Dose measurement system for a lithographic apparatus

Abstract

The present disclosure relates to a dose measurement system for a lithographic apparatus. The dose measurement system comprises: a target having a diffraction grating formed thereon, wherein the target is configured to undergo thermal expansion when illuminate irradiated with radiation, and a pitch of the diffraction grating increases with the thermal expansion of the target; a sensor configured to measure a parameter of the diffraction grating that is dependent on the pitch of the diffraction grating; and a processor configured to determine the dose of the radiation based on one or more measurements of the parameter of the diffraction grating made by the sensor.

Description

DOSE MEASUREMENT SYSTEM FOR A LITHOGRAPHIC APPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24199747.7 which was filed on 11 September 2024, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a dose measurement device for a lithographic apparatus, a dose measurement system comprising the dose measurement device, a lithographic apparatus comprising the dose measurement system, and a method of measuring a dose in a lithographic apparatus.BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., 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.

[0004] 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.

[0005] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):where X 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 X, 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 an extreme ultraviolet (EUV) radiation source. EUV radiation iselectromagnetic 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] The amount of energy, per unit area, that is received by a substrate in a lithographic apparatus is referred to as the “exposure dose”, or just the “dose”. The dose is equal to the exposure power (i.e. the power of the radiation incident on the substrate) multiplied by the time for which the exposure is performed (i.e. the time for which the substrate is irradiated with the radiation).

[0008] The throughput of a lithographic apparatus may be dependent on exposure power. Further, characteristics (e.g. the extent of) the “developing” of the photoresist (i.e. the chemical change that occurs in areas of the photoresist which are irradiated) are dependent on the exposure dose. The characteristics of the developing of the photoresist may affect the quality of the pattern that is transferred onto the substrate. Moreover, some components of a lithographic apparatus (e.g. a pellicle which is disposed in front of a patterning device to protect the patterning device from contamination) may become damaged if the exposure power is too high. Thus, it is desirable for the exposure power and exposure dose to be known (and controlled / calibrated) to a high degree of accuracy.

[0009] Currently, methods for determining exposure dose or exposure power may comprise use of photodetectors, e.g. photodiodes, which are disposed within the lithographic apparatus or embedded within a substrate. However, such photodetectors may be inaccurate. For instance, it has been observed that measurements of exposure dose / power by photodetectors in a lithographic apparatus are affected by phenomena such as the discharge of batteries in electronics surrounding the photodetectors. Further, photodetectors may become contaminated, e.g. contaminated with carbon, oxides or silicates which may be outgassed from the layer of photoresist when the photoresist is irradiated. Such contamination may have a large effect on measurements made by a photodetector. Thus, the accuracy of the photodetector may worsen over time.

[0010] Other methods for determining exposure dose or exposure power may comprise irradiating a portion of a photoresist-coated substrate, and inspecting the irradiated photoresist (e.g. using an alignment sensor a UV level sensor) to determine the exposure dose or power. Different portions of the photoresist-coated substrate may be irradiated with different doses of radiation. However, the properties of photoresists (e.g. the rate at which photoresists develop when irradiated) may vary between batches. Such variation in the properties of photoresists between batches means that photoresist-based methods for determining exposure dose or exposure power may be inaccurate.SUMMARY OF THE INVENTION

[0011] An aim of the present disclosure is to provide an improved method for determining exposure dose or exposure power.

[0012] According to the present disclosure, there is provided a dose measurement system for a lithographic apparatus. The dose measurement system comprises: a target having a diffraction grating formed thereon, wherein the target is configured to undergo thermal expansion when illuminate irradiated with radiation, and a pitch of the diffraction grating increases with the thermal expansion of the target; a sensor configured to measure a parameter of the diffraction grating that is dependent on the pitch of the diffraction grating; and a processor configured to determine the dose of the radiation based on one or more measurements of the parameter of the diffraction grating made by the sensor.

[0013] Also according to the present disclosure, there is provided a method of measuring a dose in a lithographic apparatus. The method comprises: increasing a pitch of a diffraction grating formed on a target by illuminating irradiating the target with radiation; measuring a parameter of the diffraction grating that is dependent on the pitch of the diffraction grating; and determining the dose of the radiation based on the measured parameter of the diffraction grating.

[0014] Also according to the present disclosure, there is provided a sensor device for a dose measurement system. The sensor device comprises: a target having a diffraction grating formed thereon, wherein the target is configured to undergo thermal expansion when irradiated with radiation, and a pitch of the diffraction grating increases with the thermal expansion of the target; and a base on which the target is supported. The interface between contacting surfaces of the target and the base is configured to inhibit contraction of the target after the target has undergone thermal expansion.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] 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.

[0016] Figure 1 schematically depicts a lithographic apparatus.Figure 2 schematically depicts a more detailed view of the lithographic apparatus.Figure 3 schematically depicts a dose measurement device.Figure 4 schematically depicts a dose measurement device.Figure 5 schematically depicts a method of measuring dose in a lithographic apparatus.Figure 6 schematically depicts a substrate comprising a plurality of dose measurement devices.

[0017] The features shown in the Figures are not necessarily to scale, and the size and / or arrangement depicted is not limiting. It will be understood that the Figures include optional features which may not be essential to the invention. Furthermore, not all of the features of the apparatus are depicted in each of the figures, and the Figures may only show some of the components relevant for describing a particular feature.DETAILED DESCRIPTION

[0018] Figure 1 schematically depicts a lithographic apparatus 100 including a radiation source SO according to one embodiment of the invention. The apparatus 100 comprises: an illumination system (or illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation). a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0019] The illumination system IL 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.

[0020] 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 lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable as required. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS.

[0021] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section such as to create a pattern in a target portion C of the substrate W. The pattern imparted to the radiation beam B may correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit.

[0022] Examples of patterning devices include masks, programmable mirror arrays, and programmable liquid-crystal display (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.

[0023] The projection system PS, like the illumination system IL, 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.

[0024] As here depicted, the lithographic apparatus 100 is of a reflective type (e.g., employing a reflective mask).

[0025] The lithographic apparatus 100 may be of a type having two (dual stage) or more substrate tables WT (and / ortwo or more support structures MT). In such a “multiple stage” lithographic apparatus the additional substrate tables WT (and / or the additional support structures MT) may be used in parallel, or preparatory steps may be carried out on one or more substrate tables WT (and / or one or more support structures MT) while one or more other substrate tables WT (and / or one or more other support structures MT) are being used for exposure.

