Optical demultiplexer

Abstract

A new optical demultiplexer is disclosed for use in a metrology system. The optical demultiplexer comprises: a body; an input; a plurality of outputs; and a plurality of optical filters. Each of the plurality of outputs comprises a lens supported by a surface of the body. The plurality of optical filters are supported by the body and are arranged such that when radiation comprising a plurality of spectral components is received at the input, a different spectral component is received by each of the plurality of outputs via the body. The lens of each of the plurality of outputs is arranged to couple the spectral component received by that output out of the body. Advantageously, such an arrangement can be significantly smaller than known optical demultiplexers, which typically comprise a large number of optical components.

Description

OPTICAL DEMULTIPLEXERCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 747,570 which was filed on 21 January 2025, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a new optical demultiplexer. The optical demultiplexer may, for example, form part of a metrology system. Such a metrology system may form part of an exposure apparatus, for example a lithographic apparatus or lithographic tool. Alternatively, such a metrology system may form part of a tool which is separate from such an exposure apparatus. The present invention also relates to a metrology system for determining a position of at least one target on a substrate, the metrology system comprising the new optical demultiplexer. The present invention also relates to an exposure apparatus comprising such a metrology system.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern defined by a patterning device (e.g., a mask) onto a layer of radiation- sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] Various metrology systems may be used by lithographic systems. Examples of such metrology systems include alignment measurement systems and overlay measurement systems.

[0006] Within a lithographic apparatus, the substrate (for example a resist-coated wafer) is supported by a stage and is typically clamped to the stage (for example using an electrostatic clamp). Once a substrate has been clamped to the stage, it is desirable to measure the position of a plurality of alignment marks distributed over the substrate. These alignment measurements are used during the subsequent exposure of the substrate in order to accurately form the image on the substrate. Such alignment measurements are made by an alignment measurement system. Once the pattern has been formed, an overlay measurement system may be used to measure overlay (i.e. an offset or misalignment between two or more successively formed layers on the substrate).

[0007] It may be desirable, within an alignment or overlay metrology system, to perform multiple measurements using different wavelength components. In order to make such multiple measurements simultaneously a plurality of different wavelength components illuminate the target (wafer) and the captured light at the output of the sensor may be demultiplexed.

[0008] It may be desirable to provide a new optical demultiplexer that at least partially address one or more problems associated with prior art arrangements, whether such problems are disclosed herein or otherwise.SUMMARY

[0009] According to a first aspect of the present disclosure there is provided an optical demultiplexer for use in a metrology system, the optical demultiplexer comprising: a body; an input; a plurality of outputs, each of the plurality of outputs comprising a lens supported by a surface of the body; and a plurality of optical filters supported by the body and arranged such that when radiation comprising a plurality of spectral components is received at the input, a different spectral component is received by each of the plurality of outputs via the body and wherein the lens of each of the plurality of outputs is arranged to couple the spectral component received by that output out of the body.

[0010] The optical demultiplexer according to the first aspect of the present disclosure comprises a plurality of lenses defined on the same body. Advantageously, such an arrangement can be significantly smaller than known optical demultiplexers, which typically comprise a large number of optical components. Furthermore, by providing a lens of each of the plurality of outputs that is supported by the same body, it is significantly easier to align the optical demultiplexer according to the first aspect of the present disclosure. For example, it is easier to align the demultiplexer to: an optical component (for example an optical fiber) to couple (multiplexed) radiation (for example from a sensor output) into the input of the optical demultiplexer; and / or to a plurality of optical components (for example optical fibers) to the plurality of outputs. The alignment can be achieved by moving the body in a suitable way. This is in contrast to exiting optical demultiplexers that comprise a plurality of separate optical components that may need to be independently moved to ensure alignment of the demultiplexer.

[0011] As discussed further below, the lens of each of the plurality of outputs may be supported either directly or indirectly by a surface of the body. For example, in some embodiments the lens of each of the plurality of outputs may be disposed directly on a surface of the body. Alternatively, in some embodiments, the lens of each of the plurality of outputs may be supported by a corresponding one of the plurality of filters, which, in turn, may be disposed on a surface of the body.

[0012] As discussed further below, each of the plurality of optical filters being supported by the body may mean the each of the plurality of optical filters is disposed either in or on the body.

[0013] The plurality of optical filters may be arranged in a cascade arrangement such that at least a portion of the radiation received by the input propagates to each of the plurality of optical filters sequentially.

[0014] For example, radiation received by the input may propagate to a first one of optical filters, a portion of the radiation may then passes to a second one of the plurality of optical filters and so on until the radiation is incident on a final one of the plurality of optical filters.

[0015] Each of the plurality of optical filters may be associated with a different one of the plurality of outputs.

[0016] Each of the plurality of optical filters may direct the spectral component coupled out of the body by a different one of the plurality of outputs to that output.

[0017] An optical filter that directs the spectral component coupled out of the body by one of the plurality of outputs to that output may be said to correspond to that output and, similarly, that output may be said to correspond to that optical filter. For such embodiments, each optical filter may be considered to be part of the corresponding output.

[0018] Each of the plurality of optical filters may be adjacent to the lens of a different one of the plurality of outputs.

[0019] For such embodiments each of the plurality of optical filters may be disposed on an external surface of the body and a corresponding lens from a corresponding output may be disposed on each optical filter.

[0020] For such embodiments, each optical filter may be arranged to transmit the spectral component coupled out by the corresponding lens that it is adjacent to and may be arranged to reflect other spectral components. The other spectral components may be reflected by that optical filter and may, for example, propagate to another one of the plurality of spectral filters.

[0021] Each of the plurality of optical filters may be disposed in the body.

[0022] For such embodiments each of the plurality of optical filters may be imbedded in the body and the body may comprise a plurality of portions (rather than being monolithic).

[0023] For such embodiments, each optical filter may be arranged to reflect the spectral component coupled out by the lens of a corresponding output and may be arranged to transmit other spectral components. The other spectral components may, for example, propagate to another one of the plurality of spectral filters.

[0024] For such embodiments, each of the plurality of optical filters may be separate from, or spaced apart from, a corresponding output.

[0025] Each of the plurality of optical filters may comprise non-dispersive filters.

[0026] Advantageously, such non-dispersive filters are relatively insensitive to the angle at which the radiation is incident on them. This is in contrast to dispersive filters, which can require a specific angle of incidence in order to direct different wavelength components to specific locations. Arrangements that use dispersive filters may therefore require an input radiation beam to undergo some relatively narrow angular filtering before they are incident on the dispersive filters. This can significantly reduce the intensity of the radiation and therefore may reduce the throughput of a metrology system using thedemultiplexer. Advantageously, the use of non-dispersive filters can improve the throughput of such a metrology system (relative to an arrangement that uses dispersive filters).

[0027] The optical demultiplexer may comprise a diffractive optical element arranged to receive radiation comprising a plurality of spectral components that is received by the input and to direct each of the plurality of spectral components in a different direction.

