”. “Each”, “every”, “all”, “corresponding”, “individual” and other relational terms encompass substantially “each”, “every”, “all”, etc., including cases in which each, every, all, corresponding, individual, etc. may include relationship which are not one-to-one, or do not include every possible item. For example, every may exclude items, such as items determined to be defective during testing, reference items, etc. All may exclude items, such as excess items. Corresponding may not require that items correspond in an exactly one to one manner. For example, a first item may correspond to two of a second item or vice versa. In some cases, individual may refer to multiple of an item, such as each item A has individual item B, where an item A may have two of an item B. Values should further be taken to include ranges, such as ±10%. Ranges should be taken to include endpoints.

[0044] Figure 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 projectionsystem 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).

[0045] The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising for example suitable directing mirrors and / or a beam expander. In other cases, the source may be an integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0046] 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.

[0047] The illuminator IL may comprise adjuster AD configured to adjust the (angular I 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.

[0048] 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 pupilplane of the illuminator IL. The polarization state of the radiation may be chosen in dependence on the illumination mode. For multi-pole illumination modes, the polarization of each pole of the radiation beam may be generally perpendicular to the position vector of that pole in the pupil plane of the illuminator IL. For example, for a dipole illumination mode, the radiation may be linearly polarized in a direction that is substantially perpendicular to a line that bisects the two opposing sectors of the dipole. The radiation beam may be polarized in one of two different orthogonal directions, which may be referred to as X-polarized and Y-polarized states. For a quadrupole illumination mode, the radiation in the sector of each pole may be linearly polarized in a direction that is substantially perpendicular to a line that bisects that sector. This polarization mode may be referred to as XY polarization. Similarly, for a hexapole illumination mode the radiation in the sector of each pole may be linearly polarized in a direction that is substantially perpendicular to a line that bisects that sector. This polarization mode may be referred to as TE polarization.

[0049] 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, 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.

[0050] 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” herein may be considered synonymous with the more general term “patterning device.”

[0051] The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a pattern in a target portion of the substrate. In an embodiment, a patterning device is any device that can be used to impart a radiation beam with a pattern in its crosssection to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in a target portion of the device, such as an integrated circuit.

[0052] A patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift,as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.

[0053] 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” herein may be considered as synonymous with the more general term “projection system.”

[0054] 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.

[0055] The lithographic apparatus may be of a type having two (dual stage) or more tables (e.g., two or more substrate 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.

[0056] In operation of the lithographic apparatus LA, 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 Figure 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.

[0057] The depicted apparatus may be used in at least one of the following modes. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while a pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and / or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT may be determined by the (de-) magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. In another mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed, and the programmable patterning device is updated as required after each movement of thesubstrate 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.

[0058] Combinations and / or variations on the above-described modes of use or entirely different modes of use may also be employed.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] As shown in Figure 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 chill plates CH and / or one or more bake plates BK. A substrate handler, or robot, RO picks up one or more substrates from input / output port I / Ol , I / O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus. These apparatuses, which are often collectively referred to as the track, are under the control of a track control unit TCU which is itself controlled by the supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.

[0063] 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 desirableto inspect a substrate or other object to measure or determine one or more properties such as alignment, overlay (which can be, for example, between structures in overlying layers or between structures in a same layer that have been provided separately to the layer by, for example, a double patterning process), line thickness, critical dimension (CD), focus offset, a material property, etc. Accordingly, a manufacturing facility in which lithocell LC is located also typically includes a metrology system that measures some or all of the substrates W (Figure 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 (Figure 1)).

[0064] The one or more measured parameters may include, for example, alignment, overlay between successive layers formed in or on the patterned substrate, critical dimension (CD) (e.g., critical linewidth) of, for example, features formed in or on the patterned substrate, focus or focus error of an optical lithography step, dose or dose error of an optical lithography step, optical aberrations of an optical lithography step, etc. This measurement is often performed on one or more dedicated metrology targets provided on the substrate. The measurement can be performed after-development of a resist but before etching, after-etching, after deposition, and / or at other times.

[0065] 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 metrology target such as a periodic grating).

[0066] Thus, in a device fabrication process (e.g., a patterning process or a lithography process), a substrate or other objects may be subjected to various types of measurement during or after the process. The measurement may determine whether a particular substrate is defective, may establish adjustments to the process and apparatuses used in the process (e.g., aligning two layers on the substrate or aligning the patterning device to the substrate), may measure the performance of the process and the apparatuses, or may be for other purposes. Examples of measurement include optical imaging (e.g., optical microscope), non-imaging optical measurement (e.g., measurement based on diffraction such as the ASML YieldStar metrology tool, the ASML SMASH metrology system), mechanical measurement (e.g., profiling using a stylus, atomic force microscopy (AFM)), and / or non-optical imaging (e.g., scanning electron microscopy (SEM)).

[0067] 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 ifthe 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.

[0068] 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.

[0069] To enable the metrology, often one or more metrology targets (or metrology marks) are specifically provided on the substrate. Typically, the target is specially designed and may comprise a periodic structure. For example, the metrology target on a substrate such as a semiconductor wafer 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 metrology 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).

[0070] Figure 3 depicts an example metrology system 10 that may be used to detect overlay, alignment, and / or perform other metrology operations. It comprises a radiation source 2 which projects or otherwise irradiates radiation onto a substrate W such as a semiconductor wafer (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 Figure 4. The detector may generate a metrology detection signal conveying metrology data indicative of properties of the reflected radiation. From this data, the structure or profile giving rise to the detected spectrum may be reconstructed by one or more processors PRO, a generalized example of which is shown in Figure 3.

[0071] As in the lithographic apparatus LA in Figure 1, one or more substrate tables (not shown in Figure 3 or 4) may be provided to hold the substrate W during measurement operations. The one or more substrate tables may be similar or identical in form to the substrate table WT (WTa or WTb or both) of Figure 1. In an example where metrology system 10 is integrated with the lithographic apparatus, they may 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, including continuously during metrology. 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).

[0072] For typical metrology measurements, a metrology 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. Surfaces of the features, e.g., bars, resist, fill layers, buried layers, etc., may be uneven (e.g., tilted) with respect to the Z direction.

[0073] 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., 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, including in layers which support or cover 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.

[0074] 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 the radiation, a wavelength of the radiation, a pupil size and / or shape, a resist material, and / or other process parameters.

