Detection arrangement for a level sensor

Abstract

Disclosed is a detection arrangement for a level sensor, the detection arrangement comprising: a plurality of fiber cores comprising input facets for receiving reflected measurement radiation forming one or more measurement spots at a detection plane of the level sensor, each of the measurement spots comprising a plurality of fringes; and a plurality of detectors for detecting received measurement radiation, said received measurement radiation comprising said reflected measurement radiation subsequent to it being received and carried by said plurality of fiber cores; wherein the plurality of fiber cores comprises at least a respective set of fiber cores for each of said fringes, each said set of fiber cores comprising at least a first subset of fiber cores being configured for receiving radiation from a first portion of its respective fringe and a second subset of fiber cores being configured for receiving radiation from a second portion of its respective fringe.

Description

DETECTION ARRANGEMENT FOR A LEVEL SENSORCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24197962.4 which was filed on 2 September 2024 and which is incorporated herein in its entirety by reference.FIELD OF THE INVENTION

[0002] The present invention relates to methods and apparatus usable, for example, in the manufacture of devices by lithographic techniques, and to methods of manufacturing devices using lithographic techniques. In particular the invention relates to a level sensor which may used in such apparatuses.BACKGROUND ART

[0003] A lithographic apparatus or exposure apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may 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. including part of a die, one die, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. These target portions are commonly referred to as “fields”.

[0004] In lithography, a flatness of the substrate (e.g. the wafer) may be measured and e.g. stored as a height map. The height map may be used to position a relevant target portion of the substrate at an appropriate height, in order to provide that, when projecting a pattern onto the target portion of the substrate, that target portion is positioned within a focal range (depth of focus) of a projection system (e.g., a projection lens) of the lithographic apparatus. Compiling the height map is also referred to as "level sensing" . The level sensing may be performed by a level sensor. The level sensor may be integral to a lithographic apparatus or may be a separate measurement arrangement. The level sensor may make use of an optical measurement, by means of projecting a measurement beam onto the substrate and detecting a reflection thereof. In certain detection schemes, use may be made of gratings in an optical path of the measurement beam, e.g. a projection grating upstream of the substrate and a detection grating downstream of the substrate.

[0005] Consider a level sensor having a projection grating, a detection grating and a detector. The projection grating is imaged on the substrate surface at an angle relative to the (ideal) surface normal. The image is reflected by the wafer surface and re-imaged on the detection grating. Due to the oblique incidence, a variation in the substrate's height will shift the image of the projection grating on the detection grating over a certain distance. The shifted image of the projection grating is partiallytransmited by the detection grating. The detector detects an intensity of the transmited image. The intensity is indicative of the substrate's local height. In other words, a height variation of the surface of the substrate results in a variation in the image transmited by the detection grating, allowing deriving height information from the detector signal. In a measurement principle as may be applied, the optical path of the image of the projection grating should be accurately set, so as to provide that the projection grating is correctly imaged onto the detection grating.

[0006] Present level sensors require a large detector arrangement within the sensor critical volume.

[0007] Consequently, a solution is desirable which overcomes at least some of the above-mentioned disadvantages.SUMMARY OF THE INVENTION

[0008] The invention in a first aspect provides a detection arrangement for a level sensor, the detection arrangement comprising: a plurality of fiber cores comprising input facets for receiving reflected measurement radiation at a detection plane of the level sensor, subsequent to the reflected measurement radiation having been reflected by a surface being measured, said reflected measurement radiation forming one or more measurement spots at the detection plane, each of the one or more measurement spots comprising a plurality of fringes; and a plurality of detectors for detecting received measurement radiation, said received measurement radiation comprising said reflected measurement radiation subsequent to it being received and carried by said plurality of fiber cores; wherein the plurality of fiber cores comprises at least a respective set of fiber cores for each of said fringes, each said set of fiber cores comprising at least a first subset of fiber cores being configured for receiving radiation from a first portion of its respective fringe and a second subset of fiber cores being configured for receiving radiation from a second portion of its respective fringe, the first portion being displaced from the second portion in at least a direction of fringe periodicity of said fringes.

[0009] A second aspect comprises a level sensor, comprising: a detection optical arrangement operable to collect said reflected measurement radiation; and image said reflected measurement radiation at said detection plane; and the detection arrangement according to the invention, wherein said input facets are located at said detection plane.

[0010] A third aspect comprises an exposure apparatus comprising a level sensor according to the invention.

[0011] The above and other aspects of the invention will be understood from a consideration of the examples described below.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 depicts a lithographic apparatus; and- Figure 2 depicts a schematic of a measurement and exposure processes in the apparatus of Figure 1; and- Figure 3(a) depicts a schematic view of a level sensor;- Figure 3(b) depicts a schematic view of a detection arrangement of the level sensorof Figure 3(a);- Figure 4 depicts two measurement spots as may be obtained using a level sensor as disclosed herein;- Figure 5 depicts a schematic view of a level sensor according to embodiments disclosed herein;- Figure 6 depicts a schematic view of a fiber core arrangement of a level sensoraccording to embodiments disclosed herein; and- Figure 7 depicts a schematic view of a detection arrangement for a fiber core arrangement according to embodiments disclosed herein.DETAILED DESCRIPTION OF EMBODIMENTS

[0013] Before describing embodiments of the invention in detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

[0014] Figure 1 schematically depicts an exposure apparatus or lithographic apparatus LA. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or DUV radiation), a patterning device support or support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters; two substrate tables (e.g., a wafer table) WTa and WTb each constructed to hold a substrate (e.g., a resist coated wafer) W and each connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W. A reference frame RF connects the various components, and serves as a reference for setting and measuring positions of the patterning device and substrate and of features on them.

