WO2019048295A1

WO2019048295A1 - Lithographic method and apparatus

Info

Publication number: WO2019048295A1
Application number: PCT/EP2018/073173
Authority: WO
Inventors: Paulus Hubertus Petrus KOLLER; Mark-Jan SPIJKMAN; Johannes Jacobus Matheus Baselmans
Original assignee: Asml Netherlands B.V.
Priority date: 2017-09-07
Filing date: 2018-08-29
Publication date: 2019-03-14
Also published as: KR20200030117A; KR102535147B1; CN111051992A; NL1042976A

Abstract

A method of determining a sensor contribution to a measurement of apodization. The method comprises directing a radiation beam through an aperture when the aperture is in a first configuration having a first aperture diameter, the first aperture diameter being smaller than a diameter of the radiation beam, receiving the radiation beam at the sensor, obtaining a first measurement of an amount of radiation detected by the sensor in a first region of the sensor, wherein the radiation beam is not incident on the first region and determining, based on the first measurement, a sensor contribution to a measurement of apodization.

Description

Lithographic method and apparatus

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 17189827.3 which was filed on September 7, 2017 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a lithographic method and 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 patterning device, which is alternatively referred to as a mask or a reticle, 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. 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 the beam in a given direction (the "scanning"-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.

[0004] A lithographic apparatus comprises an illumination system and a projection system. An immersion lithographic apparatus further includes a liquid layer. Both the illumination system and the projection system have inherent apodization properties. Apodization describes the angular transmission of radiation through an optical system, such as the illumination system and the projection system. It is desirable to determine the apodization properties of the projection system such that lithographic errors resulting from the apodization properties of the projection system may be accounted for. A photodetector (or camera) may be used to measure an angular intensity distribution of radiation passing through the lithographic apparatus. However, the photodetector output contains illumination system apodization contributions, projection system apodization contributions and contributions from the photodetector itself. US patent number US9261402, which is hereby incorporated by reference, describes a technique of removing the illumination system apodization contributions from the measurement. It is desirable to provide, for example, a method of determining a photodetector contribution to a measurement of apodization of a projection system of a lithography apparatus that obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere. SUMMARY

[0005] Apodization may negatively affect an imaging performance of the projection system and thereby negatively affect the performance of the lithographic apparatus. Apodization properties of the lithographic apparatus may be determined using a sensor within the lithographic apparatus. Measurements made using the sensor may include angle -dependent properties of the sensor itself which negatively affect the accuracy of the apodization measurements.

[0006] Once determined, the sensor contribution to apodization measurements may be removed from future apodization measurements of the projection system, thus enabling more accurate determination of the apodization properties of the projection system. The determined apodization properties of the projection system may then be accounted for when performing lithographic exposures, thus enabling more accurate lithographic exposures to be achieved.

[0007] According to a first aspect described herein, there is provided a method of determining a sensor contribution to a measurement of apodization. The method comprises directing a radiation beam through an aperture when the aperture is in a first configuration having a first aperture diameter. The first aperture diameter may be smaller than a diameter of the radiation beam. The radiation beam is received at the sensor and a first measurement obtained of a radiation intensity detected by the sensor in a first region of the sensor, wherein the radiation beam is not incident on the first region. There may then be determined, based on the first measurement, a sensor contribution to a measurement of apodization. By measuring an intensity of radiation detected by a sensor in an area of the sensor in which it is known that radiation is not incident, any radiation detected at those regions may be assumed to be caused by sensor artefacts and so can be used to determine a sensor contribution to measurements of apodization.

[0008] The method may further comprise adjusting the aperture such that the aperture is in a second configuration having a second aperture diameter, wherein the second aperture diameter is different from the first aperture diameter and the second aperture diameter is smaller than a beam diameter of the radiation beam, and obtaining a second measurement of an amount of radiation detected by the sensor in a second region of the sensor, wherein the second region is different to the first region and radiation beam is not incident on the second region. Determining the sensor contribution to a measurement of apodization may be based on the first and second measurements. In this way, it is possible to characterise a radial component of the sensor contribution.