[0075] Figure 5A depicts a schematic representation of an example aberration causing diffractionbased metrology errors. Figure 5A is a schematic cross-sectional view of a resist layer R titled at anangle 9tin the measurement direction (X direction) with respect to the target 30. If the refractive index of the resist is N and the average resist thickness is D, then the angular deviation d0 of the zeroth order reflection light Io from the perpendicularly incident light is expressed as follows: d9 = c (1)This angular change in the zeroth order diffraction may detected by a detector (such as the spectrometer detector 4 of Figure 3) and produce a deviation in a measurement of the target 30 (e.g., in position, overlay, etc.) in the X-axis direction by dx from the position Xo (including the imaging magnification from the angular deviation dO on the wafer W surface).

[0076] For the first order (and analogously higher) diffractions, the diffraction angles relative to the incident light (e.g., the light incident normal to the target 30 but refracted by the tilted resist R) are symmetrical and given by: a+1= sin-1A / P (2) aq= sin-12 / P (3) where P is the periodicity of the grating, and X is the wavelength of the light incident on the grating. Then the apparent diffraction angles, after further refraction by the tilted resist R surface, are given by ( sin(20 — (sin-10 / IV)))) (4)( sin(20 — (sin-10 / IV)))) (5)However, for first order (and analogously higher) diffractions relative to the target 30, the first order diffractions the diffraction angles normal to the wafer W are given byThe detected locations of the reflection (e.g., zeroth order diffraction), first order diffraction, and higher order diffractions (e.g., second order and higher diffractions) are therefore offset from their expected locations based on the tilt 0, the refractive index of the tilted resist R, and a distance of a detector (such the spectrometer detector of 4 of Figure 3) from the target 30.

[0077] In some embodiments, diffractions may be captured and combined (e.g., through intensity accumulation) with other diffractions, such as through the use of a self-referencing interferometer (SRI). A self-referencing interferometer (SRI) may obtain a reference signal (e.g., optical signal containingdiffractions) and generate a transformed signal from at least part of the reference signal, and then combine the reference and transformed signals to generate a signal reflecting a spatial overlap between the reference signal and the transformed signal — from which metrology characteristics (e.g., position, overlay, etc.) may be determined. The transformation may be a reflection, rotation (including an angular rotation), etc. of the reference signal. In some embodiments, the transformation may be a rotation by 180° (e.g., n radians) of the reference signal, such that for an on-axis diffraction, a positive diffraction of order n of a reference signal overlaps with a negative diffraction of order n of a transformed signal, and likewise a negative diffraction of the order n of the reference signal overlaps with a positive diffraction of the order n from the transformed signal. An aberration, such as caused by a tilted resist or any other sample layer or by non-idealities in the metrology system capturing the reference signal and / or generating the transformed signal, may cause deviations in the reference signal and / or transformed signal from the expected value, including deviations may be non-symmetrical. For example, a tilted resist, as depicted in Figure 5A, may shift a positive diffraction and a negative diffraction in the same or different directions, including by the same or different angles and / or distances. In some embodiments, an aberration may cause mismeasurement of a metrology parameter, such as position, overlay, etc. by causing changes to the intensity and / or position of a diffraction, from which metrology parameters are determined.

[0078] Figure 5B depicts a schematic representation of an effect of an example aberration on detection of diffractions. Figure is a plan view of example diffraction spots on a two-dimensional detector 500. Expected positions of a through beam (e.g. , 0thorder diffraction, reflected beam, etc.), positive nth order diffraction, and negative nth order diffraction are depicted as hollow circles 510, 512, and 514, respectively. The expected positions of circles 510, 512, and 514, may be determined based on the diffraction order, the wavelength of radiation subject to diffraction, the periodicity of any diffraction grating of the target (e.g., target 30 of Figure 5A), distance from the target to the two-dimensional detector, etc. Measured positions of the through beam (e.g., 0thorder diffraction, reflected beam, etc.), positive nth order diffraction, and negative nth order diffraction are depicted as filled circles 520, 522, and 524, respectively. The measured positions of the circles 520, 522, and 524 may differ from their expected positions (e.g., the positions of the circles 510, 512, and 514) due to resist tilt, metrology system aberrations (for example, lens aberrations), and / or due to target non-ideality. For example, the measured positions of the circles 520, 522, and 524 may be shifted in both the positive X and positive Y direction, from their expected positions, due to tilt in a resist layer in both the X and Y directions. In another example, the measured positions of the circles 520, 522, and 524, may be shifted in different amounts — such as if the periodicity P of the target is different than expected. In such an example, the through beam (e.g., the measure position of the circle 520) may coincide with the expected position shown by the circle 510, while the positive nth diffraction shown by circle 522 may lie outside (in the positive X direction) of the expected position shown by the circle 512 while the negative nth diffraction shown by circle 524 may lie outside (in the negative X direction) of the expected position shown by thecircle 514. In cases where target or measurement system non-ideality is present, it may be hard to deconvolve target metrology (e.g., overlay, position, etc.) from non-ideality.

[0079] For systems using SRI, the generation of a transformed signals from the reference signals may increase (for example, double or more) the amount of non-ideality captured by the metrology system which requires correction or compensation in order to obtain accurate metrology measurements. For example, for a system using SRI, transformed signals may be generated by rotating measured signals by 180°. Example transformed signals, which represent transformed measured positions of various diffractions, for the through beam (e.g., 0thorder diffraction, reflected beam, etc.), positive nth order diffraction, and negative nth order diffraction are depicted as filled circles 530, 532, and 534, respectively. In some cases, resist tilt (or other non-idealities) may be enough to cause the reference signals and transformed signals to occur in non-overlapping locations — such as depicted in Figure 5B where the transformed signals are shifted in the negative X and negative Y direction for circles 530, 532, and 534. Some systems which use an SRI may determine metrology results based on the overlap, sum, union, etc. of the reference signals and transformed signals. Resist tilt and other non-idealities may separate the reference signals and transformed signals, such as at a detector, which may cause the overlap of such signals to be reduced or even null (e.g., for disjoint signals). This may reduce the ability of the metrology system to determine metrology measurements, and reduce the accuracy of any measurements determined. Additionally, a shift in the location of the through beam — such as shown where circle 520 (and circle 530) are not coincidental with circle 510 — can cause the through beam, which may be of significantly greater intensity — to bleed into a detector field (e.g., to no longer be blocked by a through beam blocker) and cause detector saturation or other effects which may reduce the detected intensity of first or higher order diffractions. For these and other reasons, metrology based on diffraction intensity alone (including intensity summations through SRI) may be less accurate and have a smaller depth of modulation that desired and / or required.