[0015] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0016] The patterning device support MT holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the exposure apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support MT may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0017] The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0018] As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive patterning device). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask). Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” The term “patterning device” can also be interpreted as referring to a device storing in digital form pattern information for use in controlling such a programmable patterning device.

[0019] The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

[0020] The exposure apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the exposure 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.

[0021] In operation, the illuminator IL receives a radiation beam from a radiation source SO. The source and the exposure apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the exposure apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and / or a beam expander. In other cases the source may be an integral part of the exposure apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0022] The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent (commonly referred to as o-outer and o-inner, respectively) of the intensity distribution in a pupil plane of the illuminator may be adjusted. In addition, the illuminator IL may comprise various other components, such as an integratorIN and a condenser CO. The illuminator IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross section.

[0023] The radiation beam B is incident on the patterning device MA, which is held on the patterning device support MT, and is patterned by the patterning device. Having traversed the patterning device (e.g., 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. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table WTa or WTb can be moved accurately, e.g., 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 (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan.

[0024] The exposure apparatus may comprise an aberration sensor for the verification of an aberration fingerprint of the proj ection system PS . In an embodiment such an aberration fingerprint, i .e . aberrations per field point of the projection system PS, may be determined using a such wavefront aberration sensor. A wavefront aberration sensor of a known type, for instance such as described in US2002 / 0001088, incorporated herein by reference, may be used. Such a wavefront aberration sensor may be based on the principle of shearing interferometry and comprises a source module and a sensor module. The source module may comprise a patterned layer of chromium that is placed in the object plane (i.e. where during production the pattern of the patterning means is) of the projection system PS and has additional optics provided above the chromium layer. The combination provides a wavefront of radiation to the entire pupil of the projection system PS. The sensor module may comprise a patterned layer of chromium that is placed in the image plane of the projection system (i.e. where during production the substrate W is) and a camera that is placed some distance behind said layer of chromium. The patterned layer of chromium on the sensor module diffracts radiation into several diffraction orders that interfere with each other giving rise to an interferogram. The interferogram is measured by the camera. The aberrations in the projection lens can be determined by software based upon the measured interferogram. The wavefront aberration sensor may be configured to transfer information with respect to the aberration fingerprint towards the control unit.

[0025] Patterning device (e.g., mask) MA and substrate W may 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 may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g., mask) MA, the mask alignment marks may be located between the dies. Small alignment marks may also be included within dies, in amongst the device features, in which case it is desirable that the markers be as small as possible and not require any different imaging or processconditions than adjacent features. The alignment system, which detects the alignment markers is described further below.

[0026] The depicted apparatus could be used in a variety of modes. In a scan mode, the patterning device support (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The speed and direction of the substrate table WT relative to the patterning device support (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. Other types of exposure apparatus and modes of operation are possible, as is well-known in the art. For example, a step mode is known. In so-called “maskless” lithography, a programmable patterning device is held stationary but with a changing pattern, and the substrate table WT is moved or scanned.

[0027] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

[0028] Exposure apparatus LA is of a so-called dual stage type which has two substrate tables WTa, WTb and two stations - an exposure station EXP and a measurement station MEA - between which the substrate tables can be exchanged. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparatory steps carried out. This enables a substantial increase in the throughput of the apparatus. The preparatory steps may include mapping the surface height contours of the substrate using a level sensor LS and measuring the position of alignment markers on the substrate using an alignment sensor AS. If the position sensor IF is not capable of measuring the position of the substrate table while it is at the measurement station as well as at the exposure station, a second position sensor may be provided to enable the positions of the substrate table to be tracked at both stations, relative to reference frame RF. Other arrangements are known and usable instead of the dual-stage arrangement shown. For example, other exposure apparatuses are known in which a substrate table and a measurement table are provided. These are docked together when performing preparatory measurements, and then undocked while the substrate table undergoes exposure.

[0029] The exposure apparatus control unit LACU which controls all the movements and measurements of various actuators and sensors to receive substrates W and reticles MA and to implement the patterning operations. Exposure apparatus control unit LACU also includes signal processing and data processing capacity to implement desired calculations relevant to the operation of the apparatus, e.g., based on inter alia level sensor LS data, alignment sensor AS data and feedback metrology data (e.g., one or more of inter alia overlay, focus, dose, critical dimension data). In practice, control unit LACU may be realized as a system of many sub-units, each handling the real-time data acquisition, processing and control of a subsystem or component within the apparatus.

[0030] Figure 2 illustrates the steps to expose target portions (e.g. dies) on a substrate W in the dual stage apparatus of Figure 1. On the left hand side within a dotted box are steps performed at a measurement station MEA, while the right hand side shows steps performed at the exposure station EXP. From time to time, one of the substrate tables WTa, WTb will be at the exposure station, while the other is at the measurement station, as described above. For the purposes of this description, it is assumed that a substrate W has already been loaded into the exposure station. At step 200, a new substrate W’ is loaded to the apparatus by a mechanism not shown. These two substrates are processed in parallel in order to increase the throughput of the exposure apparatus.