[0009] The second region of the sensor may at least partially overlap the first region of the sensor. Alternatively, the second region of the sensor may be entirely distinct from the first region of the sensor. In each case, adjusting the aperture diameter so as to change the non-incident regions of the sensor allows for determination of a sensor contribution to an apodization measurement of different regions of the sensor.

[00010] The diameter of the aperture may define the numerical aperture in a projection system of the projection system.

[00011] The sensor may take any appropriate form and may comprise a photodetector or a camera. For example, the sensor may comprise an integrated lens interferometer at scanner (ILIAS) sensor.

[00012] The method may further comprise obtaining a measurement of apodization. Obtaining the measurement of apodization may comprise obtaining an initial measurement of apodization and subtracting from the initial measurement of apodization the determined sensor contribution. In this way, it is possible to obtain a measurement of apodization of a lithographic apparatus that does not include a sensor contribution.

[00013] The method may further comprise determining a contribution of an illumination system to a measurement of apodization. Obtaining the measurement of apodization may further comprise subtracting from the initial measurement of apodization the determined contribution of the illumination system. In this way, it is possible to obtain a measurement of apodization of the projection system of the lithographic apparatus.

[00014] The method may further comprise causing an alert to be output in response to the obtained measurement of apodization. For example, the alert may indicate a cleaning or replacement operation is to be performed. Alternatively, the alert may automatically schedule a cleaning or replacement operation.

[00015] The method may further comprise controlling a lithographic apparatus on the basis of the obtained measurement of apodization.

[00016] According to another aspect described herein, there is provided a computer program comprising computer readable instructions configured to cause a computer to carry out a method according to the first aspect. The computer program may be stored on a computer readable medium.

[00017] According to another aspect described herein, there is provided a computer apparatus for determining a sensor contribution to a measurement of apodization, the computer apparatus comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory; wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to the first aspect.

[00018] According to another aspect described herein, there is provided a lithographic apparatus comprising an illumination system, a substrate table for holding a substrate, the substrate table comprising a sensor; a projection system for projecting a radiation beam onto the sensor, the projection system comprising an aperture through which the radiation beam is directed; and an adjustable stop for controllably adjusting a diameter of the aperture.

[00019] The adjustable stop may comprise a diaphragm. [00020] The lithographic apparatus may further comprise a processor configured to perform a method according to the first aspect.

[00021] According to another aspect described herein, there is provided a projection system for a lithographic apparatus for projecting a radiation beam towards a substrate table, wherein the projection system comprises an aperture through which the radiation beam is directed and an adjustable stop for controllably adjusting a diameter of the aperture.

[00022] It will be appreciated that features described in connection with one aspect may be combined with features described in connection with other aspects. BRIEF DESCRIPTION OF THE DRAWINGS

[00023] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 schematically depicts an immersion lithographic apparatus comprising a photodetector and a processor which may be used to measure apodization in an embodiment of the invention;

- Figure 2 schematically depicts a wavefront being projected onto a sensor;

Figure 3 schematically depicts the optics of a lithographic apparatus;

Figure 4 is a flowchart showing exemplary processing that may be carried out by a processor to determine a sensor contribution to a measurement of apodization;

Figure 5 is a flowchart showing alternative exemplary processing that may be carried out by a processor to determine a sensor contribution to a measurement of apodization; and

Figure 6 is a flowchart showing exemplary processing that may be carried out to determine apodization of at least part of the lithographic apparatus of Figure 1.

DETAILED DESCRIPTION

[00024] 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.

[00025] 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 ultraviolet (EUV) radiation (e.g. having a wavelength in the range from 4-20 nm), as well as particle beams, such as ion beams or electron beams.

[00026] The term "patterning device" 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.

[00027] A patterning device may be transmissive or reflective. Examples of patterning device 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.

[00028] The support structure holds the patterning device. It holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support 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 patterning device 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 "patterning device".

[00029] 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. Any use of the term "projection lens" herein may be considered as synonymous with the more general term "projection system".

[00030] The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the radiation beam, and such components may also be referred to below, collectively or singularly, as a "lens". [00031] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more support structures). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[00032] The lithographic apparatus may 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.

