Membrane monitoring apparatus and method, and lithographic apparatus

Abstract

A monitoring subsystem for monitoring a membrane in use in a lithographic apparatus, the monitoring subsystem comprising: a light source configured to illuminate the membrane, the membrane being arranged between a first region and a second region of the lithographic apparatus; a light detector configured to measure a total power of light incident on a field of view of the light detector, the light being emitted from the light source and reflected off the membrane; and a controller configured to determine a level of degradation of the membrane at least in part based on the measured total power of light incident on the field of view of the light detector.

Description

MEMBRANE MONITORING APPARATUS AND METHOD, AND LITHOGRAPHICAPPARATUSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23196628.4 which was filed on 11 September 2023, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to techniques of detection of a level of degradation of a membrane, such as a dynamic gas lock (DGL) membrane or a pellicle, of a lithographic apparatus.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). 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] Once the EUV radiation has been generated, it is directed through the lithographic apparatus by a plurality of mirrors to a patterning surface of the patterning device, which imparts the desired pattern to the EUV radiation.

[0006] In EUV lithography, a dynamic gas lock (DGL) membrane may be employed to filter out various wavelengths, such as deep ultraviolet (DUV) and / or infrared (IR) radiation which may affect overlay or critical dimension uniformity (CDU) in a product. The DGL membrane may also prevent substrate contaminants and / or debris from entering the projection optics box of the lithographic apparatus. The DGL membrane employs a thin, nearly transparent membrane that is positioned in between the substrate and the projection system. In some embodiments, the DGL membrane replaces a dynamic gas lock construct that relied on gas flow to accomplish this same result. The DGL membrane is a self-standing membrane supported at the periphery by a border connected to a frame. The DGL membrane has a film thickness such that EUV transmission is maximized. For example, the DGL membrane has at least 65% EUV transmission, preferably at least 75%, more preferably at least 85% and even more preferably at least 95% EUV transmission.

[0007] The DGL membrane is thin as possible (e.g., about 10 to 50 nm thick) in order to minimize loss of light power. However, the nanoscale dimensions result in the DGL membrane being fragile and susceptible to rupture. Since the DGL membrane filters out various wavelengths, a ruptured membrane can affect overlay or CDU in a product. A ruptured membrane cannot prevent contaminants from entering the projection optics box, thus necessitating replacement of ruptured membrane with a new one. Furthermore, once contaminants have been allowed to spread in the region surrounding the substrate W, it may be necessary to shut down the lithographic apparatus and to conduct a lengthy clean-up procedure. A conservative countermeasure may be to establish a DGL membrane replacement schedule so that the DGL membrane is replaced sufficiently early to ensure that the DGL membrane will never be allowed to degrade to a level where the risk of rupture becomes significant. However, this would usually lead to the DGL membrane being replaced prematurely, leading to increased hardware costs and unnecessary downtime for maintenance.SUMMARY OF THE INVENTION

[0008] Therefore, an object of the present invention is to increase the availability of a lithographic apparatus. Another object of the present invention is to make more efficient use of the lifetime of a membrane while limiting the risk of rupture.

[0009] In accordance with the present invention, there is disclosed a monitoring subsystem for monitoring a membrane in use in a lithographic apparatus, the monitoring subsystem comprising: a light source configured to illuminate the membrane, the DGL membrane being arranged between a first region and a second region of the lithographic apparatus; a light detector configured to measure a total power of light incident on a field of view of the light detector, the light being emitted from the light source and reflected off the membrane; and a controller configured to determine a level of degradation of the membrane at least in part based on the measured total power of light incident on the field of view of the light detector.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] 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.

[0011] Figure 1 schematically depicts an embodiment of a lithographic apparatus.

[0012] Figure 2 schematically depicts an embodiment of a dynamic gas lock.

[0013] Figure 3 schematically depicts an embodiment of a monitoring subsystem when the DGL membrane is in a nominal position.

[0014] Figures 4a and 4b schematically depict an embodiment of a monitoring subsystem when the DGL membrane is bulged.

[0015] Figure 5 schematically depicts a possible evolution of the detector signal level over time.

[0016] Figures 6a and 6b are plots of detector signal level data obtained using a functional mock-up of an embodiment of the monitoring subsystem.

[0017] Figure 7 schematically depicts an embodiment of the monitoring subsystem.

[0018] 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

[0019] 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.

[0020] 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

[0021] 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. For that 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).

[0022] 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.

[0023] 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.

[0024] 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.

[0025] Figure 2 is a schematic diagram of an interface in between first and second regions in a lithographic apparatus. For example, the interface may be between the substrate W and projection system PS in the lithographic apparatus. As shown, the interface may include a dynamic gas lock (DGL) 22. The DGL 22 may comprise a DGL membrane 21. Another suitable interface may be that between the patterning device MA and the projection system PS in the lithographic apparatus. At this interface, a membrane similar to the DGL membrane 21 may be provided to protect the patterning device MA from contamination, in which case the membrane is also called a pellicle. Also such pellicle is a self-standing pellicle supported by a border and connected to a frame such that it can be attached to a patterning device. Furthermore, the EUV transmission of an EUV pellicle has a value of at least 65%, preferably at least 75%, more preferably at least 85% and even more preferably at least 95% EUV transmission Therefore, it is to be understood that the disclosure herein in relation to the DGL membrane 21 may also apply to other membranes used in different locations in the lithographic apparatus, such as a membrane existing at the interface between the patterning device MA and the projection system PS in the lithographic apparatus. Furthermore, the disclosure herein in relation to the DGL membrane 21 may also apply to membranes used in an inspection or metrology apparatus, or even in a transportation pod, or generally any device which requires the inspection of the degradation and / or integrity of the membrane.