[0028] The diffractive optical element may comprise a metasurface reflector. The diffractive optical element may be supported by the body. The diffractive optical element may preferentially direct each of the plurality of spectral components to a different one of the plurality of optical filters (which may, for example, be formed on a surface of the body).

[0029] Each of the plurality of optical filters may comprise a multilayer structure comprising alternating layers formed from two or more different materials and arranged to preferentially reflect or transmit a specific spectral component.

[0030] Such a multilayer structure may be referred to as a Bragg structure. Each such optical filter may be considered to be a bandpass filter. In one example embodiment, the multilayer structure may comprise alternating layers formed from silicon and silicon dioxide. The thicknesses of the layers of each such optical filter may be selected in dependence on an angle of incidence of the radiation on that filter and the wavelength of the spectral component in order to preferentially reflect or transmit that spectral component.

[0031] The body may comprise at least one substrate.

[0032] The at least one substrate may, for example, comprise any suitable type of glass such as, for example, fused silica.

[0033] The body may comprise a monolithic substrate.

[0034] With such embodiments, each of the plurality of optical filters may be formed on an external surface of the body (and a lens of a corresponding output may, for example, be formed in each optical filter).

[0035] The body may comprise a composite substrate.

[0036] For example, such a composite substrate may comprise a plurality of components rigidly joined to each other (for example via an adhesive). With such embodiments, the plurality of optical filters may be formed in the body (for example between two of the plurality of components). For example, each of the plurality of optical filters may be formed as a coating on a surface of one of the plurality of components, which may subsequently be adhered to another of the plurality of components (sandwiching the optical filter between the two components).

[0037] The optical demultiplexer may define a plurality of nominal optical paths. For example, a nominal optical path may be defined (in the body) between the input and each of the plurality of outputs. The or each nominal optical beam path may comprise at least one reflection from a surface of the body (for example total internal reflection).

[0038] The body may be provided with an anti-reflection feature on one or more portions of the body that do not correspond to a nominal optical path of radiation received at the input.

[0039] Such anti-reflection features (which may, for example, be formed by textured surfaces and / or coatings) may prevent reflection of any unwanted scattered radiation from being contained in the body. This can reduce crosstalk between output channels. That is, it can reduce a risk of radiation being directed to the wrong output (i.e. a different output to one that this radiation it is intended to be directed to).

[0040] The body may be provided with a reflective coating on one or more portions of the body that correspond to a nominal optical path of radiation received at the input.

[0041] Such reflective coatings may reduce losses within the optical demultiplexer.

[0042] The lens of at least one of the plurality of outputs may comprise a metalens.

[0043] In some embodiments, the lens of each of the outputs may comprise a metalens.

[0044] It will be appreciated that a metalens is a lens that comprises an array of nanostructures arranged to modify the phase of electromagnetic radiation so as to focus the radiation, achieving a similar result as a larger, more traditional refractive lens.

[0045] Embodiments using one or more metalens are advantageous because metalenses are small, compact, not costly and can be formed by lithographic processes.

[0046] The lens of at least one of the plurality of outputs may comprise an off-axis metalens.

[0047] For example, each of the lenses may be arranged to receive radiation at a non-zero angle to a normal of the metalens and to direct the radiation along a direction parallel to the normal of the metalens.

[0048] The lens of at least one of the plurality of outputs may be disposed on an external surface of body.

[0049] In some embodiments, the lens of each of the plurality of outputs may be defined on an external surface of the body.

[0050] The lens of at least one of the plurality of outputs may be disposed on a surface of one of the plurality of optical filters.

[0051] The lens of each of the plurality of outputs may be arranged to focus the radiation coupled out of the body at that output at an output optical branch.

[0052] The lens of each of the plurality of outputs may be arranged to focus the radiation coupled out of the body at that output at a different focal region.

[0053] The optical demultiplexer may further comprise a plurality of output optical fibers. Each of the plurality of output optical fibers may be arranged to receive radiation coupled out of the body by the lens of a different one of the plurality of outputs.

[0054] The optical demultiplexer may further comprise a second body. An input of each of the plurality of output optical fibers may be supported by the second body.

[0055] The second body may define an output optical fiber array. The second body may be rigidly fixed to the body. For example, the second body may be fixed to the body by a suitable adhesive.

[0056] The input may be defined on a surface of the body.

[0057] The input and / or the body may comprise a prism to facilitate coupling of radiation into the body (which may comprise a substrate).

[0058] The optical demultiplexer may further comprise an input fiber arranged to couple radiation into the body via the inlet.

[0059] The inlet may comprise a collimating lens arranged to collimate radiation that is coupled into the body via the inlet.

[0060] The body may comprise at least two portions and the optical demultiplexer may further comprise splitting optics arranged to receive radiation received by the input and the direct a portion thereof to each of the at least two portions.

[0061] Each of the at least two portions may comprise a substrate.

[0062] The plurality of optical filters may be such that the spectral components each have a bandwidth of 50 nm or less.

[0063] According to a second aspect of the present disclosure there is provided a metrology system for determining a position of at least one target on a substrate, the metrology system comprising an optical demultiplexer according to the first aspect of the present disclosure.

[0064] The metrology system according to the second aspect of the present disclosure may, for example, form part of an exposure apparatus such as, for example, a lithographic apparatus. The metrology system may be used to measure the position of at least one target on a substrate (for example a resist-coated silicon wafer). In so doing, the metrology system may be used to map a substrate and may be used to improve an alignment between a pattern layer that is about to be imaged onto the substrate and one or more other layers that have previously been formed on the substrate. Therefore, the metrology system according to the second aspect of the present disclosure may be referred to as an alignment sensor.

[0065] The metrology system may comprising at least one position sensor operable to: project a radiation beam onto a substrate supported by a substrate support, wherein the radiation beam comprises a plurality of spectral components; receive a portion of the radiation beam scattered from a target on the substrate wherein the portion of the radiation beam scattered from a target on the substrate is received by the input of the optical demultiplexer; and generate a measurement signal that is indicative of a position of the target relative to that position sensor.

[0066] The demultiplexer allows for a plurality of wavelength components to be projected onto the substrate (wafer) simultaneously, collected and then separated (by the demultiplexer) for analysis.

[0067] The or each at least one position sensor may comprise a self-referencing interferometer that is operable to receive at least two diffraction beams scattered from the target on the substrate and to spatially overlap these so that they mutually interfere. The measurement signal may be generated from said interference.

[0068] The radiation beam projected onto the substrate by the or each position sensor may be so projected at generally normal incidence. The target(s) may comprise a diffraction grating or the like. The target(s) may generate a plurality of angularly separate diffraction beams from the radiation beam projected there onto by one of the position sensors. It will be appreciated that the relative phases of the plurality of angularly separated diffraction beams are dependent on a position of the beam spot of the radiation beam projected onto the target relative to the target. Furthermore, as the beam spot of the radiation beam projected onto the target and / or the target move relative to each other, the relative phases of different diffraction beams will oscillate. If two or more diffraction beams are combined, an intensity of the radiation will oscillate as the beam spot of the radiation beam projected onto the target and / or the target move relative to each other. This oscillating signal may be detected by a radiation sensitive detector and a position of the target (relative to that position sensor) may be determined from a phase of this oscillating signal.