[00033] Figure 1 schematically depicts an immersion lithographic apparatus comprising a photodetector and a processor which may be used to measure apodization in an embodiment of the invention. It will of course be appreciated that a different lithographic apparatus from the one depicted in Figure 1 may be used in embodiments of the invention. For example, non-immersion lithographic apparatuses may be used in some embodiments of the invention. An EUV lithographic apparatus may be used in some embodiments of the invention.

[00034] The apparatus depicted in Figure 1 comprises:

a. an illumination system (illuminator) IL to condition a beam PB of radiation (e.g. UV radiation). b. a support structure MT to support a patterning device (e.g. a mask) MA and connected to first positioning device PM to accurately position the patterning device with respect to item PL;

c. a substrate table (e.g. a wafer table) WT for holding a substrate (e.g. a resist coated wafer) W and connected to second positioning device PW for accurately positioning the substrate with respect to item PL; and

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

[00035] 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 reflective mask or programmable mirror array of a type as referred to above).

[00036] The illuminator IL receives a radiation beam 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. In other cases the source may be integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system. [00037] 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 (commonly referred to as σ-outer and σ-inner, respectively) 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 radiation beam PB, having a desired uniformity and intensity distribution in its cross section.

[00038] The radiation beam PB is incident on the patterning device (e.g. mask) MA, which is held on the support structure MT. Having traversed the patterning device MA, the beam PB passes through the projection system PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and position sensor IF (e.g. an interferometric device), the substrate table WT 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 patterning device 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 object tables MT and WT 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 PW. 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. Patterning device MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks PI, P2.

[00039] The depicted apparatus can be used in the following preferred modes:

i. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the beam PB is projected onto a target portion C in one go (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

ii. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the beam PB is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT is determined by the (de-)magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

iii. In another mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the beam PB is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[00040] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[00041] Apodization may be defined as the angular dependency of transmission of an optical system. Apodization may negatively affect an imaging performance of the projection system PS and thereby negatively affect the performance of the lithographic apparatus. Apodization may contribute to lithographic errors which may limit an imaging performance of the lithographic apparatus. For example, a variation of the critical dimension of the lithographic apparatus may be negatively affected by effects that are attributable to apodization.

[00042] Apodization properties of the projection system PS may be determined when the optical components of the projection system PS are first manufactured. However, apodization properties of the projection system PS may change over time. For example, contaminant particles present in the projection system PS may absorb some radiation passing through the projection system PS. This may modify intensity of radiation incident on a substrate with respect to the angle of incidence of that radiation, thereby changing the apodization properties of the projection system PS. It is desirable to have a method of determining the apodization properties of the projection system PS of the lithographic apparatus. Once determined, the apodization properties of the projection system PS may be partly or fully compensated for when using the lithographic apparatus to perform a lithographic exposure.

[00043] Apodization properties of the lithographic apparatus may be determined using a sensor, comprising e.g. a photodetector, within the lithographic apparatus. The photodetector may be located at an image plane of the projection system PS, e.g. proximate the substrate W. The photodetector may, for example, be located in or on the substrate table WT. The photodetector may be configured to measure an intensity of radiation across a pupil plane of the projection system PS. For example, the photodetector may comprise an integrated lens interferometer at scanner (ILIAS) sensor. An ILIAS sensor is an interferometric wavefront measurement system that may perform static measurements on lens aberrations up to high order. An ILIAS sensor may be implemented as an integrated measurement system used for system initialization and calibration. Alternatively, an ILIAS sensor may be used for monitoring and recalibration "on-demand". US patent number US7282701B2, which is hereby incorporated by reference, discloses an ILIAS sensor that may be used to determine an intensity profile of radiation across a pupil plane of the projection system PS. The intensity profile of radiation across a pupil plane of the projection system PS may be thought of as an image of the angular distribution of radiation that is transmitted by the projection system PS. A point in the pupil plane of the projection system PS may correspond with an angle of incidence in a field plane of the projection system, and vice versa.