[0080] Figure 6 depicts a schematic representation of an example diffraction-based metrology system. Figure 6 depicts a schematic view of metrology system 600, showing beam paths within the metrology system 600. Note that system 600 is just one representative example of several different possible types of systems (which may or may not have some or all of the same components and / or may function in slightly different ways) that may provide diffraction-based metrology. Examples of such systems include ASML’s SMASH, AURORA, YieldStar system and / or other systems. System 600 is the same as or similar to system 10 described above with respect to Figure 3, with one or more components of system 600 being similar to and / or the same as one or more components of system 10 (and with Figure 6 illustrating several additional possible components of the system). In some embodiments, one or more components of system 600 may replace, be used with, and / or otherwise augment one or more components of system 10. System 600 comprises one or more radiation sources 610A-610D (e.g., similar to and / or the same as source 2 shown in Figure 3), one or more detectors 650A and / or 650B (e.g., similar to and / or the same as detector 4 shown in Figure 3), one or more processors PRO (similarto and / or the same as processor PRO shown in Figure 3), and various lenses (e.g., lenses 620A-620D), beam splitters (e.g., beam splitters 640 and 641), mirrors (e.g., mirrors 630A and 630B), apertures (e.g., aperture AP), interferometers (e.g., interferometer INF), refractive or diffractive components, and / or other components which comprise optical elements of the system 600. One or more processors PRO are operatively connected with detector 650A, detector 650B, and / or other components of system 600.

[0081] Figure 6 illustrates an illumination branch IL of system 600 including radiation sources 610A- 610D, a zeroth order ZO path including a detector 650A, a higher order HO path including a detector 650B and one or more processors PRO. The illumination from the illumination branch IL of the system 600 reaches a beamsplitter 640 and is directed at the target 30 (such as through the objective lens 620B). When the illumination arrives at the target 30, at least a portion of the illumination is diffracted by the target, such as in region 642. The diffraction may be on-axis (that is, symmetrical about an axis normal to the target 30) diffraction, or off-axis diffraction (that is, asymmetrical about an axis normal to the target 30). The diffraction may be zeroth order diffraction — e.g., reflection. The diffraction may be first order diffraction. The diffraction may be higher order diffraction (e.g., higher order than first order diffraction — that is, second or higher order diffraction). The diffraction may be diffraction of multiple different wavelengths and / or a spectrum of wavelengths simultaneously, sequentially, etc. The diffractions may include positive and negative diffractions of the same order. The diffractions may include diffractions from directionally polarized light (for example, transverse electric (TE), transverse magnetic (TM), etc. polarized light). The diffracted illumination may be passed back through the objective lens 620B (or any other appropriate optical elements) and pass through the beamsplitter 640. The beamsplitter 640 may selectively pass diffracted light through to the interferometer INF, by reflecting incoming illumination from the illumination branch IL which arrives in region 644 and by transmitting incoming illumination from the target 30 which arrives in region 644. In some embodiments, the region 644 may treat different orders of diffraction differently, such as by blocking a through beam or other operations. The interferometer INF may be a SRI or any other appropriate interferometer. The interferometer may separate various diffraction orders and / or wavelengths or may operate on different diffraction orders and / or wavelengths in the same manner.

[0082] System 600 is a diffraction-based metrology system, where diffraction The system 600 may be configured to determine one or more metrology measurement, such as position, alignment, overlay, etc., based on the target 30. In some embodiments, the components of system 600 may form a portion of an overlay and / or alignment sensor configured to be used in a semiconductor manufacturing process.

[0083] Figure 6 also illustrates a metrology target 30 which may comprise one or more metrology marks, such as diffraction grating targets, formed in a substrate W such as a semiconductor wafer. Target 30 may comprise one or more structures in the patterned substrate capable of providing a diffraction signal. One or more targets 30 may be included in a layer of a substrate in a semiconductor device structure, for example. In some embodiments, target 30 comprises a geometric feature such as a ID or 2D feature, and / or other geometric features. By way of several non-limiting examples, thefeature may comprise a grating, a line, an edge, a fine-pitched series of lines and / or edges, and / or other features.

[0084] Various lenses (example objective lens 620B is labeled in Figure 6), reflectors, mirrors, refractive or diffractive optical components, and other optical elements are configured to receive, transmit, reflect, focus, and / or perform other operations on the illumination generated by illumination sources 610A-610D, delivered to the target 30, received by the zeroth order ZO and / or higher order HO paths, and / or used by other portions of system 600. These various lenses, reflectors, and / or other optical components may comprise optical elements. The optical elements are configured for directing, shaping, focusing, or otherwise controlling the projection beam of radiation, collectively or singularly. They may include any type of lens, reflector, and / or other optical component configured to allow system 600 to function as described. For example, objective lens 620B may include one or more lenses formed from any transparent material and have curved surfaces configured to concentrate or otherwise focus one or more spots of radiation on target(s) 30. The various lenses, reflectors, optical elements, beam splitters, and other optical elements may be positioned in any location and / or at any angle relative to each other that allows system 600 to function as described herein. This may include positioning at specific relative distances between elements, specific angles between elements, etc. In some embodiments, the various lenses, reflectors, optical elements, beam splitters, and other optical components are positioned relative to each other in system 600 via structural members, clips, clamps, screws, nuts, bolts, adhesive, and / or other mechanical devices. In some embodiments, various ones of the lenses, reflectors, optical elements, beam splitters, and other optical elements are movable relative to each other. Movement may be configured to adjust locations of corresponding spots of illumination on one or more targets 30, for example. In some embodiments, movement comprises tilting, translating or otherwise changing a distance between various lenses, reflectors, and other optical components. In some embodiments, movement may encompass scanning, for example, of the measurement system 600 relative to the wafer W and / or various targets 30 on the surface of the wafer W, such as in the direction 670. Other examples of movement are contemplated.

[0085] In some embodiments, movement may be controlled electronically by a processor, such as processor PRO. Processor PRO may be included in a computing system CS (e.g., as will be described further in relation to Figure 11) and may operate based on computer or machine -readable instructions (e.g., as described below related to Figure 11). Electronic communication may occur by transmitting electronic signals between separate components, transmitting data between separate components of system 600, transmitting values between separate components, and / or other communication. The components of system 600 may communicate via wires or wirelessly via a network, such as the Internet or the Internet in combination with various other networks, like local area networks, cellular networks, or personal area networks, internal organizational networks, and / or other networks.

[0086] In some embodiments, one or more actuators (not shown in Figure 6) may be coupled to and configured to move one or more components of system 600 and / or the wafer W. The actuators may becoupled to one or components of system 600 by adhesive, clips, clamps, screws, a collar, and / or other mechanisms. The actuators may be configured to be controlled electronically. Individual actuators may be configured to convert an electrical signal into mechanical displacement. The mechanical displacement is configured to move a component of system 600. As an example, one or more of the actuators may be piezoelectric. One or more processors PRO may be configured to control the actuators. One or more processors PRO may be configured to individually control each of the one or more actuators.

