Method, computer program and assembly for pellicle monitoring

Abstract

A method of pellicle monitoring, the method comprising: directing a radiation beam onto a component configured to be covered by a pellicle; positioning the component in a first position for performing a first alignment using a first alignment mark provided on the component; after the first alignment, repositioning the component to a second position for performing a second alignment using a second alignment mark provided on the component; and measuring, using a sensor module, the intensity of the radiation beam reflected from the component and passing through a portion of the pellicle, if present, while repositioning the component from the first position to the second position.

Description

METHOD, COMPUTER PROGRAM AND ASSEMBLY FOR PELLICLE MONITORINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24188934.4 which was filed on July 16, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to techniques of monitoring a pellicle, for example detecting whether a pellicle is present / intact or absent / ruptured.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 at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] 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 can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 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 or develop holes after a number of exposure cycles. When a pellicle is no longer intact, contaminant particles may be able to reach the patterning device, leading to defects in the substrate. To prevent such defects, it may be desirable to know when the pellicle is near, or is past, its limited life span, so that the operator may replace the pellicle at an appropriate time.

[0007] More generally, components other than the patterning device may also be susceptible to particulate contamination, and may also be protected by a pellicle. To prevent performancedegradation of such components, it may be desirable to know when the pellicle is near, or is past, its limited life span, so that the operator may replace the pellicle at an appropriate time.SUMMARY OF THE INVENTION

[0008] Therefore, an object of the present invention is to provide pellicle monitoring. Another object is to provide pellicle monitoring without affecting the production throughput of the lithographic apparatus. Another object is to monitor the remaining life of a pellicle.

[0009] In accordance with the present invention, there is disclosed a method of pellicle monitoring, the method comprising: directing a radiation beam onto a component configured to be covered by a pellicle; positioning the component in a first position for performing a first alignment using a first alignment mark provided on the component; after the first alignment, repositioning the component to a second position for performing a second alignment using a second alignment mark provided on the component; and measuring, using a sensor module, the intensity of the radiation beam reflected from the component and passing through a portion of the pellicle, if present, while repositioning the component from the first position to the second position.

[0010] In accordance with the present invention, there is also disclosed a pellicle monitoring assembly comprising: a positioner; a sensor module rigidly attached to the positioner; and a controller configured to: output a control signal for directing a radiation beam onto a component configured to be covered by a pellicle; output a control signal for positioning the component in a first position for performing a first alignment using a first alignment mark provided on the component; after the first alignment, output a control signal for repositioning the component to a second position for performing a second alignment using a second alignment mark provided on the component; and receive, from the sensor module, a measurement of the intensity of the radiation beam reflected from the component and passing through a portion of the pellicle, if present, while the component is being repositioned from the first position to the second position.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference symbols indicate corresponding parts.

[0012] Figure 1 schematically depicts a lithographic apparatus.

[0013] Figure 2 schematically depicts a radiation beam reflecting off a patterning device and detected by a sensor module.

[0014] Figure 3 schematically depicts the radiation beam scanning across the patterning device.

[0015] Figure 4 schematically depicts displacing the sensor module to follow the cross-sectional shape of the radiation beam.

[0016] Figure 5 schematically depicts possible paths scanned by the sensor module of Figure 4.

[0017] Figures 6A-C schematically depict a sensor module comprising two sensors, and displacing and rotating the sensor module to follow the cross-sectional shape of the radiation beam.

[0018] Figure 7 schematically depicts possible paths scanned by the sensor module of Figures 6A- C.

[0019] Figure 8 schematically depicts a sensor module comprising a plurality of sensors arranged along curve.

[0020] Figure 9 schematically depicts possible paths scanned by the sensor module of Figure 8.

[0021] 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

[0022] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0023] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device M0 and a facetted pupil mirror device ML The faceted field mirror device M0 and faceted pupil mirror device Ml together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device M0 and faceted pupil mirror device ML

[0024] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. Forthat purpose, the projection system PS may comprise a plurality of mirrors M3, M4 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors M3, M4 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0025] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B ’ , with a pattern previously formed on the substrate W.

[0026] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS, and / or in the substrate table WT environment.

[0027] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[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 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 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 portions. 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 theother 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] In some arrangements, the patterning device stage MT may be movable substantially in one direction only, which may (by convention) be along the y-axis. The patterning device stage MT may be movable and / or rotatable in one or more directions. For example, the patterning device stage MT may be movable in the x and y directions, and / or may be rotatable in the Rz direction. The patterning device stage MT may also be movable with six degrees of freedom, i.e. movable in the x, y and z directions, and rotatable in the Rx, Ry and Rz directions, although it should be noted that the range and precision of movement in these six degrees of freedom may be unequal.

[0032] 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.

[0033] 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.

