Lithographic apparatus, calibration reticle, calibration method and device manufacturing method

Abstract

A lithographic apparatus is described, the apparatus comprising: a patterning device holder for a patterning device configured to impart a pattern to a beam of radiation; a substrate holder configured to hold a substrate; and a projection system configured to project the beam of radiation onto the substrate holder; a plurality of reference markers associated with the patterning device holder; and a sensor apparatus associated with the substrate holder, the sensor apparatus having a periodic pattern having a nominal pitch; wherein the plurality of reference markers includes a first reference marker having a first pitch corresponding to the nominal pitch and a second reference marker having a second pitch corresponding to the sum of the nominal pitch and an error offset value.

Description

LITHOGRAPHIC APPARATUS, CALIBRATION RETICLE,CALIBRATION METHOD AND DEVICE MANUFACTURING METHODCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24164108.3 which was filed on 18 March 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a lithographic apparatus, a calibration reticle, a calibration method for a lithographic apparatus and a method of manufacturing a device.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 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 , 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 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] It is desirable to measure aberrations in the projection system of a lithographic apparatus (the optical system which projects the beam patterned by the patterning device onto the substrate). A method and apparatus for measuring such aberrations is disclosed in WO2019149468A1, which document is incorporated by reference in its entirety. As disclosed therein, radiation beams diffracted by a diffraction grating at reticle level are projected onto a sensor at substrate level, the sensor including a radiation detector and a transmissive diffraction grating corresponding to the diffraction grating at reticle level. A suitable transmissive diffraction grating and a method of making it are disclosed in WO2021259745A1, which document is incorporated by reference in its entirety.SUMMARY OF THE INVENTION

[0008] It has been discovered that both the process of manufacturing the transmissive diffraction grating and exposure of it to EUV radiation in the lithographic apparatus can cause distortion of it, leading to errors in the measurement of aberrations of the projection optical system. An aim of the present invention is to improve the measurement of aberrations of the projection system by reducing or ameliorating the effects of distortions of the transmissive diffraction grating.

[0009] According to an aspect of the present invention, there is provided a lithographic apparatus comprising: a patterning device holder for a patterning device configured to impart a pattern to a beam of radiation; a substrate holder configured to hold a substrate; and a projection system configured to project the beam of radiation onto the substrate holder; a plurality of reference markers associated with the patterning device holder; and a sensor apparatus associated with the substrate holder, the sensor apparatus having a periodic pattern having a nominal pitch; wherein the plurality of reference markers includes a first reference marker having a first pitch corresponding to the nominal pitch and a second reference marker having a second pitch corresponding to the sum of the nominal pitch and an error offset value.

[0010] According to an aspect of the present invention, there is provided a method of calibrating a lithographic apparatus having a sensor apparatus associated with a substrate holder of the lithographicapparatus and a plurality of reference markers associated with a patterning device holder of the lithographic apparatus, the method comprising: measuring aberrations of a projection system of the lithographic apparatus using a selected reference marker and the sensor apparatus, wherein the reference markers have respective pitches differing by an error offset value and the selected reference marker has a pitch closely corresponding to the actual pitch of a periodic pattern of the sensor apparatus.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] 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.

[0012] Figure 1 schematically depicts a lithographic apparatus.Figure 2 schematically depicts a more detailed view of the lithographic apparatus.Figure 3 schematically depicts an aberration sensing system in a projection system lithographic apparatus.Figure 4 schematically depicts the arrangement of reference markers and a transmissive diffraction grating in the aberration sensing system of Figure 3.Figures 5A to 5C depict a process for manufacture of a transmissive diffraction grating.Figure 6 schematically depicts a method for measuring aberrations using a plurality of reference markings at reticle level.Figure 7 is a plan view of transmissive diffraction grating usable in methods of the invention.Figure 8 is a cross-sectional view of the transmissive diffraction grating of Figure 7.Figure 9 is a plan view of another diffraction optical grating usable in embodiments of the present invention.Figure 10 is a cross-sectional view of the transmissive diffraction grating of Figure 9.

[0013] 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. Like parts in different figures are indicated by like references.DETAILED DESCRIPTION

[0014] Figure 1 schematically depicts a lithographic apparatus 100 including a source collector module 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] As here depicted, the lithographic apparatus 100 is of a reflective type (e.g., employing a reflective mask).

[0021] The lithographic apparatus 100 may be of a type having two (dual stage) or more substrate tables WT (and / or two 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.

