Alignment system and lithographic apparatus

Abstract

Disclosed is a sensor arrangement for a metrology device. The arrangement comprises: multiple sensor heads comprising a first sensor head to collect first scattered radiation and define a first pupil comprising an angularly resolved representation of said first scattered radiation and a second sensor head to collect second scattered radiation and define a second pupil comprising an angularly resolved representation of said second scattered radiation; a first pupil configurator being configured to reconfigure said first pupil and / or said second pupil, such that said first scattered radiation is confined to one or more first regions of said first pupil and said second scattered radiation is confined to one or more second regions of said second pupil, wherein said first regions and second regions are substantially non-corresponding; and a first beam combiner to combine said first scattered radiation and said second scattered radiation into a first combined beam.

Description

ALIGNMENT SYSTEM AND LITHOGRAPHIC APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 688,085 which was filed on August 28, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to metrology systems, for example, an alignment system for measuring alignment mark positions in lithographic apparatuses and systems.BACKGROUND

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

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

[0005] In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters can include, for example, 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 theuse of scanning electron microscopes and various specialized tools. A fast and non-invasive form of a specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. By contrast, angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.

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

[0007] Current alignment sensors can use self-referencing interferometers to interfere plus and minus orders of diffracted light from a substrate to form an alignment signal. A detection system can measure a location of a peak of the alignment signal and can compare that peak location to an expected peak location to determine a position of an alignment mark on the substrate.

[0008] To increase measurement speed, it is proposed to provide alignment sensors with more than one measurement head or sensor head, with each sensor head comprising a respective objective lens for illuminating an alignment mark and capturing the resultant scattered radiation. This will enable multiple marks to be measured in parallel, e.g., with each sensor head able to measure a respective mark simultaneously and / or during a single measurement scan. In the latter case, the parallel measurement may measure the marks sequentially and / or a combination of sequential and simultaneous measurement during a scan.

[0009] However, collecting the illumination from multiple channels, one for each sensor head, results in significant transmission loss (e.g., a total transmission of 20% compared to a single head sensor).SUMMARY

[0010] Accordingly, it is desirable to reduce transmission loss of the collected scattered radiation from the alignment marks.

[0011] In a first aspect of the invention there is provided a sensor head arrangement for a metrology device, the sensor head arrangement comprising: a plurality of sensor heads comprising at least a first sensor head being operable to collect first scattered radiation and define a first pupil comprising an angularly resolved representation of said first scattered radiation and a second sensor head being operable to collect second scattered radiation and define a second pupil comprising an angularly resolved representation of said second scattered radiation; at least a first pupil configurator, said at leasta first pupil configurator being configured to reconfigure at least one of said first pupil and / or said second pupil, such that said first scattered radiation is confined to one or more first regions of said first pupil and said second scattered radiation is confined to one or more second regions of said second pupil, wherein said first regions and second regions are substantially non-corresponding; and at least a first beam combiner being operable to combine said first scattered radiation and said second scattered radiation into a first combined beam.

[0012] In a second aspect of the invention there is provided a method of performing a parallel measurement of two or more marks on a substrate, the method comprising: collecting first scattered radiation from a first mark and defining a first pupil comprising an angularly resolved representation of said first scattered radiation; collecting second scattered radiation from a second mark and defining a second pupil comprising an angularly resolved representation of said second scattered radiation; reconfiguring at least one of said first pupil and / or said second pupil, such that said first scattered radiation is confined to one or more first regions of said first pupil and said second scattered radiation is confined to one or more second regions of said second pupil, wherein said first regions and second regions are substantially non-corresponding; and combining said first scattered radiation and said second scattered radiation into a first combined beam, subsequent to said reconfiguring.

[0013] Further features of various aspects of the present disclosure are described in detail below with reference to the accompanying drawings. It is noted that the present disclosure is not limited to the specific aspects described herein. Such aspects are presented herein for illustrative purposes only. Additional aspects will be apparent to those skilled in the relevant art(s) based on the teachings contained herein.BRIEF DESCRIPTION OF THE DRAWINGS / FIGURES

[0014] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable those skilled in the relevant art(s) to make and use aspects described herein.Figure 1A shows a reflective lithographic apparatus, according to some aspects;Figure IB shows a transmissive lithographic apparatus, according to some aspects;Figure 2 shows more details of a reflective lithographic apparatus, according to some aspects;Figure 3 shows a lithographic cell, according to some aspects;Figures 4A and 4B are schematic illustrations of metrology apparatuses, according to some aspects.Figure 5 shows a metrology apparatus comprising multiple sensor heads;Figure 6 shows a first example of a metrology apparatus comprising multiple sensor heads and improved transmission efficiency according to some aspects;Figure 7 shows a second example of a metrology apparatus comprising multiple sensor heads and improved transmission efficiency according to some aspects;Figure 8 shows a third example of a metrology apparatus comprising multiple sensor heads and improved transmission efficiency according to some aspects;Figure 9 A shows a fourth example of a metrology apparatus comprising multiple sensor heads and improved transmission efficiency according to some aspects;Figure 9B shows a fifth example of a metrology apparatus comprising multiple sensor heads and improved transmission efficiency according to some aspects;Figure 10A shows a sixth example of a metrology apparatus comprising multiple sensor heads and improved transmission efficiency according to some aspects;Figure 10B shows a detail of an inverse spot mirror forming part of the metrology apparatus of Figure 10A; andFigure 11 shows a further example of a metrology apparatus comprising multiple sensor heads and a detector per sensor head according to some aspects.

[0015] 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

[0016] The aspects described herein, and references in the specification to “one aspect,” “an aspect,” “an exemplary aspect,” “an example aspect,” etc., indicate that the aspects described can include a particular feature, structure, or characteristic, but every aspect may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same aspect. Further, when a particular feature, structure, or characteristic is described in connection with an aspect, it is understood that it is within the knowledge of those skilled in the art to effect such feature, structure, or characteristic in connection with other aspects whether or not explicitly described.

[0017] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, can be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein can likewise be interpreted accordingly.

[0018] The terms “about,” “approximately,” or the like can be used herein to indicate the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the terms “about,” “approximately,” or the like can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

[0019] Aspects of the present disclosure can be implemented in hardware, firmware, software, or any combination thereof. Aspects of the disclosure can also be implemented as instructions stored on a computer-readable medium, which can be read and executed by one or more processors. A machine- readable medium can include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium can include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Furthermore, firmware, software, routines, and / or instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. The term “machine-readable medium” can be interchangeable with similar terms, for example, “computer program product,” “computer-readable medium,” “non-transitory computer- readable medium,” or the like. The term “non-transitory” can be used herein to characterize one or more forms of computer readable media except for a transitory, propagating signal.

[0020] Before describing such aspects in more detail, however, it is instructive to present an example environment in which aspects of the present disclosure can be implemented.

[0021] Example Lithographic Systems

[0022] FIGS. 1A and IB show a lithographic apparatus 100 and a lithographic apparatus 100’, respectively, in which aspects of the present disclosure can be implemented. Lithographic apparatus 100 and lithographic apparatus 100’ each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatus 100 and 100’ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100’, the patterning device MA and the projection system PS are transmissive.

[0023] 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.

[0024] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatus 100 and 100’, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

[0025] 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.

[0026] The patterning device MA can be transmissive (as in lithographic apparatus 100’ of Figure IB) or reflective (as in lithographic apparatus 100 of Figure 1A). Examples of patterning devices MA include reticles, masks, programmable mirror arrays, or programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase shift, or attenuated phase shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam B, which is reflected by a matrix of small mirrors.

[0027] The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0028] Lithographic apparatus 100 and / or lithographic apparatus 100’ can be of a type having two (dual stage) or more substrate tables WT (and / or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

[0029] The lithographic apparatus can also be of a type wherein at least a portion of the substrate can be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid can also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid. For example, a liquid can be located between the projection system and the substrate during exposure.

[0030] 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 Figure IB) including, for example, suitable directing mirrors and / or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus 100, 100’, for example, when the source SO is a mercury lamp. A radiation system can comprise the source SO, the illuminator IL, and / or the beam delivery system BD.

[0031] The illuminator IL can include an adjuster AD (in Figure 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 “n-outer” and “n-inner,” respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components (in Figure 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.

[0032] Referring to Figure 1 A, 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 PI, P2.

[0033] Referring to Figure 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 bythe 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.

[0034] 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 IU. 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.

[0035] The projection system PS is arranged to capture (e.g., using a lens or lens group U) the zeroth order diffracted beams, first order diffracted beams, and / or higher order diffracted beams (not shown). In some aspects, dipole illumination for imaging line patterns extending in a direction perpendicular to a line can be used to utilize the resolution enhancement effect of dipole illumination. For example, first- order diffracted beams interfere with corresponding zeroth-order diffracted beams at the level of the wafer W to create an image of the line pattern MP at highest possible resolution and process window (i.e., usable depth of focus in combination with tolerable exposure dose deviations). In some aspects, astigmatism aberration can be reduced by providing radiation poles (not shown) in opposite quadrants of the illumination system pupil IPU. Further, in some aspects, astigmatism aberration can be reduced by blocking the zeroth order beams in the pupil conjugate PPU of the projection system associated with radiation poles in opposite quadrants. This is described in more detail in US 7,511,799 B2, issued Mar. 31, 2009, which is incorporated by reference herein in its entirety.