[0031] Referring initially to the newly-loaded substrate W’, this may be a previously unprocessed substrate, prepared with a new photo resist for first time exposure in the apparatus. In general, however, the lithography process described will be merely one step in a series of exposure and processing steps, so that substrate W’ has been through this apparatus and / or other lithography apparatuses, several times already, and may have subsequent processes to undergo as well. Particularly for the problem of improving overlay performance, the task is to ensure that new patterns are applied in exactly the correct position on a substrate that has already been subjected to one or more cycles of patterning and processing. These processing steps progressively introduce distortions in the substrate that must be measured and corrected for, to achieve satisfactory overlay performance.

[0032] The previous and / or subsequent patterning step may be performed in other lithography apparatuses, as just mentioned, and may even be performed in different types of lithography apparatus. For example, some layers in the device manufacturing process which are very demanding in parameters such as resolution and overlay may be performed in a more advanced lithography tool than other layers that are less demanding. Therefore some layers may be exposed in an immersion type lithography tool, while others are exposed in a ‘dry’ tool. Some layers may be exposed in a tool working at DUV wavelengths, while others are exposed using EUV wavelength radiation.

[0033] At 202, alignment measurements using the substrate marks Pl etc. and image sensors (not shown) are used to measure and record alignment of the substrate relative to substrate table WTa / WTb. In addition, several alignment marks across the substrate W’ will be measured using alignment sensor AS. These measurements are used in one embodiment to establish a “wafer grid”, which maps very accurately the distribution of marks across the substrate, including any distortion relative to a nominal rectangular grid.

[0034] At step 204, a map of wafer height (Z) against X-Y position is measured also using the level sensor LS. Conventionally, the height map is used only to achieve accurate focusing of the exposed pattern. It may be used for other purposes in addition.

[0035] When substrate W’ was loaded, recipe data 206 were received, defining the exposures to be performed, and also properties of the wafer and the patterns previously made and to be made upon it. To these recipe data are added the measurements of wafer position, wafer grid and height map that were made at 202, 204, so that a complete set of recipe and measurement data 208 can be passed to theexposure station EXP. The measurements of alignment data for example comprise X and Y positions of alignment targets formed in a fixed or nominally fixed relationship to the product patterns that are the product of the exposure process. These alignment data, taken just before exposure, are used to generate an alignment model with parameters that fit the alignment model to the data. These parameters and the alignment model will be used during the exposure operation to correct positions of patterns applied in the current lithographic step. The model in use interpolates positional deviations between the measured positions. A conventional alignment model might comprise four, five or six parameters, together defining translation, rotation and scaling of the ‘ideal’ grid, in different dimensions. Advanced models are known that use more parameters.

[0036] At 210, wafers W’ and W are swapped, so that the measured substrate W’ becomes the substrate W entering the exposure station EXP. In the example apparatus of Figure 1, this swapping is performed by exchanging the supports WTa and WTb within the apparatus, so that the substrates W, W’ remain accurately clamped and positioned on those supports, to preserve relative alignment between the substrate tables and substrates themselves. Accordingly, once the tables have been swapped, determining the relative position between projection system PS and substrate table WTb (formerly WTa) is all that is necessary to make use of the measurement information 202, 204 for the substrate W (formerly W’) in control of the exposure steps. At step 212, reticle alignment is performed using the mask alignment marks Ml, M2. In steps 214, 216, 218, scanning motions and radiation pulses are applied at successive target locations across the substrate W, in order to complete the exposure of a number of patterns.

[0037] By using the alignment data and height map obtained at the measuring station in the performance of the exposure steps, these patterns are accurately aligned with respect to the desired locations, and, in particular, with respect to features previously laid down on the same substrate. The exposed substrate, now labeled W” is unloaded from the apparatus at step 220, to undergo etching or other processes, in accordance with the exposed pattern.

[0038] The skilled person will know that the above description is a simplified overview of a number of very detailed steps involved in one example of a real manufacturing situation. For example rather than measuring alignment in a single pass, often there will be separate phases of coarse and fine measurement, using the same or different marks. The coarse and / or fine alignment measurement steps can be performed before or after the height measurement, or interleaved.

[0039] In an exposure apparatus, it is known to control machine settings, e.g. lens settings and (reticle and substrate) stage settings, during exposures onto a substrate to optimize the projected image in terms of one or more placement parameters of interest, e.g., parameters related to placement of structures on the substrate. Such placement parameters of interest may comprise parameters on which device functionality and yield are dependent. In particular, the machine settings typically controlled include inter alia stage settings, lens control settings (e.g., to correct for lens aberration) and dose control settings. Stage settings may describe a planned or set stage movement such as described by variousstage setpoints determined by the control hardware and software (e.g., as controlled by lithographic apparatus control unit or exposure apparatus control unit LACU). These may relate to stage positioning, speed, and / or acceleration in each of the substrate plane (e.g., for placement / overlay control) and perpendicular to the substrate plane (e.g., for focus control).

[0040] In this manner, parameters of interest such as one or more of overlay, focus, and imaging (e.g., critical dimension (CD)) may be optimized. Such an optimization may comprise some trade-off between some of these parameters, but which ensure that each of these parameters of interest remain within- specification (e.g., within a tolerance indicative of a yielding or functional device). As such, “optimize” in the context of this disclosure may simply mean improve and / or ensure that the parameter is within- specification. Alternatively, or in addition, “optimize” may mean optimizing towards a target projection (e.g., an optimized or ideal projection) onto the substrate (e.g., where there is a single ideal setting rather than a specification range. Similarly “minimize” in the context of this disclosure may simply mean reduce, e.g., minimizing variation may be understood to reduce variation and / or to reduce said variation as much as is practically possible.