[0087] The quantity of the various lenses, reflectors, and / or other optical elements shown in Figure 6 is not intended to be limiting. The principles described herein may be extended such that, in some embodiments system 600 comprises additional or fewer lenses, reflectors, and / or other optical elements.

[0088] Radiation sources 610A-610D are configured to generate radiation. Although four radiation sources are depicted, more or fewer (for example, one radiation source, 12 radiation sources, etc.) may be used. The generated radiation may be in the form of an incident radiation beam (noting that an incident radiation beam described herein may simply be any radiation beam that is incident on some optical element of system 600, for example). The radiation may comprise illumination such as light and / or other radiation. In some embodiments, each individual radiation source may produce a wavelength (or range of wavelengths) or light. In some embodiments, the radiation from radiation sources 610A-610D may comprise a Gaussian radiation beam and / or other radiation. The radiation may have a target wavelength and / or wavelength range, a target intensity, and / or other characteristics. The target wavelength and / or wavelength range, the target intensity, etc., may be entered and / or selected by a user, determined by system 600 based on previous measurements, and / or determined in other ways. In some embodiments, the wavelength may be fixed based on characteristics of the radiation source (e.g., by lasing material). In some embodiments, the light comprises visible light, infrared light, near infrared light, and / or other light. In some embodiments, the radiation may be any radiation appropriate for interferometry. The radiation emitted by the radiation sources 610A-610D may be multiplexed by multiplexer MUX, such that the radiation may travel along the same optical fiber, path, etc. The radiation emitted by the radiation sources 610A-610D may be non-interfering with the radiation of other of the sources.

[0089] In operation, system 600 is configured such that an optical element is configured to receive and transmit an incident radiation beam. In this description of Figure 6, an optical element may be any of the elements depicted and described, or any additional instances of such elements. As described above, optical element may comprise a lens, aperture, a beam splitter, a mirror, a refractive or diffractive optical component, and / or other optical elements. The optical element may cause an aberration in the incident radiation beam — in addition to or instead of any aberration caused by variations in the target 30 (such as by tilted resist) as previously described. The aberration may comprise an undesired change in a target characteristic of the incident radiation beam. The target characteristic may be angle, amplitude, phase / wavefront, polarization, focus position, focal spot quality / uniformity, and / or other characteristics.

[0090] Figure 7A depicts a schematic representation of an example off-axis holography system working in concert with a diffraction-based metrology system. Figure 7 A depicts a schematic view of metrology system 700, showing beam paths within the metrology system 700 which is configured for holography. The system 700 is depicted and described with reference to the elements of metrology system 600 of Figure 6, but can instead or additionally be used with any appropriate diffraction based metrology system.

[0091] In addition to the elements described in reference to Figure 6 (or to the elements of any other appropriate diffraction-based metrology system), system 700 comprises an illumination source 710. The illumination source 710 may be any appropriate illumination source, such as a laser, which produces a relatively narrow bandwidth beam of radiation. The illumination source 710 may be connected to one or more optical fiber. Alternatively or additionally, the illumination of the illumination source 710 may be transmitted, such as through atmosphere, by any appropriate optical elements. The illumination of the illumination source 710 is split into at least two beams by a beam splitter 740. The beam splitter 740 may be any appropriate beam splitter which preservers spatial coherence of the illumination of path A and path B. Path A and path B are so labeled only for ease of description, and, as such, the labeling does not imply any relative strength, importance, ordering, etc. between the paths of the illumination of the illumination source 710. In some embodiments, the illumination of path A may be dispersed by a dispersion element DIS. In some embodiments, the dispersion element DIS may be an optical fiber termination, such that the illumination may exit an optical fiber and be diffracted by the target 30. The dispersion element DIS may be any appropriate dispersion element and / or aperture which may cause spatial coherence of the illumination of path A. The illumination of path A is diffracted by the target 30 on the wafer W. The illumination of path A may be off-axis illumination and may be asymmetrically (with respect to an axis normal to the target 30) diffracted by the target 30. The diffraction of the illumination of path A may occur in the same region as the diffraction of the illumination from the elements previously described for system 600, such as in region 742. The illumination of path A may have a different wavelength than the illumination of the radiation sources 610A-610D, and may therefore not constructively or destructively interfere, but rather be able to be multiplexed. One or more diffractions of the illumination of path A may exit the target, such as along path 746, and be captured by the objective lens 620B (including along with the diffractions previously described in reference to Figure 6). The diffraction of the illumination of path A may then be reflected, such as out of the beam path, by the beam splitter 640. In the region 744, the diffraction of the illumination of path A may be directed towards a detector DET, which may be different than the detectors 650A and / or 650B. The path 746 is depicted as passing through the mirror 630A, but may additionally or instead be routed around any mirrors or optical elements.

[0092] The illumination of path B may bypass the target 30. From the beam splitter 740, the illumination of path B (which may be a reference signal and / or analogous to a reference signal) may not interact with the target 30 — either through reflection, diffraction, refraction, etc. The illuminationof path B may be dispersed by a dispersion element DIS. In some embodiments, the dispersion element DIS may be an optical fiber termination, such that the illumination may exit an optical fiber and be received at the detector DET. The dispersion element DIS may be any appropriate dispersion element and / or aperture which may cause spatial coherence of the illumination of path B. At the detector DET, the illumination of path B and the illumination of path 746 may be recombined. The detector DET may be a two-dimensional detector. The detector DET may comprise one or more pixels. The detector DET may be a multi-color detector (e.g., may detect wavelength as well as intensity), may have a color filter such as to select for a diffraction wavelength, may be an intensity detector (e.g., may not detect color), or any other appropriate detector. The detector DET may detect a recombination of the illumination of path B and the illumination of path 746, which may constructively and destructively interfere. At the detector DET, the illumination of path B and the illumination of path 746 may be spatially coherent, such as to withing a coherence length. Path B, path A, and path 746 may comprise one or more element which may adjust path length, such as to cause spatial coherence of the illumination of path B and the illumination of path 746. The illumination of path B and the illumination of path 746 may interfer to generate a hologram on the detector DET. The interference pattern on the detector DET may be recorded by a camera CAM and sent to a processor PRO. The processor PRO, which may be the same or a different processors PRO that than of system 600, may use the recorded interference pattern (e.g., hologram) to recreate a wavefront of the diffracted illumination of path A, which may contain information about the target 30, such as tilt angle, spot size, etc.