[00044] Measurements made using the photodetector may contain contributions from the apodization properties of the projection system PS, the apodization properties of the illumination system IL and the angle-dependent properties of the photodetector itself. The angle-dependent properties of the photodetector may, for example, include ghosting effects arising from unwanted internal reflections occurring within the photodetector, electronic crosstalk within the photodetector, etc. As discussed above, US patent number US9261402 discloses a technique of removing the contribution of the illumination system IL from the apodization measurement made by the photodetector. However, the contribution from the angle -dependent properties of the photodetector may still be present in the measurement made by the photodetector. If the angle-dependent properties of the photodetector are known, then they can be removed from the measurement of apodization made by the photodetector. However, the angle -dependent properties of the photodetector are often not known and may change over the lifetime of the photodetector.

[00045] Figure 2 schematically depicts a spherical wavefront 10 being projected through an aperture 26 in the pupil plane onto a sensor 36 comprising a flat photodetector (referred to below as a flat sensor). A problem associated with projection onto a flat sensor is that the portions of the wavefront 10 that are incident on the edges of the flat sensor 36 are distributed over a larger area of the flat sensor 36 than the portions of the wavefront 10 that are incident on inner portions of the sensor 36, which results in a greater signal-to- noise ratio (SNR) at the edges of the sensor 36. It can be seen that the identical solid angles 12 and 14 of the projected wavefront 10 are spread across differently sized areas of the sensor 36 (as indicated by braces 12a and 14a). This means that a radiation intensity measured at area 12a may be different to an intensity measured at area 14a. In particular, the measured intensity will be subject to significant "roll-off at the edges of the sensor 36. That is to say that the measured intensity at the edges of the sensor 36 will be much lower than the measured intensity at the centre of the sensor 36. In addition, sensor artefacts such as internal reflections and leakage may cause undesired effects such as "ghosting", which reduce the sensor accuracy at the edge of the pupil.

[00046] Figure 3 schematically shows parts of a lithographic apparatus arranged to facilitate determination of a sensor contribution to a measurement of apodization. For clarity various components of the projection system (such as lenses) have been omitted from the schematic illustration of Figure 3. Following exit from an illumination system (not shown), a radiation beam R passes, in a direction of propagation indicated by the depicted arrow, through a reticle 22, which is generally located in the focal plane of the illumination system. The reticle may be what is referred to as a pinhole reticle, which may be used to provide a radiation beam of known dimensions. The reticle 22 is located in the plane in which, in use, the mask MA of Figure 1 would be located. After passing through the reticle 22, the radiation beam R enters the projection system PL. Within the projection system PL, the radiation beam R passes through an aperture 28. The diameter of the aperture 28 may be adjusted using an adjustable stop 26. For example, the adjustable stop 26 may take the form of a diaphragm or iris as will be apparent to the skilled person. The adjustable stop 26 may, however, be implemented in any away which allows for adjustment of the diameter of the aperture 28. The adjustable stop 26 may be operable to adjust the diameter of the aperture 28 between, for example, 1.35 and 0.85. The aperture 28 defines the numerical aperture of the projection system PL. Through use of the adjustable stop 26 to adjust the diameter of the aperture 28 the radiation beam R may be clipped. That is, the adjustable stop 26 may be used to make the diameter of the aperture 28 smaller than the diameter of the radiation beam R. As the radiation beam R may not be completely circular (e.g. because of a cross-sectional shape imparted to the radiation beam by the adjustable stop 26), it will be appreciated that as used herein, the term diameter of the radiation beam R also includes the maximum extent in a cross-section of the radiation beam R taken in a plane perpendicular to the direction of propagation of the radiation beam R (maximum cross-sectional extent).

[00047] After passing through the aperture 28, the radiation beam R directed through aperture 31 at plane 32, the plane 32 corresponding to the plane in which the substrate W would be located during use. The size of the aperture 31 may be larger than the projection of the reticle 22 at the plane 32 such that no diffraction takes place as the radiation beam R passes through the aperture 31.

