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    • 专利摘要:
    A beam measurement system, a lithographic system, and a method are disclosed. In one arrangement, the beam measurement system is for determining a property of one or more of a plasma, an image of a plasma, and a collector, of a laser produced plasma radiation source. The beam measurement system comprises at least one sensor nit configured to receive at least a portion of a radiation beam output from the collector. Each sensor unit comprises a first patterned element, a second patterned element, and a detector configured to detect radiation that has passed through the first patterned element and the second patterned element. The first patterned element and the second patterned element are each patterned with a spatially non-uniform transmittance and positioned relative to each other to provide a combined transmittance with a non-uniform angular dependence with respect to a direction of incidence of radiation on the sensor unit.
    公开号:NL2018110A
    申请号:NL2018110
    申请日:2017-01-02
    公开日:2017-07-24
    发明作者:Ni Yongfeng
    申请人:Asml Netherlands Bv；
    IPC主号:
    专利说明:

    A BEAM MEASUREMENT SYSTEM. A LITHOGRAPHIC SYSTEM. AND A METHOD
    FIELD
    [0001] The present invention relates to a beam measurement system, a lithographic system and a method. The invention relates particularly to determining performance or alignment of a radiation source.
    BACKGROUND
    [0002] 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 from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.
    [0003] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines, along with other factors such as the refractive index of material through which the radiation passes and the numerical aperture of the projection system, the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 5-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).
    [0004] When a plasma is used to generate the EUV radiation, lithographic performance may depend on the position, size or shape of an image of the plasma formed at an intermediate focus between a collector used to collect radiation emitted by the plasma and an illumination system which conditions a radiation beam output from the collector. Properties of the image of the plasma depend on the plasma itself and on the alignment between the collector and the illumination system. Prior art systems for measuring alignment between the collector and illumination system are relatively complex and may not be effective where large deviations in alignment are present. Additionally, where prior art systems depend on imaging of targets formed on the collector, contamination of the collector during use can reduce the reliability or accuracy of the measurements.
    SUMMARY
    [0005] It is an object of the invention to provide apparatus and methods for determining properties of the image of the plasma, the plasma, and/or alignment between a collector and an illumination system, more simply, more reliably and/or in a way which is effective even where large deviations in alignment are present.
    [0006] According to an aspect there is provided a beam measurement system for determining a property of one or more of a plasma, an image of a plasma, and a collector, of a laser produced plasma radiation source, the beam measurement system comprising: at least one sensor unit configured to receive at least a portion of a radiation beam output from the collector, each sensor unit comprising a first patterned element, a second patterned element, and a detector configured to detect radiation that has passed through the first patterned element and the second patterned element, the first patterned element and the second patterned element each being patterned with a spatially non-uniform transmittance and positioned relative to each other to provide a combined transmittance with a non-uniform angular dependence with respect to a direction of incidence of radiation on the sensor unit.
    [0007] Thus, a beam measurement system is provided which can accurately determine properties of the image of the plasma, the plasma, or the collector, using relatively simple components and analysis techniques. With respect to properties of the image of the plasma and the plasma, it is not necessary to rely on patterns formed on the collector. Contamination of the collector does not interfere with the measurements in this case. The approach is effective even where relatively large deviations in alignment are present.
    [0008] In an embodiment, the beam measurement system comprises a group of the sensor units, each sensor unit in the group having a first patterned element and a second patterned element having a combined transmittance with an angular dependence, and the angular dependencies for the group of sensor units are different from each other.
    [0009] In an embodiment, the different angular dependencies are obtained by providing different relative positioning between otherwise identical patterns in the first patterned element and the second patterned element in each sensor unit.
    [00010] In an embodiment, the beam measurement system comprises an array comprising a plurality of the groups of the sensor units. In an embodiment, the beam measurement system comprises a plurality of the arrays, each array being positioned to receive a different portion of the radiation beam.
    [00011] In an embodiment, each of the first patterned element and the second patterned element is patterned with a periodic arrangement of regions of high transmittance separated by regions of lower transmittance.
    [00012] In an embodiment, the first patterned element and the second patterned element are substantially planar and the first patterned element and the second patterned element are separated from each other in a direction perpendicular to the plane of the first patterned element.
    [00013] In an embodiment, the determined property of one or more of the plasma, the image of the plasma, and the collector comprises at least one of a shape of the image of the plasma at an intermediate focus, a size of the image of the plasma at the intermediate focus, and a position of the image of the plasma at the intermediate focus, the intermediate focus being a focus formed by the collector between the collector and an illumination system configured to condition the radiation beam. Optionally, one or more of the at least one sensor unit is/are positioned at far field relative to the intermediate focus.
    [00014] In an embodiment, the determined property of one or more of the plasma, the image of the plasma, and the collector comprises at least one of a shape of the plasma, a size of the plasma, and a position of the plasma.
    [00015] In an embodiment, the collector comprises a patterned region and the beam measurement system comprises at least one sensor unit positioned to receive radiation modulated by the patterned region, wherein a proportion of the modulated radiation detected by the detector is dependent on at least one of a position of the collector relative to an illumination system configured to condition the radiation beam and an orientation of the collector relative to the illumination system. Optionally, the patterned region comprises a portion of a plurality of concentric rings.
    [00016] In an embodiment, the first patterned element is patterned with a periodic arrangement, having a first pitch, of regions of high transmittance separated by regions of lower transmittance. The second patterned element is patterned with a periodic arrangement, having a second pitch different to or the same as the first pitch, of regions of high transmittance separated by regions of lower transmittance. The received radiation modulated by the patterned region of the collector is periodic and has a pitch which differs from either or both of the first pitch and the second pitch.
    [00017] In an embodiment the beam measurement system further comprises a sensor unit mounting system configured to allow at least one of the sensor units to be moved so as to selectively receive radiation modulated by one of a plurality of different patterned regions of the collector.