[0026] Referring to Figure 1, the illumination system IL receives an extreme ultraviolet radiation beam from the radiation source SO. The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation. 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 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 radiation source 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 radiation source SO. The laser and the radiation source SO may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0027] In such cases, the laser is not considered to form part of the lithographic apparatus 100 and the radiation beam B is passed from the laser to the radiation source SO 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 radiation source SO, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0028] The illumination system 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 o-outer and o-inner, respectively) of the intensity distribution in a pupil plane of the illumination system IL can be adjusted. In addition, the illumination system IL may comprise various other components, such as facetted field and pupil mirror devices. The illumination system IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross-section.

[0029] 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 MA. After beingreflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B 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 PSI can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. The patterning device (e.g., mask) MA and the substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0030] A controller 500 controls the overall operations of the lithographic apparatus 100 and in particular performs an operation process described further below. Controller 500 can be embodied as a suitably-programmed general purpose computer comprising a central processing unit, volatile and nonvolatile storage means, one or more input and output devices such as a keyboard and screen, one or more network connections and one or more interfaces to the various parts of the lithographic apparatus 100. It will be appreciated that a one-to-one relationship between controlling computer and lithographic apparatus 100 is not necessary. In an embodiment of the invention one computer can control multiple lithographic apparatuses 100. In an embodiment of the invention, multiple networked computers can be used to control one lithographic apparatus 100. The controller 500 may also be configured to control one or more associated process devices and substrate handling devices in a lithocell or cluster of which the lithographic apparatus 100 forms a part. The controller 500 can also be configured to be subordinate to a supervisory control system of a lithocell or cluster and / or an overall control system of a fab.

[0031] Figure 2 shows the lithographic apparatus 100 in more detail, including the radiation source SO, the illumination system IL, and the projection system PS. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0032] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0033] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. Forthat purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured toproject the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0034] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0035] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0036] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[0037] The lithographic apparatus LA may be of a so-called dual stage type which has two stations: an exposure station and a measurement station. The lithographic apparatus may comprise two substrate tables WT. The lithographic apparatus LA may be configured such that the two substrate tables WT can each be exchanged between the exposure station and the measurement station.

[0038] While one substrate W on one substrate table WT is being exposed at the exposure station, another substrate can be loaded onto the other substrate table WT at the measurement station and various preparatory steps carried out. This enables a substantial increase in the throughput of the apparatus. The preparatory steps may include mapping the surface height contours of the substrate W using a level sensor and / or measuring the position of alignment markers on the substrate using an alignment sensor. If the position sensor is not capable of measuring the position of the substrate table WT while it is at the measurement station as well as at the exposure station, a second position sensor may be provided to enable the positions of the substrate table to be tracked at both stations (relative to reference frame).

[0039] Other arrangements are known and usable instead of the dual-stage arrangement shown. For example, other lithographic apparatuses are known in which a substrate table and a measurement table are provided. These are docked together when performing preparatory measurements, and then undocked while the substrate table undergoes exposure.

[0040] Figure 3 depicts a sensor device 300. The sensor device 300 may be part of a dose measurement system. The dose measurement system may be configured to measure a dose (i.e. an exposure dose) in a lithographic apparatus. Some or all of the dose measurement system may be a part of (i.e. integrated within) the lithographic apparatus.

[0041] The sensor device 300 comprises a target 320. The target 320 may comprise a proximal end portion 321 and a distal end portion 322. The proximal end portion 321 and the distal end portion 322may be separated from one another in a longitudinal direction. A direction perpendicular to the longitudinal direction may be referred to as a transverse direction.

[0042] The target 320 may be a strip of material. A length of the target 320 (i.e. a dimension in the longitudinal direction) may be greater than a width of the target 320 (i.e. a dimension in the transverse direction). The target 320 may be a plate. A thickness of the target 320 (i.e. a dimension in a direction that is perpendicular to the longitudinal and transverse directions) may be less (e.g. much less) than the length and width of the target 320.

[0043] The target 320 may be configured to undergo thermal expansion when irradiated with radiation. That is, the target 320 may be configured to absorb radiation, and the resulting temperature increase of the target 320 may cause the target 320 to expand.

[0044] The target 320 (or a main body thereof) may be formed of a material with a relatively high thermal expansion coefficient. For example, the target 320 (or a main body thereof) may be formed of a metal.

[0045] The target 320 may be formed from a material which is compatible with the environment within a lithographic apparatus (i.e. which does not outgas contaminants in the near-vacuum conditions within the lithographic apparatus when exposed to EUV radiation).

[0046] In some embodiments, the target may be formed of a material comprising (optionally substantially comprising, consisting essentially of, or consisting of) aluminium, or an alloy thereof. In some embodiments, the target may be formed of a material comprising (optionally substantially comprising, consisting essentially of, or consisting of) copper, or an alloy thereof.

[0047] The length of the target 320 may be greater than or equal to 10 mm. The width of the target 320 may be greater than 3 mm. Increasing the size of the target 320 increases the amount of energy required to induce a required thermal deformation in the target 320. Thus, the length of the target may be less than 100 mm and optionally less than or equal to 20 mm, and / or the width of the target 320 may be less than 10 mm.

[0048] The target 320 may be supported on a base 310. The target 320 may be fixedly secured (e.g. fixedly coupled) to the base 320 at the proximal end portion 321. Thus, a position of the proximal end portion 321 of the target 320 may be fixed relative to the base 310.

[0049] As depicted in Figure 3, the proximal end portion 321 of the target 320 may be fixedly secured (e.g. clamped) to a mounting portion 311. The mounting portion 311 may be a part of, or fixedly secured to, the base 310. The target 320 extends distally (in the longitudinal direction) from the mounting portion 311. As will be appreciated, other methods for securing the first end portion 321 of the target 320 may be implemented. Further, in some embodiments, the first end portion 321 of the target 320 may not be fixedly secured.

