Method of pellicle monitoring

Abstract

A method of pellicle monitoring using an alignment sensor of a lithographic apparatus, wherein: the alignment sensor is configured to sense alignment using a patterning device alignment mark and a stage alignment mark; and the pellicle intersects a light path between the alignment sensor and the patterning device; wherein the method comprises: using the alignment sensor, measuring a first detection intensity; using the alignment sensor, measuring a second detection intensity, being the intensity of light reflected from the patterning device; and comparing the first and second detection intensities.

Description

METHOD OF PELLICLE DETECTION, AND COMPUTER PROGRAM FOR PELLICLE MONITORING CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23219793.9 which was filed on 22 December 2023, EP application 24161333.0 which was filed on 5 March 2024 and EP application 24176591.6 which was filed on 17 May 2024 and which are incorporated herein in their entirety by reference. FIELD

[0002] The present invention relates to techniques of detecting whether a pellicle is present / intact or absent / ruptured, and / or monitoring the remaining life of the pellicle. 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). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as “Moore’s law”. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm.

[0005] A lithographic apparatus may include an illumination system for providing a projection beam of radiation, and a support structure for supporting a patterning device. The patterning device may serve to impart the projection beam with a pattern in its cross-section. The apparatus may also include a projection system for projecting the patterned beam onto a target portion of a substrate.

[0006] Contaminant particles may be generated during exposure. If these particles are allowed to reach, and be deposited on, the patterning device, they may cast an image on the substrate, leading to defects in the substrate. As feature size continues to shrink, it becomes increasingly important to prevent such defects from forming. For this purpose, it is common to cover the patterning device with a pellicle, which is an optically transmissive membrane. Pellicles, however, have a limited life span and can rupture after a number of exposure cycles. When a pellicle ruptures, large amounts of contaminant Company Secretparticles are generated. In this scenario, it may be desirable to halt the lithographic apparatus as soon as possible. To enable this, it may be desirable to monitor the status of the pellicle. SUMMARY OF THE INVENTION

[0007] Therefore, an object of the present invention is to provide pellicle detection and / or monitoring. Another object is to provide pellicle detection without affecting the production throughput of the lithographic apparatus. Another object is to monitor the remaining life of a pellicle.

[0008] In accordance with the present invention, there is disclosed a method of pellicle monitoring using an alignment sensor of a lithographic apparatus, wherein: the lithographic apparatus comprises a patterning device stage for supporting a patterning device; the patterning device stage comprises a stage alignment mark fixedly provided thereon; the patterning device comprises a patterning device alignment mark fixedly provided thereon; the alignment sensor is configured to sense alignment using the patterning device alignment mark and the stage alignment mark; and the lithographic apparatus is configured to position a pellicle such that the pellicle intersects a light path between the alignment sensor and the patterning device; wherein the method comprises: using the alignment sensor, measuring a first detection intensity; using the alignment sensor, measuring a second detection intensity, being the intensity of light reflected from the patterning device; and comparing the first and second detection intensities. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference symbols indicate corresponding parts.

[0010] Figure 1 schematically depicts a lithographic apparatus.

[0011] Figure 2a schematically depicts light from a patterning device alignment mark passing through a pellicle.

[0012] Figure 2b schematically depicts light from a patterning device stage alignment mark not passing through the pellicle.

[0013] Figure 2c schematically depicts light travelling from the patterning device alignment mark when a pellicle is absent.

[0014] Figure 3 depicts the output signal of a shearing interferometer phase-stepping measurement sensor.

[0015] Figure 4 depicts predicting the number of exposures remaining before the pellicle should be replaced. Company Secret

[0016] Figures 5a to 5c depicts various comparison schemes.

[0017] Figures 6a and 6b depict a difference map of a pellicle that is determined to be intact.

[0018] Figures 7a and 7b depict a difference map of a pellicle that is determined to have ruptured.

[0019] Figures 8a and 8b depict a difference map of another pellicle that is determined to have ruptured.

[0020] The features shown in the Figures are not necessarily to scale, and the size and / or arrangement depicted is not limiting. It will be understood that the Figures include optional features which may not be essential to the invention. Furthermore, not all of the features of the apparatus are depicted in each of the figures, and the Figures may only show some of the components relevant for describing a particular feature. DETAILED DESCRIPTION

[0021] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., with a wavelength of 436, 405, 365, 248, 193, 157, 126 or 13.5 nm).

[0022] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0023] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation or DUV radiation), a patterning device stage or mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a substrate table or a substrate holder) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support WT in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system or a reflective optics system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0024] In operation, the illumination system IL receives the radiation beam B from a radiation source SO, e.g., via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling Company Secretradiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0025] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid 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” PS.

[0026] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate W may be covered by an immersion liquid having a relatively high refractive index, e.g., water, so as to fill an immersion space between the projection system PS and the substrate W – which is also referred to as immersion lithography. More information on immersion techniques is given in US 6,952,253, which is incorporated herein by reference.

[0027] The lithographic apparatus LA may be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0028] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage (not depicted in Figure 1). The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus LA, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0029] In operation, the radiation beam B is incident on the patterning device, e.g., mask, MA which is held on the patterning device stage MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner and a position measurement system, the substrate support WT can be moved accurately, e.g., so as to position different target portions in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using patterning device alignment mark MAF and substrate alignment marks. Although the substrate alignment marks occupy dedicated target portions, they may be located in spaces between target Company Secretportions. Substrate alignment marks are known as scribe-lane alignment marks when these are located between the target portions.

[0030] In this specification, a Cartesian coordinate system is used. The Cartesian coordinate system has three axis, i.e., an x-axis, a y-axis and a z-axis. Each of the three axes is orthogonal to the other two axes. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y-axis is referred to as an Ry-rotation. A rotation around about the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0031] As noted above, the lithographic apparatus LA may be used to expose portions of a substrate W in order to form a pattern in the substrate W. In order to improve the accuracy with which a desired pattern is transferred to a substrate W, properties such as the alignment of the patterning device MA and / or the patterning device stage MT may be measured. Such alignments may include relative alignments, including the relative alignment between the patterning device MA and the patterning device stage MT, the relative alignment between the patterning device MA and the substrate support WT, and / or the relative alignment between the patterning device stage MT and the substrate support WT. These properties may be measured on a regular basis, often before exposure of each substrate W, and / or also after exposure of each substrate W, or may be measured less frequently, for example, as part of a calibration process.

[0032] Determining the relative alignment of the patterning device stage MT and the substrate support WT assists in projecting a patterned radiation beam onto a desired portion of a substrate W. This may be particularly important when projecting patterned radiation onto a substrate W which includes portions which have already been exposed to radiation, so as to improve alignment of the patterned radiation with the previously exposed regions.

[0033] The alignment measurements described above may be performed by illuminating an alignment mark MAF fixedly provided on the patterning device MA, and / or an alignment mark MTF fixedly provided on the patterning device stage MT as required. The image produced by the alignment marks may be captured by an optical system, such as an alignment sensor WS.

[0034] The patterning device alignment mark MAF and the stage alignment mark MTF may each have a reflective geometric pattern. Different geometric patterns may be suitable for different directions or fineness of alignments. Suitable geometric patterns include: a solid square or rectangle, pinhole features, diffraction gratings, and a chequerboard pattern, for example. Furthermore, more than one alignment mark may be provided on each of the patterning device MA and the patterning device stage MT. It should also be understood that marks for other purposes than alignment (such as marks for aberration detection) may also be present. Company Secret

[0035] As shown in Figure 1, the patterning device alignment mark MAF may form part of a patterning device MA. One or more such alignment mark MAF may be provided on patterning devices MA used to perform lithographic exposures. An alignment mark MAF may be positioned outside of a patterned region of the patterning device MA, which is illuminated with radiation during a lithographic exposure. As noted above, multiple alignment marks MAF, MTF may be provided on each of the patterning device MA and the patterning device stage MT. An alignment operation may make use of one, several, or all of these alignment marks MAF, MTF. For example, each of the alignment marks MAF, MTF may comprise a dedicated piece of hardware, sometimes referred to as a “fiducial”. In some implementations, the patterning device alignment mark MAF may be etched directly into the patterning device MA, alongside the patterned region of the patterning device MA. For the purposes of this description, a fiducial is considered to be an example of an alignment mark.

[0036] As shown in Figure 1, the lithographic apparatus LA may be an EUV lithographic apparatus and therefore uses a reflective patterning device MA. The patterning device alignment mark MAF may thus be of the reflective type. In other lithography technologies, the patterning device MA may be transmissive, and the patterning device alignment mark MAF may accordingly also be transmissive. It should be understood that the present invention can be implemented with either reflective or transmissive alignment marks, depending on the lithography technology employed.

[0037] In order to measure alignment of the patterning device MA and the patterning device stage MT, an alignment sensor WS (as shown schematically in Figure 1) may be provided to measure light B’ which is output from the projection system PS. The alignment sensor WS may, for example, be provided on or attached to the substrate support WT as shown in Figure 1. In order to perform an alignment process, the patterning device stage MT may be positioned such that the stage alignment mark MTF is illuminated with light B from the illumination system IL and the image of the stage alignment mark MTF is measured by the alignment sensor WS; and, separately, the patterning device stage MT may be positioned such that the patterning device alignment mark MAF is illuminated with light B from the illumination system IL and the image of the patterning device alignment mark MAF is measured by the alignment sensor WS. The substrate support WT may be positioned such that radiation which is reflected from the alignment marks MAF, MTF is projected, by the projection system PS, onto the alignment sensor WS.

[0038] The position of the alignment feature in the radiation beam B may be measured by an alignment sensor WS positioned at a substrate W level (e.g. provided on or attached to the substrate support WT as shown in Figure 1). The alignment sensor WS may be operable to detect the position of an alignment feature in the radiation incident upon it. This may allow the alignment of the substrate support WT relative to the stage alignment mark MTF and / or the patterning device alignment mark MAF to be determined. With knowledge of the relative alignment of the patterning device MA, and / or the patterning device stage MT, and / or the substrate support WT, the patterning device MA and the substrate support WT may be moved relative to each other so as to form a pattern (using the patterned Company Secretradiation beam B’ reflected from the patterning device MA) at a desired location on the substrate W. The position of the substrate W on the substrate support WT may be determined using a separate measurement process.

[0039] During exposure cycles, various contaminant particles may be generated. These particles, if allowed to reach, and be deposited on, the patterning device MA may contaminate the patterning device MA. Such contamination may cause the pattern on the patterning device MA to be indirectly reproduced on the wafer W, leading to defects. Therefore, it is common for the operator of the lithographic apparatus LA to use a pellicle MP to protect the patterning device MA. Typically, the pellicle is a membrane which is optically transmissive to the light beam B. As shown in Figure 1, the pellicle MP may be provided just in front of the patterning device MA. The pellicle MP may be spaced away from the patterning device MA by a distance. This may prevent the image of any contaminants present on the pellicle MP from being reproduced on the substrate W. The distance between the pellicle MP and the surface of the patterning device MA may be about 1 mm, for example. The pellicle MP itself may have a thickness in the tens of nanometers. Generally, the pellicle MP is not 100% transmissive to the light B. A high level of transmission is generally desired for high illumination efficiency. Typically, the transmission of the pellicle MP may be within the range of 75% to 95%.

[0040] Pellicles MP generally have a limited life span, and may rupture after a certain number of exposure cycles. When the pellicle MP ruptures, the pellicle MP may pulverize and generate a large amount of contaminant particles, which may flow to different parts of the lithographic apparatus LA. This may result, at best, defects in the exposed substrate W and, at worst, damage to components of the lithographic apparatus LA. Therefore, in the event of rupture of the pellicle MP, it may be desirable to halt the lithographic apparatus LA as soon as possible. The surfaces of components within the lithographic apparatus LA may need to be cleaned and the components of the lithographic apparatus LA may need to be repaired or replaced. Therefore, it may be desirable to detect whether the pellicle MP is present and intact, or has ruptured, so that the lithographic apparatus can be halted if necessary. Additionally or alternatively, it may be desirable to monitor the remaining life of the pellicle MP, so that the pellicle MP may be replaced before the risk of rupture becomes too high.

[0041] In order to detect the pellicle MP, it has been contemplated to provide sensors specific for this purpose. However, in addition to taking up space within the lithographic apparatus LA, such sensors also require additional machine time to operate. As a result, although the pellicle MP can in principle be detected using such sensors, it comes at the cost of reduced production throughput. Furthermore, adding additional hardware such as dedicated sensors increases the complexity of the lithographic apparatus LA, which in turn creates an additional source of fault and makes maintenance more onerous.

[0042] However, as found by the present inventors, it is not necessary to provide dedicated sensors for detecting the presence or absence of the pellicle MP. Instead, the present inventors found that it is possible to use the alignment sensor WS described above to detect the presence or absence of the pellicle MP as well as performing alignment operations. Furthermore, by using the alignment sensor WS, it Company Secretmay be possible to determine the presence of absence of the pellicle MP at the same time as performing alignment operations. In other words, with the present invention, it may be possible to provide the additional functionality of pellicle monitoring or detection without costing machine time, so that the throughput of the lithographic apparatus LA may be maintained. Furthermore, the additional functionality of pellicle monitoring or detection may be provided without requiring new hardware (e.g. sensors).