    [00018] In an embodiment, the determined property of one or more of the plasma, the image of the plasma, and the collector comprises at least one of a position of the collector relative to the illumination system and an orientation of the collector relative to the illumination system.
    [00019] In an embodiment, the patterning of the first patterned element and the second patterned element are such that diffraction effects are negligible. Optionally, a smallest characteristic dimension of the patterning in the first patterned element and a smallest characteristic dimension of the patterning in the second patterned element are at least 10 times larger than a wavelength of radiation produced by the laser produced plasma radiation source.
    [00020] In an embodiment, the measurement system further comprises a control device configured to control the laser produced plasma radiation source based on an output from the at least one sensor unit.
    [00021] According to an aspect, there is provided a lithographic system, comprising: (a) a radiation source configured to collect radiation emitted from a plasma using a collector and output a radiation beam from the collector; and (b) a beam measurement system configured to determine a property of one or more of the plasma, an image of the plasma, and the collector by measuring a property of the radiation beam, the beam measurement system comprising at least one sensor unit configured to receive at least a portion of the radiation beam, each sensor unit comprising a first patterned element, a second patterned element, and a detector configured to detect radiation that has passed through the first patterned element and the second patterned element, wherein in each sensor unit the first patterned element and the second patterned element are each patterned with a spatially non-uniform transmittance and positioned relative to each other to provide a combined transmittance with a non-uniform angular dependence with respect to a direction of incidence of radiation on the sensor unit.
    [00022] According to an aspect, there is provided a method comprising determining a property of one or more of a plasma, an image of a plasma, and a collector in a laser produced plasma radiation source, by measuring a property of a radiation beam output by the laser produced plasma radiation source, wherein the measuring of the property of the radiation beam comprises using at least one sensor unit to receive at least a portion of the radiation beam, each sensor unit comprising a first patterned element, a second patterned element, and a detector configured to detect radiation that has passed through the first patterned element and the second patterned element, the first patterned element and the second patterned element each being patterned with a spatially non-uniform transmittance and positioned relative to each other to provide a combined transmittance with a non-uniform angular dependence with respect to a direction of incidence of radiation on the sensor unit.
    BRIEF DESCRIPTION OF THE DRAWINGS
    [00023] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:
    Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source according to an embodiment of the invention;
    Figure 2 depicts a radiation source according to an embodiment of the invention; Figure 3 depicts example rays incident on a sensor unit;
    Figure 4 depicts example rays incident on a sensor unit of the type shown in Figure 3 with a different relative position between first and second patterned elements;
    Figure 5 depicts an angular dependence of a combined transmittance through first and second patterned elements in a sensor unit;
    Figure 6 depicts an angular dependence of combined transmittance through first and second patterned elements in a sensor unit according to an alternative embodiment;
    Figure 7 depicts an array of groups of sensor units according to an embodiment (upper figure) and one of the groups in further detail (lower figure);
    Figure 8 is a schematic side view showing capture of radiation output from a collector on an array of groups of sensor units;
    Figure 9 depicts four arrays of groups of sensor elements mounted on a sensor unit mounting system;
    Figure 10 depicts an array of groups of sensor units according to an alternative embodiment (upper figure) and one of the groups in further detail (lower figure);
    Figure 11 depicts relative alignment between a patterned region on a collector mapped onto far field and first and second patterned elements of a 3x3 grid of nine sensor units;
    Figure 12 depicts the arrangement of Figure 11 after a shift in a position of the patterned region mapped onto far field caused by a change in position and/or alignment of the collector; and
    Figure 13 depicts an example pattern on a collector comprising a plurality of concentric rings.
    DETAILED DESCRIPTION
    [00024] Figure 1 shows a lithographic system including a radiation system with beam measurement system 30 according to one embodiment of the invention. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.
    [00025] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.
    [00026] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source). A laser 1, which may for example be a C02 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may for example be in liquid form, and may for example be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma.
    [00027] The EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to more generally as a normal incidence radiation collector). The collector 5 may have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm or 6.4-7.2 nm). The collector 5 may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.
    [00028] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser 1 and the radiation source SO may together be considered to be a radiation system.
    [00029] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source.
    [00030] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 11.
    [00031] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).
    [00032] Figure 2 shows a laser produced plasma (LPP) radiation source SO which has an alternative configuration to the radiation source shown in Figure 1. The radiation source SO includes a fuel emitter 3 which is configured to deliver fuel to a plasma formation region 4. The fuel may for example be tin, although any suitable fuel may be used. A pre-pulse laser 16 emits a pre-pulse laser beam 17 which is incident upon the fuel. The pre-pulse laser beam 17 acts to preheat the fuel, thereby changing a property of the fuel such as its size and/or shape. A main laser 18 emits a main laser beam 19 which is incident upon the fuel after the pre-pulse laser beam 17. The main laser beam delivers energy to the fuel and thereby coverts the fuel into an EUV radiation emitting plasma 7. The mechanism of operation so far may also be applied to the radiation source SO described above with reference to Figure 1. The radiation collector 20 of Figure 2, however, differs from the radiation collector 5 of Figure 1, as described below.
    [00033] A radiation collector 20, which may be a so-called grazing incidence collector, is configured to collect the EUV radiation and focus the EUV radiation at a point 6 which may be referred to as the intermediate focus. Thus, an image of the radiation emitting plasma 7 is formed at the intermediate focus 6. An enclosure structure 21 of the radiation source SO includes an opening 22 which is at or near to the intermediate focus 6. The EUV radiation passes through the opening 22 to an illumination system of a lithographic apparatus (e.g. of the form shown schematically in Figure 1).
    [00034] The radiation collector 20 may be a nested collector, with a plurality of grazing incidence reflectors 23, 24 and 25 (e.g. as schematically depicted). The grazing incidence reflectors 23, 24 and 25 may be disposed axially symmetrically around an optical axis O.
    The illustrated radiation collector 20 is shown merely as an example, and other radiation collectors may be used.