[0050] Thermal expansion of the target 320 when the target 320 is irradiated with radiation may be predominantly in the longitudinal direction. That is, a change in the length of the target 320 when the target 320 undergoes thermal expansion may be greater than a change in the width of the target 320when the target 320 undergoes thermal expansion. This may be due to the shape of the target 320 (i.e. the fact that the length of the target 320 is greater than a width of the target 320), and / or due to the manner in which the proximal end portion 321 of the target 320 is fixed relative to the base 310.

[0051] The target 320 may comprise a diffraction grating 340. The diffraction grating 340 may be coupled to the target 340 such that movements, deformations and / or distortions of the target 320 in the area at which the diffraction grating 340 is located are translated into the diffraction grating 340. That is, movements, deformations and / or distortions of the target 320 in the area at which the diffraction grating 340 is located result in corresponding movements, deformations and / or distortions of the diffraction grating 340.

[0052] The diffraction grating 340 may be a reflective diffraction grating. The diffraction grating 340 may formed of alternating reflective portions and absorber portions.

[0053] The diffraction grating 340 may be a linear diffraction grating 340. The reflective portions and absorber portions may each be linear, and extend in the transverse direction. The reflective portions and absorber portions may alternate in the longitudinal direction. The pitch of the diffraction grating 340 may also be referred to as the period of the diffraction grating. The pitch of the diffraction grating may be, for example, a distance from one side of an absorber portion to the same side of an adjacent absorber portion.

[0054] In some embodiments, the diffraction grating 340 may be a two-dimensional diffraction grating, e.g. a checkerboard diffraction grating. A two-dimensional diffraction grating may have a longitudinal pitch and a transverse pitch.

[0055] The target 320 and diffraction grating 340 may be configured such that a pitch of the diffraction grating increases with the thermal expansion of the target. For a linear diffraction grating, as the target 320 expands and the length of the target 320 increases in the portion of the target 320 at which the diffraction grating 340 is located, the diffraction grating 340 will be elongated in the longitudinal direction, causing the pitch of the diffraction grating 340 to increase. For a two-dimensional diffraction grating, as the target 320 expands and the length and width of the target increases in the portion of the target 320 at which the diffraction grating 340 is located, the longitudinal and transverse pitches of the diffraction grating will increase.

[0056] The target 320 may comprise a coated region 330 configured to receive and absorb radiation. The coated region 330 may be configured to absorb specific wavelength ranges, as will be described in further detail below. The coated region 330 may coat a main body of the target 320. Alternatively, the coated region 330 may be an independent component (e.g. a strip) which is assembled with the target 320 such that the independent coated region 330 and the target 320 are in thermal contact with one another. Alternatively, the coated region 330 may be provided on an intermediate component (not shown) which is in thermal contact with the target 320. In this case, the intermediate component may be considered to be a part of the target 320.

[0057] The coated region 330 may located at or near the proximal end portion of the target 320. That is, the coated region 330 may be disposed at the opposite end portion of the target 320 to the diffraction grating 340. The coated region 330 and the diffraction grating 340 may be spaced apart from one another, such that the provision of the coated region 330 does not affect the diffraction grating 340. The coated region 330 and the diffraction grating 340 may both be disposed on the same surface of the target 320, which may be referred to as a facing surface. A rear surface of the target 320 may be opposite the facing surface. A width of the coated region 330 may be substantially the same as the width of the target 320 (i.e. or the main body thereof).

[0058] An unsecured portion of the target 320 may extend distally of the portion of the target 320 at which the target 320 is fixedly secured to the base 310. The unsecured portion of the target 320 may comprise the distal end portion 322 of the target 320. The unsecured portion may be supported by the base 310 in a vertical direction (i.e. a direction that is perpendicular to the longitudinal and transverse directions). In other words, the unsecured portion ofthe target 320 may rest on the base 310. Movement of the target 320 relative to the base 310 may be permitted in some or all of the longitudinal and transverse directions, as will be described in further detail below. That is, the unsecured portion of the target 320 may be able to slide over the base 310 (e.g. when undergoing thermal expansion).

[0059] The base 310 may comprise the component on which the unsecured portion of the target 320 rests. This component may be the same as or different to the mounting portion 311. Generally, the base 310 may comprise some or all components / features which contribute to the supporting of the target 320.

[0060] A surface of the base 310 which supports the target 320 may be referred to as a supporting surface. Where the target 320 is supported by the base 310, the supporting surface of the base 310 may be in contact with the rear surface of the target 320.

[0061] The interface 350 between the supporting surface of the base 310 and the rear surface of the target 320 may be a slip-stick interface. The interface 350 may be configured to prevent or inhibit (or generally oppose) thermal contraction of the target 320. For example, the interface 350 may be configured to prevent or inhibit thermal contraction of the target 320 after the target 320 has been irradiated. For example, the interface 350 may prevent or inhibit thermal contraction of the target 320 while the target 320 is being transferred from the exposure station to the measurement station.

[0062] In some embodiments, the interface 350 between the supporting surface of the base 310 and the rear surface of the target 320 may be an anisotropic interface. That is, the interface 350 may be configured to allow relative movement between the target 320 and the base 310 in a first direction, and to inhibit (e.g. prevent, or generally oppose) relative movement between the target and the base in a second direction. The first and second directions may each be parallel to the longitudinal direction. The first direction may be a distal direction (i.e. a direction from the proximal end portion 321 to the distal end portion 322 of the target 320), and the second direction may be a proximal direction (i.e. a direction from the distal end portion 322 to the proximal end portion 321 of the target 320). The slipstick interface 350 between the supporting surface of the base 310 and the rear surface of the target 320is shown figuratively by the sawtooth profiles of the supporting surface of the base 310 and the rear surface of the target 320 in Figure 3. Such an anisotropic interface may be one example of a slip-stick interface.

[0063] As the target 320 thermally expands, the unsecured portion of the target 320 will extend in the distal direction. Moving in this direction, the teeth of the rear surface of the target 320 are able to slide over the teeth of the supporting surface of the base 310. However, movement of the unsecured portion of the target 320 in the proximal direction (e.g. during thermal contraction of the target 320) is inhibited by the slip-stick interface 350, because the teeth of the rear surface of the target 320 are unable to slide over the teeth of the supporting surface of the base 310 when the unsecured portion of the target 320 is moving in the proximal direction.