[0043] As shown in Figure 1, the pellicle MP may be positioned to cover the patterning device MA and the patterning device alignment mark MAF, but may leave the stage alignment mark MTF uncovered. As a result, light B’ from the patterning device alignment mark MAF will pass through the pellicle MP (if present) before reaching the alignment sensor WS, whereas light from the stage alignment mark MTF will not pass through the pellicle MP (even if one is present) before reaching the alignment sensor WS. In other words, the pellicle MP may be positioned such that the pellicle MP intersects a light path between the alignment sensor WS and the patterning device MA. More specifically, the pellicle MP may be positioned such that the pellicle MP intersects the light path between the alignment sensor WS and the patterning device alignment mark MAF. Furthermore, the pellicle MP may be positioned such that the pellicle MP does not intersect the light path between the alignment sensor WS and the stage alignment mark MTF. Similarly, in embodiments where the patterning device MA is of the reflective type, light beam B from the illumination system IL would pass through the pellicle MP (if present and intact) before reaching the patterning device alignment mark MAF, but would not pass through the pellicle MP (even if one is present) before reaching the stage alignment mark MTF.

[0044] As found by the inventors, the fact that the pellicle MP (if present) covers the patterning device alignment mark MAF but not the stage alignment mark MTF may enable using the alignment sensor WS to provide the additional functionality of detecting pellicle MP presence. As shown in Figure 2a, when the patterning device stage MT is positioned so that the patterning device alignment mark MAF is exposed to light B from the illumination system IL, if the pellicle MP is present and intact, the light B’ from the patterning device alignment mark MAF will pass through the pellicle MP. In case the patterning device MA is of the reflective type, such as shown in Figure 2a, the light B from the illumination system IL will also pass through the pellicle MP before reaching the patterning device alignment mark MAF. Therefore, in case the patterning device MA is of the reflective type, light B from the illumination system IL will pass through the pellicle twice before reaching the alignment sensor WS. In either case, whether the light B passes through the pellicle MP once (in case of a transmissive patterning device MA) or twice (in case of a reflective patterning device MA), the reflected light B’ will be attenuated somewhat by the pellicle MP. As a result, as shown in Figure 2a, the intensity of the light B’ detected by the alignment sensor WS may be relatively low.

[0045] By contrast, as shown in Figure 2b, when the patterning device stage MT has displaced to a position such that the stage alignment mark MTF is exposed to light B from the illumination system IL, Company Secretthe light B reaches and leaves the stage alignment mark MTF without passing through the pellicle MP. As a result, the intensity of the light B’ detected by the alignment sensor WS may be relatively high.

[0046] If the pellicle is absent or has ruptured, the light B from the illumination system IL would reach and leave the patterning device alignment mark MAF without passing through any pellicle. As a result, the intensity of the light B’ detected by the alignment sensor WS may be relatively high, and may be comparable to the intensity of light B’ from the stage alignment mark MTF.

[0047] In other words, if the pellicle MP is present and intact, it can be expected that the alignment sensor WS will measure a lower intensity of light B’ form the patterning device alignment mark MAF compared with light B’ reflected from the stage alignment mark MTF. If the pellicle MP is absent or has ruptured, it can be expected that the alignment sensor WS will measure a comparable intensity of light B’ from the patterning device alignment mark MAF and from the stage alignment mark MTF.

[0048] Therefore, in accordance with an embodiment of the present invention, a method of pellicle MP detection comprises using the alignment sensor, measuring a first detection intensity; using the alignment sensor, measuring a second detection intensity, being the intensity of light reflected from the patterning device; and comparing the first and second detection intensities. By comparing the first and second detection intensities, it is possible to detect whether the pellicle MP is present and intact, or is absent or has ruptured.

[0049] More specifically, the pellicle MP may be monitored using the stage alignment mark MTF and the patterning device alignment mark MAF. In this case, the first detection intensity, ^^^^, may be the intensity of light reflected from the stage alignment mark, and the second detection intensity, ^^^^, may be the intensity of light reflected from the patterning device alignment mark.

[0050] In order to selectively expose the patterning device alignment mark MAF and the stage alignment mark MTF, the relative position between the light beam B and the patterning device stage MT may be adjusted. That is, when measuring the first detection intensity, the relative position may be adjusted so that at least a portion of the light beam B is incident on the stage alignment mark MTF, the reflection of which being measured as the first detection intensity. When measuring the second detection intensity, the relative position may be adjusted so that at least a portion of the light beam B is incident on the patterning device alignment mark MAF, the reflection of which being measured as the second detection intensity, and the portion of the light beam passes through the pellicle in case the pellicle is present.

[0051] As with any real-life system, the first and second intensities can be affected by many different factors. A detailed mathematical model can be established to estimate what the first and second detection intensities would be when the pellicle MP is present or absent. What follows is a description of some of the factors that affect the first and second detection intensities, and how these factors may be accounted for. Of course, the more these factors are accounted for, the better the reliability of the pellicle monitoring or detection can be. It should be noted, however, that many of these factors can be simplified or neglected, and pellicle monitoring or detection can still be successfully performed by Company Secretcomparing the first and second detection intensities. More, or fewer, of these factors may be accounted for depending on the level of reliability desired. Furthermore, many of these factors can be reduced to, or approximated as, constants provided that successive measurements are performed under the same conditions (e.g. using the same settings, performed at the same or similar location, and performed at the same or similar time).

[0052] When the image of an alignment mark MAF, MTF is captured by the alignment sensor WS as discussed above, the intensity ^ that the alignment sensor WS measures may depend on: the average source power from the illumination system IL during the measurement ^^, the sensor responsivity ^^^^^^^, the specific illumination settings (e.g. the pupil transmission ^^^^^^and slit position dependency ^^^^^), the transmission of the projection system PS, ^^^^, the transmission of the dynamic gas lock membrane (DGLm), ^^^^^and the pellicle transmission, ^^^^, the optical properties of the alignment mark including the size of the projected mark image ^^, the reflectivity of the reflector mark ^^, the size of the alignment sensor WS, ^^, the reflectivity of the light absorber around the reflective portion of the mark ^^ ^(assuming that the size of the alignment sensor WS is larger than the image), and finally the measurement uncertainty !^^^^(which here includes the sensor noise !^^^^^^and the horizontal and vertical mis-alignment of the detector with respect to the mark mage). For simplicity, the alignment marks MAF, MTF may be of a square shape, but it should be understood that alignment marks of other shapes can be used. Mathematically, the intensity measured by the alignment sensor WS may be modelled as:^

[0053] The intensity defined above scales linearly with the source power and is proportional to the transmission values of the various components along the optical column (i.e. from the patterning device MA through to the substrate W, including any intervening mirrors M0, M1, M3, M4 and other optical components). More importantly, it depends on the transmission ^^^^of the pellicle MP which, when present, reduces the measured intensity and causes a drop, with respect to when it is absent. If the patterning device MA is of the reflective type (such as in EUV lithography), the pellicle MP may causean intensity drop proportional to (1 − ^^^^^), because the light beam B passes through the pellicle twice, i.e. before and after reflecting on the patterning device MA. It should be noted that the present invention can also be implemented using patterning device MA of the transmissive type, in which case the pellicleMP may cause an intensity drop proportional to (1 − ^^^^), because the light beam B passes throughthe pellicle only once.

[0054] As noted above, not all of the parameters listed in EQ1 need to be modelled in order to successfully perform pellicle detection. For example, the source intensity ^^may be assumed to be Company Secretconstant over time because the patterning device alignment mark MAF and the stage alignment mark MTF may be measured in quick succession, and / or because the spatial variation of the source intensity ^^can be treated as negligible. However, for improved reliability of pellicle detection, variations in the source intensity ^^may be compensated for.

[0055] For example, the compensation may be achieved by measuring a first source intensity, ^^,^^^, being the intensity of the light beam when measuring the first intensity; measuring a second source intensity, ^^,^^^, being the intensity of the light beam when measuring the second intensity; calculatinga compensation factor as ^^,^^^⁄ ^^,^^^ ; and before comparing the first and second detection intensities,adjusting the second detection intensity by multiplying the second detection intensity, ^^^^, by the compensation factor. In particular, the measurement of the source intensity ^^may be measured using an independent source power sensor (not shown) at the time of measuring the first source intensity and the second source intensity. The source power sensor may be placed adjacent to the patterning device MA, for example.

[0056] Another way to simplify EQ1 is to assume that the light-absorbing portion of the alignment marks MAF, MTF does not contribute to the first and second detection intensities as measured by the alignment sensor WS. That is, we can assume that ^^ ^is negligible, so that the (^^ − ^^) ∙ ^^ ^ termin EQ1 drops out. However, for improved reliability, the contribution from the light-absorbing portion can be compensated for. In particular, as pellicle transmission increases, and due to variations in the optical properties of patterning devices MA from different manufacturers, the contribution from the absorber may be non-negligible. Furthermore, most recent developments in patterning device technology such as low-n masks foresee the usage absorber with a relatively high reflectivity of up to about 15%, compared to max 0.5% for fiducials and max 2% for ordinary Tantalum reticles. Therefore, it may be desirable to compensate for the absorber contribution.

[0057] For example, the absorber contribution may be determined by separately measuring ^^^^,-., being the intensity of light reflected from the absorber portion of the stage alignment mark MTF, and measuring ^^^^,-., being the intensity of light reflected from the absorber portion of the patterning device alignment mark MAF. The absorber contribution may be estimated by displacing the patterning device stage MT so that the entire image measured by the alignment sensor WS comes from an area of light-absorbing material (assumed to have identical optical properties to the absorber portion within the alignment mark itself) adjacent to the respective alignment mark MTF, MAF. As the entire image measured by the alignment sensor WS in this operation comes from the absorber material, it is necessary to scale the intensity measurement by a geometric factor S, which represents the area of the absorber portion of the alignment mark MTF, MAF as a fraction of the total area of the alignment mark MTF, MAF, so as to obtain an estimation of the absorber contribution. When performing pellicle monitoring or detection, the estimated absorber contribution may be subtracted from the first and second intensities measured by the alignment sensor WS before comparing the first and second intensities. The absorber Company Secretcontribution need not be re-measured at every exposure cycle. Instead, it may be re-measured once for a batch of exposure cycles, so as to limit the amount of machine time needed.

[0058] In other words, the pellicle monitoring or detection method may comprise measuring ^^^^,-., being the intensity of light reflected from the absorber portion of the stage alignment mark MTF; measuring ^^^^,-., being the intensity of light reflected from the absorber portion of the patterning device alignment mark MAF; obtaining geometric factor / ^^^, being the area of the absorber portion of the stage alignment mark MTF as a fraction of the total area of the stage alignment mark MTF; obtaining geometric factor / ^^^, being the area of the absorber portion of the patterning device alignment mark MAF as a fraction of the total area of the patterning device alignment mark MAF; before comparing the first and second detection intensities, adjusting the first detection intensity, ^^^^, by subtracting / ^^^∙^^^^,-. , and adjusting the second detection intensity, ^^^^, by subtracting / ^^^ ∙ ^^^^,-..

[0059] As noted above, pellicle monitoring or detection may be achieved by comparing the first and second detection intensities. In some implementations, comparing the first and second detection intensities may include calculating a detection intensity ratio, PDR, the detection intensity ratio being a ratio between the first detection intensity, ^^^^, and the second detection intensity, ^^^^; and comparing the detection intensity ratio against a detection threshold, Th. The detection threshold, Th, may be a predetermined threshold. For example, the detection threshold may be provided as a user input. Alternatively, the detection threshold may be calculated from optical properties of the relevant components, which properties may be provided as user inputs. As noted above, many factors in EQ1 may be treated as constants over a certain timescale (e.g. for the duration of a batch of exposure cycles), provided that the measurement settings are equal. As such, when calculating the detection intensity ratio, PDR, these constants will, or can be assumed to, cancel out.

[0060] As noted above, the intensity drop due to the presence of a pellicle MP may be correlated with the transmission, ^^^^of the pellicle MP. Accordingly, the detection threshold, Th, may be set based on ^^^^. The value of ^^^^may be taken from a reference pellicle. For example, the value of ^^^^may be provided by the user, or may otherwise be predetermined. The reference pellicle may be a pellicle known to have average optical properties among a batch of pellicles.

[0061] The detection intensity ratio, PDR, may be defined as ^^^^ / ^^^^, so that the pellicle MP can be determined to be absent or ruptured if the detection intensity ratio, PDR, exceeds the detection threshold, Th. It should be understood that the reciprocal of this definition of the PDR may be used, so that the pellicle MP can be determined to be absent or ruptured if the ratio falls below the detection threshold.

[0062] As noted above, if the patterning device MA is of the reflective type, the light beam B may pass through the pellicle MP twice, and therefore be attenuated twice by the pellicle MP. Therefore,the intensity drop can be expected to be proportional to (1 − ^^^^^). Accordingly, the detection Company Secretthreshold, Th, may set to a value between ^^^^^and 1. Furthermore, the choice of the detection threshold may take into account an amount of measurement uncertainty, so as to avoid or reduce false positive or false negatives in the pellicle detection. The choice of the detection threshold may also take into account the variation in transmission from one pellicle MP to another. Therefore, the choice of the detection threshold may be obtained using a reference pellicle MP, which may have optical properties that are average within a population of pellicles MP.

