Patent Publication Number: US-10788755-B2

Title: Lithographic apparatus and device manufacturing method

Description:
PRIORITY AND BENEFIT INFORMATION 
     This application is a continuation application of U.S. patent application Ser. No. 15/653,435, entitled “Lithographic Apparatus And Device Manufacturing Method”, filed on Jul. 18, 2017, which is a continuation of U.S. patent application Ser. No. 15/178,522, entitled “Lithographic Apparatus And Device Manufacturing Method”, filed on Jun. 9, 2016, now U.S. Pat. No. 9,740,107, which is a continuation of U.S. patent application Ser. No. 14/701,236, entitled “Lithographic Apparatus And Device Manufacturing Method”, filed on Apr. 30, 2015, now U.S. Pat. No. 9,366,972, which is a divisional application of U.S. patent application Ser. No. 12/512,754, entitled “Lithographic Apparatus And Device Manufacturing Method”, filed on Jul. 30, 2009, now U.S. Pat. No. 9,057,967, which is a divisional application of U.S. patent application Ser. No. 11/710,408, entitled “Lithographic Apparatus And Device Manufacturing Method”, filed on Feb. 26, 2007, now U.S. Pat. No. 7,593,093, which is a divisional application of U.S. application Ser. No. 10/705,804, filed Nov. 12, 2003, now U.S. Pat. No. 7,199,858, which claims priority from European patent applications EP 02257822.3, filed Nov. 12, 2002, and EP 03253636.9, filed Jun. 9, 2003, each of which are incorporated herein in their entirety by reference. 
    
    
     FIELD 
     The present invention relates to immersion lithography. 
     BACKGROUND 
     The term “patterning device” as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such a patterning device include:
         A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.   A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.   A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.       

     For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth. 
     Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at one time; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally &lt;1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference. 
     In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference. 
     For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT patent application WO 98/40791, incorporated herein by reference. 
     It has been proposed to immerse the substrate in a lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final optical element of the projection lens and the substrate. The point of this is to enable imaging of smaller features because the exposure radiation will have a shorter wavelength in the liquid than in air or in a vacuum. (The effect of the liquid may also be regarded as increasing the effective NA of the system). 
     However, submersing the substrate or substrate and substrate table in a bath of liquid (see, for example, U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by reference) may mean that there is a large body of liquid that must be accelerated during a scanning exposure. This may require additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects. 
     One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection lens and the substrate (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in PCT patent application WO 99/49504, hereby incorporated in its entirety by reference. As illustrated in  FIGS. 22 and 23 , liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side.  FIG. 23  shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of  FIG. 22  the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated in  FIG. 23  in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element. 
     SUMMARY 
     Difficulties in large loss of liquid from the liquid supply system can arise with the system described above and any other systems that provide liquid on only a localized area of the substrate and between the projection system and the substrate when the localized area crosses over an edge of the substrate or other object. 
     Accordingly, it may be advantageous to provide, for example, a lithographic projection apparatus in which liquid loss from the supply system is minimized during passage over an edge portion of the substrate or other object. 
     According to an aspect, there is provided a lithographic projection apparatus comprising:
         a support structure configured to hold a patterning device, the patterning device configured to pattern a beam of radiation according to a desired pattern;   a projection system configured to project the patterned beam onto a target portion of a substrate;   a substrate table configured to hold the substrate, said substrate table comprising an edge seal member configured to at least partly surround an edge of at least one of said substrate and an object positioned on said substrate table and to provide a primary surface facing said projection system substantially co-planar with a primary surface of the said at least one of said substrate and said object; and   a liquid supply system configured to provide a liquid, through which said beam is to be projected, in a space between said projection system and said at least one of said substrate and said object, wherein said liquid supply system provides liquid to a localized area of at least one of said object, said edge seal member and said substrate.       

