Patent Publication Number: US-11036128-B2

Title: Membrane assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase entry of PCT patent application no. PCT/EP2016/079584, which was filed on Dec. 2, 2016, which claims the benefit of priority of European patent application no. 15199845.7, which was filed on Dec. 14, 2015, and of European patent application no. 16157967.7, which was filed on Mar. 1, 2016, and European patent application no. 16163962.0, which was filed on Apr. 6, 2016, each of which is incorporated herein in its entirety by reference. 
     FIELD 
     The present invention relates to a membrane assembly and a patterning device assembly for EUV lithography. 
     BACKGROUND 
     A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. 
     Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured. 
     A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
 
CD= k   1 *λ/NA  (1)
 
where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k 1  is a process-dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k 1 .
 
     In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring. 
     A lithographic apparatus includes a patterning device (e.g., a mask or a reticle). Radiation is provided through or reflected off the patterning device to form an image on a substrate. A membrane assembly may be provided to protect the patterning device from airborne particles and other forms of contamination. The membrane assembly for protecting the patterning device may be called a pellicle. Contamination on the surface of the patterning device can cause manufacturing defects on the substrate. The membrane assembly may comprise a border and a membrane stretched across the border. 
     In use the membrane is required to be fixed relative to the patterning device by mounting features, for example. It is desirable to reduce the amount of space taken up by the mounting features. It is also desirable for the membrane assembly to take up less space while it is being transported into position for mounting to the patterning device. It is also desirable to reduce the possibility of contaminant particles reaching a region between the membrane and the patterning device. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, there is provided a membrane assembly for EUV lithography, the membrane assembly comprising: a planar membrane; a border configured to hold the membrane; and a frame assembly connected to the border and configured to releasably attach to a patterning device for EUV lithography, wherein the frame assembly comprises a resilient member; wherein the frame assembly is connected to the border in a direction perpendicular to the plane of the membrane such that in use the frame assembly is between the border and the patterning device. 
     According to an aspect of the invention, there is provided a patterning device assembly for EUV lithography, the patterning device assembly comprising: a planar patterning device; at least one protrusion protruding from the patterning device; and the membrane assembly of any preceding claim, the frame assembly being connected to the patterning device via the at least one protrusion; wherein the at least one protrusion is between the border and the patterning device. 
     According to an aspect of the invention, there is provided a membrane assembly for EUV lithography, the membrane assembly comprising: a planar membrane; and a frame assembly configured to hold the membrane and to attach to a patterning device for EUV lithography; wherein the frame assembly has a locked state in which the frame assembly is locked to the patterning device such that the membrane is held a predetermined distance from the patterning device, and an unlocked state in which the membrane is less than the predetermined distance from the patterning device. 
     According to an aspect of the invention, there is provided a patterning device assembly for EUV lithography, the patterning device assembly comprising: a planar patterning device for EUV lithography; a membrane assembly comprising: a planar membrane; and a frame assembly configured to hold the membrane and to attach to the patterning device, wherein a gap is formed between opposing surfaces of the frame assembly and the patterning device; wherein the frame assembly comprises an elongate baffle configured to restrain contaminant particles from entering the gap, wherein the elongate baffle extends beyond the opposing surface of the patterning device at a location beyond the planar extent of the patterning device. 
     According to an aspect of the invention, there is provided a patterning device assembly for EUV lithography, the patterning device assembly comprising: a planar patterning device; a membrane assembly comprising a planar membrane and a border configured to hold the membrane; at least one protrusion protruding from one of the patterning device and the border, wherein the at least one protrusion is between the border and the patterning device; and a frame assembly connected to the other of the patterning device and the border, wherein the frame assembly is configured to attach to the at least one protrusion between the border and the patterning device. 
     According to an aspect of the invention, there is provided a loading apparatus for temporarily housing a membrane assembly that is mounted onto a patterning device for EUV lithography, the loading apparatus comprising protrusions at an inner surface of the loading apparatus, wherein the protrusions are configured to press a membrane holder of the membrane assembly towards the patterning device when the loading apparatus houses the membrane assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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, and in which: 
         FIG. 1  depicts a lithographic apparatus according to an embodiment of the invention; 
         FIG. 2  is a more detailed view of the lithographic apparatus; 
         FIG. 3  schematically depicts, in cross-section, part of a membrane assembly according to an embodiment of the invention; 
         FIGS. 4 to 6  schematically depict, in plan, stages of using a locking mechanism of a membrane assembly according to an embodiment of the invention; 
         FIGS. 7 to 10  schematically depict, in cross-section, membranes according to different embodiments of the invention; 
         FIGS. 11 to 14  schematically depict, in cross-section, various stages of a process of mounting a membrane assembly according to an embodiment of the invention being mounted onto a patterning device; 
         FIG. 15  schematically depicts, in cross-section, a membrane assembly according to an embodiment of the invention in a loading apparatus; 
         FIGS. 16 and 17  schematically depict, in cross-section, a membrane assembly according to an embodiment of the invention being removed from a loading apparatus; 
         FIG. 18  schematically depicts, in cross-section, a gap between a membrane assembly according to an embodiment of the invention and a patterning device; and 
         FIG. 19  schematically depicts, in cross-section, a membrane assembly according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically depicts a lithographic apparatus  100  including a source collector module SO according to one embodiment of the invention. The apparatus  100  comprises: 
     an illumination system (or illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation). 
     a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; 
     a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and 
     a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W. 
     The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. 
     The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable as required. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS. 
     The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section such as to create a pattern in a target portion C of the substrate W. The pattern imparted to the radiation beam B may correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit. 
     The patterning device MA may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable liquid-crystal display (LCD) panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable minor array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix. 
     The projection system PS, like the illumination system IL, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps. 
     As here depicted, the lithographic apparatus  100  is of a reflective type (e.g., employing a reflective mask). 
     The lithographic apparatus  100  may be of a type having two (dual stage) or more substrate tables WT (and/or two or more support structures MT). In such a “multiple stage” lithographic apparatus the additional substrate tables WT (and/or the additional support structures MT) may be used in parallel, or preparatory steps may be carried out on one or more substrate tables WT (and/or one or more support structures MT) while one or more other substrate tables WT (and/or one or more other support structures MT) are being used for exposure. 
     Referring to  FIG. 1 , the illumination system IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in  FIG. 1 , for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module SO may be separate entities, for example when a CO 2  laser is used to provide the laser beam for fuel excitation. 
     In such cases, the laser is not considered to form part of the lithographic apparatus  100  and the radiation beam B is passed from the laser to the source collector module SO with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module SO, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source. 
     The illumination system IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illumination system IL can be adjusted. In addition, the illumination system IL may comprise various other components, such as facetted field and pupil mirror devices. The illumination system IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross-section. 
     The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device MA. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS 2  (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS 1  can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. The patterning device (e.g., mask) MA and the substrate W may be aligned using mask alignment marks M 1 , M 2  and substrate alignment marks P 1 , P 2 . 
     A controller  500  controls the overall operations of the lithographic apparatus  100  and in particular performs an operation process described further below. Controller  500  can be embodied as a suitably-programmed general purpose computer comprising a central processing unit, volatile and non-volatile storage means, one or more input and output devices such as a keyboard and screen, one or more network connections and one or more interfaces to the various parts of the lithographic apparatus  100 . It will be appreciated that a one-to-one relationship between controlling computer and lithographic apparatus  100  is not necessary. In an embodiment of the invention one computer can control multiple lithographic apparatuses  100 . In an embodiment of the invention, multiple networked computers can be used to control one lithographic apparatus  100 . The controller  500  may also be configured to control one or more associated process devices and substrate handling devices in a lithocell or cluster of which the lithographic apparatus  100  forms a part. The controller  500  can also be configured to be subordinate to a supervisory control system of a lithocell or cluster and/or an overall control system of a fab. 
       FIG. 2  shows the lithographic apparatus  100  in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. An EUV radiation emitting plasma  210  may be formed by a plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the radiation emitting plasma  210  is created to emit radiation in the EUV range of the electromagnetic spectrum. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation. 
     The radiation emitted by the radiation emitting plasma  210  is passed from a source chamber  211  into a collector chamber  212 . 
