WO2017050508A1

WO2017050508A1 - Scanning measurement system

Info

Publication number: WO2017050508A1
Application number: PCT/EP2016/069921
Authority: WO
Inventors: Robbert Jan Voogd; Johannes Jacobus Matheus Baselmans; Bearrach Moest; Roland Johannes Wilhelmus STAS; Jean-Philippe Xavier VAN DAMME
Original assignee: Asml Netherlands B.V.
Priority date: 2015-09-24
Filing date: 2016-08-24
Publication date: 2017-03-30
Also published as: NL2017356A

Abstract

Controlling, based on characteristics, a lithographic apparatus having an exposure mode configured to expose a wafer held by a substrate table to an image of a pattern on a production reticle via a projection system, wherein in the exposure mode the production reticle is held at a reticle stage and is protected by a pellicle, the method comprising determining the characteristics of the projecting in a calibration mode, and the controlling comprising moving in the exposure mode at least one of the projection system, the reticle stage and the substrate table during the exposing in dependence on the characteristics.

Description

Scanning Measurement System

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 15186672.0 which was filed on 24 September 2015 and EP application 15199980.2 which was filed on 15 December 2015, and which are incorporated herein in their entirety by reference.

FIELD

[0002] The present invention relates to a measurement system capable of determining parameters such as, for example, overlay errors caused by distortion and/or translation of a reticle in an imaging system, and/or a deformation of a transparent film in an imaging system. The measurement system may be used in connection with a lithographic apparatus. BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a reticle, which is alternatively referred to as a mask or a patterning device, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation- sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed to a beam of radiation. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a beam of radiation in a given direction (the "scanning"-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.

[0004] The act of scanning the pattern through a beam of radiation may distort the reticle and/or cause the reticle to slip from its intended position. Distortions of the reticle and/or the reticle slipping from its intended position may cause overlay errors to occur at the substrate.

[0005] The reticle may be protected from contamination by a transparent film known as a pellicle. The act of scanning the pattern through a beam of radiation may deform the pellicle. Deformations of the pellicle cause overlay errors to occur at the substrate. It is desirable to determine the presence of such overlay errors, amongst other parameters, in order to correct for them.

[0006] It is an object of the present invention to provide a system and method of determining, amongst other parameters, overlay errors caused by movement of the reticle and/or a deformation of a pellicle which at least partially addresses one or more of the problems of the prior art, whether identified herein or elsewhere.

SUMMARY

[0007] According to a first aspect described herein, there is provided a method of controlling a lithographic apparatus having an exposure mode configured to expose a wafer held by a substrate table to an image of a pattern on a production reticle via a projection system, wherein in the exposure mode the production reticle is held at a reticle stage and is protected by a pellicle. The method comprises determining characteristics of the projecting in a calibration mode. The determining comprises moving the reticle stage holding a further reticle protected by a further pellicle, the further reticle having a mark, illuminating the further reticle with radiation to form a patterned radiation beam, projecting the patterned radiation beam via the projection system onto a sensor to project a further image onto the sensor, moving the sensor to keep the further image incident on the sensor, sensing the projected further image as received at the sensor, and determining the characteristics of the sensed projected further image. The controlling comprises moving, in the exposure mode, at least one of the projection system, the reticle stage and the substrate table during exposure of a wafer in dependence on the characteristics. The reticle stage may be protected by a further pellicle in the calibration mode during the determining.

[0008] Performing a scanning motion while making measurements provides an accurate representation of the effects of a scanning motion that may occur in the exposure mode, such as, for example, fading effects and/or pellicle deformation effects. Using a sensor to measure errors, such as, for example, pellicle deformation effects is more convenient and less time consuming than carrying out a measurement exposure on a test die. Fitting a pellicle to the reticle and performing measurements while scanning the reticle allows pellicle deformation effects such as overlay errors to be emulated and measured in the calibration mode of a lithographic apparatus. Making measurements in the calibration mode that are representative of effects that occur in the exposure mode is advantageous as the measurements may be subsequently used to make corrections to and improve an exposure in the exposure mode, e.g., improved overlay.

[0009] It is to be understood that the mark is not limited to any particular design and may comprise any shape or simple pattern, etc. Movement of the reticle stage and the sensor during the determining may be initiated before radiation is received at the sensor.

[00010] Moving the reticle stage and the sensor before radiation is received at the sensor allows the reticle and sensor to reach the speeds that a production reticle and a wafer would reach in the exposure mode in time for measurement. Allowing the reticle and sensor to reach pre-determined speeds before measurements are carried out may assist in the emulation and measurement of effects such as dynamic effects.

[00011] Movement of the reticle stage during the determining may be initiated before radiation is received at the sensor and movement of the sensor may be initiated only when radiation is received at the sensor.

[00012] Moving the reticle stage before radiation is received at the sensor may allow the reticle to reach a pre-determined speed that allows for the emulation and measurement of effects such as, for example, pellicle deformation effects. Only moving the sensor whilst radiation is received at the sensor may reduce the amount of movement space required for the sensor whilst still enabling effects such as pellicle deformation effects and fading effects to be emulated and measured.

[00013] Movement of the reticle stage and the sensor during the determining may be initiated only when radiation is received at the sensor.

[00014] Only moving the reticle stage and the sensor whilst radiation is received at the sensor may reduce the amount of movement space required for the reticle stage and the sensor during measurement. Only moving the reticle stage and the sensor whilst radiation is received at the sensor may also reduce the amount of time required for a measurement to take place.

[00015] The sensor may comprise a detector grating configured to receive the patterned radiation and transmit an interference pattern to the sensor.

[00016] The detector grating may comprise a checkerboard grating.

[00017] The mark may be configured to diffract radiation passing through it.

[00018] The mark may comprise two mutually orthogonal gratings.

[00019] The further reticle may be a calibration reticle. Generally a calibration reticle does not have an exposure pattern present on its image field, which allows the mark, or multiple marks, to be placed at any desired position on the calibration reticle. [00020] The further reticle may be the production reticle. Generally a production reticle has an exposure pattern present on its image field and will be used in the exposure mode of a lithographic apparatus for the production of patterned devices such as, for example, ICs. Using a production reticle for measurements in the calibration mode of a lithographic apparatus is very representative of exposures in the exposure mode of a lithographic apparatus as the production reticle may be used for an exposure after the measurements have taken place. The more representative the measurement in the calibration mode is of an exposure in the exposure mode of a lithographic apparatus the more accurate measurements and subsequent corrections may be made.

[00021] The mark may be located outside an image field of the production reticle.

[00022] It is advantageous for the marks used for measurement scans to be located outside of the image field of the production reticle in order to avoid interfering with the exposure pattern already present on the image field of the production reticle.

[00023] The determined characteristics may comprise information about fading effects.

[00024] The determined characteristics may comprise information about pellicle deformation effects.

[00025] The determining may take place outside a lithographic apparatus e.g. on an independent measurement system.

[00026] According to a second aspect described herein there is provided a lithographic apparatus comprising an illumination system configured to condition a radiation beam, a reticle stage constructed to hold a reticle, the reticle having a pattern, a substrate table constructed to hold a wafer, a projection system configured to expose the wafer to an image of the pattern, a positioning apparatus configured to move the reticle stage and the substrate table, wherein the lithographic apparatus further comprises a computer readable medium for storing computer readable code wherein the code causes the lithographic apparatus to perform a method of controlling a lithographic apparatus in accordance with the first aspect. The reticle may be protected by a pellicle.

[00027] The positioning apparatus may be configured to initiate movement of the reticle stage and the sensor before optical alignment has occurred between the mark and the sensor.

[00028] Optical alignment may be defined as when patterned radiation would be incident on the sensor if radiation was incident on the mark. [00029] The positioning apparatus may be configured to move the reticle stage and the sensor through an extent greater than an extent of a radiation beam provided to the projection system.

[00030] An extent of a radiation beam may be defined as the distance between edges of the radiation beam.

[00031] The positioning apparatus may be configured to move the reticle stage before and during optical alignment between the mark and the sensor, and move the sensor only during optical alignment between the mark and the sensor.

[00032] The positioning apparatus may be configured to move the reticle stage through an extent greater than an extent of a radiation beam provided to the projection system, and move the sensor only through the extent of the radiation beam.

[00033] The positioning apparatus may be configured to move the reticle stage and the sensor only during optical alignment between the mark and the sensor.

[00034] The positioning apparatus may be configured to move the reticle stage and the sensor only through an extent of a radiation beam provided to the projection system.

[00035] According to a third aspect described herein, there is provided a computer readable medium for storing computer readable code wherein the code causes a lithographic apparatus to perform a method in accordance with the first aspect described above.

[00036] According to a fourth aspect described herein, there is provided a method of controlling a lithographic apparatus having an exposure mode configured to expose a wafer held by a substrate table to an image of a pattern on a production reticle via a projection system, wherein in the exposure mode the production reticle is held at a reticle stage. The method comprises determining characteristics of the projecting in a calibration mode. The determining comprises moving the reticle stage holding a further reticle, the further reticle having a mark, illuminating the further reticle with radiation to form a patterned radiation beam, projecting the patterned radiation beam via the projection system onto a sensor to project a further image onto the sensor, sensing the projected further image as received at the sensor and determining the characteristics of the sensed projected further image. The controlling comprises moving in the exposure mode at least one of the projection system, the reticle stage and the substrate table during the exposing in dependence on the characteristics.

[00037] Performing a scanning motion while making measurements provides an accurate representation of the effects of a scanning motion that may occur in the exposure mode, such as, for example, dynamic effects. Using a sensor to measure errors, such as, for example, dynamic effects is more convenient and less time consuming than carrying out a measurement exposure on a test die. Making measurements in the calibration mode that are representative of effects that occur in the exposure mode is advantageous as the measurements may be subsequently used to make corrections to and improve an exposure in the exposure mode, e.g., improved overlay. It is to be understood that the mark is not limited to any particular design and may comprise any shape or simple pattern, etc.

