WO2021069147A1

WO2021069147A1 - A lithography apparatus and a method of detecting a radiation beam

Info

Publication number: WO2021069147A1
Application number: PCT/EP2020/074318
Authority: WO
Inventors: Giovanni IMPONENTE; Wouter Joep ENGELEN; Nicholas William Maria PLANTZ
Original assignee: Asml Netherlands B.V.
Priority date: 2019-10-10
Filing date: 2020-09-01
Publication date: 2021-04-15
Also published as: TW202119124A; CN114450636A

Abstract

A substrate table configured to hold a substrate, comprising: a plurality of sensor elements configured to detect a radiation beam from a projection system, the radiation beam forming an illuminated region having an elongate shape at substrate level, the elongate shape having long edges and short edges and defining a longitudinal direction and a transverse direction perpendicular to the longitudinal direction, the sensor elements arranged along the longitudinal direction, wherein the plurality of the sensor elements are arranged at different distances in the transverse direction from one of the long edges of the elongate shape.

Description

A LITHOGRAPHY APPARATUS AND A METHOD OF DETECTING A RADIATION BEAM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 19202357.0 which was filed on October 10, 2019 and which is incorporated herein in its entirety by reference.

FIELD

[0001] The present invention relates to a lithography apparatus and a method of detecting a radiation beam in a lithography apparatus.

BACKGROUND

[0002] A lithography apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation- sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0003] Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and or structures to be manufactured.

[0004] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):

[ 0005 ] where l is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, kl is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength l, by increasing the numerical aperture NA or by decreasing the value of kl .

[0006] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use a deep ultraviolet (DUV) radiation source or an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

[0007] EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.

[0008] A sensor may be provided for detecting characteristics of a radiation beam. This radiation beam may be a patterned radiation beam, i.e. a radiation beam on which a pattern has been imparted by a patterning device. For example, discrepancies between the measured radiation beam and a nominal (e.g. ideal) radiation beam can be measured. This can allow the possibility of compensating for the discrepancies.

[0009] In order to measure how the patterned radiation beam varies across an illuminated region at substrate level, it may be necessary to perform multiple measurements. Making multiple measurements increases the measurement time.

[0010] It is desirable to provide a substrate apparatus and a method of detecting a radiation beam that can allow the total measurement time to be reduced.

SUMMARY OF THE INVENTION

[0011] According to an aspect of the invention, there is provided lithography apparatus comprising: a substrate table configured to hold a substrate; and a projection system configured to project a radiation beam to form an illuminated region having an elongate shape at substrate level, the elongate shape having long edges and short edges and defining a longitudinal direction and a transverse direction perpendicular to the longitudinal direction; wherein the substrate table comprises a plurality of sensor elements configured to detect the radiation beam, the sensor elements arranged along the longitudinal direction, wherein a plurality of the sensor elements are arranged at different distances in the transverse direction from one of the long edges of the elongate shape.

[0012] According to an aspect of the invention, there is provided a method of detecting a radiation beam in a lithography apparatus, the method comprising: providing a projection beam of radiation; projecting the projection beam to form an illuminated region having an elongate shape at substrate level, the elongate shape having long edges and short edges and defining a longitudinal direction and a transverse direction perpendicular to the longitudinal direction; detecting the radiation beam with a plurality of sensor elements at the substrate level, the sensor elements arranged along the longitudinal direction, wherein a plurality of the sensor elements are arranged at different distances in the transverse direction from one of the long edges of the elongate shape.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0014] Figure 1 depicts a lithography apparatus according to an embodiment of the invention;

[0015] Figure 2 is a more detailed view of the lithography apparatus;

[0016] Figure 3 is a more detailed view of the source collector module SO of the apparatus of Figures 1 and 2;

[0017] Figure 4 is a schematic diagram of a radiation sensor;

[0018] Figure 5 is a schematic diagram of an illuminated region on a substrate;

[0019] Figure 6 is a close-up view of the illuminated region:

[0020] Figure 7 is a schematic diagram of an arrangement of sensor elements according to an embodiment of the invention;

[0021] Figure 8 is a schematic diagram of an alternative arrangement of sensor elements according to an embodiment of the invention;

[0022] Figure 9 is a schematic diagram of an arrangement of sensor elements according to a comparative example; and

[0023] Figure 10 is a graph showing the relationship between transverse position of the illuminated region and intensity of the patterned radiation beam.