[0064] The teeth extending from the rear surface of the target 320 may be substantially triangular. Each tooth of the rear surface of the target 320 may comprise, at the proximal side, a first edge protruding from the rear surface of the target 320 substantially perpendicularly to the rear surface of the target 320 to a vertex. Each tooth of the rear surface of the target 320 may further comprise a second edge extending diagonally in the distal direction from said vertex to the rear surface of the target 320.

[0065] The teeth extending from the supporting surface of the base 310 may be substantially triangular. Each tooth of the supporting surface of the base 310 may comprise, at the distal side, a first edge protruding from the supporting surface of the base 310 substantially perpendicularly to supporting surface of the base 310 to a vertex. Each tooth of the supporting surface of the base 310 may further comprise a second edge extending diagonally in the proximal direction from said vertex to the supporting surface of the base 310.

[0066] When the target 320 moves in the distal direction relative to the base, the second edges of the teeth of the rear surface 320 of the target come into contact with the second edges of the teeth of the supporting surface of the base 310. The second edges of the teeth of the rear surface of the target 320 may be able to slide over the second edges of the teeth of the supporting surface of the base 310 to allow movement of the target 320 in the distal direction relative to the base 310. When the target 320 moves in the proximal direction relative to the base 310, the first edges of the teeth of the rear surface of the target 320 come into contact with (i.e. abut) the first edges of the teeth of the supporting surface of the base 310. The first edges of the teeth of the rear surface of the target 320 may not be able to slide over the first edges of the teeth of the supporting surface of the base 310. This may be because the abutting surfaces of the teeth are substantially perpendicular to the direction of the movement of the target 320 relative to the base 310.

[0067] As will be recognized, the teeth depicted in Figure 3 and described above are intended to illustrate the concept of the slip-stick interface 350 between the base 310 and the target 320. Such teeth may not be provided in commercial embodiments of the sensor device 300. Rather, as shown in Figure 4, the base 310 may rest on the supporting surface of the base 310 which appears to be substantially flat. In Figure 4, the base 310 comprises a base portion 312. The base portion 312 forms the supportingsurface of the base 310. The functionality of the teeth may be implemented at the interface 350 between the supporting surface (of the base portion 312) and the rear surface of the target 320 in several ways, some of which are described below. It will be recognized that the fact that the dose measurement device comprises a distinct base portion 312 is not essential, and the base portion 312 may just be considered to be a part of the base 310.

[0068] In some embodiments, a sawtooth structure similar to that shown in Figure 3 is imparted into the microstructure of the rear surface of the target 320 and the supporting surface of the base portion 312. Structures other than a sawtooth structure, e.g. serrations, which achieve a similar effect may also be imparted to the microstructure of the rear surface of the target 320 and the supporting surface of the base portion 312. Such structures may mean that the coefficient of friction (static and / or dynamic) between the supporting surface of the base 310 and the rear surface of the target 320 is higher when the target 320 is moving in the second direction than the coefficient of friction (static and / or dynamic) between the supporting surface of the base 310 and the rear surface of the target 320 is higher when the target 320 is moving in the first direction. Such structures may be imparted the microstructure of the rear surface of the target 320 and the supporting surface of the base portion 312 using surface texturing techniques, such as laser surface texturing, ball burnishing, chemical etching, lithography (e.g. two- photon lithography), atomic layer deposition, ultrasonic-assisted grinding, electrical discharge machining (EDM) and micro-milling.

[0069] In some embodiments, the interface 350 may be an interface which exhibits the slip-stick phenomenon. An interface which exhibits the slip-stick phenomenon may be another example of a slipstick interface.

[0070] The slip-stick phenomenon arises as a result of the fact that the static coefficient of friction (i.e. the coefficient of friction when the two surfaces are at rest relative to one another) between two surfaces is greater than the dynamic coefficient of friction (i.e. the coefficient of friction when the surfaces are moving relative to one another) between the same two surfaces. Thus, the force required to initiate relative movement between the two surfaces is greater than the force required to sustain relative movement between the two surfaces.

[0071] At the interface 350 between the supporting surface of the base 310 and the rear surface of the target 320, the slip-stick phenomenon may prevent or inhibit thermal contraction of the target as follows.

[0072] During irradiation of the target 320, the increase in temperature may be relatively large (i.e. large compared to the decrease in temperature after irradiation of the target 320). The relatively large increase in temperature may result in athermal expansion force (i.e. a force responsible for the thermal expansion of the target 320) that is sufficient to overcome the static friction between the supporting surface of the base 310 and the rear surface of the target 320. Once the rear surface of the target 320 is moving relative to the supporting surface of the base 310, the coefficient of friction is the lower dynamic coefficient of friction. This lower dynamic coefficient of friction does not impede the full expansion of the target 320 in response to the change in temperature 320 caused by the irradiation of the target 320.

[0073] After irradiation of the target 320, the decrease in temperature may be relatively small (i.e. small relative to the increase in temperature during irradiation of the target 320). The relatively small decrease in temperature may result in a thermal contraction force (i.e. a force responsible for the thermal contraction of the target 320) that is less than the static coefficient of friction. Thus, the target may be prevented or inhibited from contracting by the frictional force at the interface 340 between the supporting surface of the base 310 and the rear surface of the target 320.

[0074] The presence of the slip-stick phenomenon at the interface may be achieved, or increased, by selecting the materials from which the supporting surface of the base portion 312 and the rear surface of the target 320 are formed. For example, when the rear surface of the target 320 is formed from aluminum and the supporting surface of the base 310 is silicon, the above-described slip-stick phenomenon may be well-exhibited. A ceramic -on-ceramic interface may also exhibit a strong slipstick phenomenon. Lubricants may also be provided to the interface (i.e. between the supporting surface of the base portion 312 and the rear surface of the target 320) to increase the strength of the slip-stick phenomenon. The morphology of the supporting surface of the base portion 312 and the rear surface of the target 320 may be modified to increase the strength of th slip-stick phenomenon. Such modifications to the morphology of the rear surface of the target 320 and the supporting surface of the base portion 312 using surface texturing techniques, such as laser surface texturing, ball burnishing, chemical etching, lithography (e.g. two-photon lithography), atomic layer deposition, ultrasonic- assisted grinding, electrical discharge machining (EDM) and micro-milling.