    [00035] A contamination trap 26 is located between the plasma formation region 4 and the radiation collector 20. The contamination trap 26 may for example be a rotating foil trap, or may be any other suitable form of contamination trap. In some embodiments the contamination trap 26 may be omitted.
    [00036] An enclosure 21 of the radiation source SO includes a window 27 through which the pre-pulse laser beam 17 can pass to the plasma formation region 4, and a window 28 through which the main laser beam 19 can pass to the plasma formation region. A mirror 29 is used to direct the main laser beam 19 through an opening in the contamination trap 26 to the plasma formation region 4.
    [00037] The radiation sources SO shown in Figures 1 and 2 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.
    [00038] Operation of the lithographic system depends on properties of the image of the plasma 7 formed at the intermediate focus 6 (e.g. one or more of the shape of the image, the size of the image, and the position of the image). The image of the plasma depends in turn on the relative alignment between the collector 5,20 and the illumination system IL, and properties of the plasma 7 itself, including the shape of the plasma 7, the size of the plasma 7, and the position of the plasma 7. Prior art methods for measuring the alignment of the collector 5,20 work reasonably well for small deviations in alignment. Larger deviations may occur, however, particularly during initial setting up of the lithographic system. Larger deviations can cause prior art systems to be driven into a working range where accuracy is lowered, for example into a non-linear working range. Larger deviations may also cause the image of the plasma 7 to be clipped by the opening 8, 22 of the radiation source SO. This clipping can further lower the accuracy of prior art measurements. Furthermore, prior art measurements may depend on imaging of patterns on the collector SO. As the collector SO becomes contaminated during use, the signal strength may be lowered or the radiation distribution from the patterns may be changed, leading to loss of accuracy or failure of the measurement.
    [00039] In an embodiment, as shown for example in Figure 1 and described below with reference to Figures 3-13, there is provided a beam measurement system 30. The radiation source SO collects radiation emitted from a plasma 7 using a collector 5,20. The collector 5,20 outputs the collected radiation as a radiation beam B. The beam measurement system 30 determines a property of one or more of the plasma 7, an image of the plasma 7, and the collector 5,20, by measuring a property of the radiation beam B.
    [00040] The beam measurement system 30 comprises at least one sensor unit 32. The sensor unit 32 receives at least a portion of the radiation beam B. Each sensor unit 32 comprises a first patterned element 34, a second patterned element 36, and a detector 38. The detector 38 detects radiation that has passed through the first patterned element 34 and the second patterned element 36. The first patterned element 34, second patterned element 36 and detector 38 may therefore form a stack. The first patterned element 34 and the second patterned element 36 are each patterned with a spatially non-uniform transmittance. The first patterned element 34 and the second patterned element 36 are positioned relative to each other to provide a combined transmittance which varies as a function of the angle of incidence of radiation on the sensor unit 32.
    [00041] In an embodiment the patterning of the first patterned element 34 comprises a pattern providing a non-uniform spatial variation in transmittance within a plane of the first patterned element 34 (i.e. when viewed perpendicular to the plane). In an embodiment the patterning of the second patterned element 36 comprises a pattern providing a non-uniform spatial variation in transmittance within a plane of the second patterned element 36 (i.e. when viewed perpendicular to the plane). Example patterns are shown schematically in Figures 3 and 4.
    [00042] In an embodiment, each of the first patterned element 34 and the second patterned element 36 is patterned with a periodic arrangement of regions 62 of high transmittance separated by regions 64 of lower transmittance, for example in the form of a grating.
    [00043] In an embodiment a first set of regions 62 and a second set of regions 64 are provided. Each of the first set of regions 62 has a first transmittance with respect to EUV radiation. Each of the second set of regions 64 has a second transmittance with respect to EUV radiation. The first transmittance is higher than the second transmittance. In an embodiment the first set of regions 62 are substantially transparent to EUV radiation (e.g. having a transmittance greater than 80%). In an embodiment the second set of regions 64 substantially block EUV radiation (e.g. having a transmittance less than 20%). In an embodiment the first set of regions 62 all have substantially equal transmittance (e.g. within 5%). In an embodiment the second set of regions 64 all have substantially equal transmittance (e.g. within 5%).
    [00044] In an embodiment, the first set of regions 62 comprises a plurality of elongate regions when viewed perpendicular to the plane of the first patterned element 34. The plurality of elongate regions may comprise a plurality of parallel elongate regions. The plurality of parallel elongate regions may comprise a plurality of straight parallel elongate regions. In an embodiment, the second set of regions 64 comprises a plurality of elongate regions when viewed perpendicular to the plane of the second patterned element 36. The plurality of elongate regions may comprise a plurality of parallel regions. The plurality of parallel elongate regions may comprise a plurality of straight parallel elongate regions.
    [00045] In an embodiment, the patterning of the first patterned element 34 and the second patterned element 34 are such that diffraction effects are negligible. Diffraction effects are negligible when the angular dependence of the combined transmittance through the first patterned element 34 and the second patterned element 36 is dominated by geometrical effects rather than diffraction effects. In an embodiment, a smallest characteristic dimension of the patterning in the first patterned element 34 is at least 10 times larger than a wavelength of radiation produced by the laser produced plasma radiation source (e.g. EUV radiation), optionally at least 25 times larger, optionally at least 50 times larger. In an embodiment, a smallest characteristic dimension of the patterning in the second patterned element 36 is at least 10 times larger than a wavelength of radiation produced by the laser produced plasma radiation source, optionally at least 25 times larger, optionally at least 50 times larger. A smallest characteristic dimension in this context is understood to mean a smallest dimension of the patterning that is relevant to transmission of radiation through the patterning. A smallest characteristic dimension may include for example a smallest spacing between regions 64 of lower transmittance. Where the patterning is periodic the smallest characteristic dimension may comprise a period or pitch 40,41 of the patterning. Arranging for the smallest characteristic dimension to be much larger than the wavelength of radiation ensures that diffraction effects are very small. In an embodiment the smallest characteristic dimension is between 0.5 microns and 5 microns, optionally about 1 micron.