     The collector chamber  212  may include a radiation collector CO. Radiation that traverses the radiation collector CO can be focused in a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector module SO is arranged such that the virtual source point IF is located at or near an opening  221  in the enclosing structure  220 . The virtual source point IF is an image of the radiation emitting plasma  210 . 
     Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device  22  and a facetted pupil mirror device  24  arranged to provide a desired angular distribution of the unpatterned beam  21 , at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the unpatterned beam  21  at the patterning device MA, held by the support structure MT, a patterned beam  26  is formed and the patterned beam  26  is imaged by the projection system PS via reflective elements  28 ,  30  onto a substrate W held by the substrate table WT. 
     More elements than shown may generally be present in the illumination system IL and the projection system PS. Further, there may be more mirrors present than those shown in the Figures, for example there may be 1-6 additional reflective elements present in the projection system PS than shown in  FIG. 2 . 
     Alternatively, the source collector module SO may be part of an LPP radiation system. 
     As depicted in  FIG. 1 , in an embodiment the lithographic apparatus  100  comprises an illumination system IL and a projection system PS. The illumination system IL is configured to emit a radiation beam B. The projection system PS is separated from the substrate table WT by an intervening space. The projection system PS is configured to project a pattern imparted to the radiation beam B onto the substrate W. The pattern is for EUV radiation of the radiation beam B. 
     The space intervening between the projection system PS and the substrate table WT can be at least partially evacuated. The intervening space may be delimited at the location of the projection system PS by a solid surface from which the employed radiation is directed toward the substrate table WT. 
     In an embodiment the lithographic apparatus  100  comprises a dynamic gas lock. The dynamic gas lock comprises a membrane assembly  80 . In an embodiment the dynamic gas lock comprises a hollow part covered by a membrane assembly  80  located in the intervening space. The hollow part is situated around the path of the radiation. In an embodiment the lithographic apparatus  100  comprises a gas blower configured to flush the inside of the hollow part with a flow of gas. The radiation travels through the membrane assembly before impinging on the substrate W. 
     In an embodiment the lithographic apparatus  100  comprises a membrane assembly  80 . As explained above, in an embodiment the membrane assembly  80  is for a dynamic gas lock. In this case the membrane assembly  80  functions as a filter for filtering DUV radiation. Additionally or alternatively, in an embodiment the membrane assembly  80  is pellicle for the patterning device MA for EUV lithography. The membrane assembly  80  of the present invention can be used for a dynamic gas lock or for a pellicle or for another purpose such as a spectral purity filter. In an embodiment the membrane assembly  80  comprises a membrane  40 , which may also be called a membrane stack. In an embodiment the membrane is configured to transmit at least 80% of incident EUV radiation. 
     In an embodiment the membrane assembly  80  is configured to seal off the patterning device MA to protect the patterning device MA from airborne particles and other forms of contamination. Contamination on the surface of the patterning device MA can cause manufacturing defects on the substrate W. For example, in an embodiment the pellicle is configured to reduce the likelihood that particles might migrate into a stepping field of the patterning device MA in the lithographic apparatus  100 . 
     If the patterning device MA is left unprotected, the contamination can require the patterning device MA to be cleaned or discarded. Cleaning the patterning device MA interrupts valuable manufacturing time and discarding the patterning device MA is costly. Replacing the patterning device MA also interrupts valuable manufacturing time. 
       FIG. 3  schematically depicts, in cross-section, part of a membrane assembly  80  according to an embodiment of the invention. The membrane assembly  80  is for EUV lithography. The membrane assembly  80  comprises a membrane  40 . The membrane  40  is emissive for EUV radiation. Of course the membrane  40  may not have 100% emissivity for EUV radiation. However, the membrane may have, for example, at least 50% emissivity, preferably at least 85%, even more preferably at least 95% emissivity for EUV radiation. As shown in  FIG. 3 , in an embodiment the membrane  40  is substantially planar. In an embodiment the plane of the membrane  40  is substantially parallel to the plane of the patterning device MA. 
     The membrane assembly  80  has a shape such as a square, a circle or a rectangle, for example. The shape of the membrane assembly  80  is not particularly limited. The size of the membrane assembly  80  is not particularly limited. For example, in an embodiment the membrane assembly  80  has a diameter in the range of from about 100 mm to about 500 mm, for example about 200 mm. 
     As depicted in  FIG. 3 , in an embodiment the membrane assembly  80  comprises a border  81 . The border  81  is configured to hold the membrane  40 . The border  81  provides mechanical stability to the membrane  40 . The border  81  is configured to reduce the possibility of the membrane  40  being deformed away from its planar shape. In an embodiment, a pre-tension is applied to the membrane  40  during its manufacture. The border  81  is configured to maintain the tension in the membrane  40  so that the membrane  40  does not have an undulating shape during use of the lithographic apparatus  100 . In an embodiment the border  81  extends along the perimeter of the membrane  40 . The outer periphery of the membrane  40  is positioned on top of the border  81  (according to the view of  FIG. 3 ). The border  81  may be at least partly formed by part of the membrane  40  which is left over from the process of manufacturing the membrane assembly  80 . Hence the border  81  may not be a separate component from the membrane  40  (i.e. is an integral part of the membrane). 
     The thickness of the border  81  is not particularly limited. For example, in an embodiment the border  81  has a thickness of at least 300 μm, optionally at least 400 μm. In an embodiment the border  81  has a thickness of at most 1,000 μm, optionally at most 800 μm. In an embodiment the border  81  has a width of at least 1 mm, optionally at least 2 mm, optionally at least 4 mm. In an embodiment the border  81  has a width of at most 10 mm, optionally at most 5 mm, optionally at most 4 mm. 
     As depicted in  FIG. 3 , in an embodiment the membrane assembly  80  comprises a frame assembly  50 . The frame assembly  50  is connected to the border  81 . In an embodiment the frame assembly  50  comprises a frame surface in contact with the border  81 . In an embodiment the frame assembly  50  is initially manufactured as a separate component from the border  81  and subsequently connected to the border  81 . For example, the combination of the membrane  40  with the border  81  may be manufactured together, while the frame assembly  50  may be manufactured separately. In a subsequent manufacturing step, the frame assembly  50  may be attached or fixed to the border  81 . 
     In an embodiment the frame assembly  50  has a width of at least 2 mm, optionally at least 5 mm, optionally at least 8 mm. In an embodiment the frame assembly  50  has a width of at most 20 mm, optionally at most 10 mm, optionally at most 8 mm. 
     In an embodiment the frame assembly  50  comprises a frame  51 . The frame  51  is the part of the frame assembly  50  that is connected to the border  81 . In an embodiment the frame  51  is made of the same material as the border  81 . For example, in an embodiment both the border  81  and the frame  51  are made of a material comprising silicon. In an embodiment the border  81  is made of silicon. In an embodiment the frame  51  is made of silicon. In an embodiment the thermal expansion of the border  81  substantially matches the thermal expansion of the frame  51 . In an embodiment the frame  51  is attached to the border  81  by an adhesive. In an embodiment the thermal expansion of the adhesive substantially matches the thermal expansion of the frame  51  and/or the border  81 . 
     As depicted in  FIG. 3 , the frame assembly  50  is configured to attach to the patterning device MA. In an embodiment the frame assembly  50  comprises a frame surface configured to contact the patterning device MA. The frame assembly  50  is for holding the position of the membrane  40  relative to the patterning device MA. Although the embodiment is described with reference to a patterning device MA, the invention is equally applicable to a membrane assembly  80  that connects to a different component other than the patterning device MA. 
     In an embodiment the frame assembly  50  is connected to the border  81  in a direction perpendicular to the plane of the membrane  40 . This is shown in  FIG. 3 . In  FIG. 3 , the plane of the membrane  40  extends left to right and into and out of the paper. The direction perpendicular to the plane of the membrane  40  corresponds to the vertical (i.e. up and down) direction in  FIG. 3 . The frame assembly  50  is connected directly below the border  81 . The frame assembly  50  being connected to the border  81  in a direction perpendicular to the plane of the membrane  40  is to be understood that the border  81  and the frame assembly  50  are aligned in a vertical direction (i.e. in a direction perpendicular to the surface of the patterning device MA and of the membrane  40 ) such that a vertical line can be drawn that extends through both the border  81  and the frame assembly  50 , as shown in  FIG. 3 . In an embodiment the interface between the border  81  and the frame assembly  50  is in a plane that is substantially parallel to the plane of the membrane  40 . 