[0093] Figure 7B depicts a schematic representation of off-axis illumination for diffraction-based holography. Figure 7B depicts a schematic view of the diffraction in the region 742 of Figure 7A. The illumination of path A approaches the target 30 at an incident angle,which is less than 90° for off- axis illumination. The illumination of path A is diffracted by the target 30 at a diffraction angle, 9d. In some embodiments, multiple diffractions of the illumination of path A are captured — such as first order diffractions, higher order diffraction, positive and negative diffractions of the same or different orders, etc. One or more diffraction of the illumination of path A is captured by the objective lens 620B and directed to the beam splitter 640. The path of the diffraction of the illumination of path A is depicted as path 746. However, it should be understood that multiple diffractions may occur and may travel along different paths towards the detector DET. At the beam splitter 640, the illumination of path 746 is selected and directed towards the detector DET, where it is combined with the illumination of path B, which bypasses the target. The illumination of path 746 and the illumination of path B are spatially coherence and constructively and destructively interfere to generate interference pattern IFP at the detector DET. From the interference pattern IFP, the wavefront on the diffraction of the illumination of path A may be reconstructed, such as by application of a Fourier transform or any other appropriate transform.

[0094] The illumination of paths A, B, and 746 does not interfere with the illumination of the system 600. In some embodiments, the illumination of the radiation source 710 is a difference wavelength thanthe illumination of the radiation sources 610A-610D. In Figure 7B, a single radiation source (e.g., radiation source 610) is depicted. The illumination generated by the source 610 is diffracted by the target 30, to generate substantially on-axis diffractions — shown as positive first order diffraction 610- 1A, positive second order diffraction 610-2A, negative first order diffraction 610-1B, and negative second order diffraction 610-2B. These diffractions are shown for illustrative purposes only, and more or fewer diffractions, including diffractions of multiple wavelengths or wavelength ranges may be used by the system 600. In some embodiments, the source 610 may generate off-axis diffractions or any other diffraction from which metrology measurements may be determined.

[0095] Figure 8A depicts a schematic representation of an example zeroth order holography system working in concert with a diffraction-based metrology system. Figure 8A depicts a schematic view of metrology system 800, showing beam paths within the metrology system 800 which is configured for holography. The system 800 is depicted and described with reference to the elements of metrology system 600 of Figure 6, but can instead or additionally be used with any appropriate diffraction based metrology system.

[0096] In addition to or instead of the elements described in reference to Figure 6 (or to the elements of any other appropriate diffraction-based metrology system), system 800 comprises a beam splitter 840 which selects at least a portion of a diffraction from the target 30 and at least a portion of a through beam from the target 30 for interference at a detector DET. The beam splitter 840 selects from a positive first order diffraction, negative first order diffraction, positive second order diffraction, negative second order diffraction, etc., at least a portion of the illumination diffracted by the target, which is depicted as the illumination of path 846. The beam splitter 840 may select 10%, 20%, 50%, or more or less of the illumination for diversion from the system 600 to the detector DET. The beam splitter 840 may select multiple diffractions. The beam splitter 840 also selects at least a portion of the through beam (e.g., the zeroth order diffraction) for diversion from the system 600 to the detector DET, which is depicted as the illumination of path 848. The illumination of path 846 and the illumination of path 848 are directed, such as by lens 820E or any other appropriate optical element, to interfere where the interference pattern is detected by detector DET and camera CAM, as previously described for system 700. Path 846 and path 848 may include one or more dispersion elements, one or more optical path length adjusters, etc., such that the illumination of path 846 and the illumination of path 848 are spatially coherent at their interference, at least to within an optical path length.

[0097] Figure 8B depicts a schematic representation of on-axis illumination for diffraction-based holography. Figure 8B depicts a schematic view of the diffraction in the region 842 of Figure 8A and beam splitting in the region 844 of Figure 8 A. The illumination of paths 846 and 848 are selected from the illumination of the system 600. In Figure 8B, a single radiation source (e.g., radiation source 610- A) is depicted. The illumination generated by the source 610- A is diffracted by the target 30, to generate substantially on-axis diffractions — shown as a zeroth order diffraction 610-0, a positive first order diffraction 610-1A, positive second order diffraction 610-2A, negative first order diffraction 610-1B,and negative second order diffraction 610-2B. These specific diffractions are shown for illustrative purposes, and more or fewer diffractions, including diffractions of multiple wavelengths or wavelength ranges may be used by the system 600. In some embodiments, the source 610-A may generate off-axis diffractions or any other diffraction from which metrology measurements may be determined.

[0098] From the diffractions of system 600, the system 800 selects a portion for diversion to the detector DET. For example, the beam splitter 840 selects a portion of the zeroth order diffraction (e.g., diffraction 610-0) for diversion to the detector DET along path 848. Likewise, the beam splitter selects a portion of the positive first order diffraction 610-1 A for diversion to the detector DET along path 846. The illumination of path 846 and the illumination of path 848 are spatially coherence and constructively and destructively interfere to generate interference pattern IFP at the detector DET. From the interference pattern IFP, the wavefront on the diffraction of the illumination of path A may be reconstructed, such as by application of a Fourier transform or any other appropriate transform.

[0099] The portion of the diffraction which is diverted to the detector DET may be limited by the diffraction intensity. In some embodiments, a portion of a diffraction may be intermittently diverted to the detector DET, and once a hologram is analyzed the diverted portion of the diffraction may be reduced such that the system 600 may operate on substantially the full diffraction strength.

[0100] Figure 9 depicts a schematic representation of an example technique for improvement of diffraction-based metrology with holography. Figure 9 is a schematic representation of digital holography microscopy (DHM) in conjunction with diffraction-based metrology. A diffraction target 30, which may have one or more tilted layers and / or other non-ideality (as previously described), is subject to illumination which generates one or more diffraction. One or more of the generated diffractions are interfered with a known (e.g., reference) signal, which may be a zeroth order diffraction, a reference signal which bypasses the target 30, etc. at an interference operation 910. The interference generates an interference pattern IFP, which is detected by a two-dimensional detector. The interference pattern IFP is subject to a transform 920 into frequency space, such as a Fourier transform, a fast Fourier transform (FFT), etc. to generate a spectrum 930 in frequency space. From the spectrum 930, various portions of the real and imaginary wavefunction may be selected and subject to another transform 940 to return from frequency space, such as a Fourier transform, an FFT, etc., to reconstruct the amplitude AMP and phase PH of the wavefront diffracted by the target 30. From the amplitude AMP and phase PH, various metrology parameters MET may be reconstructed, such as tilt angle, position, overlay, etc. In some embodiments, the amplitude AMP and phase PH may be used to determine parameter and detect non-idealities in the measurement system itself, such as lens aberration, pupil metrology, wafer mark aberrations, etc.