[0075] The provision of a slip-stick interface 350 may improve the accuracy of the dose measurement system, as will be described in below. However, in some embodiments, sufficiently accurate results may be obtained in the absence of a slip-stick interface 350. Thus, in some embodiments, the rear surface of the target 320 and the supporting surface of the base 310 may not be configured as described above, and may, for example, permit movement of the target 320 relative to the base 310 in both the proximal and distal directions. For example, the rear surface of the target 320 and the supporting surface of the base 310 may both be substantially smooth surfaces.

[0076] In some embodiments, the sensor device 300 may be comprised in a substrate W, e.g. a semiconductor substrate, e.g. a wafer. The sensor device 300 may be coupled to, integrated / embedded within or attached to the substrate W. The substrate W may be a substrate that is configured to be supported on the substrate table(s) WT of the lithographic apparatus. In some embodiments, the sensor device 300 may be a standalone device, which is disposed at substrate level in the lithographic apparatus. For example, the sensor device 300 may be mounted directly on a substrate table WT. By integrating the sensor device 300 into a substrate W, the sensor device 300 is compatible with the lithographic apparatus without requiring any modifications to be made to the lithographic apparatus.

[0077] In some embodiments, a substrate W comprises a plurality of sensor devices 300a-300h, as shown in Figure 6. The sensor devices 300a-300h may be distributed across a surface of the substrate W. The exact positioning of the sensor devices 300a-300h is not particularly limited.

[0078] The dose measurement system (of which the sensor device 300 may be a part) may comprise a sensor (not shown) configured to measure a parameter of the diffraction grating 340. The parameter of the diffraction grating 340 may be a parameter that is dependent on the pitch of the diffraction grating. The parameter may be a parameter that is indicative of the pitch (or the change in pitch) of the diffraction grating 340. Since the change in the pitch is dependent on the thermal expansion of the target 320, the parameter may be a parameter that is indicative of a magnitude of the thermal expansion of the target 320.

[0079] The parameter may be, for example, an angular spread of diffraction orders reflected from the diffraction grating 340 (e.g. the angular separation between adjacent diffraction orders). The parameter may be, for example, the pitch of the diffraction grating (i.e. the sensor may measure the pitch of the diffraction grating directly, or the sensor may determine the pitch of the diffraction grating based on other measurements).

[0080] The sensor may be an interferometric sensor. The sensor may be an alignment sensor. For example, the sensor of the dose measurement system may be Smart Alignment Sensor Hybrid (or SMASH). Information relating to SMASH may be found in U.S. patent number 6,961, 116, the entirety of which is hereby incorporated by reference. Other position measurement systems may be used. For example, the sensor in the dose measurement system may be of the type described in U.S. patent number 6,297,876, the entirety of which is hereby incorporated by reference. The measurement system described in 6,297,876 may be referred to as Advanced Technology using High order Enhancement of Alignment, or ATHENA. As a further example, the sensor in the dose measurement system may utilize the well-known "Through The Lens (TTL)" position measurement technique in which radiation diffracted by an alignment mark is formed on a detector grating to produce a periodic alignment signal.

[0081] In some embodiments, the sensor may be configured to measure the parameter when the dose sensor device 300 is at the measurement station of the lithographic apparatus. In some embodiments, the sensor may be configured to measure the parameter when the dose sensor device 300 is at the exposure station of the lithographic apparatus.

[0082] The dose measurement system may further comprise a processor (not shown). The processor may be configured to determine the dose of the radiation (i.e. the dose of the radiation with which the target 320 is irradiated) based on one or more measurements of the parameter of the diffraction grating made by the sensor.

[0083] A method of measuring a dose (i.e. exposure dose) in a lithographic apparatus will now be described with reference to Figure 5. The method may comprise use of the sensor device 300 and the dose measurement system described above. As will be recognized, some of the steps described below may be omitted in the measurement of a dose in a lithographic apparatus. The method described below is for the case where the diffraction grating 340 is a linear diffraction grating.

[0084] The sensor device 300 may begin at the measurement station of the lithographic apparatus. The sensor device 300 may be comprised within a substrate W that is supported on a substrate table WTwhich is at the measurement station of the lithographic apparatus. At step SI, the parameter of the diffraction grating that is dependent on the pitch of the diffraction grating may be measured by the sensor of the dose measurement system to obtain an initial value of that parameter. The initial value may be a standard value or a reference value. In the case that the initial value is known before the initiation of the method, step SI may be omitted. In the case that the sensor of the dose measurement system is configured to measure the parameter when the sensor device 300 is at the exposure station of the lithographic apparatus, step SI may be performed with the sensor device at the exposure station.

[0085] Once the initial value of the parameter is known, the sensor device 300 may be moved to the exposure station of the lithographic apparatus (step S2). This may comprise moving the substrate support WT on which the substrate comprising the sensor device 300 is supported from the measurement station to the exposure station. In the case that the initial value is known before the initiation of the method, the sensor device may begin the method at the exposure station of the lithographic apparatus, which means that step S2 can be omitted. Also, in the case that the step SI is performed at the exposure station, step S2 may be omitted.

[0086] At step S3, the target 320 of the sensor device 300 is irradiated (or illuminated) with a beam B of radiation. The beam B of radiation may be generated by the source SO and directed to the target 320 by the illumination system IL.

[0087] In step S3, a specific, known area of the target 320 may be irradiated. The area of the target 320 that is irradiated may be controlled using masking blades (not shown). Masking blades may be provided within the lithographic apparatus adjacent to the patterning device MA. For example, the masking blades may be provided such that they are displaced from the patterning device MA in the z- direction. The specific, known area of the target 320 that is irradiated may be the coated region 330 of the target 320.

[0088] The coated region 330 of the target absorbs energy from the beam of radiation which irradiates the target 320. In some embodiments, the coated region 330 may not be present, and a main body of the target 320 may absorb energy from the beam of radiation which irradiates the target 320.

[0089] Energy absorbed by the coated region 330 is transferred to a target 320 (or a main body thereof), e.g. by conduction. This causes a temperature of the target 320 to increase. The increase in the temperature of the target 320 causes the target 320 to expand. The expansion of the target 320 may comprise a lengthening of the target in the longitudinal direction. The lengthening of the target 320 in the longitudinal direction may cause the pitch of the diffraction grating 340 to increase.

[0090] An exposure time (i.e. the length of time for which the target 320 is irradiated) may be approximately 1 second to 10 seconds. Irradiating the target 320 for such a length of time may allow sufficient energy to be transferred to the target 320 for the change in the pitch of the diffraction grating 340 to be easily detectable (and measurable with an acceptable degree of uncertainty).