     In an embodiment the membrane assembly  80  is configured to be removable from the patterning device MA. This allows intermediate inspections of the patterning device MA to take place. In an embodiment the frame assembly  50  is configured to be repeatedly attached to and detached from the patterning device MA. 
     In use, the frame assembly  50  is between the border  81  and the patterning device MA. The term “between” herein means that the frame assembly  50  extends under the border  81  and it is positioned on the top of a surface of the patterning device. This arrangement is different from arrangements in which the frame assembly is positioned radially outwards from the border. An embodiment of the invention is expected to achieve a reduction in space around the membrane  40  required to hold the membrane  40  in position relative to the patterning device MA. 
     According to a comparative example, a membrane assembly has a frame assembly radially outwards from the border, as described for example in WO 2016/079051 A2. The frame assembly is required to be accessed in the radial direction so as to attach/detach the frame assembly to/from the patterning device. A space of about 16 mm may be required to accommodate the border, the frame assembly and space for accessing the frame assembly. 
     In contrast in an embodiment the frame assembly  50  is positioned below the border  81 , thereby reducing radial space required to accommodate the border  81  and the frame assembly  50 . For example, in an embodiment the radial space required to accommodate the border  81 , the frame assembly  50  and space for accessing the frame assembly  50  is about 12 mm. 
     An embodiment of the invention is expected to achieve a reduction in the required space in the region of the patterning device MA for mounting features. Mounting features are features that are used to mount the membrane assembly  80  onto the patterning device MA. In an embodiment a mounting feature is provided between the border  81  and the patterning device MA, it is therefore not extending radially outwards from the border. In an embodiment at least one mounting feature is in contact with the patterning device and is substantially perpendicular the surface of the patterning device. This is shown in  FIG. 3  and will be explained in further detail below. 
     In an embodiment the frame assembly  50  comprises at least one hole  52 . In an embodiment the hole  52  is a cavity or chamber or an opening within the frame  51  of the frame assembly  50 . The hole  52  is configured to receive a protrusion (e.g. a stud  60 ). The stud  60  is in direct contact with and protrudes from the patterning device MA. In an alternative embodiment the frame assembly  50  is permanently attached to the patterning device MA and the stud  60  is in direct contact with and protrudes from the border  81  of the membrane assembly  80 . 
       FIG. 3  shows the stud  60  fixed to the patterning device MA. In an embodiment the stud  60  is glued onto the patterning device MA using an adhesive. Alternatively, the stud  60  may be formed integrally with the patterning device MA. As a further alternative, the stud  60  may be initially manufactured as a separate component from the patterning device MA and subsequently fixed to the patterning device MA using means other than an adhesive, for example a screw. 
     The stud  60  and the hole  52  are mounting features. In an embodiment the stud  60  and the hole  52  are provided between the border  81  and the patterning device MA. This is different from previously known arrangements in which the mounting features are positioned radially outwards from the border  81 . 
     As depicted in  FIG. 3 , in an embodiment the hole  52  at least partially overlaps the border  81  when viewed in the direction perpendicular to the plane of the membrane  40 . This is shown in  FIG. 3 , where the hole  52  partially overlaps the border  81  when viewed in the vertical direction. Looking at  FIG. 3 , a vertical line can be drawn that extends through both the border  81  and the hole  52 . 
     In an embodiment the frame assembly  50  comprises a locking mechanism  55 . In an embodiment the locking mechanism  55  is located in hole  55 , which forms a cavity or a chamber within the frame  51 . The cavity formed in frame  55  may be partially open, or it may fully surround the locking mechanism  55 . The locking mechanism  55  is configured to lock the frame assembly  50  to the stud  60 . In an embodiment the locking mechanism  55  comprises a resilient member  53 . In an embodiment the locking mechanism  55  comprises a resilient member  53  for each hole  52 . In an embodiment the frame assembly  50  comprises a plurality of holes  52 , for example two, three, four or more holes  52 . A resilient member  53  is provided corresponding to each hole  52 . In an embodiment the resilient member  53  is coupled to the frame  51 . In an embodiment the resilient member  53  is located inside hole  52  and coupled with one section to frame  51 , whereas another section of the resilient member is available for locking and unlocking the frame assembly from the patterning device MA. In an embodiment, the direction of the forces exerted on the protrusion by the resilient member  53  and the locking mechanism  55  is parallel with the membrane  40  and the patterning device MA. In an embodiment the resilient member is arranged inside hole  52  and attached to frame  51  such that it will push onto a side surface of the protrusion, in a direction parallel to the membrane  40 , as shown in  FIG. 3 . 
     As depicted in  FIG. 3 , in an embodiment the resilient member  53  comprises a spring. For example, the spring may be a coil spring or a leaf spring. In an alternative embodiment the resilient member  53  comprises a resilient material such as rubber. In an alternative embodiment the resilient member  53  comprises a flexure. The flexure may be machined using an electrical discharge machining process, for example. 
       FIGS. 4 to 6  schematically depict stages of use of the locking mechanism  55 .  FIGS. 4 to 6  are shown in plan view.  FIG. 4  depicts an initial state in which the frame assembly  50  is positioned over the stud  60  so that the stud  60  is received into the hole  52 . The resilient member  53  is not compressed. As depicted in  FIG. 4 , the resilient member  53  extends into the hole  52 . Accordingly, the stud  60  can come into contact with the resilient member  53  when the stud  60  is received into the hole  52 . The resilient member  53  is configured to be deformable (e.g. compressible) when the stud  60  received in the hole  52  presses against the resilient member  53  in a direction within the plane of the membrane  40 . For example, in  FIG. 4  the stud  60  can press against the resilient member  53  in the direction to the right in the Figure. 
     As depicted in  FIGS. 3 to 6 , in an embodiment the locking mechanism  55  comprises a locking member  54  for each hole  52 . The locking member  54  is configured to be movable to a locking position where the locking member  54  extends into the hole  52 . In the locking position the compressed resilient member  53  exerts a force on the stud  60  received in the hole  52  towards the locking member  54 . This is shown in the sequence from  FIG. 4  to  FIG. 6 . 
     As shown in the transition from  FIG. 4  to  FIG. 5 , the stud  60  and the frame assembly  50  are moved relative to each other so that the stud  60  presses against the resilient member  53 . The stud  60  compresses the resilient member  53 , as shown in  FIG. 5 . 
     As shown in the transition from  FIG. 5  to  FIG. 6 , the locking member  54  is moved to the locking position where the locking member  54  extends into the hole  52 . For example, as shown in  FIGS. 4 to 6 , in an embodiment the frame assembly  50  comprises at least one locking aperture  56 . The locking member  54  passes through the locking apertures  56 . 
       FIG. 6  shows the locking member  54  in the locking position. The resilient member  53  exerts a force on the stud  60  in the direction of the locking member  54 . In the situation shown in  FIG. 5 , an external force is required to be exerted on the frame assembly  50  and/or on the stud  60  so that the stud  60  compresses the resilient member  53 . Once the locking member  54  is in the locking position (e.g. as shown in  FIG. 6 ), it is no longer necessary for the external force to be applied. This is because the locking member  54  holds the stud  60  and the frame assembly  50  in position relative to each other. 
     As explained above, the stud  60  is positioned under the border  81 , instead of radially outward of the border  81 . This may require an increase in the distance (also known as standoff) between the patterning device MA and the membrane  40 . The distance between the surface of the patterning device MA and the membrane  40  substantially corresponds to the combined height of the frame assembly  50  and the border  81 . In an embodiment the combined height of the frame assembly  50  and the border  81  is at least 1 mm, at least 2 mm, and optionally at least 5 mm. In an embodiment the combined height of the frame assembly  50  with the border  81  is at most 20 mm, optionally at most 10 mm, and optionally at most 5 mm. 
