Patent Publication Number: US-2023152718-A1

Title: Differential measurement system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of EP application 20168356.2 which was filed on 7 Apr. 2020, and which is incorporated herein in its entirety by reference. 
     FIELD 
     The present invention relates to systems and methods for measuring a property of targets, in particular for measuring differences in a property between two targets. A particular embodiment relates to a system and method for differential flow measurement in a lithographic apparatus. 
     BACKGROUND 
     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 (also often referred to as “design layout” or “design”) at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer). 
     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. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. 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. 
     Low-k 1  lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula may be expressed as CD = k 1 ×λ/NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case half-pitch) and k 1  is an empirical resolution factor. In general, the smaller k 1  the more difficult it becomes to reproduce the pattern on the substrate that resembles the shape and dimensions planned by a circuit designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine-tuning steps may be applied to the lithographic projection apparatus and/or design layout. These include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase shifting patterning devices, various optimization of the design layout such as optical proximity correction (OPC, sometimes also referred to as “optical and process correction”) in the design layout, or other methods generally defined as “resolution enhancement techniques” (RET). Alternatively, tight control loops for controlling a stability of the lithographic apparatus may be used to improve reproduction of the pattern at low k 1 . 
     Many lithographic apparatuses use fluids for temperature control and other purposes. For example, temperature-controlled water may be used to maintain critical parts of an apparatus at constant temperature. Any leakage of water inside the machine could cause serious damage and desirably should therefore be detected quickly. 
     SUMMARY 
     It is desirable, for example, to provide a system for measuring the difference between a property of a first target and a property of a second target, wherein the difference is not derived by the subtraction of a measurement taken by one sensor from a measurement taken by a different sensor. In particular, it is desirable to provide a system for measuring the difference between an in-flow rate and an out-flow rate that does not overly rely on the calibration of two different sensors for the accuracy of the differential measurement. 
     According to an aspect of the invention, there is provided a system for measuring the difference between a property of a first target and a property of a second target, the system comprising a first member and a second member, wherein:
     the first member comprises a first pattern, and the speed of rotation of the first member is configured to be based on the property of the first target; and   the second member comprises a second pattern wherein, the speed of rotation of the second member is configured to be based on the property of the second target,   further wherein the first and second pattern are angularly-varying and are configured to generate an interference pattern by their interaction when the first and second members have a relative difference in their rotational speeds, the interference pattern being indicative of the magnitude of this difference.   

     According to a further aspect of the invention, there is provided a method of measuring the difference between a property of a first target and a property of a second target, the method comprising:
     rotating a first member comprising a first pattern at a speed based on the property of the first target;   rotating a second member comprising a second pattern at a speed based on the property of the second target;   whereby an interference pattern between the first and second patterns is formed when the first and second members have a relative difference in their rotational speed; and,   taking a differential measurement of the property of the first and second targets based on the interference pattern.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: 
         FIG.  1    depicts a lithography apparatus according to an embodiment of the invention; 
         FIG.  2    is a more detailed view of the lithography apparatus; 
         FIG.  3    is a more detailed view of the source collector module SO of the apparatus of  FIGS.  1  and  2   ; 
         FIG.  4    depicts a first pattern that has an angular variation; 
         FIG.  5    depicts the pattern of  FIG.  4    superimposed on a second identical pattern, and the appearance of the interference generated when the second identical pattern is rotated relative to the first pattern; 
         FIG.  6    depicts the pattern of  FIG.  4    superimposed on a second identical pattern, and the appearance of the interference generated when the second identical pattern is rotated relative to the first pattern, for a small portion of the overall pattern. 
         FIG.  7    depicts the time relationship between the angular velocity of the second pattern and the change in the generated interference. 
         FIG.  8    depicts an embodiment of the measurement system according to the present invention; 
         FIG.  9    depicts another embodiment of the measurement system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    schematically depicts a lithography apparatus  100  including a source collector module SO according to one embodiment of the invention. The apparatus comprises:
     an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation).   a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device;   a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and   a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.   

     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. 
     The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithography apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. 
     The term “patterning device” 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. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. 
     The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix. 
     The projection system, like 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, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps. 
     As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask). 
     The lithography apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. 