[0022] Referring to Figure 1, the illumination system IL receives an extreme ultraviolet 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 SO may be separate entities, for example when a CO2 laser is used to provide the laser beam for fuel excitation.

[0023] 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 source collector module 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 source collector module SO, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0024] 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-outcr 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.

[0025] 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 being reflected 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 toposition 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.

[0026] 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 non-volatile 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.

[0027] Figure 2 shows the lithographic apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. An EUV radiation emitting plasma 210 may be formed by a plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the radiation emitting plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0028] The radiation emitted by the radiation emitting plasma 210 is passed from a source chamber 211 into a collector chamber 212.

[0029] The collector chamber 212 may include a radiation collector CO. Radiation that traverses the radiation collector CO can 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 SO is arranged such that the virtual source point 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.

[0030] 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 unpatterned 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 unpatterned beam 21 at the patterning device MA, held by the support structure MT, a patterned beam 26 isformed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the substrate table WT.

[0031] More elements than shown may generally be present in the illumination system IL and the projection system PS. 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.

[0032] Alternatively, the source collector module SO may be part of an LPP radiation system.

[0033] As depicted in Figure 1, in an embodiment the lithographic apparatus 100 comprises an illumination system IL and a projection system PS. The illumination system IL is configured to emit a radiation beam B. The projection system PS is separated from the substrate table WT by an intervening space. The projection system PS is configured to project a pattern imparted to the radiation beam B onto the substrate W. The pattern is for EUV radiation of the radiation beam B.

[0034] The space intervening between the projection system PS and the substrate table WT can be at least partially evacuated. The intervening space may be delimited at the location of the projection system PS by a solid surface from which the employed radiation is directed toward the substrate table WT.

[0035] Figure 3 is a schematic diagram illustrating the principle of operation of an interferometric aberration sensor system 300. A reference mark 301, 302 is imaged by the projection system (represented by reflective elements 28, 30) onto a sensor device 310 at substrate level. The reference mark may be provided on a calibration device 303, which is physically similar to a patterning device used in an exposure process and can be held by the device holder (support structure MT), or may be a reference mark 302 provided on a fiducial 304 mounted to the support structure MT. The sensor device 310 comprises a transmissive diffraction grating 311 and a sensor 312. Sensor device 310 is mounted on or incorporated in substrate table WT.

[0036] Reference marks 301, 302 are associated with the patterning device, in that they are positionable at the same location in the path of the radiation beam 21 as a patterning device is during exposure of substrates, i.e. at the object plane of the projection system. The sensor device 310 is associated with the substrate in that it is positionable at the same location in the path of the radiation beam 21 as is a substrate during the exposure of substrates, i.e. at the image plane of the projection system.

[0037] When the calibration system is used, the radiation beam is directed onto one of the reference marks 301, 302, which comprises a diffraction grating that creates three diffraction orders, a plus first order (+lst), a zeroth order (0th) and a minus first order (-1th). The three diffraction orders are then projected onto the sensor device 310 by the projection system PS (represented in Figure 3 by reflective elements 28, 30). The three diffraction orders are combined by the transmissive diffraction grating 311 so that a single image is recorded by sensor 312. Sensor 312 may be a sensor that directly detects incident EUV radiation or may be a sensor that detects radiation of another wavelength, e.g.visible light, that is covered by a scintillator. Each of the diffraction orders transverses a different path through the projection system and the image recorded by sensor 312 effectively measures the path length difference between the zero first order and the plus first order and between the minus first order and the zero first order. This measurement is referred to as the sheared wavefront.

[0038] As shown in Figure 4, each of the reference marks 301 may comprise separate orthogonal gratings 301a, 301b, e.g. an X-grating and a Y-grating, which are projected onto the transmissive diffraction grating 311 by projection system PS. The transmissive diffraction grating 311 is a two dimensional rectilinear array of holes, with the array oriented parallel to the directions of the gratings as projected. The sheared wavefront measurement is performed for both orthogonal directions.

[0039] From the sheared wavefront measurements, the aberration of the projection system can be determined, e.g. in the form of Zernike polynomials. The process for determining aberrations is described in more detail in WO2019149468A1, which document is incorporated by reference in its entirety. Other methods of measuring aberrations using two gratings, one at the object plane of the projection system and one at the image plane, may also be used.