[0026] A gas flow may be generated around DGL membrane 21 to displace or carry away contaminants and debris that may fall on the DGL membrane 21. In particular, DGL 22 may include a gas flow that flows downwards from the DGL membrane 21 to the substrate W. DGL 22 may include a gas flow above the DGL membrane 21 in order to keep the pressure at a certain level.

[0027] The DGL 22 and DGL membrane 21 may be located at a position adjacent the substrate table WT (e.g., a position separating a substrate compartment from the projection system PS). The DGL membrane 21 may thus have one side facing into the projection system PS, and the other side facing the substrate W and substrate table WT. The DGL membrane 21 can be utilized to prevent contamination of substrate W by creating a physical barrier between the interior of the projection system PS and the region surrounding the substrate W and substrate table WT. The DGL may allow the patterned beam B ’ to pass through to substrate W.

[0028] The DGL membrane 21 may be fabricated in various ways. For example, the DGL membrane 21 may be fabricated by depositing multiple layers on a polysilicon core, in which the layers are deposited (stacked) on top of each other, one at a time. For example, the core film may beabout 25 nm thick, whereas the multiple layers may each be about 2 to 4 nm thick. The multiple layers may comprise different functional layers, such as those which can block and / or allow radiation of predetermined wavelength ranges to pass through the membrane. In some aspects, the layers of the DGL membrane 21 stack may include one or more capping layers comprising an oxidation barrier, and one or more suppression or active layers for specific wavelengths, for example. In particular, the capping layers may be the outermost layers, and may provide oxidation barriers for the inner layers.

[0029] DGL membrane 21 may be fragile due to the nanoscale thickness of the DGL membrane 21 and thus susceptible to rupture. For example, in some use cases, DGL membrane 21 may have an expected lifetime of about six months and may need to be replaced because of performance degradation. Degradation of the DGL membrane 21 may result in various performance degradations of the lithographic apparatus. For example, performance degradation may occur in terms of EUV transmission loss over time and / or a decrease in EUV transmission uniformity, which may impact on- target performance and on machine throughput. In more severe scenarios, the DGL membrane 21 may degrade significantly enough to rupture. If the DGL membrane 21 is allowed to degrade to the point of rupture, the physical barrier between the projection system PS and the substrate W will be lost, and contaminants from the projection system PS may reach, and be deposited on, the substrate W, thereby causing defects. The loss of physical barrier may also allow contaminants from the region surrounding the substrate W ( e.g. the wafer stage compartment) to flow into, and contaminate, the projection system PS. Furthermore, the contaminants will spread in the region around the substrate W. Therefore, the lithographic apparatus will have to be shut down and thoroughly cleaned, leading to unnecessary downtime and maintenance cost.

[0030] Although there are known systems that can detect when the DGL membrane 21 has ruptured, it may be too late to stop contaminants from spreading into the region around the substrate W. A conservative countermeasure may be to establish a DGL membrane 21 replacement schedule so that the DGL membrane 21 is replaced sufficiently early to ensure that the DGL membrane 21 will never be allowed to degrade to a level where the risk of rupture becomes significant. However, this would usually lead to the DGL membrane 21 being replaced prematurely, leading to increased hardware costs and unnecessary downtime for maintenance.

[0031] As found by the present inventors, the DGL membrane 21 may exhibit observable changes during operation of the lithographic apparatus which may enable a determination of a level of degradation of the DGL membrane 21. Furthermore, the level of degradation of the DGL membrane 21 may be used to determine whether the DGL membrane 21 has degraded, or is in or close to an end- of-life condition, necessitating replacement. As a result, the DGL membrane 21 may be safely used until replacement is truly necessary without the risk of rupture.

[0032] When a brand new DGL membrane 21 is installed in the DGL 22, absent any dynamic or pressure forces, and neglecting the effects of gravity, the DGL membrane 21 may generally be flat and smooth, or may be in its nominal / rest position. During operation of the lithographic apparatus, apressure differential may exist across the DGL membrane 21. This pressure differential may cause the DGL membrane 21 to bulge, i.e. to assume a curved / domed shape. During different operations of the lithographic apparatus, the pressure differential across the DGL membrane 21 may vary in magnitude and / or direction, so that the DGL membrane 21 may bulge to varying degrees, either into the projection system PS or towards the substrate W. A certain degree of bulging of the DGL membrane 21 can be expected even when brand new.