[00038] The further reticle may be protected by a pellicle.

[00039] Fitting a pellicle to the reticle and performing measurements while scanning the reticle allows pellicle deformation effects such as overlay errors to be emulated and measured in the calibration mode of a lithographic apparatus.

[00040] Movement of the reticle stage during the determining may be initiated before radiation is received at the sensor.

[00041] Moving the reticle stage before radiation is received at the sensor allows the reticle to reach the speeds that a production reticle would reach in the exposure mode in time for measurement. Allowing the reticle to reach pre-determined speeds before measurements are carried out may assist in the emulation and measurement of effects such as dynamic effects and/or fading effects.

[00042] The sensor may comprise a detector grating configured to receive the projected patterned radiation beam and transmit the further image to the sensor.

[00043] The mark may be configured to diffract radiation passing through it.

[00044] The further reticle may be the production reticle. Generally a production reticle has an exposure pattern present on its image field and will be used in the exposure mode of a lithographic apparatus for the production of patterned devices such as, for example, ICs. Using a production reticle for measurements in the calibration mode of a lithographic apparatus is very representative of exposures in the exposure mode of a lithographic apparatus as the production reticle may be used for an exposure after the measurements have taken place. The more representative the measurement in the calibration mode is of an exposure in the exposure mode of a lithographic apparatus, the more accurate measurements and subsequent corrections may be made.

[00045] The further reticle may be a calibration reticle. Generally a calibration reticle does not have an exposure pattern present on its image field, which allows the mark, or multiple marks, to be placed at any desired position on the calibration reticle.

[00046] The calibration mode may be a part of the exposure mode.

[00047] The mark may be located outside an image field of the production reticle. It is advantageous for the marks used for measurement scans to be located outside of the image field of the production reticle in order to avoid interfering with the exposure pattern already present on the image field of the production reticle.

[00048] The calibration measurement may be performed to position the detector grating in linear alignment with the further image prior to the determining. Linear alignment may comprise an alignment between the aerial image and the detector grating in which the detected intensity varies linearly with the position of the detector in a direction perpendicular to the movement of the reticle.

[00049] The mark may comprise a grating line that extends along a direction parallel to the movement of the reticle.

[00050] The detector grating may comprise a first grating line that corresponds to the grating line of the mark. For example, the first grating line may be aligned with the grating line of the mark in a direction perpendicular to a scanning direction of the reticle and in a direction parallel to the scanning direction of the reticle. The first grating line and the grating line of the mark may have different lengths in the scanning direction of the reticle. The first grating line may comprise a plurality of further grating lines that extend along a direction perpendicular to the movement of the reticle.

[00051] The mark may comprise one or more second grating lines that extend along a direction perpendicular to the movement of the reticle.

[00052] The sensor may be moved during the determining to keep the further image incident on the sensor.

[00053] Movement of the reticle stage during the determining may be initiated before radiation is received at the sensor and movement of the sensor may be initiated only when radiation is received at the sensor.

[00054] Moving the reticle stage before radiation is received at the sensor may allow the reticle to reach a pre-determined speed that allows for the emulation and measurement of effects such as, for example, dynamic effects. Only moving the sensor whilst radiation is received at the sensor may reduce the amount of movement space required for the sensor whilst still enabling effects such as dynamic effects and fading effects to be emulated and measured.

[00055] Movement of the reticle stage and the sensor during the determining may be initiated only when radiation is received at the sensor.

[00056] Only moving the reticle stage and the sensor whilst radiation is received at the sensor may reduce the amount of movement space required for the reticle stage and the sensor during measurement. Only moving the reticle stage and the sensor whilst radiation is received at the sensor may also reduce the amount of time required for a measurement to take place.

[00057] The detector grating may comprise a checkerboard grating.

[00058] The mark may comprise two mutually orthogonal gratings.

[00059] The determining may further comprise forming multiple patterned radiation beams and projecting them via the projection system onto multiple sensors to project multiple further images onto the sensors; sensing the projected further images as received at the sensors and determining the characteristics of the sensed further images.

[00060] Forming multiple patterned radiation beams through multiple marks, projecting multiple further images onto the sensors and sensing the further images reduces the amount of time required to perform a measurement across the whole reticle. That is, rather than illuminating one mark per measurement scan, multiple marks may be illuminated and measured simultaneously during a single measurement scan.

[00061] According to a fifth aspect described herein, there is provided a lithographic apparatus comprising an illumination system configured to condition a radiation beam, a reticle stage constructed to hold a reticle, the reticle having a pattern and being protected by a pellicle, a substrate table constructed to hold a wafer, a projection system configured to expose the wafer to an image of the pattern and a positioning apparatus configured to move the reticle stage and the substrate table. The lithographic apparatus further comprises a computer readable medium for storing computer readable code wherein the code causes the lithographic apparatus to perform the method described herein.

[00062] According to a sixth aspect described herein, there is provided a computer readable medium for storing computer readable code wherein the code causes a lithographic apparatus to perform the method described herein.

[00063] Various aspects and features of the invention set out above or below may be combined with various other aspects and features of the invention as will be readily apparent to the skilled person.

BRIEF DESCRIPTION OF THE DRAWINGS

[00064] In the accompanying drawings equivalent elements in different drawings may be provided with like reference numerals. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: Figure 1 schematically depicts a lithographic system comprising a scanning measurement system according to a particular example embodiment of the invention;

Figure 2 schematically depicts a pellicle fitted to a reticle;

Figure 3 schematically depicts a reticle with a pellicle fitted to at three different times (a), (b) and (c) during an exposure scan;

Figure 4 schematically depicts the effect of pellicle deformation on an intended optical path of incident radiation passing through a pellicle;

Figure 5 schematically depicts a static measurement system;

Figure 6 schematically depicts a scanning measurement system according to a particular example embodiment;

Figures 7, 8 and 9 schematically depict a scanning measurement system at three different times during a measurement scan according to a particular example embodiment;

Figure 10 schematically depicts a production reticle;

Figure 11 schematically depicts a calibration reticle;

- Figure 12 schematically depicts a target patterned region;

Figure 13 schematically depicts a detector grating;

Figure 14 (a)-(e) schematically depicts five different arrangements of a grating and a detector grating for making measurements in the x direction;

Figure 15 (a)-(e) schematically depicts five different arrangements of a grating and a detector grating for making measurements in the y direction;

Figure 16 schematically depicts an alternative scanning measurement system according to a particular example embodiment of the invention;

Figure 17 (a)-(c) schematically depicts an alignment between an aerial image of a reticle mark and a detector grating at three different times during a reference measurement; - Figure 18 (a)-(c) schematically depicts an alignment between a deformed aerial image of a reticle mark and a detector grating at three different times during a measurement scan;

Figure 19 schematically depicts an arrangement of a reticle having two marks and a sensor having two detector gratings;

Figure 20 schematically depicts another reticle mark and detector grating arrangement;

Figure 21 is a flowchart indicating the processes taken in using the scanning measurement system to preemptively correct for errors at an exposure stage of a lithographic apparatus. DETAILED DESCRIPTION

[00065] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[00066] The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 4-20 nm), as well as particle beams, such as ion beams or electron beams. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.

[00067] The term "reticle" used herein should be broadly interpreted as referring to a device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[00068] A reticle may be transmissive or reflective. Examples of reticle include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.

[00069] The support structure holds the reticle. It holds the reticle in a way that depends on the orientation of the reticle, the design of the lithographic apparatus, and other conditions, such as, for example, whether or not the reticle is held in a vacuum environment. The support can use mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the reticle is at a desired position, for example with respect to the projection system. Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "reticle".

[00070] The term "projection system" used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term "projection lens" herein may be considered as synonymous with the more general term "projection system".

[00071] The term "illumination system" used herein may encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the beam of radiation, and such components may also be referred to below, collectively or singularly, as a "lens".

[00072] The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

[00073] Figure 1 schematically depicts a lithographic apparatus according to a particular example embodiment. The apparatus comprises:

a. an illuminator IL to condition a beam PB of radiation (e.g. DUV radiation. b. a support structure (which may be referred to as a mask table) MT to support a reticle (e.g. a mask) MA and connected to first positioning device PM to accurately position the reticle with respect to item PL;

c. a substrate table (which may be referred to as a wafer table) WT2 for holding a substrate (e.g. a resist coated wafer) W2 and connected to second positioning device PW2 for accurately positioning the substrate with respect to item PL, the second positioning device PW2 comprising a sensor apparatus SA located beside the substrate W2; d. another substrate table WT1 for holding a substrate Wl and connected to third positioning device PW3 for accurately positioning the substrate with respect to an alignment system AS;

e. a processor PR configured to receive measurements from the sensor apparatus SA and determine alignment, wavefront aberration and error information from the measurements;

f. a controller CN configured to receive information from the processor PR and make corresponding adjustments to item PL; and

g. a projection system (e.g. a refractive projection lens) PL configured to image a pattern imparted to the radiation beam PB by reticle MA onto a target portion C (e.g. comprising one or more dies) of the substrate W2.

[00074] As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above).

[00075] The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising for example suitable directing mirrors and/or a beam expander. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[00076] The illuminator IL may comprise adjusting means AM for adjusting the angular intensity distribution of the beam. Generally, at least the outer and/or inner radial extent of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL generally comprises various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned beam of radiation PB, having a desired uniformity and intensity distribution in its cross section.