[0024] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAIFED DESCRIPTION

[0025] Figure 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) IF 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask). [0032] 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.

[0033] Referring to Figure 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 Figure 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 C02 laser is used to provide the laser beam for fuel excitation.

[0034] 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.

[0035] 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 □ -outer and □ -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.

[0036] 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 PS2 (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 PS1 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 Ml, M2 and substrate alignment marks PI, P2.

[0037] The depicted apparatus could be used in at least one of the following modes:

[0038] 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.

[0039] 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.

[0040] 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. [0041] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0042] Figure 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.

[0043] 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.

[0044] 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. [0045] 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 radiation beam 26 is formed and the patterned radiation 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.

[0046] 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 Figure 2.

[0047] Collector optic CO, as illustrated in Figure 2, is depicted as a nested collector with grazing incidence reflectors 253, 254 and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254 and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

[0048] Alternatively, the source collector module SO may be part of an LPP radiation system as shown in Figure 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.

[0049] Figure 4 is a schematic diagram of a radiation sensor 10 of the lithography apparatus 100. Figure 4 shows the substrate table WT of the lithography apparatus 100. The substrate table WT is configured to hold a substrate W.

[0050] As shown in Figures 1 and 2, the lithography apparatus 100 comprises a projection system PS. The projection system PS is configured to project the radiation beam B to form an illuminated region 11 (shown in Figure 5) at substrate level.

[0051] As shown in Figure 4, in an embodiment the substrate table WT comprises a radiation sensor 10. The radiation sensor 10 is configured to detect the radiation beam B. The radiation sensor 10 is configured to detect characteristics of the radiation beam B at substrate level.

[0052] For example, in an embodiment the radiation sensor 10 is configured to measure how the intensity of the radiation bean B varies across the illuminated region 11. In an embodiment the radiation sensor 10 is configured to measure the wavefronts of the radiation beam B at substrate level. For example, the radiation sensor 10 may be configured to measure Zernike aberrations. In an embodiment the radiation sensor 10 is configured to measure high order Zernike aberrations and/or lower order Zernike aberrations.

[0053] As mentioned above, the radiation beam B may be EUV radiation. As shown in Figures 1 and 2, the patterning device MA, which is held by support structure MT, may be configured to reflect the radiation beam B. Alternatively, the radiation beam B conditioned by the illumination system IL may be DUV (deep ultraviolet) radiation. As shown in Figure 4, in an embodiment the patterning device MA is transmissive. The patterning device MA is configured to transmit the radiation beam B as it imparts a pattern to the radiation beam B. The present invention is applicable to a lithography apparatus 100 regardless of whether it uses EUV radiation or DUV radiation The present invention is compatible with either a transmissive patterning device MA or a reflective patterning device MA.

[0054] As mentioned above, the projection system PS is configured to project the radiation beam B to form an illuminated region 11 at substrate level. The illuminated region 11 has an elongate shape. Figure 5 is a schematic diagram of the illuminated region 11 relative to the substrate W. In an embodiment, the shape of the radiation sensor 10 in plan view approximately matches the shape of the illuminated region 11. As shown in Figure 5, in an embodiment the illuminated region 11 has long edges 12 and short edges 13. The illuminated region 11 defines a longitudinal direction 15 and a transverse direction 16 perpendicular to the longitudinal direction.

[0055] Figure 6 is a close-up view of the illuminated region 11 shown in Figure 5. In Figure 6, the longitudinal direction 15 and the transverse direction 16 are shown. In an embodiment the illuminated region 11 has a curved shape. When the illuminated region 11 has a curved shape, the transverse direction 16 at one point along the elongate shape may not be parallel to the transverse direction at another point along the elongate shape. The longitudinal direction 15 follows a curve. Figure 6 also shows a center line 14 down the middle of illuminated region 11. The center line 14 is formed by the locus of points halfway between the long edges 12 in the transverse direction 16.

[0056] The radiation sensor 10 is configured to measure how the radiation beam B varies along the length of the illuminated region 11. The radiation sensor 10 comprises a plurality of sensor elements 17 configured to detect the radiation beam B. The sensor elements 17 are arranged along the longitudinal direction 15. This allows the sensor elements 17 of the radiation sensor 10 to measure how the radiation beam B varies along the illuminated region 11.