[0091] After the step S3 of irradiating the target 320, the sensor device 300 may be moved to the measurement station of the lithographic apparatus (step S4). This may comprise moving the substratesupport WT on which the substrate comprising the sensor device 300 is supported from the exposure station to the measurement station. In the case that the dose measurement system is configured to measure the parameter when the sensor device 300 is at the exposure station of the lithographic apparatus, step S4 may be omitted.

[0092] At step S5, the parameter of the diffraction grating that is dependent on the pitch of the diffraction grating is measured. The same parameter is measured in steps SI and S5. In step S5, the parameter may be measured by the sensor of the dose measurement device.

[0093] At step S6, an increase in the pitch of the diffraction grating 340 may be determined. The increase in the pitch of the diffraction grating 340 may be based on the measurement made in step SI (i.e. the initial value for the parameter) and the measurement made in step S5 (i.e. the value for the parameter after the target 320 has been irradiated). The increase in the pitch of the diffraction grating may be used in the determination of the exposure dose. In some embodiments, the increase in the pitch of the diffraction grating 340 may not be directly determined. Rather, the dose of radiation with which the target 320 has been irradiated may be determined directly from the measurement made in step SI (i.e. the initial value for the parameter) and the measurement made in step S5 (i.e. the value for the parameter after the target 320 has been irradiated) without first determining the increase in the pitch of the diffraction grating 340 (as described below).

[0094] At step S7, the dose (i.e. the dose of the radiation with which the target 320 has been irradiated) is determined. The dose may be calculated using Equation 7, where: D is the dose [J / m2]; m is the mass of the target 320 [kg]; c is the specific heat capacity of the material from which the target 320 is formed [J / kg°C]; A is the surface area of the target 320 that is irradiated [m2]; po is the initial pitch of the diffraction grating [m]; pi is the pitch of the diffraction grating after the target 320 has been irradiated; and a is the thermal expansion coefficient. Equation 1 is derived from linear thermal expansion and specific heat capacity equations.Equation 1

[0095] Between the termination of the step S3 of irradiating the target 320 and the step S5 of measuring the parameter of the diffraction grating that is dependent on the pitch of the diffraction grating, thermal contraction of the target 320 may be inhibited. This may be achieved by inhibiting relative movement between the target 320 and the base 310 (e.g. in the proximal direction). Relative movement between target 320 and the base 310 may be inhibited by providing the slip-stick interface 350.

[0096] By inhibiting movement between target 320 and the base 310 after the irradiating is terminated, the diffraction grating 340 remains in its elongated state even if the target 320 cools down (e.g. while being transferred from the exposure station to the measurement station). That is, the change in the pitchof the diffraction grating that is induced by the dose of radiation is maintained in the period between the end of the irradiation and the measurement of the changed pitch of the diffraction grating 340. Thus, cooling of the target 320 does not reduce the extent to which the measurement performed in step S5 can be used to determine the exposure dose. In other words, by inhibiting movement between target 320 and the base 310 after the irradiating is terminated, the accuracy of the dose measurement system is improved.

[0097] In the case that the diffraction grating comprises a two-dimensional diffraction grating, the steps SI and S5 may comprise measuring a parameter relating to the longitudinal pitch, a parameter relating to the transverse pitch, or a parameter relating to the longitudinal and transverse pitches. The parameter relating to the longitudinal pitch and / or transverse pitch may be used to determine the exposure dose in a similar way to that described above for a linear diffraction grating.

[0098] The beam B of radiation may comprise radiation having different wavelengths. For example, the beam B of radiation extreme ultraviolet (EUV) radiation (i.e. radiation with a wavelength of approximately 10 nm to 20 nm, preferably 13 nm to 14 nm, e.g. 13.5 nm), deep ultraviolet (DUV) radiation (i.e. radiation with a wavelength of 190 nm to 300 nm), and infrared IR radiation (i.e. radiation with a wavelength of approximately 1 pm to 10 pm). It may be desirable for the exposure power and / or exposure dose to be known for different wavelength bands.

[0099] The exposure power and or exposure dose may be determined by performing the method of the present disclosure with a coated region 130 which is known to absorb only specific wavelength bands. For example, if only the exposure dose / power corresponding to the EUV range is of interest, the coated region 330 may be configured to absorb extreme ultra-violet radiation, and reflect deep ultra-violet radiation and infra-red radiation. Such a coated region 330 may comprise (optionally substantially comprise, consist essentially of, or consist of) chromium. If only the exposure dose / power corresponding to the EUV and DUV ranges is of interest, the coated region 330 may be configured to absorb extreme ultra-violet radiation and deep ultra-violet radiation, and to reflect infra-red radiation. Such a coated region may comprise (optionally substantially comprise, consist essentially of, or consist of) titanium oxide (TiCF). If the exposure dose / power corresponding to all wavelengths is of interest, the coated region 330 may be configured to absorb extreme ultra-violet radiation, deep ultra-violet radiation and infra-red radiation. Standard absorber coatings (e.g. carbon) may be used for a coated region 330 that is configured to absorb extreme ultra-violet radiation, deep ultra-violet radiation and infra-red radiation.

[0100] The method of the present disclosure may be performed for a plurality of (e.g. three) different sensor devices 300 (or one sensor device 300 to which a plurality of (e.g. three) different coated portions 330 are applied) to determine the exposure dose / power in different wavelength bands. In an embodiment, there is provided first, second and third sensor devices 300. The first sensor device 300 may comprise a first coated region 330 which is configured to absorb extreme ultra-violet radiation, deep ultra-violet radiation and infra-red radiation. The second sensor device 300 may comprise a secondcoated region 330 which is configured to absorb extreme ultra-violet radiation and deep ultra-violet radiation, and reflect infra-red radiation. The third sensor device may be configured to absorb extreme ultra-violet radiation, and to reflect deep ultra-violet radiation and infra-red radiation.

[0101] A first instance of the method of the present disclosure may be performed with the first sensor device 300 to obtain a first dose measurement, a second instance of the method of the present disclosure may be performed with the second sensor device 300 to obtain a second dose measurement, and a third instance of the method of the present disclosure may be performed with the third sensor device 300 to obtain a third dose measurement.