[0036] 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 Figure IB) can be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

[0037] In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short-stroke actuator or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks PI, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks can be located between the dies.

[0038] 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 can be used for various transportation operations, similar to the in-vacuum robot IVR. Both the in-vacuum and out-of-vacuum robots can be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

[0039] 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- )magnification 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 needed after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

[0040] Combinations and / or variations on the described modes of use or entirely different modes of use can also be employed.

[0041] In some aspects, lithographic apparatus 100 includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUVsource is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0042] In some aspects, lithographic apparatus 100’ includes a deep ultraviolet (DUV) source, which is configured to generate a beam of DUV radiation for DUV lithography. In general, the DUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the DUV radiation beam of the DUV source.

[0043] Figure 2 shows the lithographic apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 can be formed by a discharge produced plasma source. EUV radiation can be produced by a gas or vapor, for example Xe gas, Li vapor, or Sn vapor in which EUV radiation emitting plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The EUV radiation emitting 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 can be used for efficient generation of the radiation. In some aspects, a plasma of excited tin (Sn) (e.g., excited via a laser) is provided to produce EUV radiation.

[0044] The radiation emitted by the EUV radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 can include a channel structure. Contamination trap 230 can also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure.

[0045] 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 fdter 240 to be focused in a virtual source point INTF. The virtual source point INTF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the intermediate focus INTF is located at or near an opening 219 in the enclosing structure 220. The virtual source point INTF is an image of the EUV radiation emitting plasma 210. Grating spectral filter 240 is used in particular for suppressing infra-red (IR) radiation.

[0046] Subsequently the radiation traverses the illumination system IL, which can include a faceted field mirror device 222 and a faceted pupil mirror device 224 arranged to provide a desired angular distribution of the radiation beam 221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation 221 at the patterning device MA, held by the support structure MT, a patterned beam 226 is formed and thepaterned 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.

[0047] More elements than shown can generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 can optionally be present, depending upon the type of lithographic apparatus. Further, there can be more mirrors present than those shown in the Figure 2, for example there can be one to six additional reflective elements present in the projection system PS than shown in Figure 2.

[0048] Collector optic CO, as illustrated in Figure 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.

[0049] Example Lithographic Cell

[0050] Figure 3 shows a lithographic cell 300, also sometimes referred to a lithocell or cluster, according to some aspects. Lithographic apparatus 100 or 100’ can form part of lithographic cell 300. Lithographic cell 300 can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. Conventionally these include spin coaters SC to deposit resist layers, developers DE 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 can be operated to maximize throughput and processing efficiency.

[0051] Example Inspection Apparatus

[0052] 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 alignment apparatuses and / or 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 all). Generally marks are measured separately to obtain X- and Y-positions. A combined X- and Y-measurement can be performed using the techniques described in U.S. Publication No. 2009 / 195768 A (Bijnen et al.), however. The full contents of both of these disclosures are incorporated herein by reference.

[0053] Figure 4A shows a cross-sectional view of an inspection apparatus 400 that can be implemented as a part of lithographic apparatus 100 or 100’, according to some aspects. In some aspects, inspectionapparatus 400 can be configured to align a substrate (e.g., substrate W) with respect to a patterning device (e.g., patterning device MA). Inspection apparatus 400 can be further configured to detect positions of alignment marks on the substrate and to align the substrate with respect to the patterning device or other components of lithographic apparatus 100 or 100’ using the detected positions of the alignment marks. Such alignment of the substrate can ensure accurate exposure of one or more patterns on the substrate.

[0054] The terms “inspection apparatus,” “metrology system,” or the like can be used herein to refer to, e.g., a device used for measuring a property of a structure (e.g., overlay sensor, critical dimension sensor, or the like), a device or system used in a lithographic apparatus to inspect an alignment of a wafer (e.g., alignment sensor), or the like.

[0055] In some aspects, inspection apparatus 400 can include an illumination system 412, a beam splitter 414, an interferometer 426, a detector 428, a beam analyzer 430, and a processor 432. Illumination system 412 can be configured to provide an electromagnetic narrow band radiation beam 413 having one or more passbands. In an example, the one or more passbands can be within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, the one or more passbands can be discrete narrow passbands within a spectrum of wavelengths between about 500 nm to about 900 nm. Illumination system 412 can be further configured to provide one or more passbands 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 can improve long-term stability and accuracy of alignment systems (e.g., inspection apparatus 400) compared to the current alignment apparatuses.

[0056] In some aspects, beam splitter 414 can be configured to receive radiation beam 413 and split radiation beam 413 into at least two radiation sub-beams. For example, radiation beam 413 can be split into radiation sub-beams 415 and 417, as shown in Figure 4A. Beam splitter 414 can be further configured to direct radiation sub-beam 415 onto a substrate 420 placed on a stage 422. In one example, the stage 422 is movable along direction 424. Radiation sub-beam 415 can be configured to illuminate an alignment mark or a target 418 located on substrate 420. Alignment mark or target 418 can be coated with a radiation sensitive film. In some aspects, alignment mark or target 418 can have one hundred and eighty degrees (i.e., 180°) symmetry. That is, when alignment mark or target 418 is rotated 180° about an axis of symmetry perpendicular to a plane of alignment mark or target 418, rotated alignment mark or target 418 can be substantially identical to an unrotated alignment mark or target 418. The target 418 on substrate 420 can be (a) a resist layer grating comprising bars that are formed of solid resist lines, or (b) a product layer grating, or (c) a composite grating stack in an overlay target structure comprising a resist grating overlaid or interleaved on a product layer grating. The bars can alternatively be etched into the substrate. This pattern is sensitive to chromatic aberrations in the lithographic projection apparatus, particularly the projection system PL, and illumination symmetry and the presenceof such aberrations will manifest themselves in a variation in the printed grating. One in-line method used in device manufacturing for measurements of line width, pitch, and critical dimension makes use of a technique known as “scatterometry”. Methods of scatterometry are described in Raymond et al., “Multiparameter Grating Metrology Using Optical Scatterometry”, J. Vac. Sci. Tech. B, Vol. 15, no. 2, pp. 361-368 (1997) and Niu et al., “Specular Spectroscopic Scatterometry in DUV Lithography”, SPIE, Vol. 3677 (1999), which are both incorporated by reference herein in their entireties. In scatterometry, light is reflected by periodic structures in the target, and the resulting reflection spectrum at a given angle is detected. The structure giving rise to the reflection spectrum is reconstructed, e.g. using Rigorous Coupled-Wave Analysis (RCWA) or by comparison to a library of patterns derived by simulation. Accordingly, the scatterometry data of the printed gratings is used to reconstruct the gratings. The parameters of the grating, such as line widths and shapes, can be input to the reconstruction process, performed by processing unit PU, from knowledge of the printing step and / or other scatterometry processes.

[0057] In some aspects, beam splitter 414 can be further configured to receive diffraction radiation beam 419 and split diffraction radiation beam 419 into at least two radiation sub-beams, according to an aspect. Diffraction radiation beam 419 can be split into diffraction radiation sub-beams 429 and 439, as shown in Figure 4A.

[0058] It should be noted that even though beam splitter 414 is shown to direct radiation sub-beam 415 towards alignment mark or target 418 and to direct diffracted radiation sub-beam 429 towards interferometer 426, the disclosure is not so limiting. Other optical arrangements can be used to obtain the similar result of illuminating alignment mark or target 418 on substrate 420 and detecting an image of alignment mark or target 418.

[0059] As illustrated in Figure 4A, interferometer 426 can be configured to receive radiation sub-beam 417 and diffracted radiation sub-beam 429 through beam splitter 414. In an example aspect, diffracted radiation sub-beam 429 can be at least a portion of radiation sub-beam 415 that can be reflected from alignment mark or target 418. In an example of this aspect, interferometer 426 comprises any appropriate set of optical -elements, for example, a combination of prisms that can be configured to form two images of alignment mark or target 418 based on the received diffracted radiation sub-beam 429. It should be appreciated that a good quality image need not be formed. It can be enough to have the features of alignment mark 418 resolved. Interferometer 426 can be further configured to rotate one of the two images with respect to the other of the two images 180° and recombine the rotated and unrotated images interferometrically.

[0060] In some aspects, detector 428 can be configured to receive the recombined image via interferometer signal 427 and detect interference as a result of the recombined image when alignment axis 421 of inspection apparatus 400 passes through a center of symmetry (not shown) of alignment mark or target 418. Such interference can be due to alignment mark or target 418 being 180° symmetrical, and the recombined image interfering constructively or destructively, according to anexample aspect. Based on the detected interference, detector 428 can be further configured to determine a position of the center of symmetry of alignment mark or target 418 and consequently, detect a position of substrate 420. According to an example, alignment axis 421 can be aligned with an optical beam perpendicular to substrate 420 and passing through a center of image rotation interferometer 426. Detector 428 can be further configured to estimate the positions of alignment mark or target 418 by implementing sensor characteristics and interacting with wafer mark process variations.

