WO2025067799A1

WO2025067799A1 - Lithographic apparatus, metrology systems, digital holographic microscopy alignment sensor, and method thereof

Info

Publication number: WO2025067799A1
Application number: PCT/EP2024/073972
Authority: WO
Inventors: Sebastianus Adrianus GOORDEN; Simon Reinald HUISMAN; Arie Jeffrey Den Boef; Alexander Prasetya Konijnenberg; Teunis Willem Tukker
Original assignee: Asml Netherlands B.V.
Priority date: 2023-09-27
Filing date: 2024-08-27
Publication date: 2025-04-03

Abstract

A system includes an illumination system, an optical system, a detector system, and a processor. The illumination system directs a first illumination beam and a second illumination beam towards a target structure and a first reference beam towards the detector system. The optical system directs a first scattered beam and a second scattered beam from the target structure to a detector system. The detector system captures an interference pattern and outputs a measurement signal. The first scattered beam, the second scattered beam, and the first reference beam generate the interference pattern. The processor analyzes the measurement signal to determine a characteristic of the target structure.

Description

LITHOGRAPHIC APPARATUS, METROLOGY SYSTEMS, DIGITAL HOLOGRAPHIC MICROSCOPY ALIGNMENT SENSOR, AND METHOD THEREOF

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63/585,762 which was filed on September 27, 2023 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present disclosure relates to a lithographic apparatus. For example, the present disclosure relates to digital holographic microscopy based metrology sensors.

BACKGROUND

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

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

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

[0006] As discussed above, the alignment apparatus or a metrology system may detect positions of the alignment marks for aligning the substrate. However, optical aberrations in the alignment apparatus or the metrology system may lead to errors in the alignment measurement. Errors in alignment of wafers in the lithographic apparatus result in reduced quality, unreliable performance, and reduced yield rates of fabricated devices, which in turn increases time and cost of fabrication of devices.

SUMMARY

[0007] Accordingly, it is desirable to improve the performance of metrology systems. For example, there is a desire to provide alignment measurements in an extended wavelength range as discussed in embodiments described herein.

[0008] In some embodiments, a system can include an illumination system, an optical system, a detector system, and a processor. The illumination system directs a first illumination beam and a second illumination beam towards a target structure and a first reference beam towards the detector system. The optical system directs a first scattered beam and a second scattered beam from the target structure to the detector system. The detector system captures an interference pattern and outputs a measurement signal. The first scattered beam, the second scattered beam, and the first reference beam generate the interference pattern. The processor analyzes the measurement signal to determine a characteristic of the target structure.

[0009] In some embodiments, a method includes directing a first illumination beam and a second illumination beam towards a target structure to produce a first scattered beam and a second scattered beam. The method can also include directing a reference beam towards a detector system and detecting an interference pattern of the first scattered beam, the second scattered beam, and the reference beam. The method can also include determining, using a processor, a characteristic of the target structure based at least on the detected interference pattern.

[0010] In some embodiments, a lithographic apparatus can include an illumination system, a projection system, and an inspection system. The illumination system illuminates a pattern of a patterning device. The projection system projects an image of the pattern onto a substrate. The inspection system includes an illumination system, an optical system, a detector system, and a processor. The illumination system directs a first illumination beam and a second illumination beam towards a target structure and a first reference beam towards the detector system. The optical system directs a first scattered beam and a second scattered beam from the target structure to the detector system. The detector system captures an interference pattern and outputs a measurement signal. The first scattered beam, the second scattered beam, and the first reference beam generate the interference pattern. The processor analyzes the measurement signal to determine a characteristic of the target structure.

[0011] In some embodiments, a system can include an illumination system, an optical system, a detector system, and a processor. The illumination system directs a first illumination beam and a second illumination beam towards a target structure and a first reference beam and a second reference beam towards the detector system. The first illumination beam and the first reference beam are coherent. The second illumination beam and the second reference beam are coherent, and the first illumination beam and the first reference beam are incoherent with the second illumination beam and the second reference beam. The optical system can be configured to direct a first scattered beam and a second scattered beam from the target structure to the detector system. The first scattered beam includes at least two positive diffraction orders and the second scattered beam includes at least two negative diffraction orders. The detector system captures a first interference pattern and a second interference pattern and generates at least a measurement signal. A processor analyzes the measurement signal to determine an alignment characteristic of the target structure.

[0012] Further features of the present disclosure, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. It is noted that the present disclosure is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

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

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

[0015] FIG. IB shows a schematic of a transmissive lithographic apparatus, according to some embodiments.

[0016] FIG. 2 shows a more detailed schematic of the reflective lithographic apparatus, according to some embodiments. [0017] FIG. 3 shows a schematic of a lithographic cell, according to some embodiments.

[0018] FIGS. 4A and 4B show schematics of inspection apparatuses, according to some embodiments. [0019] FIG. 5 shows a schematic of an inspection system, according to some embodiments.

[0020] FIG. 6 A shows a schematic of an inspection system, according to some embodiments.

[0021] FIG. 6B shows a schematic of a diffraction pattern at a pupil plane of the inspection system, according to some embodiments.

[0022] FIG. 7 A shows a schematic of an inspection system, according to some embodiments.

[0023] FIG. 7B shows a schematic of an inspection system, according to some embodiments.

[0024] FIG. 7C shows a schematic of a diffraction pattern at a pupil plane of the inspection system, according to some embodiments.

[0025] FIG. 8 shows a schematic of an inspection system, according to some embodiments.

[0026] FIG. 9A shows a schematic of a process to determine a characteristic of a target, according to some embodiments.

[0027] FIG. 9B shows a schematic of a process to determine a characteristic of a target, according to some embodiments.

[0028] FIG. 9C shows a schematic of a diffraction pattern at a pupil plane of the inspection system, according to some embodiments.

[0029] FIG. 10 is a flowchart of a method for determining a characteristic of a target, according to some embodiments.

[0030] FIG. 11 is an example computer system useful for implementing various embodiments.

[0031] The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. Additionally, generally, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to-scale drawings.

DETAILED DESCRIPTION

[0032] This specification discloses one or more embodiments that incorporate the features of the present disclosure. The disclosed embodiment(s) are provided as examples. The scope of the present disclosure is not limited to the disclosed embodiment(s). Claimed features are defined by the claims appended hereto.

[0033] The embodiment(s) described, and references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0034] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, can be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

[0035] The term “about” as used herein indicates the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the term “about” can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

[0036] Embodiments of the disclosure can be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the disclosure may also be implemented as instructions stored on a machine -readable medium, which can be read and executed by one or more processors. A machine- readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, and/or instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.

[0037] Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present disclosure can be implemented.

Example Lithographic Systems

[0038] FIGS. 1 A and IB show schematic illustrations of a lithographic apparatus 100 and lithographic apparatus 100’, respectively, in which embodiments of the present disclosure may be implemented. Lithographic apparatus 100 and lithographic apparatus 100’ each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatus 100 and 100’ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100’, the patterning device MA and the projection system PS are transmissive.

[0039] The illumination system IL can include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B.

[0040] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatus 100 and 100’, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which can be fixed or movable, as desired. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

[0041] The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

[0042] The terms “inspection apparatus,” “metrology system,” or the like may be used herein to refer to, e.g., a device or system used for measuring a property of a structure (e.g., overlay error, critical dimension parameters) or used in a lithographic apparatus to inspect an alignment of a wafer (e.g., alignment apparatus).

[0043] The patterning device MA can be transmissive (as in lithographic apparatus 100’ of FIG. IB) or reflective (as in lithographic apparatus 100 of FIG. 1A). Examples of patterning devices MA include reticles, masks, programmable mirror arrays, or programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase shift, or attenuated phase shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which may be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam B, which is reflected by a matrix of small mirrors.

[0044] The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment may be used for EUV or electron beam radiation since other gases may absorb too much radiation or electrons. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0045] Lithographic apparatus 100 and/or lithographic apparatus 100’ can be of a type having two (dual stage) or more substrate tables WT (and/or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

[0046] The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

[0047] Referring to FIGS. 1A and IB, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus 100, 100’ can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus 100 or 100’, and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (in FIG. IB) including, for example, suitable directing mirrors and/or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus 100, 100’, for example, when the source SO is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD, if desired, may be referred to as a radiation system.