[0041] A level sensor arrangement and / or height measurement arrangement can determine a height by measuring an intensity imbalance between scattered radiation (e.g., between a pair of complementary diffraction orders) from a grating. A first grating (projection grating) projects a patterned beam onto a wafer, and a second grating (detection grating) forms part of a detection arrangement which measures the intensity of the transmitted light. A projection of the fringes generated by the projection grating onto the detection grating is used to determine asymmetry of the scattered radiation: if the detection grating is not symmetrically illuminated, caused by a change of wafer height, an intensity imbalance is measured. The reference grating may comprise wedges to enhance asymmetry detection.

[0042] Figure 3(a) schematically depicts a known level sensor 10 according to an embodiment of the present invention, which may be used in a lithographic apparatus as described earlier (e.g., level sensor LS described in relation to Figure 1). More detail on this level sensor 10 may be found in patent publication US2010 / 0233600, which is hereby incorporated by reference. A radiation source 16 is used (e.g., having a specific radiation spectrum), which emits radiation towards the substrate W. The radiation from the radiation source 16 is directed at a projection grating 17 positioned between the radiation source 16 and substrate W. Using a reflective projection system comprising mirrors 14 and 15 in the example shown, the image formed by the projection grating 17 is projected on the substrate W. The reflected radiation from the substrate W is again optically focused using further reflective elements 14 and 15 to a detector unit 18. The detector unit 18 is e.g. provided with a UV sensor to measure the radiation reflected from the substrate W and to determine the substrate height (profile) from the reflected radiation in operation.

[0043] Figure 3(b) schematically depicts a known type of detector unit 18 or detection arrangement which may be used with the level sensor 10 of Figure 3(a). The detector unit 18 may comprise a (relatively shallow ruled) detection grating 31 with a triangular grating profile (e.g., comprisingwedges). The ruled grating 31 may function as a phase grating. The pitch P of the ruled grating 31 is larger than the wavelength of the radiation emitted by the radiation source 16. The pitch P is e.g., typically equal to the period of the image of the projection grating 17 (e.g., the projection grating and detection gratings are matched gratings). In Figure 3(b) a transmission grating 31 is shown, but as an alternative a ruled reflection grating maybe used.

[0044] The triangular grating profde of the ruled grating 31 acts as a series of wedges 3 la, 3 lb, which redirects the radiation beam 20 according to the well-known Snell's law. The image that is on a positive wedge 31a is redirected in an upward direction (‘u’) and the image that is on a negative wedge 31b is redirected in a downward direction (‘d’). If the image is exactly centered on the ruled grating 31 then the ‘u’ and ‘d’ images have equal intensity and the output of a processing unit 37 connected to two detectors 35, 36 will be zero. If the substrate height changes then the image of the projection grating 31 will also shift and this will result in an unbalance between the ‘u’ and ‘d’ signals.

[0045] The level sensor projects a series of measurement spots on the wafers, e.g., using an optical Offner column. Each measurement spot is an area of projected patterned radiation (as patterned by the projection grating) on the substrate which is in turn reflected and detected. A second Offner column is used to re-image these spots on the detector unit 18, i.e., on the (ruled) detector grating 31. The detector performs a ‘knife-edge’ measurement (i.e., as has been described, whereby the incident radiation component deflected by the positive wedge is compared to the incident radiation component deflected by the negative wedge) on each fringe of a series of conjugate fringes which form part of the same measurement spot.

[0046] Figure 4 is a schematic illustration of two such measurement spots 400a, 400b. Each measurement spot 400a, 400b comprises multiple, here four, fringes 410.

[0047] Methods disclosed herein aim to improve on known level sensor arrangements such as illustrated in Figure 3(a). For example, in the present arrangement, the detector unit 18, comprising the reference grating (ruled grating 31), splitting optics and detectors 35, 36 (i.e., the data acquisition module) are fairly large / bulky and / or emit heat and are comprised within the critical volume. It is desirable to reduce the detector unit size and / or reduce the component volume and / or reduce the heat emitted within the critical volume.

[0048] An improved level sensor detector arrangement is proposed, which comprises a plurality of fiber cores having their input facets in a detection plane (e.g., the location of the present detection grating). These fiber cores are arranged in sets of fiber cores and positioned such that each set of said plurality of fiber cores samples a respective fringe being measured. As such, each of these sets of fiber cores is a subset of the plurality of fiber cores.

[0049] Each set comprises at least two paired subsets of fiber cores, wherein each paired subset comprises a first subset and a second subset located symmetrically (either side) with respect to a center of the fringe (i.e., along the spot direction). Each set may comprise only the at least two paired subsetsof fiber cores, or else may comprise one or more additional (not necessarily paired) subsets of fiber cores.

[0050] Each of the paired subsets may comprise a single core, and as such, at its most basic the proposed arrangement may sample each fringe using only two fiber cores (i.e., each set of fiber cores may comprise only two fiber cores).

[0051] Imbalance measurements, e.g., knife-edge type measurements, may be performed per fringe to determine the height of the surface being measured, the imbalance measurements comparing a measurement parameter (e.g., intensity) collected by the first subset of fiber cores with that of the second subset of fiber cores.