[0040] It will be appreciated that for the transmissive diffraction grating 311 to recombine the diffraction orders that are created by the reference marks 301, 302, the transmissive diffraction grating 311 should have a pitch that corresponds to the pitch of the reference mark. This means that if the projection system PS has a magnification M other than 1 (e.g. 1 / 4 or 1 / 5), the pitch of the transmissive diffraction grating 311 is desirably equal to M x the pitch of the reference grating 301, 302. If the projection system PS is anamorphic (that is has different magnifications in different directions), the pitches of the X-grating and Y-grating forming the reference mark 301, 302 may be different and / or the transmissive diffraction grating 311 may have different pitches in different directions.

[0041] Figures 5 A to 5C depict a method of manufacturing a transmissive diffraction grating 311. The transmissive diffraction grating is ultimately defined by an aluminum nitride (AIN) layer 401 which is attached to a silicon 406 via an Si sN i layer 405. Other materials may be used in place of AIN. A buffer layer 402 is applied above AIN layer 401. The grating pattern is defined e.g. through lithography, in a chrome layer 403. 404 depicts the remnants of resist used to pattern the chrome layer 403. The assembly as depicted in Figure 5A Reference is subjected to a first etch process which removes the buffer layer and the AIN layer in the areas not protected by the patterned chrome layer 403. This achieves the situation illustrated in Figure 5B. A second etch is then performed, the second etch being selective to etch the chrome layer 403 and buffer layer 402 but not the AIN layer 401. This then arrives at the situation depicted in Figure 5C, in which the original pattern of the chrome layer has been transferred into the AIN layer as an array of through-holes.

[0042] During the process of forming the transmissive diffraction grating, the AIN layer 401 is placed under tension in order to ensure that it is flat. The tension applied to the AIN layer 401 may be very high, e.g. 650 MPa or more. The process of forming the holes to define the transmissive diffraction grating 310 reduces the local stiffness of the AIN membrane. Because material is removedfrom the AIN layer 401, its stiffness is reduced. The open area ratio of the transmission diffractive grating 311 (that is the ratio of the holes to the total area across which the holes are disposed) is quite high, e.g. up to 50%, so the reduction in stiffness is significant. Due to the application of the tensile stress, the change in stiffness of the AIN layer 401 causes a distortion. In particular, the AIN layer 401 may expand (be stretched) in the area where the holes have been formed. This may result in a deviation in the pitch of the transmissive diffraction grating of the order of 1.5 parts per 1000 (i.e. 0.15%). Although this seems a small amount, it has a significant and undesirable effect on the measurements made by the interferometric aberration sensor system 300. Furthermore, the actual deviation in the pitch of the transmissive diffraction grating 311 from its nominal pitch is not easily predictable. A further issue is that the inventors have determined that the transmissive diffraction grating 311 changes with time and exposure to EUV radiation. Thus, errors in the measurements of the interferometric aberration sensor 300 may increase with time due to increasing distortion of the transmissive diffraction grating 311. Following exposure to EUV, distortion of the transmissive diffraction grating 311 may be 3 parts per 1000 or more.

[0043] The present invention proposes to provide multiple reference marks associated with the patterning device holder that have different pitches. The different pitches of the reference marks include at least one reference mark having a pitch corresponding to the nominal pitch of the transmission diffraction grating as well as additional reference marks having pitches corresponding to expected or possible actual pitches of the transmissive diffraction grating. For example, the gratings may have pitches corresponding to Pn. + N.E where Pnis the nominal pitch of the transmissive diffraction grating, N is an integer (e.g. in the range of from 0 to 10) and E is an error offset value. E may be in the range of from 0.1 per 1000 to 10 per 1000 of the nominal pitch Pn. In an embodiment, E is less than or equal to about 10 / 1000, desirably less than or equal to about 5 / 1000, more desirably less than or equal to about 2.5 / 1000, of the nominal pitch of the periodic pattern. For example E may be 5 / 10,000 or 25 / 10,000 of the nominal pitch of the periodic pattern.

[0044] As mentioned above, the pitches of the X-grating and Y-grating making up each reference mark may be different. Furthermore, the distortion of the transmissive diffraction grating may be different in X and Y directions, e.g. because the membrane is not square. Therefore, the pitches of the X-grating and Y-grating making up the additional reference marks may be different. In the case where the gratings additional reference marks have pitches corresponding to Pn. + N.E, there may be different values of Pnand E for X-gratings and Y-gratings. These may be denoted P„x, Pny, Ex, and EY.

[0045] It should be noted that the interferometric aberration sensor may also have reference markers associated with the patterning device holder that have pitches corresponding to integer multiples of the pitch of the transmissive diffraction grating 311 as well as those reference marks which correspond to slight deviations from the nominal pitch.