[0063] It may however occur that, for a pellicle MP on the edge of population, the PDR is not too far from the chosen detection threshold, Th, so that measurement uncertainty fluctuation may occasionally cause the PDR to exceed the detection threshold, Th, leading to a false rupture detection and unnecessary system shutdown. Similarly, the optical properties of the patterning device alignment mark MAF may also vary among patterning devices MA. For example, with certain manufacturers, the variation in reflectivity can be up to about 10%.

[0064] To reduce false rupture detections, a solution is to use a relative PDR check. That is, in a plurality of exposure cycles in which the detection intensity ratio, PDR, is measured and calculated, the detection method may comprise calculating a difference between the detection intensity ratio in exposure cycle n + 1, ^1^^23, and that in the previous exposure cycle n, ^1^^; and determining thatthe pellicle is intact in exposure cycle n + 1 if (^1^^23 − ^1^^) < 53 ∙ !^^^^, where 53 is apredetermined constant equal to or greater than √2. A determination that the pellicle is intact may override any determination that the pellicle is absent or ruptured. False rupture detections may thus be reduced or prevented.

[0065] As noted above, the detection threshold, Th, may be set to a value between ^^^^^and 1, and the measurement of the detection intensities may have a degree of measurement uncertainty !^^^^. As 3 such, the detecti threshold, Th, may be set to be− 5^ ∙ ! ),5^is a predetermined constant equal to or greater than √2. In other words, the detection threshold, Th, may be set to be half way between maximum possible PDR that the alignment sensor WS could measure when the pellicle MP is present and the minimum possible when the pellicle MP is absent. It should be noted that the values of 53and 5^may be unrelated and may be different or equal. In some embodiments, both 53and 5^may be set to √2.

[0066] In some situations, alignment marks MAF, MTF may have different manufacturing processes, and may sometimes be damaged or contaminated, or it may be that the alignment marks MAF, MTF have different mark geometries, etc. The result is that, in these cases, the nominal signal for a naked (i.e. not covered by a pellicle MP) patterning device MA may not be 100%, and similarly the nominal signal for a patterning device MA with a pellicle MP may not be ^^^^^, so that detection threshold above,^ℎ =31 − 5^ ∙ ! ), may not yield perfectly reliable detection. As afurther refinement, one can e.g. determine the PDR on a reference patterning device MA, possibly naked Company Secretand purposely chosen to be in the average of the patterning device MA population. The resulting intensity ratio value, ^1^^^^^, may be used to set the detection threshold, Th, to be ^1^^^^^∙ 329:= ;<^ . This value corresponds to the mean value between the measured PDR for the naked patterning device MA and the corresponding PDR obtained if the patterning device MA is provided with a pellicle MP of a given nominal transmission ^^^^.

[0067] In addition or as an alternative to detecting when the pellicle MP is likely to have ruptured (or be absent), it may be useful to have some warning in advance, so that the pellicle MP may be replaced before the risk of rupture becomes too high. In particular, it may be useful to be able to have a measure of the amount of life left in a pellicle MP. For example, it may be useful to have a prediction of the number of exposures remaining before rupture is likely.

[0068] Accordingly, with reference to Figure 4, the method may further comprise measuring and recording a transmission value of the pellicle at each of a plurality of time points, recording an exposure count, x, of the pellicle when each of the plurality of transmission values is measured and, using the plurality of transmission values and the plurality of exposure counts as input data to a prediction model, predicting the number of exposures remaining, X, before the transmission value of the pellicle is expected to reach a replacement transmission threshold, ^ℎ^^^.

[0069] The transmission value of the pellicle MP may be any measure of transmission of the pellicle MP to the radiation beam B, which may comprise EUV radiation. For example, the transmission Tpelmay be used directly as the measured transmission value of the pellicle MP at each time point. Alternatively, the detection intensity ratio, PDR, calculated as ^^^^ / ^^^^, may be used as the measured transmission value of the pellicle MP at each time point. Alternatively, noting that the detection intensity ratio, PDR, may be approximately correlated to T2, the square root of PDR may be used as the measured transmission value of the pellicle MP at each time point.

[0070] As noted above, it may be beneficial to replace the pellicle MP before the risk of rupture becomes too high. Accordingly, a replacement transmission threshold, ^ℎ^^^, may be defined. The replacement transmission threshold, ^ℎ^^^, may correspond to a threshold on the transmission value of the pellicle MP at which replacement is recommended. The replacement transmission threshold, ^ℎ^^^, may be a predefined threshold set by the operator of the lithographic apparatus LA. The value of the replacement transmission threshold, ^ℎ^^^, may be determined based on a trade-off between making maximal use of the available life of the pellicle MP and the risk of rupture.

[0071] With certain types of pellicle material, the pellicle MP may become increasingly transmissive to the radiation beam B and B’ as the number of exposures increases. Different mechanisms may lead to the increase in transmission. For example, the pellicle MP material may become thinner due to repeated exposure to the radiation beam B. That is, the radiation beam B may etch away some of the material of the pellicle MP at each exposure. Alternatively or additionally, the material of the pellicle Company SecretMP may become less dense due to repeated exposure to the radiation beam B, leading to increased transmission. The pellicle MP may become more transmissive due to exposure to the radiation beam B in the presence of hydrogen.

[0072] For example, the pellicle MP may be made of carbon nanotubes (CNT). Carbon nanotubes may be a desirable material choice for pellicles MP because it is able to withstand high intensities of EVU radiation. CNT pellicles MP may exhibit increasing transmission as the number of exposures increases. This may be due to etching by the radiation beam B, leading to reducing thickness over time. Alternatively or additionally, the carbon nanotubes may shrink due to exposure to the radiation beam B, leading to increased transmission. In particular, the wall thickness of the carbon nanotubes may shrink. The carbon nanotubes may shrink without substantially affecting the thickness of the pellicle MP. In essence, CNT pellicles MP may have a porous structure, and may become optically less dense due to repeated exposure to the radiation beam B.

[0073] In an arrangement, the replacement transmission threshold, ^ℎ^^^, is set to a value less than 100%, so as to allow a safety margin. For example, replacement transmission threshold, ^ℎ^^^, may be set to a value less than the detection threshold, Th. This may be particularly appropriate for pellicles MP whose transmission increases with the number of exposures to the radiation beam B. By setting the replacement transmission threshold, ^ℎ^^^, this way, the operator may be prompted to replace the pellicle MP before the pellicle MP has, or is presumed to have, ruptured. In particular, the operator may take note of the number of exposures remaining, X, and schedule a replacement of the pellicle MP at an appropriate time. The operator may not necessarily need to wait until the number of exposures remaining, X, falls to zero before replacing the pellicle MP. For example, the operator may opt to replace the pellicle MP early in order to avoid interrupting the operation of the lithographic apparatus LA during a batch of exposures. Similarly, for the same reason, the operator may not necessarily need to replace the pellicle MP immediately as soon as the number of exposures remaining, X, has fallen to zero.

[0074] By way of an example only, a typical CNT pellicle MP may have a transmission of around 90% to 96%, typically greater than 95%, when brand new. The replacement transmission threshold, ^ℎ^^^, may be set to correspond to a transmission of around 98% to 99%, for example.

[0075] As noted above, the prediction model may take the transmission values and the exposure counts as input data. However, the input data may comprise further parameters. That is, the input data space may have more than two dimensions (not shown in Figure 4). For example, the input data may comprise three, four, or more parameters. With more input parameters taken into consideration, the accuracy of the prediction of the number of exposures remaining, X, may be improved.

[0076] For example, some operation regimes may involve varying the hydrogen pressure from one exposure cycle to another. Therefore, the method may further comprise measuring and recording the hydrogen pressure around the pellicle MP when each of the plurality of transmission values is measured, Company Secretand the input data may further comprise the plurality of hydrogen pressure measurements. Hydrogen pressure may be a useful input parameter because it may have an effect on the erosion rate of the pellicle MP. Generally, it is expected that a higher pressure of hydrogen leads to a higher erosion rate. The hydrogen pressure may be taken to be equal to the main chamber pressure of the lithographic apparatus LA. Alternatively, a dedicated pressure sensor may be provided in the environment surrounding the pellicle MP in order to measure the hydrogen pressure.

[0077] Similarly, some operation regimes may involve varying the temperature of the pellicle MP or of the environment surrounding the pellicle MP from one exposure cycle to another. Therefore, the method may further comprise measuring and recording the temperature of the pellicle MP or of the environment surrounding the pellicle MP when each of the plurality of transmission values is measured, and the input data may further comprise the plurality of temperature measurements. Temperature may also be a useful input parameter because it may have an effect on the erosion rate of the pellicle MP. Generally, it is expected that a higher temperature results in a lower erosion rate.

[0078] Referring again to Figure 4, irrespective of the number of input parameters, the prediction model may predict the number of exposures remaining, X, by performing regression analysis on the input data, and extrapolating therefrom. That is, the regression analysis may extrapolate the input data to predict the cumulative number of exposures when the transmission value of the pellicle MP will equal the replacement threshold, ^ℎ^^^, and the number of exposures remaining, X, may be calculated as the difference between the predicted cumulative number of exposures and the current exposure count (i.e. the number of times the pellicle MP has already been exposed to the radiation beam B).

[0079] Without being bound by any specific theory, it is observed that the relationship between the number of exposures and the transmission of the pellicle MP may be approximately linear or slightly accelerating (hence Figure 4 shows an extrapolation line which curves upwards, although the extent of the acceleration is exaggerated for illustrative purposes). This is particularly true of CNT pellicles MP. As such, the regression analysis may comprise linear regression. Linear regression may provide sufficiently accurate fitting of the input data. In order to better account for the acceleration effect, quadratic regression and / or exponential regression may be used.

[0080] Regression analysis comprising linear, quadratic and / or exponential regression may also be used when the input data comprises more than two parameters (i.e. the number of exposures, x, and the transmission values of the pellicle MP), such as hydrogen pressure and / or temperature as noted above. However, as the dimensionality of the input data increases, and due to potential non-linear effect of various input parameters, machine learning may be more effective at predicting the number of exposures remaining, X. Therefore, the prediction model may comprise a trained machine learning model. It should be noted, of course, that machine learning may be used even when the input parameters comprise only the number of exposures, x, and the transmission values of the pellicle MP.

[0081] As the pellicle MP can be expected to degrade gradually, the transmission value of the pellicle MP may also be expected to increase gradually. Nevertheless, as with any real-world measurements, Company Secretthe input data may comprise unexpected fluctuations such as measurement noise. Therefore, a noise filter may be applied to the input data. Specifically, the noise filter may be applied to the input data before applying the prediction model. For example, the noise filter may comprise a low-pass filter.

[0082] During the operation of the lithographic apparatus LA, it may also occur that, for instance due to contamination, the reference intensity provided by the stage alignment mark MTF may drift over time, making the threshold, Th, determined above invalid over time. This can be solved by: at an initial time point t0, measuring a reference first detection intensity, ^^^^,^^^(^^), being the first detection intensity measurable when a reference stage alignment mark is installed in the lithographic apparatus, and measuring the first detection intensity, ^^^^(^^); at a subsequent time point t1, re-measuring the reference first detection intensity, ^^^^,^^^(^3), and re-measuring the first detection intensity, ^^^^(^3); calculating a drift factor, 1^^^subsequent time point t1, adjustingthe detection intensity ratio, PDR, by multiplying it by the drift factor, 1^^^(>3), before comparing it against the detection threshold, Th. The re-measurement of the reference first detection intensity, ^^^^,^^^(^3), and the first detection intensity, ^^^^(^3)may be repeated from time to time, and the drift factor 1 be updated accordingly. It should also be noted that, in the definition of 1^^^(>3) by dividing the intensity measurements at time t1by those at time t0, common mode effects (e.g. transmission of the optical column) may be cancelled.

[0083] As with the stage alignment mark MTF, the optical properties of a given patterning device alignment mark MAF can also drift over time, for example due to contamination. As such, as second drift factor may be calculated and applied to account for this drift. For example, the pellicle detection method may comprise, at an initial time point t2, measuring a reference first detection intensity, ^^^^,^^^(^^), being the first detection intensity measurable when a reference stage alignment mark is installed in the lithographic apparatus, and measuring the second detection intensity, ^^^^(^^). Then, at a subsequent time point t3, which may be a number of exposure cycles later, the pellicle detection method may comprise re-measuring the reference first detection intensity, ^^^^,^^^(^G), and re-measuring ? the second detection intensity, ^@AB,E;@(CH)^^^(^^); calculating a second drift factor, 1^^^(>G), as ?E;C(CH) / ?@AB,E;@(C=)?E;C(C=); and at the subsequent time point t3, adjusting the detection intensity ratio, PDR, by multiplying it by the second drift factor, 1^^^(>G), before comparing it against the detection threshold, Th.

[0084] It should be understood that the initial time point t2 mentioned in the context of the second drift factor, 1^^^, may have any temporal relationship with the initial time point t0mentioned above in the context of the first drift factor 1^^^. For example, the initial time point t2may coincide with the initial time point t0. Similarly, the subsequent time point t3mentioned in the context of the second drift factor, 1^^^, may have any temporal relationship with the subsequent time point t1mentioned above in Company Secretthe context of the first drift factor 1^^^. For example, the subsequent time point t3may coincide with the subsequent time point t1.