[0069] According to a third aspect of the present disclosure there is provided an exposure apparatus comprising the metrology system of the second aspect of the present disclosure.

[0070] The exposure apparatus according to the third aspect of the present disclosure may, for example, comprise a lithographic apparatus.

[0071] Features of different aspects of the invention may be combined together, where the person skilled in the art deems it to be appropriate.BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0073] Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and a substrate table according to an embodiment of the present disclosure;

[0074] Figure 2 schematically shows an alignment system for analyzing alignment marks;

[0075] Figure 3 schematically shows an arrangement wherein an alignment system (which may, for example, be of the form of the alignment system shown in Figure 2) may be used to determine a position of alignment marks on a substrate relative to a substrate stage WT;

[0076] Figure 4 schematically shows a new optical demultiplexer for use in a metrology system according to an embodiment of the present disclosure

[0077] Figure 5 schematically shows a first embodiment of the optical demultiplexer shown in Figure 4;

[0078] Figure 6 schematically shows a second embodiment of the optical demultiplexer shown in Figure 4;

[0079] Figure 7 schematically shows a third embodiment of the optical demultiplexer shown in Figure 4;

[0080] Figure 8 schematically shows a fourth embodiment of the optical demultiplexer shown in Figure 4; and

[0081] Figure 9 schematically shows how anti-reflection features and reflective coatings may be formed on a body by a suitable combination of coatings on the body.DETAILED DESCRIPTION

[0082] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS, a measurement system MS, and a substrate loading system SL. The support structure MT may be referred to as a patterning device support MT. The lithographic system further comprises two substrate tables WT1, WT2 which are each configured to support a substrate W. Each substrate table WT1 , WT2 may be referred to as a substrate support WT1 , WT2. The support structure MT and the two substrate tables WT1, WT2 may each comprise a clamp, for example an electrostatic clamp.

[0083] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11. In general, the illumination system IL is configured to control an angular and spatial intensity distribution at the patterning device MA.

[0084] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto a substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto a substrate W held by one or the substrate tables WT1, WT2 when that substrate table is disposed in an exposure region (in Figure 1 substrate table WT2 is positioned in the exposure region). The projection system PS is configured to form an image of the patterning device (for example a diffraction limited image) on a substrate W supported by one of the two substrate tables WT1, WT2 disposed in an exposure region. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B ’ , thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0085] The faceted field mirror device 10 and the faceted pupil mirror device 11 are arranged to provide a desired angular distribution of the radiation beam B, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. The illumination system IL may be arranged to provide Kohler illumination of an illumination region IR (that the patterning device MA may be moved through during exposure of a substrate W) such that the plasma at the plasma formation region 4 is out of focus (and therefore does not influence properties of the radiation beam) at the patterning device MA and in the conjugate plane of the substrate W. As used herein, the illumination region IR may also be referred to as the illumination slit or the slit.

[0086] The illumination region IR is in a field plane in which the reticle MA is disposed during a lithographic exposure. Therefore, the illumination region IR may be referred to as a reticle-level or object-level illumination region IR. The reticle-level illumination region IR may be referred to as a first illumination region IR. It will be appreciated that projection system PS forms an image IR’ of the illumination region IR in the plane of the substrate W. The image IR’ of the illumination region IR in the plane of the substrate W may be referred to as a wafer-level or image-level illumination region IR’. As used herein, the wafer-level illumination region IR’ may also be referred to as the slit. The waferlevel illumination region IR’ may be referred to as a second illumination region IR.

[0087] The lithographic apparatus LA may be a scanning lithographic apparatus (also referred to as a scanner) and the image (of a reticle MA) may be formed during a scanning exposure. In such an exposure, the reticle MA may be moved in a scanning direction through the reticle-level illumination region IR. It will be appreciated that, as a result of this movement of the reticle MA, the image of the reticle MA will move at wafer level. Therefore, during the scanning exposure, the wafer W is also moved through the wafer-level illumination region IR’ . In particular, the wafer W is moved so that the image of reticle MA is substantially static with respect to the wafer W. The movement of the reticle MA and the substrate W are therefore synchronized. The movement (speed and direction) of the substrate W will be dependent on the movement of the reticle and the imaging performed by the projection system PS. In some embodiments, the projection system PS may form an inverted image of the reticle MA and therefore the substrate W may be moved in an opposite direction to the reticle MA. In some embodiments, the projection system PS may apply a reduction factor to the patterned EUV radiation beam B’ in the scanning direction and therefore the substrate W may move at a speed that is smaller than the speed of the reticle MA by this reduction factor.

[0088] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0089] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the environment of the projection system PS.

[0090] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1 , which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of electrons with ions of the plasma.

[0091] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0092] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1 , the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0093] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at the intermediate focus 6 to form an image at the intermediate focus 6 of the plasma 7 present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0094] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source or a free electron laser (FEL) may be used to generate EUV radiation.

[0095] The measurement system MS of the lithographic apparatus is configured to perform measurements of properties of a substrate W held on one of the two substrate tables WT1, WT2 when that substrate table is disposed in a metrology region (in Figure 1 substrate table WT1 is positioned inthe metrology region). The metrology region may alternatively be referred to as an intermediate metrology region. The measurement system MS may comprises an alignment system, which is configured to measure the positions of alignment marks on the substrate with reference to alignment marks on the substrate table WT1. Additionally or alternatively, the measurement system MS may comprise a level sensor which is configured to measure the topography of the substrate W. These measured properties are used to ensure accuracy when a pattern is subsequently projected onto the substrate W.

[0096] The two substrate tables WT1, WT2 may both be disposed in a wafer environment WE. Each of the two substrate tables WT1, WT2 may be movable within the wafer environment WE, as discussed further below. In particular, the substrate tables WT1, WT2 may each be positionable in an exposure region (see, for example, substrate table WT2 in Figure 1) and a metrology region (see, for example, substrate table WT1 in Figure 1).

[0097] The lithographic apparatus LA is a dual- stage lithographic apparatus. That is, the lithographic apparatus includes two substrate tables WT1, WT2 and is configured such that a substrate W held on one of the substrate tables WT1 is measured by the measurement system MS simultaneously with a pattern being applied to a substrate W held on the other substrate table WT2. For ease of terminology, one of the substrate tables WT1 may be referred to as a first substrate table, and the other substrate table WT2 may be referred to as a second substrate table. Dual-stage lithographic apparatus advantageously provide higher throughput (i.e. exposure of substrates per hour) than single stage lithographic apparatus. Once the substrate W held on the first substrate table WT1 has been measured, and the substrate W held on the second substrate table WT2 has been exposed, the substrate table WT1 supporting the measured substrate W is moved to be beneath the projection system PS. At the same time, the exposed substrate W supported by the second substrate table WT2 is moved to the substrate loading system SL.