[0033] As found by the present inventors, as the length of use or cumulative exposure to the patterned beam B’ increases, the DGL membrane 21 may degrade. The degradation may be due to various causes. For example, repeated exposure to the patterned beam B’, chemical reactions with gasses present on either side of the DGL membrane 21, and mechanical stresses due to repeated changes in the pressure differential across the DGL membrane 21 may each contribute to the degradation of the DGL membrane 21. As the DGL membrane 21 degrades, it is found that the DGL membrane 21 may become less taut. Therefore, in the presence of a pressure differential, the DGL membrane 21 may exhibit an increasing degree of bulging as the degradation advances. The increasing degree of bulging of the DGL membrane 21 may be regarded as a primary effect that is observable as the degradation of the DGL membrane 21 advances.

[0034] As found by the present inventors, in addition to or instead of bulging, the DGL membrane 21 may become wrinkled as the degradation advances. Furthermore, it is found that the level of wrinkling (in a qualitative sense) may also be affected by the magnitude of pressure differential across the DGL membrane 21. Specifically, wrinkles appear to more likely appear when the pressure differential across the DGL membrane 21 is small or absent, and the DGL membrane 21 may become less wrinkled as the pressure differential across the DGL membrane 21 increases. This is believed to be due to a tendency for the pressure differential across the DGL membrane 21 to stretch out the DGL membrane 21, which may in turn flatten the wrinkles to a greater or lesser degree.

[0035] In view of the observable effects of degradation of the DGL membrane 21 mentioned above, a level of degradation of the DGL membrane 21 may be measured.

[0036] As shown in Figure 3, in an embodiment of the present invention, a monitoring subsystem 1 may be provided. The monitoring subsystem 1 may be for use in the lithographic apparatus. As shown, the monitoring subsystem 1 may comprise a light source 11. The light source 11 may be configured to illuminate the DGL membrane 21. The light source 11 may be configured to emit visible light. For example, the light source 11 may be configured to emit light comprising light within a spectral band of 430 nm to 630 nm, or 480 nm to 580 nm, or 500nm to 550nm such as 530 nm. It is to be understood that the light source 11 may additionally or alternatively emit light outside the visible spectrum, such as infrared light. The light source 11 may employ an LED source, or other suitable types of light generation technologies.

[0037] As noted above, the DGL membrane 21 may be arranged between the substrate W (not shown in Figure 3) and the projection optics in the projection system PS (also not shown in Figure 3) of the lithographic apparatus.

[0038] The monitoring subsystem 1 may further comprise a light detector 12. The light detector 12 may be configured to measure light that has been emitted from the light source 11 and reflected off the DGL membrane 21. The light detector 12 may have a field of view 120. The light detector 12 may be configured to measure a total power of light incident on the field of view 120 of the light detector 12.

[0039] Any type of light detector 12 may be used as long as it is capable of measuring a total power of light incident on its field of view 120. Suitable light detecting technologies include photodiodes, phototransistors, photoresistors, photomultiplier tubes, Charge-Coupled Devices (CCDs), and Complementary Metal-Oxide-Semiconductor (CMOS) sensors. Furthermore, the light detector 12 may comprise light collecting optics, such as one or more lens and / or one or more aperture. For simplicity, the light detector 12 may be non-spatially resolved (i.e. it does not have multiple pixels). Specifically, the light detector 12 may have a single voltage output representing the measured total power of light incident on the field of view 120 of the light detector 12. It should be understood that spatially-resolved light detectors (such as image sensors) may also be used to the extent that pixel values could be added up to provide a measurement of the total power of light incident on the field of view 120 of the light detector 12, but the spatial resolution is not required for the implementation of certain embodiments of the present invention.

[0040] The monitoring system 1 may further comprise a controller 15 (see Figure 7). The controller 15 may be configured to determine a level of degradation of the DGL membrane 21. The controller 15 may determine the level of degradation at least in part based on the measured total power of light incident on the field of view 120 of the light detector 12. The controller 15 may be placed anywhere in the lithographic apparatus. The controller 15 may be placed in an atmospheric-pressure area of the lithographic apparatus. This may facilitate maintenance.

[0041] The light source 11 may be operated at a known or predetermined power level. For example, the controller 15 may control the power level of the light source 11. Because the power level of the light source 11 may be known or predetermined, the measured total power of light incident on the field of view 120 of the light detector 12 may be compared against the power level of the light source 11. The power level of the light source 11 may also be assumed constant, so that the measured total power of light incident on the field of view 120 of the light detector 12 can be used directly without being compared with the power level of the light source 11. For example, the level of degradation of the DGL membrane 21 may be at least in part based on the absolute value of the measured total power of light incident on the field of view 120 of the light detector 12.