[0057] It is possible for sensor elements to be arranged to follow the center line 14 or one of the long edges 12 of the illuminated region 11. When the illuminated region has a curved elongate shape, then sensor elements can be arranged to follow the same curve.

[0058] As shown in Figures 5 and 6, in an embodiment the elongate shape is curved. In an alternative embodiment, the projection system PS is configured to project a radiation beam B to form an illuminated region 11 having an elongate rectangular shape. For example, when the radiation beam B is DUV radiation, a rectangular illuminated region 11 may be provided. [0059] Figure 7 is a schematic diagram showing an arrangement of sensor elements 17 of a radiation sensor 10 according to the present invention, Figure 7 also shows how the arrangement of said elements 17 compares to a comparative example. In Figure 7, comparative dots 27 show the position of sensor elements according to a comparative example. In the comparative example, sensor elements are arranged to closely follow the curve of the elongate shape of the illuminated region 11.

[0060] As shown in Figure 7, in an embodiment a plurality of sensor elements 17 are arranged at different distances in the transverse direction 16 from one of the long edges 12 of the elongate shape of the illuminated region 11. A plurality of the sensor elements 17 are offset (in the transverse direction) from the positions shown in the comparative example. In Figure 7, arrows 18 shows how the position of each sensor elements 17 varies relative to the comparative dot 27.

[0061] An embodiment of the invention is expected to reduce the time taken to measure how the radiation beam B varies throughout the illuminated region 11. It is desirable to measure how the radiation beam B varies along the transverse direction 16 (i.e. in the width direction) of the illuminated region 11. This requires taking measurements at a variety of different positions along the transverse direction 16. By providing sensor elements 17 at different transverse positions, the radiation beam B is measured at different transverse positions simultaneously. In contrast, the comparative example shown in Figure 7 would require more measurements with the whole radiation sensor 10 shifted in the transverse direction relative to the illuminated region 11. The greater number of measurements would take a longer time.

[0062] It is desirable to measure fading of the radiation beam B . The fading of the radiation beam B is related to how the radiation beam varies (e.g. in intensity or other characteristics) along the transverse direction of the illuminated region 11. With a layout as shown in the comparative example in Figure 7, fading measurements require two or more scans at different transverse positions in the illuminated region 11. This leads to a throughput penalty. An embodiment of the invention is expected to capture fading with a smaller number of measurements (e.g. with only a single scan).

[0063] As shown in Figure 7, in an embodiment at least one of the sensor elements 17 is offset in the transverse direction 16 towards a concave side of the elongate shape. In the embodiment shown in Figure 7, the first, fourth and seventh sensor elements 17 (going from left to right in Figure 7) are offset towards the concave side. This is indicated by the downward arrows 18 . As shown in Figure 7, in an embodiment at least one of the sensor elements 17 is offset in the transverse direction 16 towards a convex side of the elongate shape. In the embodiment shown in Figure 7, the second, third, fifth and sixth (going from left to right in Figure 7) sensor elements 17 are offset towards the convex side. This is indicated by the upward arrows 18 in Figure 7. The offsets are relative to the comparative example which follows the center line 14 of the illuminated region 11.

[0064] By providing offsets towards both the concave and convex sides, a single scan includes measurements from at least three transverse positions in the illuminated region 11. [0065] In the example shown in Figure 7, measurements are made up of two different transverse positions. The second, third, fifth and sixth sensor elements 17 corresponds to a first transverse position. The first, fourth and seventh elements 17 correspond to a second transverse position. Instead of measuring seven different longitudinal positions at a single transverse position, at least one measurement is made at a plurality of different transverse positions. In an embodiment at least one of the sensor elements 17 is not offset. This makes it possible to make measurements of three different transverse positions simultaneously.

[0066] The measurements corresponding to the different sensor elements 17 can be interpolated so as to provide information about the variation of the radiation beam B across both the longitudinal and transverse directions of the illuminated shape 11.

[0067] Figure 8 shows an arrangement of the sensor elements 17 of a radiation sensor 10. The arrangement shown in Figure 8 is alternative to the arrangement shown in Figure 7. Figure 8 also shows the comparative dots 27 showing the arrangement of sensor elements in a comparative example.

[0068] The arrangements shown in Figures 7 and 8 differ from each other in that the arrangement shown in Figure 7 is symmetrical, whereas the arrangement shown in Figure 8 is sparse or asymmetrical. As shown in Figure 7, in an embodiment the sensor elements 17 are arranged symmetrically about an axis of symmetry that extends in the transverse direction 16. In the example shown in Figure 7 the axis of symmetry is in the transverse direction cutting through the central (fourth) sensor element 17 shown in Figure 7.