[00077] The radiation beam PB is incident on the reticle (e.g. mask) MA, which is held on the support structure MT. Having traversed the reticle MA, the beam PB passes through the lens PL, which focuses the beam onto a target portion C of the substrate W2. With the aid of the second positioning device PW2 and position sensor IF (e.g. an interferometric device), the substrate table WT2 can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted in figure 1) can be used to accurately position the reticle MA with respect to the path of the beam PB, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the tables MT and WT2 will be realized with the aid of a long-stroke module (coarse positioning) and a short- stroke module (fine positioning), which form part of the positioning device PM and PW2. However, in the case of a stepper (as opposed to a scanner) the support structure MT may be connected to a short stroke actuator only, or may be fixed.

[00078] The lithographic apparatus may for example move the reticle MA and the substrate W2 with a scanning motion when projecting the pattern from the reticle onto a target portion C. Cartesian coordinates are indicated in Figure 1. As is conventional, the z- direction corresponds with an optical axis of the radiation beam PB. In an embodiment in which the lithographic apparatus is a scanning lithographic apparatus, the y-direction corresponds with the direction of scanning motion.

[00079] As depicted, the lithographic apparatus may be of a type having two (dual stage) or more substrate tables WT1, WT2. In a dual stage lithographic apparatus two substrate tables WT1, WT2 are provided in order to allow properties of one substrate Wl to be measured while exposure of another substrate W2 is taking place ("exposure of a substrate" means projection of patterned radiation onto the substrate as described above).

[00080] The reticle MA may be protected from contaminant particles by a thin transparent film known as a pellicle. The pellicle is positioned at a distance from the reticle MA that is sufficient that any particles that are incident upon the surface of the pellicle are not in the focal plane of the radiation beam PB. This separation between the pellicle and the mask MA, acts to reduce the extent to which any particles on the surface of the pellicle impart a pattern to the radiation beam PB. It will be appreciated that where a particle is present in the beam of radiation PB, but at a position that is not in a focal plane of the beam of radiation PB (i.e., not at the surface of the mask MA), then any image of the particle will not be in focus at the surface of the substrate W. In some embodiments, the separation between the pellicle and the mask MA may, for example, be between 2 mm and 3mm (e.g. around 2.5 mm).

[00081] Figure 2 is a schematic diagram of a pellicle 2 fitted to a reticle MA. The reticle MA has a patterned surface 4. A pellicle frame 6 which supports the pellicle 2 is provided with an attachment mechanism 8. The attachment mechanism 8 may be configured to allow the pellicle frame 6 to be removably attachable to the mask MA (i.e. to allow the pellicle frame 6 to be attachable to and detachable from the mask MA). The attachment mechanism 8 is configured to engage with an attachment feature (not shown) provided on the mask MA. The attachment feature may, for example, be a protrusion which extends from the mask MA. The attachment mechanism 8 may, for example, comprise a locking member which engages with the protrusion and secures the pellicle frame 6 to the mask MA. A contamination particle 10 was incident upon the pellicle 2 and is held by the pellicle 2. The pellicle 2 holds the contamination particle 10 sufficiently far from the patterned surface 4 of the mask MA that it is not imaged onto substrates by the lithographic apparatus.

[00082] It will be appreciated that the pellicle 2 may be formed from different material depending upon the characteristics of the lithographic apparatus and that the choice of pellicle may affect the distortion induced image errors. For example, the lithographic apparatus of Figure 1 is a deep ultraviolet (DUV) lithography apparatus which uses DUV radiation to illuminate a reticle MA then the pellicle 2 used may be a DUV transparent pellicle, e.g. an amorphous fluoropolymer such as Cytop. Another type of lithography apparatus uses Extreme Ultraviolet (EUV) radiation (an example of which is described in, and depicted in Figures 1 and 2 of, US20150253679, "Lithographic Method and Apparatus", the entire contents of which are incorporated herein by reference). If a lithographic apparatus uses EUV radiation to illuminate a reticle MA then the pellicle of that lithographic apparatus may be an EUV transparent pellicle, and other suitable materials may be used to form the EUV transparent pellicle.

[00083] The pellicle 2 may, for example, be formed from a material such as polysilicon (pSi) film. Polysilicon (pSi) film is substantially transparent to EUV radiation. The pellicle 2 may alternatively be formed from some other material which is substantially transparent to EUV radiation, for example graphene, silicene, etc. By the terms EUV transparent pellicle or a film substantially transparent for EUV radiation herein it is meant that the pellicle 2 transmits at least 65% of incident EUV radiation, preferably at least 80% and more preferably at least 90% of incident EUV radiation. A capping layer which may help to reduce the effect of hydrogen radicals, plasma and traces of oxygen on the pellicle 2 may be provided. The capping layer may be provided both on the pellicle 2 and on the pellicle frame 6.

[00084] When the reticle MA is moved during a photolithographic exposure scan the reticle and/or other components of the lithographic apparatus may experience unwanted effects. For example, the reticle MA may distort as a result of the force exerted on it by the support structure MT when the support structure accelerates. The distortion of the reticle MA may distort the image of the reticle pattern on the wafer. The distortion of the reticle may result in unwanted magnification of the image of the reticle pattern on the wafer, i.e. magnification errors. The reticle MA may slip from its intended position as a result of the force exerted on it by the support structure MT when the support structure accelerates. The reticle slip may cause a translation of the image of the reticle pattern on the wafer from its intended position. The presence of contamination on the reticle MA may increase the amplitude and/or frequency of such reticle slips. These and other unwanted effects resulting from movement of the reticle MA may be known as dynamic effects.

[00085] Dynamic effects may include, for example, the induced oscillation of optics, such as projection lens PL components, due to forces generated by an acceleration of the support structure MT. The support structure MT holding the reticle MA may experience deformations as a result of its own acceleration. The positioning device PM comprises motors which enable movement of the reticle MA. The motors generate magnetic fields when accelerating the reticle MA. The magnetic fields may cause nearby metal to deform. That is, depending on the amplitude and direction of the magnetic field generated by the motors, nearby metallic components may deform, e.g. they may contract or expand. Magnetic deformation of metallic components caused by motors used to move the reticle MA may be considered to be a dynamic reticle effect. For example, magnetic deformation of metal components in a positioning device PM may affect the accuracy of the positioning device, which in turn may lead to imaging errors occurring at the substrate W2. Affected metal may remain deformed when the magnetic field is removed.

[00086] Referring to the DUV lithographic apparatus of figure 1 , when the reticle MA is moved during a photolithographic exposure scan the pellicle 2 may deform due to differences in local air pressures. When the reticle MA moves during a photolithographic exposure scan the local air pressures around the pellicle 2 may fluctuate and act on different parts of the pellicle 2. The fluctuating air pressures acting on the pellicle 2 cause deformation of the pellicle 2. Pellicle deformations may also be considered to be a dynamic reticle effect as they result from movement of the reticle MA. The deformation of the pellicle 2 depends on parameters such as the speed and direction of movement of the pellicle 2 during a photolithographic exposure scan. For EUV lithography apparatuses, pellicle deflections may be induced by causes other than gas pressure gradients, e.g., by the pellicle's inertia. For example, the pellicle of an EUV lithography apparatus may be attached along its sides to a frame that is moved together with the reticle. Owing to the stiffness of the pellicle's material the free area of the pellicle is pulled by the parts of the pellicle attached to the frame. Due to gravity and thermal expansion the pellicle may sag. In some embodiments, the reticle of an EUV lithography apparatus may be of a reflective type. Accordingly, pressure waves in the pellicle's material are induced by forces and torques that dynamically distort the radiation path twice: the incident light via the pellicle to the reticle and then the reflected light from the reticle via the pellicle to the downstream components of the lithography apparatus.

[00087] Figure 3 schematically depicts a reticle MA with a pellicle 2 fitted to it via a pellicle frame 6 at three different times (a), (b) and (c) during a photolithographic exposure scan. The reticle MA is scanned through a radiation beam PB in the positive y direction in the example of figure 3. The radiation beam PB is imparted with a pattern on passing through the reticle MA, and is then incident on a projection system PL configured to image the pattern imparted to the radiation beam PB onto a target portion of a substrate (not shown).

[00088] The images on the right-hand side of figure 3 are magnified schematic depictions of the reticle MA and pellicle 2 shown on the corresponding images on the left- hand side of figure 3. The images on the right-hand side of figure 3 provide examples of how the pellicle 2 may deform during an exposure scan. The deformation of the pellicle 2 may change throughout the exposure scan due to changing air pressures over the surface of the pellicle 2. As can be seen in figure 3, the deformation of the pellicle 2 is not uniform throughout the depicted exposure scan. A tangent line 5 to a portion of the pellicle 2 located beneath the radiation beam PB changes angle with respect to the radiation beam PB in each of the images (a), (b) and (c). In general the pellicle 2 deformation is neither uniform nor stable throughout an exposure scan. Deformation of the pellicle 2 is more likely to occur when the scan motion is relatively fast, such as, for example, when the reticle MA and pellicle frame 6 move at a speed of approximately 4 ms^"1.

[00089] Pellicle deformation results in the surface of the pellicle 2 tilting with respect to incident radiation PB. That is, the angle of the normal of the deformed pellicle surface with respect to the z axis (shown in figure 3) is different to that of a non-deformed pellicle 2. Radiation PB incident on a deformed pellicle has a different angle of incidence than it would have with a non-deformed pellicle. As can be seen in the images on the right-hand side of figure 3, the tangent line 5 to the portion of the pellicle 2 beneath the radiation beam PB is not perpendicular to the incident radiation PB.

[00090] Figure 4 schematically depicts the effect of pellicle deformation on an intended optical path 11 of incident radiation 7 passing from a pellicle cavity 13 (i.e. the space between the patterned surface of the reticle and the pellicle) through the pellicle 2 of the DUV lithography apparatus of figure 1. The incident radiation 7 refracts as it passes through the deformed pellicle 2. The refraction causes the incident radiation 7 to deviate from its intended optical path 11. The extent of the optical path deviation depends on the refractive index of the pellicle 2, the refractive index of the pellicle cavity 13 and the angle of incidence of the radiation. As can be seen in the magnified portion of figure 4, the optical path of the refracted radiation 9 has shifted with respect to its intended position 11 as a result of the pellicle 2 deformation.