[0061] In a further aspect, detector 428 determines the position of the center of symmetry of alignment mark or target 418 by performing one or more of the following measurements:1. measuring position variations for various wavelengths (position shift between colors);2. measuring position variations for various orders (position shift between diffraction orders);3. measuring position variations for various polarizations (position shift between polarizations); and4. measuring intensity difference between opposite orders of a diffraction order pair (e.g., to characterize and correct for asymmetry).

[0062] This data can be obtained using any type of alignment sensor, for example, a SMASH (SMart Alignment Sensor Hybrid) sensor, as described in U.S. Patent No. 6,961,116 that employs a selfreferencing interferometer with a single detector and four different wavelengths, and extracts the alignment signal in software, or Athena (Advanced Technology using High order ENhancement 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.

[0063] In some aspects, beam analyzer 430 can be configured to receive and determine an optical state of diffracted radiation sub-beam 439. The optical state can be a measure of beam wavelength, polarization, or beam profile. Beam analyzer 430 can be further configured to determine a position of stage 422 and correlate the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420 can be accurately known with reference to stage 422. Alternatively, beam analyzer 430 can be configured to determine a position of inspection apparatus 400 or any other reference element such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400 or any other reference element. Beam analyzer 430 can be a point or an imaging polarimeter with some form of wavelength-band selectivity. In some aspects, beam analyzer 430 can be directly integrated into inspection apparatus 400, or connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other aspects.

[0064] In some aspects, beam analyzer 430 can be further configured to determine the overlay data between two patterns on substrate 420. One of these patterns can be a reference pattern on a reference layer. The other pattern can be an exposed pattern on an exposed layer. The reference layer can be an etched layer already present on substrate 420. The reference layer can be generated by a reference pattern exposed on the substrate by lithographic apparatus 100 and / or 100’. The exposed layer can be aresist layer exposed adjacent to the reference layer. The exposed layer can be generated by an exposure pattern exposed on substrate 420 by lithographic apparatus 100 or 100’. The exposed pattern on substrate 420 can correspond to a movement of substrate 420 by stage 422. In some aspects, the measured overlay data can also indicate an offset between the reference pattern and the exposure pattern. The measured overlay data can be used as calibration data to calibrate the exposure pattern exposed by lithographic apparatus 100 or 100’, such that after the calibration, the offset between the exposed layer and the reference layer can be minimized.

[0065] In some aspects, beam analyzer 430 can be further configured to determine a model of the product stack profile of substrate 420, and can be configured to measure overlay, critical dimension, and focus of target 418 in a single measurement. The product stack profile contains information on the stacked product such as alignment mark, target 418, or substrate 420, and can include mark process variation-induced optical signature metrology that is a function of illumination variation. The product stack profile can also include product grating profile, mark stack profile, and mark asymmetry information. An example of beam analyzer 430 is Yieldstar™, manufactured by ASML, Veldhoven, The Netherlands, as described in U.S. Patent No. 8,706,442, which is incorporated by reference herein in its entirety. Beam analyzer 430 can be further configured to process information related to a particular property of an exposed pattern in that layer. For example, beam analyzer 430 can process an overlay parameter (an indication of the positioning accuracy of the layer with respect to a previous layer on 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.

[0066] In some aspects, an array of detectors (not shown) can be connected to beam analyzer 430, and allows the possibility of accurate stack profile detection as discussed below. For example, detector 428 can be an array of detectors. For the detector array, a number of options are possible: a bundle of multimode fibers, discrete pin detectors per channel, or CCD or CMOS (linear) arrays. The use of a bundle of multimode fibers enables any dissipating elements to be remotely located for stability reasons. Discrete PIN detectors offer a large dynamic range but each need separate pre-amps. The number of elements is therefore limited. CCD linear arrays offer many elements that can be read-out at high speed and are especially of interest if phase -stepping detection is used.

[0067] In some aspects, a second beam analyzer 430’ can be configured to receive and determine an optical state of diffracted radiation sub-beam 429, as shown in Figure 4B. The optical state can be a measure of beam wavelength, polarization, or beam profile. Second beam analyzer 430’ can be identical to beam analyzer 430. Alternatively, second beam analyzer 430’ can be configured to perform one or more of the functions of beam analyzer 430, such as determining a position of stage 422 and correlating the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420,can be accurately known with reference to stage 422. Second beam analyzer 430’ can also be configured to determine a position of inspection apparatus 400, or any other reference element, such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400, or any other reference element.

[0068] In some aspects, second beam analyzer 430’ can be directly integrated into inspection apparatus 400, or it can be connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other aspects. Alternatively, second beam analyzer 430’ and beam analyzer 430 can be combined to form a single analyzer (not shown) configured to receive and determine the optical states of both diffracted radiation sub-beams 429 and 439.

[0069] In some aspects, processor 432 receives information from detector 428 and beam analyzer 430. Processor 432 can create a basic correction algorithm based on the information received from detector 428 and beam analyzer 430, including but not limited to the optical state of the illumination beam, the alignment signals, associated position estimates, and the optical state in the pupil, image, and additional planes. The pupil plane is the plane in which the radial position of radiation defines the angle of incidence and the angular position defines the azimuth angle of the radiation. Processor 432 can utilize the basic correction algorithm to characterize the inspection apparatus 400 with reference to wafer marks and / or alignment marks 418.

[0070] To increase measurement speed and therefore throughput / productivity, it has been proposed to perform parallel alignment measurements, where more than one alignment mark is measured in parallel. One way of doing this would be to provide multiple alignment apparatuses (hereafter alignment sensors); however this may be prohibitively expensive in terms of cost and / or space.

[0071] A more cost effective proposal is to provide an alignment sensor with multiple sensor heads. Each sensor head can measure a respective alignment mark in parallel.

[0072] Figure 5 is a schematic illustration of an alignment sensor comprising multiple sensor heads (in this example, five). An illumination branch 500 comprises an illumination source 504, illumination objective 508 and mirror / prism 512. The input illumination 514 is directed by a beam splitter 516 (or spot mirror or other suitable arrangement) to a sensor head module 520 comprising beam expander 524, beam splitting arrangement 528a-528f and sensor heads 532a-532e. The beam splitting arrangement may comprise multiple (e.g., non-polarizing) beam splitters and reflectors (e.g., prisms or mirrors). In this specific example, the beam splitting arrangement comprises four beam splitters 528a, 528b, 528c, 528e and two reflectors / prisms 528d, 528f arranged as illustrated. In the specific example shown beam splitting arrangement 528a-528f, splits the input illumination into five paths, one for each sensor head 532a-532e.

[0073] The sensor heads (or at least the respective measurement spots from the sensor heads) may be arranged such that they are all mutually offset, e.g., such that the sensor heads do not need to literally measure multiple marks simultaneously. As such, a parallel measurement may measure multiple marks in parallel, i.e., in a single scan, but where the marks are actually measured sequentially during the scan.This is illustrated at the bottom of Figure 5, where measurement spots 533a-533e are shown staggered (e.g., offset in Y), such that only a single measurement spot 533e is measuring an alignment mark 535e in the position illustrated. Each of the alignment marks 535a-535e will be measured by a respective measurement spot 533a-533e sequentially as the sensor head arrangement 532a-532e is scanned in the direction indicated by the arrows. Such an arrangement may help with the signal processing (e.g., demultiplexing of the resultant measurements).

[0074] It can be appreciated that input illumination may instead be directed towards the substrate (portion) 536 via respective spot mirrors (or other beam directors) located immediately before each sensor head (e.g., between each beam splitter / prism 528b-528f and its respective sensor head 532a- 532e) such that the beam splitting arrangement 528a-528f is not used to split the input illumination (this is done elsewhere).

[0075] More specifically, a first beam splitter splits the input beam into two paths, one for sensor heads 532a, 532d, 532e and the other for the remaining sensor heads. Second beam splitter 532b splits the transmitted beam from the first beam splitter 532a into a path for the sensor head 532a and a path for sensor heads 532d, 532e. Third beam splitter 528c splits the reflected beam from the first beam splitter 532a into a path for sensor head 532b and, via prism / reflector 532d, a path for sensor head 532c. Fourth beam splitter 528e splits the reflected beam from the second beam splitter 532b into a path for sensor head 532d and, via prism / reflector 532f, a path for sensor head 532e.

[0076] In a specific example, the splitting arrangement 528a-528f may have characteristics (e.g., respective reflect / transmit ratios per beamsplitter) to ensure that the transmission loss (transmission efficiency) of each illumination path (i.e., between beam expander 524 and each respective sensor head 532a-532e is substantially the same or similar. For example reflectiomtransmission ratios may be 0.4:0.6, 0.667:0.333, 0.5:0.5 and 0.5:0.5 for the first beam splitter 532a, second beam splitter 532b, third beam splitter 532c and fourth beam splitter 532e respectively (the prisms 528d, 528f provide substantially 100% reflection).