[0034] The alignment measurements described above may include illuminating one or more alignment marks MAF fixedly provided on the patterning device MA. The image produced by the alignment marks may be captured by an optical system, such as an alignment sensor.

[0035] The patterning device alignment mark MAF may 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 beunderstood that marks for other purposes than alignment (such as marks for aberration detection) may also be present.

[0036] 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 may be provided on the patterning device MA. An alignment operation may make use of one, several, or all of these alignment marks MAF. For example, each of the alignment marks MAF 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.

[0037] 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.

[0038] In order to measure alignment of the patterning device MA and the patterning device stage MT, an alignment sensor may be provided to measure light B’ which is output from the projection system PS. The alignment sensor may, for example, be provided on or attached to the substrate support WT. In order to perform an alignment process, 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. The substrate support WT may be positioned such that radiation which is reflected from the alignment marks MAF is projected, by the projection system PS, onto the alignment sensor.

[0039] The position of the alignment feature in the radiation beam B may be measured by an alignment sensor positioned at a substrate W level (e.g. provided on or attached to the substrate support WT). The alignment sensor 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 patterning device alignment mark MAF to be determined. With knowledge of the relative alignment of the patterning device MA, 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 radiation 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.

[0040] 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 patterningdevice 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 to the radiation beam B (which may be in the EUV range) may be within the range of 75% to 98%.

[0041] Pellicles MP generally have a limited life span, and may develop holes or rupture after a certain number of exposure cycles. When the pellicle MP breaks, debris may be created, which may contaminate various surfaces of the lithographic apparatus LA, including the patterning device MA surface. Other contaminant particles may also flow to the patterning device MA and be deposited thereon. Furthermore, the loss of the pellicle MP may result in a sudden increase in the intensity of the reflected radiation beam B ’ . All these factors may result in defects in the exposed substrate W. In this event, the lithographic apparatus LA may need to be halted, and surfaces of the lithographic apparatus LA may need to be cleaned, repaired or replaced. Therefore, in the event that the pellicle MP is no longer intact, or before the pellicle MP develops holes or ruptures, it may be desirable to halt the lithographic apparatus LA, and replace the pellicle MP. Therefore, it may be desirable to detect whether the pellicle MP is present and intact, or has developed holes or ruptured, or is close to developing holes or rupturing, so that the pellicle MP can be replaced 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 or developing holes becomes too high.

[0042] Depending on the material of the pellicle MP, the pellicle MP may exhibit different degradation behavior over time. Pellicles MP made of certain materials have the tendency to rupture catastrophically, so that the pellicle MP tends to be either present and intact, or essentially absent.

[0043] Alternatively, 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 EUV radiation. CNT pellicles MP used in EUV lithography may have a thickness less than 100 nm, for example, and can be fragile. 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 thethickness 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.

[0044] Furthermore, CNT pellicles MP tend not to rupture catastrophically. Instead, CNT pellicles MP tend to develop holes locally. Therefore, with CNT pellicles MP, the status of the pellicle MP as a whole may not be reliably inferred from a detection that a localized portion of the pellicle MP is intact. For example, it may be detected that a portion of the pellicle MP covering a patterning device alignment mark MAF is intact, but it cannot be reliably inferred from this detection alone that a central portion of the pellicle MP is also intact. This is because a hole could have developed in the central portion of the CNT pellicle MP, and this hole would not be detectable by monitoring the status of the portion of the pellicle MP covering the patterning device alignment mark MAF. Instead, it may be necessary to directly monitor the status of the central portion of the pellicle MP. More generally, to obtain a full picture of the status of the pellicle MP, it may be desirable to sample the whole surface of the pellicle MP. That is, the more sample points are used, the more it can be confidently concluded that there are no holes present in the pellicle MP. Nevertheless, it should be understood that any sampling coverage of e.g. a central portion of the pellicle MP may be better than no coverage at all. Therefore, some improvement in the confidence in pellicle monitoring would still be achieved with scarce sampling of the pellicle MP surface.

[0045] As shown in Figure 2, one technique of determining whether a portion of the pellicle MP is intact is by measuring the intensity drop of the radiation beam B as it passes through the pellicle MP. Generally, with reflective-type patterning devices MA, the radiation beam B will pass through the pellicle MP twice. Therefore, if the pellicle MP has a nominal transmission T, then the intensity of the radiation beam B may be attenuated by a factor of T2when the pellicle MP is brand new. As the pellicle MP degrades over time, its transmission may gradually increase until a hole is formed. As a result, the radiation beam B may be less and less attenuated by the pellicle MP over time. This change in attenuation may be used to infer whether a portion of the pellicle MP is intact.

[0046] In order to determine the status of the pellicle MP, it is possible to perform a dedicated scan of the surface of the pellicle MP as an additional operation to be performed at each cycle of substrate exposure. However, any such additional operation will add to the duration of the cycle, and thus reduce the manufacturing throughput of the lithographic apparatus LA.