     Where applied to a substrate, the edge seal member surrounds a position on the substrate table where, in use, the substrate is to be placed, e.g., surrounding the chuck or pimple table on which the substrate is held. In this way the substrate can be positioned closely adjacent to the edge of the edge seal member such that as an edge of the substrate moves under the projection system there is no sudden loss of liquid from the space because there is no large gap for the liquid to flow through. The edge seal member may be an integral part of the substrate table or may be moveably mounted relative to the remainder of the substrate table. In the latter case, it can be arranged such that the gap between the edge seal member and the substrate can be varied and/or the height of the primary surface of the edge seal member can be varied to accommodate variations in substrate height or thickness, i.e., to ensure that the primary surface of the edge seal member is substantially coplanar with the primary surface of the substrate. The above may also be applied to an object on the substrate table such as a sensor, e.g., a projection beam sensor. 
     In an embodiment, the substrate table comprises a gap seal member configured to abut or at least partly overlap, in the direction of the optical axis, both the edge seal member and said at least one of said substrate and said object. For example, in this way the gap between the edge seal member and a substrate (or an object), which is due to the size mismatch between the inner edge of the edge seal member and the outer edge of the substrate or object (which is necessary to accommodate slight variations in the diameter of the substrate or object), can be covered by the gap seal member. This further reduces the amount of liquid loss into the gap between the edge seal member and the substrate or object. In an embodiment, the gap seal member is configured to be in contact with the primary surfaces, thereby spanning the gap between the edge seal member and said at least one of said substrate and said object. 
     If the gap seal member has inner and outer edges, at least one of the edges may be tapered such that the thickness of the gap seal member facing away from the edge seal member or said at least one of said substrate and said object decreases towards the edge of the gap seal member. This helps the liquid supply system move smoothly over the gap between the substrate or object and the edge seal member. 
     A way to hold the gap seal member removably in place is to provide the substrate table with a vacuum port in the primary surface of said edge seal member. 
     Another way to minimise the amount of liquid which escapes into the gap between the edge seal member and the substrate or object is to provide the substrate table with a hydrophobic layer facing edge portions of said edge seal member and the substrate or object on an opposite side of the edge seal member and the substrate or object to the projection system. Such a hydrophobic layer may be any material which exhibits hydrophobic properties, for example Teflon, silicon rubber or other plastics materials. Inorganic coatings are generally desired because they have better radiation resistance than organic coatings. In an embodiment, the liquid has a contact angle of greater than 90° with the hydrophobic layer. This reduces the chances of liquid seeping into the gap. 
     According to an aspect, there is provided a lithographic projection apparatus comprising:
         a support structure configured to hold a patterning device, the patterning device configured to pattern a beam of radiation according to a desired pattern;   a projection system configured to project the patterned beam onto a target portion of a substrate;   a substrate table configured to hold the substrate, said substrate table comprising:   an edge seal member configured to at least partly surround an edge of at least one of said substrate and an object positioned on said substrate table, and   a further edge seal member configured to extend across the gap between said edge seal member and said at least one of said substrate and said object and to be in contact with said at least one of said substrate and said object; and   a liquid supply system configured to provide a liquid, through which said beam is to be projected, in a space between said projection system and said at least one of said substrate and said object.       

     In this way the gap between the edge seal member and the substrate or object is closed off so that there is no gap between the edge seal member and the substrate or object through which liquid from the liquid supply system can pass. This is particularly so if the further edge seal member is flexible in which case a better seal between the further edge seal member and the substrate or object is achievable. 
     In an embodiment, the flexible further edge seal member is attached to the edge seal member and has a port, connected to a vacuum source, adjacent its end distal from said edge seal member, such that on actuation of said vacuum source, said flexible further edge seal member is deflectable upwards to contact against the substrate or object to form a seal between said flexible further edge seal member and the substrate or object due to the force generated by the vacuum source acting on the substrate or object. This allows the flexible further edge seal member to be actuatable to contact with the substrate or object and to be deactuatable so that it falls away from the substrate or object. The application of the vacuum ensures a good seal between the flexible further edge seal member and the substrate or object. 
     In another embodiment, the flexible further edge seal member is disposed between the edge seal member and the substrate or object and with a surface substantially co-planar with the primary surfaces of the edge seal member and the substrate or object. In this way the gap between the edge seal member and the substrate or object can be sealed such that only small amounts of liquid can pass into the gap. In an embodiment, the flexible further edge seal member is shaped to contact the substrate or object on the surface opposite its primary surface and may be effective to apply a force to the substrate or the object away from the substrate table when the substrate or object is held on the substrate table. In the case of the substrate, the flexible further edge seal member in this way may help in the removal of the substrate from the substrate table after exposure of the substrate. 
     According to an aspect, there is provided a lithographic projection apparatus comprising:
         a support structure configured to hold a patterning device, the patterning device configured to pattern a beam of radiation according to a desired pattern;   a projection system configured to project the patterned beam onto a target portion of a substrate;   a substrate table configured to hold the substrate, said substrate table comprising:   an edge seal member configured to at least partly surround an edge of at least one of said substrate and an object positioned on said substrate table, and   at least one of a vacuum port and a liquid supply port positioned to provide respectively a vacuum or liquid to the gap between said edge seal member and said at least one of said substrate and said object on a side opposite said projection system;   a liquid supply system configured to provide a liquid, through which said beam is to be projected, in a space between said projection system and said at least one of said substrate and said object.       