[0052] The plurality of fiber cores are connected at an output end to a plurality of detectors, such as a plurality of photodiodes. In an embodiment, the fiber cores are connected such that corresponding subsets of the fiber cores, optionally per measurement spot, are connected to a common detector. This reduces the number of photodiodes required. At least two photodiodes are required per fringe, one for each of the paired subsets. In this respect, it can be appreciated that is not necessary to analyze individual fringes or fringe portions separately, and in fact the present arrangement described in relation to Figure 3 is unable to do so. Corresponding subsets of fiber optics may comprise subsets which sample substantially the same portion or region of their respective fringes.

[0053] For example, all the first subsets which sample a first portion (e.g., first side) of its respective fringe may be connected to at least one first group common detector and all the second subsets which sample a second portion (e.g., second side) of its respective fringe may be connected to at least one second group common detector. This may be done per measurement spot such that there is a different first group common detector per measurement spot and a different second group common detector per measurement spot. Alternatively, the first subsets of all the measurement spots may be read by a single first group photodiode and the second subsets of all the measurement spots may be read by a single second group photodiode. Other intermediate arrangements are also possible.

[0054] Figure 5 is a simplified schematic drawing of a level sensor 500 according to concepts disclosed herein. The only difference between this arrangement 500 and the level sensor 10 of Figure 3(a) is in the detection arrangement 518, which replaces detection unit 18. As such, other aspects of the level sensor 500 will not be described again.

[0055] Detection arrangement 518 comprises a plurality of fiber cores 520 having input facets arranged to measure the measurement spots at a detection plane DP. Note that this detection arrangement 518 does not have any detection grating or ruled grating; the input facets are located where the detection grating would be on a more convention level sensor such as that of Figure 3(a). The plurality of fiber cores 520 transport the measured radiation to a plurality of detectors 522 (e.g., photodiodes such as a photo diode array). As such, the plurality of fiber cores replaces the sorting optics and detection grating of a conventional level sensor, while the plurality of detectors 522 replaces the present data acquisition module.

[0056] It can be appreciated that the plurality of detectors 522 may be located inside or outside of the critical volume. In the latter example, the critical volume which would otherwise be consumed by the detection grating, sorting optics and data acquisition module is now freed, other than the volume taken up by the portion of the plurality of fiber cores 520 inside of the critical volume.

[0057] The respective arrangement of the plurality of fiber cores 520 and plurality of detectors 522 may be better understood with reference to Figure 6. Figure 6 shows four fringes 410 for each of two measurement spots 400a, 400b, incident on a plurality of fiber core input facets. The plurality of fiber core input facets are arranged into sets 600 of fiber cores, each set 600 of fiber cores comprising two or more paired subsets 620a, 620b of fiber cores, i.e., first subset 620a and second subset 620b. Each first subset 620a and second subset 620b respectively comprises at least one fiber core, and in this specific example four fiber cores.

[0058] Optionally, as shown in this example, there may be one or more additional subsets per set 600 of fiber cores, whether paired or unpaired. In this specific example, each set 600 comprises a third subset 630 which samples a center area of each fringe 410, i.e., it is located between the paired subsets 620a, 620b. In this specific example the third subset comprises five fiber cores, i.e., such that each set 600 in this specific example comprises 13 fiber cores. Such an addition subset 630 center provides additional data, e.g., it provides an extra data point to fit. It also makes it possible to measure total intensity (intensity magnitude, e.g., as measured by the subset 630 alone or as measured by all fiber cores 610 of a set 600, may be used to determine corrections for the height measurement). Of course, where there is no additional or center subset 630, the paired subset(s) may be positioned more centrally (e.g., each moved an equal distance towards the center compared to the arrangement shown) and / or may comprise larger diameter fiber cores.

[0059] Figure 7 illustrates an example detector arrangement corresponding to the fiber core arrangement of Figure 6. Shown is the detection plane DP, i.e., an image plane onto which the reflected measurement spots are imaged. The array of fiber core input facets illustrated in Figure 6 are located at this plane. Also shown is a detector array 522 comprising a plurality of detectors (e.g., photodiodes). Shown specifically are six detectors 720a-720f, although this may be more or less depending on the number of spots measured simultaneously, the number of fringes per spot sampled and the specific sampling strategy employed.

[0060] In the specific example, the subsets 620a, 620b, 630 are grouped into groups of corresponding subsets per measurement spot, with each group being connected to a respective detector 720a-720f. The groups are indicated by the numerals 1 to 6 on Figures 6 and 7.

[0061] By way of further elaboration, fiber cores 710a are sampling measurement spot 400a and fiber cores 710b are sampling measurement spot 400b. Considering the fiber cores 710a of measurement spot 400a, all of the fiber cores of the first paired subset 620a for all of the fringes are grouped into a first group of fiber cores 730a, all of the fiber cores of the second paired subset 620b for all of the fringes are grouped into a second group of fiber cores 730b and all of the fiber cores of the subset 630 for allof the fringes are grouped into a third group of fiber cores 730c. Similarly, for the fiber cores 710b of measurement spot 400b, all of the fiber cores of the first paired subset 620a for all of the fringes are grouped into a fourth group of fiber cores 730d (an additional first group in the context of the claims), all of the fiber cores of the second paired subset 620b for all of the fringes are grouped into a fifth group of fiber cores 730e (an additional second group in the context of the claims) and all of the fiber cores of the subset 630 for all of the fringes are grouped into a sixth group of fiber cores 730f (an additional third group in the context of the claims).