[0098] The substrate loading system SL includes a substrate handler (not depicted) which is configured to remove the patterned substrate W from one of the substrate tables WT1, WT2 when that substrate table is disposed in a loading / unloading region and then load a new substrate to be patterned onto that substrate table. Once the substrate has been loaded onto the substrate table, the measurement system MS may be used to measure alignment mark positions and the topology of the substrate. Simultaneously, the substrate W held on the other substrate table (which is disposed in an exposure region) is exposed by the lithographic apparatus LA.

[0099] The above method may be repeated many times in order to expose many substrates W using the lithographic apparatus LA.

[0100] Integrated circuits (ICs) are built up layer by layer, and modern ICs can have 30 or more layers. On Product Overlay (OPO) is a measure of a system's ability to print these layers accurately on top of each other. Successive layers or multiple processes on the same layer must be accurately aligned to the previous layer. Otherwise, electrical contact between structures will be poor and the resulting devices will not perform to specification. Good overlay improves device yield and enables smaller productpatterns to be printed. The overlay error between successive layers formed in or on the patterned substrate is controlled by various parts of the exposure apparatus of the lithographic apparatus.

[0101] Process-induced wafer errors are a significant impediment to OPO performance. Process-induced wafer errors are attributable to the complexity of printed patterns as well as an increase of the number of printed layers. This error is of relatively high spatial variation that is different from wafer to wafer, and within a given wafer.

[0102] In order to control the lithographic process to place device features accurately on the substrate, one or more alignment marks are generally provided on, for example, the substrate, and the lithographic apparatus includes one or more alignment sensors by which the position of the mark may be measured accurately. The alignment sensor may be effectively a position measuring apparatus. Different types of marks and different types of alignment sensors are known from different times and different manufacturers. Measurement of the relative position of several alignment marks within the field can correct for process-induced wafer errors. Alignment error variation within the field can be used to fit a model to correct for OPO within the field

[0103] Lithographic apparatus are known to use various alignment systems to align the substrate with respect to the lithographic apparatus. The data can for example be obtained with any type of alignment sensor. A first example is a SMASH (SMart Alignment Sensor Hybrid) sensor, as described in U.S. Pat. No. 6,961,116, issued Nov. 1, 2005 and titled “Lithographic Apparatus, Device Manufacturing Method, and Device Manufactured Thereby,” which is hereby incorporated by reference herein in its entirety, and which employs a self-referencing interferometer with a single detector and four different wavelengths and extracts the alignment signal in software. A second example is an ATHENA (Advanced Technology using High order ENhancement of Alignment) sensor, as described in U.S. Pat. No. 6,297,876, issued Oct. 2, 2001 and titled “Lithographic Projection Apparatus with an Alignment System for Aligning Substrate on Mask,” which is hereby incorporated by reference in its entirety, and which directs each of seven diffraction orders to a dedicated detector. A third example is an ORION sensor, which uses multiple polarizations per available signal (color).

[0104] Reference is made in particular to the European application No. EP 1 372040 Al, granted Mar.5, 2008 and titled “Lithographic Apparatus and Device Manufacturing Method”, which document is hereby incorporated by reference in its entirety. EP 1 372040 Al describes an alignment system using a self-referencing interferometer that produces two overlapping images of an alignment marker. These two images are rotated over 180° with respect to each other. EP 1 372 040 Al further describes the detection of the intensity variation of the interfering Fourier transforms of these two images in a pupil plane. These intensity variations correspond to a phase difference between different diffraction orders of the two images, and from this phase difference positional information is derived, which is required for the alignment process. Reference is also made to U.S. Pat. No. 8,610,898, “Self-Referencing Interferometer, Alignment System, and Lithographic Apparatus” issued Dec. 17, 2013, the entire contents of which are hereby incorporated by reference in their entirety.

[0105] Such alignment sensors typically measure alignment marks in the form of scribe lane marks to determine a position of each mark. A wafer grid is determined from these mark positions. An accurate wafer grid reduces the potential for overlay errors.

[0106] Figure 2 shows a schematic overview of a known alignment system 50. A light source 51 emits a spatially coherent beam of radiation which illuminates an alignment marker WM on a substrate (e.g., a wafer), which scatters the radiation into positive and negative diffraction orders +n and -n. These diffraction orders are collimated by an objective lens 52 and enter a self-referencing interferometer (SRI) 53. The self-referencing interferometer outputs two images of the input, which have a relative rotation of 180°, and which overlap and which can therefore be made to interfere. In a pupil plane 54, the overlapping Fourier transforms of these images can be seen and be made to interfere (the different diffraction orders being separated in this pupil plane 54). Detectors 55 in the pupil plane detect the interfered diffraction orders to provide position information. Based on this position information a substrate can be aligned accurately with respect to a lithographic apparatus. The right-hand part of Figure 2 shows the formation of two overlapping images in the pupil plane 54; for one image +n' and -n' are rotated by +90° relative to the input diffraction orders +n and -n; for the other image +n" and -n" are rotated by -90° relative to the input diffraction orders -i-n and -n. In the pupil plane the orders of respectively (+n' and -n"), and (+n" and -n') interfere.

[0107] The output of a sensor such as that just described is typically relayed to a detector that detects the pattern of intensities in the sensor output. In the alignment sensor the output sensor (a single pixel detector) detects the intensity variation arising from the interference between the + order and - order. This interference creates two channels, a SUM channel in which the electric fields are added and a DIFF channel in which the electric fields are subtracted. These two channels are 180 degrees out of phase with each other. The alignment position of the mark is computed by measuring the phase of the signal at the DIFF or SUM or a combination of the two channels. In such systems all of the light is directed to the detector.

[0108] Figure 3 schematically shows an arrangement wherein an alignment system AS (which may, for example, be of the form of the alignment system 50 shown in Figure 2) may be used to determine a position of alignment marks on a substrate W relative to a substrate stage WT. The alignment system AS may be supported by a main support MS. The alignment system AS may be used to determine a position of a mark (on either a substrate W or a fiducial F) relative to the alignment sensor AS The alignment system AS may be used to determine a position of a mark (on either a substrate W or a fiducial F) relative to the alignment sensor AS.

[0109] A first position sensor PSI is operable to determine an orientation or pointing direction of the alignment sensor AS relative to the main support MS. For example, the first position sensor PSI may be operable to determine an orientation of an objective lens of the alignment sensor AS relative to the main support MS. Such a measurement (made by the first positon sensor PSI) may be referred to as an alignment reference axis (ARA) measurement.

[0110] A second position sensor PS2 is operable to determine a position and / or an orientation of the substrate support WT relative to the main support MS. The second position sensor PS2 may comprise one or more interferometers. For example, the one or more interferometers may be supported by the main support MS. The substrate support WT may comprise one or more reflective surfaces or mirrors and one arm or branch of the or each interferometer may be comprise a radiation beam (for example a laser beam) that is reflected from one of the one or more reflective surfaces or mirrors. Alternatively, the second position sensor PS2 may comprise one or more grid plates and encoders. Such a measurement (made by the second positon sensor PS2) may be referred to as stage position measurement (SPM).