     In an embodiment the protrusion (stud  60 ) and the locking mechanism  55  are located inside the hole (cavity)  52 . In an embodiment the protrusion and the locking mechanism  55  comprising the locking member  54  and the resilient member  53  are located inside hole  52 . In an embodiment a surface of the cavity formed in frame  51  has a surface in direct contact with the border  81 . In an embodiment the frame assembly  50 , comprising frame  51  which defines hole  52  encompassing mounting features to attach the frame  51  to the patterning device MA, is in direct contact with and below border  81  such that the frame assembly comprising the mounting features is arranged in between the border  81  and the patterning device MA. In an embodiment the hole  52  is formed by the frame which is more rigid than the resilient member. Therefore, the resilient member  53  is more deformable than the frame  51  in which hole  52  is formed, such that when in locking position, it is the resilient member  53  (and thus not frame  51 ) which is deformed under compression. Thus, the cavity will not resiliently deform (i.e. it keep its rigid shape) when the resilient member  53  and the locking mechanism  54  are in contact with protrusion  60 . 
     In an embodiment the resilient member  53  comprises a spring made of a material such as stainless steel. In an embodiment the resilient member  53  is connected to a contact pad  57  made of a different material from the resilient member  53 . For example, the contact pad  57  may be made of the same material as the stud  60  and/or the locking member  54 . In an embodiment the contact pad  57  comprises titanium. In an embodiment the locking member  54  comprises titanium. In an embodiment the stud  60  comprises titanium. Titanium is known to provide a ductile contact. However, in an alternative embodiment, other materials can be used for the contact pad  57 , the stud  60  and the locking member  54 . 
     As shown in  FIGS. 4 to 6 , in an embodiment the cross-sectional area of the hole  52  is greater than the cross-sectional area of the stud  60  in plan view. The hole  52  is oversized relative to the stud  60 . In an embodiment the resilient member  53  is provided against an end stop (not shown in the Figures). The resilient member  53  protrudes into the hole  52  when viewed in plan view (as shown in  FIG. 4 ). Accordingly, the resilient member  53  effectively reduces the cross-sectional area of the hole  52  in plan view. The remaining cross-sectional dimensions of the hole  52  are larger than the dimensions of the stud  60 . Accordingly, the stud  60  can be received into the hole  52  when the frame assembly  50  is moved vertically over the stud  60 . The frame assembly  50  is pushed sideways against the resilient member  53  so that the resilient member  53  is deflected inwards. The locking member  54  is placed preventing the frame assembly  50  from bending back. In an embodiment the locking member  54  is a pin. The locking member  54  can be inserted from the side or from the top. After the locking member  54  has been inserted, the frame assembly  50  is locked to the patterning device MA. 
     In an embodiment the frame assembly  50  comprises four holes  52  evenly distributed around the frame assembly  50 . In an embodiment the frame assembly  50  has a similar shape to the border  81 , following the perimeter of the membrane  40 .  FIG. 3  depicts the resilient member  53  radially inward of the hole  52 . However, this is not necessarily the case. The resilient member  53  may be radially outward of the hole  52  or neither radially inward nor outward relative to the hole  52 . The hole  52  is positioned between the resilient member  53  and the locking member  54 . 
     In an embodiment a resilient member  53  is radially inward of a hole  52  at one side of the membrane assembly  80 , whereas another resilient member  53  is radially outward of another hole  52  at the opposite side of the membrane assembly  80 . This allows the studs  60  at opposite sides of the patterning device MA to compress both resilient members  52  with one movement of the membrane assembly  80  relative to the patterning device MA. In an embodiment the membrane assembly  80  is configured such that all of the studs  60  received in corresponding holes  52  compress corresponding resilient members  52  with one movement of the membrane assembly  80  relative to the patterning device MA. 
     As shown in  FIGS. 4 to 6 , in an embodiment the locking member  54  is provided as a loose part. In an alternative embodiment the locking member may be formed to be integral with the rest of the frame assembly  50 , provided that the locking member  54  can be slid into the locking position. 
     In an embodiment the stud  60  has a diameter (in plan view) of at least 1 mm, optionally at least 2 mm, and optionally at least 3 mm. In an embodiment the stud  60  has a diameter of at most 10 mm, optionally at most 5 mm, and optionally at most 3 mm. 
     As explained above, in an embodiment the resilient member  53  extends into the hole  52  when it is not compressed. In an embodiment the resilient member  53  extends into the hole  52  by a distance of at least 0.1 mm, optionally at least 0.2 mm, and optionally at least 0.5 mm. In an embodiment the resilient member  53  extends into the hole  52  by a distance of at most 2 mm, optionally at most 1 mm, and optionally at most 0.5 mm. 
     As mentioned above, the hole  52  has a diameter that is larger than the diameter of the stud  60 . In an embodiment the diameter of the hole is greater than the diameter of the stud  60  by at least 0.2 mm, optionally at least 0.5 mm, and optionally at least 1 mm. In an embodiment the diameter of the hole  52  is greater than the diameter of the stud  60  by at most 5 mm, optionally at most 2 mm, and optionally at most 1 mm. In an embodiment the locking member  54  has a length of at least 1 mm, optionally at least 2 mm, and optionally at least 4 mm. 
     In an embodiment the locking member  54  has a length of at most 10 mm, optionally at most 5 mm, and optionally at most 4 mm. In an embodiment the locking member  54  has a width of at least 0.2 mm, optionally at least 0.5 mm, and optionally at least 1 mm. In an embodiment the locking member  54  has a width of at most 5 mm, optionally at most 2 mm, and optionally at most 1 mm. 
     An embodiment of the invention is expected to achieve a reduction in tooling steps required for attaching/detaching the member assembly  80  to/from the patterning device MA. 
       FIG. 11  schematically depicts, in cross-section, a membrane assembly  80  according to an alternative embodiment of the invention. The membrane assembly  80  is for EUV lithography. 
     The membrane assembly  80  comprises a planar membrane  40 . Only part of the membrane  40  is shown in  FIG. 11 . The membrane assembly  80  comprises a frame assembly  50 .  FIG. 11  shows a cross-sectional view of the frame assembly  50  on one side of the membrane  40 . The frame assembly  50  is configured to hold the membrane  40 . The frame assembly  50  is configured to attach to a patterning device MA for EUV lithography. As depicted in  FIG. 11 , in an embodiment the frame assembly  50  is configured to hold the membrane  40  via a border  81 . The construction of the membrane  40  and the border  81  can be the same as in any of the other embodiments described in this document. 
       FIGS. 11 to 14  schematically depict various stages of a process of attaching the frame assembly  50  to the patterning device MA.  FIG. 13  schematically depicts the frame assembly  50  attached to the patterning device MA. As depicted in  FIG. 13 , in an embodiment the frame assembly  50  has a locked state. In the locked state, the frame assembly  50  is locked to the patterning device MA such that the membrane  40  is held a predetermined distance D 1  from the patterning device MA. The predetermined distance is shown in  FIG. 13 . The predetermined distance D 1  is measured in the direction perpendicular to the plane of the membrane  40  and the plane of the patterning device MA. The frame assembly  50  is biased into the locked state by the resilient member  53 . 
     As depicted in  FIG. 14 , in an embodiment the frame assembly  50  has an unlocked state. In the unlocked state the membrane assembly  50  is not locked to the patterning device MA. In the unlocked state, the membrane  40  is less than the predetermined distance D 1  from the patterning device MA. For example, as shown in  FIG. 14 , in the unlocked state the membrane  40  is an unlocked state distance D 2  from the patterning device MA. The unlocked state distance D 2  is less than the predetermined distance D 1 . As shown from a comparison between  FIG. 13  and  FIG. 14 , in the unlocked state the resilient member  53  is compressed, with a membrane holder  58  of the frame assembly  50  forced closer to the patterning device MA. 
     The membrane  40  is used for mitigating any defect in the front side of the patterning device MA. The membrane  40  reduces the possibility of contaminant particles reaching the patterning device MA. In an embodiment the membrane assembly  80  is loaded into the lithographic apparatus  100  in a loading apparatus  70  (shown in  FIGS. 15 to 17 ). As shown in  FIGS. 15 to 17 , in an embodiment the loading apparatus  70  comprises a cutout portion for accommodating the membrane  40 . It is desirable for the loading apparatus  70  to be as thin as possible. The thickness of the loading apparatus  70  is shown in the up and down directions in  FIGS. 15 to 17 . 