[0085] In some embodiments, the pellicle MP and the patterning device MA may be fixed together to form an assembly, and may be installed and uninstalled together. Furthermore, in some embodiments, the pellicle MP and the patterning device MA may be installed in the lithographic apparatus LA for a number of exposure cycles, then swapped out for different pellicles and patterning devices for a number of exposure cycles, and then reinstalled for further exposure cycles. In some situations, the swapping out and the reinstallation may be several months apart. When the pellicle MP and the patterning device MA are reinstalled, the last known value of the second drift factor 1^^^may be reused without further measurements. More generally, the last known value of the second drift factor 1^^^may be stored for each pellicle MP and patterning device MA pair before it is swapped out, and may be retrieved when it is swapped back in. A database may be established to store the known value of the second drift factor 1^^^for each of a collection of pellicle MP and patterning device MA pairs.

[0086] Specifically, the pellicle detection method may comprise associating the second drift factor, 1^^^(>G), with the patterning device MA and pellicle MP installed in the lithographic apparatus at time point t3, and storing the second drift factor, 1^^^(>G); uninstalling the patterning device MA and pellicle MP; and, at a subsequent time point t4after time point t3, reinstalling the patterning device MA and pellicle MP and retrieving the second drift factor, 1^^^(>G) associated with the patterning device and pellicle. After retrieving the second drift factor, 1^^^(>G), for exposure cycles from this point on, the detection intensity ratio, PDR, may be adjusted by multiplying it by the retrieved second drift factor, 1^^^(>G), before comparing it against the detection threshold, Th.

[0087] By reusing the last known value of the second drift factor 1^^^, the intensity detection ratio, PDR, can be compensated appropriately before comparing it against the detection threshold, Th, so that the likelihood of false detections can be reduced. Without using the last known value of the second drift factor 1^^^, when the patterning device MA and pellicle MP pair is reinstalled, it may be necessary to allow a large degree of uncertainty in the optical properties of the patterning device MA and pellicle MP pair. By reusing the last known value of the second drift factor 1^^^, this uncertainty can be reduced or eliminated. Of course, as in any real-life systems, some uncertainty may remain. For example, the measurement uncertainty, !^^^^, mentioned above may persist.

[0088] Furthermore, the PDRt3 at time point t3 may also be stored. At time point t4, when PDRt4 is calculated, the relative PDR check as described above may be performed using PDRt3 and PDRt4. That is, a relative PDR check can be performed right from the first exposure cycle after reinstallation as if the patterning device MA and pellicle MP pair had never been removed and reinstalled. This may result in improved accuracy.

[0089] As with the alignment marks MAF, MTF, the lithographic apparatus LA as a whole may also drift over time due to a number of contributing factors, including but not limited to degradation of the Company Secretprojection system PS and the illumination system IL, or any of the intervening mirrors M0, M1, M3 and M4. As shown in EQ1, the intensities measured by the alignment sensor WS are in part influenced by the source power intensity, P0. The system drift of the lithographic apparatus LA as a whole may be tracked by calculating a system transmission factor. For example, the pellicle detection method may further comprise, at an initial time point t5, measuring ^^,^I, being the intensity of the light beam at time point t5, and ^^^^,^I, being the second detection intensity at time point t5; calculate a system transmission ? factor at time point tE;C,CK5, ^^J^,^I, as at a subsequent time point t6, measuring ^^,^L, being the intensityof the light beam at time point t6, and ^^^^,^L, being the second detection intensity at time point t6; calculate the system transmission factor at time point t6, ^^J^,^L, asbefore comparing the first and second detection intensities after the subsequent time point t6, adjusting the second detectionintensity by multiplying the second detection intensity, ^^^^, by ^^J^,^I⁄ ^^J^,^L .

[0090] Again, it should be understood that the initial time point t5mentioned here may have any temporal relationship with the initial time points t0and t2mentioned above in the context of the first and second drift factors 1^^^and 1^^^. For example, the initial time point t5may coincide with the initial time point t0 and / or the initial time point t2. Similarly, the subsequent time point t6 mentioned here may have any temporal relationship with the subsequent time points t1 and t3 mentioned above in the context of the first and second drift factors 1^^^and 1^^^. For example, the subsequent time point t6 may coincide with the subsequent time point t1 and / or with the subsequent time point t3.

[0091] In some embodiments of the lithographic apparatus LA, there may be more than one substrate support WT, so that the next substrate W can be loaded while the current substrate W is being exposed. Furthermore, each substrate support WT may be associated with, or equipped with, its own alignment sensor WS. For example, the lithographic apparatus LA may comprise alternating substrate supports WT. That is, the lithographic apparatus LA may comprise a first and second substrate supports WT. The lithographic apparatus LA may comprise a second alignment sensor WS, in addition to the first alignment sensor WS. The first alignment sensor WS may be associated with the first substrate support WT, and the second alignment sensor WS may be associated with the second substrate support WT.

[0092] As a result, every other exposure step may be effectively blind for pellicle presence. Furthermore, as patterning device MA alignment operations become more and more time-aggressive to increase machine throughput, it may occur that the alignment sensor WS used for alignment operations may not be the same each time, e.g. different sensor on the same substrate support WT, or sensors from different substrate supports WT. Therefore, it may be desirable to compensate for differences between different alignment sensors WS. For example, the pellicle detection method may further compriseobtaining a sensitivity ratio, ^N⁄ ^^ , being a ratio between the sensitivity of the first alignment sensor,^N, and that of the second alignment sensor, ^^; using the second alignment sensor, measuring ^^^^,^, Company Secretbeing the intensity of light reflected from the patterning device alignment mark; adjusting ^^^^,^bymultiplying it with the sensitivity ratio, ^N⁄ ^^ ; and comparing ^^^^ and ^^^^,^.

[0093] The sensitivity ratio, ^N⁄ ^^ , may be obtained using a priori calibration. However, this mayrequire a specific measurement scheme and additional machine time to perform the calibration. Furthermore, sensors responsivities may vary over time, e.g. due to sensor contamination or aging, so that the initial a priori calibration may become inaccurate, and re-calibration may become necessary.

[0094] However, in an exposure sequence, there are naturally measurements that are performed in practically identical conditions. For example, consecutive alignment operations may be performed using the same patterning device alignment mark MAF by the first and second alignment sensors WSin turn. The sensitivity ratio, ^N⁄ ^^ , can be readily computed for every batch of exposures, and can beupdated over time. In other words, if ^^^^is measured in exposure cycle m, then ^^^^,^may measuredin exposure cycle m + 1 immediately following the exposure cycle m, and the sensitivity ratio, ^N⁄ ^^ ,may be set to be ^^^^ / ^^^^,^.

[0095] However, this requires assuming that the pellicle MP is intact in both exposure cycles m and m + 1, which assumption may occasionally break down. If the pellicle MP does rupture between cyclem and cycle m + 1, then the calculated sensitivity ratio, ^N⁄ ^^ , may be incorrect. To improve therobustness of the setting of sensitivity ratio, ^N⁄ ^^ , the calculation of the sensitivity ratio may bedelayed until exposure cycle m + 2 when the lithographic apparatus LA alternates back to using the first alignment sensor WS (and the first substrate support WT), and the pellicle MP is detected still to be intact. That is, if the pellicle MP is intact in cycle m + 2, the pellicle MP must also be intact in cycle m + 1. Furthermore, it is possible to confirm that the pellicle MP is intact in cycle m + 2 because the measurements in cycles m and m + 1 were both made by the first alignment sensor WS. In other words, the pellicle detection method may further comprise using the first alignment sensor WS to detect the pellicle MP in exposure cycle m + 2 following the exposure cycle m + 1, wherein the sensitivity ratio,^N⁄ ^^ , is set to be ^^^^ / ^^^^,^ (using values measure in cycles m and m + 1) only if the pellicle isdetected to be intact in exposure cycle m + 2. The sensitivity ratio, ^N⁄ ^^ , may be stored in a cachefor use in a subsequent batch of exposure cycles. Furthermore, by storing the sensitivity ratios, ^N⁄ ^^ ,statistical averaging techniques may be applied to improve detection accuracy.

[0096] Of course, this scheme works also if measurements are not made in two consecutive exposure cycles (provided the pellicle MP has not ruptured). More generally, it works for any combination of sensors: different sensors in the same substrate support WT, sensors from different substrate supports WT, different pairings of alignment sensor WS and patterning device alignment mark MAF, and even different types of sensors.

[0097] It is finally worth noting that, the sensitivity ratio, ^N⁄ ^^ , assuming that all othermeasurement conditions are equal, effectively depends on the sensor responsivity only, and not on specific conditions and illumination settings for a specific batch of exposures. Therefore, any measured Company Secretsensitivity ratio, ^N⁄ ^^ , may be reused in subsequent batches to enable earlier detection of pelliclefailure. Furthermore, the measured sensitivity ratios can be stored in a database to enable statistical averaging methods to improve in accuracy.

[0098] Analogous to the concept of sensor sensitivity ratio explained above, where multiple patterning device alignment marks MAF are present on the patterning device MA, the difference in the intensity of light reflected by the patterning device alignment marks MAF can also be compensated for. Specifically, the pellicle detection method may further comprise obtaining an alignment mark intensity ratio ^^^^ON / ^^^^O^, ^^^^ONbeing the intensity of reflected light measurable from the first patterning device alignment mark and ^^^^O^being the intensity of reflected light measurable from the second patterning device alignment mark under equal conditions; using the alignment sensor, measuring ^^^^,^^^O^, being the intensity of light reflected from the second patterning device alignment mark; ? substituting ^^,EP Q^^^,^^^O^∙ ? as the second detection intensity before the comparing the first andsecond detection intensities. This may be beneficial because different patterning device alignment marks MAF may be used in different steps in an alignment operation, and different alignment operations may also use different patterning device alignment marks MAF. This approach may be particularly effective if the patterning device alignments marks MAF are located closely together on the patterning device MA, so that ^^^^^(see EQ1) is approximately constant.

[0099] It should be understood that the various adjustments and compensations described above may be freely combined as desired. Furthermore, each of the contributing factors identified in EQ1 that are not explicitly accounted for in the above disclosure may also be measured and calibrated. The multiplicative nature of these factors means that any such further refinements can be included, as required, by multiplying the PDR by further compensation factors.

[0100] As mentioned above, an advantage of using the alignment sensor WS and alignment marks MAF, MTF to detect the pellicle MP is that it may cost little to no extra machine time, as all the raw measurements required may already be obtained as part of various routine alignment operations. Therefore, the steps of measuring the second detection intensity and comparing the first and second detection intensities may be repeated at each exposure cycle. Furthermore, the lithographic apparatus LA may be halted upon detecting that the pellicle is absent or ruptured.

[0101] In addition to detecting the pellicle MP, the method may be extended to detect other optically transmissive membranes along the optical column between the patterning device MA and the alignment sensor WS. For example, the method may be extended to detect the dynamic gas lock membrane (DGLm). As known in EUV lithography, a DGLm may be employed to prevent substrate contaminants and / or debris from entering the projection system PS of the lithographic apparatus LA.

[0102] In the context of the present invention, whereas the pellicle MP covers the patterning device MA but not the patterning device MAF, the DGLm (if present) always intersects the light path leading to the alignment sensor WS. Thus, the light beam B’ reaching the alignment sensor WS will be Company Secretattenuated by a factor of (1 − ^^^^^), ^^^^^ being the transmission of the DGLm (see EQ1). Inparticular, both the first and second detection intensities will include the attenuation. As such, the state of the DGLm cannot be detected by comparing the first and second detection intensities. However, inthe event of DGLm rupture, a jump in the first and / or second detection intensities by the factor of (1 −^^^^^) can be expected. Therefore, the pellicle detection method may further comprise obtaining the transmission of the DGLm, ^^^^^; determining that the DGLm has ruptured upon detection that thefirst and / or second detection intensity in an exposure cycle has increased by a factor of (1 − ^^^^^)compared with a previous exposure cycle. Furthermore, in the event that the DGLm ruptures between the measurements of the first and second detection intensities, a jump in the intensity detection ratio,PDR, by a factor of (1 − ^^^^^) can be expected. This can also be used to detect DGLm rupture. Thetransmission of the DGLm, ^^^^^, may be measured in a calibration step, e.g. at the beginning of an exposure batch.

[0103] The present method may be performed using various types of alignment sensors WS. For example, the alignment sensor WS may comprise a transmission image sensor (TIS). Detailed disclosure of TIS may be found in US 6888151 B2, WO 2022207259 A1 and WO 2017207512 A2, for example, all of which are incorporated herein by reference.

[0104] Additionally or alternatively, the alignment sensor WS may comprise a shearing interferometer phase-stepping measurement sensor (PARIS). Detailed disclosure of PARIS may be found in : WO 2021069147 A1, WO 2022248154 A1 and WO 2022268679 A1, all of which are incorporated herein by reference. Furthermore, each of the stage alignment mark MTF and the patterning device alignment mark MAF may comprise orthogonal gratings, for use with the PARIS.