[00091] The nature of dynamic effects depend on the scanning motion of the reticle MA. That is, the acceleration, speed, distance travelled and direction of movement of the reticle MA may all contribute to the dynamic effects. In contemporary lithographic apparatus there are two directions in which the reticle MA is scanned during exposure. The two scan directions may be referred to as scan-up (positive y direction) and scan-down (negative y direction) respectively as the scans typically take place in opposite directions along the same axis. The dynamic effects experienced by a reticle MA moving in the scan-up direction are different to the dynamic effects experienced by the reticle moving in the scan-down direction. The dynamic effects may also depend on the type of reticle MA that is being used.

[00092] The alignment of an image to its intended position on a substrate may be referred to as overlay. Inaccuracies in the alignment of an image to its intended position on a substrate W2 may be known as overlay errors. The misalignment caused to a projected image by dynamic effects results in overlay errors at an exposed die. The overlay error at an exposed point of a die caused by dynamic effects may depend on the region of the reticle MA through which the radiation beam PB passed before illuminating that point of the die. For example, a reticle MA region which has undergone a large distortion will typically cause a large overlay error for a die exposed to radiation passing through that reticle region.

[00093] The optical path deviations caused to radiation as a result of the pellicle deformation may result in overlay errors occurring at the substrate W2. That is, the image imparted to the radiation beam PB by the reticle MA is misaligned with its intended position on the substrate W2 due to optical path deviations caused by the deformed pellicle 2.

[00094] The misalignment caused to a projected image by pellicle deformation results in overlay errors at an exposed die. The overlay error at an exposed point of a die caused by pellicle deformation depends on the region of the pellicle 2 through which the radiation beam PB passed before illuminating that point of the die. For example, a pellicle 2 region which has undergone a large deformation will typically cause a large overlay error for a die exposed to radiation passing through that pellicle region.

[00095] The pellicle 2 deformation and its associated overlay error are also dependent on the direction in which the exposure scan is taking place. In contemporary lithographic apparatus there are two directions in which the reticle MA is scanned during exposure. The two scan directions may be referred to as scan-up and scan-down respectively as the scans typically take place in opposite directions along the same axis.

[00096] Dual-stage lithographic apparatus, such as the one depicted in figure 1, comprise a measurement stage WT1 as well as an exposure stage WT2. The measurement stage WT1 is used to determine various characteristics of a substrate W, such as its topography and positioning. A number of errors associated with the lithographic apparatus such as, for example, magnification errors, wavefront aberrations, focus errors and overlay errors, as well as the positioning of the substrate W2 with respect to the reticle MA may be determined at the exposure stage WT2.

[00097] Figure 5 schematically depicts a static measurement system. Measurements at the exposure stage WT2 may take place using a combination of a reticle 20 with a plurality of patterned regions 22, including a target patterned region 22', and a sensor apparatus 26, such as those schematically depicted in figure 5. The target patterned region 22' may also be referred to as a mark. The sensor apparatus 26 may, for example, be an integrating sensor apparatus such as a camera. The sensor apparatus 26 may, for example, be a non-integrating sensor apparatus, such as a diode. Pulses of radiation 30 are provided through an exposure aperture 14. A diffuser (not shown) may be provided to provide diffuse radiation to the target patterned region 22' of the reticle 20. The pulses of radiation 30 are imparted with a pattern and become patterned radiation 32 on passing through the target patterned region 22'. The pulses of radiation 30 undergo diffraction on passing through the target patterned region 22', and as a result, interference takes place within the patterned radiation 32. The patterned radiation 32 passes through projection optics 24 that project an image of an interference pattern onto a detector grating 28 of a sensor apparatus 26. The detector grating 28 is situated above a detector region 27 such that the detector grating 28 transmits a modified interference pattern to the detector region 27 when illuminated with radiation. The detector region 27 detects the modified interference pattern and measurements are made from the detected modified interference pattern. The measurements are made with both the reticle 20 and the sensor apparatus 26 kept substantially stationary in a first position. The patterned region 22 and the sensor apparatus 26 may act as a lateral shear interferometer when measuring parameters such as, for example, wavefront aberrations present in the radiation pulses. The measurements may be integrated by the sensor apparatus 26 to form a single exposure image. The detector region 27 may comprise a camera such as, for example, a CCD camera or a CMOS camera. The detector region 27 may, for example, comprise a diode. In general, any detector that is capable of collecting photons may be used at the detector region 27. [00098] Once one set of measurements has been collected from the first position, the radiation pulses are stopped and the reticle 20 is stepped to a second position. Once the reticle 20 is in the second position the radiation pulses are started again to obtain a new set of measurements for a second exposure image. Once the new set of measurements has been collected the pulses of radiation are stopped and the reticle 20 is stepped to a third position where radiation pulses are provided again. This step-measure process repeats across a number of different reticle 20 positions until a desired number of exposure images have been captured. Information about the alignment of the reticle with respect to the sensor apparatus 26, as well as focus errors, overlay errors and wavefront aberrations, may be extracted from the collected exposure images. This method of measurement may be referred to as static measurement.

[00099] The measurements may be used to correct for errors that occur during a photolithographic exposure at the exposure stage WT2. For example, wavefront aberration measurements may be provided to the processor PR that analyses the wavefront aberration measurements. The processor PR may then provide information to the controller CN that adjusts components and/or parameters of the lithographic apparatus in order to correct for the measured wavefront aberrations in preparation for an exposure scan. The controller CN may, for example, make adjustments to the projection system PL in order to correct for the measured wavefront aberrations. As another example, the controller CN may make adjustments to the position of the substrate table WT2 by providing instructions to the second positioning device PW2. As a further example, the controller CN may make adjustments to parameters of the radiation beam PB such as, for example, adjusting the wavelength of the radiation beam PB by providing instructions to the radiation system. In general, the controller CN may adjust one or more components of a lithographic apparatus in response to the information provided by the processor PR.

[000100] Static measurement does not account for the errors introduced by dynamic effects such as, for example, pellicle 2 deformation during exposure scans. This is because dynamic effects such as distortion of the reticle and/or pellicle 2 deformation is a result of a scanning motion of the reticle MA during an exposure scan, and the reticle does not undergo a scanning motion during static measurement.

[000101] During exposure of a substrate W2, radiation pulses PB are directed through the illuminator IL and exit the illuminator IL through an exposure aperture. When an exposure scan takes place, the reticle MA is scanned across the exposure aperture in at least one direction in order to illuminate the reticle MA with pulses of radiation PB. Each exposed point on the substrate W2 receives an amalgamation of radiation pulses PB that exited the exposure aperture at different positions along the exposure aperture. Radiation pulses PB exiting the exposure aperture at different positions along the exposure aperture experience different optical effects such as focus errors, overlay errors and wavefront aberrations. Each optical effect experienced by a radiation pulse depends on the position through which the radiation pulse exited the exposure aperture. The exposed points on the substrate W2 experience the focus errors, overlay errors and wavefront aberrations of radiation passing through different exposure aperture positions as an average. This averaging effect may be referred to as fading.

[000102] Static measurement does not account for fading because the reticle MA is kept substantially stationary during the measurement. That is, the detector region 27 receives radiation that has only passed through a single exposure aperture position during static measurement.

[000103] One method of accounting for the scanning motions of the reticle MA made during an exposure scan and the related dynamic effects, such as pellicle deformation, reticle slip, reticle distortion, etc., and fading effects involves carrying out a similar scanning motion of the reticle MA during measurement at the exposure stage WT2. That is, the dynamic effects and/or fading effects may be accounted for by performing a measurement scan that replicates an exposure scan. The measurement scan that determines fading effects and/or dynamic effects may be performed within a lithographic apparatus. The measurement scan that determines fading effects and/or dynamic effects may be performed outside of a lithographic apparatus such as, for example, in an independent scanning measurement system. The results of the measurement scan may be analyzed to provide instructions for controlling a lithographic apparatus. The set of instructions may be stored on a computer readable medium storing computer readable code. The computer readable medium storing the instructions for controlling a lithographic apparatus may be provided to the lithographic apparatus in order to control the lithographic apparatus. The measurement scan may take place as part of a calibration mode of a lithographic apparatus. The measurement scan involves movement of both the reticle MA and the substrate table WT2. The more accurately the measurement scan at the exposure stage WT2 can replicate the exposure scan, the more accurately overlay errors, focus errors and wavefront aberrations may be determined and corrected for.

[000104] Figure 6 is a schematic diagram of a scanning measurement system 12. The scanning measurement system 12 comprises an exposure aperture 14 of an illuminator IL. In the example embodiment depicted by figure 6, the extent of the exposure aperture 14 in the y direction is defined by the separation of the edges of first and second blades 15, 16. The blades 15, 16 are not essential to the invention. In general, the exposure aperture 14 may be any area through which radiation is provided to the scanning measurement system 12. A reticle 20 is located beneath the exposure aperture 14 and is offset from the exposure aperture 14 in the negative y direction. The reticle 20 has patterned regions 22, and a target patterned region 22', located across its surface. The patterned regions 22 and target patterned region 22' may be referred to as marks. A diffuser (not shown) may be provided to provide diffuse radiation to the target patterned region 22' of the reticle 20, however this is not essential to the invention. Projection optics 24 (in the form of two convex lenses in the example of figure 6) are located beneath the reticle 20. The projection optics 24 may correspond with the projection system PL. An integrating sensor apparatus 26 is located beneath the projection optics 24. The integrating sensor apparatus 26 comprises a detector region 27 located beneath a detector grating 28 which is substantially aligned with the edge of the second blade 16 along the y axis. Although an integrating sensor apparatus 26 is depicted in the example of figure 6, any sensor apparatus capable of detecting radiation may be used e.g. a non- integrating sensor apparatus such as, for example, a diode. The second positioning device PW2 may comprise the integrating sensor apparatus 26. The scanning measurement system 12 also comprises a processor PR. A positioning apparatus (not shown) is provided. If used as part of a lithographic apparatus, the components of the scanning measurement system 12 correspond with some of the components of the lithographic apparatus schematically depicted in figure 1. The reticle 20 would take the place of the reticle MA. The projection system PL would take the place of the projection optics 24. The integrating sensor apparatus 26 would take the place of the S A in figure 1.