     In the case of a liquid supply port, no liquid can find its way into the gap between the edge seal member and the substrate or object from the space between the projection system and the substrate or object, because that gap is already filled with liquid. If the vacuum alternative is used, any liquid which does find its way into that gap will be removed and may be recycled. The provision of vacuum is advantageous when a gas seal member of a liquid supply system is used to keep the liquid in the space between the projection system and the substrate or object. This is because it can not only remove any liquid passing into the gap but also any gas from the gas seal member. 
     Further, there may be provided a channel positioned radially inwardly of the vacuum port, the channel being connected to a gas source such that on actuation of the vacuum a flow of gas radially outwardly from said channel toward said vacuum can be established. Such a flow of gas can be used to ensure that any liquid which does reach the non-immersed side of the substrate or object is caught in the gas flow and transported away towards the vacuum. 
     According to an aspect, there is provided a lithographic projection apparatus comprising:
         a support structure configured to hold a patterning device, the patterning device configured to pattern a beam of radiation according to a desired pattern;   a projection system configured to project the patterned beam onto a target portion of a substrate;   a substrate table configured to hold the substrate, said substrate table comprising a support surface configured to support an intermediary plate between said projection system and at least one of said substrate and an object positioned on said substrate table and not in contact with said at least one of said substrate and said object; and   a liquid supply system configured to provide a liquid, through which said beam is to be projected, in a space between said projection system and said at least one of said substrate and said object.       

     In this way an intermediary plate can be used which is of an overall size larger than the substrate or object so that, for example, during imaging of edge portions of the substrate or object, the liquid supply system is situated at a medial portion of the intermediary plate such that no or few problems with loss of liquid through gaps at edges exist. With such a system it is also possible to provide the substrate table with a transmission image sensor (TIS) configured to sense a beam and wherein the intermediary plate is positionable between the sensor and said projection system. Thus it is possible for the transmission image sensor to detect a beam under the same conditions that a substrate is to be imaged. It will therefore be possible to more accurately position the substrate table so that the projection beam is correctly focused on the substrate. 
     According to an aspect, there is provided a lithographic apparatus comprising:
         a support structure configured to hold a patterning device, the patterning device configured to pattern a beam of radiation according to a desired pattern;   a projection system configured to project the patterned beam onto a target portion of a substrate;   a substrate table configured to hold the substrate; and   a liquid supply system configured to provide a liquid, through which said beam is to be projected, in a space between said projection system and at least one of said substrate and an object positioned on said substrate table, wherein a structure of the liquid supply system extends along at least part of the boundary of said space between said projection system and said substrate table and capillaries extend away from said substrate table and are positioned between said structure and said projection system.       

     In this way, a larger gap may be spanned over the edge of the substrate or object before catastrophic liquid loss occurs as capillary action aids in the liquid spanning gaps. 
     In an embodiment, the inner coating of the capillary is hydrophobic and the apparatus comprises an electric device configured to apply a potential difference between said liquid in said space and said capillaries. In this way, an even larger gap may be spanned for liquid loss. 
     According to an aspect, there is provided a device manufacturing method comprising:
         providing a liquid in a space between a projection system and at least one of a substrate and an object positioned on a substrate table;   projecting a patterned beam of radiation, through said liquid, onto a target portion of the substrate using the projection system; and       

     either: providing an edge seal member surrounding at least part of an edge of said at least one of said substrate and said object and with a primary surface substantially co-planar to a primary surface of said at least one of said substrate and said object, wherein said liquid is provided to a localized area of at least one of said substrate, said object, and said edge seal member, or 
     providing an edge seal member at least partly surrounding an edge of said at least one of said substrate and said object and a further edge seal member extending across the gap between the edge seal member and said at least one of said substrate and said object and in contact with said least one of said substrate and said object, or 
     providing an edge seal member at least partly surrounding an edge of said least one of said substrate and said object and providing at least one of a vacuum or liquid to the gap between the edge seal member and said least one of said substrate and said object on a side of said least one of said substrate and said object opposite to said projection system, or 
     positioning an intermediary plate in the space between said least one of said substrate and said object and said projection system and not in contact with said least one of said substrate and said object, or 
     providing a structure extending along at least part of the boundary of the space between said projection system and said substrate table and providing capillaries extending away from the substrate table between the structure and said projection system. 
     Although specific reference may be made in this text to the use of the apparatus described herein in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle”, “wafer” or “die” in this text should be considered as being replaced by the more general terms “mask”, “substrate” and “target portion”, respectively. 
     In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm). 
    