[0042] As shown in Figure 3, the light source 11 may be positioned to emit light towards the DGL membrane 21 at an angle of incidence 0. As shown, the angle of incidence 0 may be measured fromthe normal to the DGL membrane 21. Specifically, the angle of incidence 0 may be defined assuming a perfectly flat and smooth DGL membrane 21. The light detector 12 may also be correspondingly positioned to receive light reflected off the DGL membrane 21. The positioning of the light source 11 and the light detector 12, including the angle of incidence 0, may be chosen according to the space available in the lithographic apparatus in the vicinity of the DGL 22. For example, the angle of incidence 0 may be about 30°, 40°, 50°, or 60°. In one arrangement, the angle of incidence 0 may be about 48°. The light source 11 and the light detector 12 may be positioned on the same side of the DGL membrane 21. For example, the light source 11 and the light detector 12 may be placed within the projection system PS, so that the light from the light source 11 impinges on the side of the DGL membrane 21 facing inside the projection system PS. Alternatively, the light source 11 and the light source 12 may be placed on the opposite side of the DGL membrane 21, i.e. outside the projection system PS, so that the light from the light source 11 impinges on the side of the DGL membrane 21 facing the substrate W and the substrate table WT.

[0043] In Figure 3, the DGL membrane 21 is shown to be smooth and flat, or in its nominal / resting position. Therefore, the DGL membrane 21 may behave as a (partially reflective) flat mirror to the light emitted by the light source 11. As shown in Figure 3, the light emitted by the light source 11 may have a certain beam profile 110, for example a circular profile. The light beam emitted by the light source 11 may be parallel, convergent or divergent. Therefore, the beam profile may vary, or remain constant, as light is emitted from the light source 11, reflected off the DGL membrane 21, and as it travels towards the light detector 12. If the light beam is parallel and the DGL membrane 21 is flat and smooth, the beam profile may remain constant throughout, so that the beam profile 119 at the light detector 12 may be substantially identical to the beam profile 110 at the light source 11.

[0044] Referring to Figure 4a, when the DGL membrane 21 ’ bulges towards the light source 11 and the light detector 12, the beam profile 119’ at the light detector 12 may change in size and / or shape. The DGL membrane 21’ may transform into a (partially reflective) convex mirror when the pressure on the side of the DGL membrane 21’ facing the light source 11 and the light detector 12 is lower than the pressure on the opposite side of the DGL membrane 21 ’ . The DGL membrane 21 ’ may have a generally rectangular or oval shape. As a result, when the DGL membrane 21’ bulges under a pressure differential, the DGL membrane 21’ may exhibit different curvatures along the major and minor axes of the generally rectangular or oval shape. In particular, due to the generally rectangular or oval shape of the DGL membrane 21’, the DGL membrane 21’ may behave as an astigmatic mirror, i.e. it may have non-equal optical powers in different directions. As a result, as shown, after the light from the light source 11 has reflected off the DGL membrane 21’, the beam profile may become increasingly distorted until the light reaches the detector 12. This is illustrated by beam profiles 115’ (immediately after the reflection), 117’ (approximately half way between the DGL membrane 21’ and the light detector 12), and 119’ (at the light detector 12). As illustrated, the beam profile becomes increasingly oval. It should be understood that the distortion of the beam profileshown in Figure 4a is only one possibility out of many, and the actual distortion will depend on various factors, including the geometry of the DGL membrane 21’, the level of degradation, the instantaneous pressure differential, the angle of incidence 0, material properties, etc. The DGL membrane 21’ may also degrade (and its mechanical properties may change) in a spatially non- uniform manner (e.g. the membrane and its mechanical properties may change in a non-uniform way over its area), which may also contribute to the distortion.

[0045] In addition to beam profile distortion, the bulging of the DGL membrane 21’ may also shift the reflected light beam. For example, whereas the light detector 12 may be positioned so that the beam profile 119 is centered with the field of view 120 of the light detector 12 when the DGL membrane 21 is flat and smooth or in its nominal / resting position (see Figure 3), when the DGL membrane 21’ bulges towards the light source 11 and the light detector 12 (see Figure 4a), the beam profile 119’ at the light detector 12 may become off-center from the field of view 120 of the light detector 12. Specifically, the beam profile 119’ at the light detector 12 may shift relative to the field of view 120 of the light detector 12 in the same direction as the bulge of the DGL membrane 21’. If the bulging of the DGL membrane 21’ continues to increase, the shifting of the light beam may become so severe that the light beam misses the field of view 120 of the light detector 12 entirely. In lesser degrees of bulging of the DGL membrane 21’ , the beam profile 119’ at the light detector 12 may partially fall within the field of view 120 of the light detector 12.

[0046] In the scenario shown in Figure 4a, where the DGL membrane 21’ bulges to become a convex mirror, the beam profile may generally increase in cross-sectional area as the light travels from the DGL membrane 21’ towards the light detector 12. Therefore, as the beam profile grows bigger, the light intensity may decrease. This decrease in light intensity may translate to a reduction in the measured total power of light instant on the field of view 120 of the light detector 12. The shifting of the light beam, as explained above, may cause less of the reflected light beam to fall within the field of view 120 of the light detector 12, and may thus also contribute to a reduction in the measured total power of light incident on the field of view 120 of the light detector 12.

[0047] As noted above, it has been found that the degree of bulging of the DGL membrane 21’ may increase with the degree of degradation of the DGL membrane 21’. Therefore, for a given pressure differential across the DGL membrane 21’, the degree of bulging may be correlated with the level of degradation, and the degree of bulging may affect the measured total power of light incident on the field of view 120 of the light detector 12.