[0111] The alignment system AS may be used to determine a position of a mark (on either a substrate W or a fiducial F) relative to the alignment sensor AS. Using the measurements from the first and second positions sensors, this may be converted into a position of the mark relative to the substrate support WT.

[0112] Some embodiments of the present disclosure relate to a new optical demultiplexer for use in a metrology system. An example of such an optical demultiplexer 100 is shown schematically in Figure 4.

[0113] The optical demultiplexer 100 comprises: a body 110; an input 120; a plurality of outputs 130a-13 On; and plurality of optical filters 140a-140n.

[0114] Each of the plurality of outputs 130a-130n comprises a lens 132a-132n supported, either directly or indirectly, by a surface of the body 110.

[0115] The plurality of optical filters 140a-140n are supported, either directly or indirectly, by the body 110. As explained below, the plurality of optical filters 140a-140n may, for example, be supported by a surface of the body 110 (for example, each adjacent to a lens 132a-132n of one of the plurality of outputs 130a- 130n); see, for example, the embodiments shown in Figures 5, 6 and 7. Alternatively, each of the plurality of optical filters 140a-140n may be disposed in the body 110 (for example imbedded in the body 110; see, for example the embodiments shown in Figure 8).

[0116] The plurality of optical filters 140a-140n are arranged such that when radiation 150 comprising a plurality of spectral components is received at the input 120, a different spectral component 150a-150n is received by each of the plurality of outputs 130a-130n via the body 110. The lens 132a-132n of each of the plurality of outputs 130a-130n is arranged to couple the spectral component 150a-150n received by that output 130a-130n out of the body 110.

[0117] The optical demultiplexer 100 shown schematically in Figure 4 comprises a plurality of lenses 132a-130n defined on the same body 110. Advantageously, such an arrangement can be significantly smaller than known optical demultiplexers, which typically comprise a large number of optical components. Furthermore, by providing a lens 132a-132n of each of the plurality of outputs 130a-130n that is supported by the same body 110, it is significantly easier to align the new optical demultiplexer 100. For example, it is easier to align the demultiplexer to: an optical component (for example anoptical fiber) to couple (multiplexed) radiation 150 (from a sensor output) into the input 120 of the demultiplexer 100; and / or to a plurality of optical components (for example optical fibers) to the plurality of outputs 130a-130n. The alignment can be achieved by moving the body 110 in a suitable way. This is in contrast to exiting optical demultiplexers that comprise a plurality of separate optical components that may need to be independently moved to ensure alignment of the demultiplexer.

[0118] As discussed further below, the lens 132a-132n of each of the plurality of outputs 130a-130n may be supported either directly or indirectly by a surface of the body 110. For example, in some embodiments the lens 132a-132n of each of the plurality of outputs 130a-130n may be disposed directly on a surface of the body 110. Alternatively, in some embodiments, the lens 132a-132n of each of the plurality of outputs 130a-130n may be supported by a corresponding one of the plurality of filters 140a-140n, which, in turn, may be disposed on a surface of the body 110.

[0119] As discussed further below, each of the plurality of optical filters 140a-140n being supported by the body 110 may mean the each of the plurality of optical filters 140a-140n is disposed either in or on the body 110.

[0120] Figures 5 to 8 each show an embodiment of an optical demultiplexer 100 of the type shown schematically in Figure 4.

[0121] In some embodiments of the optical demultiplexer 100, the plurality of optical filters 140a-140n may be arranged in a cascade arrangement such that at least a portion of the radiation 150 received by the input propagates to each of the plurality of optical filters 140a- 140n sequentially. For example, radiation 150 received by the input 120 may propagate to a first one of optical filters 140a, a portion of the radiation may then passes to a second one of the plurality of optical filters 140b and so on until the radiation is incident on a final one of the plurality of optical filters 140n. Figures 5, 6 and 8 all show arrangements wherein at least some of the plurality of optical filters 140a-140n are arranged in such a cascade arrangement.

[0122] Each of the plurality of optical filters 140a-140n may be associated with a different one of the plurality of outputs 130a-130n.

[0123] In some embodiments of the optical demultiplexer 100, each of the plurality of optical filters 140a-140n may direct the spectral component 150a- 150n coupled out of the body by a different one of the plurality of outputs 130a-130n to that output 130s-130n (see Figures 5, 6, 7 and 8).

[0124] An optical filter 140a-140n that directs the spectral component 150a- 150n coupled out of the body by one of the plurality of outputs 130a-130n to that output 130a-130n may be said to correspond to that output 130a-130n and, similarly, that output 130a-130n may be said to correspond to that optical filter 140a- 140n. For such embodiments, each optical filter 140a- 140n may be considered to be part of the corresponding output 130a-130n.

[0125] In some embodiments of the optical demultiplexer 100, each of the plurality of optical filters 140a-140n may be adjacent to the lens 132a-132n of a different one of the plurality of outputs 130a-130n (see Figures 5, 6 and 7). For such embodiments each of the plurality of optical filters 140a-140nmay be disposed on an external surface of the body 110 and a corresponding lens 132a-132n from a corresponding output 130a-130n may be disposed on each optical filter 140a-140n.

[0126] For such embodiments, each optical filter 140a- 140m may be arranged to transmit the spectral component 150a-150n coupled out by the corresponding lens 132a-132n that it is adjacent to and may be arranged to reflect other spectral components. The other spectral components may be reflected by that optical filter 140a-140n and may, for example, propagate to another one of the plurality of spectral filters 140a- 140n.

[0127] In some embodiments of the optical demultiplexer 100, each of the plurality of optical filters 140a- 140n may be disposed in the body 110 (see, for example, Figure 8).

[0128] For such embodiments each of the plurality of optical filters 140a-140n may be imbedded in the body 110 and the body 110 may comprise a plurality of portions rather than being monolithic (see Figure 8).

[0129] For such embodiments, each optical filter 140a- 140n may be arranged to reflect the spectral component 150a-150n coupled out by the lens 132a-132n of a corresponding output 130a-130n and may be arranged to transmit other spectral components. The other spectral components may, for example, propagate to another one of the plurality of spectral filters 140a- 140n.

[0130] For such embodiments, each of the plurality of optical filters 140a- 140n may be separate from, or spaced apart from, a corresponding output 130a-130n.

[0131] In some embodiments of the optical demultiplexer 100 each of the plurality of optical filters 140a- 140 may comprise a non-dispersive filter. Advantageously, such non-dispersive filters are relatively insensitive to the angle at which the radiation is incident on them. This is in contrast to dispersive filters, which can require a specific angle of incidence in order to direct different wavelength components to specific locations. Arrangements that use dispersive filters may therefore require an input radiation beam to undergo some relatively narrow angular filtering before they are incident on the dispersive filters. This can significantly reduce the intensity of the radiation and therefore may reduce the throughput of a metrology system using the demultiplexer. It can also cause non-uniform transmission across the spectrum, which can lead to a reduction in the metrology accuracy. Advantageously, the use of non-dispersive filters can improve the throughput of such a metrology system 100 (relative to an arrangement that uses dispersive filters).