[0048] The illuminator IL can include an adjuster AD (in FIG. IB) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as “o-outer” and “o-inner,” respectively) of the intensity distribution in a pupil plane of the illuminator may be adjusted. In addition, the illuminator IL can comprise various other components (in FIG. IB), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

[0049] Referring to FIG. 1A, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask) MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0050] Referring to FIG. IB, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. The projection system has a pupil conjugate PPU to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at the mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

[0051] The projection system PS projects an image of the mask pattern MP, where the image is formed by diffracted beams produced from the mark pattern MP by radiation from the intensity distribution, onto a photoresist layer coated on the substrate W. For example, the mask pattern MP can include an array of lines and spaces. A diffraction of radiation at the array and different from zeroth order diffraction generates diverted diffracted beams with a change of direction in a direction perpendicular to the lines. Undiffracted beams (i.e., so-called zeroth order diffracted beams) traverse the pattern without any change in propagation direction. The zeroth order diffracted beams traverse an upper lens or upper lens group of the projection system PS, upstream of the pupil conjugate PPU of the projection system PS, to reach the pupil conjugate PPU. The portion of the intensity distribution in the plane of the pupil conjugate PPU and associated with the zeroth order diffracted beams is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The aperture device PD, for example, is disposed at or substantially at a plane that includes the pupil conjugate PPU of the projection system PS.

[0052] The projection system PS is arranged to capture, by means of a lens or lens group L, not only the zeroth order diffracted beams, but also first-order or first- and higher-order diffracted beams (not shown). In some embodiments, dipole illumination for imaging line patterns extending in a direction perpendicular to a line may be used to utilize the resolution enhancement effect of dipole illumination. For example, first-order diffracted beams interfere with corresponding zeroth-order diffracted beams at the level of the wafer W to create an image of the line pattern MP at highest possible resolution and process window (i.e., usable depth of focus in combination with tolerable exposure dose deviations). In some embodiments, astigmatism aberration may be reduced by providing radiation poles (not shown) in opposite quadrants of the illumination system pupil IPU. Further, in some embodiments, astigmatism aberration may be reduced by blocking the zeroth order beams in the pupil conjugate PPU of the projection system associated with radiation poles in opposite quadrants. This is described in more detail in US 7,511,799 B2, issued Mar. 31, 2009, which is incorporated by reference herein in its entirety. [0053] With the aid of the second positioner PW and position sensor IFD (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor (not shown in FIG. IB) may be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

[0054] In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator only or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks Pl, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they may be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

[0055] Mask table MT and patterning device MA can be in a vacuum chamber V, where an in-vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum chamber. Alternatively, when mask table MT and patterning device MA are outside of the vacuum chamber, an out-of- vacuum robot may be used for various transportation operations, similar to the in- vacuum robot IVR. Both the in- vacuum and out-of- vacuum robots need to be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

[0056] The lithographic apparatus 100 and 100’ can be used in at least one of the following modes:

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

2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT can be determined by the (de- jmagnification and image reversal characteristics of the projection system PS. 3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation may be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

[0057] Combinations and/or variations on the described modes of use or entirely different modes of use may also be employed.

[0058] In a further embodiment, lithographic apparatus 100 includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0059] FIG. 2 shows the lithographic apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment may be maintained in an enclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 can be formed by a discharge produced plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor, or Sn vapor in which the very hot plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma 210 is created by, for example, an electrical discharge causing at least a partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor, or any other suitable gas or vapor may be required for efficient generation of the radiation. In some embodiments, a plasma of excited tin (Sn) is provided to produce EUV radiation. [0060] The radiation emitted by the hot plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 can include a channel structure. Contamination trap 230 can also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure.

[0061] The collector chamber 212 can include a radiation collector CO, which can be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point INTF. The virtual source point INTF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the intermediate focus INTF is located at or near an opening 219 in the enclosing structure 220. The virtual source point INTF is an image of the radiation emitting plasma 210. Grating spectral filter 240 is used in particular for suppressing infra-red (IR) radiation. [0062] Subsequently the radiation traverses the illumination system IL, which can include a faceted field mirror device 222 and a faceted pupil mirror device 224 arranged to provide a desired angular distribution of the radiation beam 221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the radiation beam 221 at the patterning device MA, held by the support structure MT, a patterned beam 226 is formed and the patterned beam 226 is imaged by the projection system PS via reflective elements 228, 229 onto a substrate W held by the wafer stage or substrate table WT.

[0063] More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 can optionally be present, depending upon the type of lithographic apparatus. Further, there may be more mirrors present than those shown in the FIG. 2, for example there may be one to six additional reflective elements present in the projection system PS than shown in FIG. 2.

[0064] Collector optic CO, as illustrated in FIG. 2, is depicted as a nested collector with grazing incidence reflectors 253, 254, and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254, and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

Exemplary Lithographic Cell

[0065] FIG. 3 shows a lithographic cell 300, also sometimes referred to a lithocell or cluster, according to some embodiments. Lithographic apparatus 100 or 100’ can form part of lithographic cell 300. Lithographic cell 300 can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. In some examples, these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH, and bake plates BK. A substrate handler, or robot, RO picks up substrates from input/output ports I/Ol, I/O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus 100 or 100’. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses may be operated to maximize throughput and processing efficiency.

Exemplary Inspection Apparatus

[0066] In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more inspection apparatuses for accurate positioning of marks on a substrate. These alignment apparatuses are effectively position measuring apparatuses. Different types of marks and different types of alignment apparatuses and/or systems are known from different times and different manufacturers. A type of system widely used in current lithographic apparatus is based on a self-referencing interferometer as described in U.S. Patent No. 6,961,116 (den Boef et al.). Generally marks are measured separately to obtain X- and Y-positions. A combined X- and Y-measurement may be performed using the techniques described in U.S. Publication No. 2009/195768 A (Bijnen et al.) and in U.S. Patent No. 11,360,399 B2 (Goorden et al.). The full contents of which is incorporated herein by reference., however. The full contents of both of these disclosures are incorporated herein by reference. [0067] FIG. 4A shows a schematic of a cross-sectional view of an inspection apparatus 400 that can be implemented as a part of lithographic apparatus 100 or 100’, according to some embodiments. In some embodiments, inspection apparatus 400 can be configured to align a substrate (e.g., substrate W) with respect to a patterning device (e.g., patterning device MA). Inspection apparatus 400 can be further configured to detect positions of alignment marks on the substrate and to align the substrate with respect to the patterning device or other components of lithographic apparatus 100 or 100’ using the detected positions of the alignment marks. Such alignment of the substrate may ensure accurate exposure of one or more patterns on the substrate.

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

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

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

[0071] It should be noted that even though beam splitter 414 is shown to direct radiation sub-beam 415 towards alignment mark or target 418 and to direct diffracted radiation sub-beam 429 towards interferometer 426, the disclosure is not so limiting. It would be apparent to a person skilled in the relevant art that other optical arrangements may be used to obtain the similar result of illuminating alignment mark or target 418 on substrate 420 and detecting an image of alignment mark or target 418. [0072] As illustrated in FIG. 4A, interferometer 426 can be configured to receive radiation sub-beam 417 and diffracted radiation sub-beam 429 through beam splitter 414. In an example embodiment, diffracted radiation sub-beam 429 can be at least a portion of radiation sub-beam 415 that can be reflected from alignment mark or target 418. In an example of this embodiment, interferometer 426 comprises any appropriate set of optical-elements, for example, a combination of prisms that may be configured to form two images of alignment mark or target 418 based on the received diffracted radiation sub-beam 429. It should be appreciated that a good quality image need not be formed, but that the features of alignment mark 418 should be resolved. Interferometer 426 can be further configured to rotate one of the two images with respect to the other of the two images 180° and recombine the rotated and unrotated images interferometrically. [0073] In some embodiments, detector 428 can be configured to receive the recombined image via interferometer signal 427 and detect interference as a result of the recombined image when alignment axis 421 of inspection apparatus 400 passes through a center of symmetry (not shown) of alignment mark or target 418. Such interference may be due to alignment mark or target 418 being 180° symmetrical, and the recombined image interfering constructively or destructively, according to an example embodiment. Based on the detected interference, detector 428 can be further configured to determine a position of the center of symmetry of alignment mark or target 418 and consequently, detect a position of substrate 420. According to an example, alignment axis 421 can be aligned with an optical beam perpendicular to substrate 420 and passing through a center of image rotation interferometer 426. Detector 428 can be further configured to estimate the positions of alignment mark or target 418 by implementing sensor characteristics and interacting with wafer mark process variations.