[0046] A method according to the invention is described with reference to Figure 6. First, the actual pitch of the transmissive diffraction grating 311 is determined. One way of determining the actual pitch of the transmissive diffraction grating 311 is to measure it when manufactured or when installed in the lithographic apparatus. Approaches to measuring the diffraction grating in the lithographic apparatus are described below. Before installation in in the lithographic apparatus the actual pitch may be measured using a scanning electron microscope (SEM). The actual pitch of the transmissive diffraction grating 311 may be stored in the lithographic apparatus as a reference value which is updated periodically. In many cases, the actual pitch of the transmissive diffraction grating 311 will change much more slowly than aberrations of the projection system are to be measured so that determination of the actual pitch of the transmissive diffraction grating 311 may comprise retrieving a stored reference value.

[0047] Based on the determined actual pitch of the transmissive diffraction grating 311, one reference mark (e.g. comprising a pair of orthogonal gratings) is selected from a plurality of reference marks 301-1 to 301-4 of different pitches provided on a calibration device and used for measurement of aberrations in the known method. An X-grating may be selected independently of the Y-grating, i.e. The X-grating of a first reference mark is selected and the Y-grating of a second, different, reference mark is selected. In an embodiment, all of the plurality of reference marks 301-1 to 301-4 are illuminated by the EUV radiation beam and their images are projected to the substrate plane and the sensor 310 is positioned at the position of the image of the selected one of the plurality of reference marks 301-1 to 301-4. Alternatively, the position of the sensor 310 may be fixed and the calibration device is positioned so that an image of the selected one of the plurality of reference marks 301-1 to 301-4 is projected to the position of the sensor 310. Measurements are taken using the known process for determining aberrations.

[0048] This results in a more accurate determination of aberrations than if a reference mark having a pitch corresponding to the nominal pitch of the transmissive diffraction grating 311 were to be used. It will be appreciated that the number of marks is finite so that it may not be possible to choose a reference mark that has pitch exactly corresponding to the actual pitch of transmissive diffraction grating 311. Therefore, some error in aberration measurement may remain, but that error will be less than would be the case where only a reference mark corresponding to the nominal pitch of the transmissive diffraction grating is available.

[0049] A further advantage of the present invention is that the determined actual pitch of the transmissive diffraction grating may be used in the method for calculating aberrations based on the measurement results of the interferometric aberration sensor. This can result in a more accurate calculation of aberrations than if the nominal pitch is used.

[0050] Figures 7 and 8 depict a transmissive diffraction grating 311 that is usable in embodiments of the invention in plan and cross-section respectively. As can be seen in Figure 7, the transmissive diffraction grating 311 has a regular array of holes 315 disposed within a circular area 316. Thecircular area 316 corresponds to the size of the projected image of a reference mark plus a suitable margin. No holes are formed in the area outside the circular region 316 in order to reduce manufacturing costs.

[0051] Figure 8 depicts a cross-section of the grating and a representation of the elasticity of different parts of the grating as a series connection of three springs with elastic modulus El, E2, El. The removal of material from the array in which the transmissive diffraction grating is formed reduces the effective modulus of the membrane within the circular region 316. The modulus of the membrane without holes is El, the modulus of the region 316 after holes 315 have been formed is E2 where E2 less than El. This means that when a tension force is applied to the membrane, as is necessary to maintain the membrane flat during manufacture, the membrane in the region 316 will stretch more than the membrane outside the region 316. This is the source of the problem described above.

[0052] Figures 9 and 10 depict an alternative approach to solution of the problem of distortion of the transmissive diffraction grating 311. This approach can reduce the deformation of the sensor grating, in particular due to the applied strain in manufacture, and can therefore be used with the above described approach to reduce the number of gratings with different pitches that are needed at reticle level. In the transmissive diffraction grating 311’ illustrated in Figure 9, holes 315 are formed throughout the area of the membrane 318 that is unsupported in frame 319, that is the area indicated by dashed square 317. Desirably, holes 315 are formed in the entirety of the membrane 318. Because holes 315 are uniformly arrayed across the entirety of membrane 318, the membrane has the same modulus of elasticity, E2, across its whole unsupported area. Therefore, there is no, or very much reduced, distortion of the array of holes forming the transmissive diffraction grating 311 as a result of the tension applied to the membrane 318.

[0053] The transmissive diffraction grating 311’ illustrated in Figure 9 may be incorporated into a sensor that is incorporated in, or mounted on, a substrate stage or substrate holder. The transmissive diffraction grating 311’ may be used as a calibration reference.