[0102] The third dose measurement is a dose measurement corresponding to the EUV wavelength range only. By subtracting the third dose measurement from the second dose measurement, a dose measurement corresponding to the DUV wavelength range only can be obtained. By subtracting the second dose measurement from the first dose measurement, a dose measurement corresponding to the IR wavelength range can be obtained.

[0103] In the dose measurement system / method of the present disclosure, the sensor device 300 is a passive sensor device, i.e. it has no electronics. Thus, there is no electronic noise or power source noise introduced into the dose measurement system / method by the sensor device 300, e.g. by the discharge of batteries in the surrounding electronics. Further, there is no need to provide power infrastructure or data infrastructure (e.g. cables) to the sensor device 300. Moreover, the sensor device of the present disclosure may be more accurate because it is less susceptive to contamination (e.g. compared to photodetector-based methods). Moreover, the system / method of the present disclosure may be more reliable than, e.g., photoresist-based methods of dose measurement, because the uncertainty introduced in exposure measurements by the variation in characteristics of the photoresist in photoresist-based methods is not present in the method / system of the present disclosure.

[0104] The dose measurement device 300 of the present disclosure may be manufactured using processing suitable for manufacturing microelectromechanical systems (MEMS). MEMS are miniaturized mechanical and electromechanical elements that are made using semiconductor fabricating techniques, e.g. lithography.

[0105] A lithographic apparatus in accordance with the present invention may be used for the manufacture of ICs.

[0106] Although specific reference may be made in this text to the use of a 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, liquidcrystal displays (LCDs), thin -film magnetic heads, etc.

[0107] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented by instructions stored on a machine-readable medium, which may be read and executedby 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 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. and in doing that may cause actuators or other devices to interact with the physical world.

[0108] 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.

[0109] 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, where the context allows, is not limited to optical lithography.

[0110] The methods of the present invention may be performed by computer systems comprising one or more computers. A computer used to implement the invention may comprise one or more processors, including general purpose CPUs, graphical processing units (GPUs), tensor processing units (TPU) or other specialised processors. A computer used to implement the invention may be physical or virtual. A computer used to implement the invention may be a server, a client or a workstation. Multiple computers used to implement the invention may be distributed and interconnected via a network such as a local area network (EAN) or wide area network (WAN). Individual steps of the method may be carried out by a computer system but not necessarily the same computer system. The methods of the invention may be carried out on a computer system located on the same vehicle as the sensor unit or remote from the vehicle. Results of a method of the invention may be displayed to a user or stored in any suitable storage medium. The present invention may be embodied in a non-transitory computer- readable storage medium that stores instructions to carry out a method of the invention. Any suitable programming language may be used to implement the invention. The present invention may be embodied in a computer system comprising one or more processors and memory or storage storing instructions to carry out a method of the invention.

[0111] Aspects of the invention are described in the following numbered clauses.1. A dose measurement system for a lithographic apparatus, the dose measurement system comprising:a target having a diffraction grating formed thereon, wherein the target is configured to undergo thermal expansion when irradiated with radiation, and a pitch of the diffraction grating increases with the thermal expansion of the target; a sensor configured to measure a parameter of the diffraction grating that is dependent on the pitch of the diffraction grating; and a processor configured to determine the dose of the radiation based on one or more measurements of the parameter of the diffraction grating made by the sensor.2. The dose measurement system of clause 1, wherein the parameter of the diffraction grating that is dependent on the pitch of the diffraction grating is the pitch of the diffraction grating.3. The dose measurement system of clause 1 or 2, wherein the target is supported on a base, and wherein an interface between contacting surfaces of the target and the base is configured to inhibit thermal contraction of the target.4. The dose measurement system of any of the preceding clauses, wherein the interface is configured to allow relative movement between the target and the base in a first direction, and to inhibit relative movement between the target and the base in a second direction, wherein the second direction is opposite to the first direction.5. The dose measurement system of clause 3 or 4, wherein the contacting surfaces of the target and the base comprise a surface formed from silicon and a surface formed from aluminium.6. The dose measurement system of any of clauses 3 to 5, wherein a topology of one or both of the contacting surfaces is modified.7. The dose measurement system of any of the preceding clauses, wherein the target is formed of a metal, optionally wherein the target is formed of aluminium or an alloy thereof.8. The dose measurement system of any of the preceding clauses, wherein the target comprises a coated region configured to receive the radiation.9. The dose measurement system of clause 8, wherein the coated region is configured to: absorb extreme ultra-violet radiation, deep ultra-violet radiation and infra-red radiation; absorb extreme ultra-violet radiation and deep ultra-violet radiation, and reflect infra-red radiation; or absorb extreme ultra-violet radiation, and reflect deep ultra-violet radiation and infra-red radiation.10. The dose measurement system of any of the preceding clauses, wherein the dose measurement system comprises a plurality of targets, the plurality of targets comprising two or more of: a first target comprising a coated region that is configured to absorb extreme ultra-violet radiation, deep ultra-violet radiation and infra-red radiation; a second target comprising a coated region that is configured to absorb extreme ultra-violet radiation and deep ultra-violet radiation, and reflect infra-red radiation; anda third target comprising a coated region that is configured to absorb extreme ultra-violet radiation, and reflect deep ultra-violet radiation and infra-red radiation.11. The dose measurement system of any of the preceding clauses, wherein the sensor is an alignment sensor.12. The dose measurement system of any of the preceding clauses, wherein the sensor is an interferometric sensor.13. A method of measuring a dose in a lithographic apparatus, the method comprising: increasing a pitch of a diffraction grating formed on a target by irradiating the target with radiation; measuring a parameter of the diffraction grating that is dependent on the pitch of the diffraction grating; and determining the dose of the radiation based on the measured parameter of the diffraction grating.14. The method of clause 13, wherein the parameter of the diffraction grating that is dependent on the pitch of the diffraction grating is the pitch of the diffraction grating.15. The method of clause 13 or 14, further comprising determining an increase in the pitch of the diffraction grating, and wherein the determining of the dose of the radiation is based on the determined increase in the pitch of the diffraction grating.16. The method of clause 15, wherein the determining of the increase in the pitch of the diffraction grating comprises measuring the pitch of the diffraction grating after the irradiating of the target with the radiation.17. The method of any of clauses 13 to 16, further comprising measuring the pitch of the diffraction grating before the irradiating of the target with the radiation.18. The method of any of clauses 13 to 17, wherein the irradiating of the target with radiation comprises irradiating a coated portion of the target with radiation.19. A sensor device for a dose measurement system, the sensor device comprising: a target having a diffraction grating formed thereon, wherein the target is configured to undergo thermal expansion when irradiated with radiation, and a pitch of the diffraction grating increases with the thermal expansion of the target; and a base on which the target is supported, wherein an interface between contacting surfaces of the target and the base is configured to inhibit contraction of the target after the target has undergone thermal expansion.20. The sensor device of clause 19, wherein the interface is configured to allow relative movement between the target and the base in a first direction, and to inhibit relative movement between the target and the base in a second direction, wherein the second direction is opposite to the first direction.21. The sensor device of clause 20, wherein the target moves in the first direction relative to the base when the target undergoes thermal expansion.22. The sensor device of any of clauses 19 to 21, wherein the target is fixedly coupled to the base at a first end portion of the target.23. The sensor device of clause 22, wherein the coated region is provided at the first end portion of the target.24. The sensor device of clause 22 or 23, wherein the diffraction grating is formed at a portion of the target other than the first end portion.25. A semiconductor substrate comprising the sensor device of any of clauses 19 to 24.26. The dose measurement system of any of clauses 1 to 12, wherein the dose measurement system comprises the sensor device of any of clauses 19 to 24.27. A lithographic apparatus comprising: the dose measurement system of any of clauses 1 to 12 or 26; an illumination system configured to direct radiation towards the target of the dose measurement system.28. The lithographic apparatus of clause 27, wherein the lithographic apparatus comprises an exposure station and a measurement station, and the illumination system is configured to direct radiation towards the target of the dose measurement system when the target is at the exposure station.29. The lithographic apparatus of clause 28, wherein the sensor is configured to measure the parameter of the diffraction grating when the target is at the measurement station.30. The lithographic apparatus of clause 29, wherein the sensor is configured to measure the parameter of the diffraction grating when the target is at the exposure station.31. A method of manufacturing a device comprising use of the lithographic apparatus of clause 24.32. A method of measuring a dose in a lithographic apparatus, the method comprising: increasing a pitch of a diffraction grating formed on a target by irradiating the target with radiation; inhibiting thermal contraction of the target; measuring a parameter of the diffraction grating that is dependent on the pitch of the diffraction grating; determining the dose of the radiation based on the measured parameter of the diffraction grating.33. The method of clause 32, wherein the inhibiting of the thermal contraction of the target comprises inhibiting relative movement between the target and a base on which the target is supported.34. The method of clause 32 or 33, further comprising, during the irradiating of the target with the radiation, allowing relative movement between the target and the base.35. The method of any of clauses 32 to 34, further comprising, after the irradiating of the target with radiation and before the determining of the increase in the pitch of the diffraction grating, moving the target from an exposure station to a measurement station.36. A computer program for a lithographic apparatus, the computer program comprising instructions which, when executed by a processor, cause the lithographic apparatus to perform the method of any of clauses 13 to 18 or 31 to 35.