[0074] In a further embodiment, detector 428 determines the position of the center of symmetry of alignment mark or target 418 by performing one or more of the following measurements:

1. measuring position variations for various wavelengths (position shift between colors);

2. measuring position variations for various orders (position shift between diffraction orders); and

3. measuring position variations for various polarizations (position shift between polarizations).

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

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

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

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

[0079] In some embodiments, an array of detectors (not shown) may be connected to beam analyzer 430, and allows the possibility of accurate stack profile detection as discussed below. For example, detector 428 can be an array of detectors. For the detector array, a number of options are possible: a bundle of multimode fibers, discrete pin detectors per channel, or CCD or CMOS (linear) arrays. The use of a bundle of multimode fibers enables any dissipating elements to be remotely located for stability reasons. Discrete PIN detectors offer a large dynamic range but each need separate pre-amps. The number of elements is therefore limited. CCD linear arrays offer many elements that may be read-out at high speed and are especially of interest if phase-stepping detection is used. [0080] In some embodiments, a second beam analyzer 430’ can be configured to receive and determine an optical state of diffracted radiation sub-beam 429, as shown in FIG. 4B. The optical state may be a measure of beam wavelength, polarization, or beam profile. Second beam analyzer 430’ can be identical to beam analyzer 430. Alternatively, second beam analyzer 430’ can be configured to perform at least all the functions of beam analyzer 430, such as determining a position of stage 422 and correlating the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420, can be accurately known with reference to stage 422. Second beam analyzer 430’ can also be configured to determine a position of inspection apparatus 400, or any other reference element, such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400, or any other reference element. Second beam analyzer 430’ can be further configured to determine the overlay data between two patterns and a model of the product stack profile of substrate 420. Second beam analyzer 430’ can also be configured to measure overlay, critical dimension, and focus of target 418 in a single measurement.

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

[0082] In some embodiments, processor 432 receives information from detector 428 and beam analyzer 430. For example, processor 432 can be an overlay calculation processor. The information may comprise a model of the product stack profile constructed by beam analyzer 430. Alternatively, processor 432 can construct a model of the product mark profile using the received information about the product mark. In either case, processor 432 constructs a model of the stacked product and overlay mark profile using or incorporating a model of the product mark profile. The stack model is then used to determine the overlay offset and minimizes the spectral effect on the overlay offset measurement. Processor 432 can create a basic correction algorithm based on the information received from detector 428 and beam analyzer 430, including but not limited to the optical state of the illumination beam, the alignment signals, associated position estimates, and the optical state in the pupil, image, and additional planes. The pupil plane is the plane in which the radial position of radiation defines the angle of incidence and the angular position defines the azimuth angle of the radiation. Processor 432 can utilize the basic correction algorithm to characterize the inspection apparatus 400 with reference to wafer marks and/or alignment marks 418.

[0083] In some embodiments, processor 432 can be further configured to determine printed pattern position offset error with respect to the sensor estimate for each mark based on the information received from detector 428 and beam analyzer 430. The information includes but is not limited to the product stack profile, measurements of overlay, critical dimension, and focus of each alignment marks or target 418 on substrate 420. Processor 432 can utilize a clustering algorithm to group the marks into sets of similar constant offset error, and create an alignment error offset correction table based on the information. The clustering algorithm may be based on overlay measurement, the position estimates, and additional optical stack process information associated with each set of offset errors. The overlay is calculated for a number of different marks, for example, overlay targets having a positive and a negative bias around a programmed overlay offset. The target that measures the smallest overlay is taken as reference (as it is measured with the best accuracy). From this measured small overlay, and the known programmed overlay of its corresponding target, the overlay error may be deduced. Table 1 illustrates how this may be performed. The smallest measured overlay in the example shown is -1 nm. However, this is in relation to a target with a programmed overlay of -30 nm. The process may have introduced an overlay error of 29 nm.

The smallest value may be taken to be the reference point and, relative to this, the offset may be calculated between measured overlay and that expected due to the programmed overlay. This offset determines the overlay error for each mark or the sets of marks with similar offsets. Therefore, in the Table 1 example, the smallest measured overlay was -1 nm, at the target position with programmed overlay of 30 nm. The difference between the expected and measured overlay at the other targets is compared to this reference. A table such as Table 1 may also be obtained from marks and target 418 under different illumination settings, the illumination setting, which results in the smallest overlay error, and its corresponding calibration factor, may be determined and selected. Following this, processor 432 can group marks into sets of similar overlay error. The criteria for grouping marks may be adjusted based on different process controls, for example, different error tolerances for different processes.

[0084] In some embodiments, processor 432 can confirm that all or most members of the group have similar offset errors, and apply an individual offset correction from the clustering algorithm to each mark, based on its additional optical stack metrology. Processor 432 can determine corrections for each mark and feed the corrections back to lithographic apparatus 100 or 100’ for correcting errors in the overlay, for example, by feeding corrections into the inspection apparatus 400. In addition, a digital holographic microscope used for overlay metrology is described in U.S. Patent Publication No. 2023/0044632 (Coene et. al ) which is incorporated by reference herein in its entirety.

[0085] In some aspects, it is desired to use an extended wavelength range for an illuminating beam (towards infra-range (IR) wavelengths) during alignment measurements to extend measurements through opaque layers. In some aspects, alignment sensors have tight aberration requirements over a large wavelength range which result in complex optics. Moreover, some alignment sensors use polarizing optics that are difficult to manufacture over the extended wavelength range.

[0086] Embodiments of the present disclosure provide apparatus and function for performing alignment measurements in the extended wavelength range. For example, a full complex field (phase and amplitude) is measured that enables the ability to computationally correct for hardware imperfections (e.g., optical aberrations) in the apparatus. Thus, the apparatus can have an extended wavelength range. In addition, the apparatus may be miniaturized compared to other alignment sensors because the correction of aberrations is done using a processor instead of using additional optical elements.

Example Inspection Systems

[0087] FIG. 5 shows a schematic of an inspection system 500, according to some embodiments. In some embodiments, inspection system 500 can also represent a more detailed view of beam analyzer 430. Note that for the sake of simplicity, FIG. 5 only shows some of the components of inspection system 500.

[0088] As illustrated in FIG. 5, a target 506 on a substrate 508 can be illuminated by two illumination beams of radiation, i.e., a first illumination beam 502 and a second illumination beam 504. In some aspects, first illumination beam 502 and second illumination beam can simultaneously illuminate target 506. Target 506 can be alignment mark or target 418 of FIGS. 4A and 4B. In some aspects, target 506 can be a micro diffraction based overlay (pDBO) grating or a bottom grating used in overlay measurements.

[0089] In some aspects, a target 506 can be illuminated by a single illumination beam of radiation. The illumination beams (e.g., first illumination beam 502 and second illumination beam 504) may be off- axis beams (as shown in FIG. 5). However, in other embodiments, the illumination beam may be an on- axis beam.

[0090] First illumination beam 502 can be obliquely incident onto target 506 at a first angle of incidence in a first direction with respect to the optical axis OA of system 500. Second illumination beam 504 can be incident on target 506 at a second angle of incidence in a second direction with respect to the optical axis OA. In some aspects, the incident angle of second illumination beam 504 can be the same as that of first illumination beam 502. In some aspects, target 506 can be illuminated at a larger oblique angle of incidence (e.g., in the range between 70° and 90°, in the range between 60° and 90°, in the range between 70° and 90°). Target 506 can scatter radiation. Depending on the structure of target 506, the scattered radiation can comprise reflected radiation, diffracted radiation, and/or transmitted radiation.