[0048] Figure 4b shows a scenario when the pressure differential is reversed, i.e. the pressure on the side of the DGL membrane 21 ” facing the light source 11 and the light detector 12 is greater than on the opposite side of the DGL membrane 21 ” . In this scenario, the DGL membrane 21 ” may transform into a (partially reflective) concave mirror. As before, if the DGL membrane 21” has a generally rectangular / oval shape, the DGL membrane 21” may behave as a concave mirror with different optical powers in different directions.

[0049] As shown, because the DGL membrane 21” behaves as a concave mirror, the light beam reflecting off the DGL membrane 21” may become convergent (or less divergent than the beam before the reflection). For example, as shown, the beam profile 115” immediately after the reflection may be smaller than the original beam profile 110. When the degree of bulging is mild, the beam profile may continue to decrease as the light travels from the DGL membrane 21” towards the light detector 12. In some scenarios, the light beam may have converged sufficiently so that it falls entirely within the field of view 120 of the light detector 12, so that the measured total power of light incident on the field of view 120 of the light detector 12 may be at a maximum at this point.

[0050] As the degree of bulging increases, however, as illustrated in Figure 4b, the light beam may converge to a focal point before it reaches the light detector 12. As shown, at some distance away from the DGL membrane 21’ ’ , the beam profile 116” has converged to a focal point in one direction. Beyond this point, the light beam may become divergent in one direction, so that the beam profile 117” at a position further down the light path becomes wider. By the time that the light reaches the light detector, the beam profile 119” may have become wider than the field of view 120 of the light detector 12. At the same time, due to the astigmatism, the beam profile may remain convergent in the other direction up to the light detector 12.

[0051] As with the scenario shown in Figure 4a, when the DGL membrane 21” transforms into a concave mirror as shown in Figure 4b, the reflected light beam may also become shifted relative to the light detector 12. As shown, the beam profile 119” at the light detector 12 may be shifted relative to the field of view 120 of the light detector 12 in the same direction as the bulging of the DGL membrane 21”.

[0052] Therefore, starting from a flat and smooth DGL membrane 21 as shown in Figure 3, as the degree of bulging increases in the direction shown in Figure 4b, it can be expected that the measured total power of light incident on the field of view 120 of the light detector 12 may initially increase (slightly) or remain constant and, as the degree of bulging continues to increase so that the focal point in at least one optical axis becomes increasingly close to the DGL membrane 21”, it can be expected that the measured total power of light incident on the field of view 120 of the light detector 12 will decrease. The shifting of the reflected light beam may also contribute to a reduction in the measured total power of light incident on the field of view 120 of the light detector 12.

[0053] As mentioned above, the bulging of the DGL membrane 21 may be a primary observable effect of degradation, and that a secondary observable effect may come from the wrinkling of the DGL membrane 21. Wrinkles in the DGL membrane 21 may cause light scattering. Therefore, when the DGL membrane 21 becomes more wrinkled, the reflected light beam may become more diffuse and the reflected beam profile may become less sharp. This may be expected to result in a decrease in the measured total power of light incident on the field of view 120 of the light detector 12.

[0054] As it will be appreciated, although the effect of degradation of the DGL membrane 21 on the degree of bulging and / or wrinkling can be qualitatively described, the precise behavior of the DGLmembrane 21 may depend on a number of factors such as geometries, dimensions, pressure differentials, temperatures and material properties of the DGL membrane 21. Similarly, the precise effect of the bulging and / or wrinkling on the reflected light beam, and therefore the measured total power of light incident on the field of view 120 of the light detector 12, may also depend on a number of factors which may be difficult to quantify. However, it is generally not necessary to model each of these effects from first principles. Instead, the determination of the level of degradation of the DGL membrane 21 may be based on empirical relationships, which may be established for a particular design of the lithographic apparatus, a particular operation regime, as well as the properties of the DGL membrane 21 used, etc.

[0055] Nevertheless, some general trends may be established. Referring to Figure 5, as the DGL membrane 21 degrades due to stress or changes in material properties over time, in some embodiments, it is expected that the detector signal (i.e. the measured total power of light incident on the field of view 120 of the light detector 12) will generally decrease. For example, the detector signal may steadily decrease initially, and then decrease more sharply before the point of rupture of the DGL membrane 21. Once the DGL membrane 21 has ruptured, it is expected that little or none of the light emitted by the light source 11 will reach the light detector 12 because the DGL membrane 21 may no longer be present.

[0056] In particular, through empirical measurements, it may be observed that the total power of light incident on the field of view 120 of the light detector 12 may fall to a certain low level before rupture. Accordingly, as shown in Figure 5, a predetermined threshold may be defined to indicate that the DGL membrane 21 has degraded but not ruptured. Specifically, the predetermined threshold may be defined at a value above the detector signal level at the point of rupture. As the prediction of rupture may be statistical in nature, if the predetermined threshold is defined at a level which is too close to the detector signal at the point of rupture, it is possible that the DGL membrane 21 may sometimes rupture before the detector signal falls below the predetermined threshold. Nevertheless, in order to make optimal use of the lifetime of the DGL membrane 21, it may be generally desirable to define the predetermined threshold close to the detector signal level at the point of rupture. One choice of the predetermined threshold may for example be a balance between a maximum use of the lifetime of the DGL membrane 21 and the risk of rupture of the DGL membrane 21 during operation of the lithographic apparatus, or alternatively the predetermined threshold can be chosen such as to keep a balanced threshold in operation and minimize the risk of rupture at the cost of a shorter use of the DGL membrane lifetime.