[00048] Following the aperture 31, the radiation beam is directed to a sensor 36, which may comprise, e.g. a photodetector. As described above, the sensor 36 may, for example, comprise an ILIAS sensor or other suitable sensor. The sensor 36 may comprise other components in addition to a photodetector. For example, where the sensor 36 comprises a photodetector, the photodetector may be placed in a far field plane without imaging optics, or may be located in a pupil plane with imaging optics. While the sensor 36 may comprise other components, for convenience herein, the term sensor is generally used to refer to the photodetector (or other similar component) on which the radiation beam is finally incident.

[00049] The radiation beam R has a smaller cross-sectional area (in a plane perpendicular to the direction of propagation) when it is incident on the sensor 36 than an area of a detection area of the sensor 36. When the aperture 28 is at a first diameter (depicted in Figure 3 by solid lines outlining the radiation beam R below the aperture 28), the radiation beam R is not incident on regions 38 at the edges of the sensor 36 (also referred to below as non-incident regions). When the adjustable stop 26 is operated to reduce the diameter of the aperture 28, the radiation beam R is clipped (as depicted in Figure 3 by the dashed lines outlining the radiation beam R). As a result, the non-incident regions on the sensor 36 become larger to include the area 40. The first diameter may be a widest diameter of the aperture 28 such that even at the maximum diameter of the aperture 28, the radiation beam R fully falls within the detection area of the sensor 36. In other embodiments, however, the aperture 28 may be adjustable such that the cross-sectional area of the eradiation beam R fills the detection area of the sensor 36 (i.e. such that there are no non-incident regions).

[00050] A sensor contribution to a measurement of apodization obtained from measurements taken by the sensor 36 may be determined by measuring an intensity of radiation detected at the non-incident regions 38 of the sensor 36. Since the radiation beam R is not incident on those regions 38, any radiation which is detected at the regions 38 can be attributed to sensor artefacts. The contribution of the determined sensor artefacts may then be used to determine the contribution of the sensor 36 to the overall measured apodization. The determined sensor contribution to the measured apodization may then be subtracted from future measurements of the overall apodization in order to remove the effect of the sensor artefacts from the overall measurement of apodization.

[00051] In some embodiments, the diameter of the aperture is varied using the adjustable stops 26 and the measurement of radiation at the (now changed) non-incident areas is repeated. In this way, it may be possible to characterize the radial component of the sensor artefacts. For example, a first measurement may be performed when the aperture 28 is at its largest diameter and a measurement made of how much radiation is detected in the currently non-incident region of the sensor 36. The currently non-incident region may be thought of as a first region of the sensor 36. The adjustable stops 26 may then be used to reduce the diameter of the aperture 28 . When the aperture is smaller, the non-incident region(s) become larger because more of the radiation beam R is blocked from passing through the aperture 28.

[00052] A second measurement may then be made of how much radiation is received in a second region of the sensor 36. The second region may be that part of the (now larger) non-incident region which does not overlap with the first region (i.e. that part of the non-incident region where the first measurement was performed in the preceding step). Alternatively, the second region may overlap, at least partially, the first region. In some embodiments, the second region may fully overlap the first region such that the second region comprises the whole of the larger non-incident region.

[00053] In an embodiment, the sensor contribution may be calculated using:

M = 7

where M is the measurement vector, having an element for each pixel of the sensor 26, / is the ideal result (e.g. the intensity that is expected at each pixel of the sensor 26) and T is a transfer matrix. In an ideal situation, the transfer matrix T is a unit matrix. Upon changing the diameter of the aperture 28, M and / will change, thereby allowing T to be determined.

[00054] During such processing to determine a sensor contribution to a measurement of apodization, it may be desirable to ensure that only the size of the aperture 28 is modified (e.g. and not an illumination setting), since the sensor artefacts which may be observed can be different for different illumination modes. In other embodiments, a process of taking intensity distribution measurements for different aperture sizes (as described above) may be performed for each of a plurality of illumination modes. In some embodiments, it may be possible to use the measurements obtained using the techniques described above with reference to Figure 3 to calibrate an angular sensitivity difference between different sensors. For example, if the angular distribution of radiation received at the sensor pupil 34 of a lithographic apparatus is known, it may be possible to calibrate a sensor 36 by varying the angular distribution using the adjustable stop 26 for adjusting the aperture 28 and recording the output from the sensor 36 to be calibrated.