[0047] As found by the present inventors, it is possible to incorporate a pellicle monitoring scan into an exposure cycle without adding to the duration of the cycle.

[0048] As shown in Figure 2, the patterning device MA may be provided with two or more patterning device alignment marks (two are shown in Figure 2, labelled MAF1 and MAF2). These alignment marks MAF1, MAF2 may be separated by some distance. For example, the alignment marks MAF1, MAF2 may be provided near opposite edges or opposite corners of the patterning device MA. Typically, the patterning device MA alignment operation may include performing alignments using the alignments marks MAF1, MAF2 in turn. For example, a first alignment may beperformed using a first alignment mark MAF1, followed by a second alignment using a second alignment mark MAF2. During the first alignment, the patterning device stage MT may be positioned so that the radiation beam B is incident on the first alignment mark MAF1. Similarly, during the second alignment, the patterning device stage MT may be positioned so that the radiation beam B is incident on the second alignment mark MAF2. In between the two alignments, the patterning device stage MT may displace from a position where the radiation beam B is incident on the first alignment mark MAF1, to a position where the radiation beam B is incident on the second alignment mark MAF2.

[0049] In known lithographic operations, while the patterning device stage MT is being displaced between the first and second alignments, the radiation beam B is switched off and no intensity measurements are taken during this time. In other words, there is typically some “dead time” between the first and second alignments when the patterning device stage MT must displace from one position to another. However, as found by the present inventors, this dead time may provide a convenient opportunity for sampling the surface of the pellicle MP.

[0050] Therefore, in a method of pellicle monitoring, the method may comprise directing a radiation beam B onto a component (e.g. a patterning device MA) configured to be covered by a pellicle MP; positioning the component in a first position for performing a first alignment using a first alignment mark MAF1 provided on the component; after the first alignment, repositioning the component to a second position for performing a second alignment using a second alignment mark MAF2 provided on the component; and while repositioning the component from the first position to the second position, measuring, using a sensor module WS, the intensity of the radiation beam B’ reflected from the component and passing through a portion of the pellicle MP, if present.

[0051] As can be seen, by keeping the radiation beam B on while re-positioning the component from the first position to the second position, and by measuring the intensity of the reflected radiation beam B’ during this time, a portion of the pellicle MP between the first and second alignment marks MAF1, MAF2 can be sampled. From the intensity measurements, it can be inferred whether the sample portion of the pellicle MP is intact. In the simplest case, as shown in Figure 5, intensity measurements may be taken along a single straight-line path Pl 1 running between the first and second alignment marks MAF1, MAF2. Therefore, some sampling coverage of an interior portion of the pellicle MP can be achieved without adding time to the exposure cycle.

[0052] In more detail, although the radiation beam B is depicted schematically in Figure 2 with a single line, it should be understood that the radiation beam B may have a cross-sectional profile. The cross-sectional profile may take any suitable shape as determined by the optics of the lithographic apparatus LA. For example, the cross-sectional profile may be circular or oval. In some lithographic apparatuses, in order to make efficient use of lenses and / or mirrors, the radiation beam B may have a curved / crescent shape. For example, the radiation beam B may pass through a curved slit before reaching the sensor module WS. Generally, the curved slit may be located within the illuminationsystem IL. That is, the radiation beam B may assume the shape of the curved slit before reaching the patterning device MA.

[0053] Figure 3 shows the curved cross-section profile of the radiation beam B incident on the patterning device MA. Specifically, during the first alignment, the patterning device MA (or a different component) may be positioned so that the first alignment mark MAF1 coincides with the cross-section profile of the radiation beam B 1. Similarly, during the second alignment, the patterning device MA (or the component) may be positioned so that the second alignment mark MAF2 coincides with the cross-section profile of the radiation beam B3. While repositioning between the first and second alignment, the patterning device MA (or the component) may be at an intermediate position so that the cross-section profile of the radiation beam B2 falls on an interior portion of the patterning device MA. Therefore, while the cross-section profile of the radiation beam B2 falls on this intermediate position, the intensity of the reflected radiation beam B ’ may be measured to estimate the attenuation caused by the pellicle MP.

[0054] Furthermore, as shown in Figure 3, more than two alignment marks may be provided on the patterning device MA (or the component). For example, six alignment marks may be provided. For example, three alignment marks may be provided along one edge of the patterning device MA (or the component), and another three alignment marks may be provided along an opposite edge. The alignment operations may make use of any of these alignment marks. For example, as shown in Figure 3, the first and second alignments may be performed using alignment marks MAF1, MAF2 placed near the middle of opposite edges of the patterning device MA (or the component). However, it should be understood that the first and second alignments may be performed using other alignment marks, such as those at the corners of the patterning device MA (or the component), and the patterning device MA (or the component) may be positioned accordingly so that the relevant alignment marks coincide with the cross-sectional profile of the radiation beam B (which may be curved) during the alignments.