[0105] For each alignment mark MAF, MTF, the PARIS may perform the phase stepping measurement first on one grating orientation (e.g. U grating) then on the other, orthogonal grating (e.g. V grating). This leads to two wavefront data so that, for each of the pixels in the sensor, the measuredintensity, I, may described as ^ = 1S + T UVW(X).

[0106] 1S is an offset component which is linearly proportional to the incoming light on the sensor and thus proportional to: the source power ^^, the average of the reflectivity of grating, ^Y^^^^, and of the absorber, ^Y^^N ^, of the grating structure of the patterning device alignment mark MAF, and / or the gratings of the stage alignment mark MTF, ^Z^^^^, surface transmission of pellicle MP, Tpel2, transmission of projection system PS, ^^^^, sensitivity of the PARIS, ^, etc. Therefore, the DC component may be modelled as:which in the typical case of a binary mask grating (negligible absorber reflectivity) becomes:1S~ ^ ∙ ^ ^^ ^^^ ∙ ^Y^^^^ ∙ ^^^^ ∙ ^ ∙…

[0107] T is the magnitude of the phase-modulated component of the signal, which has similar dependency towards source power, projection box PS transmission, sensitivity of the PARIS, etc., Company Secretwhile, with respect to grating surface properties, it is proportional to the contrast between gratingreflectivity and background absorber ~(^Y^^^^ − ^Y^^N ^). Thus, M may be modelled as:which in the typical case of a binary mask grating becomes:

[0108] Finally, X is the phase describing the aberration status of the lens, and is not relevant for pellicle detection.

[0109] As can be seen, due to their common dependency on the various parameters, the DC and M are equally sensitive to pellicle presence. On the other hand, they are differently sensitive to measurement disturbance (for example, the modulated component is also sensitive to the aberration perceived by the sensor, e.g. out-of-focus will blur the image and reduce the modulation, while offset is not). As such, the redundancy offered by these two measurements may provide further robustness to systematic errors than a single measurement.

[0110] Each of DC and M can be used to detect the transmission change due to rupture of the pellicle MP. That is, each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, may be derived from the offset component, DC, of the same pixel or group of pixels. Alternatively or additionally, each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, may be derived from the phase-modulated component, T UVW(X), of the same pixel or group of pixels. Specifically, each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, may be derived from the magnitude of the phase-modulated component, M, of the same pixel or group pixels.

[0111] Furthermore, for improved statistical confidence, each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, may be derived from both the offset component, DC, and the magnitude of the phase-modulated component, M, of the same pixel or group pixels. For example, each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, may be derived from a linear combination of the offset component, DC, and the magnitude of the phase-modulated component, M, of the same pixel or group of pixels. More specifically, each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, may be calculated as the average of the offset component, DC, and the magnitude of the phase-modulated component, M, of the same pixel or group of pixels.

[0112] The detection intensity ratio, PDR, may be calculated as discussed above as ^^^^ / ^^^^. As can be seen, all the terms ^^, ^Y^^^^, ^^^^, and ^ cancel out in the PDR, except ^^^^^, which will cancel out only if the pellicle MP is absent / ruptured. Furthermore, all of the adjustment and compensations disclosed above can equally be applied when a PARIS is used.

[0113] Furthermore, as found by the inventors, the ratio 1S^^^^^⁄ T^^^^^ , where:Company Secret1S^^^and T^^^are, respectively, the offset component and the magnitude of the phase- modulated component of the pixel or group of pixels measurable from the stage alignment mark MTF, and 1S^^^and T^^^are, respectively, the offset component and the magnitude of the phase- modulated component of the pixel or group of pixels resulting from light reflected from the patterning device alignment mark MAF, may be invariant whether or not a pellicle MP is present / intact.

[0114] Therefore, the pellicle detection method may further comprise using the PARIS, measuring1S^^^, T^^^, 1S^^^ and T^^^, calculating and monitoring the value of 1S^^^^^⁄ T^^^^^ , and upondetection that 1S^^^^^⁄ T^^^^^ has changed by more than a predetermined amount, determining that themeasurements are unreliable. This may indicate an instability in the system, an incorrect measurement or an error, and any apparent detection of pellicle rupture may be disregarded.

[0115] As noted above, the PARIS outputs an array of pixels. The above mathematical description of the output of PARIS is true of individual pixels. Therefore, all of the techniques above can be performed on the basis of a single pixel alone. However, for improved statistical confidence and measurement uncertainty suppression, the above techniques can be performed by averaging a group of the pixels. In particular, the group of pixels may be selected from the best performing subset of pixels. For example, the pixels at the center of the field of view of the PARIS may be selected, as they may typically produce a stronger signal thus have a better signal-to-noise. Alternatively, the group of pixels may be selected from those which are less sensitive to noise sources.

[0116] Furthermore, in an embodiment of PARIS, also known as Parallel ILIAS, multiple pairs (e.g. seven pairs) of U and V detectors may be used simultaneously. In this embodiment, the PARIS can measure a corresponding plurality of (e.g. seven) alignment marks MAF fixedly provided on the patterning device MA. Furthermore, the patterning device alignment marks MAF may be dispersed across the full width of the light bean B’. Therefore, the PARIS may generate one set of wavefront data for each of the U and V detector in each of the multiple detector pairs, and each set of wavefront data can be broken down into the DC and M terms. For a PARIS with seven pairs of U and V detectors, 2 x 2 x 7 = 28 simultaneous measurements of the pellicle may be produced at a time. These simultaneous measurements may be used collectively for correlation analysis and for a statistically robust detection. Specifically, each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, may be derived from an average of the outputs from the plurality of detectors, which outputs may be produced simultaneously.

[0117] Pellicle detection using PARIS may bring another performance advantage compared with TIS. The performance of TIS in pellicle detection can be limited by the residual reflectivity of the absorber portion of the alignment marks MAF, MTF, and by manufacturing tolerances of the sensor which can lead to up to 15-20% systematic bias in the detection intensity ratio, PDR, measurement. Company SecretWhen using TIS in pellicle detection, accuracy can be improved by performing additional measurements, but at the cost of machine time and thus throughput.

[0118] Using PARIS, in the case of a non-binary (i.e. having high absorber reflectivity, e.g. up to 13%) patterning device alignment mark MAF, assuming that the geometric design which is still binary, the DC and M ratios may be modelled respectively as:

[0119] Averaging the above two terms gives:which is independent from the absorber reflectivity. This may make absorber reticle-to-reticle error contribution to detection accuracy effectively negligible.

[0120] Therefore, to take advantage this, the pellicle monitoring method may further comprise: using the PARIS, measuring 1S^^^and T^^^being, respectively, the offset component and the magnitude of the phase-modulated component of the pixel or group of pixels resulting from light reflected from the stage alignment mark; using the PARIS, measuring 1S^^^and T^^^being, respectively, the offset component and the magnitude of the phase-modulated component of the pixel or group of pixels resulting from light reflected from the patterning device alignment mark; calculating the detectionintensity ratio, PDR, as (1S + T^^^^^) / 2, wherein 1S^^^^^

[0121] With certain pellicle materials, including but not limited to CNT pellicles, the pellicle MP may degrade gradually rather than catastrophically. For example, the pellicle MP may tear locally at first, and the area of failure may expand over time, folding and wrapping around until the entire pellicle MP has failed. It may be desirable to detect the failure during early stages of the process, before total failure of the pellicle MP has occurred.

[0122] However, early stages of failure of the pellicle MP may be difficult to detect using localised measurements of the second intensity ^^^^. That is, unless the point of measurement of the second intensity ^^^^happens to coincide with a point of failure of the pellicle MP, early stages of failure may go undetected.

[0123] One proposal was to monitor the entire surface of the pellicle MP at many sample points. However, such a process would require a large amount of measurement time. Furthermore, the patterning device MA would have to incorporate a large number of measurement marks (one for each sample point), which would take up a large amount of surface area that could otherwise be used for exposure features. In particular, if the entire surface of the pellicle MP is to be checked before every exposure of a substrate W, each exposure cycle may be lengthened by a corresponding amount, which may significantly reduce the throughput of the lithographic apparatus LA. In some scenarios, even sampling the entire surface of the pellicle MP once per batch of exposures may result in an unacceptable Company Secretreduction in the throughput of the lithographic apparatus LA. For reference, in one proposed measurement scheme, it takes about 30 to 60 seconds to sample the entire surface of the pellicle MP.

[0124] Therefore, it may be desirable to monitor the entire surface of the pellicle MP without costing too much throughput of the lithographic apparatus LA.

[0125] As noted above, the output of the PARIS provides two wavefront data so that, for each of thepixels in the sensor, the measured intensity, I, may described as ^ = 1S + T UVW(X). As found by thepresent inventors, although the phase term, X, is not sensitive to the pellicle transmission ^^^^as such, it carries information about the wavefront across the pixel array of the PARIS, from which aberrations in the optical system may be derived.

[0126] It is important to note that, however, when a pellicle MP is mounted on a patterning device MA, even if it has a high transmission ^^^^, its presence may still contribute to the overall aberration content measurable by the PARIS. In particular, local inhomogeneity in the pellicle material, defects, variations in transmission etc., can be expected to have an influence on the higher order aberrations.

[0127] Therefore, as the pellicle MP progressively wears and a crack opens on its membrane structure, while still being mounted over the patterning device MA, its mechanical structure can become severely altered. This can result in a different mechanical equilibrium, different asymmetric tension and membrane stresses, wrinkles, etc., each of which may modify the state of the pellicle MP from its original intact state. That is, localised damages may produce globally observable changes to the pellicle MP. These changes may in turn alter the aberrations measurable by the PARIS. More specifically, these changes may impart a different “fingerprint” in terms of Zernike polynomials that can be extracted from the measurements by the PARIS.

[0128] Therefore, as found by the present inventors, the output of the PARIS may enable detection of early stages of failure of the pellicle MP. In particular, the Zernike aberration terms derivable from the output of the PARIS may be used. Furthermore, although the PARIS may make localised measurements of the pellicle MP, the global status of the pellicle MP may be inferred from the output of the PARIS. Therefore, by using the output of the PARIS, it may not be necessary to sample the entire surface of the pellicle MP. In addition, it may be possible to extract Zernike aberration terms from PARIS measurements that are already performed during alignment of the patterning device MA, so that no additional measurements may be necessary, and no time penalty may be added to the exposure cycles.

[0129] Specifically, when using a PARIS, each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, may be a field of intensity values corresponding to the array of pixels output by the PARIS. The method may thus comprise determining one or more non-zeroeth order term of the Zernike polynomials of the aberrations present in ^^^^and / or ^^^^. For example, the method may comprises determining at least one Zernike term of 2ndor 3rdorder (the 2ndand 3rdorder being 90° rotated with respect to each other, but otherwise having the same angular and radial frequencies) or Company Secretgreater of aberrations present in ^^^^and / or ^^^^, the order of a Zernike aberration term being defined according to the Noll ordering scheme (Noll, R. J. (1976). “Zernike polynomials and atmospheric turbulence”. J. Opt. Soc. Am. 66 (3): 207. Bibcode:1976JOSA...66..207N. doi:10.1364 / JOSA.66.000207.). Several Zernike terms of 2ndor 3rdorder or greater may be determined. For example, at least one Zernike term of the 5thor 6thorder or greater may be calculated.

[0130] Although deteriorations of the pellicle MP may manifest themselves in all Zernike terms, Zernike terms of different orders may be affected more or less by different factors. For example, lower- order Zernike terms (up to the 4thorder, for example) may be more affected by translational alignment errors (e.g. caused by other parts of the lithographic apparatus LA, in particular the patterning device stage MT), and higher-order Zernike terms may be more indicative of deteriorations of the pellicle MP. For example, for the purpose of the present invention, “higher-order” Zernike terms may comprise the 5thor 6thorder and above, or the 7thor 8thorder and above, or the 9thor 10thorder and above, or the 11thorder and above, or the 12thor 13thorder and above, or the 14thor 15thorder and above.

[0131] The method may further comprise comparing the Zernike aberrations in the second detection intensity, k^^^, of the current exposure with the Zernike aberrations in the first detection intensity, k^^^. For example, the Zernike aberrations in the first detection intensity, k^^^, which is measured from the stage alignment mark MTF, may be subtracted from the second detection intensity, k^^^, of the current exposure, which is measured from the patterning device alignment mark MAF. As noted above, the first detection intensity ^^^^may not necessarily be measured at every exposure of the substrate W. For example, in some scenarios, the first detection intensity ^^^^may be measured once per batch of exposures. This comparison may cancel out any aberration contributions from the various components along the optical column (i.e. from the patterning device MA through to the substrate W, including any intervening mirrors M0, M1, M3, M4 and other optical components), the alignment marks MAF and MTF, as well as contributions from the PARIS itself. As a result, it can be assumed that any remaining contributions may come from the patterning device MA and the pellicle MP.

[0132] It should be understood that the Zernike aberrations in the second detection intensity, k^^^, may be subjected to other additional or alternative normalization processes or checks.

[0133] For example, as shown in Figure 5a, the method may further comprise comparing the Zernike aberrations in the second detection intensity, k^^^, of the current exposure with those of the first exposure within the same batch of exposures. This may, for example, be done within each batch of exposures. This comparison may enable any contribution from the patterning device MA to be cancelled out.