     In practice, the membrane  40  can sag under its own weight. It is desirable for the membrane  40  to avoid touching the inside of the loading apparatus  70 , so as to avoid any damage to the membrane  40 . It is desirable for the membrane  40  to be thin so as to be able to transmit a higher proportion of EUV radiation. However, thinner membranes  40  tend to sag more. Hence, there is a trade-off between the thinness of the membrane  40  and the thinness of the loading apparatus  70 . 
     It is desirable for there to be a large standoff between the membrane  40  and the patterning device MA in use of the lithographic apparatus  100 . The standoff is the distance between the patterning device MA and the membrane  40  when the lithographic apparatus  100  is in use. Accordingly, it is not desirable to increase the space for the membrane  40  to sag simply by reducing the standoff between the membrane  40  and the patterning device MA. 
     Instead, the embodiment shown in  FIGS. 11 to 14  allows the membrane  40  to be temporarily pressed towards the patterning the device MA while the membrane  40  is being transported in the loading apparatus  70 . For example,  FIGS. 15 and 16  show the membrane  40  in the temporary state of being pressed upwards towards the patterning device MA. This allows the membrane  40  to sag more without touching the inside of the loading apparatus  70 . Meanwhile,  FIG. 17  shows the membrane assembly  80  out of the loading apparatus  70 . The membrane  40  is no longer pressed upwards towards the patterning device MA such that the membrane  40  has the predetermined distance D 1  from the patterning device MA. Such arrangement enables the variation of the distance between the membrane  40  and the patterning device MA on a direction perpendicular to the patterning device. Increasing the distance between the membrane  40  adds clearance for the membrane, which allows venting times and pump down times to be shorter. 
     Accordingly, an embodiment of the invention is expected to allow the membrane  40  to be made according to a greater level of design freedom (because a greater level of sagging is allowed), without increasing the thickness of the loading apparatus  70 . 
     As depicted in  FIG. 11 , in an embodiment the frame assembly  50  comprises a resilient member  53 . In an embodiment the resilient member  53  comprises a preload spring. The resilient member  53  may be a torsion spring or a leaf spring, for example. As depicted in  FIG. 11 , in an embodiment the frame assembly  50  comprises a membrane holder  58 . The membrane holder  58  is configured to hold the membrane  40 . The membrane holder  58  makes up most of the frame assembly  50 . The membrane holder  58  has a fixed position relative to the membrane  40 . When the membrane holder  58  moves, the membrane  40  also moves together with the membrane holder  58 . 
     As depicted in  FIG. 11 , in an embodiment the membrane holder  58  comprises an end stop surface  33 . The end stop surface  33  is configured to contact a locking surface  34  of the stud  52  protruding from the patterning device MA. When the frame assembly  50  is in the locked state, the end stop surface  33  of the membrane holder  58  abuts the locking surface  34  of the stud  52 . 
     In an embodiment the end stop surface  33  is on top of the locking surface  34  of the stud  52 . This is different from previously known mechanisms in which a fixed end stop is positioned below the stud. According to the present invention, the membrane holder  58  can be pressed upward, against the resilient member  53 . 
     As depicted in  FIG. 11 , in an embodiment the frame assembly  50  comprises a clamping member  59 . The clamping member  59  is movable relative to the membrane holder  58 . In an embodiment the frame assembly  50  comprises a resilient member  53 . The resilient member  53  connects the membrane holder  58  to the clamping member  59 . The membrane holder  58  can move relative to the clamping member  59  by compression of the resilient member  53 . The membrane holder  58  is movable relative to the clamping member  59  in a direction perpendicular to the plane of the membrane  40 . 
       FIG. 11  depicts a point in time in which the frame assembly  50  is in an initial state. In the initial state, the resilient member  53  is in a substantially uncompressed state. The frame assembly  50  cannot fit over the stud  52  because there is insufficient space between the end stop surface  33  of the membrane holder  58  and the abutment surface  31  of the clamping member  59 . The abutment surface  31  is configured to contact an engagement surface  32  of the stud  52 . The engagement surface  32  of the stud  52  faces away from the patterning device MA. 
       FIG. 12  schematically depicts the frame assembly  50  in a preparatory state. In the preparatory state, the resilient member  53  is compressed such that the distance between the end stop surface  33  of the membrane holder  58  and the abutment surface  31  of the clamping member  59  is increased. A special tool may be used to perform this operation. In particular, the tool may be used to hold the clamping member  59 . With the clamping member  59  held in position, the tool is used to apply a force pressing the membrane holder  58  in the direction of the patterning device MA. Accordingly, the resilient member  53  is compressed and the membrane holder  58  is forced towards the patterning device MA. The frame assembly  50  and the patterning device MA are then maneuvered with respect to each other such that the stud  52  (or part of the stud  52 ) enters between the end stop surface  33  of the membrane holder  58  and the abutment surface  31  of the clamping member  59 . 
       FIG. 13  depicts the frame assembly  50  in the locked state. In the transition from the preparatory state shown in  FIG. 12  to the locked state shown in  FIG. 13 , the force on the membrane holder  58  is released. As a result, the resilient member  53  expands. The abutment surface  31  of the clamping member  59  comes into contact with the engagement surface  32  of the stud  52 . The end stop surface  33  of the membrane holder  58  comes into contact with the locking surface  34  of the stud  52 . The locking surface  34  of the stud  52  faces towards the patterning device MA. 
       FIG. 14  schematically depicts the frame assembly  50  in the unlocked state. In the unlocked state, the abutment surface  31  of the clamping member  59  remains in contact with the engagement surface  32  of the stud  52 . The membrane holder  58  is forced towards the patterning device MA such that the end stop surface  33  of the membrane holder  58  moves away from the locking surface  34  of the stud  52 . Accordingly, the frame assembly  52  is no longer locked to the patterning device MA. The membrane  40  is closer to the patterning device MA compared to in the locked state shown in  FIG. 13 . When the resilient member  53  is compressed, the membrane holder  58  moves towards the patterning device MA. 
       FIG. 15  schematically depicts the membrane assembly  80  attached to the patterning device MA and stored in the loading apparatus  70 . The loading apparatus  70  may alternatively be called a load lock or an inner pod. As shown in  FIG. 15 , in an embodiment the loading apparatus  70  comprises at least one protrusion  71 . The protrusions  71  are for pressing up the membrane holder  58  of the frame assembly  50 . The protrusions  71  protrude from the inside surface (or the base plate) of the loading apparatus  70 . 
     As can be seen from  FIG. 15 , by pressing the resilient members  53  by the protrusions  71 , the membrane holder  58  can be pressed towards the patterning device MA. This adds clearance for the membrane  40  towards the inner surface of the loading apparatus  70 . 
     In an embodiment the distance between the membrane  40  and the inner surface of the loading apparatus  70  is increased from about 0.5 mm to about 1.5 mm by compressing the resilient members  53  by the protrusions  71 . In an embodiment the clearance of the membrane  40  towards the patterning device MA is temporarily reduced from about 2.5 mm to about 1.5 mm. The clearance of the membrane  40  towards the patterning device MA is subsequently increased back up to about 2.5 mm when the membrane assembly  80  is released from the loading apparatus  70 . 
     An embodiment of the invention is expected to reduce the risk of electrostatic discharge between the loading apparatus  70  and the membrane  40 . This is because of the increased clearance between the membrane  40  and the inner surface of the loading apparatus  70 . 
     As depicted in  FIG. 16 , in an embodiment the membrane assembly  80  comprises a sealing frame  72 . The sealing frame  72  is provided to control the opening of the volume between the membrane  40  and the patterning device MA. In an embodiment the sealing frame  72  is positioned around the membrane  40 . In an embodiment the sealing frame  72  comprises a plurality of seal openings  73 . When the membrane holder  58  is pressed upwards towards the patterning device MA, frame openings  74  in the frame assembly  50  will line up with the seal openings  73  to allow high speed pressure equalization, as shown in  FIG. 16 . The frame openings  74  line up with the seal openings  73  so that gas (e.g. air) can pass through. 