[0054] To measure the actual pitch of the transmissive diffraction grating 311 when installed in the lithographic apparatus, the following process can be used. An image, e.g. of the reference mark associated with the patterning device holder is projected onto the transmissive diffraction grating 311. The projected image is smaller than the transmissive diffraction grating 311, at least in the direction in which the pitch is to be measured. The signal output by sensor 312 is measured whilst the sensor device 310 is moved in the direction in which the pitch of the transmissive diffraction grating 311 is to be measured. The signal output by sensor 312 will vary cyclically as the sensor device 310 moves, due to changing alignments between light and dark areas of the projected image and the holes of the transmissive diffraction grating. By determining the period of the cyclic variation output by the sensor 312 in terms of distance moved by the sensor 310, the actual pitch of the transmissive diffraction grating 311 can be determined straightforwardly. This measurement may be repeated intwo orthogonal directions to determine the pitch of the transmissive diffraction grating 311 in two orthogonal directions.

[0055] 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.

[0056] 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 executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic 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.

[0057] 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.

[0058] 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.

[0059] Aspects of the invention are described in the following numbered clauses.1. A lithographic apparatus comprising: a patterning device holder for a patterning device configured to impart a pattern to a beam of radiation; a substrate holder configured to hold a substrate; and a projection system configured to project the beam of radiation onto the substrate holder; a plurality of reference markers associated with the patterning device holder; and a sensor apparatus associated with the substrate holder, the sensor apparatus having a periodic pattern having a nominal pitch;wherein the plurality of reference markers includes a first reference marker having a first pitch corresponding to the nominal pitch and a second reference marker having a second pitch corresponding to the sum of the nominal pitch and an error offset value.2. The lithographic apparatus of clause 1, wherein the plurality of reference markers includes a plurality of further reference markers each having a pitch corresponding to the sum of the nominal pitch and a respective integer multiple of the error offset value.3. The lithographic apparatus of clause 1 or 2, wherein the reference markers are formed on a patterning device configured to be held by the patterning device holder.4. The lithographic apparatus of clause 1 or 2, wherein the reference markers are formed on a fiducial mounted to a stage supporting the patterning device holder.5. The lithographic apparatus of any one of the preceding clauses, wherein the periodic pattern of the sensor apparatus comprises a membrane having an array of holes extending across substantially all of an unsupported area thereof.6. A calibration device for a lithographic apparatus, the calibration device being configured to be held by a patterning device holder and having a plurality of reference markers, wherein the plurality of reference markers includes a first reference marker having a first pitch corresponding to a nominal pitch of a periodic pattern of a sensor apparatus and a second reference marker having a second pitch corresponding to the sum of the nominal pitch and an error offset value.7. The calibration device of clause 6, wherein the plurality of reference markers includes a plurality of further reference markers each having a pitch corresponding to the sum of the nominal pitch and a respective integer multiple of the error offset value.8. A method of calibrating a lithographic apparatus having a sensor apparatus associated with a substrate holder of the lithographic apparatus and a plurality of reference markers associated with a patterning device holder of the lithographic apparatus, the method comprising: measuring aberrations of a projection system of the lithographic apparatus using a selected reference marker and the sensor apparatus, wherein the reference markers have respective pitches differing by an error offset value and the selected reference marker has a pitch closely corresponding to the actual pitch of a periodic pattern of the sensor apparatus.9. The method of clause 8, wherein the plurality of reference markers includes reference markers each having a pitch corresponding to the sum of the nominal pitch and a respective integer multiple of the error offset value.10. The method of clause 8 or 9, wherein the reference markers are formed on a patterning device configured to be held by the patterning device holder.11. The method of clause 8 or 9, wherein the reference markers are formed on a fiducial mounted to a stage supporting the patterning device holder.12. The method of any of clauses 8 to 11, further comprising determining the actual pitch of a periodic pattern of a sensor apparatus and selecting the selected reference marker on the basis of the determined actual pitch.13. The method of clause 12, wherein determining the actual pitch comprises: making a first measurement using one of the reference markers and the sensor apparatus whilst the sensor apparatus is at a first position; moving the sensor apparatus to a second position; and making a second measurement using the one of the reference markers and the sensor apparatus whilst the sensor apparatus is at the second position.14. The method of clause 13, wherein determining the actual pitch comprises retrieving a predetermined value.15. The method of any of clauses 8 to 14, wherein measuring aberrations comprises measuring a sheared wavefront and calculating a Zernike fitted wavefront with reference to the determined actual pitch.16. The method of any of clauses 8 to 14, wherein measuring aberrations of the projection system comprises making a plurality of measurements using different reference markers and the sensor apparatus; and selecting one of the measurements.17. The apparatus, device or method according to any one of the preceding clauses wherein the error offset value is less than or equal to about 10 / 1000, desirably less than or equal to about 5 / 1000, more desirably less than or equal to about 2.5 / 1000, of the nominal pitch of the periodic pattern.18. A device manufacturing method comprising: calibrating a lithographic apparatus using the method of any of clauses 8 to 17 to obtain a measurement of aberrations; adjusting an operating parameter of the lithographic apparatus on the basis of the measurement of aberrations; using the lithographic apparatus to impart a pattern to a substrate.While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A lithographic apparatus comprising: a patterning device holder for a patterning device configured to impart a pattern to a beam of radiation; a substrate holder configured to hold a substrate; and a projection system configured to project the beam of radiation onto the substrate holder; a plurality of reference markers associated with the patterning device holder; and a sensor apparatus associated with the substrate holder, the sensor apparatus having a periodic pattern having a nominal pitch; wherein the plurality of reference markers includes a first reference marker having a first pitch corresponding to the nominal pitch and a second reference marker having a second pitch corresponding to the sum of the nominal pitch and an error offset value.