[0091] In some embodiments, target 506 can diffract first illumination beam 502 into a number of diffraction orders. In some aspects, target 506 can comprise a diffractive structure (e.g., a grating(s)). Target 506 can scatter multiple beams. At least a first scattered beam 510 including at least one non- zeroth diffraction order, e.g., negative first diffraction order can be collected by an objective lens 514 (e.g., one or more lenses). First scattered beam 510 can be focused on an image sensor 518 (e.g., capture device, camera).

[0092] In some embodiments, target 506 can diffract second illumination beam 504 into a number of diffraction orders. A second scattered beam 512 including at least one non-zeroth diffraction order, e.g., positive first diffraction order can be collected by objective lens 514 and subsequently focused onto image sensor 518. In some aspects, the Oth diffraction order (= the specular reflection) is not captured by objective lens 514 so the 4-lst and -1st diffraction orders form a dark-field image of target 506 on image sensor 518. The zeroth diffraction order and other undesired orders may either be blocked by a beam blocking element (not shown) or configured to completely fall outside the NA of objective lens 514. In some aspects, objective lens 514 can be used only in the detection path (as shown) and not used for illumination (e.g., focusing illumination beams 502 and 504 onto target 506). Thus, the illumination does not necessarily have to go through the same objective as the scattered light. In some embodiments, objective lens 514 can be used to collect the scattered radiation and to focus illumination beams 502 and 504 onto target 506. In some aspects, first scattered beam 510 and second scattered beam 512 can be simultaneously incident at a common position of image sensor 518.

[0093] A first off-axis reference beam 516 is coherently added to first scattered beam 510 and a second off-axis reference beam 520 is coherently added to second scattered beam 512. For example, first off- axis reference beam 516 can be incident at the same position of image sensor 518 as first scattered beam 510 and second scattered beam 512.

[0094] The path difference between the -1st order and -4-lst order beams is deliberately made larger than the coherence length of the light beam (which is of the order of 100 pm) so that the +1 st and -1st order beams do not coherently interfere on image sensor 518.

[0095] As described previously herein, target 506 can comprise a grating. In some aspects, the pitch of the grating is selected such that only the 4-lst and the -1st diffraction orders are captured by objective lens 514. In this way, two overlapping off-axis digital holograms are recorded on image sensor 518 from which the complex -1st order and 4-lst order images of the grating can be retrieved with Fourier Transform methods.

[0096] In order to extract an alignment signal, the complex 4-lst and -1st order images £+1 and E-l are added to obtain a synthetic intensity image from which the grating position Xg could be inferred:

[0097] Here Kx is the spatial frequency of the interference pattern between the +lst and -1st order beams and P is an (unknown) phase factor that described the phase difference between the two illumination and reference beams. As can be seen from equation (1), any unknown drift in the phase difference f> between the illumination and reference beams can lead to uncorrectable alignment errors. [0098] However, a grating with a pitch that is sufficiently large so that the second diffraction order is also captured can be used. The grating is designed in such that two diffraction orders are generated and captured by objective lens 514. In order to also capture a 2nd diffraction order the pitch P of the grating is selected such that:

[0099] In system 500, sin(0i//)~O.95 and NAlens=0.8 where 9111 corresponds the angle of incidence of first illumination beam 502 and second illumination beam 504 with respect to OA. Assuming, for example, a longest wavelength of 1600 nm, this yields a lower pitch limit of about 2 pm. Under that condition the intensity of the two retrieved +1 st and -1st images is given by:

[0100] So two computed intensity images of target 506 are obtained. It should be noted that the position of the fringe pattern is also impact by the Z-position (i.e., the grating height) but from these two images the Z-induced shift can be separated from the actual grating position Xg.

[0101] In some embodiments, a third scattered beam 522 and a fourth scattered beam 524 can also be collected by objective lens 514. In some aspects, third scattered beam 522 can comprise a negative second diffraction order. Fourth scattered beam 524 can comprise a positive second diffraction order.

[0102] In order to generate an interference pattern between two or more beams, the two or more beams should be at least partially coherent to each other, to a degree that is sufficient to form an interference pattern. In addition, the coherence between the two or more beams should be temporal and spatial coherence. Thus, first illumination beam 502 and first off-axis reference beam 516 are coherent to each other and second illumination beam 504 and second off-axis reference beam 520 are coherent to each other.

[0103] The characteristic of target 506 can be determined using a processing unit of inspection system 500 (not shown) (e.g., computer system 1100). The processing unit uses the interference pattern recorded by image sensor 518. The processing unit may be coupled to image sensor 518 to receive a signal comprising information about the interference pattern recorded by image sensor 518. The processing unit may correct for aberrations in inspection system 500. In some aspects, the processing unit may correct for aberrations of objective lens 514 of inspection system 500. The processing unit uses the interference pattern to calculate a complex field of radiation at image sensor 518. Complex field refers to when both amplitude and phase information are present. The complex field calculation is known in the field of holography as would be understood by one of ordinary skill in the art. Knowledge of the complex field provides additional information about the characteristic of target 506 compared to techniques in which phase and amplitude information are not both available.

[0104] However, in some aspects, a large mark pitch may be undesirable because of worse results in reproducibility tests (repro), less averaging over line edge roughness, and other advantages that could be obtained if alignment on overlay mark bottom gratings (which have small pitches) are used.

[0105] FIG. 6A shows a schematic of an inspection system 600, according to some embodiments. In some embodiments, inspection system 600 can also represent a more detailed view of beam analyzer 430. Note that for the sake of simplicity, FIG. 6A only show some of the components of inspection system 600 for the purpose of describing working principle of the system.

[0106] A first illumination beam 602 can be obliquely incident onto a target 606 on a substrate 608. In some aspects, a first scattered beam 610 is collected by a lens 614. First scattered beam 610 can be focused on a sensor 618 (e.g., an image sensor). Depending on the structure of target 606 the scattered radiation may comprise reflected radiation, diffracted radiation, or transmitted radiation. Target 606 can diffract first illumination beam 602 into a number of diffraction orders. For example, first scattered beam 610 can comprise at least one non-zeroth diffraction order, e.g., positive first diffraction order (+lst diffraction order). Similarly, a second illumination beam 604 can be obliquely incident onto target 606. In some aspects, the incident angle of second illumination beam 604 can be the same as that of first illumination beam 602. In some aspects, target 606 can be illuminated at a larger oblique angle of incidence (typically between 70° and 80°). In some aspects, second illumination beam 604 is incident on target 606 from an opposite side of system 600. A second scattered beam 612 is collected by lens 614. Second scattered beam 612 can comprise at least one non-zeroth diffraction order, e.g., negative first diffraction order (-1st diffraction order).

[0107] In some aspects, lens 614 can be used only in the detection path (as shown) and not used for illumination (e.g., focusing illumination beams 602 and 604 onto target 606). Thus, the illumination does not necessarily have to go through the same objective as the scattered light. In some embodiments, lens 614 can be shared and be used to collect the scattered radiation and to focus illumination beams 602 and 604 onto target 606.

[0108] A reference beam 616 can be interfered with first scattered beam 610 and second scattered beam 612. Thus, a two dimensional (2D) interference pattern may be formed on sensor 618. The 2D interference pattern may represent a digital hologram that includes information about the objective lens of inspection system 600 (e.g., lens 614). Thus, the information may be used to correct for aberrations of the wavefront of the beams. Reference beam 616 can be incident at the same position of sensor 618 as first scattered beam 610 and second scattered beam 612. In some aspects, first illumination beam 602, second illumination beam 604, and reference beam 616 are coherent with respect to each other. [0109] Corrections for hardware and optical imperfections (e.g., defocus) may be performed on the 2D interference pattern. The position may be obtained using the corrected 2D interference pattern. In some aspects, an algorithm may be used to fit the aligned position in a first dimension and in a second dimension (e.g., in a x direction and in a y direction). Because the aberrations are corrected digitally, some aberrations in the objective lens or lens 614 are tolerated and alignment measurements may be performed over the extended wavelength range (e.g., 400 nm to 1600 nm) without a decrease in the accuracy and with a decrease in the number of elements of the objective lens or lens 614. Due to the decrease in the number of elements, more alignment sensors may be used which increase the throughput of the lithography apparatus.