[0069] Alternatively as shown in Figure 8, in an embodiment the sensor elements 17 are arranged asymmetrically about an axis extending in the transverse direction halfway along the sensor elements 17 in the longitudinal direction. For example, the third sensor element 17 is offset towards to convex side, whereas the fifth sensor element 17 is not offset.

[0070] The arrangements shown in Figure 7 and 8 are examples of how the sensor elements 17 may be arranged. However, other arrangements are possible while providing the advantage of measuring the radiation beam B at multiple transverse positions simultaneously.

[0071] In an embodiment, the sensor elements 17 are arranged in a zig-zag pattern. For example, when the elongate shape is rectangular, the sensor element 17 may be arranged in a simple zig-zag pattern so as to measure at two different transverse positions simultaneously.

[0072] Figure 9 shows a radiation sensor 10 with comparative dots 27 corresponding to sensor elements. The comparative example shown in Figure 9 may be a sensor for an illuminated region 11 which has a rectangular shape. The sensor elements are all positioned at the same transverse position. This makes it necessary to perform multiple measurements to measure out multiple transverse positions. [0073] In comparison, providing sensor elements 17 that are arranged in a zig-zag pattern allows multiple transverse positions to be measured simultaneously.

[0074] In an embodiment, the central sensor element 17 is either not offset or it is offset towards to the concave side. In an embodiment, one of the sensor elements 17 positioned centrally along the longitudinal direction is more offset in the transverse direction along the concave side of the elongate shape from said one of the long edges 12 compared to another one of the sensor elements 17. Figure 7 shows the central sensor element 17 being offset towards the concave side (downwards in Figure 7). Meantime, Figure 8 shows the central sensor element 17 not offset. By providing no offset or an offset towards the concave side for the central sensor element 17, the lateral extent (in the up and down direction in Figure 7 and 8) of the sensor elements 17 is not increased by the offsetting. An embodiment of the invention is expected to reduce the lateral extent of the radiation sensor 10.

[0075] However, in an alternative embodiment, the central sensor element 17 may be offset towards the convex side.

[0076] As mentioned above, in an embodiment the lithography apparatus comprises an illuminator IL. The illuminator IL is configured to provide the projection beam of radiation. In an embodiment, the illuminator IL is configured to provide the radiation beam B so that its intensity varies nominally trapezoidally in the transverse direction 16 of the elongate shape. Figure 10 is a graph showing the relationship between the transverse position and the intensity of the radiation beam. The x-axis corresponds to the position along the transverse direction 16 of the illuminated region 11. The y-axis represents the intensity of the radiation beam B. The transverse positions of the long edges 12 of the elongate shape are shown in Figure 10. Figure 10 shows the trapezoidal shape of the radiation beam B. [0077] Figure 10 shows a plateau 18 of intensity of the radiation beam B in the transverse direction 16. A central region 19 of the illuminated region 11 corresponds to the plateau 18 of intensity. In an embodiment, the sensor elements 17 are arranged so that they are all arranged in the nominal plateau 18 of intensity of the trapezoid. An embodiment of the invention is expected to improve the accuracy of the measurements made at different transverse positions simultaneously.

[0078] Of course, the radiation beam B may not have a perfectly trapezoidal shape in the transverse direction. In an embodiment the radiation beam B has a Gaussian distribution in the transverse direction. The radiation beam B may have a target shape corresponding to the nominal trapezoidal shape. Differences from the nominal trapezoidal shape can be detected by the measurements made by the radiation sensor 10. The lithography apparatus 100 can be adjusted to compensate for the discrepancies from the nominal shape of the radiation shape B. For example, the position and/or orientation of optical elements may be adjusted based on the measurements made by the radiation sensor 10. In an embodiment the projection system PS is configured to correct for the aberrations of the wavefront measured with the radiation sensor 10.

[0079] It is not essential for the radiation beam B to have a trapezoid shape in the transverse direction. Some other shapes of beams are also possible. In an embodiment, the radiation beam B has a plateau of intensity in the transverse direction.