[0062] Each of the groups of fiber cores 730a-730f is read by a respective detector 720a-720f of the detector array 522, i.e., first group common detectors 720a, 720d, second group common detectors 720b, 720e and third group common detectors 720c, 720f.

[0063] There are a number of variations on the specific examples shown which fall within the scope of the concepts disclosed herein.

[0064] As illustrated, the fiber cores can be grouped according to position within a fringe, so as to group together corresponding areas (e.g., with respect to the fringe) on different fringes within a spot. In the example given, each measurement spot has a respective group of detectors, while which means that information per fringe cannot be determined. In another embodiment, each fringe may be read by a respective dedicated group of detectors.

[0065] Alternatively, or in addition, embodiments may monitor the intensity ‘along’ the fringe (e.g., top versus bottom as represented in Figure 6), requiring more detectors. This may comprise for example, per subset 620a, 620b 620c or for only one (or more) subsets per set, e.g., the central subset 630: respective detectors for a first or upper sub-subset, a second or lower sub-subset and optionally further one or more intermediate sub-subsets with the fiber cores grouped accordingly. Ultimately, it is possible that each fiber core connects to an individual pixel, becoming essentially a camera (i.e., each group of fiber cores comprises a single fiber core).

[0066] While all the fiber cores are shown to have a circular cross-section, this is not essential nor necessary. One, more or all of the fiber cores may have a different shape (e.g., square, hexagonal or other polygon. The shape may be varied over the plurality of fiber cores, and / or within each set of fiber cores. The specific arrangement of fiber cores can be tailored to their shape.

[0067] There may be variation in fiber core size (e.g., diameter or other dimension depending on shape) over the plurality of fiber cores, and / or within each set of fiber cores.

[0068] The arrangement of Figure 6 shows regions between fringes where there are no fiber cores. These regions may also comprise fiber cores, e.g., to fill these regions either completely or partially.

[0069] For example, depending on calibration strategy and design, the fiber cores do not need to be periodic with the fringes. Arranging the fiber cores to be non-periodic with the fringes can allow for flexibility in the pitch of projection grating. In such an example the fiber cores may be arranged continuously in the fringe periodicity direction (e.g., left to right in Figure 6) per spot, e.g., to form a ID or 2D array of fiber cores extending along at least the fringe periodicity direction, with eachfiber / fiber subset along the fringe periodicity direction being connected to a respective detector to effectively form a ID camera.

[0070] Each fiber core may be arranged to interface with a color module filter before projection onto the detector array, e.g., to provide demultiplexing according to color. For example, each fiber core may have the same type of color filter. Alternatively, each fiber core or group of fiber cores may have a respective different color (if combined with sufficient detectors).

[0071] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0072] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may 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.

[0073] The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 1-100 nm), as well as particle beams, such as ion beams or electron beams.

[0074] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components. Reflective components are likely to be used in an apparatus operating in the UV and / or EUV ranges.

[0075] The breadth and scope of the present invention should not be limited by any of the abovedescribed exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