2. The lithographic apparatus of claim 1, wherein the plurality of reference markers includes a plurality of further reference markers each having a pitch corresponding to the sum of the nominal pitch and a respective integer multiple of the error offset value.

3. The lithographic apparatus of claim 1 or 2, wherein the reference markers are formed on a patterning device configured to be held by the patterning device holder.

4. The lithographic apparatus of claim 1 or 2, wherein the reference markers are formed on a fiducial mounted to a stage supporting the patterning device holder.

5. The lithographic apparatus of any one of the preceding claims, wherein the periodic pattern of the sensor apparatus comprises a membrane having an array of holes extending across substantially all of an unsupported area thereof.

6. A calibration device for a lithographic apparatus, the calibration device being configured to be held by a patterning device holder and having a plurality of reference markers, wherein the plurality of reference markers includes a first reference marker having a first pitch corresponding to a nominal pitch of a periodic pattern of a sensor apparatus and a second reference marker having a second pitch corresponding to the sum of the nominal pitch and an error offset value.

7. The calibration device of claim 6, wherein the plurality of reference markers includes a plurality of further reference markers each having a pitch corresponding to the sum of the nominal pitch and a respective integer multiple of the error offset value.

8. A method of calibrating a lithographic apparatus having a sensor apparatus associated with a substrate holder of the lithographic apparatus and a plurality of reference markers associated with a patterning device holder of the lithographic apparatus, the method comprising: measuring aberrations of a projection system of the lithographic apparatus using a selected reference marker and the sensor apparatus, wherein the reference markers have respective pitches differing by an error offset value and the selected reference marker has a pitch closely corresponding to the actual pitch of a periodic pattern of the sensor apparatus.

9. The method of claim 8, wherein the plurality of reference markers includes reference markers each having a pitch corresponding to the sum of the nominal pitch and a respective integer multiple of the error offset value.

10. The method of claim 8 or 9, wherein the reference markers are formed on a patterning device configured to be held by the patterning device holder or wherein the reference markers are formed on a fiducial mounted to a stage supporting the patterning device holder.

11. The method of any of claims 8 to 10, further comprising determining the actual pitch of a periodic pattern of a sensor apparatus and selecting the selected reference marker on the basis of the determined actual pitch.

12. The method of any of claims 8 to 11, wherein measuring aberrations comprises measuring a sheared wavefront and calculating a Zernike fitted wavefront with reference to the determined actual pitch.

13. The method of any of claims 8 to 11, wherein measuring aberrations of the projection system comprises making a plurality of measurements using different reference markers and the sensor apparatus; and selecting one of the measurements.

14. The apparatus, device or method according to any one of the preceding claims wherein the error offset value is less than or equal to about 10 / 1000, desirably less than or equal to about 5 / 1000, more desirably less than or equal to about 2.5 / 1000, of the nominal pitch of the periodic pattern.

15. A device manufacturing method comprising: calibrating a lithographic apparatus using the method of any of claims 8 to 13 to obtain a measurement of aberrations;adjusting an operating parameter of the lithographic apparatus on the basis of the measurement of aberrations; using the lithographic apparatus to impart a pattern to a substrate.

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