[0077] Each sensor head 532a-532e illuminates a portion of the substrate 536, e.g., a respective alignment mark on the substrate 536, and captures the resultant scattered radiation (e.g., +1 and -1 diffraction orders 538-, 538+). The scattered (e.g., diffracted) radiation follows the same (return) paths as the illumination back through beam splitting arrangement 528a-528f, beam expander 524 and beam splitter 516, which transmits the scattered radiation to intensity channel arrangement 540. The intensity channel arrangement is an optional arrangement for monitoring intensity of the scattered (e.g., diffracted) radiation (e.g., the intensity of each diffraction order of interest). The resultant intensity information may be used to correct positional information measured by the phase channels according to known methods. The intensity channel arrangement 540 will not be described in detail, but may comprise an intensity channel pickoff 541, e.g., a partial polarizer which directs a small part of light to the intensity channels, and a pupil divider 543 which separates the negative and positive diffraction orders.

[0078] The radiation which is not picked off for intensity measurement, is introduced into feed optics 560 which feeds the radiation into interferometer arrangement 544. In this specific embodiment, the feed optics 560 separates the x-polarized and y-polarized components of the scattered radiation from the mark and feeds each component into a respective self-referencing interferometer SRI 544a, 544b for interference, so as to analyze alignment position from each of these components. This is only an example arrangement, and the concepts disclosed herein are also applicable to single interferometer (e.g., single SRI) embodiments.

[0079] Beam splitters 548a, 548b and objective lenses 552a, 552b, 552c, 552d project interfered signals from the SRIs (i.e., phase channels for determining the position of the mark from each sensor), and the intensity signals of the intensity channels (e.g., for performing mark asymmetry corrections and / or other corrections) onto the detector or detection arrangement 556.

[0080] The multiple head sensor arrangement 520 described suffers from low transmission, with only 20% transmission in detection branch compared to single head sensor (the same is also true in illumination, although this can be overcome by increasing illumination intensity or providing the illumination via a separate spot mirror arrangement thereby obviating the beam splitting arrangement 528a-528f in illumination). Mechanical solutions for improving transmission, such as moving mirrors, are impractical due to their necessary complexity, power consumption, thermal issues (heat generated) and dynamic issues. The accuracy required would be very difficult to achieve.

[0081] As such, a number of methods and apparatuses for improving the transmission of the scattered radiation in the detection branch will be described. The proposed methods will replace one or more of the beam splitters, which combine the scattered radiation from two or more sensor heads, with a pupil division arrangement which divides the detection pupil into separated respective detection pupil regions for each sensor head of at least two sensor heads. A pupil in this context may comprise an angularly resolved spectrum or representation of the scattered radiation, e.g., at a pupil plane or angularly resolved plane in the detection branch. Each pupil region may be contiguous or non-contiguous within the pupil space. There pupil space may be divided into pupil regions comprising a respective pupil region for each pupil which is to be combined into a single beam (e.g., losslessly) using mirror apertures.

[0082] As such, a sensor head arrangement for a metrology device, such as an alignment sensor, may comprise a plurality of sensor heads comprising at least a first sensor head being operable to collect first scattered radiation and a second sensor head being operable to collect second scattered radiation; at least one pupil configurator, said at least one pupil configurator being configured to reconfigure at least one of said first scattered radiation and said second scattered radiation, such that said first scattered radiation is confined to one or more first regions of an angularly resolved plane of said first sensor head and said second scattered radiation is confined to one or more second regions of an angularly resolved plane of said second sensor head, wherein said first regions and second regions are substantially noncorresponding (e.g., do not overlap and / or do not comprise common pupil coordinates); and at least one beam combiner being operable to combine said first scattered radiation and said second scatteredradiation subsequent to said at least one pupil configurator directing at least one of said first scattered radiation and said second scattered radiation.

[0083] A first example takes advantage of the fact that alignment marks typically only diffract radiation to angles which lie on one of two mutually perpendicular lines within the pupil (e.g., either side of the pupil center in each of two mutually perpendicular directions). There are two main types of alignment mark, for example, a first type comprises periodic structures which are aligned respectively to X and Y (where X and Y are the two mutually perpendicular directions of the substrate plane). This first type of target diffracts radiation to angles which lie along the pupil coordinate system axes (the kxand kydirections where kxis the pupil direction which corresponds to X and kyis the pupil direction which corresponds to Y). The second type of target comprises periodic structures which are aligned respectively to 45 degrees and -45 degrees with respect to X. This second type of target diffracts radiation to angles which lie along the 45 degree and -45 degree directions with respect to kx. The same basic concept is equally applicable to these first and second types of targets (and other types of targets which diffract to only two mutually perpendicular lines in the pupil), with the only difference being the orientation of certain components as will be described.

[0084] The proposed method comprises imposing a relative rotation between at least a first pupil of scattered radiation from a first sensor head and a second pupil of scattered radiation from a second sensor head and combining these pupils using a suitable spatial pupil combiner (or spatial pupil divider). The spatial pupil combiner may substantially losslessly combine the first and second pupils by reflecting and transmitting the portions of the respective pupils comprising the scattered (e.g., diffracted) radiation (i.e., the information carrying regions of each pupil).

[0085] For example, the spatial pupil combiner may comprise a mirror aperture comprising mirrored regions for reflecting first regions (first radial regions, i.e., regions which extend along a radial direction of the pupil) of the first pupil (or angularly resolved plane) comprising the diffracted radiation captured from the first sensor head and aperture regions for transmitting second regions (second radial regions) of the second pupil comprising the diffracted radiation captured from the second sensor head. The positions of the first radial pupil regions and second radial pupil regions are displaced angularly due to the relative pupil rotation between the first and second pupil.

[0086] In this way, a more efficient use of the pupil space (angularly resolved space) is achieved, with the diffracted orders from two or more sensor heads comprised within a single pupil, such that the beams from two or more sensor heads can be substantially losslessly combined.

[0087] Where there are two pupils being combined, the imposed relative rotation between the first pupil and second pupil may be, for example, 45 degrees (or more generally between 30 and 60 degrees or between 40 and 50 degrees) to maximize separation of the diffraction orders comprised within the two pupils. This may be achieved by rotating either (or both) of the first and second pupils.

[0088] The beam combiner or mirror aperture may comprise a cross mirror, wherein the mirrored regions form a cross shape. Where more than two pupils are being combined, they may each havedifferent respective rotations relative to the other pupils (e.g., all but one pupil may be rotated, each respective rotation being different), and may be combined e.g., two at a time, using suitably shaped mirror apertures which respectively reflect and pass the pupil regions which carry the diffraction orders as appropriate.

[0089] The pupil rotation may be achieved using any suitable pupil configurator such as a pupil rotator. For example, the pupil rotator may comprise any prism capable of pupil rotation, e.g., a Pechan prism, dove prism or K-prism. These are passive 3D prisms which can rotate an input pupil by an arbitrary fixed angle.

[0090] In another embodiment, the pupil may be separated into different regions circumferentially. For example, the pupil may be divided into two or more concentric circumferential regions, e.g., an first region or inner region and a second region or outer region. As before each region may relate to respective scattered radiation from a respective different sensor head. An annular prism element, mirror arrangement or magnifier may distribute one of a first pupil or second pupil around the other of the first pupil or second pupil.

[0091] These two concepts can be combined, e.g., such that one or more of the circumferential pupil regions is / are also separated into radial pupil sub-regions. This enables even more information (e.g., from multiple sensor heads) to be combined into a single pupil.

[0092] Figure 6 is an example implementation of the concepts disclosed herein. It shows a similar sensor head arrangement as sensor head module 520 in Figure 5. However, a first pupil rotator 600a (e.g., a Pechan, dove or K prism) has been added which rotates the pupil comprising the diffracted radiation captured by the sensor head 532c and a second pupil rotator 600b (e.g., a Pechan, dove or K prism) has been added which rotates the pupil comprising the diffracted radiation captured by the sensor head 532e. The other difference is that the beam splitters 528c, 528e have each been replaced by a respective spatial pupil combiner (e.g., mirror aperture or cross mirror) 604a, 604b.

[0093] The first type of alignment mark will diffract incident illumination to pupil locations 622 (a single arbitrary diffraction order location is labeled) on the kxand kyaxes as illustrated by unrotated pupil representation 620. This will be the case (at least initially) for all of the sensor heads when they measure the first type of alignment mark. The pupil rotators 600a, 600b rotate its respective pupil (i.e., the pupils of sensor heads 532c, 532e), e.g., by 45 degrees, as illustrated by rotated pupil representation 624.

[0094] Where the alignment marks to be measured comprise the first type of alignment mark as described above, the spatial pupil combiners 604a, 604b may each comprise cross mirrors 606. These cross mirrors 606 comprise mirror regions 608, aperture regions 612 and a central (aperture) region 616. Here the mirror regions 608 are aligned with the kxand kyaxes. The mirror regions 608 will reflect the portions of the non-rotated pupils comprising the scattered radiation captured by of sensor heads 532b, 532c while the aperture regions 612 will pass / transmit the portions of the rotated pupils comprising the scattered radiation captured by sensor heads 532c, 532e.