       FIG. 17  schematically depicts the membrane assembly  80  outside of the loading apparatus  70 . The seal openings  73  do not line up with the frame openings  74  such that the path for gas and particles is blocked. Accordingly, the sealing frame  72  seals the space between the membrane  40  and the patterning device MA. 
       FIG. 18  schematically depicts a gap G between the membrane assembly  80  and the patterning device MA. In positions where the studs  52  are not provided, contaminant particles can potentially pass through the gap G and enter into the space or region between the patterning device MA and the membrane  40 . The gap G may have a thickness of about 300 μm, for example. 
     The membrane assembly  80  is vulnerable to allowing particles that are smaller than the size of the gap G to reach the space between the membrane  40  and the patterning device MA. For example, contaminant particles that originate from outside of the space and have a size of less than about 200 μm can pass through the gap G either in a straight line or with some bouncing between the patterning device MA and the frame assembly  50 . 
       FIG. 19  schematically depicts, in cross-section, a variation of a membrane assembly  80 . This variation shown in  FIG. 19  can be applied to any of the embodiments of the membrane assembly  80  described in this document. 
     The variation of the membrane assembly  80  shown in  FIG. 19  is for reducing the possibility of particles entering the region between the membrane  40  and the patterning device MA from outside that region. Additionally, variation of the membrane assembly  80  shown in  FIG. 19  is for reducing the possibility of particles that originate in the gap G exiting the region between the membrane  40  and the patterning device MA. As depicted in  FIG. 19 , in an embodiment the membrane assembly  80  comprises a frame assembly  50  configured to hold the membrane  40  and to attach to the patterning device MA. A gap G is formed between opposing surfaces of the frame assembly  50  and the patterning device MA. 
     In an embodiment the frame assembly  50  comprises an elongate baffle  75 . The elongate baffle  75  is configured to restrain contaminant particles from entering the gap G. As shown in  FIG. 19 , the elongate baffle  75  extends beyond the opposing surface of the patterning device MA. In an embodiment the elongate baffle  75  extends beyond the opposing surface of the patterning device MA at a location beyond the planar extent of the patterning device MA. 
     The membrane assembly  80  reduces the possibility of particles reaching the space between the patterning device MA and the membrane  40 . As depicted in  FIG. 19 , in an embodiment the frame assembly  50  comprises at least one further baffle  76 . Each further baffle  76  is configured to restrain contaminant particles from entering the gap G. Each further baffle  76  extends towards the patterning device MA. 
     In an embodiment the frame assembly  50  comprises three or four grooves (i.e. spaces between the elongate baffle  75  and each of the further baffles  76 ). In an embodiment the grooves do not have equal widths. In other words the elongate baffle  75  and the further baffles  76  may not be equally spaced. Particles that are released within the region between the membrane  40  and the patterning device MA (e.g. particles released from the side of the membrane  40  facing the patterning device MA) and travel towards the frame assembly  50  are less likely to be reflected back to the patterning device MA. The particles can become trapped within the grooves. 
     In an embodiment the elongate baffle  75  has a height of greater than or equal to about 1 mm. In an embodiment the distance between the elongate baffle  75  (and further baffles  76 ) and the patterning device MA is selected so as to encourage pressure equalization during operation of the lithographic apparatus  100 . In an embodiment the distance between the elongate baffle  75  and the nearest further baffle  76  is about 300 μm. In an embodiment the distance between the further baffle  76  closest to the elongate baffle  75  and the next further baffle  76  is about 1.2 to 1.5 times greater than the distance between the elongate baffle  75  and the nearest further baffle  76 . In an embodiment the distance between the further baffle  76  in the middle of the three shown in  FIG. 19  and the further baffle  76  that is furthest from the elongate baffle  75  is about 1.2 to 1.5 times the distance between the two further baffles  76  in the center and to the right of the three further baffles  76  shown in  FIG. 19 . In an embodiment each further baffle  76  has a height of about 600 μm. In an embodiment the width of the grooves between baffles increases by about 20 to 50 percent in the direction from the elongate baffle  75 . 
     In an embodiment the elongate baffle  75  and the further baffles  76  are positioned between the center of the patterning device MA and the studs  52 . Accordingly, the baffle structures can constrain particles of, for example, titanium alloy, released by the studs  52 . Hence, in an embodiment the elongate baffle  75  extends beyond the opposing surface of the patterning device MA at a location that is not beyond the planar extent of the patterning device MA. 
     In an embodiment the elongate baffle  75  and/or the further baffles  76  are made of a material that has a high Hamaker constant. In an embodiment the frame assembly  50  is made of a material that has a high Hamaker constant. An embodiment of the invention is expected to reduce the possibility of particles with a large range of sizes, materials, travel speeds and angles of incidence from reaching the patterning device MA. 
       FIG. 7  schematically depicts, in cross-section, a membrane  40  according to an embodiment of the invention. As shown in  FIG. 7 , the membrane  40  comprises a stack. The stack comprises a plurality of layers. 
     In an embodiment the stack comprises at least on silicon layer  41 . The silicon layer  41  comprises a form of silicon. In an embodiment the stack comprises at least one silicon compound layer  43 . The silicon compound layer  43  is made of a compound of silicon and another element selected from the group consisting of boron, phosphorus, bromine and sulphur. However, other elements may also be used. In particular, in an embodiment the element that combines with the silicon to form the silicon compound layer  43  is any element that can be used as a dopant material for doping the silicon layer  41 . The embodiment will be described with boron as the element that combines with the silicon, merely for convenience. The embodiment is not limited to the element being boron. 
     In an embodiment the silicon compound layer  43  comprises a silicon boride. Silicon boride has the chemical formula SiB x , where x can be 3, 4, 6, 14, 15, 40 etc. Silicon boride has metallic properties. In particular, the silicon compound layer  43  has the property of metal that it increases the emissivity for EUV radiation of the membrane  40 . A membrane made of only the silicon layer  41  would have a low emissivity, perhaps of the order of 3%. The emissivity dramatically increases if a metal or a compound that has metallic properties is added to the membrane  40 . 
     Metals are known to limit the practical thickness of the membrane due to EUV absorption. By providing the silicon membrane layer  43 , an embodiment of the invention is expected to achieve an increase in possible thickness of a membrane  40  that has sufficient emissivity for use in the lithographic apparatus  100 . 
     As depicted in  FIG. 7 , in an embodiment the silicon compound layer  43  is formed as an interlayer between the silicon layer  41  and a non-metallic layer  42  comprising the element that combines with silicon to form the silicon compound layer  43 . For example, in an embodiment the non-metallic layer  42  comprises boron. In an embodiment the boron is provided in the form of boron carbide. However alternative forms of boron can be used. 
     In an embodiment the silicon layer  41  is initially provided adjacent to the non-metallic layer  42 . The boron in the non-metallic layer  42  locally dopes the silicon in the silicon layer  41 . The boron dopes the silicon to the extent that silicon boride is produced to form the silicon compound layer  43 . The boron dopes the silicon such that there are more boron atoms than silicon atoms in the doped silicon, i.e. forming silicon boride. 
     In an embodiment, silicon layers  41  and non-metallic layers  42  are provided as multilayers. Locally, boron silicide can strengthen the membrane  40  (by a laminate effect and by radiation hardening of boron in silicon) so that the membrane  40  can withstand higher temperatures. 
     As depicted in  FIG. 7 , in an embodiment the stack comprises a plurality of silicon layers  41 , a plurality of non-metallic layers  42  and a silicon compound layer  43  between each pair of silicon layers  41  and a non-metallic layer  42 . 
     As depicted in  FIG. 7 , in an embodiment the stack comprises layers in the following order: a non-metallic layer  42 , a silicon compound layer  43 , a silicon layer  41 , a silicon compound layer  43 , a non-metallic layer  42 , a silicon compound layer  43 , a silicon layer  41 , a silicon compound layer  43 , a non-metallic layer  42 , a silicon compound layer  43 , a silicon layer  41 , a silicon compound layer  43  and a non-metallic layer  42 . This is a multilayer stack. In an embodiment the stack may comprise a non-metallic layer  42  and then repeated cycles of a set of four layers comprising a silicon compound layer  43 , a silicon layer  41 , a silicon compound layer  43  and a non-metallic layer  42 . 