[0132] As shown in Figure 7, in some embodiments, the optical demultiplexer 100 may comprise a diffractive optical element 160 arranged to receive radiation 150 comprising a plurality of spectral components that is received by the input and to direct each of the plurality of spectral components 150a-150d in a different direction.

[0133] The diffractive optical element 160 may comprise a metasurface reflector. The diffractive optical element 160 may be supported by the body 110. The diffractive optical element may preferentially direct each of the plurality of spectral components 150a-150d to a different one of the plurality of optical filters 140a- 140d (which may, for example, be formed on a surface of the body 110).

[0134] In some embodiments of the optical demultiplexer 100 each of the plurality of optical filters 140a- 140n may comprise a multilayer structure comprising alternating layers formed from two or more different materials and arranged to preferentially reflect or transmit a specific spectral component.

[0135] Such a multilayer structure may be referred to as a Bragg structure or, in general, a multi-layer interference. Each such optical filter 140a-140n may be considered to be a bandpass filter. In one example embodiment, the multilayer structure may comprise alternating layers formed from silicon and silicon dioxide. The thicknesses of the layers of each such optical filter 140a-140n may be selected in dependence on an angle of incidence of the radiation on that filter 140a-140n and the wavelength of the spectral component in order to preferentially reflect or transmit that spectral component.

[0136] In some embodiments, the body 110 may comprise at least one substrate. The at least one substrate may, for example, comprise any suitable type of glass such as, for example, fused silica.

[0137] In some embodiments of the optical demultiplexer 100, the body 110 may comprise a monolithic substrate. With such embodiments, each of the plurality of optical filters 140a- 140n may be formed on an external surface of the body 110 (and a lens 132a-132n of a corresponding output 130a- 130n may, for example, be formed in each optical filter 140a-140n). Figures 5 and 7 show examples wherein the body 110 may comprises a monolithic substrate.

[0138] In some embodiments of the optical demultiplexer 100, the body 110 may comprise a composite substrate. For example, such a composite substrate 110 may comprise a plurality of components rigidly joined to each other (for example via an adhesive). With such embodiments, the plurality of optical filters 140a-140n may be formed in the body 110 (for example between two of the plurality of components). For example, each of the plurality of optical filters 140a- 140n may be formed as a coating on a surface of one of the plurality of components, which may subsequently be adhered to another of the plurality of components (sandwiching the optical filter 140a-140n between the two components). Figure 8 shows an example wherein the body 110 may comprises a composite substrate.

[0139] The optical demultiplexer 100 may define a plurality of nominal optical paths. For example, a nominal optical path may be defined (in the body 110) between the input 120 and each of the plurality of outputs 130a-130n. The or each nominal optical beam path may comprise at least one reflection from a surface of the body 110 (for example total internal reflection). The plurality of nominal optical paths defined by the embodiments of optical demultiplexers 100 shown in Figures 5 to 8 are indicated schematically in these Figures by arrows.

[0140] In some embodiments of the optical demultiplexer 100, the body 110 may be provided with an anti-reflection features 112 on one or more portions of the body 110 that do not correspond to a nominal optical path of radiation received at the input. Such anti-reflection features 112 (for example textures and / or coatings) may prevent reflection of any unwanted scattered radiation from being contained in the body 110. Advantageously, this can help to reduce crosstalk between output channels. That is, it can reduce a risk of radiation being directed to the wrong output 130a- 130n (i.e. a different output 130a-130n to one that this radiation it is intended to be directed to). The embodiments shown in Figures 5 and 6 show example of such anti-reflection features 112 on one or more portions of the body 110.

[0141] In some embodiments of the optical demultiplexer 100 the body 110 may be provided with a reflective coating 114 on one or more portions of the body 110 that correspond to a nominal optical path of radiation 150 received at the input 120. Such reflective coatings 114 may reduce losses within the optical demultiplexer 100. The embodiments shown in Figures 5 and 6 show example of such reflective coatings 114 on one or more portions of the body 110.

[0142] In some embodiments, the anti-reflection features 112 and the reflective coatings 114 may be formed by a suitable combination of coatings and / or textures on the body 110, as now discussed with reference to Figure 9. In one example, a coating 116 which can be textured may be formed on the body 110. Portions 116a of the coating 116 which correspond to the regions of the anti-reflection features 112 (i.e. portions of the body 110 that do not correspond to a nominal optical path of radiation received at the input) may be textured to reduce total internal reflection from such portions. This texturing may be formed using any surface structuring techniques such as, for example, laser ablation. Optionally, these textured portions 116a of the coating 116 which correspond to the regions of the anti-reflection features 112 may also be provided with a coating, for example a coating of titanium (Ti). Portions of the coating 116 which correspond to the regions of the reflective coatings 114 (i.e. portions of the body 110 that correspond to a nominal optical path of radiation received at the input) may be provided with a reflective coating 118, for example a coating of aluminium oxide (AI2O3).

[0143] In some embodiments of the optical demultiplexer 100 the lens 132a-132n of at least one of the plurality of outputs 130a-130n may comprise a metalens. In some embodiments, the lens 132a-132n of each of the outputs 130a-130n may comprise a metalens.

[0144] It will be appreciated that a metalens is a lens that comprises an array of nanostructures arranged to modify the phase of electromagnetic radiation so as to focus the radiation, achieving a similar result as a larger, more traditional refractive lens.

[0145] Embodiments using one or more metalens are advantageous because metalenses are small, compact, not costly, can be formed by lithographic processes, and can support a high numerical aperture for a narrow radiation band.

[0146] In some embodiments of the optical demultiplexer 100 the lens 132a-132n of at least one of the plurality of outputs 130a-130n may comprise an off-axis metalens. For example, each of the lenses 132a-132n may be arranged to receive radiation at a non-zero angle to a normal of the metalens 132a-132n and to direct the radiation 150a-150n along a direction parallel to the normal of the metalens 132a-132n. Examples of such embodiments are shown in Figures 5, 6 and 7.

[0147] In some embodiments of the optical demultiplexer 100, the lens 132a-132n of at least one of the plurality of outputs 130a-130n may be disposed on an external surface of body 110. In some embodiments, the lens 132a-132n of each of the plurality of outputs 130a-130n may be defined on an external surface of the body 110.

[0148] In some embodiments of the optical demultiplexer 100 the lens 132a-132n of at least one of the plurality of outputs 130a-130n may be disposed on a surface of one of the plurality of optical filters 140a-140n. Figures 5, 6 and 7 show examples wherein the 100 the lenses 132a-132n of the plurality of outputs 130a-130n are each disposed on a surface of a different one of the plurality of optical filters 140a-140n.