[0095] It can be appreciated that, where the alignment marks are the second type of alignment marks which diffract incident illumination to pupil locations along lines 45 and -45 to the kxaxis (e.g., as illustrated by pupil representation 624, then the arrangement illustrated is essentially the same, other than the cross mirror 606’ being orientated at 45 degrees with respect to the cross mirror 606.

[0096] It can be further appreciated that at least the concept described is applicable for a sensor head module comprising any number of sensor heads (greater than one). Also, in the specific sensor head arrangement shown, some benefit will accrue when only one pupil rotator and corresponding spatial pupil combiner is provided for just one of the sensor head pairs on either side of the central sensor head 532a, and therefore the concepts disclosed include such examples.

[0097] The central region 616 of each mirror aperture or cross mirror may comprise an illumination aperture to accommodate the illumination beam. A small beam splitter or suitable coating may be provided within this central region 616 to provide illumination to each sensor head of the relevant pair (sensor heads 532b, 532c or sensor heads 532d, 532e). Alternatively, the input illumination may be directed towards the substrate 536 via respective spot mirrors (or other beam directors) located immediately before each sensor head as has been described.

[0098] The arrangement of Figure 6 may increase efficiency of the collection optics to 33% from 20% compared to the arrangement of single head sensor (e.g., as illustrated in Figure 4B).

[0099] Figure 7 illustrates a further embodiment, implemented as a “clean slate” design. The arrangement of Figure 6 comprises an adaption of a sensor head arrangement presently under development, and there are obvious practical benefits in only minimally adapting this arrangement. However, it is also apparent that further improvement can be obtained by designing from scratch.

[0100] Figure 7 is a highly simplified schematic of only the detection branch of a metrology device comprising a four sensor head arrangement 732a-732d. Note that only the detection path is shown here, i.e., from substrate 738 to detector arrangement 756; the illumination path to the substrate 738 is not shown. The scattered radiation from (e.g., respective marks) on substrate 736 is captured by the sensor heads 732a-732d. More specifically, the scattered radiation from sensor head 732a is reflected by reflector 770a to a first pupil rotator 772a which is configured to rotate the pupil 45 degrees. The information carrying regions of the rotated pupil are then reflected by mirror aperture (cross mirror) 775a, comprising mirrored sections at 45 degrees to the pupil coordinate system. The combined beam is then directed by feed optics 760 (e.g., a polarizing beam splitter).

[0101] Similarly, the scattered radiation from sensor head 732d is reflected by reflector 770b to a second pupil rotator 772b which is configured to rotate the pupil 45 degrees. The information carrying regions of the rotated pupil are then transmitted through the apertures of mirror aperture (cross mirror) 775b, comprising mirrored sections aligned with the pupil coordinate system. The combined beam is then directed to feed optics 760 or polarizing beam splitter.

[0102] The feed optics 760 may divide the scattered radiation based on polarization direction to a respective interferometer (e.g., SRI) 774a. 744b of interferometer arrangement 744, with the interferedbeams detected by detector arrangement 756 (e.g., via lens arrangements 552a-552d or similar as before). The detector arrangement 756 may comprise separate detectors per interferometer or otherwise.

[0103] As before, a single interferometer or SRI example is possible, which measures only a single polarization state.

[0104] This arrangement may be extended to comprise more (e.g., 5 or 6) sensor heads, with one or both arms comprising an additional pupil rotator at an intermediate angle, with an additional mirror aperture orientated appropriately. For example, either or both arms may comprise a first pupil rotators at 45 degrees and a second pupil rotator at 22.5 degree with an additional mirror aperture orientated accordingly. The basic concept may be better understood with reference to the example of Figure 8 below (i.e., this arrangement may be a combination of the Figure 7 and Figure 8 examples.

[0105] It can be appreciated that this Figure 7 arrangement may additionally comprise the intensity channel arrangement 540 of Figure 5.

[0106] This Figure 7 example results in substantially no transmission loss in the detection path. Note that it is possible to support an intensity channel using a non-polarizing pickoff above the feed optics if desired. A relay can also be added to make the pupil smaller.

[0107] Figure 8 is a further example, which provides for 5 sensor heads and a transmission efficiency of 50%. The pupil comprising scattered radiation from substrate 836 captured by sensor head 832e is rotated by a first angle (e.g., between 40 and 50 degrees, between 44 and 46 degrees or 45 degrees) by pupil rotator 872a to obtain pupil 820a. Pupil 820a is directed by reflector 870 to mirror aperture 875a orientated at a second angle (e.g., between 20 and 25 degrees, between 22 and 23 degrees or 22.5 degrees), such that the information carrying portion passes through the mirror’s apertures. The pupil comprising scattered radiation captured by sensor head 832d is rotated by the second angle by pupil rotator 872b to obtain pupil 820b, and the information carrying portion is reflected by mirror aperture 875a and combined with pupil 820a. The information carrying portions of combined pupils 820a, 820b pass through the apertures of mirror aperture 875b orientated at a third angle (e.g., between 65 and 70 degrees, between 67 and 68 degrees or 67.5 degrees). The pupil comprising scattered radiation captured by sensor head 832c is rotated by the third angle by pupil rotator 872c to obtain pupil 820c and the information carrying portion is reflected by mirror aperture 875b and combined with pupils 820a, 820b. The first, second and third angles are all different and non-zero. The information carrying portions of combined pupils 820a, 820b, 820c pass through the apertures of mirror aperture 875c orientated at zero degrees (mirrors aligned with the pupil coordinate system). The information carrying portion of pupil 820d comprising the scattered radiation captured by sensor head 832d is reflected (without prior rotation) by mirror aperture 875c and combined with pupils 820a, 820b 820c. This combined beam is then combined with the pupil captured by sensor head 832a by beam splitter 872.

[0108] This arrangement provides 50% transmission. A four head variation, which does not include sensor head 832a and beam splitter 872 can provide 100% transmission (e.g., as does the similar arrangement of Figure 7). Of course, this arrangement may be adapted to provide fewer or possiblymore sensor heads, although there is a limit to the number of rotated pupils at respective different angles which can be accommodated in a single pupil space due to the finite size of the diffraction orders (in this example the pupil space is divided into four separate pupil regions to accommodate the four pupils 820a, 820b, 820c, 820d). As before, this example relates to the first type of alignment mark; the arrangement can be adapted for the second type of alignment mark as has been described (e.g., by rotating mirror 845c by 45 degrees).

[0109] In all the described examples, the mirror apertures may comprise a mechanism (e.g., an actuator and corresponding control) so as to be rotatable to accommodate the different mark types (e.g., actuatable at least between a first orientation and second orientation). This mechanism does not need to be particularly fast or dynamically stable as the mark types are virtually never mixed on a single substrate, or both used for a single application. As such, it is envisaged that the mirror apertures would only require actuation in a setup stage.

[0110] In all relevant examples described herein, the shapes of the mirror regions, the aperture regions and / or the central region of the mirror apertures may differ from those illustrated. For example, the mirror regions may increase in width radially towards the pupil edge. The central region may be circular or any suitable shape.

[0111] Figure 9A is a highly simplified schematic drawing which illustrates another embodiment, where the respective pupil regions comprise circumferential pupil regions, e.g., such that a pupil of one sensor head is placed around or inside of the pupil of another sensor head. A first pupil from a first sensor head 932a is annulated using an axiconic optical arrangement such as a suitable axicon pair 980 or axicon prism. Such an axiconic optical arrangement or axicon pair is an example of a pupil configurator. The annular beam is then combined by beam combiner 988 with a second pupil of second sensor head 932b. In this manner, the first pupil is comprised within a first circumferential region 982 around the second pupil, which is comprised within a second circumferential region 984. For example, the second circumferential region 984 may represent a standard NA (e.g., 0.6-0.7), with the first pupil shaped to surround the second pupil.

[0112] The axicon pair 980 may comprise two complementary axicons or axicon surfaces, a first axicon to provide a diverging annular beam and a second axicon to parallelize the beam to provide a substantially parallel annular beam. The axicons or axicon surfaces of an axicon pair may each comprise separate elements or complementary surfaces on a single element (the latter example being illustrated).

[0113] The axicons may be conical (circular cross section), although this may deform the diffraction orders slightly. However, this is (in first order) not a problem because symmetry is maintained. The axiconic optical arrangement or axicon pair 980 may instead comprise straight segments with 8-fold or 4-fold symmetry. The 8-fold symmetry ensures both mark types can be accommodated. In this case, the first circumferential region 982 will take a different form, comprising discrete wedges rather than a continuous annular shape. The straight segments may prevent the diffraction order distortion, although at the cost of pitch compatibility.