     Referring to  FIG.  1   , the illuminator IL receives an extreme ultra violet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in  FIG.  1   , for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation. 
     In such cases, the laser is not considered to form part of the lithography apparatus and the radiation beam is passed from the laser to the source collector module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source. 
     The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross section. 
     The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from 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 PS 2  (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT 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 PS 1  can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. Patterning device (e.g., mask) MA and substrate W may be aligned using mask alignment marks M 1 , M 2  and substrate alignment marks P 1 , P 2 . 
     The depicted apparatus could be used in at least one of the following modes:
     1. In step mode, the support structure (e.g., mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.   2. In scan mode, the support structure (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 velocity and direction of the substrate table WT relative to the support structure (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.   3. In another mode, the support structure (e.g., mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.   

     Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. 
       FIG.  2    shows the apparatus  100  in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure  220  of the source collector module SO. An EUV radiation emitting plasma  210  may be formed by a discharge produced plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the very hot plasma  210  is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma  210  is created by, for example, an electrical discharge causing an at least partially ionized plasma. Partial pressures of, for example,  10  Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation. 
     The radiation emitted by the hot plasma  210  is passed from a source chamber  211  into a collector chamber  212  via an optional gas barrier or contaminant trap  230  (in some cases also referred to as contaminant barrier or foil trap) that is positioned in or behind an opening in source chamber  211 . The contaminant trap  230  may include a channel structure. Contamination trap  230  may also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier  230  further indicated herein at least includes a channel structure, as known in the art. 
     The collector chamber  211  may include a radiation collector CO, which may be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side  251  and a downstream radiation collector side  252 . Radiation that traverses collector CO can be reflected off a grating spectral filter  240  to be focused in a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector module is arranged such that the intermediate focus IF is located at or near an opening  221  in the enclosing structure  220 . The virtual source point IF is an image of the radiation emitting plasma  210 . 
     Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device  22  and a facetted pupil mirror device  24  arranged to provide a desired angular distribution of the radiation beam  21 , at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation  21  at the patterning device MA, held by the support structure MT, a patterned beam  26  is formed and the patterned beam  26  is imaged by the projection system PS via reflective elements  28 ,  30  onto a substrate W held by the wafer stage or substrate table WT. 
     More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filter  240  may optionally be present, depending upon the type of lithography apparatus. Further, there may be more mirrors present than those shown in the Figures, for example there may be 1- 6 additional reflective elements present in the projection system PS than shown in  FIG.  2   . 
     Collector optic CO, as illustrated in  FIG.  2   , is depicted as a nested collector with grazing incidence reflectors  253 ,  254  and  255 , just as an example of a collector (or collector mirror). The grazing incidence reflectors  253 ,  254  and  255  are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source. 
     Alternatively, the source collector module SO may be part of an LPP radiation system as shown in  FIG.  3   . A laser LA is arranged to deposit laser energy into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li), creating the highly ionized plasma  210  with electron temperatures of several 10’s of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near normal incidence collector optic CO and focused onto the opening  221  in the enclosing structure  220 . 
     In a lithographic apparatus such as that described above, various components must be cooled or maintained at a very precise temperature. Often, temperature-controlled water is circulated through parts of the lithographic apparatus in order to perform cooling or temperature control. The lithographic apparatus is provided with a fluid inlet and a fluid outlet in order to receive fluid and expel fluid, e.g. water, from an external supply. It will be appreciated that a leak of a fluid, especially water, inside the lithographic apparatus is highly undesirable. As well as the possibility of costly damage and downtime to clean up a leak, a leak might affect the humidity within the lithographic apparatus which can affect sensors, such as interferometric displacement sensors, and lead to erroneous imaging. 
     A conventional approach to leak-detection in a lithographic apparatus is to provide a first flow sensor and a second flow sensor at a fluid inlet and a fluid outlet respectively. The presence of a leak can be detected by subtracting the measured flow value of the first sensor from the measured flow value of the second sensor. This approach of calculating a difference between two measurements in order to determine the presence or magnitude of a leak is prone to inaccuracies. If the first sensor and the second sensor are not calibrated accurately, the offset will give a large error in the difference measurement. Sensor calibration can involve correcting for sensor drift, signal de-noising, phase-shift compensation, digitization, or other conversion. The presence or magnitude of a leak in the compartment will therefore be inaccurately determined. 