     In an embodiment each non-metallic layer  42  has a thickness of at least 0.5 nm, optionally at least 1 nm and optionally at least 2 nm. In an embodiment each non-metallic layer  42  has a thickness of at most 10 nm, optionally at most 5 nm, and optionally at most 2 nm. 
     In an embodiment each silicon compound layer  43  has a thickness of at least 0.5 nm, optionally at least 1 nm, and optionally at least 2 nm. In an embodiment each silicon compound layer  43  has a thickness at most 10 nm, optionally at most 5 nm, and optionally at most 2 nm. 
     In an embodiment each silicon layer  41  has a thickness of at least 2 nm, optionally at least 5 nm, and optionally at least 8 nm. In an embodiment each silicon layer  41  has a thickness of at most 20 nm, optionally at most 10 nm, optionally at most 8 nm. 
     The embodiment depicted in  FIG. 7  with silicon layers  41  of 8 nm thickness, non-metallic layers  42  of 2 nm thickness and silicon compound layers  43  of 2 nm thickness is expected to be achieve an emissivity for EUV radiation of about 90%. 
       FIG. 8  depicts an alternative embodiment in which the stack comprises layers in the following order: a non-metallic layer  42 , a silicon compound layer  43 , a silicon layer  41 , a silicon compound layer  43  and a non-metallic layer  42 . 
     As depicted in  FIG. 8  in an embodiment the membrane  40  comprises only one silicon layer  41 . In such an embodiment the silicon layer  41  can have a thickness of at least 10 nm, optionally at least 20 nm, and optionally at least 38 nm. In an embodiment the single silicon layer  41  has a thickness of at most 100 nm, optionally at most 50 nm, and optionally at most 38 nm. The embodiment shown in  FIG. 8  and having a silicon layer  41  of 38 nm thickness, non-metallic layer  42  of 4 nm thickness and silicon compound layer  43  of 2 nm thickness is expected to achieve an emissivity for EUV radiation of about 90%. 
     In an embodiment a total combined thickness of silicon compound layers  43  in the stack is at most about 20 nm. Metal and compounds having metallic properties improve the emissivity of the membrane  40  provided that the combined thickness is not too thick. For layers of metal or compounds having metallic properties that are too thick, the emissivity can be reduced. 
       FIG. 9  schematically depicts an alternative embodiment of a membrane  40 . As depicted in  FIG. 9 , in an embodiment the stack comprises at least one silicon layer  41 , at least one capping layer  46  and at least one anti-migration layer  47 . In an embodiment the capping layer  46  comprises ruthenium. The capping layer  46  is provided at an external surface of the membrane  40 . The anti-migration layer  47  comprises at least one of molybdenum and titanium. The anti-migration layer  47  is adjacent to each capping layer  46 . 
     The capping layer  46  comprising ruthenium improves the emissivity of the membrane  40 . The capping layer  46  reduces the possibility of the membrane  40  oxidizing. The capping layer  46  is configured to protect the membrane  40  from hydrogen gas. 
     During use of the lithographic apparatus  100 , the membrane  40  can heat up due to absorbing radiation. When the capping layer  46  heats up, the material (e.g. ruthenium) of the capping layer  46  can migrate. The migration is the transport of the material caused by the gradual movement of the ions in the capping layer  46 . When the material starts to migrate, the material can form islands in the capping layer  46 . When the material starts to migrate, the effectiveness of the capping layer  46  in reducing oxidation, protecting from hydrogen gas and improving emissivity is reduced. Hence, during use of the lithographic apparatus  100 , the membrane  40  can start to oxidize and the emissivity can decrease. 
     By providing the anti-migration layer  47 , migration of the capping layer  46  is reduced. Molybdenum and titanium are metals that have relatively high melting temperatures and good emissivity for UV radiation. Titanium and molybdenum do not migrate as much has ruthenium when they are heated. Titanium and molybdenum have good metal to metal contact with ruthenium. By providing the anti-migration layer  47  adjacent to the capping layer  46 , migration of the capping layer  46  is reduced. As a result, even when the capping layer  46  is heated during use of the lithographic apparatus  100 , the good properties of the capping layer  46  are retained at higher temperatures. 
     As depicted in  FIG. 9 , in an embodiment the stack comprises layers in the following order: a capping layer  46  comprising ruthenium at an external surface of the membrane  40 , an anti-migration layer  47  comprising at least one of molybdenum and titanium, a silicon layer  41 , an anti-migration layer  47  comprising at least one of molybdenum and titanium, and a capping layer  46  comprising ruthenium at the other external surface of the membrane  40 . In an embodiment a capping layer  46  comprising ruthenium is provided at both external surfaces of the membrane  40 . 
       FIG. 10  depicts an alternative embodiment of a membrane in which the use of the anti-migration layer  47  is combined with the idea of using the silicon compound layer  43 . 
     As depicted in  FIG. 10  in an embodiment the stack comprises layers in the following order: a capping layer  46  comprising ruthenium at an external surface of the membrane  40 , an anti-migration layer  47  comprising at least one of molybdenum and titanium, a silicon layer  41 , a silicon compound layer  43  and a non-metallic layer  42 . 
     During manufacture of the membrane assembly  80 , the boron carbide layer can protect the silicon layer  41  chemically from etching processes. In an embodiment the membrane  40  comprises a periodic structure. In an embodiment the period is not set to be equal to 6.6 nm or 6.7 nm. If the period is at or close to 6.7 nm the membrane may act as a mirror for the EUV radiation. 
     Silicon can crystallise in a diamond cubic crystal structure. In an embodiment the border  81  comprises a cubic crystal of silicon. In an embodiment the border  81  has a &lt;100&gt; crystallographic direction. 
     In an embodiment the silicon layer  41  is formed from polycrystalline or nanocrystalline silicon. Polycrystalline or nanocrystalline silicon has a brittle nature. Hence, a membrane  40  that comprises a silicon layer  41  formed from polycrystalline or nanocrystalline silicon can shatter into many particles when the membrane assembly  80  breaks. An embodiment of the invention is expected to achieve an improvement in the mechanical properties of the membrane assembly  80 . 
     Polycrystalline silicon and nanocrystalline silicon each have high transmission for EUV radiation. Polycrystalline silicon and nanocrystalline silicon each have good mechanical strength. 
     However, it is not essential for the membrane of the silicon layer  41  to be formed from polycrystalline or nanocrystalline silicon. For example, in an alternative embodiment the silicon layer  41  is formed from a multi-lattice membrane or a silicon nitride. 
     In a further alternative embodiment the silicon layer  41  is formed from monocrystalline silicon. In such an embodiment the monocrystalline silicon membrane can be formed by a silicon on insulator (SOI) technique. The starting material for this product is a so-called SOI substrate. An SOI substrate is a substrate comprising a silicon carrier substrate with a thin, monocrystalline silicon layer on top of a buried isolating SiO 2  layer. In an embodiment the thickness of the monocrystalline silicon layer can range between about 5 nm to about 5 μm. In an embodiment the silicon layer  41  is present on the SOI substrate before the SOI substrate is used in the method of manufacture. 
     In an embodiment the silicon layer  41  comprises silicon in one of its allotrope forms such as amorphous, monocrystalline, polycrystalline or nanocrystalline silicon. A nanocrystalline silicon means a polycrystalline silicon matrix containing a certain amorphous silicon content. In an embodiment polycrystalline or nanocrystalline silicon is formed by crystallising amorphous silicon in the silicon layer  41 . For example, in an embodiment a silicon layer  41  is added to the stack as an amorphous silicon layer. The amorphous silicon layer crystallises into a polycrystalline or nanocrystalline silicon layer when a certain temperature is exceeded. For example, the silicon layer  41  as an amorphous silicon layer transforms into the silicon layer  41  as a polycrystalline or nanocrystalline silicon layer. 