[0112] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A dose measurement system for a lithographic apparatus, the dose measurement system comprising: at least one target having a diffraction grating formed thereon, wherein the target is configured to undergo thermal expansion when irradiated with radiation, and a pitch of the diffraction grating increases with the thermal expansion of the at least one target; a sensor configured to measure a parameter of the diffraction grating that is dependent on the pitch of the diffraction grating; and a processor configured to determine the dose of the radiation based on one or more measurements of the parameter of the diffraction grating made by the sensor.

2. The dose measurement system of claim 1, wherein the parameter of the diffraction grating that is dependent on the pitch of the diffraction grating is the pitch of the diffraction grating.

3. The dose measurement system of claim 1 or 2, wherein the target is supported on a base, and wherein an interface between contacting surfaces of the target and the base is configured to inhibit thermal contraction of the at least one target.

4. The dose measurement system of any of the preceding claims, wherein the interface is configured to allow relative movement between the target and the base in a first direction, and to inhibit relative movement between the at least one target and the base in a second direction, wherein the second direction is opposite to the first direction.

5. The dose measurement system of claim 3 or 4, wherein the contacting surfaces of the at least one target and the base comprise a surface formed from silicon and a surface formed from aluminium.

6. The dose measurement system of any of claims 1 to 4, wherein the target moves in the first direction relative to the base when the target undergoes thermal expansion.

7. The dose measurement system of any of the preceding claims, wherein the at least one target is formed of a metal, optionally wherein the target is formed of aluminium or an alloy thereof.

8. The dose measurement system of any of the preceding claims, wherein the at least one target comprises a coated region configured to receive the radiation configured to: absorb extreme ultra-violet radiation, deep ultra-violet radiation and infra-red radiation;absorb extreme ultra-violet radiation and deep ultra-violet radiation, and reflect infra-red radiation; or absorb extreme ultra-violet radiation, and reflect deep ultra-violet radiation and infra-red radiation.

9. The dose measurement system of any of the preceding claims, wherein the sensor is an alignment sensor.

10. The dose measurement system of any of the preceding claims, wherein the sensor is an interferometric sensor.

11. A method of measuring a dose in a lithographic apparatus, the method comprising: increasing a pitch of a diffraction grating formed on a target by irradiating the target with radiation; measuring a parameter of the diffraction grating that is dependent on the pitch of the diffraction grating; and determining the dose of the radiation based on the measured parameter of the diffraction grating.

12. The method of claim 11, wherein the parameter of the diffraction grating that is dependent on the pitch of the diffraction grating is the pitch of the diffraction grating.

13. The method of claim 12, further comprising determining an increase in the pitch of the diffraction grating by measuring the pitch of the diffraction grating after the irradiating of the at least one target with the radiation before the irradiating of the at least one target with the radiation, and wherein the determining of the dose of the radiation is based on the determined increase in the pitch of the diffraction grating.

14. The method of any of claims 12 and 13, wherein the irradiating of the target with radiation comprises irradiating a coated portion of the at least one target with radiation.

15. A lithographic apparatus comprising : the dose measurement system of any of claims 1 to 10; an illumination system configured to direct radiation towards the target of the dose measurement system.

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