    [00046] In an embodiment, the first set of regions 62 are irregularly spaced apart from each other. In such an embodiment the first set of regions 62 are optionally spaced apart by distances which are much larger than the wavelength of the radiation (e.g. EUV), optionally at least 10 times larger, optionally at least 25 times larger, optionally at least 50 times larger. In another embodiment, the first set of regions 62 are equally spaced apart from each other with a pitch 40,41. In such an embodiment the pitch 40,41 is optionally arranged to be much larger than the wavelength of radiation (e.g. EUV), optionally at least 10 times larger, optionally at least 25 times larger, optionally at least 50 times larger. In an embodiment the pitch 40,41 is between 0.5 microns and 5 microns, optionally about 1 micron.
    [00047] In an embodiment, the first set of regions 62 in the first patterned element 34 are spaced apart with a first pitch 40 and the first set of regions 62 in the second patterned element 36 are spaced apart with a second pitch 41. The first pitch 40 may be the same as the second pitch 41 (as in the examples of Figures 3 and 4) or different from the second pitch 41. Providing patterns with different pitch allows for increased flexibility in the angular variation of the combined transmittance of the first patterned element 34 and the second patterned element 36.
    [00048] In the examples of Figures 3 and 4, the first set of regions 62 comprises a plurality of straight parallel elongate regions orientated perpendicularly to the plane of the page (i.e. into the page) and spaced apart from each other with pitch 40,41 · The second set of regions 64 comprises a corresponding plurality of straight parallel elongate regions interposed between the first set of regions 62 to form a grating structure.
    [00049] In an embodiment the first patterned element 34 and the second patterned element 36 are substantially planar. In an embodiment the first patterned element 34 and the second patterned element 36 are separated from each other in a direction perpendicular to the plane of the first patterned element 34 by a distance 42. In an embodiment the distance 42 is at least 50 microns, optionally at least 100 microns, optionally at least 500 microns, optionally at least 1mm. In any of these embodiments the distance 42 may be less than 10mm, optionally less than 5mm, optionally less than 3mm.
    [00050] The pitch 40,41 and the distance 42 are arranged so that the second set of regions 64 intersect incident rays 51-54 in different ways depending on the angle of incidence 72-74 (see Figure 3). This geometrical effect provides the desired angular variation in the combined transmittance.
    [00051] In Figure 3 example ray 51 is incident on the sensor unit 32 at 0 radians relative to normal incidence. At this angle of incidence the ray 51 can pass maximally through one or more regions in the first set of regions 62 (i.e. regions of relatively high transmittance) of both of the first patterned element 34 and the second patterned element 36. Example ray 54 is incident on the sensor unit 32 at an oblique angle of incidence 74 but can also pass maximally through one or more regions in the first set of regions 62 of both of the first patterned element 34 and the second patterned element 36. The combined transmittance (of the first patterned element 34 and the second patterned element 36) is therefore similar and maximal at 0 radians and at angle 74. At intermediate angles the combined transmittance is lower because rays cannot reach the detector 38 without at least partially encountering one or more regions of the second set of regions 64 (i.e. regions of relatively low transmittance). Example ray 52 is incident on the sensor unit 32 at an oblique angle 72 and partially encounters a region of the second set of regions 64 in the second patterned element 36. Example ray 53 is incident on the sensor unit 32 at an oblique angle 73 and encounters a region of the second set of regions 64 in the second patterned element 36 more directly than the example ray 52. The combined transmittance with respect to rays incident at angle 72 is therefore higher than at angle 73, but lower than at 0 radians and at angle 74.
    [00052] In Figure 4 the first patterned element 34 and the second patterned element 36 have the same patterns as in the arrangement of Figure 3 but are shifted relative to each other in a direction parallel to the plane of the first patterned element 34 and perpendicular to the elongate regions 62 and 64 (the second patterned element 36 is shifted upwards relative to the first patterned element 34 in the orientation shown in Figure 4). The shift in relative position causes a corresponding shift in the angular dependence of the combined transmittance of the first patterned element 34 and second patterned element 36. In the arrangement of Figure 4 the combined transmittance with respect to example rays 51 and 54 is now minimal (similar to the combined transmittance with respect to example ray 53 in Figure 3). The combined transmittance with respect to ray 53 is maximal (similar to the combined transmittance with respect to rays 51 and 54 in Figure 3). The combined transmittance with respect to ray 52 is intermediate between the combined transmittance with respect to rays 51 and 54 and the combined transmittance with respect to ray 53.
    [00053] In arrangements of the type shown in Figures 3 and 4 the combined transmittance through the first patterned element 34 and the second patterned element 36 will vary continuously as a function of angle of incidence. In this embodiment the continuous variation is periodic. A periodic variation may be convenient because it provides repeating features (e.g. maxima and minima) which have a simple angular positional relationship relative to each other (the angular separation is constant). Such repeating features may facilitate the interpretation of changes in output from individual sensor units 32 and/or differences in output from different sensor units 32. In other embodiments a continuous variation is provided which is not periodic. Such a continuous variation may or may not contain repeating features (e.g. maxima or minima). Where repeating features are present and the angular positions of the repeating features are known, the repeating features may facilitate the interpretation of changes in output from individual sensor units 32 and/or differences in output from different sensor units 32.
    [00054] In the example of Figures 3 and 4 the combined transmittance has peaks corresponding to the directions of rays such as example rays 51 and 54 in Figure 3 and ray 53 in Figure 4. The combined transmittance has troughs corresponding to the directions of rays such as example ray 53 in Figure 3 and example rays 51 and 54 in Figure 4. The periodicity will depend on the ratio of the distance 42 to the pitch 40,41. Increasing the ratio will decrease the period (the angular distance between adjacent peaks).