     The present disclosure provides an apparatus and a method for measuring the difference between a property of a first target and a property of a second target using an interference pattern, e.g. a Moiré pattern. A Moiré pattern is an interference pattern generated by similar patterns that are superimposed at a small offset. In particular, the present disclosure provides an apparatus and a method for measuring the leakage flow in a compartment of a lithography apparatus by observing the generation of a Moiré pattern. 
     The present disclosure is directed towards directly measuring the difference between the property of the first target, and the property of the second target. It is desirable to measure the difference between the property of the first target and the property of the second target directly. In particular, it may be desirable to measure the leakage flow in a compartment without subtracting an inflow measurement taken by a first flow sensor, from an outflow measurement taken by a second flow sensor. 
       FIG.  4    and  FIG.  5    illustrate the principle of generating a Moiré pattern by rotation, as used in the present invention. 
       FIG.  4    depicts a single pattern  400 . The pattern  400  has an angular variation with respect to the axis  403 . The pattern may comprise two components. In  FIG.  4   , the pattern has a first component  401  and a second component  402 . Desirably, the pattern comprises plurality of first components  401  and a plurality of second components regularly arranged in an alternating manner around the axis  403 . 
     In  FIG.  4   , the first component  401  is depicted in black and the second component  402  is depicted in white. In practice, the first component  401  and the second component  402  are distinguished by any difference in an optical property, or other property. For example, the first component  401  may have a lower transmissivity than the second component  402 . Alternatively, the first component  401  may have a lower reflectivity than the second component  402 . Other configurations of the present disclosure may distinguish the first and second component with any of the following properties: different refractive indices; concentric patterns with a different number of lines/spokes. 
     While pattern  400  has the appearance of a spoked wheel, any other angularly-varying pattern could be used in the present invention. The pattern  400  may have rotational symmetry. It may be preferable for the pattern  400  to have as many axes of rotational symmetry as is practical to manufacture. It may be preferable for the pattern  400  to have a particular number of axes of rotational symmetry, as will be described in more detail. Conversely, the pattern may not have any axes of rotational symmetry. 
     While not shown in  FIG.  4   , pattern  400  may have a third or more components. These additional components are distinguished by having different optical or other properties to the first component and the second component. 
     While the first component and the second component are shown in  FIG.  4    to be binary in nature, i.e. either “black” or “white”, it will be appreciated that the principle of the present disclosure does not exclude non-binary pattern arrangements. The pattern  400  may comprise gradients, or other intermediate components. 
     The axis  403  may preferably be disposed at the centroid of the pattern  400 . In the embodiment of  FIG.  4   , the pattern is circular, and the axis  403  is disposed at the center of the circle. While it may be preferable that the axis  403  is at the centroid of the pattern  400 , non-centroid axes are possible and can still function under the principle of the present disclosure. 
       FIG.  5    is a schematic depiction of how a Moiré pattern can be generated according to the present disclosure. In  FIG.  5   , and in the present disclosure generally, the offset between patterns is generated by rotation. 
     Schematics  510 ,  520  and  530  illustrate the interference pattern generated by two superimposed copies of pattern  400 , the two copies referred to as pattern  400 A and pattern  400 B. In these schematics, pattern  400 B is rotated clockwise with respect to pattern  400 A. For the purposes of illustration, the first component  401  of both patterns  400 A and  400 B is opaque, and the second component  402  of both patterns  400 A and  400 B is transparent. 
     In the foregoing, the term “Moiré pattern” refers to this interference pattern produced by superimposing pattern  400 A and pattern  400 B. The Moiré pattern resulting from pattern  400 A and pattern  400 B depends on the relative placement of the first component  401 A of pattern  400 A and the second component  402 A of pattern  400 A, with the first component  401 B of pattern  400 B and the second component  402 B of pattern  400 B. 
     The resulting Moiré pattern can be inferred using binary logic. Pattern  400 A can be considered as a first input, and pattern  400 B can be considered as a second input. The first component  401  of both patterns  400 A and  400 B can be considered a [1] input, and the second component  402  of both patterns  400 A and  400 B can be considered a [0] input. The resulting Moiré pattern can therefore be considered an “ 400 A OR  400 B” operation in binary logic, applied across the whole 2-dimensional pattern. 