[0149] The lens 132a-132n of each of the plurality of outputs 130a-130n may be arranged to focus the radiation 150a-150n coupled out of the body 110 at that output 130a-130n at an output optical branch (which may comprise an optical fiber, as discussed below).

[0150] In some embodiments of the optical demultiplexer 100, the lens 132a-132n of each of the plurality of outputs 130a-130n may be arranged to focus the radiation 150a-150n coupled out of the body 110 at that output 130a-130n at a different focal region 170a-170n.

[0151] In some embodiments the optical demultiplexer 100 may further comprise a plurality of output optical fibers 180a-180n (see Figures 5, 6 and 8). Each of the plurality of output optical fibers 180a-180n may be arranged to receive radiation 150a-150n coupled out of the body 110 by the lens 132a-132n of a different one of the plurality of outputs 130a-130n.

[0152] In some embodiments the optical demultiplexer 100 may further comprise a second body 190. An input of each of the plurality of output optical 180a-180n may be supported by the second body 190. The second body 190 may define an output optical fiber array. The second body 190 may be rigidly fixed to the body 110. For example, the second body 190 may be fixed to the body 110 by a suitable adhesive.

[0153] The input 120 may be defined on a surface of the body 110.

[0154] The input 120 and / or the body 110 may comprise a prism 122 to facilitate coupling of radiation 150 into the body 110 (which may comprise a substrate). Examples of such embodiments are shown in Figures 5 and 6.

[0155] In some embodiments the optical demultiplexer 100 may further comprise an input fiber 124 arranged to couple radiation 150 into the body 110 via the inlet 120.

[0156] In some embodiments of the optical demultiplexer 100 the inlet 120 may comprise a collimating lens 126 arranged to collimate radiation 150 that is coupled into the body 110 via the inlet 120.

[0157] In some embodiments of the optical demultiplexer 100 the body 110 may comprise at least two portions 110a, 110b. The optical demultiplexer 100 may further comprise splitting optics arranged to receive radiation 150 received by the input 120 and the direct a portion thereof to each of the at least two portions 110a, 110b. Each of the at least two portions 110a, 110b may comprise a substrate. An example of such an embodiment is shown in Figure 6. In the embodiment of Figure 6, the splitting optics is provided by an interface between the prism 120 and one of the two portions 110a that is arranged to transmit a first portion of the radiation 150 received by the input 120 to the first portion 110a of the body 110 and to reflect a second portion of the radiation 150 received by the input 120 to the second portion 110b of the body 110. Note that the splitting optics may separate shorter wavelengthsto one portion of the body 110 and longer wavelength to another portion of the body 110. For example, the first portion of the radiation 150 (which is transmitted to the first portion 110a of the body 110) may comprise shorter wavelengths than the second portion of the radiation 150 (which is transmitted to the second portion 110b of the body 110). Note that in some embodiments further splitting can be done as desired or required. For example, the splitting optics may comprise a plurality of optics or interfaces that may be arranged in a cascade arrangement.

[0158] Note that in the arrangement shown in Figure 6, the body 110 comprises two portions 110a, 110b that are mutually adjacent in a first direction (a vertical direction in Figure 6). Each portion 110a, 110b of the body 110 may be considered to be a demultiplexer module. Additionally or alternatively, in some embodiments, a parallel arrangement of demultiplexer modules may be provided in a second direction (for example a direction that is perpendicular to the plane of Figure 6). Advantageously, such co-packaging of multiple demultiplexer moduless helps to reduce the cost of the demultiplexer 100.

[0159] In some embodiments of the optical demultiplexer 100, the plurality of optical filters 140a-140n may be such that the spectral components 150a- 150n each have a bandwidth of 50 nm or less.

[0160] Some embodiments of the present disclosure relate to a new metrology system for determining a position of at least one target on a substrate, the metrology system comprising an optical demultiplexer 100 of the type shown in any of Figures 4 to 8 and as described above.

[0161] The new metrology system may, for example, form part of an exposure apparatus such as, for example, a lithographic apparatus LA of the type shown in Figure 1. For example, the new metrology system may form part of the measurement system MS of the lithographic apparatus LA. The new metrology system may comprise any combination of features of the alignment system 50 shown in Figure 2 and / or the arrangement comprising an alignment system AS as shown in Figure 3. For example, the new metrology system may be used to measure the position of at least one target on a substrate (for example a resist-coated silicon wafer W). In so doing, the metrology system may be used to map a substrate and may be used to improve an alignment between a pattern layer that is about to be imaged onto the substrate and one or more other layers that have previously been formed on the substrate. Therefore, the new metrology system may be referred to as an alignment sensor.

[0162] In some embodiments, the new metrology system may comprise at least one position sensor (for example position sensor 50 shown in Figure 2) that is operable to: project a radiation beam onto a substrate W, WM supported by a substrate support WT, wherein the radiation beam comprises a plurality of spectral components; receive a portion of the radiation beam scattered from a target on the substrate W, WM wherein the portion of the radiation beam scattered from a target on the substrate W, WM is received by the input of the optical demultiplexer 100; and generate a measurement signal that is indicative of a position of the target relative to that position sensor 50.

[0163] The demultiplexer 100 allows for a plurality of wavelength components to be projected onto the substrate W (wafer) simultaneously, collected and then separated (by the demultiplexer) for analysis.

[0164] In some embodiments of the new metrology system, the or each at least one position sensor 50 comprises a self-referencing interferometer that is operable to receive at least two diffraction beams (-n, -i-n) scattered from the target on the substrate W, WM and to spatially overlap these so that they mutually interfere and wherein the measurement signal is generated from said interference.

[0165] The radiation beam projected onto the substrate by the or each position sensor may be so projected at generally normal incidence. The target(s) may comprise a diffraction grating or the like. The target(s) may generate a plurality of angularly separate diffraction beams from the radiation beam projected there onto by one of the position sensors. It will be appreciated that the relative phases of the plurality of angularly separated diffraction beams are dependent on a position of the beam spot of the radiation beam projected onto the target relative to the target. Furthermore, as the beam spot of the radiation beam projected onto the target and / or the target move relative to each other, the relative phases of different diffraction beams will oscillate. If two or more diffraction beams are combined, an intensity of the radiation will oscillate as the beam spot of the radiation beam projected onto the target and / or the target move relative to each other. This oscillating signal may be detected by a radiation sensitive detector and a position of the target (relative to that position sensor) may be determined from a phase of this oscillating signal.