    [00055] Figures 5 and 6 show two illustrative examples. The vertical axis in each figure is combined transmittance through the first patterned element 34 and second patterned element 36. The horizontal axis in each figure is incidence angle relative to normal incidence in mrad. In Figure 6 the ratio of distance 42 to pitch 40,41 is 2000 (achievable for example with distance 42 = 2mm and pitch 40,41 = 1 micron). In Figure 7 the ratio of distance 42 to pitch 40,41 is 1000 (achievable for example with distance 42 = 1 mm and pitch 40,41 = 1 micron). The solid curves show the variation of combined transmittance with incidence angle in the case where the pattern of the first patterned element 34 is aligned with the pattern of the second patterned element 36 (as in Figure 3 for example). The broken line curves show the variation of combined transmittance with incidence angle in the case where the pattern of the first patterned element 34 is shifted relative to the pattern of the second patterned element 36 by half a pitch 40,41 (as in Figure 4 for example).
    [00056] Figure 5 shows separation between peaks of 0.5 mrad. In the case where the sensor unit 32 is being used to measure the position of an image of the plasma 7 at an intermediate focus located 1.5m from the sensor unit 32, a change in incidence angle of 0.5 mrad would correspond to a shift in the position of the image of the plasma 7 of 750 microns. Figure 6 shows separation between peaks of 1.0 mrad, which would correspond to a shift in the position of the image of the plasma 7 of 1,5mm. The distance 42 and/or pitch 40,41 can be adjusted as desired so that the angular variation provides appropriate sensitivity for the particular property being measured.
    [00057] In an embodiment, the beam measurement system 30 comprises a group (comprising a plurality) of the sensor units 32. Each sensor unit 32 in the group has a first patterned element 34 and a second patterned element 36 having a combined transmittance with an angular dependence. The angular dependencies for the group of sensor units 32 are different from each other. Thus, for radiation incident on multiple sensor units 32 in the group from a common direction, a corresponding multiple of different output levels is expected from the corresponding multiple detectors 38. By comparing the output levels it is possible accurately to deduce the direction of incidence of the radiation beam, even where the overall intensity of the radiation incident on the detectors varies (for example where the radiation is derived from a pattern formed on the collector 5,20 and the signal level is affected by contamination of the collector 5,20).
    [00058] In an embodiment the different angular dependencies are obtained by providing different relative positioning between otherwise identical patterns in the first patterned element 34 and the second patterned element 36 in each sensor unit 32. Figures 3 and 4 show example arrangements of this type with the relative positions of the first patterned element 34 and the second patterned element 36 being shifted by half a pitch in the arrangement of Figure 4 relative to the arrangement of Figure 3. In an embodiment, a set of n sensor units 32 are provided with the relative positions of the first patterned element 34 and the second patterned element 36 being shifted progressively by 1/n times the pitch from each other. For example, in the case where four sensor units 32 are provided, the second sensor unit 32 is shifted by 1/4 times the pitch relative to the first sensor unit 32, the third sensor unit 32 is shifted by 1/2 times the pitch relative to the first sensor unit 32, and the fourth sensor unit 32 is shifted by 3/4 times the pitch relative to the first sensor unit 32. Many other arrangements are possible.
    [00059] In an embodiment, the beam measurement system comprises an array 37 comprising a plurality of the groups of the sensor units 32.
    [00060] Figure 7 shows an arrangement in which the beam measurement system 30 comprises a 2D array 37 (in this particular example an 8x8 array) of groups 35 of sensors units 32. In this example, each group 35 comprises a first set of sensor units 32 which are oriented so as to be sensitive to an angle of incidence of radiation varying within a first plane (the angle being obtained for example by obtaining the angle of incidence of a component parallel to the first plane of a vector representing the radiation) and a second set of sensor units 32 which are orientated so as to be sensitive to an angle of incidence of radiation varying within a second plane (the angle being obtained for example by obtaining the angle of incidence of a component parallel to the second plane of the vector representing the radiation). The second plane is non-parallel to the first plane. In an embodiment, the second plane is perpendicular to the first plane. In the example of Figure 7, the first plane is horizontal and perpendicular to the plane of the page and the first set of sensor units 32 is the uppermost set of four sensor units 32 shown in the expanded example group 35 shown in the lower part of Figure 7. The second plane is vertical and perpendicular to the plane of the page and the second set of sensor units 32 is the lowermost set of four sensor units 32 shown in the expanded example group 35 shown in the lower part of the Figure 7. Signals output from the detectors 38 from the sensor units 32 of each group 35 allow the direction of incidence of radiation on that group 35 to be determined accurately and reliably. Providing an array 37 of groups 35 allows spatial variations in the direction of incidence of radiation to be detected, thereby providing the possibility to measure properties of an incident radiation beam in detail. Where the incident radiation beam depends on a property of the image of the plasma 7, the plasma 7, or the collector 5,20, the property of the image of the plasma 7, the plasma 7, or collector 5,20, may also therefore be derived accurately and reliably.
    [00061] In an embodiment, the beam measurement system 30 comprises a plurality of the arrays 37, each array 37 being positioned to receive a different portion of the radiation beam. Example embodiments comprising such plural arrays 37 are described below with reference to Figures 8-10.
    [00062] In an embodiment, the property of one or more of the plasma 7, the image of the plasma 7, and the collector 5 being determined by the beam measurement system 30 comprises at least one of the shape, size, and position of the image of the plasma 7 at the intermediate focus 6. Alternatively or additionally, in an embodiment the property of one or more of the plasma 7, the image of the plasma 7, and the collector 5 being determined by the beam measurement system 30 comprises at least one of the shape, size, and position of the plasma 7 itself.