     Where the first component  401 A of pattern  400 A overlaps with the first component  401 B of pattern  400 B, this component of the Moiré pattern has the appearance of a first component  401  (in the Figure, it is opaque/black). Where the first component  401 A of pattern  400 A overlaps with the second component  402 B of pattern  400 B, this component of the Moiré pattern also has the appearance of a first component  401  (in the Figure, it is opaque/black). Where the second component  402 A of pattern  400 A overlaps with the first component  401 B of pattern  400 B, this component of the Moiré pattern also has the appearance of a first component  401  (in the Figure, it is opaque/black). However, where the second component  402 A of pattern  400 A overlaps with the second component  402 B of pattern  400 B, this component of the Moiré pattern has the appearance of a second component  402  (in the Figure, it is transparent/white). 
     In schematic  510 , the two patterns  400 A and  400 B are superimposed on each other with an identical axis  403 , and are aligned by a common axis of rotational symmetry. The resulting interference pattern has the appearance of either one of pattern  400 A or  400 B. In other words, no interference pattern is apparent. 
     In schematic  520 , pattern  400 B has been rotated clockwise about the shared axis  403 , while pattern  400 A has been kept stationary. Applying the “OR” binary logic operation between the two patterns, in the regions where: the first component  401 A of pattern  400 A overlaps with the first component  401 B of pattern  400 B; or, the first component  401 A of pattern  400 A overlaps with the second component  402 B of pattern  400 B; or, the second component  402 A of pattern  400 A overlaps with the first component  401 B of pattern  400 B; the corresponding region of the Moiré pattern is black, or opaque. Conversely, where the second component  402 A of pattern  400 A overlaps with the second component  402 B of pattern  400 B, the corresponding region of the Moiré pattern is white, or transparent. In the example of schematic  520 , this Moiré pattern gives the appearance that the opaque/black spokes on the pattern have thickened compared to pattern  400  and Moiré pattern  510 . 
     Schematic  530  shows a further clockwise rotation of pattern  400 B with respect to pattern  400 A. In this schematic, the two patterns are perfectly “out of phase” with respect to their axes of rotational symmetry. At this amount of relative rotation between patterns  400 A and  400 B, there is the highest possible proportion of a first component  401  (in the Figure, opaque/black) in the Moiré pattern overall. In other words, the overall Moiré pattern is as “dark”, or “opaque” as possible. 
     The resulting Moiré patterns  510 ,  520  and  530  have different proportions of a first component  401  and a second component  402 , wherein the first component in the Figure is black/opaque and the second component in the Figure is transparent/white. 
     The present apparatus and method uses the phenomenon of these different Moiré patterns in order to measure a property difference between two targets, explained in detail below. 
     In  FIG.  5   , curve  550  indicates how the proportion of a second component  402  on the Moiré pattern varies as the patterns  400 A and  400 B are rotated relative to each other. The vertical axis “I” denotes the total area of the second component  402  in the Moiré pattern. Axis “I” can also be considered to denote the amount of light that is able to pass through the Moiré pattern if the second component  402  is transparent. The horizontal axis “α” denotes the angle of rotation of pattern  400 B relative to pattern  400 A. The curve  550  corresponds to the Moirés chematics  510 ,  520  and  530  above it. 
     In practice, the first component  401  and the second component  402  are distinguished by any difference in an optical property, or other property. Therefore, the “value I” can generally be considered to be the transmissivity, of the Moiré pattern, where the second component  402  has a higher transmissivity than the first component  401 . 
     As depicted in  FIG.  5   , schematic  510 , and by the corresponding point in curve  550 , the value of I is highest when patterns  400 A and  400 B have no difference in angular rotation. In other words, their points of rotational symmetry correspond and no interference pattern is apparent. 
     As depicted in  FIG.  5   , schematic  520 , and by the corresponding point in curve  550 , the value of I is intermediate when patterns  400 A and  400 B have a slight difference in angular rotation. In other words, their points of rotational symmetry are slightly out of phase and the interference pattern is apparent. 
     As depicted in  FIG.  5   , schematic  530 , and by the corresponding point in curve  550 , the value of I is lowest when patterns  400 A and  400 B are completely out of phase with respect to their points of rotational symmetry. 