[000105] All of the components depicted in figure 6 are in a "pre-scan-up" position. The scan-up direction is the positive y direction for the reticle 20 in the depicted example of figure 6. In the example of figure 6 the reticle 20 has a pellicle 2 fitted to it in order to replicate pellicle deformation effects that occur during an exposure scan. The pellicle 2 is not essential to the scanning measurement system 12. That is, a measurement scan may be performed with the scanning measurement system 12 without the pellicle 2 being present. The positioning apparatus is able to move each of the reticle 20, the blades 15, 16 and the integrating sensor apparatus 26 in scanning motions. The positioning apparatus may move the reticle 20, the blades 15, 16 and the integrating sensor apparatus 26 along the x, y and z axes. The positioning apparatus may comprise the first positioning device PM and the second positioning device PW2. The scanning measurement system 12 is capable of determining parameters such as alignments, wavefront aberrations and errors, such as focus errors and overlay errors.

[000106] A number of measurement scan options are possible. A first measurement scan option involves the scanning motion of the reticle 20 corresponding with the scanning motion of a reticle MA during a photolithographic exposure scan, and the scanning motion of the integrating sensor apparatus 26 corresponding with the scanning motion of a substrate W2 during a photolithographic exposure scan. That is, the reticle 20 and the integrating sensor apparatus 26 move the same distance, in the same direction, with the same acceleration and at the same speed as the reticle MA and substrate W2 would be moved during a photolithographic exposure scan. Movement of the reticle 20 may involve movement of a reticle stage. Movement of the integrating sensor apparatus 26 may involve movement of a substrate table. The first measurement scan may, for example, involve moving the reticle 20 and the integrating sensor apparatus 26 before radiation is received at the sensor in order to reach speeds that enable accurate emulation of the dynamic effects, such as pellicle deformation, reticle slip, reticle distortion, etc.. The first measurement scan option may be referred to as a complete measurement scan. The complete measurement scan allows the dynamic effects and/or fading effects to be emulated (and therefore determined and corrected) at the exposure stage WT2. The complete measurement scan provides the most realistic representation of the dynamic effects and/or fading effects that occur during a photolithographic exposure scan.

[000107] A second measurement scan option involves moving the reticle 20 by the same distance, in the same direction and at the same speed as a reticle MA would be moved during a photolithographic exposure scan, but limiting an extent of travel of the integrating sensor apparatus 26 to a distance less than an extent of travel of a substrate W2 during a photolithographic exposure scan. That is, the scanning motion of the reticle 20 corresponds to the scanning motion of a reticle MA during a photolithographic exposure scan and the scanning motion of the integrating sensor apparatus 26 is restricted to a portion of the scanning motion of a substrate W2 during a photolithographic exposure scan. The integrating sensor apparatus 26 only begins to scan once a predetermined target patterned region 22' of the reticle 20 has become aligned with the exposure aperture 14 along the y axis. The second measurement scan option may involve moving the reticle 20 before radiation is received at the integrating sensor apparatus 26 and moving the integrating sensor apparatus 26 only whilst radiation is received at the integrating sensor apparatus 26. The second measurement scan option requires less space for moving the integrating sensor apparatus 26 than is required by the complete measurement scan, as movement is only along the extent of the exposure aperture 14. The second measurement scan option may be referred to as a partial measurement scan. The partial measurement scan allows the dynamic effectsand/or fading effects to be measured while requiring less space for movement of the integrating sensor apparatus 26 than the complete measurement scan. A partial measurement scan would take the same time to complete as a complete measurement scan.

[000108] A third measurement scan option involves limiting the extent of travel of the reticle to a distance less than the extent of travel of a reticle during a photolithographic exposure scan and to limit the extent of travel of the sensor to a distance less than the extent of travel of a substrate during a photolithographic exposure scan. That is, the scanning motion of the reticle 20 and the scanning motion of the integrating sensor apparatus 26 are restricted to a portion of the scanning motion of the reticle MA and a portion of the scanning motion of the substrate W2 during a photolithographic exposure scan respectively. The third measurement scan option may involve only moving the reticle 20 and the integrating sensor apparatus 26 only whilst radiation is received at the integrating sensor apparatus 26. The third measurement scan option requires less total movement space than either the first or second scan options. The third measurement scan option may be referred to as a restricted measurement scan. The restricted measurement scan allows the fading effects to be measured at the exposure stage WT2; however, the pellicle 2 may not deform as it would do during a photolithographic exposure scan as the scanning motion of the reticle 20 may not accurately emulate the scanning motion of a reticle MA during a photolithographic exposure scan. That is, the dynamic effects measured during a restricted measurement scan may not be representative of the dynamic effects experienced during an exposure scan. For example, the pellicle 2 deformation effects measured during a restricted measurement scan may not be representative of pellicle deformation effects during an exposure scan. If a diffuser is present for any of the first, second and third scans detailed above then the scanning motion of the diffuser may correspond with the scanning motion of the integrating sensor apparatus 26. The restricted measurement scan would take less time to complete than the partial measurement scan or the complete measurement scan.

[000109] The partial measurement scan will now be described in further detail with reference to figures 6-9. A partial measurement scan-up begins with the reticle 20 shown in figure 6 moving in the positive y direction. Both the integrating sensor apparatus 26 and the blades 15, 16 remain stationary in their pre-scan-up positions. It is advantageous for radiation to not be provided through the exposure aperture 14 until the target patterned region 22' of the reticle 20 that is to be measured has become substantially aligned with the edge of the first blade 15 along the y axis in order to avoid, for example, unwanted heating of system components.

[000110] Figure 7 schematically depicts the scanning measurement system 12 once the target patterned region 22' that is to be measured has become substantially aligned with the exposure aperture 14 along the y axis. At this point in the partial measurement scan, pulses of radiation 30 are provided through the exposure aperture 14. The radiation pulses 30 pass through the exposure aperture 14 and are incident on the target patterned region 22' of the reticle 20. The exposure aperture 14 follows the reticle 20 as it scans and stays aligned with the target patterned region 22' in order to keep the target patterned region 22' illuminated with radiation 30 while avoiding unwanted illumination of other patterned regions 22. The pulses of radiation 30 are imparted with a pattern to form patterned radiation 32 as a result of passing through the target patterned region 22'. The pulses of radiation may undergo diffraction when passing through the target patterned region 22'. The patterned radiation 32 then passes though projection optics 24 which project a further image onto the detector grating 28 of the integrating sensor apparatus 26. The detector grating 28 transmits a modified interference pattern to the detector region 27 of the integrating sensor apparatus 26. The detector region 27 receives the modified interference pattern and produces an output signal indicative of the intensity of the modified interference pattern.

[000111] The integrating sensor apparatus 26 scans across the exposure aperture 14 in order to keep the image of the interference pattern incident on the detector grating 28. The integrating sensor apparatus 26 is shown to move in the negative y direction in the example of figure 7. The direction in which the integrating sensor apparatus 26 is moved depends on the direction in which the reticle 20 is moved and also on the projection optics 24 present. In general, the integrating sensor apparatus 26 is moved such that the image of the interference pattern is incident on the detector grating 28 of the integrating sensor apparatus 26 throughout measurement.

[000112] The scanning motion (i.e. the distance travelled, direction, acceleration and speed) of the reticle 20, the integrating sensor apparatus 26 and the blades 15, 16 that define the exposure aperture 14 may be adjusted by providing instructions to the positioning apparatus. For example, the scanning motion of the reticle 20 may be adjusted to emulate a number of different photolithographic exposure scans to recreate the dynamic effects for a number of different photolithographic exposure scans. That is, the reticle 20, the integrating sensor apparatus 26 and the exposure aperture 14 are not restricted to performing a single scanning motion. The use of blades 15, 16 to define an exposure aperture 14 is not essential to the invention. However the blades 15, 16 provide the advantage of, for example, limiting unwanted stray radiation passing through non-target patterned regions 22. As another example, the blades 15, 16 also provide the advantage of avoiding unwanted heating of system components.

[000113] Figure 8 schematically depicts the scanning measurement system 12 approximately half way through a partial measurement scan-up. The integrating sensor apparatus 26 and the exposure aperture 14 have maintained their alignment along the y direction in order to keep the image of the interference pattern incident on the detector grating 28.

[000114] Figure 9 schematically depicts the scanning measurement system 12 near the end of a partial measurement scan-up. Pulses of radiation 30 are no longer provided though the exposure aperture 14 once the integrating sensor apparatus 26 has travelled the desired portion of the extent of travel of a substrate W2 during a photolithographic exposure scan. The reticle 20 may continue to scan in the positive y direction until it reaches a distance that a reticle MA would have reached during a photolithographic exposure scan, or the reticle 20 may stop scanning once the integrating sensor apparatus 26 stops scanning. A processor PR receives the output signal from the detector region 27 and determines alignments, wavefront aberrations and errors such as focus errors, overlay errors (including those resulting from dynamic effects) and fading effects from the output signal by comparing the output signal to an ideal calibration signal. The ideal calibration signal may define a desired alignment position. Both the integrating sensor apparatus 26 and the exposure aperture 14 are now in their pre-scan-down positions ready for a partial measurement scan in the negative y- direction.