    [00063] Figure 8 is a schematic side view illustrating how the measurement system 30 may be positioned relative to an example radiation system. In this example the plasma 7 emits EUV radiation which is collected by collector 5. EUV radiation is emitted from different positions over a three-dimensional volume occupied by the plasma 7. The volume occupied by the plasma 7 is indicated by a circular black region in Figure 8, but the volume need not necessarily be spherical. The collector 5 forms an intermediate focus 6 in or near an opening 8 in an enclosing structure 9 of a radiation source SO. The beam measurement system 30 is arranged so that the sensor units 32 receive at least a portion of the radiation beam at a far field position (i.e. where the radiation beam has a substantially plane-wave form) relative to the intermediate focus 6. In this embodiment arrays 37 of sensor units 32 are provided to allow a shape of the image of the plasma 7 at intermediate focus to be determined. Each array 37 samples a portion of the radiation beam, thereby providing information about a corresponding portion of the image of the plasma 7. With multiple arrays 37 it is possible to determine the shape of a large portion of the image of the plasma 7 or the shape of the whole of the image of the plasma 7. Each of the arrays 37 may be as shown in Figure 7 for example or may take other forms. It is not essential for the sensor units 32 to be provided at far field relative to the intermediate focus 6. One or more of the sensor units 32 may be provided closer to the intermediate focus. The cross-section of the radiation beam is smaller at positions nearer to the intermediate focus 6 relative to far field positions. A sensor unit 32 of a particular size may therefore sample a larger proportion of the radiation beam at positions nearer to the intermediate focus 6. It may be desirable in this situation for the sensor units 32 to occupy regions of the radiation beam which contribute to the patterned radiation beam projected onto the substrate W. In this case the measurement system 30 may be configured so that the sensor units 32 are not located permanently within the radiation beam. One or more of the sensor units 32 can be configured to be located in the radiation beam only while the determination of the property of one or more of the plasma 7, the image of the plasma 7, and the collector 5,20 is being performed. In an embodiment, the light intensity at the intermediate focus is reduced while one or more of the sensor units 32 are in the radiation beam at positions close to the intermediate focus 6 to avoid damage to those sensor units 32. Alternatively or additionally, a filter may be provided over the one or more sensor units 32 to reduce the intensity of radiation reaching the sensor units 32. In an embodiment one or more arrays 37 of sensor units 32 are provided in a single unit which covers the majority or all of the radiation beam. This allows the beam measurement system 30 to obtain detailed information about the image of the plasma 7, including for example full anisotropy of the image of the plasma 7, using the single unit. The size of the single unit will depend on the position relative to the intermediate focus 6. In one particular embodiment where the opening 8 in the enclosing structure 9 of the radiation source, where the image of the plasma 7 is located, is about 6.5mm in diameter, the single unit comprising the one or more arrays 37 may be of the order of 10mm to 20mm in diameter.
    [00064] In an embodiment one or more of the sensor units 32 are connected to and/or positioned directly adjacent to a facetted field mirror device 10 of an illumination system IL. Relative to an axis representing an average direction of incidence of radiation onto the facetted field mirror device 10, the one or more sensor units 32 may be positioned radially within a region where radiation is received by the facetted field mirror device 10 (where there is room), radially outside of the region, or both. Alternatively or additionally, one or more of the sensor units 32 may be connected to and/or positioned directly adjacent to other elements of the illumination system IL, such as a faceted pupil mirror device 11. Relative to an axis representing an average direction of incidence of radiation onto the facetted pupil mirror device 11, the one or more sensor units 32 may be positioned radially within a region where radiation is received by the facetted pupil mirror device 11 (where there is room), radially outside of the region, or both. It is beneficial for the sensor units 32 to sample as much as possible of the radiation beam to obtain the most information about the plasma, plasma image or collector, subject to not interfering with the functionality of elements of the illumination system (e.g. facetted field mirror device 10 or facetted pupil mirror device 11).
    [00065] In an alternative embodiment the plurality of sensor units 32 are provided in plural arrays 37 and all of the sensor units 32 have the same orientation in each of the plural arrays 37. An example of this type is shown in Figures 9 and 10. In this example, the beam measurement system 30 comprises four arrays 37, but fewer than four arrays 37 or more than four arrays 37 could alternatively be provided. Each of the arrays 37 comprises plural groups 35 of sensor units 32. All of the sensor units 32 in each group 35 and in the array 37 have the same orientation. Sensor units 32 within a given group 35 differ from each other by having different relative shifts between the first patterned element 34 and the second patterned element 36. In the orientation shown in Figure 10, it can be seen from the expanded example group 35 shown in the lower part of Figure 10 that the orientation of the sensor units 32 is such as to be sensitive to an angle of incidence of radiation varying within a plane that is vertical and perpendicular to the page. In this example each of the four arrays 37 are aligned as shown in Figure 9 so as to be parallel to a circumferential direction of a circular path and located at different positions on the circumference. The orientations of sensor units 32 in nearest neighbour arrays 37 are aligned perpendicularly relative to each other in this particular example. When taken in combination the outputs from all of the arrays 37 allows the direction of incidence of a radiation beam to be determined in three dimensions.
    [00066] In an embodiment the determined property of one or more of the plasma 7, the image of the plasma 7, and the collector 5,20, comprises at least one of a position of the collector 5,20 relative to the illumination system IL and an orientation of the collector 5,20 relative to the illumination system IL.
    [00067] In an embodiment a control device 110 is provided that controls the radiation source SO based on the property of one or more of the plasma 7, the image of the plasma 7, and the collector 5,20, determined by the beam measurement system 30. For example, the control device 110 may modify operation of the radiation source to change a property of one or more of the plasma 7, the image of the plasma 7, and the collector 5,20 or to compensate for a deviation in the property of one or more of the plasma 7, the image of the plasma 7, and the collector 5,20 from a target state, in response to an output from the beam measurement system 30.