[0110] In some embodiments, first illumination beam 602 and second illumination beam 604 are relatively stable (i.e., less than 0.1 nm). In order to maintain stability between first illumination beam 602 and second illumination beam 604, illumination beam 602 and second illumination beam 604 can have mostly a common path from a light source (not shown) to target 606. In some aspects, reference beam 616 can have a stability from 10 nm to 100 nm relative to first illumination beam 602 and second illumination beam 604. The global phase difference between reference beam 616 and the illumination beams (e.g., first illumination beam 602 and second illumination beam 604) is not used. The phase gradient of reference beam 616 across sensor 618 is known and used in the digital holography calculation to determine the alignment as would be understood by one of ordinary skill in the art.

[0111] FIG. 6B shows a schematic of a diffraction pattern at a pupil plane (PP) of system 600, according to some embodiments.

[0112] In some aspects, target 606 can be a micro diffraction based overlay (pDBO) grating. Insert 620 shows an exemplary pDBO bottom grating. For example, target 606 can be a pDBO bottom grating. Target 606 can comprise one or more x-grating segments and one or more y-grating segments. Each segment can comprise a series of bars or grating lines. The x-grating segment can comprise bars that are parallel to the y-axis to provide periodicity in the x-direction. The y-segment can comprise bars that are parallel to the x-axis to provide periodicity in the y-direction. FIG. 6B shows the -1 and +1 diffraction spots for the x and y. Generally, for x-grating segments, mostly the x diffraction orders and the reference beam interfere. Similarly, for y-grating segments, mostly y diffraction orders and the reference beam interfere. However, cross talk may be an issue when there is a large wafer quality (WQ) difference between x- and y- segments.

[0113] In some embodiments, a second reference beam (not shown) may be interfered with first scattered beam 610 and second scattered beam 612. The second reference beam may have a different incident angle on sensor 618 compared to reference beam 616. Second reference beam may be coherent with respect to first illumination beam 602, second illumination beam 604, and reference beam 616. In some aspects, second reference beam may have an orthogonal polarization compared to reference beam 616 (e.g., p- and s- polarization, respectively). Both polarization components of the signal beams (or measurement signal) are obtained using a single image captured by sensor 618.

[0114] In some embodiments, x-measurements and y-measurements are decoupled. In some aspects, decoupling between the measurements (e.g., x-measurement and y-measurement) is achieved by making the beam incoherent with respect to each other in the x-direction and y-direction.

[0115] FIGS. 7A and 7B show schematics of an inspection system 700, according to some embodiments. FIG. 7A shows a front view of inspection system 700. FIG. 7B shows a side view of inspection system 700. In some embodiments, inspection system 700 can also represent a more detailed view of beam analyzer 430. Note that for the sake of simplicity, FIGS. 7A and 7B only show some of the components of inspection system 700.

[0116] In some embodiments, a first group of beams is used to obtain the characteristic of the target in a first direction (e.g., the x-direction) and a second group of beams is used to obtain the characteristic of the target in a second direction different than the first direction (e.g., the y-direction). The first group of beams is incoherent with respect to the second group of beams. First group of beams may comprise a first illumination beam 702a, a second illumination beam 704a, and a first reference beam 716a. Second group of beams may comprise a third illumination 702b, a fourth illumination beam 704b, and a second reference beam 716b. First illumination beam 702a, second illumination beam 704a, and first reference beam 716a are coherent with respect to each other and incoherent with respect to the second group of beams (third illumination 702b, fourth illumination beam 704b, and second reference beam 716b). Third illumination 702b, fourth illumination beam 704b, and second reference beam 716b are coherent with respect to each other and incoherent with respect to first illumination beam 702a, second illumination beam 704a, and first reference beam 716a.

[0117] Interference occurs between the beams within the same group and is suppressed between different beam groups. Thus, only desired interference patterns are formed on an image sensor 718.

[0118] In some embodiments, first illumination beam 702a and second illumination beam 704a can be obliquely incident onto a target 706a (e.g., x-grating segment of a pDBO bottom grating) on a substrate 708. In some aspects, a first scattered beam 710a and a second scattered beam 712a can be collected by a lens 714. First scattered beam 710a and second scattered beam 712a can be focused on a sensor 718 (e.g., an image sensor). Depending on the structure of target 706a the scattered radiation may comprise reflected radiation, diffracted radiation, or transmitted radiation. Target 706a can diffract first illumination beam 702a and second illumination beam 704a into a number of diffraction orders. For example, first scattered beam 710a can comprise at least one non-zeroth diffraction order, e.g., positive first diffraction order (+lst diffraction order). Second scattered beam 712a can comprise at least one non-zeroth diffraction order, e.g., negative first diffraction order (-1st diffraction order).

[0119] In some embodiments, third illumination beam 702b and fourth illumination beam 704b can be obliquely incident onto a target 706b (e.g., y-grating segment of the pDBO bottom grating) on substrate 708. In some aspects, a third scattered beam 710b and a fourth scattered beam 712b can be collected by lens 714. Third scattered beam 710b and fourth scattered beam 712b can be focused on sensor 718. Depending on the structure of target 706b the scattered radiation may comprise reflected radiation, diffracted radiation, or transmitted radiation. Target 706b can diffract third illumination beam 702b and fourth illumination beam 704b into a number of diffraction orders. For example, third scattered beam 710b can comprise at least one non-zeroth diffraction order, e.g., positive first diffraction order (+lst diffraction order). Fourth scattered beam 712b can comprise at least one non-zeroth diffraction order, e.g., negative first diffraction order (-1st diffraction order).

[0120] In some embodiments, a first interference pattern is formed by scattered beam 710a, scattered beam 712a, and first reference beam 716a. A second interference pattern is formed by scattered beam 710b, scattered beam 712b, and second reference beam 716b. The first interference pattern can be used to determine the alignment in the x-direction. The second interference pattern can be used for alignment measurement in the y-direction.

[0121] FIG. 7C shows a schematic of a diffraction pattern at a pupil plane (PP) of system 700, according to some embodiments. The -1 and +1 diffraction spots for the x and y measurements are shown. The diffraction spots 4-ly, -ly, and Ref-y (shown with a fill pattern) are coherent with each other. The diffraction spots +1X, -lx, and Ref-x (shown with a solid pattern) are coherent with each other but incoherent with the other diffractions spots.

[0122] In some embodiments, the first group of beams can also include another reference beam (i.e., in addition to first reference beam 716a). In some aspects, the another reference beam may have a polarization that is orthogonal to the first reference beam 716a. The another reference beam may be coherent with respect to first illumination beam 702a, second illumination beam 704a, and first reference beam 716a and incoherent with respect to the second group of beams. In addition, the another reference beam may be incident at a different angle compared to first reference beam 716a on sensor 718. The second group of beams can also include another reference beam (i.e., in addition to second reference beam 716b). In some aspects, the another reference beam of the second group of beams may have a polarization orthogonal to second reference beam 716b. The another reference beam may be coherent with respect to third illumination beam 702b, fourth illumination beam 704b, and second reference beam 716b and incoherent with respect to the first group of beams. In addition, the another reference beam may be incident at a different angle compared to second reference beam 716b on sensor 718.

[0123] FIG. 8 shows a schematic of an inspection system 800, according to some embodiments. In some embodiments, inspection system 800 can also represent a more detailed view of beam analyzer 430. Note that for the sake of simplicity, FIG. 8 only shows some of the components of inspection system 800.

[0124] In some embodiments, inspection system 800 can comprise an illumination source 834, an image sensor 818, and an optical module 836. Optical source 834 can generate a coherent radiation beam for the illumination of a target 806 on a substrate 808. An optical element 822 (e.g., beamsplitter) receives the radiation beam from illumination source 834 and split radiation beam into a first radiation sub-beam 802 and a second radiation sub-beam 816. First radiation sub-beam 802 can be directed towards a single mode fiber using a reflective element 824 (e.g., mirror). Second radiation sub-beam 816 is directed to a delay stage 820 and then coupled to a single mode fiber.