[0080] In the example shown in Figure 7 and Figure 8, some sensor elements 17 are offset in one direction while other sensor elements 17 are offset in the opposite direction. In an embodiment, the amount of offset is the same for all of the sensor elements 17 that are offset. In the example shown in Figure 7, all of the sensor elements except for the central sensor elements 17 are offset. In an embodiment, the size of the offset is the same for each of the offset sensor elements 17. In other words, the distance between the sensor element 17 and the comparative dot 27 is the same for the first, second, third, fifth, sixth and seventh sensor elements 17.

[0081] In the example shown in Figure 8, each of the first, second, third, fourth, sixth and seventh sensor element’s 17 is offset by the same distance from its corresponding comparative dot 27. In an embodiment, the possible positions of the sensor elements 17 are discretized. For example, a grid of points separated by consistently spaced steps are provided. Each sensor element 17 is offset by only one step (or is not offset at all). This helps to simplify the arrangement of the sensor elements 17. In an embodiment, each of the sensor elements 17 that is offset in the transverse direction 16 from the long edge 12 of the elongate shape is offset by substantially the same amount. In an alternative embodiment each sensor element 17 is offset by two or more steps.

[0082] As shown in Figure 7 and Figure 8 in an embodiment the sensor elements are 17 evenly spaced along the longitudinal direction 15. An embodiment of the invention is expected to achieve a better fit quality and/or a reduced sensitivity to sensor noise. However, this is not necessarily the case. The sensor elements 17 may be spaced at different intervals along the longitudinal direction.

[0083] In an embodiment the radiation sensor 10 comprises an imaging device. The imaging device may be, for example, a charge-coupled device (CCD). A single imaging device may be used for a plurality of sensor elements 17. The sensor elements 17 correspond to different positions that can be measured simultaneously. In an embodiment the radiation sensor 10 comprises one or more gratings. In an embodiment, each sensor element 17 corresponds to a separate grating. In an embodiment, each sensor element corresponds to a separate opening within a cover of the radiation sensor 10. Each opening allows the radiation beam B to reach the grating and subsequently the imaging device.

[0084] Although specific reference may be made in this text to the use of lithography apparatus in the manufacture of ICs, it should be understood that the lithography 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 multiple processed layers. [0085] The term “lens,” where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0086] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the sensor elements 17 may be arranged differently from what is shown in Figures 7 and 8.

[0087] Clauses:

Clause 1. A lithography apparatus comprising: a substrate table configured to hold a substrate; and a projection system configured to project a radiation beam to form an illuminated region having an elongate shape at substrate level, the elongate shape having long edges and short edges and defining a longitudinal direction and a transverse direction perpendicular to the longitudinal direction; wherein the substrate table comprises a plurality of sensor elements configured to detect the radiation beam, the sensor elements arranged along the longitudinal direction, wherein a plurality of the sensor elements are arranged at different distances in the transverse direction from one of the long edges of the elongate shape.

Clause 2. The lithography apparatus of clause 1, wherein the elongate shape is rectangular.

Clause 3. The lithography apparatus of clause 1, wherein the elongate shape is curved.

Clause 4. The lithography apparatus of clause 3, wherein one of the sensor elements positioned centrally along the longitudinal direction is more offset in the transverse direction towards a concave side of the elongate shape from said one of the long edges compared to another one of the sensor elements.

Clause 5. The lithography apparatus of clause 3 or 4, wherein at least one of the sensor elements is offset in the transverse direction towards a concave side of the elongate shape and at least one of the sensor elements is offset in the transverse direction towards a convex side from said one of the long edges of the elongate shape.

Clause 6. The lithography apparatus of any preceding clause, wherein at least one of the sensor elements is offset in the transverse direction, differently compared to another of the sensor elements, relative to a locus of points halfway between the long edges of the elongate shape.

Clause 7. The lithography apparatus of any preceding clause, wherein the sensor elements are arranged in a zig-zag pattern.

Clause 8. The lithography apparatus of any preceding clause, wherein the sensor elements are arranged asymmetrically about an axis extending in the transverse direction half way along the sensor elements in the longitudinal direction.

Clause 9. The lithography apparatus of any of clauses 1 to 7, wherein the sensor elements are arranged symmetrically about an axis of symmetry that extends in the transverse direction.

Clause 10. The lithography apparatus of any preceding clause, wherein each of the sensor elements that is offset in the transverse direction from said one of the long edges of the elongate shape is offset by substantially the same amount. Clause 11. The lithography apparatus of any preceding clause, wherein the sensor elements are equally spaced along the longitudinal direction.