    [00068] In an embodiment the collector 5,20 comprises a patterned region 94. Examples of patterned regions 94 are shown in Figure 13, discussed below. Where the collector is a normal incidence collector 5, as shown in Figure 1 for example, the patterned region 94 may be formed on an existing surface of the collector 5 for example (without any additional element being provided solely to support the patterned region 94). Where the collector is a grazing incidence collector 20, as shown in Figure 2 for example, the patterned region 94 may be formed as an additional element, mounted for example at an exit of the collector 20. The beam measurement system 30 comprises at least one sensor unit 32 positioned to receive radiation modulated by the patterned region 94. In such an embodiment a proportion of the modulated radiation detected by the detector 38 of the sensor unit 32 is dependent on at least one of the position and orientation of the collector 5,20 relative to the illumination system IL. The patterned region 94 may take any form. In an embodiment the patterned region 94 comprises a plurality of elongate elements forming a grating. In an embodiment the patterned region 94 comprises a portion of a plurality of concentric rings.
    [00069] Figure 13 depicts an example pattern 96 on the collector 5,20 comprising a plurality of concentric rings. The diameter of the rings is not particularly limited. In an embodiment, the diameter is in the range of 400mm to 800mm, optionally in the range of 550mm to 650mm. Example patterned regions 94 are shown surrounded by broken lines. The patterned regions 94 of Figure 13 could be respectively imaged for example onto arrays 37 of sensor units 32 such as those shown in Figure 9. The patterned regions 94 may be small enough that the parallel lines of the concentric rings are approximately straight, such that parallel lines in each patterned region 94 resemble a grating. A shift in the orientation of the collector 5,20 relative to the illumination system IL will cause a shift in a far field image of each patterned region 94 on an array 37 of sensor units 32. The shift in the far field image will cause a corresponding change in the amount of light reaching the detector 38 of each sensor unit 32. The outputs from sensor units 32 of the arrays therefore provide a measure of the shift in the orientation of the collector 5,20.
    [00070] The principle of operation is illustrated schematically in Figures 11 and 12. In each of these figures, the uppermost series of elongate regions 102 represent a mapping of a patterned region 94 onto a 3x3 array 37 of nine sensor units 32. The elongate regions 102 represent regions of low reflectivity on the collector 5,20 and therefore correspond to regions of low radiation beam intensity on the sensor units 32 at far field. The low transmittance regions 64 of the first patterned element 34 of each sensor unit 32 are shown as open rectangles. The low transmittance regions 64 of the second patterned element 36 of each sensor unit 32 are shown as cross-hatched rectangles. For clarity the 3x3 array 37 of sensor units 32 is shown underneath the series of elongate regions 102 but would in reality be positioned in an overlapping position. Thus, the signal output from each sensor unit 32 will depend on the proportion of the regions 104 in between the elongate regions 102 (three representative examples of which are shown by arrows in Figures 11 and 12) that overlap with regions outside of the low transmittance regions 64 of both of the first and second patterned elements 34,36.
    [00071] Thus, in the example of Figure 11, it can be seen that the output of the central sensor unit 32 will be a maximum, with the output of all other sensor units 32 taking lower values. In this embodiment this represents a state where the collector 5,20 is aligned as desired.
    [00072] In Figure 12, by contrast, the collector 5,20 alignment has shifted and a maximum output now occurs in the sensor unit 32 at the top left, with the output of all other sensor units 32, including the central sensor unit 32, taking lower values. In the top left sensor unit 32 it can be seen that the regions 104 do not overlap with any low transmittance region 64 in that sensor unit 32. In all other sensor units 32 there is at least partial overlap between the regions 104 and the low transmittance regions 64. In the sensor unit 32 at bottom right, for example, it can be seen that the regions 104 overlap completely with the low transmittance region 64 of the first patterned element 34 and the low transmittance region 64 of the second patterned element 36, thereby providing a minimum output from that sensor unit 32.
    [00073] In arrangements of this type, the collector 5,20 can be aligned quickly and reliably by adjusting the collector 5,20 until output from the central sensor unit 32 in each of the arrays 37 of sensor units 32 takes a maximal value (e.g. relative to the outputs from the other sensor units 32).
    [00074] In the embodiment described above with reference to Figure 11 and 12, the patterned region 94 comprises a periodic arrangement of elongate regions 102. The pitch 95 of the elongate regions 102, when mapped onto the sensor units 32, is the same as the pitch 40 of the periodic arrangement of low transmittance regions 64 in the first patterned element 34 and the pitch 41 of the periodic arrangement of low transmittance regions 64 in the second patterned element 36. This is not essential. In other embodiments, the pitch 95 may be different to either or both of the pitch 40 and the pitch 41. Arranging for the pitch 95 to be different to either or both of the pitch 40 and the pitch 41 can increase the sensitivity with which rotation of the patterned region 94 can be detected due to the formation of moiré fringes. Relative to the case where the pitch 95 is the same as the pitches 40 and 41, the rate at which an angle of inclination of the moiré fringes changes as a function of rotation of the patterned region 94 in increased. Detection of the change in angle of the moiré fringes can therefore provide a sensitive measure of the change in rotational position of the patterned region 94, and therefore of the collector 5,20.
    [00075] In an embodiment, the beam measurement system 30 further comprises a sensor unit mounting system 90 that allows at least one of the sensor units 32 to be moved so as to selectively receive modulated radiation from one of a plurality of different patterned regions 94 on the collector 5,20. An example is illustrated in Figure 9 where the sensor unit mounting system 90 is capable of moving the arrays 37 along a circular curved path 92. The sensor units 32 can therefore be moved to a different position in the event that contamination on the collector 5,20 compromises measurements based on patterned regions 94 that are aligned with a current position of the arrays 37.
    [00076] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.
    [00077] The term “EUV radiation” may be considered to encompass electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.
    [00078] Although Figures 1 and 2 depict the radiation source SO as a laser produced plasma LPP source, any suitable source may be used to generate EUV radiation. For example, EUV emitting plasma may be produced by using an electrical discharge to convert fuel (e.g. tin) to a plasma state. A radiation source of this type may be referred to as a discharge produced plasma (DPP) source. The electrical discharge may be generated by a power supply which may form part of the radiation source or may be a separate entity that is connected via an electrical connection to the radiation source SO.