[0057] Given a predetermined threshold, the controller 15 may be configured to compare the measured total power of light incident on the field of view 120 of the light detector 12 against the predetermined threshold. Specifically, the controller 15 may be configured to indicate that the DGL membrane 21 has degraded when the measured total power of light incident on the field of view 120 of the light detector 12 has fallen below the predetermined threshold. Based on this indication, theoperator of the lithographic apparatus may decide to shut down the lithographic apparatus and replace the DGL membrane 21.

[0058] Although the present invention may help prevent DGL membrane 21 ruptures, ruptures can, in principle, still happen. For example, if the operator decides to continue the manufacture of devices despite warnings that the DGL membrane 21 may be degraded, the DGL membrane 21 may eventually rupture. Other abnormal external circumstances may also cause the DGL membrane 21 to rupture suddenly. Therefore, the controller 15 may be usefully configured to detect whether the DGL membrane 21 is intact or present, as opposed to ruptured or absent. The controller 15 may do so similarly based on the measured total power of light incident on the field of view of the light detector 12. In particular, when the DGL membrane 21 has ruptured or is entirely absent, little to no light from the light source 11 will be picked up by the light detector 12. Any light that remains picked up will generally be light that has scattered around the vicinity of the light source 11 and detector 12. Therefore, when the measured total power of light incident on the field of view of the light detector 12 is zero (or less than a nominal zero), it may be concluded that the DGL membrane 21 has ruptured or is absent.

[0059] As noted above, the degree of bulging and / or wrinkling of the DGL membrane 21 may depend not just on the degree of degradation of the DGL membrane 21, but also on the pressure difference across the DGL membrane 21 at any given moment. Therefore, the controller 15 may be further configured to adjust the predetermined threshold based on a measurement of a pressure difference between the two sides of the DGL membrane 21. For example, the controller 15 may store an empirical relationship between the predetermined threshold and the measured pressure difference across the DGL membrane 21.

[0060] The pressure difference across the DGL membrane 21 may be measured or obtained using any suitable means. For example, as shown in Figure 7, the monitoring subsystem 1 may comprise pressure sensors 13 and 14 on either side of the DGL membrane 21, and the pressure difference may be obtained by subtracting the signals of the pressure sensors 13 and 14 one from the other. Alternatively, a single pressure sensor may be provided across the interface between the interior of the projection system PS and the region surrounding the substrate W and the substrate table WT, and the pressure difference across the DGL membrane 21 may be determined directly from the signal of this pressure sensor. It is to be understood that the monitoring subsystem 1 does not necessarily require dedicated pressure sensors such as shown in Figure 7. Instead, the pressure sensors may be shared with other subsystems of the lithographic apparatus. Alternatively or additionally, the pressure difference across the DGL membrane 21 may be obtained by means other than real-time measurements. For example, the pressure difference may be known in advance for one or more states of the lithographic apparatus, so that the pressure difference can be inferred from the state of the lithographic apparatus at any given time. The pressure difference in various states of the lithographic apparatus may be measured using pressure sensors during system verification (i.e. before thelithographic apparatus is put into operation), or may be determined by flow modelling (e.g. during design of the lithographic apparatus). It should be understood that the pressure difference may be obtained by any one of real-time pressure measurements, measurements in advance, and determination by flow modelling, or any combination of these means.

[0061] Figures 6a and 6b are plots of data points obtained from a functional mock-up of the monitoring subsystem 1. These plots show the detector signal level (i.e. the measured total power of light incident on the field of view 120 of the light detector 12) and various values of pressure differential across the DGL membrane 21. In each of Figures 6a and 6b, data series 31 is obtained using a brand new DGL membrane 21; data series 32 is obtained using a degraded DGL membrane 21; and data series 39 is obtained with no DGL membrane present. As confirmed by experiment, the absence of a DGL membrane 21 is an accurate representation of a ruptured DGL membrane 21. Figure 6b shows the same data as Figure 6a, but is zoomed in to a pressure difference between -10 Pa and +10 Pa. In Figures 6a and 6b, by definition, the pressure differential is positive when the DGL membrane 21’ bulges to be a convex mirror, and the pressure differential is negative when the DGL membrane 21” bulges to be a concave mirror.

[0062] As shown in Figure 6a, when the DGL membrane 21 is brand new and substantially free of wrinkles, the detector signal decreases as the pressure difference increases from 0 to 150 Pa and beyond. As the pressure differential decreases from 0 to -150 Pa, the detector signal is observed to be approximately constant down to about -35 Pa, and decrease thereafter. As shown by data series 32, when the DGL membrane 21 has degraded, the detector signal is lower than that of a brand new DGL membrane across all pressure differential values. In this example, the predetermined threshold mentioned above may be set according to data series 32, and may further be adjusted as a function of the pressure difference according to the data series 32. That is, the controller 15 may monitor both the detector signal and the pressure difference continuously, and if the detector signal falls below the data series 32 at the instantaneous pressure difference value, the controller may indicate that the DGL membrane 21 has degraded.