[0055] The sensor module WS may be positioned anywhere downstream from the patterning device MA (or the component) and the pellicle MP, as long as it is able to coincide with the reflected radiation beam B ’ while the patterning device MA (or the component) is at an intermediate position between the first and second positions. For example, the sensor module WS may be rigidly attached to a positioner. By attaching the sensor module WS to a positioner, the position of the sensor module WS may be adjusted so as to coincide with the reflected radiation beam B’. For example, referring to Figure 4, depending on the x-position of the sensor module WS, the y-position of the sensor module WS may need to be adjusted accordingly in order for the sensor module WS to remain coincidental with the cross-sectional profile of the radiation beam B’ (which, as explained above, may be curved).

[0056] Conveniently, the sensor module WS may be attached to the positioner (i.e. the “second positioner” mentioned above) that moves the substrate table WT. As noted above, positioner of the substrate table WT may have several degrees of freedom of movement, which may enable the sensormodule WS to follow the cross-sectional profile of the radiation beam B’. Attaching the sensor module WS to the positioner of the substrate table WT may be advantageous because the position of the sensor module WS can be controlled without requiring new hardware components.

[0057] The sensor module WS may, in the simplest case, comprise a single sensor. As a result, the path scanned by the sensor module WS may comprise a single path, such as path Pl 1 shown in Figure 5. Using a single sensor, especially if it has a large field of view, may have the advantage that it is less susceptible to signal noise. However, generally, the sensor module WS may comprise one or more sensor.

[0058] For example, as shown in Figure 6A, the sensor module WS may comprise two sensors WS1, WS2. Correspondingly, as shown in Figure 7, the two sensors WS1, WS2 may scan two paths P21 intermediate the first and second alignment marks MAF1, MAF2. The sensor module WS may be positioned and / or oriented so that the two sensors WS1, WS2 are both coincident with the reflected radiation beam B’. By scanning two paths P21, a greater sampling coverage of the pellicle MP may be achieved.

[0059] For another example, as shown in Figure 8, the sensor module WS may comprise at least three sensors, WS1, WS2, WS3. For example, Figure 8 shows a sensor module WS comprising five sensors. It should be understood that more or fewer sensors than five may be provided.Correspondingly, as shown in Figure 9, each of the at least three sensors WS1, WS2, WS3 may scan a separate path P3 intermediate the first and second alignment marks MAF1, MAF2. The at least three sensors WS1, WS2, WS3 may be arranged along a curve. This may enable the reflected radiation beam B’ to be incident on the at least three sensors WS1, WS2, WS3 simultaneously. That is, the at least three sensors WS1, WS2, WS3 may be arranged a long a curve that follows the curved shape of the cross-sectional profile of the reflected radiation beam B ’ . More specifically, the at least three sensors WS1, WS2, WS3 may be arranged so that, in at least one position and orientation of the positioner, the radiation beam is incident on the at least three sensors WS1, WS2, WS3 simultaneously.

[0060] It should be noted that, although the sensors WS1, WS2, WS3 are depicted in Figures 6A and 8 as having a relatively small circular field of view, they can have a field of view of any suitable shape and any suitable size. For example, the sensors WS1, WS2, WS3 may have a field of view that is relatively wide in the x-direction, so that each of the paths P21 or P3 may correspond to an equally wide strip of area of the pellicle MP scanned by the respective sensor WS1, WS2, WS3. This may increase the sampling coverage of the pellicle MP. Additionally or alternatively, the sensors WS1, WS2, WS3 may have a field of view that has a width in the y-direction greater than the y-direction width of the cross-sectional shape of the reflected radiation bean B’. This may ensure that the maximum flux of the radiation beam B’ is captured by the sensors WS1, WS2, WS3, which may in turn improve the signal-to-noise ratio of the measurements made by the sensors WS1, WS2, WS3.

[0061] The sensors WS1, WS2, WS3 may be of any suitable type capable of measuring light intensity. For example, the sensor module WS may comprise one or more transmission image sensor (TIS), one or more parallel integrated lens interferometer (PARIS), one or more energy sensor, one or more spot sensor, one or more slit sensor (SLS), or one or more photodetector. The sensor module WS may comprise a combination of these different sensor types. Conveniently, some sensors of these types may already be attached to the substrate support WT (or the measurement stage) for other existing operations of the lithographic apparatus. For example, TIS or PARIS sensors may already be present for the first and second alignments; the same sensors may be used for pellicle monitoring. For example, an SLS sensor may already be present for measuring the illumination intensity uniformity along the curved slit (i.e. in the x-direction); the same sensor may be used for pellicle monitoring.