[0125] In some embodiments, the illumination beams are generated close to target 806 in order to maintain stability between the two illumination beams. In some aspects, first radiation sub-beam 802 can be split into multiple sub-beams using a first diffractive element 828. Second radiation sub-beam 816 can be split into multiple sub-beams using diffractive element 826. Diffractive elements 826 and 828 can include a two-dimensional grating. Diffractive elements 826 and 828 can be used to generate off-axis illumination beams. Diffractive element 826 can generate two off-axis reference beams. The two off-axis reference beams may be directed toward image sensor 818. Diffractive element 828 can generate two off-axis illumination beams that are used to illuminate target 806. In other embodiments second radiation sub-beam 816 and first radiation sub-beam 802 can be split into multiple beams using beamsplitters.

[0126] In some embodiments, optical module 836 can include optical elements that impose incoherence between the illumination beams used for x-measurements and the illumination beams used for y-measurement. For example, optical module 836 can include coherence retarders to make x- and y- measurements incoherent with respect to each others. First radiation sub-beam 802 can be collimated using a lens 812 and passed through a retarder 832. Retarder 832 can be a coherence retarder configured to delay the x-beams with respect to the y-beams. Similarly, second radiation sub-beam 816 can be collimated using a lens 810 and passed through a retarder 830. In some aspects, retarder 830 can also block undesired diffraction orders. In some aspects, retarder 830 and retarder 832 are matched such as a respective reference beam is coherent with respective illumination beams. Then, second radiation subbeam 816 can be passed via a beamsplitter 838 and focused using a lens 804 onto image sensor 818. Second radiation sub-beam 816 can comprise two beams that are incoherent with respect to each other. The two beams are used as the reference beams for the X and Y measurements, respectively, as discussed further below.

[0127] First radiation sub-beam 802 is passed to beamsplitter 840 and focused using an objective lens 814 onto target 806 on substrate 808. First radiation sub-beam 802 comprises a first pair of beams comprising two beams coherent with each other and a second pair of beams comprising two beams coherent with each other but incoherent with the first pair of beams. The first pair of beams may be used for x-measurements. The second pair of beams may be used for the y-measurements.

[0128] Scattered beams can be collected by objective lens 814 and focused using lens 804 on image sensor 818. In some embodiments, optical module 836 can include an optical element 842. Optical element 842 can stop the zero-order diffraction beam of the scattered beams. Two interference patterns are formed on image sensor 818. A first interference pattern is formed by the scattered beams of the first pair of beams and the reference beam for the x-measurement. A second interference pattern is formed by scattered beams of the second pair of beams and the reference beam for the y-measurements. The x and y alignment positions may be determined using the first interference beam and the second interference beam as discussed previously herein.

[0129] FIG. 9A is a flowchart of a method for analyzing an interference pattern, according to some embodiments. In 912, the interference pattern of the reference beam and the scattered beam (holographic image) is Fourier transformed into an image spectrum in the spatial frequency domain as further described in WO 2022/200014 (Coene et al. ) incorporated herein by reference in its entirety.

[0130] In 914, aberration correction is performed to correct for any aberration in the optical system using techniques described in WO 2019197117A1 (De Boer et al.) incorporated herein by reference in its entirety. Note that the two dimensional interference pattern captured on sensor 818 is transformed to a one dimensional pattern when target 806 include a one dimensional grating. In some aspects, phase information may be discarded. In some aspects, both amplitude and phase information are used to determine the corrected image.

[0131] In 916, inverse Fourier transform (IFT) is performed to obtain a corrected image. In 918, the aligned position is obtained from the one dimensional interference pattern. The aligned position can be obtained from the complex-valued (aberration-corrected) image or from the corresponding intensity image. When using the intensity image, for example methods described in WO 2023126174A1 (Goorden et al.) and WO 2023030832A1 (Goorden et al.) incorporated herein by reference in their entirety can be used to obtain the aligned position, including local fits within the mark and corrections for finite size and other effects. These methods can be adapted and applied to complex-valued images as well.

[0132] In some aspects, alternatively or in addition, the complex image can be digitally cropped to only contain a part within the mark. The complex field can be propagated to the Fourier plane. The first positive and the first negative diffraction orders can be selected. Respective phases of these orders can be determined. The aligned position can be determined from the phase difference.

[0133] In some embodiments, the reference beam illuminates the image sensor under a significantly larger angle compared to other beams (e.g., scattered beams). However, in some cases the angle of the reference beam and the other beams are comparable. FIG. 9C shows the diffraction pattern at a pupil plane (PP) of an inspection system (e.g., inspection system 800, inspection system 700). As shown in FIG. 9A diffraction spots from the reference beam and scattered beams are close to each other. An adjusted data analysis approach may be used as shown in FIG. 9B.

[0134] FIG. 9B is a flowchart for analyzing an interference pattern, according to some embodiments. In some aspects, a fast Fourier transform may be applied on an interference pattern 900. Interference pattern 900 can correspond to the diffraction pattern shown in FIG. 9C. A Fourier representation 902 (Fourier spectrum) is obtained via a 2D Fourier transformation. Fourier representation is cropped and move to the origin as shown by 904 in a first dimension (e.g., x-direction). Similarly, an edge of the Fourier representation is cropped along a second dimension (e.g., y-direction) and moved to the origin as shown by 906. In 908, an inverse fast Fourier transform is applied on 904 to obtain the x-position. In 910, the inverse fast Fourier transform is applied on modified Fourier representation 906 to obtain the y-position.

[0135] FIG. 10 shows method steps (e.g., using one or more processors) for performing a method 1000 including functions described herein, according to some embodiments. The method 1000 of FIG. 10 can be performed in any conceivable order and it is not required that all steps be performed. Moreover, the method steps of FIG. 10 described above merely reflect an example of steps and are not limiting.

[0136] In some embodiments, method 1000 illustrates a method for determining a characteristic of a target.

[0137] In some embodiments, in 1002 a first illumination beam and a second illumination beam are directed towards a target structure to produce a first scattered beam and a second scattered beam. In some aspects, a third illumination beam and a fourth illumination beams are directed toward the target structure.

[0138] In some embodiments, in 1004 a reference beam, the first scattered beam, and the second scattered beam are directed towards a detector system. In some aspects, another reference beam is directed towards the detector system. In some aspects, the third illumination beam, the fourth illumination beam, and the another reference beam are at least partially coherent, and wherein the first illumination beam, the second illumination beam, and the reference beam are at least partially coherent. In some aspects, incoherence between the first illumination beam and the third illumination beam is imposed using an optical element (e.g., coherence retarder).

[0139] In some embodiments, in 1006 an interference pattern (holographic image) of the first scattered beam, the second scattered beam, and the reference beam is detected.

[0140] In some embodiments, in 1008 a characteristic of the target structure may be determined. In some aspects, a complex- valued image field may be extracted from the holographic image and is computationally corrected for hardware imperfections.

[0141] The corrected complex field (amplitude and phase) may be used to fit the aligned position. In some embodiments, a Fourier transform is applied to the detected interference pattern to obtain a Fourier representation. First diffraction orders along a first dimension of the Fourier representation are cropped and shifted to a center of the Fourier representation. An inverse Fourier transform is applied to the cropped Fourier representation to obtain the characteristic of the target structure in the first dimension. First diffraction orders along a second dimension of the Fourier representation are cropped and shifted to the center of Fourier representation. An inverse Fourier transform to the cropped Fourier representation along the second dimension is applied to obtain the characteristic of the target structure in the second dimension.

[0142] In some embodiments, the systems and methods described herein may be used for overlay measurements. In some aspects, a target including a grating is provided in two or more different layers next to each other. Inspection system 500, inspection system 600, inspection system 700, or inspection system 800 may be used to capture an image of the target. In some aspects, the aligned position of each of the grating is determined as described previously herein. The difference between the positions of the gratings correspond to the overlay.

[0143] In some embodiments, the systems and methods described herein may be used for focus and dose metrology. A grating structure may be designed, exposed, and developed such that a measured position of the grating structure correlates to a defocus and/or dose used during the exposure. Inspection system 500, inspection system 600, inspection system 700, or inspection system 800 may be used to determine the defocus and/or dose.