[0134] For example, as shown in Figure 5b, the method may further comprise comparing the Zernike aberrations in the second detection intensity, k^^^, of the current exposure with those of the immediately preceding exposure. This comparison may enable the detection of small variations from one exposure to the next exposure, and may increase the detection frequency. Company Secret

[0135] For example, as also shown in Figure 5b, the method may further comprise comparing the Zernike aberrations in the second detection intensity, k^^^, of the first exposure of the current batch of exposures with those of the last exposure of the immediately preceding batch of exposures. This comparison may enable the detection of small variations mentioned above to include the first exposure in a batch of exposures. This comparison may be particular effective when the same patterning device MA and / or the same pellicle MP is used in both the current batch of exposures and the previous batch of exposures.

[0136] For example, as shown in Figure 5c, the method may further comprise comparing the Zernike aberrations in the second detection intensity, k^^^, of the current exposure of the current batch of exposures with those of the corresponding exposure of the immediately preceding batch of exposures.

[0137] It should be understood that the above comparisons may be freely combined as necessary.

[0138] It should also be understood that the above comparisons may be based on any Zernike term of 2ndor 3rdorder or greater. Furthermore, the above comparisons may be based on several Zernike terms of 2ndor 3rdorder or greater. For example, the comparisons may be performed term against term. Additionally or alternatively, two or more of the Zernike terms can be combined together (e.g. adding the coefficients as a weighted sum) before the comparison.

[0139] Each of the above comparisons may be used to determine if the pellicle MP is worn. For example, for each of the above comparisons, the method may comprise indicating that the pellicle is worn if the difference in the at least one Zernike term of 2ndor 3rdorder or greater exceeds a predetermined threshold.

[0140] It should be understood that Zernike aberrations may also be used for monitoring the pellicle MP with Parallel ILIAS (as explained above). With Parallel ILIAS, since measurements are made at multiple locations simultaneously, it may be possible to construct an aberration map of the entire pellicle MP surface using the Zernike terms extracted from each of the PARIS sensors in the Parallel ILIAS stem. Furthermore, with Parallel ILIAS, further metrics may be defined by observing the differences in the measurements from the different PARIS (in particular the PARIS sensors corresponding to the patterning device alignment marks MAF located at the extreme ends of the slit, or those located at geometrical nodes or any location over the pellicle MP area that is significant for specific Zernike “fingerprint”).

[0141] Furthermore, in addition or as an alternative to monitoring early stages of failure of the pellicle MP, the Zernike aberrations may be used for monitoring the health of the pellicle MP or for estimating the remaining lifetime (due to ageing and degradation) of the pellicle MP.

[0142] In addition to or as an alternative to using the stage alignment mark MTF and the patterning device alignment mark MAF, pellicle monitoring may be performed using light reflected from the patterning device MA. Specifically, pellicle monitoring may be performed using maps of detection intensities of the patterning device MA. Specifically, the measuring the first detection intensity may comprise measuring the intensity of light reflected from a plurality of locations of the patterning device Company SecretMA, thereby producing a first detection map. The measuring the second detection intensity may be performed after the measuring the first detection intensity, and may comprise measuring the intensity of light reflected from the plurality of locations of the patterning device MA, thereby producing a second detection map. The comparing the first and second detection intensities may comprise comparing the first and second detection maps.

[0143] The values of the first and second detection maps may be offset by the attenuation caused by the patterning device MA itself. That is, the first and second detection maps may be offset by a reference detection map measured on a “bare” patterning device MA. That is, the reference detection map may be measured without a pellicle MP intersecting the light path between the alignment sensor and the patterning device MA.

[0144] The comparing the first and second detection maps may comprise calculating a difference map by subtracting the first and second detection maps one from the other.

[0145] In embodiments where the first and second detection maps are offset by the reference detection map, the values in the difference map may be assumed to be entirely or predominantly determined by the local transmission value of the pellicle MP at the plurality of locations. Furthermore, the values in the difference map may be normalized by the corresponding values in the reference detection map (obtained from a bare patterning device MA), so that the difference map may contain changes in the local transmission values of the pellicle MP (e.g. in terms of percent change of the values in the first detection map, or as dimensionless numbers).

[0146] In embodiments where the first detection map is obtained in the presence of a pellicle MP, the values in the difference map may represent changes in the detection intensity at the plurality of locations of the pellicle MP.

[0147] Various metrics may be determined from the first and second detection maps, and may be used in isolation or in combination for determining the state of the pellicle MP.

[0148] For example, the average difference may be calculated from the difference map. It may be determined that a pellicle MP is absent or ruptured if the average difference exceeds a predetermined average difference threshold. The predetermined average difference threshold may be variously determined. For example, the predetermined average difference threshold may be determined experimentally. Alternatively, the predetermined average difference threshold may be determined from historical data. For example, the predetermined average difference threshold may be set to correspond to the envelope of transmission values of a population of pellicles MP that are known to be intact.

[0149] Figure 6a depicts the difference map of a pellicle MP that is determined to be intact. This difference map is produced by subtracting the first detection map from the second detection map. In this example, the first detection map and second detection map have each been offset by a reference detection map obtained for a bare patterning device MA (i.e. in the absence of a pellicle MP). Therefore, the first and second detection maps contain values, taken at two time points, are assumed to measure the contribution from the pellicle MP and assumed to exclude any attenuation caused by the patterning Company Secretdevice MA. The values in the difference map have also been normalized so that they, effectively, are a measure of the change in local transmission values at various location of the pellicle MP (i.e. a value of 0 corresponds to no change in transmission; a value of +1% corresponds to a 1 percentage point increase in transmission). In Figure 6a, the vertical axis shows the % change in transmission. Figure 6b is a histogram of the difference map of Figure 6a.

[0150] As can be seen visually in Figure 6b, the average difference is approximately 0% (dRT = 0.1%), which is below the predetermined average difference threshold, which is set to 2.0% (i.e. a 2.0 percentage point increase in transmission) in this example.

[0151] Figures 7a and 7b show an example in which the pellicle MP is determined to have ruptured. These plots are produced in the same way as Figures 6a and 6b, except with data from another scenario. As with Figures 6a and 6b, Figure 7a is a difference map showing changes in local transmission values, and Figure 7b is a histogram of the difference map of Figure 7a.

[0152] As can be seen From Figure 7b, the average difference (dRT) is around 2.4%, which exceeds the 2.0% predetermined average difference threshold. As a result, the pellicle MP is determined to have ruptured.

[0153] Although the average difference is an effective metric, there are scenarios where a ruptured pellicle MP may escape detection based on the average difference alone. For example, if the rupture of the pellicle MP is very localized, the contribution of the rupture to the average difference may be relatively small, and the average difference may not exceed the predetermined average difference threshold. Nevertheless, the pellicle MP may require immediate replacement even if the rupture is localized.

[0154] Therefore, other metrics may be used in addition or as alternatives. In particular, metrics that measure the distribution of the values in the detection maps and / or difference map may be used.

[0155] For example, a metric may be defined to measure how widely spread the values in the difference map are. Specifically, the difference map may be transformed into a histogram of difference values. From the histogram of difference values, a distribution width of the difference values may be calculated. Different definitions of the distribution width may be used. For example, the width may be defined as the width of the smallest range of difference values that encompasses a majority (e.g. 70%, 80%, 90%, 95% or 98%) of data points. For another example, the distribution width may be the standard deviation in the values of the difference map. It may be determined that a pellicle is absent or ruptured if the distribution width exceeds a predetermined distribution width threshold. The predetermined distribution width threshold may be determined experimentally, or may be determined from historical data. For example, the predetermined distribution width threshold may be determined as a multiple of the standard deviation in the transmission values of a population of pellicles MP that are known to be intact.

[0156] Figures 8a and 8b depict using the distribution width to detect a ruptured pellicle MP. Figures 8a and 8b are generated in the same way as Figures 6a, 6b, 7a and 7b, except using data from a different Company Secretscenario. Figure 8a shows the difference map of this example, and Figure 8b is a histogram of the difference map. In the example shown in Figures 8a and 8b, the pellicle MP is not determined to have ruptured according to the average difference metric discussed above, because the average difference (dRT) is only 1.8%, which is below the 2.0% threshold. As can be seen visually from Figure 8a, the rupture is relatively localized, and this results in a relatively small change to the average difference. However, the distribution width (here defined as the width of the smallest range of difference values that encompasses 90% of the data points) is 6.8%, which is above the predetermined distribution width threshold, in this example set to 4.0% (i.e.4 percentage points in terms of transmission).

[0157] As noted above, the distribution width may be used alone for detecting a ruptured pellicle MP. Therefore, the example shown in Figures 7a and 7b would also result in a determination of a rupture pellicle MP because the width of distribution in the histogram of Figure 7b (defined in the same way as in Figure 8b, as described above) is 7.6%, which exceeds the 4.0% threshold.

[0158] Alternatively, the distribution width metric may be used in combination with other metrics, such as the average difference mentioned above. More particularly, the average difference may be used as a primary metric, and the distribution width may be used as a secondary metric. That is, the distribution width metric may be used only when the pellicle has not been determined as ruptured using the average difference metric. More specifically, the step of determining that the pellicle is absent or ruptured if the distribution width exceeds the predetermined distribution width threshold may be performed only if the average difference is determined not to exceed the predetermined average difference threshold. For example, as noted above, whereas the scenario in Figures 8a and 8b does not result in a determination of a ruptured pellicle MP according to the average difference metric, it does under the distribution width metric. Conversely, because the scenario in Figures 7a and 7b results in a determination of a ruptured pellicle MP according to the average difference metric, it is not necessary to also apply the distribution width metric.

[0159] As yet another metric that may be used on its own or in combination with the average difference metric and / or the distribution width metric, the state of the pellicle MP may be determined based on the mode of distribution of the detection values. For example, the first and second detection maps may be transformed, respectively, into first and second histograms of detection values. Thereafter, a first distribution mode may be determined from the first histogram of detection values, and a second distribution mode may be determined from the second histogram of detection values. Generally, although the transmission values of an intact pellicle MP will exhibit some spread, the pellicle MP tends to be homogeneous and the transmission values tend to exhibit a monomodal distribution. On the contrary, a damaged or ruptured pellicle MP may be less homogeneous, and the transmission values tend to exhibit a multimodal distribution. In particular, in cases where the pellicle MP has ruptured in such a way that there are regions where the pellicle MP is absent and other regions where the pellicle MP is intact, a bimodal distribution in the transmission values can be expected. Company Secret

[0160] Therefore, it may be determined that a pellicle is ruptured if the first distribution mode is a monomodal distribution, and the second distribution mode is a multimodal distribution (or, more specifically, a bimodal distribution).

[0161] Although Figures 6a, 7a and 8a are difference maps rather than maps of detection values, and Figures 6b, 7b and 8b are histograms of the corresponding ones of Figures 6a, 7a and 8a, the histograms of detection values exhibit the same distribution modes as the corresponding histograms of difference values.

[0162] As can be seen in Figure 6b (for an intact pellicle MP), the histogram shows a monomodal distribution. Therefore, the pellicle MP is not determined to have ruptured according to the distribution mode metric.

[0163] In each of the examples of Figures 7b and 8b, the pellicle MP is determined to have ruptured according to the distribution mode metric. In each of Figures 7b and 8b, the histogram shows a multimodal (specifically, bimodal) distribution. A comparison of either Figure 7b or 8b with Figure 6b shows that the distribution mode has changed from monomodal to multimodal, and the pellicle MP is determined to have ruptured according to the distribution mode metric. More broadly, a change away from a monomodal distribution (i.e. a change from monomodal to multimodal) may indicate rupture.

[0164] In addition to a method of pellicle detection, there is also disclosed a method of manufacturing devices comprising the method of pellicle detection disclosed above.

[0165] There is also disclosed a computer program comprising instructions to cause a lithographic apparatus to perform the method of pellicle detection disclosed above.

[0166] There is also disclosed a lithographic apparatus configured to perform the method of pellicle detection disclosed above.

[0167] 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, flat-panel displays, liquid-crystal displays (LCDs), thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and / or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains one or multiple processed layers. Company Secret

[0168] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications.