[0062] The paths Pl 1, P21 and P3 in Figures 5, 7 and 9 are shown as straight paths. This corresponds to an operation in which the positioner (e.g. the positioner for the substrate support WT) is stationary during pellicle monitoring. These straight paths provide some sampling coverage of the pellicle MP. However, it is possible to increase the sampling coverage of the pellicle MP. As noted above, the repositioning the component (e.g. the patterning device MA) from the first position to the second position may comprise displacing the component along a straight trajectory in a first direction (e.g. the y-direction, which may be the scanning direction of the patterning device MA). During this time, the sensor module WS may be displaced back and forth in a second direction (e.g. the x- direction) non-parallel to the first direction. The first direction may be substantially perpendicular to the second direction. As a result, instead of scanning straight paths Pl 1, P21, the sensor module WS may scan meandering paths Pl 2, P22 (the actual meandering amplitude may be less than depicted in Figures 5 and 7). For example, the paths P12, P22 may be approximately sinusoidal, which may avoid acceleration spikes in the movement of the positioner (e.g. the positioner of the substrate support WT). It should be understood that other path shapes are possible, such as sawtooth shapes or triangular waves.

[0063] As can be seen from Figures 5 and 7, a greater sampling coverage of the pellicle MP can be achieved by displacing the sensor module WS back and forth in the second direction while the component (e.g. the patterning device MA) is displaced in the first direction during repositioning from the first alignment to the second alignment. Furthermore, because the movements of the sensor module WS (and the positioner) may be entirely independent from the movements of the component (e.g. the patterning device MA), the back-and-forth movement of the sensor module WS will not slow down the repositioning of the component (e.g. the patterning device MA) from the first position to the second position. That is, while the back-and-forth movement creates meandering scanning paths P12, P22, which are longer than the corresponding straight paths Pl 1, P21 and provide greater sampling coverage of the pellicle MP, it is possible to avoid adding time to the duration of the exposure cycle. As a result, greater sampling coverage of the pellicle MP can be achieved without costing manufacturing throughput of the lithographic apparatus LA.

[0064] It should be understood that the scanning paths may be varied from one exposure cycle to another. This may allow different portions of the pellicle MP to be scanned, which may improve the cumulative sampling coverage of the pellicle MP over time.

[0065] In one implementation, it takes about 120 ms to reposition the patterning device MA from the first position to the second position. During this time, the sensor module WS, comprising TIS sensors, makes intensity measurements at a frequency of 60 kHz, resulting in up to 7200 measurement points per TIS sensor. The sensor module WS can complete one cycle of back-and-forth movement in the x-direction with an amplitude of about 40 mm. Alternatively, the sensor module WS can complete two cycles of back-and-forth movement in the x-direction with an amplitude of about 11 mm. Alternatively, the sensor module WS can complete three cycles of back-and-forth movement in the x-direction with an amplitude of about 5 mm. In each case, the peak acceleration of the sensor module WS is less than 100 ms2.

[0066] As noted above, the reflected radiation beam B’ may have a curved cross-sectional profile. Therefore, while displacing the sensor module WS back and forth in the second direction (e.g. the x- direction), the field of the view of the sensors WS1, WS2 may move away from the locus of maximum intensity B’max(see Figure 6A) along the length of the cross-sectional profile of the reflected beam B’. If the amplitude of the back-and-forth movement of the sensor module WS is sufficiently large, the field of view of the sensors WS1, WS2 may even move entirely outside the cross-sectional profile of the reflected beam B’. This may result in a drop in measured light intensity that does not correspond to the status of the pellicle MP.

[0067] To compensate for this effect, as shown in Figures 6B and 6C, while displacing the sensor module WS back and forth in the second direction (e.g. the x-direction), it may be useful to continuously adjust the position of the sensor module WS in the first direction (e.g. the y-direction) and / or the rotational orientation of the sensor module WS about an axis perpendicular to both the first and second directions (e.g. in the Rz-direction), so that each of the one or more sensor WS1, WS2 remains in the path of the reflected radiation beam B’. The same compensation may be useful also when the sensor module WS comprises a single sensor, except that rotational adjustments in the Rz- direction may not be necessary (see Figure 4). More precisely, the continuously adjusting the position and / or rotational orientation of the sensor module WS may cause the one or more sensor WS1, WS2 to follow the cross-sectional shape of the reflected radiation beam B’.

[0068] As noted above, any sampling of an interior portion of the pellicle MP may be better than no sampling at all. As such, useful information regarding the status of the pellicle MP may be obtained even with only a single sample point of an interior portion of the pellicle MP. However, while the sensor module WS has the potential to make sample measurements at any point along paths Pl 1, P12, P21, P22, P3, it may be beneficial to make multiple measurements along these paths. That is, the measuring the intensity of the radiation beam B’ reflected from the component (e.g. the patterning device MA) may comprise making a plurality of measurements of the intensity of the reflectedradiation beam B ’ when the component is at a corresponding plurality of positions between the first and second positions. This may provide a greater number of samples of the surface of the pellicle MP, and thus greater confidence in the determination of the status of the pellicle MP.