     In an embodiment the amorphous silicon layer is in-situ doped during its growth. In an embodiment the amorphous silicon layer is doped after its growth. By adding a p- or n-type dopant the silicon conductivity increases, which has a positive effect on the thermomechanical behavior due to the power of the EUV source. 
     In an embodiment the membrane  40  is thin enough that its transmission for EUV radiation is sufficiently high, for example greater than 50%. In an embodiment the thickness of the membrane  40  is at most about 200 nm, and optionally at most about 150 nm. A 150 nm Si membrane would transmit about 77% of incident EUV radiation. In an embodiment the thickness of the membrane  40  is at most about 100 nm. A 100 nm Si membrane would transmit about 84% of incident EUV radiation. A 60 nm Si membrane would transmit about 90% of incident EUV radiation. 
     In an embodiment the membrane  40  is thick enough that it is mechanically stable when the membrane assembly  80  is fixed to the patterning device MA of the lithographic apparatus  100  and during use of the lithographic apparatus  100 . In an embodiment the thickness of the membrane  40  is at least about 10 nm, optionally at least about 20 nm, and optionally at least about 35 nm. In an embodiment the thickness of the membrane  40  is about 55 nm. 
     In an embodiment the membrane assembly  80  is applied as a pellicle or as part of a dynamic gas lock. Alternatively, the membrane assembly  80  can be applied in other filtration areas such as identification, or for beam splitters. 
     Embodiments of the invention are described in the clauses set out below: 
     1. A membrane assembly for EUV lithography, the membrane assembly comprising:
         a planar membrane;   a border configured to hold the membrane; and   a frame assembly connected to the border and configured to releasably attach to a patterning device for EUV lithography, wherein the frame assembly comprises a resilient member;   wherein the frame assembly is connected to the border in a direction perpendicular to the plane of the membrane such that in use the frame assembly is between the border and the patterning device.       

     2. The membrane assembly of clause 1, wherein a mounting feature is provided between the border and the patterning device. 
     3. The membrane assembly of any of clauses 1 and 2, wherein the frame assembly comprises at least one hole configured to receive a protrusion protruding from the patterning device, wherein the hole at least partially overlaps the border when viewed in the direction perpendicular to the plane of the membrane. 
     4. The membrane assembly of clause 3, wherein the frame assembly comprises a locking mechanism configured to lock the frame assembly to the protrusion. 
     5. The membrane assembly of clause 4, wherein the locking mechanism comprises a resilient member for each hole, wherein each resilient member extends into the hole and is configured to be deformable when the protrusion received in the hole presses against the resilient member. 
     6. The membrane assembly of clause 5, wherein the locking mechanism comprises a locking member for each hole, wherein the locking member is configured to be moveable to a locking position where the locking member extends into the hole such that the compressed resilient member exerts a force on the protrusion received in the hole towards the locking member. 
     7. A patterning device assembly for EUV lithography, the patterning device assembly comprising:
         a planar patterning device;   at least one protrusion protruding from the patterning device; and   the membrane assembly of any preceding clause, the frame assembly being connected to the patterning device via the at least one protrusion;   wherein the at least one protrusion is between the border and the patterning device.       

     8. A patterning device assembly for EUV lithography, the patterning device assembly comprising:
         a planar patterning device;   a membrane assembly comprising a planar membrane and a border configured to hold the membrane;   at least one protrusion protruding from one of the patterning device and the border, wherein the at least one protrusion is between the border and the patterning device; and   a frame assembly connected to the other of the patterning device and the border, wherein the frame assembly is configured to attach to the at least one protrusion between the border and the patterning device.       

     9. A membrane assembly for EUV lithography, the membrane assembly comprising:
         a planar membrane; and   a frame assembly configured to hold the membrane and to attach to a patterning device for EUV lithography;   wherein the frame assembly has a locked state in which the frame assembly is locked to the patterning device such that the membrane is held a predetermined distance from the patterning device, and an unlocked state in which the membrane is less than the predetermined distance from the patterning device.       

     10. The membrane assembly of clause 9, wherein the frame assembly comprises:
         a membrane holder configured to hold the membrane;   a clamping member; and   a resilient member connecting the membrane holder to the clamping member;   wherein the membrane holder is movable relative to the clamping member in a direction perpendicular to the plane of the membrane via compression of the resilient member.       

     11. The membrane assembly of clause 10, wherein the clamping member comprises an abutment surface configured to contact an engagement surface of a protrusion protruding from the patterning device, wherein the engagement surface faces away from the patterning device. 
     12. The membrane assembly of clause 10 or 11, wherein the membrane holder comprises an end stop surface configured to contact a locking surface of a protrusion protruding from the patterning device when the frame assembly is in the locked state, wherein the locking surface faces towards the patterning device. 
     13. The membrane assembly of any of clauses 10 to 12, wherein the resilient member is configured to bias the frame assembly to the locked state. 
     14. The membrane assembly of any of clauses 10 to 13, wherein the frame assembly is configured such that when the resilient member is compressed, the membrane holder moves towards the patterning device. 
     15. A patterning device assembly for EUV lithography, the patterning device assembly comprising:
         a planar patterning device for EUV lithography;   a membrane assembly comprising:
           a planar membrane; and   a frame assembly configured to hold the membrane and to attach to the patterning device,   
           wherein a gap is formed between opposing surfaces of the frame assembly and the patterning device;   wherein the frame assembly comprises an elongate baffle configured to restrain contaminant particles from entering the gap, wherein the elongate baffle extends beyond the opposing surface of the patterning device at a location beyond the planar extent of the patterning device.       

     16. A patterning device assembly for EUV lithography, the patterning device assembly comprising:
         a planar patterning device; and   the membrane assembly of any of clauses 1 to 6 and 9 to 14, wherein a gap is formed between opposing surfaces of the frame assembly and the patterning device to which the frame assembly is attached;   wherein the frame assembly comprises an elongate baffle configured to restrain contaminant particles from entering the gap, wherein the elongate baffle extends beyond the opposing surface of the patterning device at a location beyond the planar extent of the patterning device.       

     17. The patterning device assembly of clause 8 or 9, wherein a gap is formed between opposing surfaces of the frame assembly and the patterning device to which the frame assembly is attached;
         wherein the frame assembly comprises an elongate baffle configured to restrain contaminant particles from entering the gap, wherein the elongate baffle extends beyond the opposing surface of the patterning device at a location beyond the planar extent of the patterning device.       

     18. The patterning device assembly of any of clauses 15 to 17, wherein the frame assembly comprises:
         at least one further baffle configured to restrain contaminant particles from entering the gap, wherein each further baffle extends towards the patterning device.       

     19. A loading apparatus for temporarily housing a membrane assembly that is mounted onto a patterning device for EUV lithography, the loading apparatus comprising protrusions at an inner surface of the loading apparatus, wherein the protrusions are configured to press a membrane holder of the membrane assembly towards the patterning device when the loading apparatus houses the membrane assembly. 
     20. A patterning device assembly for EUV lithography, the patterning device assembly comprising:
         a planar patterning device;   the membrane assembly of any of clauses 9 to 14 mounted onto the patterning device; and   a loading apparatus for temporarily housing the membrane assembly, the loading apparatus comprising protrusions at an inner surface of the loading apparatus, wherein the protrusions are configured to press the membrane holder of the membrane assembly towards the patterning device when the loading apparatus houses the membrane assembly.       

     21. The patterning device assembly of clause 20, wherein the membrane assembly comprises a sealing frame positioned around the membrane and configured to control opening of a volume between the membrane and the patterning device. 
     22. The patterning device assembly of clause 21, wherein the sealing frame comprises a plurality of seal openings, wherein when the membrane holder is pressed towards the patterning device, frame openings in the frame assembly line up with the seal openings to allow a flow of gas through the seal openings and the frame openings. 
     23. The patterning device assembly of clause 22, wherein the seal openings are arranged such that the seal openings do not line up with the frame openings when the membrane assembly is not housed by the loading apparatus such that the membrane holder is not pressed towards the patterning device. 
     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, LCDs, thin-film magnetic heads, etc. 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 multiple processed layers. 
     While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the various photo resist layers may be replaced by non-photo resist layers that perform the same function. 
     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.