[0114] The beam combiner 988 may comprise a reflector arrangement comprising a pair of reflectors operable to reflect said first scattered radiation and second scattered radiation along a common axis. For example, the beam combiner 988 may comprise a pair of mirrors / reflectors as illustrated, e.g., a smaller 45 degree orientated mirror inside of (e.g., centered on the same optical axis) a larger 45 degree orientated mirror.

[0115] Figure 9B illustrates a variation of the arrangement of Figure 9A where the axiconic optical arrangement comprises an axiconic mirror arrangement 986. The axiconic mirror arrangement 986 directs one of the pupils (here the pupil of second sensor head 932b, although this is arbitrary) to the first circumferential region 982’ around second circumferential region 984’. Using mirrors instead of glass may have advantages. For example, the path lengths may be better matched (the increased path length through reflector arrangement or mirror arrangement 986 may be compensated by the smaller path length to the sensor (being the “inside” path). Having matched path lengths is not necessary, but may have advantages. Like the prism example, the axiconic mirror arrangement 986 may also have rotational symmetry or be segmented.

[0116] The axiconic mirror arrangement 986 may comprise a reflective axicon or reflaxicon, e.g., as described in The Reflaxicon, a New Reflective Optical Element, and Some Applications, Applied Optics Vol. 12, Issue 8, pp. 1940-1945 (1973), which is hereby incorporated by reference. This device comprises a primary conical mirror 986a and a larger secondary conical mirror 986b located coaxially with respect to the primary mirror. The secondary mirror is truncated at such a location that the inner diameter of the secondary mirror (the diameter of the hole) exceeds the base diameter of the primary mirror. The divergence / convergence of the beam may be configured by selection of the half-angles of each conical mirror 986a, 986b (e.g., a parallel beam may be obtained by appropriate selection of the half-angles).

[0117] In this example, the beam combiner 990 may again comprise a pair of mirrors / reflectors, but where the inner mirror is the larger diameter, annular shaped, mirror.

[0118] The pupil configurator may comprise other suitable pupil configurators than illustrated and described here. Another possibility may comprise using a beam magnifier to magnify one beam around the other.

[0119] These arrangements may be combined with the other arrangements and examples described. For example, the arrangements of Figures 9A and / or 9B may be implemented in combination with the sensor head arrangement of Figure 5 or Figure 6. For example, considering Figure 5, the rotator 600b may be replaced with prism 980 (Figure 9A) and reflector 528f and beam splitter 528e replaced with beam combiner 988. A similar approach can be followed on the other side (e.g., replacing components 600a, 528d, 528c with prism 980 and beam combiner 988 in the same way).

[0120] Figure 10A illustrates an example of a specific combined arrangement which combines the concept of Figure 7 with the concept of Figure 9A (or Figure 9B), thereby splitting the pupil into regionsboth circumferentially and radially, while combining the pupils from (in this example) four sensor heads losslessly. Similarly to Figure 7, only the detection path is shown.

[0121] The sensor heads 1032a, 1032b, 1032c, 1033d correspond to sensor heads 732a, 732b, 732c, 733d of Figure 7 and reflectors 1070a, 1070b, pupil rotators 1072a 1072b and mirror apertures (cross mirrors) 1075a, 1075b respectively correspond to, and operate the same as, reflectors 770a, 770b, pupil rotators 772a 772b and mirror apertures (cross mirrors) 775a, 775b of Figure 7. As such, they will not be described in detail again. Briefly, the scattered radiation from sensor head 1032a is rotated (e.g., by 45 degrees) by first pupil rotator 1072a and combined with the scattered radiation captured by sensor head 1032b using mirror aperture 1075ato form first combined beam 1082 (comprising a pupil divided into regions radially). The scattered radiation from sensor head 1032d is rotated (e.g., by 45 degrees) by second pupil rotator 1072b and combined with the scattered radiation captured by sensor head 1032c using mirror aperture 1075b to form combined beam 1084 (also comprising a pupil divided into regions radially). One of the combined beams, here the second combined beam 1084 propagating parallel to the substrate plane, is annulated to form an annulated beam 1086 (i.e., having an annular cross section) using an axiconic optical arrangement. This axiconic optical arrangement may comprise a suitable axicon pair or axicon prism 1080 as illustrated (e.g., the same as axicon pair or axicon prism 980 in Figure 9A) or as an alternative example, an axiconic mirror arrangement 986 as illustrated in Figure 9B.

[0122] The first combined beam 1082 and annular (second combined) beam 1086 are then in turn combined using a different form of mirror aperture as has been described, e.g., an inverse spot mirror 1060. The inverse spot mirror 1060 may be formed by a suitable prism arrangement, for example.

[0123] An example inverse spot mirror 1060 is shown in Figure 10B. The inverse spot mirror may comprise a partially reflective surface 1062 or hypotenuse surface, e.g., a 45 degree oriented surface with respect to the propagation directions of each of the beams 1082, 1086 being combined. The partially reflective surface 1062 comprises a transmissive surface portion 1064 in a central area of the surface and reflective surface portion 1066 in a peripheral area (i.e., surrounding and around the transmissive surface portion 1064). The transmissive surface portion 1064 may, for example, be substantially oval or circular (e.g., appearing substantially circular in a plane parallel to the substrate plane) thereby forming an effective aperture in the otherwise mirrored surface 1066. As such, “aperture” in the context of a mirrored aperture may include a substantially transmissive surface in addition to a strict definition of aperture.

[0124] It should be appreciated that the transmissive surface portion and reflective surface portion may be switched from the example shown, i.e., the transmissive surface portion may surround a central reflective surface portion, with the beams arranged accordingly (i.e., combined beam 1032b would then be annulated instead of combined beam 1084).

[0125] The transmissive surface portion 1064 is configured to reflect the annular beam 1086 and the transmissive surface portion 1064 is configured to transmit the first combined beam 1082, such that inthe pupil plane, the annular beam 1086 surrounds the first combined beam 1082. As such, the pupil is divided into regions both circumferentially and radially.

[0126] The remaining elements of the sensor are represented by element 1090. This may comprise, for example, feed optics 760, interferometer / SRI arrangement 744 and sensor arrangement 756 as with Figure 7. Optionally, intensity channels may also be provided.

[0127] It can be appreciated that the number of sensor heads may be increased from the four shown, e.g., using the concepts described in relation to Figure 8. In this way, one or both of the annular beam 1086 and first combined beam 1082 may comprise scattered radiation from more than two sensor heads, by dividing the pupil into more radial pupil regions. This can accommodate a sensor head arrangement with (at least close to) 100% efficiency for anything up to 8 or 10 sensor heads, possibly more.

[0128] It can be appreciated that the inverse spot mirror 1060 may be used in embodiments similar to those of Figure 9A and 9B, where the pupil is divided only circumferentially; i.e., to combine beams from two sensor heads (sensor head 1032b, 1032d) without any radial combination. This equates to Figure 10A without sensor heads 1032a, 1032c and without elements 1072a, 1072b, 1075a, 1075b.

[0129] It can be appreciated that the Figure 9A, 9B, 10A examples effectively increase the etendue of the light going into the interferometer collection fiber. If needed, the etendue accepted by elements such as collection fiber can be increased by increasing the diameter and / or NA of the fiber to prevent light loss. All subsequent elements, such as photodetector, may also be assessed and matched in etendue (e.g. increased in diameter), where required. Some of the proposed embodiments have magnifying optics (e.g., beam expander 524 in Figure 5) between the sensor heads and the rest of the sensor, to optimally match the sensor heads (microscope objects) to the rest of the optics. These magnifying optics can be re-optimized to ensure the light is optimally transmitted through the rest of the optics and optimally coupled into the collection fiber.

[0130] There may be more than two circumferential pupil regions, with the basic concept essentially being repeated (e.g., using a prism or mirror arrangement to place a further pupil around the outermost of the two previously combined pupils, e.g., around first circumferential region 982, 982’). Alternatively, or in addition, the circumferential pupil region examples may be combined with radial pupil region examples previously described. For example, one or both of the first circumferential region 982, 982’ and / or second circumferential region 984, 984’ can be sub-divided into radial regions by combining the optical arrangements described. If only one is to be sub-divided, it is preferable that it be the outermost circumferential region (as it is larger). When a prism or mirror is used using only straight segments, there will be even more space for accommodating different radial sub-regions.

[0131] For example, a combined beam (by way of a specific illustrative example, the beam reflected by beam splitter 528a being a combination of the pupils from sensor heads 532b, 532c (the latter rotated) can be fed into axicon pair / prism 980 or mirror arrangement 986 and combined with another beam (comprising scattered radiation from one or more other sensor heads).

[0132] In all the above embodiments, a single detector or detector arrangement may be used, in which case the information from the different sensor heads will require demultiplexing into a single output channel. As such, a demultiplexer may be provided to demultiplex the data per sensor head. Alternatively, all embodiments described may implement respective separate output channels and detectors for each sensor head (e.g., downstream of the interferometer(s)).