[0069] As noted above, the intensity of the reflected radiation beam B’ may be attenuated by the presence of the pellicle MP. However, the local reflectivity of different portions of the component (e.g. the patterning device MA) may also affect the intensity of the reflected radiation bean B’. To compensate for this, it may be useful to offset variations among the plurality of measurements caused by reflectivity variations across the component against the plurality of measurements of the intensity of the reflected radiation beam B’. The reflectivity variations across the component (e.g. the patterning device MA) may be obtained using any suitable means, for example by measurement. That is, the variations among the plurality of measurements caused by reflectivity variations across the component may be measured from the component in the absence of a pellicle MP. Alternatively or additionally, the reflectivity variations across the component (e.g. the patterning device MA) may be obtained by calculation / simulation. That is, the variations among the plurality of measurements caused by reflectivity variations across the component may be calculated from the design of a pattern of the component.

[0070] In addition to reflectivity variations across the component (e.g. the patterning device MA), the intensity of the incoming radiation beam B may also drift over time. Therefore, to compensate for any drift, it may be useful to measure a reference intensity of the radiation beam B, the reference intensity being the intensity of the radiation beam B before the radiation beam reaches the component. For example, the reference intensity of the radiation beam B may be measured at the exit of the illumination system IL, and may be achieved using an energy sensor. The measured intensity of the reflected radiation beam B’ may thus be normalized by the reference intensity.

[0071] Based on the measured intensity of the reflected radiation beam B’, it can be determined whether at least a portion of the pellicle MP is worn and / or has ruptured. As noted above, the presence of the pellicle MP will result in attenuation of the radiation beam B. Therefore, if a portion of the pellicle MP is worn and / or has ruptured, an increase in the intensity of the reflected radiation beam B’ measured at that portion of the pellicle MP can be expected. Therefore, it can be determined that at least a portion of the pellicle MP is worn and / or has ruptured if the measured intensity of the reflected radiation beam B’ exceeds a predetermined threshold.

[0072] Upon initial determination that at least a portion of the pellicle MP is worn and / or has ruptured, a dedicated scan of the pellicle MP surface may be performed to determine the actual breakage before aborting the exposure cycle.

[0073] Certain overrides may additionally be applied to reduce false positives. For example, as found by the present inventors, when a hole has developed in the pellicle MP, an abrupt change in the measured intensity of the reflected radiation beam B’ can generally be expected across the edge of the hole. Conversely, a lack of such abrupt change may indicate that a hole is not present. Therefore, itmay be useful to calculate a gradient of the intensity of the reflected radiation beam B’ measured while repositioning the component from the first position to the second position, and to override the determination that at least a portion of the pellicle MP is worn and / or has ruptured as a false positive if the gradient is below a second predetermined threshold. The second predetermined threshold may be adjusted / corrected from time to time to account for an increase in transmission of the pellicle MP due to normal wear.

[0074] In addition or as an alternative to determining that at least a portion of the pellicle MP is worn and / or has ruptured if the measured intensity of the reflected radiation beam B ’ exceeds a predetermined threshold, it can also be determined that at least a portion of the pellicle MP is worn and / or has ruptured if the measured intensity of the reflected radiation beam B ’ has changed relative to the same measurement made at a previous exposure cycle by more than a predetermined amount. This is because pellicles MP are generally expected to increase in transmission gradually while it is intact. Therefore, a sudden jump in transmission may indicate a ruptured pellicle MP.

[0075] In addition to methods of pellicle monitoring, the present disclosure also contemplates a method of manufacturing devices comprising the method of pellicle monitoring disclosed above.

[0076] There is also disclosed a computer program comprising instructions to cause a lithographic apparatus LA to execute the steps of the method of pellicle monitoring disclosed above.

[0077] It should be understood that the present invention may also be embodied in a pellicle monitoring assembly. The pellicle monitoring assembly may comprise the positioner (e.g. the positioner for moving the substrate table WT); the sensor module WS rigidly attached to the positioner; and a controller. The controller may be configured to output a control signal for directing a radiation beam onto a component (e.g. a patterning device MA) configured to be covered by a pellicle MP; output a control signal for positioning the component in a first position for performing a first alignment using a first alignment mark provided on the component; after the first alignment, output a control signal for repositioning the component to a second position for performing a second alignment using a second alignment mark provided on the component; and receive, from the sensor module, a measurement of the intensity of the radiation beam reflected from the component and passing through a portion of the pellicle, if present, while the component is being repositioned from the first position to the second position.

[0078] The control signal for repositioning the component from the first position to the second position may comprise a control signal for displacing the component along a straight trajectory in a first direction (e.g. the y-direction).

[0079] The controller may be further configured to displace the sensor module WS back and forth in a second direction (e.g. the x-direction) non-parallel to the first direction. The first direction may be substantially perpendicular to the second direction.