[0144] Various embodiments may be implemented, for example, using one or more well-known computer systems, such as computer system 1100 shown in FIG. 11. One or more computer systems 1100 can be used, for example, to implement any aspect of the disclosure discussed herein, as well as combinations and sub-combinations thereof.

[0145] Computer system 1100 can include one or more processors (also called central processing units, or CPUs), such as a processor 1104. Processor 1104 may be connected to a communication infrastructure or bus 1106.

[0146] Computer system 1100 can also include customer input/output device(s) 1103, such as monitors, keyboards, pointing devices, etc., which may communicate with communication infrastructure 1106 through customer input/output interface(s) 1102.

[0147] One or more of processors 1104 can be a graphics processing unit (GPU). In an embodiment, a GPU may be a processor that is a specialized electronic circuit designed to process mathematically intensive applications. The GPU may have a parallel structure that is efficient for parallel processing of large blocks of data, such as mathematically intensive data common to computer graphics applications, images, videos, etc.

[0148] Computer system 1100 can also include a main or primary memory 1108, such as random access memory (RAM). Main memory 1108 may include one or more levels of cache. Main memory 1108 may have stored therein control logic (i.e., computer software) and/or data.

[0149] Computer system 1100 can also include one or more secondary storage devices or memory 1110. Secondary memory 1110 may include, for example, a hard disk drive 1112 and/or a removable storage device or drive 1114. Removable storage drive 1114 can be a floppy disk drive, a magnetic tape drive, a compact disk drive, an optical storage device, tape backup device, and/or any other storage device/drive.

[0150] Removable storage drive 1114 can interact with a removable storage unit 1118. Removable storage unit 1118 can include a computer usable or readable storage device having stored thereon computer software (control logic) and/or data. Removable storage unit 1118 can be a floppy disk, magnetic tape, compact disk, DVD, optical storage disk, and/ any other computer data storage device. Removable storage drive 1114 can read from and/or write to removable storage unit 1118. [0151] Secondary memory 1110 can include other means, devices, components, instrumentalities or other approaches for allowing computer programs and/or other instructions and/or data to be accessed by computer system 1100. Such means, devices, components, instrumentalities or other approaches may include, for example, a removable storage unit 1122 and an interface 1120. Examples of the removable storage unit 1122 and the interface 1120 can include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM or PROM) and associated socket, a memory stick and USB port, a memory card and associated memory card slot, and/or any other removable storage unit and associated interface.

[0152] Computer system 1100 can further include a communication or network interface 1124. Communication interface 1124 can enable computer system 1100 to communicate and interact with any combination of external devices, external networks, external entities, etc. (individually and collectively referenced by reference number 1128). For example, communication interface 1124 can allow computer system 1100 to communicate with external or remote devices 1128 over communications path 1126, which may be wired and/or wireless (or a combination thereof), and which may include any combination of LANs, WANs, the Internet, etc. Control logic and/or data may be transmitted to and from computer system 1100 via communication path 1126.

[0153] Computer system 1100 may also be any of a personal digital assistant (PDA), desktop workstation, laptop or notebook computer, netbook, tablet, smart phone, smart watch or other wearable, appliance, part of the Internet-of-Things, and/or embedded system, to name a few non-limiting examples, or any combination thereof.

[0154] Computer system 1100 may be a client or server, accessing or hosting any applications and/or data through any delivery paradigm, including but not limited to remote or distributed cloud computing solutions; local or on-premises software (“on-premise” cloud-based solutions); “as a service” models (e.g., content as a service (CaaS), digital content as a service (DCaaS), software as a service (SaaS), managed software as a service (MSaaS), platform as a service (PaaS), desktop as a service (DaaS), framework as a service (FaaS), backend as a service (BaaS), mobile backend as a service (MBaaS), infrastructure as a service (laaS), etc.); and/or a hybrid model including any combination of the foregoing examples or other services or delivery paradigms.

[0155] Any applicable data structures, file formats, and schemas in computer system 1100 may be derived from standards including but not limited to JavaScript Object Notation (JSON), Extensible Markup Language (XML), Yet Another Markup Language (YAML), Extensible Hypertext Markup Language (XHTML), Wireless Markup Language (WML), MessagePack, XML User Interface Language (XUL), or any other functionally similar representations alone or in combination. Alternatively, proprietary data structures, formats or schemas may be used, either exclusively or in combination with known or open standards.

[0156] In some embodiments, a tangible, non-transitory apparatus or article of manufacture comprising a tangible, non-transitory computer useable or readable medium having control logic (software) stored thereon may also be referred to herein as a computer program product or program storage device. This includes, but is not limited to, computer system 1100, main memory 1108, secondary memory 1110, and removable storage units 1118 and 1122, as well as tangible articles of manufacture embodying any combination of the foregoing. Such control logic, when executed by one or more data processing devices (such as computer system 1100), may cause such data processing devices to operate as described herein.

[0157] Based on the teachings contained in this disclosure, it will be apparent to persons skilled in the relevant art(s) how to make and use embodiments of this disclosure using data processing devices, computer systems and/or computer architectures other than that shown in FIG. 11. In particular, embodiments can operate with software, hardware, and/or operating system implementations other than those described herein.

[0158] The embodiments may further be described using the following clauses:

1. A system comprising: an illumination system configured to direct at least a first illumination beam and a second illumination beam towards a target structure and a first reference beam towards a detector system; an optical system configured to direct a first scattered beam and a second scattered beam from the target structure to the detector system; the detector system configured to capture an interference pattern and to output a measurement signal, wherein the first scattered beam, the second scattered beam, and the first reference beam generate the interference pattern; and a processor configured to analyze the measurement signal to determine a characteristic of the target structure.

2. The system of clause 1 , wherein the first scattered beam is a first diffracted beam and the second scattered beam is a second diffracted beam.

3. The system of clause 1, wherein the first illumination beam, the second illumination beam, and the first reference beam are at least partially coherent.

4. The system of clause 1, wherein the illumination system is further configured to direct a third illumination beam and a fourth illumination beam towards the target structure and a second reference beam towards the detector system, the third illumination beam, the fourth illumination beam, and the second reference beam being coherent; wherein the optical system is further configured to direct a third scattered beam and a fourth scattered beam from the target structure to the detector system; wherein the detector system is further configured to capture another interference pattern formed by the third scattered beam, the fourth scattered beam, and the second reference beam, the first illumination beam and the third illumination beam being incoherent with respect to each other and to generate another measurement signal; and wherein the processor is further configured to analyze the another measurement signal to determine another characteristic of the target structure. 5. The system of clause 4, wherein the target structure comprises first periodic structures having a first pitch along a first direction and second periodic structures having a second pitch along a second direction that is different from the first direction; and wherein the characteristic is associated with the first direction and the another characteristic is associated with the second direction.

6. The system of clause 4, further comprising: an optical element configured to impose incoherence between the first illumination beam and the third illumination beam.

7. The system of clause 1, wherein the processor is further configured to: crop and move to a center in a Fourier domain a portion of a Fourier representation of the interference pattern along a first dimension; and obtain the characteristic of the target structure in the first dimension by at least performing an inverse Fourier transform operation.

8. The system of clause 1, wherein the characteristic of the target structure comprises an alignment characteristic.

9. The system of clause 1, wherein the first scattered beam comprises positive diffraction orders in a first direction and in a second direction, and the second scattered beam comprises negative diffraction orders in the first direction and the second direction; and wherein the target structure includes grating segments in the first direction and in the second directions.

10. The system of clause 1, wherein the illumination system is further configured to direct a second reference beam toward the detector system; and wherein a polarization of the first reference beam is orthogonal to the polarization of the second reference beam.

11. A method comprising: directing a first illumination beam and a second illumination beam towards a target structure to produce a first scattered beam and a second scattered beam; directing a reference beam towards a detector system; detecting, using the detector system, an interference pattern of the first scattered beam, the second scattered beam, and the reference beam; and determining, using a processor, a characteristic of the target structure based at least on the detected interference pattern.

12. The method of clause 11, further comprising: directing a third illumination beam and a fourth illumination beam towards the target structure and another reference beam towards the detector system, wherein the third illumination beam, the fourth illumination beam, and the another reference beam are at least partially coherent, and wherein the first illumination beam, the second illumination beam, and the reference beam are at least partially coherent.