[0076] Various embodiments of the present systems and methods are disclosed in the subsequent list of numbered clauses. In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of clauses that may be optionally claimed in any combination:1. A detection arrangement for a level sensor, the detection arrangement comprising: a plurality of fiber cores comprising input facets for receiving reflected measurement radiation at a detection plane of the level sensor, subsequent to the reflected measurement radiation having been reflected by a surface being measured, said reflected measurement radiation forming one or more measurement spots at the detection plane, each of the one or more measurement spots comprising a plurality of fringes; anda plurality of detectors for detecting received measurement radiation, said received measurement radiation comprising said reflected measurement radiation subsequent to it being received and carried by said plurality of fiber cores; wherein the plurality of fiber cores comprises at least a respective set of fiber cores for each of said fringes, each said set of fiber cores comprising at least a first subset of fiber cores being configured for receiving radiation from a first portion of its respective fringe and a second subset of fiber cores being configured for receiving radiation from a second portion of its respective fringe, the first portion being displaced from the second portion in at least a direction of fringe periodicity of said fringes.2. A detection arrangement as claimed in clause 1, wherein: at least per measurement spot, at least a sub-subset of the first subset of fiber cores for all of the fringes of said measurement spot are grouped into a first group of fiber cores; at least per measurement spot, at least a sub-subset of the second subset of fiber cores for all of the fringes of said measurement spot are grouped into a second group of fiber cores; said first group of fiber cores are connected to at least one first group common detector of said plurality of detectors; and said second group of fiber cores are connected to at least one second group common detector of said plurality of detectors.3. A detection arrangement as claimed in clause 2, wherein: at least per measurement spot, all of the fiber cores of the first subset of fiber cores for all the fringes of said measurement spot are grouped into the first group of fiber cores; and at least per measurement spot, all of the fiber cores of the second subset of fiber cores for all the fringes of said measurement spot are grouped into the second group of fiber cores.4. A detection arrangement as claimed in clause 2 or 3, wherein: each first group of fiber cores respectively for each measurement spot is connected to a respective different first group common detector; and each second group of fiber cores respectively for each measurement spot is connected to a respective different second group common detector.5. A detection arrangement as claimed in clause 2 or 3, wherein: each first group of fiber cores for all of said measurement spots is connected to a single first group common detector; and each second group of fiber cores for all of said measurement spots is connected to a single second group common detector.6. A detection arrangement as claimed in clause 2, 3 or 4 wherein: said at least a sub-subset of the first subset of fiber cores comprises at least a first sub-subset of the first subset of fiber cores being configured for receiving radiation from a first sub-portion of said first portion of its respective fringe and a second sub-subset of the first subset of fiber cores beingconfigured for receiving radiation from a second sub-portion of said first portion of its respective fringe; and said at least a sub-subset of the second subset of fiber cores comprises at least a first sub-subset of the second subset of fiber cores being configured for receiving radiation from a first sub-portion of said second portion of its respective fringe and a second sub-subset of the second subset of fiber cores being configured for receiving radiation from a second sub-portion of said second portion of its respective fringe, wherein, in each case, said first sub-portion is displaced from the second sub-portion in at least a direction perpendicular to the direction of fringe periodicity in said detection plane; and wherein each said sub-subset is connected to at least one different detector of said plurality of detectors.7. A detection arrangement as claimed in any preceding clause, wherein each said set of fiber cores comprises at least a third subset of fiber cores is configured for receiving radiation from a third portion of its respective fringe.8. A detection arrangement as claimed in clause 7, wherein: at least per measurement spot, at least a sub-subset of the third subset of fiber cores for all of the fringes of said measurement spot are grouped into a third group of fiber cores; and said third group of fiber cores are connected to at least one third group common detector of said plurality of detectors.9. A detection arrangement as claimed in clause 8, wherein at least per measurement spot, all of the fiber cores of the third subset of fiber cores for all the fringes of said measurement spot are grouped into the third group of fiber cores.10. A detection arrangement as claimed in clause 8 or 9, wherein each third group of fiber cores respectively for each measurement spot is connected to a respective different third group common detector.11. A detection arrangement as claimed in clause 8 or 9, wherein each third group of fiber cores for all of said measurement spots is connected to a single third group common detector.12. A detection arrangement as claimed in clause 9, 10 or 11, wherein: said at least a sub-subset of the third subset of fiber cores comprises at least a first sub-subset of the third subset of fiber cores being configured for receiving radiation from a first sub-portion of said third portion of its respective fringe and a second sub-subset of the third subset of fiber cores being configured for receiving radiation from a second sub-portion of said third portion of its respective fringe; wherein, said first sub-portion is displaced from the second sub-portion in at least a direction perpendicular to the direction of fringe periodicity in said detection plane; and wherein each said sub-subset of the third subset is connected to at least one different detector of said plurality of detectors.13. A detection arrangement as claimed in any preceding clause, wherein said first portion of each fringe and said second portion of each fringe are located symmetrically around a center of said fringe in at least said direction of fringe periodicity.14. A detection arrangement as claimed in clause 1, wherein said plurality of detectors comprises a respective detector connected to each said subset of fiber cores.15. A detection arrangement as claimed in clause 1, wherein said plurality of detectors comprises a respective detector connected to each said fiber core of said plurality of fiber cores.16. A detection arrangement as claimed in any preceding clause, wherein each detector of said plurality of detectors comprises a photodiode.17. A detection arrangement as claimed in any preceding clause, wherein one or more fiber cores of said plurality of fiber cores are circular in cross-section.18. A detection arrangement as claimed in any preceding clause, wherein one or more fiber cores of said plurality of fiber cores are polygonal in cross-section.19. A detection arrangement as claimed in any preceding clause, wherein a diameter and / or other dimension of said fiber cores is varied over said plurality of fiber cores.20. A detection arrangement as claimed in any preceding clause, wherein said plurality of fiber cores are arranged non-periodically with said fringes in said direction of fringe periodicity for at least one of said one or more measurement spots.21. A detection arrangement as claimed in any preceding clause, wherein said plurality of fiber cores are arranged continuously in said direction of fringe periodicity for at least one of said one or more measurement spots.22. A detection arrangement as claimed in any preceding clause, comprising a color filter module between said plurality of fiber cores and said plurality of detectors.23. A level sensor, comprising: a detection optical arrangement operable to collect said reflected measurement radiation; and image said reflected measurement radiation at said detection plane; and the detection arrangement as claimed in any preceding clause; wherein said input facets are located at said detection plane.24. A level sensor as claimed in clause 23, further comprising at least one linear phase modulating projection grating element being configured to scatter measurement radiation to generate scattered radiation; and projection optics configured to direct said scattered radiation onto said surface to be measured to generate said reflected measurement radiation.25. A level sensor as claimed in clause 23 or 24, wherein said plurality of detectors are located outside of a critical volume of said level sensor.26. A level sensor as claimed in clause 23, 24 or 25, comprising at least one processor unit being operable to determine a height of said surface from said reflected measurement radiation.27. An exposure apparatus comprising the level sensor of any of clauses 23 to 26.28. An exposure apparatus as claimed in clause 27, comprising: a substrate support for supporting a substrate; and wherein said level sensor is configured for measuring height data describing the height of the surface of said substrate.