[0080] There is also disclosed a lithographic apparatus LA configured to perform the method of pellicle monitoring disclosed above.

[0081] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0082] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatuses may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0083] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0084] Aspects of the invention are described in the following numbered clauses.1. A method of pellicle monitoring, the method comprising: directing a radiation beam onto a component configured to be covered by a pellicle; positioning the component in a first position for performing a first alignment using a first alignment mark provided on the component; after the first alignment, repositioning the component to a second position for performing a second alignment using a second alignment mark provided on the component; and measuring, using a sensor module, the intensity of the radiation beam reflected from the component and passing through a portion of the pellicle, if present, while repositioning the component from the first position to the second position.2. The method of clause 1 , wherein the radiation beam passes through a curved slit before reaching the sensor module.3. The method of clause 1 or clause 2, wherein the sensor module is rigidly attached to a positioner.4. The method of any one of the preceding clauses, wherein the sensor module comprises one or more sensor.5. The method of clause 4, wherein the sensor module comprises two sensors.6. The method of clause 4, wherein the sensor module comprises at least three sensors.7. The method of clause 6, wherein the at least three sensors are arranged along a curve so that, in at least one position and orientation of the positioner, the radiation beam is incident on the at least three sensors simultaneously.8. The method of any one of clauses 4 to 7, wherein the sensor module comprises one or more transmission image sensor, one or more parallel integrated lens interferometer, one or more energy sensor, one or more spot sensor, one or more slit sensor, or one or more photodetector.9. The method of any one of the preceding clauses, wherein the repositioning the component from the first position to the second position comprises displacing the component along a straight trajectory in a first direction.10. The method of clause 9, further comprising displacing the sensor module back and forth in a second direction non-parallel to the first direction.11. The method of clause 10, wherein the first direction is substantially perpendicular to the second direction.12. The method of clause 10 or clause 11, further comprising, while displacing the sensor module back and forth in the second direction, continuously adjusting the position of the sensor module in the first direction and / or the rotational orientation of the sensor module about an axis perpendicular to both the first and second directions, so that each of the one or more sensor remains in the path of the reflected radiation beam.13. The method of clause 12, wherein the continuously adjusting the position and / or rotational orientation of the sensor module causes the one or more sensor to follow the cross-sectional shape of the reflected radiation beam.14. The method of any one of the preceding clauses, wherein the measuring the intensity of the radiation beam reflected from the component comprises making a plurality of measurements of the intensity of the reflected radiation beam when the component is at a corresponding plurality of positions between the first and second positions.15. The method of clause 14, further comprising offsetting variations among the plurality of measurements caused by reflectivity variations across the component against the plurality of measurements of the intensity of the reflected radiation beam.16. The method of clause 15, wherein the variations among the plurality of measurements caused by reflectivity variations across the component are measured from the component in the absence of a pellicle.17. The method of clause 15, wherein the variations among the plurality of measurements caused by reflectivity variations across the component are calculated from the design of a pattern of the component.18. The method of any one of the preceding clauses, further comprising measuring a reference intensity of the radiation beam, the reference intensity being the intensity of the radiation beam before the radiation beam reaches the component.19. The method of clause 18, further comprising normalising the measured intensity of the reflected radiation beam by the reference intensity.20. The method of any one of the preceding clauses, further comprising determining whether at least a portion of the pellicle is worn and / or has ruptured based on the measured intensity of the reflected radiation beam.21. The method of clause 20, further comprising determining that at least a portion of the pellicle is worn and / or has ruptured if the measured intensity of the reflected radiation beam exceeds a predetermined threshold.22. The method of clause 21, further comprising calculating a gradient of the intensity of the reflected radiation beam measured while repositioning the component from the first position to the second position, and overriding the determination that at least a portion of the pellicle is worn and / or has ruptured as a false positive if the gradient is below a second predetermined threshold.23. The method of any one of clauses 20 to 22, further comprising determining that at least a portion of the pellicle is worn and / or has ruptured if the measured intensity of the reflected radiation beam has changed relative to the same measurement made at a previous exposure cycle by more than a predetermined amount.24. The method any one of clauses 3 to 23, wherein the positioner further configured to move a substrate support.25. The method of any one of the preceding clauses, wherein the component is a patterning device.26. The method of any one of the preceding clauses, wherein the component is provided within a lithographic apparatus.27. A method of manufacturing devices comprising the method of pellicle monitoring of any one of the preceding clauses.28. A computer program comprising instructions to cause a lithographic apparatus to execute the steps of the method of any one of the preceding clauses.29. A pellicle monitoring assembly comprising: a positioner; a sensor module rigidly attached to the positioner; and a controller configured to: output a control signal for directing a radiation beam onto a component configuredto be covered by a pellicle; output a control signal for positioning the component in a first position for performing a first alignment using a first alignment mark provided on the component; after the first alignment, output a control signal for repositioning the component to a second position for performing a second alignment using a second alignment mark provided on the component; and receive, from the sensor module, a measurement of the intensity of the radiation beam reflected from the component and passing through a portion of the pellicle, if present, while the component is being repositioned from the first position to the second position.30. The assembly of clause 29, wherein the radiation beam passes through a curved slit before reaching the sensor module.31. The assembly of clause 29 or 30, wherein the control signal for repositioning the component from the first position to the second position comprises a control signal for displacing the component along a straight trajectory in a first direction.32. The assembly of clause 31 , wherein the controller is further configured to displace the sensor module back and forth in a second direction non-parallel to the first direction.33. The assembly of clause 32, wherein the first direction is substantially perpendicular to the second direction.34. The assembly of any one of clauses 29 to 33, wherein the controller is configured to perform the method of any one of clauses 4 to 8 and 12 to 26.35. A lithographic apparatus configured to perform the method of any one of clauses 1 to 26.