     This variation in the value I can occur with respect to relative rotation, even when the whole area of the Moiré pattern is not observed. A variation in I may be observed when only a small portion of the total pattern area is observed. Accordingly in the foregoing, a variation in I may refer to this property over a whole pattern, or refer to this property over a smaller area of the pattern. 
       FIG.  6    depicts a portion of the Moiré pattern that is generated due to the relative rotation of patterns, similarly to as in  FIG.  5   . Pattern  400  is depicted with an area of observation  600 . The area of observation  600  only encompasses the ends of several spokes in the pattern  400 . 
     Subfigures  610 ,  620  and  630  show the area of observation  600  for the schematics  510 ,  520  and  530 . Subfigures  610 ,  620  and  630  show that while the whole pattern is not under observation, a change in the ratio between the first opaque/black component  401  and a second transparent/white component  402 . Therefore in the foregoing it should be understood that the change in the ratio between the first opaque/black component  401  and a second transparent/white component  402 , or the change in the value of I occurs not only over the whole Moiré pattern, but also occurs independently in small portions of the Moiré pattern. 
     Schematics  530  and  630  portray the Moiré pattern arising when patterns  400 A and  400 B are completely out of phase. If pattern  400 B was further rotated clockwise from its position in schematics  530  and  630 , a Moiré pattern similar to  520  and  620  would arise. If pattern  400 B was even further rotated clockwise, the points of rotational symmetry of  400 A and  400 B would at some point align again, as in  510  and  610 . 
     Therefore, rotating pattern  400 B with respect to pattern  400 A at a constant angular velocity yields a periodic fluctuation in the value of I. The peaks in the value of I correspond to where the points of rotational symmetry of patterns  400 A and  400 B align. The troughs in the value of I correspond to where the axes of rotational symmetry of patterns  400 A and  400 B are completely out of phase. 
       FIG.  7    illustrates the periodic fluctuation in the value of I for two different relative angular speeds between the patterns  400 A and  400 B. The vertical axis denotes the value of I and the horizontal axis denotes time. The curves  710  and  720  denote the variation of the value of I over time. 
     A first angular speed ω1 is lower than a second angular speed ω2. The curve  710  depicts the time-dependent variation of I when  400 B rotates at the angular velocity ω1. The curve  720  depicts the time-dependent variation of I when  400 B rotates at the angular velocity ω2. Curve  710  has a lower frequency than curve  720 . The period T 1  is larger than the period T 2 . 
     Therefore, if pattern  400 A is kept stationary, and pattern  400 B is rotated relative to pattern  400 A, the speed of angular rotation ω of the pattern  400 B can be inferred from the frequency of oscillation in the value of I. A higher frequency of oscillation in the value of I denotes a higher speed of angular rotation ω. 
     It will be appreciated that while the above has only discussed examples where the first pattern  400 A is kept stationary, the absolute rotation of patterns  400 A and  400 B do not influence the Moiré pattern. For example, if patterns  400 A and  400 B were rotated in the same direction and at the same speed, no interference pattern would emerge, and there would be no variance in the value of I. 
     In other words, the Moiré patterns depicted in  FIGS.  5  and  6    only emerge due to a relative angular displacement between patterns  400 A and  400 B. Consequently, the frequency of variation in the value I depends only on the relative angular speeds of  400 A and  400 B. 
     It is for this reason that the Moiré patterns of the present disclosure are particularly suitable for taking a difference measurement. In the present invention, the speed of rotation of a first member containing a pattern that has an angular variation is based on a property of a first target; and the speed of rotation of a second member containing a pattern that has an angular variation is based on a property of a second target. The patterns on the first and second members are superimposed to generate a Moiré pattern. 
     Therefore, only the difference between the property of the first target and the property of the second target generates change in the superimposed patterns thereby generating the Moiré pattern. If the property of the first target and the second target are the same, no change in the Moiré pattern will be detected. 
     It is useful to note that any change in the Moiré pattern depends on the magnitude of the difference between the property of the targets, and not the magnitude of the property of the targets. This gives rise to a higher accuracy. In essence, the “common mode signal” between the property of the first target and the property of the second target is eliminated. 