[00055] In some embodiments, the stop 26 may comprise an iris comprising a plurality of blades, such that in at least one position of the stop 26, the outer edge of the aperture 28 is not completely circular. Where the outer edge of the aperture 28 is not circular, this will affect the shape of the radiation beam R. Any change in shape of the radiation beam R may be taken into account when determining which regions of the sensor 36 should be considered to be non-incident regions.

[00056] Figure 4 shows a flow chart illustrating steps of an exemplary method of determining a sensor contribution to a measurement of apodization. The processing of Figure 4 may be carried out by, for example, a controller MP operable to receive signals from the sensor 36 and to determine an apodization contribution of the sensor 36. The method comprises, at a first step S 1 , directing a radiation beam R through an aperture 28, with the aperture 28 having a first diameter which is smaller than a beam diameter of the radiation beam R and smaller than a width of a detection area of the sensor 36. At a second step S2, the radiation beam R is received at the sensor 36. At a third step S3, a first measurement of an amount of radiation detected by the sensor 36 in a first region (e.g. a first non-incident region) of the sensor 36 is obtained. At a fourth step S4, the first measurement is used by the controller to determine a contribution of the sensor 36 (i.e. a sensor contribution) to a measurement of apodization.

[00057] Figure 5 shows a flow chart illustrating another exemplary method of determining a sensor contribution to a measurement of apodization. Steps SI to S3 are the same as in the method described above with respect to Figure 4. However, in the method of Figure 5, at a step S5, the adjustable stops 26 are used to adjust the diameter of the aperture 28 is adjusted to a second diameter, different from the first diameter. Like the first diameter, the second diameter is smaller than a beam diameter of the radiation beam R and smaller than a width of a detection area of the sensor 36. At a step S6, a second measurement, of an amount of radiation detected by the sensor 36 in a second region (i.e. the now changed non-incident region) of the sensor 36 is performed. At a step S7, the sensor contribution to a measurement of apodization of a projection system of a lithographic apparatus is determined on the basis of the first and the second measurements. As described above, the first and second measurements may be used to determine the sensor contribution at different radial portions of the sensor 36. [00058] Figure 6 is a flowchart showing processing that may be carried out to determination an apodization of the optical column (including the illumination system IL and the projection system PL) or of the projection system PL. At a step S10 an initial apodization measurement is obtained using the sensor 36. At a step S12, the contribution of the sensor 36 to the apodization measurement (determined using the processing of, e.g., Figure 4 or 5) is subtracted from the initial apodization measurement. The processing of step S12 provides a "sensor contribution-free" apodization measurement. Processing optionally passes to step S14 (depicted in dashed outline) at which a contribution of the illumination system IL is also subtracted from the sensor contribution-free apodization measurement obtained at step SI 2. As described above, determination a contribution of the illumination system IL may be performed using the techniques described in US patent number US9261402, which is hereby incorporated by reference. The processing of step S14 provides a measurement of the projection system contribution to apodization.

[00059] At a step SI 6, the controller MP may further be operable to perform an action in response to determining the apodization contribution of the sensor 36 or in response to determining an apodization of the optical column (e.g. including the illumination system IL and the projection system PL) or in response to determining an apodization of the projection system. For example, in an embodiment, the controller MP may be operable to output an alert. For example, the controller MP may output an alert to a indicate that a maintenance operation is required, such as a cleaning or lens-replacement operation. In another embodiment, the projection system PL may comprise one or more actuators operable to adjust an apodization of the projection system PL in response to the determination made by the controller MP. In another embodiment, the controller MP may be operable to control other components of the lithographic apparatus LA responsive to the apodization measurements obtained at steps S 12 or S 14.

[0040] 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.