[000115] A variety of reticles 20 may be used within the scanning measurement system 12. For example, figure 10 schematically depicts a production reticle 34. The production reticle 34 has an exposure pattern (not shown) present on its image field 36 and will be used at the exposure stage WT2 for the production of patterned devices such as, for example, ICs. It is advantageous for the patterned regions 22 used for measurement scans to be located outside of the image field 36 of the production reticle 34 in order to avoid interfering with the exposure pattern already present on the image field 36 of the production reticle 34. Having patterned regions 22 present in the image field 36 of the production reticle 34 is possible, however this will reduce the amount of space available for an exposure pattern on the production reticle 34. The first two rows of patterned regions present on the production reticle are labeled as Rl and R2 respectively. The number, spacing and shape of patterned regions 22 present on the production reticle 34 may vary.

[000116] Carrying out measurement scans at the exposure stage WT2 is representative of the effects present during an exposure scan at the exposure stage WT2 because the same radiation PB exits the exposure aperture of the same illuminator IL and is incident on the same reticle MA and the same projection system PL for both scans. Use of a production reticle 34 for measurement scans at the exposure stage WT2 is highly representative of exposure scans at the exposure stage WT2 as the same reticle 34 is used at both stages. The measurement scan may be carried out with the production reticle 34 at the exposure stage WT2 and then the second positioning device PW2 may move the exposure stage WT2 such that a subsequent exposure scan may take place with the substrate W2 using corrections determined from the results of the measurement scan. The dynamic effects present outside the image field 36 of the production reticle 34 may be representative of the dynamic effects present across the image field of the production reticle. The dynamic effects present across the image field 36 of the production reticle 34 may be calculated from measurements made outside the image field of the production reticle. The pellicle deformation effects present outside the image field 36 of the production reticle 34 are representative of the pellicle deformation effects present across the image field 36 of the production reticle 34. The pellicle deformation effect present across the image field 36 of the production reticle 34 may be calculated from the measurements made outside the image field 36 of the production reticle 34 by the scanning measurement system 12. Parameters such as wavefront aberrations, focus errors, overlay errors and the alignment of the production reticle 34 with respect to the integrating sensor apparatus 26 may be determined from the results of the measurement scans and any subsequent image field 36 calculations.

[000117] Figure 11 is a schematic diagram of a calibration reticle 38 which may be used as the reticle 20 in the scanning measurement system 12. The patterned regions 22 of the calibration reticle 38 that are used for measurement scans may be located across the entire calibration reticle 38 as no exposure pattern exists on the image field 36 of the calibration reticle 38. The number, location, spacing and shape of patterned regions 22 present on the calibration reticle 38 may vary.

[000118] Using a calibration reticle 38 for measurement scans performed by the scanning measurement system 12 has the advantage of having patterned regions 22 across the entire reticle 20 rather than just having patterned regions 22 outside the image field 36 of the calibration reticle 38. The dynamic effects may be measured across the entire calibration reticle 38 rather than being measured outside its image field 36 and subsequently calculated for the image field 36. In general, any reticle 20 of any suitable shape or size with patterned regions 22 having any suitable shape, size or pattern may be used within the scanning measurement system 12.

[000119] Figure 12 is a schematic depiction of an example target patterned region 22' which may be applied to a reticle 20 for a measurement scan. The target patterned region 22' in the example of figure 12 comprises two mutually orthogonal gratings 40 and 42; however other configurations of gratings may be used. The two gratings 40, 42 may be used to determine parameters such as overlay errors caused by dynamic effects and wavefront aberrations. One grating 40 may be used to determine parameters such as overlay errors in the x-direction and the other grating 42 may be used to determine parameters such as overlay errors in the y-direction.

[000120] Figure 13 is a schematic depiction of an example detector grating 28 which may be provided above the detector region 27 in the integrating sensor apparatus 26. The detector grating 28 causes diffraction of the patterned radiation 32 incident upon it and transmits a modified interference pattern that is incident on the detector region 27 of the integrating sensor apparatus 26. The modified interference pattern incident on the detector region 27 depends on parameters such as, for example, the alignment of the reticle 20 with the detector region 27, wavefront aberrations, fading effects and errors such as focus errors and overlay errors caused by dynamic effects. The detector grating 28 shown in the example of figure 13 comprises a checkerboard pattern; however other patterns may be used. The pitch of the patterned region gratings 40, 42 may be the same as the pitch of the detector grating 28.

[000121] The scanning motion schematically depicted in figures 6-9 takes place with the target patterned region 22' and the detector grating 28 in a first arrangement with respect to each other. Figure 14 (a) depicts a first arrangement of the grating 40 of the target patterned region 22' and the detector grating 28 of the integrating sensor apparatus 26. A scan-down measurement may take place with the target patterned region 22' and the detector grating 28 kept in this first arrangement after the scan-up has finished. Once a desired number of scans have taken place under the first arrangement, the positioning of the target patterned region 22' with respect to the detector grating 28 is shifted slightly and the measurement scan is repeated in a second arrangement. [000122] One or more measurement scans may take place with the target patterned region 22' and the detector grating 28 in the first arrangement. An intensity detected by the detector region 27 of the integrating sensor apparatus 26 as a result of a measurement scan while in the first arrangement is indicated via the dashed arrow underneath figure 14 (a). The target patterned region 22' and the detector grating 28 are then shifted to a second arrangement depicted in figure 14 (b). One or more measurement scans take place in the second arrangement. The intensity detected by the detector region 27 of the integrating sensor apparatus 26 while in the second arrangement is indicated via the dashed arrow underneath figure 14 (b). The intensity detected in the second arrangement is greater than the intensity detected in the first arrangement as the target patterned region 22' and the detector grating 28 are better aligned with each other in the second arrangement compared to the first arrangement. The target patterned region 22' and the detector grating 28 are then shifted to a third arrangement depicted in figure 14 (c). One or more measurement scans take place in the third arrangement. The intensity detected in the third arrangement is indicated via the dashed arrow underneath figure 14 (c). The intensity detected in the third arrangement is greater than the intensity detected for both the first and second arrangements as the target patterned region 22' and detector grating 28 have a better alignment in the third arrangement. Figures 14 (d) and 14 (e) schematically depict the grating 40 and the detector grating 28 in fourth and fifth arrangements respectively, along with their associated detected intensities. The arrangements depicted in figure 14 may be used to determine parameters in one direction such as, for example, overlay errors and wavefront aberrations in the x direction.

[000123] Figure 15 schematically depicts five different arrangements (a), (b,) (c), (d) and (e) between the grating 42 of the target patterned region 22' and the detector grating 28 of the integrating sensor apparatus 26, along with their associated detected intensities after measurement scans. The detected intensities are indicated underneath the depicted arrangements via the dashed arrows. The arrangements depicted in figure 15 may be used to determine parameters in another direction such as, for example, overlay errors and wavefront aberrations in the y direction.

[000124] The detected intensities may be represented as a sinusoidal signal. The sinusoidal signal may be compared to a calibration sinusoidal signal that defines a desired alignment position. The calibration sinusoidal signal and the detected sinusoidal signal may be compared and parameters such as a misalignment of an image caused by dynamic effects may be determined. A distortion map may be determined from the x and y alignment information gathered from measurement scans carried out in the arrangements depicted in figures 14 and 15. As the dynamic effects, such as deformation experienced by the pellicle 2, are different for the scan-up and scan-down directions, multiple measurement scans may take place in the scan-up and scan-down directions for sufficient measurement of each target patterned region 22' .

[000125] The scanning measurement system is not limited to measuring a single target patterned region 22' per measurement scan. Multiple target patterned regions 22' may be illuminated with pulses of radiation. Multiple gratings 28 and associated detector regions 27 may be used to measure the multiple illuminated target patterned regions 22' during a single measurement scan. Multiple exposure apertures 14 may be used to illuminate the target patterned regions 22' or a single large exposure aperture may be used.

[000126] For example, a partial measurement scan-up in which multiple target patterned regions 22' are measured would be similar to the partial measurement scan-up depicted in figures 6-9. Pulses of radiation 30 would be provided when the first target patterned region 22' to be measured became aligned with the edge of the first blade 15. Multiple gratings 28 and detector regions 27 would be provided such that each target patterned region 22' would be measured separately. Pulses of radiation 30 would stop once the last target patterned region 22' to be measured became aligned with the edge of the second blade 16.

[000127] Some or all of the patterned regions 22 in a single row of a reticle 20 may be measured during a single measurement scan. For example, seven patterned regions 22 in the first row Rl of the production reticle 34 depicted by figure 10 may be measured during the same scan-up measurements for each of the ten arrangements depicted in figures 14 and 15. That is, the scanning motion of figures 6-9 may be repeated for each of the ten arrangements depicted in figures 14 and 15. Once the integrating wavefront sensor 26 has collected images for each of the ten arrangements across the first row Rl of patterned regions 22 the production reticle 34 may be moved such that the patterned regions 22 in the next row R2 may be measured for each of the ten arrangements depicted in figures 14 and 15. Successive rows of patterned regions 22 may be measured in consecutive measurement scans until each row of patterned regions 22 on a reticle 20 has been measured. It may be beneficial to follow a scan-up measurement with a scan-down measurement along the same row of patterned regions 22. Other scan sequences may be performed. A different number of patterned region and detector grating arrangements may be used.