[0133] Figure 11 illustrates a metrology arrangement comprising separate output channels and detectors for each sensor head. The metrology apparatus up to and including the SRI(s) is represented by metrology device 1101. This may comprise any of the metrology devices disclosed herein. The combined beam(s), e.g., one per SRI, can be separated into the respective different detection branches using a suitable splitting arrangement or spatial demultiplexer arrangement 1155 such as the mirror apertures (e.g., cross mirrors for the radial embodiments or annular mirrors for the circumferential embodiments) already described. In this case the mirror apertures can be used as beam dividers rather than beam combiners. The separated beams are then detected by a respective separate detector region 1156a, 1156b, 1156c, 1156d for each of the sensor heads. The separate detector regions may comprise respective separate detectors or different spaced apart regions on one or more detectors.

[0134] In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of clauses that may be optionally claimed in any combination:1. A sensor head arrangement for a metrology device, the sensor head arrangement comprising: a plurality of sensor heads comprising at least a first sensor head being operable to collect first scattered radiation and define a first pupil comprising an angularly resolved representation of said first scattered radiation and a second sensor head being operable to collect second scattered radiation and define a second pupil comprising an angularly resolved representation of said second scattered radiation; at least a first pupil configurator, said at least a first pupil configurator being configured to reconfigure at least one of said first pupil and / or said second pupil, such that said first scattered radiation is confined to one or more first regions of said first pupil and said second scattered radiation is confined to one or more second regions of said second pupil, wherein said first regions and second regions are substantially non-corresponding; and at least a first beam combiner being operable to combine said first scattered radiation and said second scattered radiation into a first combined beam.2. A sensor head arrangement as claimed in clause 1, wherein said first beam combiner is located downstream of said at least one pupil configurator.3. A sensor head arrangement as claimed in clause 1 or 2, wherein said first beam combiner comprises a substantially lossless beam combiner.4. A sensor head arrangement as claimed in any preceding clause, wherein one or more of said at least a first pupil configurator comprises a pupil rotator being operable to impose a relative rotation between said first pupil and said second pupil.5. A sensor head arrangement as claimed in clause 4, wherein said at least a first pupil configurator is operable to rotate said first pupil such that the first scattered radiation is confined to said first regions and / or said at least one pupil configurator is operable to rotate said second pupil such that the second scattered radiation is confined to said second regions.6. A sensor head arrangement as claimed in clause 4 or 5, wherein at least one of said at a first pupil configurator comprises a Pechan prism, dove prism or K-prism.7. A sensor head arrangement as claimed in any preceding clause, wherein each said at least one beam combiner comprises a respective mirror aperture comprising reflective regions corresponding to said first regions and aperture regions corresponding to said second regions, said at least one beam combiner being operable to reflect the first scattered radiation and to pass the second scattered radiation onto a common path.8. A sensor head arrangement as claimed in clause 7, wherein each said mirror aperture comprises a respective actuator for orienting each said mirror aperture between at least a first orientation and second orientation.9. A sensor head arrangement as claimed in any preceding clause, wherein said first regions and second regions each comprise radial regions, which extend radially within its respective pupil.10. A sensor head arrangement as claimed in any preceding clause, wherein said plurality of sensor heads comprises a third sensor head being operable to collect third scattered radiation and define a third pupil comprising an angularly resolved representation of said third scattered radiation and a fourth sensor head being operable to collect fourth scattered radiation and define a fourth pupil comprising an angularly resolved representation of said second scattered radiation; a second pupil configurator, said second pupil configurator being operable to impose a relative rotation between said third pupil and said fourth pupil; and a second beam combiner being operable to combine said third scattered radiation and said fourth scattered radiation downstream of said second pupil configurator into a second combined beam.11. A sensor head arrangement as claimed in clause 10, further comprising at least one additional beam combiner being operable to combine said first combined beam and said second combined beam.12. A sensor head arrangement as claimed in clause 11, wherein said plurality of sensor heads comprises a fifth sensor head and said at least one additional beam combiner is operable to combine said first combined beam and said second combined beam with fifth scattered radiation from the fifth sensor head.13. A sensor head arrangement as claimed in clause 11 or 12, wherein said at least one additional beam combiner comprises one or more one beam splitters.14. A sensor head arrangement as claimed in any of clauses 1 to 9, wherein: said plurality of sensor heads comprises at least a third sensor head, operable to collect third scattered radiation and define a third pupil comprising an angularly resolved representation of said third scattered radiation; and said sensor head arrangement further comprises at least a second pupil configurator being operable to rotate said third pupil, such that said first pupil, said second pupil and third pupil all have respective different orientations.15. A sensor head arrangement as claimed in clause 14, further comprising at least a second beam combiner, such that said first beam combiner and second beam combiner together combine said first scattered radiation, second scattered radiation and third scattered radiation.16. A sensor head arrangement as claimed in clause 15, wherein said first beam combiner and second beam combiner each comprise a mirror aperture having a respective different orientation corresponding to the imposed rotations to said second pupil and said third pupil.17. A sensor head arrangement as claimed in clause 16, wherein said plurality of sensor heads comprises one or more additional sensor heads and said sensor head arrangement further comprises a further pupil configurator and further mirror aperture for each additional sensor head, each pupil configurator imposing a respective different rotation and each mirror aperture having a respective different orientation such that respective scattered radiation captured by each sensor head is confined to a respective different non-corresponding pupil region.18. A sensor head arrangement as claimed in clause 1, 2 or 3, wherein said at least one pupil configurator is operable to configure said first pupil around said second pupil, such that said first region comprises a first circumferential region which surrounds said second region, said second region comprising a second circumferential region.19. A sensor head arrangement as claimed in clause 18, wherein said at least one pupil configurator comprises an axiconic optical arrangement.20. A sensor head arrangement as claimed in clause 19, wherein said axiconic optical arrangement comprises an axicon pair.21. A sensor head arrangement as claimed in clause 19, wherein said axiconic optical arrangement comprises a reflaxicon.22. A sensor head arrangement as claimed in clause 21, wherein said axiconic optical arrangement comprises primary reflector and a secondary reflector located coaxially with respect to the primary reflector.23. A sensor head arrangement as claimed in any of clauses 19 to 22, wherein said axiconic optical arrangement comprises substantially straight sections with four-fold or eight-fold symmetry.24. A sensor head arrangement as claimed in any of clauses 19 to 23, wherein said axiconic optical arrangement is operable to direct said first scattered radiation and second scattered radiation along a common axis.25. A sensor head arrangement as claimed in any of clauses 18 to 24, wherein said plurality of sensor heads comprises at least a third sensor head operable to collect third scattered radiation and define a third pupil comprising an angularly resolved representation of said third scattered radiation and said sensor head arrangement further comprises at least a second pupil configurator being operable to rotate said third pupil with respect to one of said first pupil or second pupil and a mirror aperture operable to combine the third pupil with said first pupil or second pupil, thereby subdividing one of said first circumferential region or second circumferential region radially.26. A sensor head arrangement as claimed in clause 25, wherein said plurality of sensor heads comprises at least a fourth sensor head operable to collect fourth scattered radiation and define a fourth pupil comprising an angularly resolved representation of said fourth scattered radiation and said sensor head arrangement further comprises at least a third pupil configurator being operable to rotate said fourth pupil with respect to the other of said first pupil or second pupil and a mirror aperture operable to combine the fourth pupil with said other of the first pupil or second pupil thereby subdividing the other of said first circumferential region or second circumferential region radially.27. A sensor head arrangement as claimed in any of clauses 18 to 26, wherein at least said first pupil and at least said second pupil are combined using an inverse spot mirror, said inverse spot mirror comprises a partially transmissive service having a central first region and a surrounding second region; wherein the first region is transmissive and the second region is reflective, or the first region is reflective and the second region is transmissive.28. An alignment system comprising: the sensor head arrangement of any preceding clause; at least one interferometer configured to receive at least said first scattered radiation and second scattered radiation and, per sensor head of said plurality of sensor heads, generate an alignment signal, thereby generating a plurality of alignment signals comprising a respective alignment signal per sensor head; and at least one detector configured to determine a respective position of each of a plurality of alignment marks based on said plurality of alignment signals.29. An alignment system as claimed in clause 28, wherein said at least one interferometer comprises a self-referencing interferometer.30. An alignment system as claimed in clause 28 or 29, further comprising a demultiplexer to demultiplex said plurality of alignment signals.31. An alignment system as claimed in clause 28, 29 or 30 wherein said at least one detector comprises a plurality of detectors or detector regions, each detector or detector region being operable to receive a respective alignment signal of said plurality of alignment signals.32. An alignment system as claimed in clause 31, further comprising a splitting arrangement being operable to split at least said first scattered radiation and second scattered radiation prior to said at least one detector.33. An alignment system as claimed in any of clauses 28 to 32, further comprising a radiation source configured to produce one or more illumination beams and direct the one or more illumination beams toward said plurality of alignment marks.34. An alignment system as claimed in any of clauses 28 to 33, comprising a polarizing beam splitter configured to receive at least said first scattered radiation and second scattered radiation and split each into a first polarization component and a second polarization component; and wherein said at least one interferometer comprises a first interferometer for receiving each first polarization component and a second interferometer for receiving each second polarization component.35. A lithographic apparatus comprising: a patterning device support for supporting a patterning device; a substrate support for supporting a substrate; a projection system being operable to project a patterned beam onto the substrate, subsequent to the beam being patterned by the patterning device; and the alignment system of any of claims 28 to 34, being operable to measure positional information relating to a position of said substrate.36. A method of performing a parallel measurement of two or more marks on a substrate, the method comprising: collecting first scattered radiation from a first mark and defining a first pupil comprising an angularly resolved representation of said first scattered radiation; collecting second scattered radiation from a second mark and defining a second pupil comprising an angularly resolved representation of said second scattered radiation; reconfiguring at least one of said first pupil and / or said second pupil, such that said first scattered radiation is confined to one or more first regions of said first pupil and said second scattered radiation is confined to one or more second regions of said second pupil, wherein said first regions and second regions are substantially non-corresponding; and combining said first scattered radiation and said second scattered radiation into a first combined beam, subsequent to said reconfiguring.37. A method as claimed in clause 36, wherein said reconfiguring comprises rotating at least one of said first pupil and / or said second pupil to impose a relative rotation between said first pupil and said second pupil.38. A method as claimed in clause 37, wherein said rotating comprises rotating said first pupil such that the first scattered radiation is confined to said first regions and / or rotating said second pupil such that the second scattered radiation is confined to said second regions.39. A method as claimed in any of clauses 36, 37 or 38, wherein said combining step comprises reflecting the first scattered radiation and passing the second scattered radiation onto a common path.40. A method as claimed in any of clauses 36 to 39, wherein said first regions and second regions each comprise radial regions, which extend radially within its respective pupil.41. A method as claimed in any of clauses 36 to 40, further comprising: collecting third scattered radiation from a third mark and defining a third pupil comprising an angularly resolved representation of said third scattered radiation; collecting fourth scattered radiation from a fourth mark and defining a fourth pupil comprising an angularly resolved representation of said second scattered radiation; imposing a relative rotation between said third pupil and said fourth pupil; and combining said third scattered radiation and said fourth scattered radiation downstream of said second pupil configurator into a second combined beam.42. A method as claimed in clause 41, comprising: collecting fifth scattered radiation from a fifth mark and defining a fifth pupil comprising an angularly resolved representation of said fifth scattered radiation combining said first combined beam and said second combined beam with said fifth scattered radiation.43. A method as claimed in clause 41, comprising a beam directing arrangement operable to direct said first combined beam to a first interferometer and said second combined beam to a second interferometer.44. A method as claimed in any of clauses 36 to 40, wherein: collecting third scattered radiation from a third mark and defining a third pupil comprising an angularly resolved representation of said third scattered radiation; and rotating said third pupil, such that said first pupil, said second pupil and third pupil all have respective different orientations.45. A method as claimed in clause 44, wherein said combining step comprises combining said first scattered radiation, second scattered radiation and third scattered radiation.46. A method as claimed in clause 45, comprising performing a parallel measurement of more than said first mark, second mark and third mark, and imposing a respective different rotation to each respective scattered radiation captured from each mark such that it is confined to a respective different non-corresponding pupil region.47. A method as claimed in clause 46, wherein said configuring step comprises configuring said first pupil around said second pupil, such that said first region comprises a first circumferential region which surrounds said second region, said second region comprising a second circumferential region.48. A method as claimed in clause 47, further comprising: collecting third scattered radiation from a third mark and defining a third pupil comprising an angularly resolved representation of said third scattered radiation; rotating said third pupil with respect to one of said first pupil or second pupil; andcombining the third pupil with said first pupil or second pupil, thereby subdividing said first circumferential region or second circumferential region radially.