[0166] Some embodiments of the present invention relate to an exposure apparatus comprising the new metrology system (which comprises an optical demultiplexer 100 of the type shown in any of Figures 4 to 8 and as described above). The exposure apparatus may, for example, comprise a lithographic apparatus LA of the type shown in Figure 1.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. An optical demultiplexer for use in a metrology system, the optical demultiplexer comprising:a body;an input;a plurality of outputs, each of the plurality of outputs comprising a lens supported by a surface of the body; anda plurality of optical filters supported by the body and arranged such that when radiation comprising a plurality of spectral components is received at the input, a different spectral component is received by each of the plurality of outputs via the body and wherein the lens of each of the plurality of outputs is arranged to couple the spectral component received by that output out of the body.2. The optical demultiplexer of clause 1 wherein the plurality of optical filters are arranged in a cascade arrangement such that at least a portion of the radiation received by the input propagates to each of the plurality of optical filters sequentially.3. The optical demultiplexer of any preceding clause wherein each of the plurality of optical filters directs the spectral component coupled out of the body by a different one of the plurality of outputs to that output.4. The optical demultiplexer of any preceding clause wherein each of the plurality of optical filters is adjacent to the lens of a different one of the plurality of outputs.5. The optical demultiplexer of any preceding clause wherein each of the plurality of optical filters is disposed in the body.6. The optical demultiplexer of any preceding clause wherein each of the plurality of optical filters comprises non-dispersive filters.7. The optical demultiplexer of any one of clauses 1 to 6 comprising a diffractive optical element arranged to receive radiation comprising a plurality of spectral components that is received by the input and to direct each of the plurality of spectral components in a different direction.8. The optical demultiplexer of any preceding clause wherein each of the plurality of optical filters comprises a multilayer structure comprising alternating layers formed from two or more different materials and arranged to preferentially reflect or transmit a specific spectral component.9. The optical demultiplexer of any preceding clause wherein the body comprises at least one substrate.10. The optical demultiplexer of any preceding clause wherein the body comprises a monolithic substrate.11. The optical demultiplexer of any preceding clause wherein the body comprises a composite substrate.12. The optical demultiplexer of any preceding clause wherein the body is provided with an antireflection feature on one or more portions of the body that do not correspond to a nominal optical path of radiation received at the input.13. The optical demultiplexer of any preceding clause wherein the body is provided with a reflective coating on one or more portions of the body that correspond to a nominal optical path of radiation received at the input.14. The optical demultiplexer of any preceding clause wherein the lens of at least one of the plurality of outputs comprises a metalens.15. The optical demultiplexer of any preceding clause wherein the lens of at least one of the plurality of outputs comprises an off-axis metalens.16. The optical demultiplexer of any preceding clause wherein the lens of at least one of the plurality of outputs is disposed on an external surface of body.17. The optical demultiplexer of any preceding clause wherein the lens of at least one of the plurality of outputs is disposed on a surface of one of the plurality of optical filters.18. The optical demultiplexer of any preceding clause wherein the lens of each of the plurality of outputs is arranged to focus the radiation coupled out of the body at that output at a different focal region.19. The optical demultiplexer of any preceding clause further comprising a plurality of output optical fibers, wherein each of the plurality of output optical fibers is arranged to receive radiation coupled out of the body by the lens of a different one of the plurality of outputs.20. The optical demultiplexer of clause 19 further comprising a second body wherein an input of each of the plurality of output optical fibers is supported by the second body.21. The optical demultiplexer of any preceding clause further comprising an input fiber arranged to couple radiation into the body via the inlet.22. The optical demultiplexer of any preceding clause wherein the inlet comprises a collimating lens arranged to collimate radiation that is coupled into the body via the inlet.23. The optical demultiplexer of any preceding clause wherein the body comprises at least two portions and wherein the optical demultiplexer further comprises splitting optics arranged to receive radiation received by the input and the direct a portion thereof to each of the at least two portions. 24. The optical demultiplexer of any preceding clause wherein the plurality of optical filters are such that the spectral components each have a bandwidth of 50 nm or less.25. A metrology system for determining a position of at least one target on a substrate, the metrology system comprising an optical demultiplexer according to any preceding clause.26. The metrology system of clause 25 comprising at least one position sensor operable to:project a radiation beam onto a substrate supported by a substrate support, wherein the radiation beam comprises a plurality of spectral components;receive a portion of the radiation beam scattered from a target on the substrate wherein the portion of the radiation beam scattered from a target on the substrate is received by the input of the optical demultiplexer; andgenerate a measurement signal that is indicative of a position of the target relative to that position sensor.27. The metrology system of clause 26 wherein the or each at least one position sensor comprises a self-referencing interferometer that is operable to receive at least two diffraction beams scattered from the target on the substrate and to spatially overlap these so that they mutually interfere and wherein the measurement signal is generated from said interference.28. An exposure apparatus comprising the metrology system of one of clauses 25 to 27.

[0167] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0168] Although specific reference has been made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus,or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions. A substrate clamp according to an embodiment of the invention may form part of a lithographic tool.

[0169] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. An optical demultiplexer for use in a metrology system, the optical demultiplexer comprising:a body;an input;a plurality of outputs, each of the plurality of outputs comprising a lens supported by a surface of the body; anda plurality of optical filters supported by the body and arranged such that when radiation comprising a plurality of spectral components is received at the input, a different spectral component is received by each of the plurality of outputs via the body and wherein the lens of each of the plurality of outputs is arranged to couple the spectral component received by that output out of the body.

2. The optical demultiplexer of claim 1 wherein the plurality of optical filters are arranged in a cascade arrangement such that at least a portion of the radiation received by the input propagates to each of the plurality of optical filters sequentially.

3. The optical demultiplexer of any preceding claim wherein each of the plurality of optical filters directs the spectral component coupled out of the body by a different one of the plurality of outputs to that output.

4. The optical demultiplexer of any preceding claim wherein each of the plurality of optical filters is adjacent to the lens of a different one of the plurality of outputs.

5. The optical demultiplexer of any preceding claim wherein each of the plurality of optical filters is disposed in the body.

6. The optical demultiplexer of any preceding claim wherein each of the plurality of optical filters comprises non-dispersive filters.

7. The optical demultiplexer of any one of claims 1 to 6 comprising a diffractive optical element arranged to receive radiation comprising a plurality of spectral components that is received by the input and to direct each of the plurality of spectral components in a different direction.

8. The optical demultiplexer of any preceding claim wherein each of the plurality of optical filters comprises a multilayer structure comprising alternating layers formed from two or more different materials and arranged to preferentially reflect or transmit a specific spectral component.

9. The optical demultiplexer of any preceding claim wherein the body comprises at least one substrate.

10. The optical demultiplexer of any preceding claim wherein the body comprises a monolithic substrate.

11. The optical demultiplexer of any preceding claim wherein the body comprises a composite substrate.

12. The optical demultiplexer of any preceding claim wherein the body is provided with an antireflection feature on one or more portions of the body that do not correspond to a nominal optical path of radiation received at the input.

13. The optical demultiplexer of any preceding claim wherein the body is provided with a reflective coating on one or more portions of the body that correspond to a nominal optical path of radiation received at the input.

14. The optical demultiplexer of any preceding claim wherein the lens of at least one of the plurality of outputs comprises a metalens.

15. The optical demultiplexer of any preceding claim wherein the lens of at least one of the plurality of outputs comprises an off-axis metalens.

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