[000128] The detector region 27 of the integrating sensor apparatus 26 collects a number of pulses of radiation during measurement (e.g. the scanning motion depicted in figures 6-9) before integrating the detected radiation into a single exposure image. In this way, the integrating sensor apparatus 26 resembles a substrate W2 during exposure. That is, during an exposure scan the substrate W2 receives multiple pulses of radiation and effectively integrates these radiation pulses to form a single exposure image. The fading effect is therefore accounted for during the measurement scan as the different overlay errors, focus errors and wavefront aberrations resulting from radiation pulses 30 passing through different positions of the exposure aperture 14 are integrated by the integrating sensor apparatus 26. As a result, the integrating sensor apparatus 26 experiences the same fading effect that is experienced by a substrate W2 during an exposure scan. Fading effects may be measured by the scanning measurement system 12. It will be appreciated that some dynamic effects and fading effects may be measured without a pellicle present. That is, a pellicle need not be present during a measurement operation where it is only desired to measure some dynamic effects and fading effects.

[000129] The method of accounting for the scanning motions of the reticle MA made during an exposure scan described above may be adjusted for use with an alternative scanning measurement system. That is, the dynamic effects may be accounted for by performing an alternative measurement scan with an alternative scanning measurement system. The alternative measurement scan may be performed within a lithographic apparatus. The alternative measurement scan may be performed outside of a lithographic apparatus such as, for example, in an independent scanning measurement system. The results of the alternative measurement scan may be analyzed to provide instructions for controlling a lithographic apparatus. The set of instructions may be stored on a computer readable medium storing computer readable code. The computer readable medium storing the instructions for controlling a lithographic apparatus may be provided to the lithographic apparatus in order to control the lithographic apparatus. The alternative measurement scan may take place as part of a calibration mode of a lithographic apparatus. The alternative measurement scan involves movement of the substrate table WT2. The more accurately the alternative measurement scan at the exposure stage WT2 can replicate the exposure scan, the more accurately imaging errors such as overlay errors, focus errors and wavefront aberrations may be determined and corrected for.

[000130] Figure 16 is a schematic depiction of an alternative scanning measurement system 44. The alternative scanning measurement system 44 comprises a reticlereticle 20. The reticle 20 has a mark 46. The mark 46 may comprise a grating line. The length and orientation of the grating line with respect to the reticle 20 may depend on what quantity is being measured during a measurement scan. For example, to determine overlay errors present along the x direction of the reticle 20, the mark 46 may comprise a grating line extending along a substantial portion of the reticle's length in the y direction. As another example, to determine overlay errors present along the y direction of the reticle 20, the mark 46 may comprise a grating line extending along a small portion of the reticle's length in the x direction. The width of the grating line may be configured to diffract radiation incident on it. The mark 46 is exposed to radiation during measurement. A projection system PS projects an image of the mark 46 onto a substrate table 48. The substrate table 48 holds a sensor that comprises a detector grating 50 above a detector 52. The detector 52 may, for example, be a photodiode. The detector grating 50 is similar in width and orientation to the mark 46 on the reticle 20. The image of the mark 46 at the sensor on the substrate table 48 may be known as an aerial image. In the example of figure 16, the reticle 20 is protected from contamination by a pellicle 2 held by a pellicle frame 6. The pellicle 2 and pellicle frame 6 are not essential to the alternative scanning measurement system 44. That is, measurement scans may be performed by the alternative scanning measurement system 44 without a pellicle 2 or pellicle frame 6 being present.

[000131] If used as part of a lithographic apparatus, the components of the alternative scanning measurement system 44 correspond with some of the components of the lithographic apparatus schematically depicted in figure 1. The reticle 20 would take the place of the reticle MA. The projection system PS would take the place of the projection optics PL. The sensor that comprises a detector grating 50 above a detector 52 would take the place of the sensor apparatus SA.

[000132] A calibration measurement may be performed to move the mark 46 and the detector grating 50 into a desired alignment before a measurement scan takes place. During a calibration measurement the substrate table 48 is stepped in the x direction whilst pulses of radiation originating from, for example, a laser pass through the alternative scanning measurement system 44. The intensity of radiation detected by the detector 52 is dependent on the alignment between the aerial image of the mark 46 and the detector grating 50 on the substrate table 48.

[000133] The graph to the right of figure 16 shows the intensity of radiation detected by the detector 52 as the substrate table 48 is stepped in the x direction underneath the length of the reticle 20 during a calibration measurement. The more aligned the aerial image and the detector grating 50 are, the higher the detected intensity is. The circled region of the graph is the point of alignment between the aerial image and the detector grating 50 at which the detected intensity varies linearly with the position of the detector 52 in the x direction. This alignment may be known as linear alignment. Having the aerial image and the detector grating 50 linearly aligned allows errors such as overlay errors that are perpendicular to the scan-up or scan-down directions to be determined in a relatively simple manner. This is because the misalignment of the aerial image with the detector grating 50 in the x direction varies linearly with the detected intensity of radiation. For example, as the reticle 20 is scanned along the y axis during a measurement scan, errors present along the x direction may be determined in a relatively simple manner from the detected intensity of radiation due to its linear relationship with aerial image misalignment in the x direction. Having the aerial image and the detector grating 50 in other alignments is possible. However, linear alignment allows for relatively simple extraction of imaging error information such as, for example, overlay errors via its linear relationship with the detected intensity.

[000134] A reference measurement may take place once the aerial image and the detector grating 50 have been moved into linear alignment. The reference measurement involves stepping the reticle 20 in the scan-up or scan-down direction whilst keeping the substrate table 48 stationary. The reticle 20 is stepped to the positions that measurements will take place during a subsequent measurement scan. The purpose of the reference measurement is to determine the intensity of radiation detected at each position along the reticle mark 46 without any dynamic effects present. This allows a direct comparison to be made with the intensity signal detected during a measurement scan with dynamic effects present.

[000135] Figure 17 is a schematic depiction of the alignment between an aerial image 54 of a reticle mark 46 and the detector grating 50 at three different times a), b) and c) during a reference measurement. The aerial image 54 of the reticle mark 46 is in linear alignment with the detector grating 50 in the example of figure 17. The graph in figure 17 shows the detected intensity of radiation throughout the reference measurement. In the example of figure 17 the alignment between the aerial image 54 and the detector grating 50 does not change throughout the reference measurement. The graph shows that the detected intensity of radiation remains constant when the alignment between the aerial image 54 and the detector grating 50 does not change. As the reticle 20 is stepped rather than scanned during the reference measurement there are insignificant dynamic effects present throughout the reference measurement.

[000136] Once the reference measurement has been made a measurement scan may take place to determine imaging errors in a direction perpendicular to the scan-up or scan-down directions. During the measurement scan the substrate table 48 remains stationary and the reticle 20 is scanned in the directions in which it would be scanned during a photolithographic exposure. The scanning motion of the reticle 20 during the measurement scan may be selected to provide an accurate replication of a photolithographic exposure scan. That is, the distance travelled by the reticle 20, the acceleration, the speed and/or the direction of reticle 20 may be varied. The reticle 20 may be protected using a pellicle 2 that will be used (or is the same or similar to a pellicle that will be used) during a subsequent photolithographic exposure scan. Pulses of radiation are fired through the reticle mark 46 throughout the measurement scan. With a reticle 20 having a length of, for example, 140mm, the speed of the reticle during the scan being, for example, 3.2 ms^"1 and a laser pulse frequency of, for example, 6kHz, it would be possible to take approximately 250 measurements across the full length of the reticle 20 during a measurement scan.

[000137] Figure 18 is a schematic depiction of an alignment between a deformed aerial image 56 of the reticle mark 46 and the detector grating 50 at three different times a), b) and c) during a measurement scan. The aerial image 56 of the mark 46 may have become deformed due to, for example, dynamic effects such as reticle distortion. The graph in the lower portion of figure 18 shows the detected intensity throughout the measurement scan. The detected intensity depends on the alignment between the aerial image 56 of the reticle mark 46 and the detector grating 50. As can be seen in figure 18, the degree of alignment between the aerial image 56 and the detector grating 50 changes throughout the measurement scan due to the deformed shape of the aerial image 56. At time a) the aerial image 56 and the detector grating 50 are close to being linearly aligned, and the detected intensity is relatively high. At time b) the aerial image 56 and the detector grating 50 are not well aligned, hence the detected intensity is low. At time c) the aerial image 56 and the detector grating 50 are close to being linearly aligned again and the detected intensity has increased to its original value.

[000138] Figure 19 is a schematic diagram of an arrangement of a reticle 20 having two marks 46 and a sensor 58 having two detector gratings 50, each having a detector (not visible in figure 19) underneath them. A greater or smaller number of marks 46, detector gratings 50 and detectors may be used. The positions of the marks 46 on the reticle 20 and the position of detector gratings 50 on the sensor 58 may be different to those shown in the example of figure 19. The arrangement depicted in figure 19 may be used to measure imaging errors in a direction perpendicular to the scanning direction of the reticle 20 such as, for example, distortions of the aerial image 56 in the x direction. The arrangement comprises semi- isolated reticle marks 46 (i.e. the marks 46 are separated in the x direction by a distance that is significantly larger than the width of the marks in the x direction) that traverse a substantial portion of the length of the reticle 20 in the y direction. Correspondingly positioned detector gratings 50 are located on the sensor 58. The reticle 20 is scanned in the y direction during a measurement scan and the detected intensity of radiation at the sensor 58 may be used to determine imaging errors in the x direction. Errors present in the y direction may be determined mathematically from the imaging errors determined in the x direction by way of a simulation using relevant physical theorems and mathematics, such as, for example, a computer model. Alternatively the imaging errors in the y direction may be measured independently.