    [00079] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.
    [00080] 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.
    [00081] Embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.
    [00082] 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 clauses set out below. Other aspects of the invention are set-out as in the following numbered clauses. 1. A beam measurement system for determining a property of one or more of a plasma, an image of a plasma, and a collector, of a laser produced plasma radiation source, the beam measurement system comprising: at least one sensor unit configured to receive at least a portion of a radiation beam output from the collector, each sensor unit comprising a first patterned element, a second patterned element, and a detector configured to detect radiation that has passed through the first patterned element and the second patterned element, the first patterned element and the second patterned element each being patterned with a spatially non-uniform transmittance and positioned relative to each other to provide a combined transmittance with a non-uniform angular dependence with respect to a direction of incidence of radiation on the sensor unit. 2. The beam measurement system of clause 1, comprising a group of the sensor units, each sensor unit in the group having a first patterned element and a second patterned element having a combined transmittance with an angular dependence, and the angular dependencies for the group of sensor units are different from each other. 3. The beam measurement system of clause 2, wherein the different angular dependencies are obtained by providing different relative positioning between otherwise identical patterns in the first patterned element and the second patterned element in each sensor unit. 4. The beam measurement system of clause 2 or 3, comprising an array comprising a plurality of the groups of the sensor units. 5. The beam measurement system of clause 4, comprising a plurality of the arrays, each array being positioned to receive a different portion of the radiation beam. 6. The beam measurement system of any preceding clause, wherein each of the first patterned element and the second patterned element is patterned with a periodic arrangement of regions of high transmittance separated by regions of lower transmittance. 7. The beam measurement system of any preceding clause, wherein the first patterned element and the second patterned element are substantially planar and the first patterned element and the second patterned element are separated from each other in a direction perpendicular to the plane of the first patterned element. 8. The beam measurement system of any preceding clause wherein the determined property of one or more of the plasma, the image of the plasma, and the collector comprises at least one of a shape of the image of the plasma at an intermediate focus, a size of the image of the plasma at the intermediate focus, and a position of the image of the plasma at the intermediate focus, the intermediate focus being a focus formed by the collector between the collector and an illumination system configured to condition the radiation beam. 9. The beam measurement system of clause 8, wherein one or more of the at least one sensor unit is/are positioned at far field relative to the intermediate focus. 10. The beam measurement system of any preceding clause, wherein the determined property of one or more of the plasma, the image of the plasma, and the collector comprises at least one of a shape of the plasma, a size of the plasma, and a position of the plasma. 11. The beam measurement system of any preceding clause, wherein the collector comprises a patterned region and the beam measurement system comprises at least one sensor unit positioned to receive radiation modulated by the patterned region, wherein a proportion of the modulated radiation detected by the detector is dependent on at least one of a position of the collector relative to an illumination system configured to condition the radiation beam and an orientation of the collector relative to the illumination system. 12. The beam measurement system of clause 11, wherein the patterned region comprises a portion of a plurality of concentric rings. 13. The beam measurement system of clause 11 or 12, wherein: the first patterned element is patterned with a periodic arrangement, having a first pitch, of regions of high transmittance separated by regions of lower transmittance; the second patterned element is patterned with a periodic arrangement, having a second pitch different to or the same as the first pitch, of regions of high transmittance separated by regions of lower transmittance; and the received radiation modulated by the patterned region of the collector is periodic and has a pitch which differs from either or both of the first pitch and the second pitch. 14. The beam measurement system of any of clauses 11-13, further comprising a sensor unit mounting system configured to allow at least one of the sensor units to be moved so as to selectively receive radiation modulated by one of a plurality of different patterned regions of the collector. 15. The beam measurement system of any of clauses 11-14, wherein the determined property of one or more of the plasma, the image of the plasma, and the collector comprises at least one of a position of the collector relative to the illumination system and an orientation of the collector relative to the illumination system. 16. The beam measurement system of any preceding clause, wherein the patterning of the first patterned element and the second patterned element are such that diffraction effects are negligible. 17. The beam measurement system of clause 16, wherein a smallest characteristic dimension of the patterning in the first patterned element and a smallest characteristic dimension of the patterning in the second patterned element are at least 10 times larger than a wavelength of radiation produced by the laser produced plasma radiation source. 18. The beam measurement system of any preceding clause, further comprising: a control device configured to control the laser produced plasma radiation source based on an output from the at least one sensor unit. 19. A lithographic system, comprising: (a) a radiation source configured to collect radiation emitted from a plasma using a collector and output a radiation beam from the collector; and (b) a beam measurement system configured to determine a property of one or more of the plasma, an image of the plasma, and the collector by measuring a property of the radiation beam, the beam measurement system comprising at least one sensor unit configured to receive at least a portion of the radiation beam, each sensor unit comprising a first patterned element, a second patterned element, and a detector configured to detect radiation that has passed through the first patterned element and the second patterned element, wherein in each sensor unit the first patterned element and the second patterned element are each patterned with a spatially non-uniform transmittance and positioned relative to each other to provide a combined transmittance with a non-uniform angular dependence with respect to a direction of incidence of radiation on the sensor unit. 20. A method, comprising: determining a property of one or more of a plasma, an image of a plasma, and a collector in a laser produced plasma radiation source, by measuring a property of a radiation beam output by the laser produced plasma radiation source, wherein the measuring of the property of the radiation beam comprises using at least one sensor unit to receive at least a portion of the radiation beam, each sensor unit comprising a first patterned element, a second patterned element, and a detector configured to detect radiation that has passed through the first patterned element and the second patterned element, the first patterned element and the second patterned element each being patterned with a spatially non-uniform transmittance and positioned relative to each other to provide a combined transmittance with a non-uniform angular dependence with respect to a direction of incidence of radiation on the sensor unit.
    权利要求:
    Claims (1)
    [1]
    A lithography device comprising: an illumination device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.
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    法律状态:
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