[0101] Figure 10 depicts a flowchart illustrating an exemplary method 1000 for diffraction-based holography. Each of these operations is described in detail below. The operations of method 1000 presented below are intended to be illustrative. In some embodiments, method 1000 may be accomplished with one or more additional operations not described, and / or without one or more of theoperations discussed. Additionally, the order in which the operations of method 1000 are illustrated in Figure 10 and described below is not intended to be limiting. In some embodiments, one or more portions of method 1000 may be implemented (e.g., by simulation, modeling, etc.) in one or more processing devices (e.g., one or more processors). The one or more processing devices may include one or more devices executing some or all of the operations of method 1000 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 1000, for example.

[0102] At an operation 1002, an illumination beam is diffracted by a target. The illumination beam may be a narrow bandwidth beam of radiation, in any appropriate wavelength regime, such as visible, infrared, ultraviolet, optical, etc. The illumination beam may be emitted by a laser or any other appropriate narrow band source. In some embodiments, the illumination may be emitted by a broadband illumination source and filtered, e.g., spatially, by wavelength filters, etc., to become a more narrow band illumination.

[0103] The illumination may be diffracted by any appropriate target. The target may be a diffraction grating. The target may comprise multiple diffraction gratings. The target may be a point source emitter. The target may be a target designed for position determination. The target may be a multilayer target and / or a target designed for overlay determination. The target may be etched into a substrate, such as a semiconductor wafer. The target may be fabricated in one or more photoresist layer. The target may be covered by one or more layer or be exposed, at least in part, on the surface of a device. The target may have any appropriate periodicity. The diffraction angles and positions may be dependent on the target’s periodicity.

[0104] The illumination beam may be diffracted in any appropriate manner, such as on-axis (e.g., by illumination normal to the surface of the target), off-axis (e.g., by illumination substantially oblique to the surface of the target), etc. The illumination beam may be polarized and may be diffracted differently based on said polarization. The illumination beam may be diffracted into multiple diffraction orders. The illumination beam may be diffracted into both positive and negative diffraction orders.

[0105] At an operation 1004, the diffracted illumination is combined with a reference beam. The reference beam may be any appropriate reference beam, which is spatially coherent with the diffracted illumination of the illumination beam which produces the diffracted illumination. The reference beam may be emitted by the same source as the illumination beam, and split from the illumination beam by a beam splitter or other optical element. The reference beam may be an undiffracted portion of the illumination beam which produces the diffracted illumination. The reference beam may be made spatially coherent with the diffracted illumination by application of wavelength filtering, spatial filtering, etc.

[0106] The diffracted illumination and the reference beam may be combined through interference, including constructive and destructive interference. The diffracted illumination and the reference beammay be combined by any appropriate method which preserves phase information — e.g., not by intensity summation along. The diffracted illumination and the reference beam may be combined in a hologram. The diffracted illumination and the reference beam may be combined by methods associated with DHM.

[0107] At an operation 1006, the combination of the diffracted illumination and the reference beam is detected, such as by a two-dimensional detector. In some embodiments, the combination may be detected by a one-dimensional detector, such as a scanning one-dimensional detector. The detector may be a camera. The detector may be a screen on which light in incident, where the screen is filmed by a camera. The detector may detect individual photons. The detector may be a time lapse detector and / or a detector with an accumulation time. The detector may detect the hologram created by the combination of the diffracted illumination and the reference beam. The detector may be focusable or moveable such that the hologram may be focused on the detector. The detector may be a multi-color detector, which may be configured to detect holograms of different wavelengths at substantially the same time.

[0108] At an operation 1008, one or more characteristics of the target may be determined based on the detected combination. The characteristics may include position, overlay, tilt, etc. The characteristics may be determined by a processor based on the detected hologram. The characteristics may be determined by comparing the detected hologram to a library of pre-determined holograms for various arrangements of the target. The characteristics may be determined by calculating target characteristics from the detected hologram, such as through the use of one or more transform. The characteristics may be determined based on the measured intensity of the detected hologram. The characteristics may be determined based on a curve fitting to the measured intensity of the detected hologram. The detected hologram may be used to determine a wavefront for the diffracted illumination of the combination, and the characteristics may be determined based on the determined wavefront. The characteristics may include information about the measurement system, in addition to or instead of information about the target.

[0109] Once the characteristics of the target and / or system are determined, a correction may be applied to diffraction-based metrology. For example, if the resist is known to be tilted at a angle 0, then the overlap between the reference signals and transformed signals of the SRI may be adjusted to correct for the tilt angle (e.g., to increase the signal which is lost due to non-overlapping of the reference and transformed signals generated by the interferometer). In other embodiments, a wafer and / or target may be excluded from metrology, such as if the tilt angle is too large. In another embodiment, a pupil aberration may be determined and then used to correct measured diffraction locations, such as for position and overlay measurements.

[0110] As described above, method 1000 (and / or the other methods and systems described herein) is configured to determine target and / or system characteristics for a diffraction-based metrology system using digital microscopy holography.

[0111] Figure 11 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 communicationmechanism for communicating information, and a processor PRO (or multiple processors) 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.

[0112] Computer system CS may be coupled via bus BS to a display DS, such as a cathode ray tube (CRT) or flat panel or touch panel display 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.