[0169] Aspects of the invention are described in the following numbered clauses. 1. A method of pellicle monitoring using an alignment sensor of a lithographic apparatus, wherein: the lithographic apparatus comprises a patterning device stage for supporting a patterning device; the patterning device stage comprises a stage alignment mark fixedly provided thereon; the patterning device comprises a patterning device alignment mark fixedly provided thereon; the alignment sensor is configured to sense alignment using the patterning device alignment mark and the stage alignment mark; and the lithographic apparatus is configured to position a pellicle such that the pellicle intersects a light path between the alignment sensor and the patterning device; wherein the method comprises: using the alignment sensor, measuring a first detection intensity; using the alignment sensor, measuring a second detection intensity, being the intensity of light reflected from the patterning device; and comparing the first and second detection intensities. 2. The method of clause 1, wherein: the lithographic apparatus is configured to position a pellicle such that the pellicle intersects the light path between the alignment sensor and the patterning device alignment mark, and does not intersect the light path between the alignment sensor and the stage alignment mark the first detection intensity, ^^^^, is the intensity of light reflected from the stage alignment mark; the second detection intensity, ^^^^, is the intensity of light reflected from the patterning device alignment mark. 3. The method of clause 2, further comprising adjusting the relative position between a light beam and the patterning device stage such that: when measuring the first detection intensity, at least a portion of the light beam is incident on the stage alignment mark, the reflection of which being measured as the first detection intensity; and when measuring the second detection intensity, at least a portion of the light beam is incident on the patterning device alignment mark, the reflection of which being measured as the second detection intensity, and the portion of the light beam passes through the pellicle in case the pellicle is present. 4. The method of clause 3, further comprising: measuring a first source intensity, ^^,^^^, being the intensity of the light beam when measuring the first intensity; measuring a second source intensity, ^^,^^^, being the intensity of the light beam when measuring the second intensity; Company Secretcalculating a compensation factor as ^^,^^^⁄ ^^,^^^ ; andbefore comparing the first and second detection intensities, adjusting the second detection intensity by multiplying the second detection intensity, ^^^^, by the compensation factor. 5. The method of clause 3 or clause 4, wherein each of the patterning device alignment mark and the stage alignment mark is defined by a geometric arrangement of a reflective portion and an absorber portion, and the method further comprises: measuring ^^^^,-., being the intensity of light reflected from the absorber portion of the stage alignment mark; measuring ^^^^,-., being the intensity of light reflected from the absorber portion of the patterning device alignment mark; obtaining geometric factor / ^^^, being the area of the absorber portion of the stage alignment mark as a fraction of the total area of the stage alignment mark; obtaining geometric factor / ^^^, being the area of the absorber portion of the patterning device alignment mark as a fraction of the total area of the patterning device alignment mark; before comparing the first and second detection intensities, adjusting the first detection intensity, ^^^^,by subtracting / ^^^ ∙ ^^^^,-. , and adjusting the second detection intensity, ^^^^, by subtracting / ^^^ ∙^^^^,-.. 6. The method of any one of clauses 2 to 5, wherein the comparing the first and second detection intensities comprises: calculating a detection intensity ratio, PDR, the detection intensity ratio being a ratio between the first detection intensity, ^^^^, and the second detection intensity, ^^^^; and comparing the detection intensity ratio against a detection threshold, Th. 7. The method of clause 6, wherein the detection threshold, Th, is a predetermined threshold. 8. The method of clause 6 or clause 7, further comprising: obtaining the transmission of a reference pellicle to the measured light, ^^^^, and setting the detection threshold, Th, based on ^^^^. 9. The method of clause 8, wherein the detection intensity ratio, PDR, is calculated as ^^^^ / ^^^^, and the method further comprises determining that a pellicle is absent or ruptured if the detection intensity ratio, PDR, exceeds the detection threshold, Th. 10. The method of clause 9, wherein the detection intensity ratio, PDR, is measured and calculated in each of a plurality of exposure cycles, and the method further comprises: obtaining the measurement uncertainty value !^^^^associated with the measurement of the detection intensities; calculating a difference between the detection intensity ratio in exposure cycle n + 1, ^1^^23, and that in the previous exposure cycle, ^1^^; Company Secretdetermining that the pellicle is intact in exposure cycle n + 1 if (^1^^23 − ^1^^) < 53 ∙ !^^^^, where53is a predetermined constant equal to or greater than√2. 11. The method of clause 10, wherein the determining that the pellicle is intact comprises overriding any determination that the pellicle is absent or ruptured. 12. The method of clause 9, wherein the detection threshold, Th, is set to a value between ^^^^^and 1. 13. The method of any one of clauses 8 to 12, further comprising obtaining a measurement uncertainty value !^^^^associated with the measurement of the detection intensities, wherein the detection threshold, Th, is set to bewhere 5^ is apredetermined constant equal to or greater than√2. 14. The method of any one of clauses 8 to 12, further comprising: obtaining a reference second detection intensity, ^^^^,^^^, being the second detection intensity measurable when a reference patterning device and no pellicle are installed in the lithographic apparatus; and calculating a reference detection intensity ratio, ^1^^^^^, as ^^^^,^^^ / ^^^^; 329 wherein the detection threshold, Th, is set to ^1^:= ;<^^^^∙ ^ . 15. The method of any one of clauses 2 to 14, further comprising: measuring and recording a transmission value of the pellicle at each of a plurality of time points; recording an exposure count, x, of the pellicle when each of the plurality of transmission values is measured; using the plurality of transmission values and the plurality of exposure counts as input data to a prediction model, predicting the number of exposures remaining, X, before the transmission value of the pellicle is expected to reach a replacement transmission threshold, ^ℎ^^^. 16. The method of clause 15, wherein the detection intensity ratio, PDR, calculated as ^^^^ / ^^^^, is used as the measured transmission value of the pellicle at each time point. 17. The method of clause 16, wherein the replacement transmission threshold, ^ℎ^^^, is set to a value less than the detection threshold, Th. 18. The method of any one of clauses 15 to 17, further comprising measuring and recording the hydrogen pressure around the pellicle when each of the plurality of transmission values is measured; wherein the input data further comprises the plurality of hydrogen pressure measurements. 19. The method of any one of clauses 15 to 18, further comprising measuring and recording the temperature of the pellicle or of the environment surrounding the pellicle when each of the plurality of transmission values is measured; wherein the input data further comprises the plurality of temperature measurements. Company Secret20. The method of any one of clauses 15 to 19, wherein the prediction model predicts the number of exposures remaining by performing regression analysis on the input data, and extrapolating therefrom. 21. The method of clause 20, wherein the regression analysis comprises one or more of linear regression, quadratic regression, and exponential regression. 22. The method of any one of clauses 15 to 19, wherein the prediction model comprises a trained machine learning model. 23. The method of any one of clauses 15 to 22, further comprising applying a noise filter to the input data. 24. The method of clause 23, wherein the noise filter comprises a low-pass filter. 25. The method of any one of clauses 6 to 24, further comprising: at an initial time point t0, measuring a reference first detection intensity, ^^^^,^^^(^^), being the first detection intensity measurable when a reference stage alignment mark is installed in the lithographic apparatus, and measuring the first detection intensity, ^^^^(^^); at a subsequent time point t1, re-measuring the reference first detection intensity, ^^^^,^^^(^3), and re- measuring the first detection intensity, ^^^^(^3); ? culating a drift factor, 1( )? cal@AB CD^^^(>3), as?@AB,E;@(CD) / ?@AB,E;@(CF); and at the subsequent time point t1, adjusting the detection intensity ratio, PDR, by multiplying it by the drift factor, 1^^^(>3), before comparing it against the detection threshold, Th. 26. The method of any one of clauses 6 to 25, further comprising: at an initial time point t2, measuring a reference first detection intensity, ^^^^,^^^(^^), being the first detection intensity measurable when a reference stage alignment mark is installed in the lithographic apparatus, and measuring the second detection intensity, ^^^^(^^); at a subsequent time point t3, re-measuring the reference first detection intensity, ^^^^,^^^(^G), and re- measuring the second detection intensity, ^^^^(^G); calculating a second drift factor, 1^^^at the subsequent time point t3, adjusting the detection intensity ratio, PDR, by multiplying it by the second drift factor, 1^^^(>G), before comparing it against the detection threshold, Th. 27. The method of clause 26, further comprising: associating the second drift factor, 1^^^(>G), with the patterning device and pellicle installed in the lithographic apparatus at time point t3, and storing the second drift factor, 1^^^(>G); uninstalling the patterning device and pellicle; at a subsequent time point t4after time point t3, reinstalling the patterning device and pellicle and retrieving the second drift factor, 1^^^(>G) associated with the patterning device and pellicle. 28. The method of any one of clauses 3 to 27, further comprising: Company Secretat an initial time point t5, measuring ^^,^I, being the intensity of the light beam at time point t5, and ^^^^,^I, being the second detection intensity at time point t5; ? calculate a system transmission factor at time point tE;C,CK5, ^^J^,^I, as ^F,CK; at a subsequent time point t6, measuring ^^,^L, being the intensity of the light beam at time point t6, and ^^^^,^L, being the second detection intensity at time point t6; ? calculate the system transmission factor at time point tE;C,CM6, ^^J^,^L, as ^F,CM; and before comparing the first and second detection intensities after the subsequent time point t6, adjustingthe second detection intensity by multiplying the second detection intensity, ^^^^, by ^^J^,^I⁄ ^^J^,^L .29. The method of any one of clauses 2 to 28, wherein the lithographic apparatus comprises a second alignment sensor, wherein the method further comprises:obtaining a sensitivity ratio, ^N⁄ ^^ , being a ratio between the sensitivity of the first alignment sensor,^N, and that of the second alignment sensor, ^^; using the second alignment sensor, measuring ^^^^,^, being the intensity of light reflected from the patterning device alignment mark;adjusting ^^^^,^ by multiplying it with the sensitivity ratio, ^N⁄ ^^ ; andcomparing ^^^^and ^^^^,^. 30. The method of clause 29, wherein ^^^^is measured in exposure cycle m, ^^^^,^is measured inexposure cycle m + 1 immediately following the exposure cycle m, and the sensitivity ratio, ^N⁄ ^^ , isset to be ^^^^ / ^^^^,^. 31. The method of clause 30, further comprising using the first alignment sensor to detect the pelliclein exposure cycle m + 2 following the exposure cycle m + 1, wherein the sensitivity ratio, ^N⁄ ^^ , isset to be ^^^^ / ^^^^,^only if the pellicle is detected to be intact in exposure cycle m + 2. 32. The method of any one of clauses 2 to 31, wherein the patterning device comprises a second patterning device alignment mark fixedly provided thereon, wherein the method further comprises: obtaining an alignment mark intensity ratio ^^^^ON / ^^^^O^, ^^^^ONbeing the intensity of reflected light measurable from the first patterning device alignment mark and ^^^^O^being the intensity of reflected light measurable from the second patterning device alignment mark under equal conditions; using the alignment sensor, measuring ^^^^,^^^O^, being the intensity of light reflected from the second patterning device alignment mark; ? substituting ^^^^,^^^O^∙^,EP Q?^,EPRas the second detection intensity before the comparing the first and second detection intensities. 33. The method of any one of clauses 2 to 32, wherein the first and second detection intensities are measured during a patterning device alignment operation. Company Secret34. The method of any one of clauses 2 to 33, wherein at least the steps of measuring the second detection intensity and comparing the first and second detection intensities are repeated at each exposure cycle. 35. The method of any one of clauses 2 to 34, further comprising halting the lithographic apparatus upon detecting that the pellicle is absent or ruptured. 36. The method of any one of clauses 2 to 35, wherein the lithographic apparatus further comprises a dynamic gas lock membrane, DGLm, that intersects both the light path between the alignment sensor and the patterning device alignment mark, and the light path between the alignment sensor and the stage alignment mark, the method further comprising: obtaining the transmission of the DGLm, ^^^^^; determining that the DGLm has ruptured upon detection that the first and / or second detection intensityin an exposure cycle has increased by a factor of (1 − ^^^^^) compared with a previous exposure cycle.37. The method of any one of clauses 2 to 36, wherein the alignment sensor comprises a transmission image sensor. 38. The method of any one of clauses 2 to 37, wherein: each of the stage alignment mark and the patterning device alignment mark comprises orthogonal gratings; the alignment sensor comprises a shearing interferometer phase-stepping measurement sensor, PARIS; and the PARIS outputs an array of pixels, each pixel having an offset component, DC, and a phase- modulated component, T UVW(X). 39. The method of clause 38, wherein each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, is derived from the offset component, DC, of the same pixel or group of pixels. 40. The method of clause 38 or clause 39, wherein each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, is derived from the phase-modulated component, T UVW(X), of the same pixel or group of pixels. 41. The method of clause 40, wherein each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, is derived from the magnitude of the phase-modulated component, M, of the same pixel or group pixels. 42. The method of clause 39, wherein each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, is derived from a linear combination of the offset component, DC, and the magnitude of the phase-modulated component, M, of the same pixel or group of pixels. 43. The method of any one of clauses 38 to 42, further comprising: using the PARIS, measuring 1S^^^and T^^^being, respectively, the offset component and the magnitude of the phase-modulated component of the pixel or group of pixels measurable from the stage alignment mark; Company Secretusing the PARIS, measuring 1S^^^and T^^^being, respectively, the offset component and the magnitude of the phase-modulated component of the pixel or group of pixels resulting from light reflected from the patterning device alignment mark; calculating and monitoring the value ofupon detection that_E,CAghas changed by more than a predetermined amount, determining that the measurements are unreliable. 44. The method of any one of clauses 38 to 43, further comprising averaging the group of pixels. 45. The method of any one of clauses 38 to 44, wherein the group of pixels is located at the centre of the field of view of the PARIS. 46. The method of any one of clauses 38 to 45, wherein: the patterning device stage comprises a plurality of stage alignment marks fixedly provided thereon, and / or the patterning device comprises a plurality of patterning device alignment marks fixedly provided thereon; the PARIS comprises a plurality of detectors for measuring light reflected from the plurality of alignment marks on the patterning device stage and / or the patterning device; and each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, is derived from an average of the outputs from the plurality of detectors. 47. The method of any one of clauses 38 to 46, comprising: using the PARIS, measuring 1S^^^and T^^^being, respectively, the offset component and the magnitude of the phase-modulated component of the pixel or group of pixels resulting from light reflected from the stage alignment mark; using the PARIS, measuring 1S^^^and T^^^being, respectively, the offset component and the magnitude of the phase-modulated component of the pixel or group of pixels resulting from light reflected from the patterning device alignment mark;calculating the detection intensity ratio, PDR, as (1S^^E;C^^^^^ + T^^^^^) / 2, wherein 1S^^^^^ =^^@ABand T = _E;C^^^^^_@AB. 48. The method of any one of clauses 38 to 47, wherein each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, is a field of intensity values corresponding to the array of pixels output by the PARIS, and the method further comprises determining at least one Zernike term of the 2ndor 3rdorder or greater of aberrations present in ^^^^and / or ^^^^, the order of a Zernike aberration term being the defined according to the Noll ordering scheme. 49. The method of clause 48, further comprising comparing the Zernike aberrations in the second detection intensity, k^^^, of the current exposure with the Zernike aberrations in the first detection intensity, k^^^. Company Secret50. The method of clause 48 or clause 49, further comprising comparing the Zernike aberrations in the second detection intensity, k^^^, of the current exposure with those of the first exposure within the same batch of exposures. 51. The method of any one of clauses 48 to 50, further comprising comparing the Zernike aberrations in the second detection intensity, k^^^, of the current exposure with those of the immediately preceding exposure. 52. The method of any one of clauses 48 to 51, further comprising comparing the Zernike aberrations in the second detection intensity, k^^^, of the first exposure of the current batch of exposures with those of the last exposure of the immediately preceding batch of exposures. 53. The method of any one of clauses 48 to 52, further comprising comparing the Zernike aberrations in the second detection intensity, k^^^, of the current exposure of the current batch of exposures with those of the corresponding exposure of the immediately preceding batch of exposures. 54. The method of any one of clauses 49 to 53, wherein the comparing the Zernike aberrations comprises calculating a difference in at least one Zernike term of the 2ndor 3rdorder or greater, optionally at least one Zernike term of the 5thor 6thorder or greater 55. The method of clause 54, further comprising indicating that the pellicle is worn if the difference in the at least one Zernike term of the 2ndor 3rdorder or greater exceeds a predetermined threshold. 56. The method of any one of the preceding clauses, wherein: the measuring the first detection intensity comprises measuring the intensity of light reflected from a plurality of locations of the patterning device, thereby producing a first detection map; the measuring the second detection intensity is performed after the measuring the first detection intensity, and comprises measuring the intensity of light reflected from the plurality of locations of the patterning device, thereby producing a second detection map; the comparing the first and second detection intensities comprises comparing the first and second detection maps. 57. The method of clause 56, wherein each of the first and second detection maps is offset by a reference detection map measured without a pellicle intersecting the light path between the alignment sensor and the patterning device. 58. The method of clause 56 or clause 57, wherein comparing the first and second detection maps comprises calculating a difference map by subtracting the first and second detection maps one from the other. 59. The method of clause 58, further comprising calculating the average difference from the difference map. 60. The method of clause 59, further comprising determining that a pellicle is absent or ruptured if the average difference exceeds a predetermined average difference threshold. 61. The method of any one of clauses 58 to 60, further comprising transforming the difference map into a histogram of difference values. Company Secret62. The method of clause 61, further comprising calculating a distribution width of the difference values from the histogram of difference values. 63. The method of clause 62, further comprising determining that a pellicle is absent or ruptured if the distribution width exceeds a predetermined distribution width threshold. 64. The method of clause 63, wherein the step of determining that the pellicle is absent or ruptured if the distribution width exceeds the predetermined distribution width threshold is performed only if the average difference is determined not to exceed the predetermined average difference threshold. 65. The method of any one of clauses 56 to 64, further comprising transforming the first and second detection maps respectively into first and second histograms of detection values. 66. The method of clause 65, further comprising: determining a first distribution mode from the first histogram of detection values; and determining a second distribution mode from the second histogram of detection values. 67. The method of clause 66, further comprising determining that a pellicle is ruptured if: the first distribution mode is a monomodal distribution; and the second distribution mode is a multimodal distribution. 68. The method of clause 66, further comprising determining that a pellicle is ruptured if: the first distribution mode is a monomodal distribution; and the second distribution mode is a bimodal distribution. 69. The method of any one of the preceding clauses, wherein the pellicle is made of a material that increases in transmission with the number of exposures. 70. The method of any one of the preceding clauses, wherein the pellicle is made of a carbon nanotube material. 71. A method of manufacturing devices comprising the steps of the method of any one of the preceding clauses. 72. A computer program comprising instructions to cause a lithographic apparatus to perform the method of any one of clauses 1 to 70. 73. A lithographic apparatus configured to perform the method of any one of clauses 1 to 70.