[0063] As can be seen in Figure 6b, in the data series 32, a local minimum is observed at zero pressure difference. This is believed to be caused by wrinkles in the DGL membrane 21. When there is no pressure difference acting on the DGL membrane 21, there is no force to flatten out the wrinkles. As the pressure difference increases in magnitude (irrespective of direction), it is believed that the wrinkles are gradually flattened out, so that the detector signal initially increases when the pressure difference moves away form zero. As can be seen, other than the local minimum observed at zero pressure difference, the data series 32 follows the general shape of data series 31, except that it is shifted downwards. For completeness, it can be seen from data series 39 that the detector signal is effectively zero when the DGL membrane is absent / ruptured.

[0064] The light source 11 and the light detector 12 may be configured so that the light incident on the light detector 12 has a spot size which is comparable to the field of view 120 of the light detector12 when the DGL membrane is smooth and flat, or when the DGL membrane is in its nominal / rest position. The spot size may be larger, or slightly larger, than the field of view 120 of the light detector 12, such a shown by the beam profile 119 in Figure 3.

[0065] In addition to the monitoring subsystem 1, the present disclosure also contemplates a lithographic apparatus comprising the monitoring subsystem 1. The present disclosure also contemplates a method of monitoring the condition of the DGL membrane 21 using the techniques described above, and methods of manufacturing devices including the method of monitoring the DGL membrane 21. The present disclosure also contemplates a method of manufacturing devices using the lithographic apparatus described above.

[0066] 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.

[0067] 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.

[0068] Aspects of the invention are described in the following numbered clauses.1. A monitoring subsystem for monitoring a membrane in use in a lithographic apparatus, the monitoring subsystem comprising: a light source configured to illuminate the membrane, the membrane being arranged between a first region and a second region of the lithographic apparatus; a light detector configured to measure a total power of light incident on a field of view of the light detector, the light being emitted from the light source and reflected off the membrane; and a controller configured to determine a level of degradation of the membrane at least in part based on the measured total power of light incident on the field of view of the light detector.2. The subsystem of clause 1 , wherein the first region is configured to contain a substrate, the second region comprises projection optics of the lithographic apparatus, and the membrane is a dynamic gas lock (DGL) membrane.3. The subsystem of clause 1 or clause 2, wherein the light source is configured to emit visible light.4. The subsystem of clause 1 or clause 2, wherein the light source is configured to emit light comprising light within a spectral band of 430 nm to 630 nm, or 480 nm to 580 nm, or 500nm to 550 nm.5. The subsystem of any one of the preceding clauses, wherein the controller is configured to compare the measured total power of light incident on the field of view of the light detector against a predetermined threshold indicative of a membrane that has degraded but not ruptured.6. The subsystem of clause 5, wherein the controller is configured to indicate that the membrane has degraded when the measured total power of light incident on the field of view of the light detector has fallen below the predetermined threshold.7. The subsystem of clause 5, wherein the controller is configured to adjust the predetermined threshold based on a measurement of a pressure difference between the two sides of the membrane.8. The subsystem of any one of the preceding clauses, wherein the controller is configured to determine whether the membrane is i) intact or present, or ii) ruptured or absent, based at least in part based on the measured total power of light incident on the field of view of the light detector.9. The subsystem of any one of the preceding clauses, wherein the light detector comprises light collecting optics.10. The subsystem of any one of the preceding clauses, wherein the light detector is non-spatially resolved.11. The subsystem of any one of the preceding clauses, wherein the light detector has a single voltage output representing the measured total power of light incident on the field of view of the light detector.12. The subsystem of any one of the preceding clauses, wherein the light incident on the light detector has a spot size larger than the field of view of the light detector when the membrane is smooth and flat.13. A lithographic apparatus comprising the subsystem of any one of the preceding clauses.14. A method comprising: illuminating, using a light source, a membrane arranged between a first region and second a region of the lithographic apparatus; measuring a total power of light incident on a field of view of a light detector, the light being emitted from the light source and reflected off the membrane; and determining a level of degradation of the membrane at least in part based on the measured total power of light incident on the field of view of the light detector.15. The method of clause 13, wherein the first region is configured to contain a substrate, the second region comprises projection optics of the lithographic apparatus, and the membrane is a dynamic gas lock (DGL) membrane.16. The method of clause 14 or clause 15, wherein the light source emits visible light.17. The method of clause 14 or clause 15, wherein the light source emits light comprising light within a spectral band of 430 nm to 630 nm, or 480 nm to 580 nm, or 500nm to 550 nm.18. The method of any one of clauses 14 to 17, further comprising comparing the measured total power of light incident on the field of view of the light detector against a predetermined threshold indicative of a membrane that has degraded but not ruptured.19. The method of clause 18, further comprising indicating that the membrane has degraded when the measured total power of light incident on the field of view of the light detector has fallen below the predetermined threshold.20. The method of clause 19, further comprising adjusting the predetermined threshold based on a measurement of a pressure difference between the two sides of the membrane.21. The method of any one of clauses 14 to 20, further comprising determining whether the membrane is i) intact or present, or ii) ruptured or absent, based at least in part based on the measured total power of light incident on the field of view of the light detector.22. The method of any one clauses 14 to 21, wherein the light detector has a single voltage output representing the measured total power of light incident on the field of view of the light detector.23. The method of any one of clauses 14 to 22, wherein the light incident on the light detector has a spot size larger than the field of view of the light detector when the membrane is smooth and flat.24. A method of manufacturing devices comprising the method of any one of clauses 12 to 19.25. A method of manufacturing devices using the lithographic apparatus of clause 11.