[0113] In some embodiments, portions of one or more methods described herein may be performed by computer system CS in response to processor PRO executing one or more sequences of one or more instructions contained in main memory MM. Such instructions may be read into main memory MM from another computer-readable medium, such as storage device SD. Execution of the sequences of instructions included in main memory MM causes processor PRO to perform the process steps (operations) described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory MM. In some embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0114] The embodiments may further be described using the following clauses:1. A metrology system comprising: an illumination source, the illumination source configured to produce a beam of illumination; wherein at least a first portion of the beam is diffracted by a target and wherein at least a second portion of the beam is not diffracted by the target; a detector, the detector configured to detect a recombination of the first portion of the beam diffracted by the target and the second portion of the beam not diffracted by the target; anda processor operatively connected with the detector, the processor configured to determine a phase and / or amplitude of a waveform of the first portion of the beam after it has interacted with the target based on the recombination detected by the detector.2. The system of clause 1, the processor further configured to determine a tilt of a layer of the target based on the phase and / or amplitude of the waveform.3. The system of clause 2, the processor further configured to correct a determined location of the target based on the determined tilt.4. The system of clause 2, wherein the tilt arises from an uneven thickness of the layer, wherein the target comprises a diffraction grating and the layer comprises a process layer coating the diffraction grating.5. The system of any preceding clause, the processor further configured to determine a location of the target based on the phase and / or amplitude of the waveform.6. The system of any preceding clause the processor further configured to correct a determined location of the target based on the phase and / or amplitude of the waveform of the first portion of the beam after it has interacted with the target.7. The system of clause 6, wherein the determined location of the target comprises a location determined by the processor and / or another processor based on diffraction from the target detected by an additional detector.8. The system of clause 7, wherein the diffraction from the target is comprises multiple diffraction orders.9. The system of clause 7, wherein the diffraction from the target used to determine the location of the target has a different source than the first portion of the beam diffracted by the target.10. The system of clause 7, wherein the first portion of the beam diffracted by the target used to determine the location of the target has the same source as at least a subset of the diffraction from the target.11. The system of any preceding clause, wherein the first portion of the beam diffracted by the target passes through a first wavelength filter and / or slit filter and wherein the second portion of the beam not diffracted by the target passes through a second wavelength filter and / or slit filter.12. The system of clause 11 , wherein the first wavelength filter and / or slit filter is located in a path of the first portion of the beam after the diffraction and before the recombination.13. The system of clause 11, wherein the first wavelength filter and / or slit filter is the second wavelength filter and / or slit filter.14. The system of any preceding clause, wherein the recombination of the first portion of the beam diffracted by the target and the second portion of the beam not diffracted by the target comprises a digital hologram.15. The system of any preceding clause, wherein the first portion of the beam diffracted by the target and the second portion of the beam not diffracted by the target are substantially temporally coherent.16. The system of any preceding clause, wherein, at the recombination, the first portion of the beam and the second portion of the beam are within a coherence length of one another.17. The system of any preceding clause, wherein the second portion of the beam not diffracted by the target comprises a portion of the beam which bypasses the target.18. The system of clause 17, further comprising an upstream beam splitter, the upstream beam splitter configured to generate, from the beam of illumination, the first portion of the beam configured to diffract from the target and the second portion of the beam configured to bypass the target.19. The system of clause 18, wherein the upstream beam splitter is configured to split the beam of illumination before interaction with the target.20. The system of clause 17, wherein the first portion of the beam diffracted by the target comprises off-axis illumination.21. The system of any preceding clause, wherein the second portion of the beam not diffracted by the target comprises a portion of the beam reflected by the target.22. The system of clause 21, wherein the second portion of the beam comprises a zeroth order return from the target.23. The system of clause 21, wherein the second portion of the beam not diffracted by the target comprises an on-axis reflection resulting from illumination normal to the target.24. The system of clause 21 , wherein the first portion of the beam diffracted by the target comprises a diffracted beam resulting from illumination normal to the target.25. The system of clause 24, wherein the target is illuminated by incoherent illumination, the incoherent illumination giving rise to illumination diffracted by the target, and wherein the first portion of the beam diffracted by the target comprises a subset of the illumination diffracted by the target.26. The system of clause 25, wherein the first portion of the beam diffracted by the target comprises a subset of the illumination diffracted by the target selected by a downstream beam splitter.27. The system of clause 26, wherein the downstream beam splitter is configured to split the illumination diffracted by the target after interaction with the target.28. The system of any preceding clause, further comprising a recombiner configured to focus the first portion of the beam diffracted by the target and the second portion of the beam not diffracted by the target at a spot on the detector.29. The system of clause 28, wherein the recombiner comprises a lens and / or slit recombiner.30. The system of any preceding clause, wherein the detector comprises a camera.31. The system of any preceding clause, wherein the detector detects intensity of the recombination in two or more dimensions.32. The system of any preceding clause, wherein the detector is configured to detect intensity of illumination of substantially the same wavelength as the first portion of the beam diffracted by the target and the second portion of the beam not diffracted by the target.33. The system of any preceding clause, the processor further configured to determine a phase aberration in the waveform.34. The system of any preceding clause, the processor further configured to determine a beam and / or pupil distortion based on the determined phase and / or amplitude of the waveform.35. The system of any preceding clause, the processor further configured to determine a target variation based on the determined phase and / or amplitude of the waveform.36. The system of any preceding clause, wherein the first portion of the beam diffracted by the target comprises a first order diffraction.37. The system of any preceding clause, wherein the first portion of the beam diffracted by the target comprises a positive and negative first order diffraction.38. The system of any preceding clause, wherein the first portion of the beam diffracted by the target comprises multiple diffraction orders.39. A metrology system comprising: an illumination source, the illumination source configured to produce one or more wavelengths of illumination which are diffracted by a target; one or more detectors configured to detect locations of one or more diffraction orders of the illumination diffracted by the target; a beam splitter configured to select, for deflection to an additional detector, a subset of the one or more diffraction orders of the illumination; a beam conditioner to generate substantially temporally coherent illumination from the selected subset; the additional detector configured to detect a recombination of the selected subset of the substantially temporally coherent illumination and an additional substantially temporally coherent illumination; and a processor operatively connected with the one or more detectors and the additional detector, the processor configured to determine a location of the target based on the detected locations of the one or more diffraction orders as detected by the one or more detectors, the processor configured to determine a non-ideality in the determined location based on the recombination detected by the additional detector.40. The metrology system of clause 39, wherein the additionally substantially temporally coherent illumination comprises zeroth order illumination diffracted by the target.41. The metrology system of clause 39 or 40, wherein the beam conditioner generates substantially coherent illumination from a zeroth order illumination diffracted by the target.42. The metrology system of any clause 39 to 41, wherein the beam splitter selects a subset of illumination from a first order diffraction(s) of the target.43. The metrology system of any clause 39 to 42, wherein the beam splitter selects, as a subset, between approximately 5% and 50% from one or more diffraction order for diversion to the additional detector.44. The metrology system of any clause 39 to 43, wherein the processor is further configured to correct the determined location of the target based on the determined non-ideality.45. The metrology system of any clause 39 to 44, wherein the additional detector comprises a camera.46. The metrology system of any clause 39 to 45, wherein the one or more detectors comprise point and / or linear detectors.47. The metrology system of any clause 39 to 46, wherein the target and / or metrology system are scanned with respect to one another.48. A metrology system comprising: an illumination source, the illumination source configured to produce one or more wavelengths of illumination which are diffracted by a target; one or more detectors configured to detect locations of one or more diffraction orders of the illumination diffracted by the target; an additional illumination source, the additional illumination source configured to generate substantially coherent light, wherein a first portion of the substantially coherent light is diffracted by the target and wherein a second portion of the substantially coherent light is undiffracted by the target; an additional detector configured to detect a recombination of the first portion of the substantially coherent light diffracted by the target and the second portion of the substantially coherent light undiffracted by the target; and a processor operatively connected with the one or more detectors and the additional detector, the processor configured to determine a location of the target based on the detected locations of the one or more diffraction orders as detected by the one or more detectors, the processor configured to determine a non-ideality in the determined location based on the recombination detected by the additional detector.49. The metrology system of clause 48, further comprising a beam splitter configured to generate the first portion of the substantially coherent light diffracted by the target and the second portion of the substantially coherent light undiffracted by the target from the substantially coherent light of the additional illumination source.50. The metrology system of clause 48 or 49, wherein the second portion of the substantially coherent light undiffracted by the target comprises coherent light which bypasses the target.51. The metrology system of any clause 48 to 50, wherein the processor is further configured to correct the determined location of the target based on the determined non-ideality.52. The metrology system of any clause 48 to 51, wherein the additional detector comprises a camera.53. The metrology system of any clause 48 to 52, wherein the one or more detectors comprise point and / or linear detectors.54. The metrology system of any clause 48 to 53, wherein the target and / or metrology system are scanned with respect to one another.55. A method for metrology, comprising: illuminating a diffraction target with a first beam of illumination, the illumination generating at least one diffraction order; recombining the at least one diffraction order with a source of substantially coherent illumination; detecting the recombination by a two-dimensional detector; and determining, by a processor, a phase and / or amplitude of a waveform of the at least one diffraction order after it has interacted with the target based on the recombination detected by the two-dimensional detector.56. The method of clause 55, further comprising determining a tilt of a layer of the target based on the phase and / or amplitude of the waveform.57. The method of clause 55 or 56, wherein determining the phase and / or amplitude of the waveform comprises determining the phase and / or amplitude based on a frequency transform of intensity of the recombination detected by the two-dimensional detector.58. The method of clause 57, wherein the frequency transform comprises a fast Fourier transform (FFT).59. A method for metrology, comprising: illuminating a diffraction target with multiple wavelengths of illumination, the illumination generating multiple diffraction orders; detecting, by one or more detectors, locations of the multiple diffraction orders; determining, by a processor, a location of the diffraction target based on the detected location of the multiple diffraction orders; selecting, by a beam splitter, a portion of at least one diffraction order for deflection to an additional detector; recombining the selected portion of the at least one diffraction order with a substantially temporally coherent illumination source; detecting, by the additional detector, the recombination; determining, by the processor, a phase and / or amplitude of a waveform corresponding to the at least one diffraction order after it has interacted with the target based on the recombination detected by the additional detector; and correcting, by the processor, the determined location of the diffraction target based on the phase and / or amplitude of the waveform corresponding to the at least one diffraction order.60. The method of clause 59, wherein determining a phase and / or amplitude of a waveform of the at least one diffraction order after it has interacted with the target based on the recombination comprisingdetermining a non-ideality in at least one of the following: the diffraction target, the illumination, the one or more detectors, a pupil of the one or more detectors, and the additional detector.61. The method of clause 60, wherein correcting the determined location of the diffraction target comprises correcting subsequent determined locations of the diffraction target based on the determined non-ideality.62. The method of any clause 59 to 61, further comprising conditioning, by a conditioner, the selected portion of the at least one diffraction order to be substantially temporally coherent.63. The method of clause 62, wherein the multiple wavelengths of illumination are not substantially temporally coherent when incident on the diffraction target.64. The method of any clause 59 to 63, wherein the additional detector comprises a two- dimensional detector.65. The method of any clause 59 to 64, wherein the additional detector comprises a camera and wherein the one or more detectors comprise point and / or linear detectors.