[0041] In an embodiment, the invention may form part of a mask inspection apparatus. The mask inspection apparatus may use EUV radiation to illuminate a mask and use an imaging sensor to monitor radiation reflected from the mask. Images received by the imaging sensor are used to determine whether or not defects are present in the mask. The mask inspection apparatus may include optics (e.g. mirrors) configured to receive EUV radiation from an EUV radiation source and form it into a radiation beam to be directed at a mask. The mask inspection apparatus may further include optics (e.g. mirrors) configured to collect EUV radiation reflected from the mask and form an image of the mask at the imaging sensor. The mask inspection apparatus may include a processor configured to analyse the image of the mask at the imaging sensor, and to determine from that analysis whether any defects are present on the mask. The processor may further be configured to determine whether a detected mask defect will cause an unacceptable defect in images projected onto a substrate when the mask is used by a lithographic apparatus.

[0042] In an embodiment, the invention may form part of a metrology apparatus. The metrology apparatus may be used to measure alignment of a projected pattern formed in resist on a substrate relative to a pattern already present on the substrate. This measurement of relative alignment may be referred to as overlay. The metrology apparatus may for example be located immediately adjacent to a lithographic apparatus and may be used to measure the overlay before the substrate (and the resist) has been processed.

[0043] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0044] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0045] 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.

[0046] 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 determining a sensor contribution to a measurement of apodization, the method comprising:

directing a radiation beam through an aperture while the aperture is in a first configuration having a first aperture diameter, the first aperture diameter being smaller than a diameter of the radiation beam ; receiving the radiation beam at the sensor;

obtaining a first measurement of a radiation intensity detected by the sensor in a first region of the sensor, wherein the radiation beam is not incident on the first region;

determining, based on the first measurement, a sensor contribution to a measurement of apodization.

2. A method according to claim 1 , further comprising:

adjusting the aperture such that the aperture is in a second configuration having a second aperture diameter, wherein the second aperture diameter is different from the first aperture diameter and the second aperture diameter is smaller than a beam diameter of the radiation beam, and

directing a radiation beam through the aperture while the aperture is in the second configuration; obtaining a second measurement of an amount of radiation detected by the sensor in a second region of the sensor, wherein the second region is different to the first region and radiation beam is not incident on the second region;

wherein determining the sensor contribution to a measurement of apodization is based on the first and second measurements.

3. A method according to claim 2, wherein the second region of the sensor at least partially overlaps the first region of the sensor.

4. A method according to any of the preceding claims, wherein the diameter of the aperture defines the numerical aperture in a projection system of the projection system.

5. A method according to any of the preceding claims, wherein the sensor comprises a photodetector or a camera.

6. A method according to any preceding claim, further comprising obtaining a measurement of apodization, wherein obtaining the measurement of apodization comprises obtaining an initial measurement of apodization and subtracting from the initial measurement of apodization the determined sensor contribution.

7. A method according to claim 6, further comprising:

determining a contribution of an illumination system to a measurement of apodization; and wherein obtaining the measurement of apodization further comprises subtracting from the initial measurement of apodization the determined contribution of the illumination system.

8. A method according to claims 6 or 7, further comprising controlling a lithographic apparatus on the basis of the obtained measurement of apodization.

9. A computer program comprising computer readable instructions configured to cause a computer to carry out a method according to any one of claims 1 to 8.

10. A non-transitory computer readable medium carrying a computer program according to claim 9.

11. A computer apparatus for determining a sensor contribution to a measurement of apodization, the computer apparatus comprising:

a memory storing processor readable instructions; and

a processor arranged to read and execute instructions stored in said memory;

wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any one of claims 1 to 8.

12. A lithographic apparatus comprising:

an illumination system;

a substrate table for holding a substrate, the substrate table comprising a sensor;

a projection system for projecting a radiation beam onto the sensor, the projection system comprising an aperture through which the radiation beam is directed; and

an adjustable stop for controllably adjusting a diameter of the aperture.

13. A lithographic apparatus according to claim 12, wherein the adjustable stop comprises a diaphragm.

14. A lithographic apparatus according to claim 12, further comprising a processor configured to perform a method according to any of claims 1 to 8.

15. A projection system for a lithographic apparatus for projecting a radiation beam towards a substrate table, wherein the projection system comprises an aperture through which the radiation beam is directed and an adjustable stop for controUably adjusting a diameter of the aperture.