[0135] A combination of temporal and spatial demultiplexing is also possible, e.g., where each (or at least one) detector region is shared between a subset of the sensor heads.

[0136] The terms “radiation,” “beam,” “light,” “illumination,” or the like can be used herein to refer to one or more types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength X of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-100 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G- line 436 nm; H-line 405 nm; and / or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in some aspects, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm.

[0137] Although some aspects of the present disclosure are described in the context of lithographic apparatuses in the manufacture of ICs, it should be understood that lithographic apparatuses described herein can be used in other applications, for example, in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, ECDs, thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein can be considered as specific examples of the more general terms “substrate” or “target portion”, respectively. A substrate can be processed before or after exposure in, for example, a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) and / or a metrology unit. Where applicable, aspects disclosed herein can be applied to such and other substrate processing tools. Furthermore, a substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains multiple processed layers.

[0138] Furthermore, although some aspects of the present disclosure are described in the context of optical lithography, it should be understood that aspects of the present disclosure are not limited to optical lithography. For example, 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.

[0139] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0140] The present disclosure has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. The foregoing description of specific aspects will so fully reveal the 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 aspects, without undue experimentation and without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed aspects, based on the teaching and guidance presented herein.

[0141] It is to be understood that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections can set forth one or more, but not necessarily all, aspects of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the present disclosure and the appended claims in any way. The breadth and scope of the protected subject matter should not be limited by any of the above -de scribed aspects, but should be defined in accordance with the following claims and their equivalents.

Claims

CLAIMS1. A sensor head arrangement for a metrology device, the sensor head arrangement comprising: a plurality of sensor heads comprising at least a first sensor head being operable to collect first scattered radiation and define a first pupil comprising an angularly resolved representation of said first scattered radiation and a second sensor head being operable to collect second scattered radiation and define a second pupil comprising an angularly resolved representation of said second scattered radiation; at least a first pupil configurator, said at least a first pupil configurator being configured to reconfigure at least one of said first pupil and / or said second pupil, such that said first scattered radiation is confined to one or more first regions of said first pupil and said second scattered radiation is confined to one or more second regions of said second pupil, wherein said first regions and second regions are substantially non-corresponding; and at least a first beam combiner being operable to combine said first scattered radiation and said second scattered radiation into a first combined beam.

2. A sensor head arrangement as claimed in claim 1, wherein said first beam combiner is located downstream of said at least one pupil configurator.

3. A sensor head arrangement as claimed in claim 1 or 2, wherein said first beam combiner comprises a substantially lossless beam combiner.

4. A sensor head arrangement as claimed in any preceding claim, wherein one or more of said at least a first pupil configurator comprises a pupil rotator being operable to impose a relative rotation between said first pupil and said second pupil.

5. A sensor head arrangement as claimed in claim 4, wherein said at least a first pupil configurator is operable to rotate said first pupil such that the first scattered radiation is confined to said first regions and / or said at least one pupil configurator is operable to rotate said second pupil such that the second scattered radiation is confined to said second regions.

6. A sensor head arrangement as claimed in claim 4 or 5, wherein at least one of said at a first pupil configurator comprises a Pechan prism, dove prism or K-prism.

7. A sensor head arrangement as claimed in any preceding claim, wherein each said at least one beam combiner comprises a respective mirror aperture comprising reflective regions corresponding to said first regions and aperture regions corresponding to said second regions, said at least one beamcombiner being operable to reflect the first scattered radiation and to pass the second scattered radiation onto a common path.

8. A sensor head arrangement as claimed in claim 7, wherein each said mirror aperture comprises a respective actuator for orienting each said mirror aperture between at least a first orientation and second orientation.

9. A sensor head arrangement as claimed in any preceding claim, wherein said first regions and second regions each comprise radial regions, which extend radially within its respective pupil.

10. A sensor head arrangement as claimed in any preceding claim, wherein said plurality of sensor heads comprises a third sensor head being operable to collect third scattered radiation and define a third pupil comprising an angularly resolved representation of said third scattered radiation and a fourth sensor head being operable to collect fourth scattered radiation and define a fourth pupil comprising an angularly resolved representation of said second scattered radiation; a second pupil configurator, said second pupil configurator being operable to impose a relative rotation between said third pupil and said fourth pupil; and a second beam combiner being operable to combine said third scattered radiation and said fourth scattered radiation downstream of said second pupil configurator into a second combined beam.

11. A sensor head arrangement as claimed in claim 10, further comprising at least one additional beam combiner being operable to combine said first combined beam and said second combined beam.

12. A sensor head arrangement as claimed in claim 11, wherein said plurality of sensor heads comprises a fifth sensor head and said at least one additional beam combiner is operable to combine said first combined beam and said second combined beam with fifth scattered radiation from the fifth sensor head.

13. A sensor head arrangement as claimed in claim 11 or 12, wherein said at least one additional beam combiner comprises one or more one beam splitters.

14. A sensor head arrangement as claimed in any of claims 1 to 9, wherein: said plurality of sensor heads comprises at least a third sensor head, operable to collect third scattered radiation and define a third pupil comprising an angularly resolved representation of said third scattered radiation; andsaid sensor head arrangement further comprises at least a second pupil configurator being operable to rotate said third pupil, such that said first pupil, said second pupil and third pupil all have respective different orientations.

15. A sensor head arrangement as claimed in claim 14, further comprising at least a second beam combiner, such that said first beam combiner and second beam combiner together combine said first scattered radiation, second scattered radiation and third scattered radiation.

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