[000139] Figure 20 is a schematic diagram of another reticle mark 46 and detector grating 50 arrangement. The arrangement shown in figure 20 may be used to measure imaging errors in a direction parallel to the scanning direction of the reticle 20 such as, for example, translations of the aerial image 54 in the y direction due to the reticle slipping from its intended position. The arrangement depicted in the example of figure 20 comprises three columns of multiple small marks 46 on the reticle 20 and three columns of detector gratings 50 on a sensor 58. The detector grating 50 comprises two gratings in each column. A greater or smaller number of marks 46, columns and detector gratings 50 may be used. Each of the three columns of marks 46 is at a different respective x position along the reticle 20. In the example of figure 20, the detector grating 50 spacing in the y direction is larger than the mark 46 spacing in the y direction in order to ensure that an intensity signal is present at all times throughout the measurement scan. That is, there will be an intensity signal present regardless of the reticle 20 position at any time at which a laser pulse is fired.

[000140] The magnitude of the detected intensity signal depends on the alignment between the detector gratings 50 and the marks 46. Performing a reference measurement (i.e. stepping the reticle 20 in the scan-up or scan-down direction whilst keeping the substrate table 48 stationary) with the reticle mark 46 and detector grating 50 arrangement shown in figure 20 would provide a sinusoidal signal of detected intensity. The results of a measurement scan may be compared to the sinusoidal signal resulting from the reference measurement in order to determine imaging errors in a direction parallel to the scan-up and scan-down directions. It is not essential to have the marks 46 and the detector gratings 50 in linear alignment to measure imaging errors in a direction parallel to the scan-up and scan- down directions.

[000141] The alternative scanning measurement system 44 is not limited to measuring a single aerial image 56 per measurement scan. Multiple reticle marks 46 may be illuminated with pulses of radiation. Multiple detector gratings 50 and associated detectors 52 may be used to measure the multiple aerial images 56 during a single measurement scan. A variety of reticles may be used within the alternative scanning measurement system 44. For example, a production reticle 34 may be used within the alternative scanning measurement system 44. In the example of figure 10, patterned regions 22 suitable for the scanning measurement system 12 are present on the production reticle 34. Reticle marks 46 suitable for the alternative scanning measurement system 44 may instead be present on the production reticle 34. A calibration reticle 38 may be used within the alternative scanning measurement system 44. In the example of figure 11 , patterned regions 22 suitable for the scanning measurement system 12 are present on the calibration reticle 38. Reticle marks 46 suitable for the alternative scanning measurement system 44 may instead be present on the calibration reticle 38. In general, any reticle 20 of any suitable shape or size with marks 46 having any suitable shape, size or pattern may be used within the alternative scanning measurement system 44.

[000142] Figure 21 is a flowchart indicating an example process which may be used to preemptively correct for errors at the exposure stage WT2 of a lithographic apparatus. With reference also to figure 1, the process begins at step SI with the scanning measurement system 12 performing one or more measurement scans in a calibration mode. Once the integrating sensor apparatus 26 has integrated the radiation pulses detected during a measurement scan it may pass the output signal to the processor PR. At step S2, the processor PR may receive a number of measurements from the integrating sensor apparatus 26. At step S3, the processor PR may determine one or more parameters from the measurements such as, for example, focus errors, overlay errors, wavefront aberrations, pellicle deformation effects, fading effects and the alignment of the reticle 20 with respect to the integrating sensor apparatus 26. At step S4, the processor PR may provide the determined parameters to a controller CN which is configured to receive the parameters and make adjustments to the lithographic apparatus to correct for the parameters at step S5. Alternatively the controller CN may comprise a processor capable of determining parameters from measurements made by the scanning measurement system 12. The adjustments made to the lithographic apparatus by the controller CN may preemptively correct for the errors and wavefront aberrations determined by the processor PR for an exposure scan at the exposure stage WT2. Once the adjustments have been made to the lithographic apparatus an exposure scan in an exposure mode may take place at the exposure stage WT2 at step S6. The process depicted by figure 21 may be performed using the alternative scanning measurement system 44. [000143] Either scanning measurement system 4, 12 may be used as part of a reticle contamination warning system. The presence of contamination on the reticle MA may increase the amplitude and/or frequency of reticle 20 slips. If the scanning measurement system 4, 12 detects a reticle slip across a distance that is larger than a predetermined value then a warning signal may be provided to a user of the scanning measurement system. The warning signal may warn the user that the reticle 20 is likely to be contaminated. The warning signal may be provided to the user via a read out such as, for example, a monitor.

[000144] The frequency with which the scanning measurement system 12 and/or the alternative scanning measurement system 44 performs measurement scans in a lithographic apparatus may be determined by the stability of the dynamic effects. That is, the less stable the dynamic effects, are, the more advantageous it is to carry out measurement scans frequently. If, for example, the pellicle deformation effect is significantly different for each substrate W2 then it would be preferable to perform a measurement scan before each exposure scan takes place. However, the greater the frequency of measurement scans the less time is available for carrying out exposure scans, which will negatively affect the throughput of the lithographic apparatus. Wafers may be provided to the lithographic apparatus in batches. It may be preferable to perform a measurement scan each time a new batch of wafers is provided to the lithographic apparatus. It may be preferable to perform a measurement scan each time a new pellicle is fitted to reticle MA. It may be preferable to perform a measurement scan each time a different reticle MA is to be used in the lithographic apparatus. Alternatively, it may be beneficial to perform measurement scans daily, or weekly, or monthly etc. In general, the measurement scans may be performed at a desired frequency. The results of measurement scans may be stored for future use. An alternative method of determining dynamic effects involves performing a calibration exposure on a substrate and comparing the printed pattern to an intended pattern. Calibration exposures often require a reticle with dense marks present in order to accurately measure characteristics of the printed pattern, e.g. the overlay error of the printed pattern. The comparison between the printed pattern and the intended pattern is often performed as part of an automated calibration process that involves iteratively reducing unwanted differences between the intended pattern and the printed pattern. The automated calibration process may involve performing multiple calibration exposures on multiple substrates. The automated calibration process may require a substantial amount of time to perform and thus may negatively affect the throughput of a lithographic apparatus. The automated calibration process may be expensive as it requires the use of multiple substrates that cannot then be used for the production of integrated circuits. The scanning measurement systems 12, 44 described herein provide faster methods of determining dynamic effects that do not require the use of expensive calibration substrates or time-consuming automated calibration loops. Nor does the invention described herein require a reticle with dense marks.

[000145] 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 reticle). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[000146] Embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.

[000147] 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

CLAIMS:

1. A method of controlling a lithographic apparatus having an exposure mode configured to expose a wafer held by a substrate table to an image of a pattern on a production reticle via a projection system, wherein in the exposure mode the production reticle is held at a reticle stage and is protected by a pellicle;

the method comprises determining characteristics of the projecting in a calibration mode;

the determining comprises:

moving the reticle stage holding a further reticle protected by a further pellicle, the further reticle having a mark;

illuminating the further reticle with radiation to form a patterned radiation beam; projecting the patterned radiation beam via the projection system onto a sensor to project a further image onto the sensor;

moving the sensor to keep the further image incident on the sensor;

sensing the projected further image as received at the sensor;

determining the characteristics of the sensed projected further image; and

the controlling comprises moving in the exposure mode at least one of the projection system, the reticle stage and the substrate table during the exposing in dependence on the characteristics.

2. The method of claim 1, wherein movement of the reticle stage and the sensor during the determining is initiated before radiation is received at the sensor.

3. The method of claim 1, wherein movement of the reticle stage during the determining is initiated before radiation is received at the sensor and movement of the sensor is initiated only when radiation is received at the sensor.

4. The method of claim 1, wherein movement of the reticle stage and the sensor during the determining is initiated only when radiation is received at the sensor.

5. A method of controlling a lithographic apparatus having an exposure mode configured to expose a wafer held by a substrate table to an image of a pattern on a production reticle via a projection system, wherein in the exposure mode the production reticle is held at a reticle stage;

the determining comprises:

moving the reticle stage holding a further reticle, the further reticle having a mark; illuminating the further reticle with radiation to form a patterned radiation beam; projecting the patterned radiation beam via the projection system onto a sensor to project a further image onto the sensor;

sensing the projected further image as received at the sensor;

determining the characteristics of the sensed projected further image; and

6. The method of claim 5, wherein movement of the reticle stage during the determining is initiated before radiation is received at the sensor.

7. The method of any one of claims 5 to 6, wherein the sensor comprises a detector grating configured to receive the projected patterned radiation beam and transmit the further image to the sensor.

8. The method of any one of claims 5 to 7, wherein the mark is configured to diffract radiation passing through it.

9. The method of claim7, wherein a calibration measurement is performed to position the detector grating in linear alignment with the further image prior to the determining.

10. The method of claim 7 or any claim dependent thereon, wherein the mark comprises a grating line that extends along a direction parallel to the movement of the reticle and wherein the detector grating comprises a first grating line that corresponds to the grating line of the mark.

11. The method of any one of claims 5 to 8, wherein the sensor is moved during the determining to keep the further image incident on the sensor.

12. The method of claim 11, wherein movement of the reticle stage during the determining is initiated before radiation is received at the sensor and movement of the sensor is initiated only when radiation is received at the sensor.

13. The method of any one of claims 5 to 12, wherein the determining further comprises forming multiple patterned radiation beams and projecting them via the projection system onto multiple sensors to project multiple further images onto the sensors; sensing the projected further images as received at the sensors and determining the characteristics of the sensed further images.

14. A lithographic apparatus comprising:

an illumination system configured to condition a radiation beam;

a reticle stage constructed to hold a reticle, the reticle having a pattern and being protected by a pellicle;

a substrate table constructed to hold a wafer;

a projection system configured to expose the wafer to an image of the pattern;

a positioning apparatus configured to move the reticle stage and the substrate table, wherein the lithographic apparatus further comprises:

a computer readable medium for storing computer readable code wherein the code causes the lithographic apparatus to perform the method of any one of claims 5 to 13.

15. A computer readable medium for storing computer readable code wherein the code causes a lithographic apparatus to perform the method of any one of claims 5 to 13.