Claims

CLAIMS1. A detection arrangement for a level sensor, the detection arrangement comprising: a plurality of fiber cores comprising input facets for receiving reflected measurement radiation at a detection plane of the level sensor, subsequent to the reflected measurement radiation having been reflected by a surface being measured, said reflected measurement radiation forming one or more measurement spots at the detection plane, each of the one or more measurement spots comprising a plurality of fringes; and a plurality of detectors for detecting received measurement radiation, said received measurement radiation comprising said reflected measurement radiation subsequent to it being received and carried by said plurality of fiber cores; wherein the plurality of fiber cores comprises at least a respective set of fiber cores for each of said fringes, each said set of fiber cores comprising at least a first subset of fiber cores being configured for receiving radiation from a first portion of its respective fringe and a second subset of fiber cores being configured for receiving radiation from a second portion of its respective fringe, the first portion being displaced from the second portion in at least a direction of fringe periodicity of said fringes.

2. A detection arrangement as claimed in claim 1, wherein: at least per measurement spot, at least a sub-subset of the first subset of fiber cores for all of the fringes of said measurement spot are grouped into a first group of fiber cores; at least per measurement spot, at least a sub-subset of the second subset of fiber cores for all of the fringes of said measurement spot are grouped into a second group of fiber cores; said first group of fiber cores are connected to at least one first group common detector of said plurality of detectors; and said second group of fiber cores are connected to at least one second group common detector of said plurality of detectors.

3. A detection arrangement as claimed in claim 2, wherein: at least per measurement spot, all of the fiber cores of the first subset of fiber cores for all the fringes of said measurement spot are grouped into the first group of fiber cores; and at least per measurement spot, all of the fiber cores of the second subset of fiber cores for all the fringes of said measurement spot are grouped into the second group of fiber cores.

4. A detection arrangement as claimed in claim 2 or 3, wherein: each first group of fiber cores respectively for each measurement spot is connected to a respective different first group common detector; andeach second group of fiber cores respectively for each measurement spot is connected to a respective different second group common detector.

5. A detection arrangement as claimed in claim 2 or 3, wherein: each first group of fiber cores for all of said measurement spots is connected to a single first group common detector; and each second group of fiber cores for all of said measurement spots is connected to a single second group common detector.

6. A detection arrangement as claimed in claim 2, 3 or 4 wherein: said at least a sub-subset of the first subset of fiber cores comprises at least a first sub-subset of the first subset of fiber cores being configured for receiving radiation from a first sub-portion of said first portion of its respective fringe and a second sub-subset of the first subset of fiber cores being configured for receiving radiation from a second sub-portion of said first portion of its respective fringe; and said at least a sub-subset of the second subset of fiber cores comprises at least a first sub-subset of the second subset of fiber cores being configured for receiving radiation from a first sub-portion of said second portion of its respective fringe and a second sub-subset of the second subset of fiber cores being configured for receiving radiation from a second sub-portion of said second portion of its respective fringe, wherein, in each case, said first sub-portion is displaced from the second sub-portion in at least a direction perpendicular to the direction of fringe periodicity in said detection plane; and wherein each said sub-subset is connected to at least one different detector of said plurality of detectors.

7. A detection arrangement as claimed in any preceding claim, wherein each said set of fiber cores comprises at least a third subset of fiber cores is configured for receiving radiation from a third portion of its respective fringe; at least per measurement spot, at least a sub-subset of the third subset of fiber cores for all of the fringes of said measurement spot are grouped into a third group of fiber cores; said third group of fiber cores are connected to at least one third group common detector of said plurality of detectors, and at least per measurement spot, all of the fiber cores of the third subset of fiber cores for all the fringes of said measurement spot are grouped into the third group of fiber cores.

8. A detection arrangement as claimed in claim 7, wherein each third group of fiber cores respectively for each measurement spot is connected to a respective different third group common detector.

9. A detection arrangement as claimed in claim 7, wherein each third group of fiber cores for all of said measurement spots is connected to a single third group common detector.

10. A detection arrangement as claimed in claim 7, wherein: said at least a sub-subset of the third subset of fiber cores comprises at least a first sub-subset of the third subset of fiber cores being configured for receiving radiation from a first sub-portion of said third portion of its respective fringe and a second sub-subset of the third subset of fiber cores being configured for receiving radiation from a second sub-portion of said third portion of its respective fringe; wherein, said first sub-portion is displaced from the second sub-portion in at least a direction perpendicular to the direction of fringe periodicity in said detection plane; and wherein each said sub-subset of the third subset is connected to at least one different detector of said plurality of detectors.

11. A detection arrangement as claimed in any preceding claim, wherein said first portion of each fringe and said second portion of each fringe are located symmetrically around a center of said fringe in at least said direction of fringe periodicity, wherein said plurality of fiber cores are arranged non-periodically with said fringes in said direction of fringe periodicity for at least one of said one or more measurement spots, or wherein said plurality of fiber cores are arranged continuously in said direction of fringe periodicity for at least one of said one or more measurement spots.

12. A level sensor, comprising: a detection optical arrangement operable to collect said reflected measurement radiation; and image said reflected measurement radiation at said detection plane; and the detection arrangement as claimed in any preceding claim; wherein said input facets are located at said detection plane.

13. A level sensor as claimed in claim 12, further comprising at least one linear phase modulating projection grating element being configured to scatter measurement radiation to generate scattered radiation; and projection optics configured to direct said scattered radiation onto said surface to be measured to generate said reflected measurement radiation.

14. An exposure apparatus comprising the level sensor of any of claims 12 and 13.

15. An exposure apparatus as claimed in claim 14, comprising: a substrate support for supporting a substrate; and wherein said level sensor is configured for measuring height data describing the height of the surface of said substrate.

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