[0085] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method of pellicle monitoring, the method comprising: directing a radiation beam onto a component configured to be covered by a pellicle; positioning the component in a first position for performing a first alignment using a first alignment mark provided on the component; after the first alignment, repositioning the component to a second position for performing a second alignment using a second alignment mark provided on the component; and measuring, using a sensor module, the intensity of the radiation beam reflected from the component and passing through a portion of the pellicle, if present, while repositioning the component from the first position to the second position.

2. The method of claim 1 , wherein the repositioning the component from the first position to the second position comprises displacing the component along a straight trajectory in a first direction.

3. The method of claim 2, further comprising displacing the sensor module back and forth in a second direction non-parallel to the first direction.

4. The method of claim 3, wherein the first direction is substantially perpendicular to the second direction.

5. The method of claim 3 or claim 4, further comprising, while displacing the sensor module back and forth in the second direction, continuously adjusting the position of the sensor module in the first direction and / or the rotational orientation of the sensor module about an axis perpendicular to both the first and second directions, so that each of the one or more sensor remains in the path of the reflected radiation beam.

6. The method of claim 5, wherein the continuously adjusting the position and / or rotational orientation of the sensor module causes the one or more sensor to follow the cross-sectional shape of the reflected radiation beam.

7. The method of any one of the preceding claims, wherein the measuring the intensity of the radiation beam reflected from the component comprises making a plurality of measurements of the intensity of the reflected radiation beam when the component is at a corresponding plurality of positions between the first and second positions.

8. The method of claim 7, further comprising offsetting variations among the plurality of measurements caused by reflectivity variations across the component against the plurality of measurements of the intensity of the reflected radiation beam.

9. The method of claim 8, wherein the variations among the plurality of measurements caused by reflectivity variations across the component are measured from the component in the absence of a pellicle.

10. The method of claim 9, wherein the variations among the plurality of measurements caused by reflectivity variations across the component are calculated from the design of a pattern of the component.

11. The method of any one of the preceding claims, further comprising measuring a reference intensity of the radiation beam, the reference intensity being the intensity of the radiation beam before the radiation beam reaches the component.

12. The method of claim 11, further comprising normalising the measured intensity of the reflected radiation beam by the reference intensity.

13. The method of any one of the preceding claims, further comprising determining whether at least a portion of the pellicle is worn and / or has ruptured based on the measured intensity of the reflected radiation beam.

14. The method of claim 13, further comprising determining that at least a portion of the pellicle is worn and / or has ruptured if the measured intensity of the reflected radiation beam exceeds a predetermined threshold.

15. The method of claim 14, further comprising calculating a gradient of the intensity of the reflected radiation beam measured while repositioning the component from the first position to the second position, and overriding the determination that at least a portion of the pellicle is worn and / or has ruptured as a false positive if the gradient is below a second predetermined threshold.

16. The method of any one of claims 13 to 15, further comprising determining that at least a portion of the pellicle is worn and / or has ruptured if the measured intensity of the reflected radiation beam has changed relative to the same measurement made at a previous exposure cycle by more than a predetermined amount.

17. A pellicle monitoring assembly comprising: a positioner; a sensor module rigidly attached to the positioner; and a controller configured to: output a control signal for directing a radiation beam onto a component configured to be covered by a pellicle; output a control signal for positioning the component in a first position for performing a first alignment using a first alignment mark provided on the component; after the first alignment, output a control signal for repositioning the component to a second position for performing a second alignment using a second alignment mark provided on the component; and receive, from the sensor module, a measurement of the intensity of the radiation beam reflected from the component and passing through a portion of the pellicle, if present, while the component is being repositioned from the first position to the second position.

18. The assembly of claim 17, wherein the control signal for repositioning the component from the first position to the second position comprises a control signal for displacing the component along a straight trajectory in a first direction.

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