       FIG.  8    depicts a practical example of the above principle applied to an apparatus for detecting leaks in a lithographic apparatus. A first flow rotor  802 A is coupled to a first point in a channel of the fluid circulation system, e.g. at the inlet, and configured to rotate in accordance with the rate of flow at the first point. Desirably the first flow rotor  802 A is configured to rotate at a rate that is proportional to the mass flow rate of fluid at the first point. A second flow rotor  802 B is coupled to a second point in the channel of the fluid circulation system, e.g. at the outlet, and configured to rotate in accordance with the rate of flow at the second point. Desirably the second flow rotor  802 B is configured to rotate at a rate that is proportional to the mass flow rate of fluid at the second point. Desirably the relationship between rate of rotation of each of rotors  802 A,  802 B and flow at the respective points in the channel is the same. 
     It will be appreciated that the flow rotors do not have to be disposed at the inlet and outlet but can be disposed anywhere convenient in a fluid circulation system. Multiple pairs of flow rotors may be disposed within a fluid circulation system so as to allow the location of any leak to be determined. 
     Each of flow rotors  802 A,  802 B is coupled to a respective one of first and second transparent disks  801 A,  802 B. Transparent disks  801 A,  801 B are examples of rotating members. First pattern  400 A is provided on first transparent disk  801 A and second pattern  400 B is provided on second transparent disk  801 B. A light source  803  (e.g. a laser, LED or laser diode) emits a first beam  810  toward transparent disk, the light is modulated by the first pattern and propagates in a second beam  820  to the second transparent disk  801 B. The light is further modulated by the second pattern  400 B and propagates to a detector  804  as a third beam  830 . The combination of the modulation of the light by the first and second patterns  400 A,  400 B represents or encodes the Moiré pattern defined by the relative angular positions of the first and second transparent disks  801 A,  801 B. 
     Light source  803  may include focusing, collimating, directing or other beam forming components. Detector  804  may include light collecting elements and filters to reject stray light. 
     In operation, an intensity signal measured by the detector  804  will vary with time. The time variation will comprise two signal components: a first signal component determined by the angular frequency of the first pattern  400 A and the rate of rotation of second transparent disk  801 A and a second signal component determined by the angular frequency of the second pattern  400 B and the rate of rotation of second transparent disk  801 B. Since it is expected that any leak in the fluid circulation system is small compared to the flow rate through the system, the difference in frequency between the first and second signal components will be small, the sum of these components will provide a signal that is the combination of a fundamental signal and a beat signal. The beat signal represents the Moiré pattern and can be detected by various means such as: a frequency analyzer, a low-pass filter or a fast Fourier transform (FFT). 
     It will be appreciated that  FIG.  8    depicts a very simple embodiment of the invention and many variations thereon are possible. 
     For example, gearing can be provided between the flow rotors and the transparent disks to increase the rate of rotation of the transparent disks. Such gearing increases the frequency of the fundamental signal and beat signal which can make them easier to separate and increase accuracy of measurement of the beat signal and hence the leak rate. 
     It is desirable for accuracy that the two flow rotor and disk combinations are identical but if for any reason a change in one parameter of one half of the system is necessary a compensatory change can be made in another parameter of the same or the other half of the system. For example, space considerations might require that one of the rotors has to be made smaller so that it rotates with a different constant of proportionality to the mass flow rate in which case a compensatory gearing can be introduced in the other half of the system or the patterns can have a different angular frequency. 
     If it is not convenient to provide a beam path directly from source  803  to detector  804  through disks  801 A,  801 B, an optical system can be provided to conduct the beam as required. The optical system may comprise: optical fibers; free space optics such as lenses and mirrors; or any combination thereof. Optical fibers may be especially advantageous if the two points in the fluid circulation system are far apart. 
     The light beam can be conducted through the two transparent disks multiple times. Such an arrangement may be advantageous in increasing the variation in the intensity signal if the patterns have low contrast. 
     The relative sizes of the cross-section of the beam and the pattern components can be selected for various different effects. If the beam is small relative to the pattern components then the fundamental signal will have a large amplitude. This can be useful for noise rejection or to enable an absolute flow rate measurement to be obtained in addition to the differential flow rate. If the beam is large compared to the pattern components the amplitude of the fundamental signal will be reduced and can be made negligible. This can avoid the need to electronically separate the beat signal representing the Moiré pattern from the fundamental signal. 