[0115] The term “computer-readable medium” and / 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.

[0116] 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.

[0117] Computer system CS may also include a communication interface CI coupled to bus BS. Communication interface CI provides a two-way data communication coupling to a network link NDL that is connected to a local network LAN. For example, communication interface CI may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface CI may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface CI sends and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information.

[0118] 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.

[0119] 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.

[0120] While the concepts disclosed herein may be used for manufacturing with a substrate such as a silicon wafer, it shall be understood that the disclosed concepts may be used with any type of manufacturing system (e.g., those used for manufacturing on substrates other than silicon wafers).

[0121] In addition, the combination and sub-combinations of disclosed elements may comprise separate embodiments. For example, one or more of the operations described above may be included in separate embodiments, or they may be included together in the same embodiment.

[0122] 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: an illumination source, the illumination source configured to produce a beam of illumination; wherein at least a first portion of the beam is diffracted by a target and wherein at least a second portion of the beam is not diffracted by the target; a detector, the detector configured to detect a recombination of the first portion of the beam diffracted by the target and the second portion of the beam not diffracted by the target; and a processor operatively connected with the detector, the processor configured to determine a phase and / or amplitude of a waveform of the first portion of the beam after it has interacted with the target based on the recombination detected by the detector.

2. The system of claim 1, the processor further configured to determine a tilt of a layer of the target based on the phase and / or amplitude of the waveform.

3. The system of claim 2, the processor further configured to correct a determined location of the target based on the determined tilt.

4. The system of claim 2, wherein the tilt arises from an uneven thickness of the layer, wherein the target comprises a diffraction grating and the layer comprises a process layer coating the diffraction grating.

5. The system of any preceding claim, the processor further configured to determine a location of the target based on the phase and / or amplitude of the waveform.

6. The system of any preceding claim the processor further configured to correct a determined location of the target based on the phase and / or amplitude of the waveform of the first portion of the beam after it has interacted with the target.

7. The system of claim 6, wherein the determined location of the target comprises a location determined by the processor and / or another processor based on diffraction from the target detected by an additional detector.

8. The system of claim 7, wherein the diffraction from the target is comprises multiple diffraction orders.

9. The system of claim 7, wherein the diffraction from the target used to determine the location of the target has a different source than the first portion of the beam diffracted by the target.

10. The system of claim 7, wherein the first portion of the beam diffracted by the target used to determine the location of the target has the same source as at least a subset of the diffraction from the target.

11. The system of any preceding claim, wherein the first portion of the beam diffracted by the target passes through a first wavelength filter and / or slit filter and wherein the second portion of the beam not diffracted by the target passes through a second wavelength filter and / or slit filter.

12. The system of claim 11, wherein the first wavelength filter and / or slit filter is located in a path of the first portion of the beam after the diffraction and before the recombination.

13. The system of claim 11, wherein the first wavelength filter and / or slit filter is the second wavelength filter and / or slit filter.

14. The system of any preceding claim, wherein the recombination of the first portion of the beam diffracted by the target and the second portion of the beam not diffracted by the target comprises a digital hologram.

15. The system of any preceding claim, wherein the first portion of the beam diffracted by the target and the second portion of the beam not diffracted by the target are substantially temporally coherent.

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