Clause 12. The lithography apparatus of any preceding clause comprising: an illuminator configured to provide a projection beam of radiation, wherein the projection beam is the radiation beam projected by the projection system.

Clause 13. The lithography apparatus of clause 12, wherein the illuminator is configured to provide the projection beam such that its intensity varies nominally trapezoidally in the transverse direction of the elongate shape.

Clause 14. The lithography apparatus of clause 13, wherein the sensor elements are arranged so that they are all arranged within a nominal plateau of intensity of the trapezoid.

Clause 15. The lithography apparatus of any preceding clause comprising: a support structure configured to support a patterning device that patterns the radiation beam according to a desired pattern, wherein the patterned beam is the radiation beam projected by the projection system.

Clause 16. The substrate table of the lithography apparatus of any preceding clause. Clause 17. A method of detecting a radiation beam in a lithography apparatus, the method comprising: providing a projection beam of radiation; projecting the projection beam to form an illuminated region having an elongate shape at substrate level, the elongate shape having long edges and short edges and defining a longitudinal direction and a transverse direction perpendicular to the longitudinal direction; detecting the radiation beam with a plurality of sensor elements at the substrate level, the sensor elements arranged along the longitudinal direction, wherein a plurality of the sensor elements are arranged at different distances in the transverse direction from one of the long edges of the elongate shape.

Claims

1. A substrate table configured to hold a substrate, comprising: a plurality of sensor elements configured to detect a radiation beam from a projection system, the radiation beam forming an illuminated region having an elongate shape at substrate level, the elongate shape having long edges and short edges and defining a longitudinal direction and a transverse direction perpendicular to the longitudinal direction, the sensor elements arranged along the longitudinal direction, wherein the plurality of the sensor elements are arranged at different distances in the transverse direction from one of the long edges of the elongate shape.

2. The substrate table of claim 1, wherein the elongate shape is rectangular or is curved.

3. The substrate table of claim 1, wherein the elongate shape is curved.

4. The substrate table of claim 3, wherein one of the sensor elements positioned centrally along the longitudinal direction is more offset in the transverse direction towards a concave side of the elongate shape from said one of the long edges compared to another one of the sensor elements.

5. The substrate table of claim 3 or 4, wherein at least one of the sensor elements is offset in the transverse direction towards a concave side of the elongate shape and at least one of the sensor elements is offset in the transverse direction towards a convex side from said one of the long edges of the elongate shape.

6. The substrate table of any preceding claim, wherein at least one of the sensor elements is offset in the transverse direction, differently compared to another of the sensor elements, relative to a locus of points halfway between the long edges of the elongate shape.

7. The substrate table of any preceding claim, wherein the sensor elements are arranged in a zig zag pattern or/and wherein the sensor elements are arranged asymmetrically about an axis extending in the transverse direction half way along the sensor elements in the longitudinal direction.

8. The substrate table of any of claims 1 to 6, wherein the sensor elements are arranged symmetrically about an axis of symmetry that extends in the transverse direction.

9. The substrate table of any preceding claim, wherein each of the sensor elements that is offset in the transverse direction from said one of the long edges of the elongate shape is offset by substantially the same amount.

10. The substrate table of any preceding claim, wherein the sensor elements are equally spaced along the longitudinal direction.

11. A lithography apparatus comprising a substrate table according to any preceding claim.

12. The lithography apparatus of claim 11 comprising: an illuminator configured to provide a projection beam of radiation, wherein the projection beam is the radiation beam projected by the projection system.

13. The lithography apparatus of claim 12, wherein the illuminator is configured to provide the projection beam such that its intensity varies nominally trapezoidally in the transverse direction of the elongate shape.

14. The lithography apparatus of claim 13, wherein the sensor elements are arranged so that they are all arranged within a nominal plateau of intensity of the trapezoid.

15. A method of detecting a radiation beam in a lithography apparatus, the method comprising: providing a projection beam of radiation; projecting the projection beam to form an illuminated region having an elongate shape at substrate level, the elongate shape having long edges and short edges and defining a longitudinal direction and a transverse direction perpendicular to the longitudinal direction; detecting the radiation beam with a plurality of sensor elements at the substrate level, the sensor elements arranged along the longitudinal direction, wherein a plurality of the sensor elements are arranged at different distances in the transverse direction from one of the long edges of the elongate shape.