     In an embodiment, the patterns  400 A,  400 B are each divided into two corresponding concentric annular regions. The angular frequency of the two components  401 ,  402  in a first annular region is different than the angular frequency of the two components in a second annular region. Desirably the angular frequencies of the two annular regions are mutually prime. The light beam is arranged to pass through both annular regions, e.g. by being large enough to overlap both annular regions. Alternatively two light beams, one passing through each annular region, can be used. The two light beams can be directed onto one sensor or have separate sensors whose outputs are combined electronically. This arrangement can address an issue that if the two transparent discs develop a constant offset whereby their patterns are exactly out of phase no light reaches the detector  804  which can make signal acquisition difficult and result in a low signal to noise ratio such that dark noise in the detector can lead to false positives. Since, in this arrangement the patterns in the two annular regions have different angular frequencies there will be no relative position of the two discs in which both annular regions are completely opaque. 
     In another embodiment, shown in  FIGS.  9 A and  9 B , the discs are replaced by drums  901 A,  901 B and the radial patterns are replaced by patterns  410 A,  410 B comprising dark bands  411  and light bands  412 . Drums  901 A,  901 B are examples of rotating members. Other parts of this embodiment are the same as those of the embodiment of  FIG.  8    and have like reference numerals. 
     Dark bands  411  and light bands  412  can be parallel to the axis of the respective drum or at an angle so as to form spirals. Like the regions  401 ,  402  the duty ratio of the dark and light bands is desirably 50% but other values are possible. 
     Various different configurations are possible with drums  901 A,  901 B. In the illustrated configuration the light bands of both drums are transparent and drum  901 B is located inside drum  901 A, e.g. concentrically. Light source  803  is outside drum  901 B and detector  804  is inside drum  901 B so that light passes through both drums. In another configuration, the light bands of drum  901 B are reflective and detector  804  is outside drum  901 A, e.g. adjacent light source  803 . In another configuration drums  901 A and  901 B are spaced apart rather than nested and the light bands on both drums are reflective. The light source is arranged to direct light onto the drums so that it reflects off both to reach the detector. The drums need not be cylindrical, for example conical drums can be convenient in some configurations, nor do both drums need to have the same shape. Other configurations are possible. 
     It will be appreciated that a “light region” or “light band” of a pattern is one which allows light to reach the detector whereas a “dark region” or “dark band” is one which prevents or reduces light reaching the detector. Depending on the configuration of the system and the nature of the member on which the pattern is provided, a light region might be transmissive or reflective. Similarly a dark region may be transmissive, reflective or absorbing. 
     It will also be appreciated that the variations described in relation to  FIG.  8    can be combined with each other in various ways and can also be applied to the arrangement of  FIGS.  9 A and  9 B . 
     As mentioned above, for leak detection it is desirable that the flow rotors measure mass flow rate. It is also possible to use flow rotors that measure volume flow rate. In that case, if there is a significant temperature difference between the fluid at the first point and the fluid at the second point and the fluid has a non-negligible coefficient of thermal expansion, it is possible to measure the temperatures of the fluid at the two points and apply a correction to the apparent leakage measured by the flow differential. 
     In addition to the detection of leaks, it will be appreciated that the present invention can be used to measure intentional differences between two flows. For example it might be useful to know the flow rate in a small branch off a main flow where it is difficult to measure the flow in the branch to be measured directly. The present invention can be used to measure the difference in flows in a main channel before and after a branch and thereby infer the flow in the branch. 
     Whilst the invention has been described in relation to measurement of flow rates of fluid it will be appreciated that the principle of the invention can be applied to measurement of any other property that can be converted to a rotatory motion. 
     Embodiments of the invention have been described in the context of a lithographic apparatus employing EUV radiation. It will be appreciated that the invention may also be applied in other type of lithography apparatus — such as DUV lithographic apparatus, e-beam lithographic apparatus or imprint lithographic apparatus — and to other tools used in semiconductor manufacture such as assessment tools (e.g. metrology tools, inspection tools and scanning electron microscopes). Lithographic apparatus and these other tools may be collectively referred to as lithographic tools and have the common characteristics that fluids, e.g. water, are circulated through the tool, e.g. for temperature control, and that leaks of fluid may lead to very costly damage and downtime. 
     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, such as 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. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains one or multiple processed layers. 
     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.