[0069] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0070] 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 monitoring subsystem for monitoring a membrane in use in a lithographic apparatus, the monitoring subsystem comprising: a light source configured to illuminate the membrane, the membrane being arranged between a first region and a second region of the lithographic apparatus; a light detector configured to measure a total power of light incident on a field of view of the light detector, the light being emitted from the light source and reflected off the membrane; and a controller configured to determine a level of degradation of the membrane at least in part based on the measured total power of light incident on the field of view of the light detector.

2. The subsystem of claim 1, wherein a) the first region is configured to contain a substrate, the second region comprises projection optics of the lithographic apparatus, and the membrane is a dynamic gas lock (DGL) membrane, and / or b) the first region is configured to contain a patterning device, the second region comprises projection optics of the lithographic apparatus, and the membrane is a pellicle.

3. The subsystem of claim 1 or claim 2, wherein the light source is configured to emit visible light.

4. The subsystem of claim 1 or claim 2, wherein the light source is configured to emit light comprising light within a spectral band of 430 nm to 630 nm, or 480 nm to 580 nm, or 500nm to 550 nm.

5. The subsystem of any one of the preceding claims, wherein the controller is configured to compare the measured total power of light incident on the field of view of the light detector against a predetermined threshold indicative of a membrane that has degraded but not ruptured.

6. The subsystem of claim 5, wherein the controller is configured to indicate that the membrane has degraded when the measured total power of light incident on the field of view of the light detector has fallen below the predetermined threshold.

7. The subsystem of claim 5, wherein the controller is configured to adjust the predetermined threshold based on a measurement of a pressure difference between the two sides of the membrane.

8. The subsystem of any one of the preceding claims, wherein the controller is configured to determine whether the membrane is i) intact or present, or ii) ruptured or absent, based at least in part based on the measured total power of light incident on the field of view of the light detector.

9. The subsystem of any one of the preceding claims, wherein the light detector comprises light collecting optics.

10. The subsystem of any one of the preceding claims, wherein the light detector is non-spatially resolved.

11. The subsystem of any one of the preceding claims, wherein the light detector has a single voltage output representing the measured total power of light incident on the field of view of the light detector.

12. The subsystem of any one of the preceding claims, wherein the light incident on the light detector has a spot size larger than the field of view of the light detector when the membrane is smooth and flat.

13. A lithographic apparatus comprising the subsystem of any one of the preceding claims.

14. A method comprising: illuminating, using a light source, a membrane arranged between a first region and second a region of the lithographic apparatus; measuring a total power of light incident on a field of view of a light detector, the light being emitted from the light source and reflected off the membrane; and determining a level of degradation of the membrane at least in part based on the measured total power of light incident on the field of view of the light detector.

15. The method of claim 13, wherein a) the first region is configured to contain a substrate, the second region comprises projection optics of the lithographic apparatus, and the membrane is a dynamic gas lock (DGL) membrane, and / or b) the first region is configured to contain a patterning device, the second region comprises projection optics of the lithographic apparatus, and the membrane is a pellicle..

16. The method of claim 14 or claim 15, wherein the light source emits visible light.

17. The method of claim 14 or claim 15, wherein the light source emits light comprising light within a spectral band of 430 nm to 630 nm, or 480 nm to 580 nm, or 500 to 550 nm.

18. The method of any one of claims 14 to 17, further comprising comparing the measured total power of light incident on the field of view of the light detector against a predetermined threshold indicative of a membrane that has degraded but not ruptured.

19. The method of claim 18, further comprising indicating that the membrane has degraded when the measured total power of light incident on the field of view of the light detector has fallen below the predetermined threshold.

20. The method of claim 19, further comprising adjusting the predetermined threshold based on a measurement of a pressure difference between the two sides of the membrane.

21. The method of any one of claims 14 to 20, further comprising determining whether the membrane is i) intact or present, or ii) ruptured or absent, based at least in part based on the measured total power of light incident on the field of view of the light detector.

22. The method of any one claims 14 to 21, wherein the light detector has a single voltage output representing the measured total power of light incident on the field of view of the light detector.

23. The method of any one of claims 14 to 22, wherein the light incident on the light detector has a spot size larger than the field of view of the light detector when the membrane is smooth and flat.

24. A method of manufacturing devices comprising the method of any one of claims 12 to 19.

25. A method of manufacturing devices using the lithographic apparatus of claim 11.

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