13. The method of clause 12, further comprising: imposing, using an optical element, incoherence between the first illumination beam and the third illumination beam.

14. The method of clause 11, further comprising: applying a Fourier transform to the detected interference pattern to obtain a Fourier representation; cropping and shifting to a center first diffraction orders along a first dimension of the Fourier representation; applying an inverse Fourier transform to the cropped Fourier representation to obtain the characteristic of the target structure in the first dimension; cropping and shifting to the center first diffraction orders along a second dimension of the Fourier representation; and applying an inverse Fourier transform to the cropped Fourier representation along the second dimension to obtain the characteristic of the target structure in the second dimension.

15. A lithographic apparatus comprising: an illumination system configured to illuminate a pattern of a patterning device; a projection system configured to project an image of the pattern onto a substrate; and an inspection system comprising: an optical system configured to: direct a first illumination beam and a second illumination beam towards a target structure and a first reference beam towards a detector system; and direct a first scattered beam and a second scattered beam from the target structure to the detector system; the detector system configured to capture an interference pattern and to output a measurement signal, wherein the first scattered beam, the second scattered beam, and the first reference beam generate the interference pattern; and a processor configured to analyze the measurement signal to determine a characteristic of the target structure.

16. The lithographic apparatus of clause 15, wherein the first scattered beam is a first diffracted beam and the second scattered beam is a second diffracted beam.

17. The lithographic apparatus of clause 15, wherein the first illumination beam, the second illumination beam, and the first reference beam are at least partially coherent.

18. The lithographic apparatus of clause 15, wherein the illumination system is further configured to direct a third illumination beam and a fourth illumination beam towards the target structure and a second reference beam towards the detector system, the third illumination beam, the fourth illumination beam, and the second reference beam being at least partially coherent; wherein the optical system is further configured to direct a third scattered beam and a fourth scattered beam from the target structure to the detector system; wherein the detector system is further configured to capture another 2D interference pattern formed by the third scattered beam, the fourth scattered beam, and the second reference beam, the first illumination beam and the third illumination beam being incoherent with respect to each other and to generate another measurement signal; and wherein the processor is further configured to analyze the another measurement signal to determine another characteristic of the target structure.

19. The lithographic apparatus of clause 18, wherein the target structure comprises first periodic structures having a first pitch along a first direction and second periodic structures having a second pitch along a second direction that is different from the first direction; and wherein the characteristic is associated with the first direction and the another characteristic is associated with the second direction.

20. The lithographic apparatus of clause 18, wherein the inspection system further comprises: an optical element configured to impose incoherence between the first illumination beam and the third illumination beam. 21. The lithographic apparatus of clause 15, wherein the characteristic of the target structure comprises to an alignment characteristic.

22. A system comprising: an illumination system configured to direct a first illumination beam and a second illumination beam towards a target structure and a first reference beam and a second reference beam towards a detector system, wherein the first illumination beam and the first reference beam are coherent, the second illumination beam and the second reference beam are coherent, and the first illumination beam and the first reference beam are incoherent with the second illumination beam and the second reference beam; an optical system configured to direct a first scattered beam and a second scattered beam from the target structure to the detector system, wherein the first scattered beam comprises at least two positive diffraction orders and the second scattered beam comprises at least two negative diffraction orders; the detector system configured to capture a first interference pattern and a second interference pattern and to output at least a measurement signal; and a processor configured to analyze the measurement signal to determine an alignment characteristic of the target structure.

23. A system comprising: an illumination system configured to direct an illumination beam towards a target structure and a reference beam towards a detector system; an optical system configured to direct at least a first scattered beam and a second scattered beam from the target structure to the detector system, wherein the first scattered beam and second scattered beam comprises two or more diffraction orders; the detector system configured to capture an interference pattern and to output a measurement signal, wherein the first scattered beam, the second scattered beam, and the reference beam generate the interference pattern; and a processor configured to analyze the measurement signal to determine a characteristic of the target structure.

[0159] Although specific reference can be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as specific examples of the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein can be processed, before or after exposure, in for example a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) and/or a metrology unit. Where applicable, the disclosure herein can be applied to such and other substrate processing tools. Further, the substrate can 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 contains multiple processed layers.

[0160] Although specific reference may have been made above to the use of embodiments of the present disclosure in the context of optical lithography, it will be appreciated that the present disclosure can be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device can be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0161] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present disclosure is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0162] The terms “radiation,” “beam of radiation” or the like as used herein can encompass all types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength I of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-20 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as matter beams, such as ion beams or electron beams. The terms “light,” “illumination,” or the like can refer to non-matter radiation (e.g., photons, UV, X-ray, or the like). Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G- line 436 nm; H-line 405 nm; and/or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in some embodiments, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a desired wavelength band, of which at least part is in the range of 5-20 nm.

[0163] It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the present disclosure and the appended claims in any way.

[0164] The present disclosure has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0165] While specific embodiments of the disclosure have been described above, it will be appreciated that embodiments of the present disclosure may be practiced otherwise than as described. The descriptions are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the disclosure as described without departing from the scope of the claims set out below.

[0166] The foregoing description of the specific embodiments will so fully reveal the general nature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein.

[0167] The breadth and scope of the protected subject matter should not be limited by any of the above- described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

2. The system of claim 1 , wherein the first scattered beam is a first diffracted beam and the second scattered beam is a second diffracted beam.

3. The system of claim 1, wherein the first illumination beam, the second illumination beam, and the first reference beam are at least partially coherent.

4. The system of claim 1 , wherein the illumination system is further configured to direct a third illumination beam and a fourth illumination beam towards the target structure and a second reference beam towards the detector system, the third illumination beam, the fourth illumination beam, and the second reference beam being coherent; wherein the optical system is further configured to direct a third scattered beam and a fourth scattered beam from the target structure to the detector system; wherein the detector system is further configured to capture another interference pattern formed by the third scattered beam, the fourth scattered beam, and the second reference beam, the first illumination beam and the third illumination beam being incoherent with respect to each other and to generate another measurement signal; and wherein the processor is further configured to analyze the another measurement signal to determine another characteristic of the target structure.

5. The system of claim 4, wherein the target structure comprises first periodic structures having a first pitch along a first direction and second periodic structures having a second pitch along a second direction that is different from the first direction; and wherein the characteristic is associated with the first direction and the another characteristic is associated with the second direction.

6. The system of claim 4, further comprising: an optical element configured to impose incoherence between the first illumination beam and the third illumination beam.

7. The system of claim 1, wherein the processor is further configured to: crop and move to a center in a Fourier domain a portion of a Fourier representation of the interference pattern along a first dimension; and obtain the characteristic of the target structure in the first dimension by at least performing an inverse Fourier transform operation.

8. The system of claim 1, wherein the characteristic of the target structure comprises an alignment characteristic.

9. The system of claim 1, wherein the first scattered beam comprises positive diffraction orders in a first direction and in a second direction, and the second scattered beam comprises negative diffraction orders in the first direction and the second direction; and wherein the target structure includes grating segments in the first direction and in the second directions.

10. The system of claim 1, wherein the illumination system is further configured to direct a second reference beam toward the detector system; and wherein a polarization of the first reference beam is orthogonal to the polarization of the second reference beam.

12. The method of claim 11, further comprising: directing a third illumination beam and a fourth illumination beam towards the target structure and another reference beam towards the detector system, wherein the third illumination beam, the fourth illumination beam, and the another reference beam are at least partially coherent, and wherein the first illumination beam, the second illumination beam, and the reference beam are at least partially coherent.

13. The method of claim 12, further comprising: imposing, using an optical element, incoherence between the first illumination beam and the third illumination beam.

14. The method of claim 11, further comprising: applying a Fourier transform to the detected interference pattern to obtain a Fourier representation; cropping and shifting to a center first diffraction orders along a first dimension of the Fourier representation; applying an inverse Fourier transform to the cropped Fourier representation to obtain the characteristic of the target structure in the first dimension; cropping and shifting to the center first diffraction orders along a second dimension of the Fourier representation; and applying an inverse Fourier transform to the cropped Fourier representation along the second dimension to obtain the characteristic of the target structure in the second dimension.