[0170] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0171] 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. Company Secret

Claims

CLAIMS 1. A method of pellicle monitoring using an alignment sensor of a lithographic apparatus, wherein: the lithographic apparatus comprises a patterning device stage for supporting a patterning device; the patterning device stage comprises a stage alignment mark fixedly provided thereon; the patterning device comprises a patterning device alignment mark fixedly provided thereon; the alignment sensor is configured to sense alignment using the patterning device alignment mark and the stage alignment mark; and the lithographic apparatus is configured to position a pellicle such that the pellicle intersects a light path between the alignment sensor and the patterning device; wherein the method comprises: using the alignment sensor, measuring a first detection intensity; using the alignment sensor, measuring a second detection intensity, being the intensity of light reflected from the patterning device; and comparing the first and second detection intensities.

2. The method of claim 1, wherein: the lithographic apparatus is configured to position a pellicle such that the pellicle intersects the light path between the alignment sensor and the patterning device alignment mark, and does not intersect the light path between the alignment sensor and the stage alignment mark the first detection intensity, ^^^^, is the intensity of light reflected from the stage alignment mark; the second detection intensity, ^^^^, is the intensity of light reflected from the patterning device alignment mark.

3. The method of claim 2, further comprising adjusting the relative position between a light beam and the patterning device stage such that: when measuring the first detection intensity, at least a portion of the light beam is incident on the stage alignment mark, the reflection of which being measured as the first detection intensity; and when measuring the second detection intensity, at least a portion of the light beam is incident on the patterning device alignment mark, the reflection of which being measured as the second detection intensity, and the portion of the light beam passes through the pellicle in case the pellicle is present.

4. The method of claim 3, further comprising: measuring a first source intensity, ^^,^^^, being the intensity of the light beam when measuring the first intensity; Company Secretmeasuring a second source intensity, ^^,^^^, being the intensity of the light beam when measuring the second intensity; calculating a compensation factor as ^^,^^^⁄ ^^,^^^ ; andbefore comparing the first and second detection intensities, adjusting the second detection intensity by multiplying the second detection intensity, ^^^^, by the compensation factor.

5. The method of claim 3 or claim 4, wherein each of the patterning device alignment mark and the stage alignment mark is defined by a geometric arrangement of a reflective portion and an absorber portion, and the method further comprises: measuring ^^^^,-., being the intensity of light reflected from the absorber portion of the stage alignment mark; measuring ^^^^,-., being the intensity of light reflected from the absorber portion of the patterning device alignment mark; obtaining geometric factor / ^^^, being the area of the absorber portion of the stage alignment mark as a fraction of the total area of the stage alignment mark; obtaining geometric factor / ^^^, being the area of the absorber portion of the patterning device alignment mark as a fraction of the total area of the patterning device alignment mark; before comparing the first and second detection intensities, adjusting the first detection intensity,^^^^, by subtracting / ^^^ ∙ ^^^^,-., and adjusting the second detection intensity, ^^^^, by subtracting / ^^^ ∙^^^^,-..

6. The method of any one of claims 2 to 5, wherein the comparing the first and second detection intensities comprises: calculating a detection intensity ratio, PDR, the detection intensity ratio being a ratio between the first detection intensity, ^^^^, and the second detection intensity, ^^^^; and comparing the detection intensity ratio against a detection threshold, Th.

7. The method of claim 6, further comprising: obtaining the transmission of a reference pellicle to the measured light, ^^^^, and setting the detection threshold, Th, based on ^^^^.

8. The method of claim 7, wherein the detection intensity ratio, PDR, is calculated as ^^^^ / ^^^^, and the method further comprises determining that a pellicle is absent or ruptured if the detection intensity ratio, PDR, exceeds the detection threshold, Th. Company Secret9. The method of claim 8, wherein the detection threshold, Th, is set to a value between ^^^^^and 1.

10. The method of any one of claims 2 to 9, wherein the replacement transmission threshold, ^ℎ^^^, is set to a value less than the detection threshold, Th.

11. The method of claim 10, further comprising measuring and recording the hydrogen pressure around the pellicle when each of the plurality of transmission values is measured; wherein the input data further comprises the plurality of hydrogen pressure measurements.

12. The method of any one of claims 10 or 11, further comprising measuring and recording the temperature of the pellicle or of the environment surrounding the pellicle when each of the plurality of transmission values is measured; wherein the input data further comprises the plurality of temperature measurements.

13. The method of any one of claims 2 to 12, wherein the lithographic apparatus comprises a second alignment sensor, wherein the method further comprises: obtaining a sensitivity ratio, ^N⁄ ^^ , being a ratio between the sensitivity of the first alignmentsensor, ^N, and that of the second alignment sensor, ^^; using the second alignment sensor, measuring ^^^^,^, being the intensity of light reflected from the patterning device alignment mark; adjusting ^^^^,^ by multiplying it with the sensitivity ratio, ^N⁄ ^^ ; andcomparing ^^^^and ^^^^,^.

14. The method of any one of claims 2 to 13, wherein the lithographic apparatus further comprises a dynamic gas lock membrane, DGLm, that intersects both the light path between the alignment sensor and the patterning device alignment mark, and the light path between the alignment sensor and the stage alignment mark, the method further comprising: obtaining the transmission of the DGLm, ^^^^^; determining that the DGLm has ruptured upon detection that the first and / or second detectionintensity in an exposure cycle has increased by a factor of (1 − ^^^^^) compared with a previousexposure cycle.

15. The method of any one of claims 2 to 14, wherein the alignment sensor comprises a transmission image sensor. Company Secret16. The method of any one of claims 2 to 15, wherein: each of the stage alignment mark and the patterning device alignment mark comprises orthogonal gratings; the alignment sensor comprises a shearing interferometer phase-stepping measurement sensor, PARIS; and the PARIS outputs an array of pixels, each pixel having an offset component, DC, and a phase- modulated component, T UVW(X).

17. The method of claim 16, further comprising: using the PARIS, measuring 1S^^^and T^^^being, respectively, the offset component and the magnitude of the phase-modulated component of the pixel or group of pixels measurable from the stage alignment mark; using the PARIS, measuring 1S^^^and T^^^being, respectively, the offset component and the magnitude of the phase-modulated component of the pixel or group of pixels resulting from light reflected from the patterning device alignment mark; calculating and monitoring the value ofupon detection that_E,CAgchanged by more than a predetermined amount, determining that the measurements are unreliable.

18. The method of any one of claims 16 or 17, wherein: the patterning device stage comprises a plurality of stage alignment marks fixedly provided thereon, and / or the patterning device comprises a plurality of patterning device alignment marks fixedly provided thereon; the PARIS comprises a plurality of detectors for measuring light reflected from the plurality of alignment marks on the patterning device stage and / or the patterning device; and each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, is derived from an average of the outputs from the plurality of detectors.

19. The method of any one of claims 16 to 18, wherein each of the first detection intensity, ^^^^, and the second detection intensity, ^^^^, is a field of intensity values corresponding to the array of pixels output by the PARIS, and the method further comprises determining at least one Zernike term of the 2ndor 3rdorder or greater of aberrations present in ^^^^and / or ^^^^, the order of a Zernike aberration term being the defined according to the Noll ordering scheme. Company Secret20. The method of claim 19, further comprising comparing the Zernike aberrations in the second detection intensity, k^^^, of the current exposure with the Zernike aberrations in the first detection intensity, k^^^.

21. The method of any one of the preceding claims, wherein: the measuring the first detection intensity comprises measuring the intensity of light reflected from a plurality of locations of the patterning device, thereby producing a first detection map; the measuring the second detection intensity is performed after the measuring the first detection intensity, and comprises measuring the intensity of light reflected from the plurality of locations of the patterning device, thereby producing a second detection map; the comparing the first and second detection intensities comprises comparing the first and second detection maps.

22. The method of claim 21, wherein comparing the first and second detection maps comprises calculating a difference map by subtracting the first and second detection maps one from the other.

23. The method of any one of claims 21 or 22, further comprising transforming the difference map into a histogram of difference values and calculating a distribution width of the difference values from the histogram of difference values.

24. The method of any one of claims 21 to 23, further comprising determining that a pellicle is absent or ruptured if a) the distribution width exceeds a predetermined distribution width threshold and / or b) the average difference exceeds a predetermined average difference threshold.

25. The method of any one of claims 21 to 24, further comprising transforming the first and second detection maps respectively into first and second histograms of detection values, then determining a first distribution mode from the first histogram of detection values; and determining a second distribution mode from the second histogram of detection values.

26. The method of claim 25, further comprising determining that a pellicle is ruptured if: the first distribution mode is a monomodal distribution; and the second distribution mode is a multimodal distribution. Company Secret

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