    
     
       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: 
         FIG. 1  depicts a lithographic projection apparatus according to an embodiment of the invention; 
         FIG. 2  depicts the liquid reservoir of a first embodiment of the invention; 
         FIG. 3  is similar to  FIG. 2  showing an edge seal member on the substrate table according to an embodiment of the invention; 
         FIG. 4  illustrates a second embodiment of the invention; 
         FIG. 5  illustrates an alternative form of the second embodiment of the present invention; 
         FIG. 6  illustrates a detail of the second embodiment of the present invention; 
         FIGS. 7 a - d    illustrate four versions of a third embodiment of the present invention; 
         FIG. 8 a    illustrates a first version of a fourth embodiment of the present invention; 
         FIG. 8 b    illustrates a second version of the fourth embodiment; 
         FIG. 8 c    illustrates a third version of the fourth embodiment; 
         FIG. 9  illustrates in detail further aspects of the first version of the fourth embodiment of the present invention; 
         FIG. 10  illustrates a fifth embodiment of the present invention; 
         FIG. 11  illustrates a sixth embodiment of the present invention; 
         FIG. 12  illustrates in plan the substrate and edge seal member of a seventh embodiment of the present invention; 
         FIG. 13  illustrates a cross section through the seventh embodiment of the present invention; 
         FIG. 14  illustrates a detail of the seventh embodiment of the present invention; 
         FIG. 15  illustrates in detail a further arrangement of the seventh embodiment; 
         FIG. 16  illustrates an eighth embodiment of the present invention; 
         FIG. 17  illustrates a ninth embodiment of the present invention; 
         FIG. 18  illustrates a tenth embodiment of the present invention; 
         FIG. 19  illustrates an eleventh embodiment of the present invention; 
         FIG. 20  illustrates a twelfth embodiment of the present invention; 
         FIG. 21  illustrates a thirteenth embodiment of the present invention; 
         FIG. 22  illustrates an alternative liquid supply system according to an embodiment of the invention; and 
         FIG. 23  illustrates, in plan, the system of  FIG. 22 . 
     
    
    
     In the Figures, corresponding reference symbols indicate corresponding parts. 
     DETAILED DESCRIPTION 
     Embodiment 1 
       FIG. 1  schematically depicts a lithographic projection apparatus according to a particular embodiment of the invention. The apparatus comprises:
         a radiation system Ex, IL, for supplying a projection beam PB of radiation (e.g. DUV radiation), which in this particular case also comprises a radiation source LA;   a first object table (mask table) MT provided with a mask holder for holding a mask MA (e.g. a reticle), and connected to first positioning means for accurately positioning the mask with respect to item PL;   a second object table (substrate table) WT provided with a substrate holder for holding a substrate W (e.g. a resist-coated silicon wafer), and connected to second positioning means for accurately positioning the substrate with respect to item PL;   a projection system (“lens”) PL (e.g. a refractive system) for imaging an irradiated portion of the mask MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.       

     As here depicted, the apparatus is of a transmissive type (e.g. has a transmissive mask). However, in general, it may also be of a reflective type, for example (e.g. with a reflective mask). Alternatively, the apparatus may employ another kind of patterning device, such as a programmable mirror array of a type as referred to above. 
     The source LA (e.g. an excimer laser) produces a beam of radiation. This beam is fed into an illumination system (illuminator) IL, either directly or after having traversed conditioning means, such as a beam expander Ex, for example. The illuminator IL may comprise adjusting means AM for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the beam. In addition, it will generally comprise various other components, such as an integrator IN and a condenser CO. In this way, the beam PB impinging on the mask MA has a desired uniformity and intensity distribution in its cross-section. 
     It should be noted with regard to  FIG. 1  that the source LA may be within the housing of the lithographic projection apparatus (as is often the case when the source LA is a mercury lamp, for example), but that it may also be remote from the lithographic projection apparatus, the radiation beam which it produces being led into the apparatus (e.g. with the aid of suitable directing mirrors); this latter scenario is often the case when the source LA is an excimer laser. The current invention and claims encompass both of these scenarios. 
     The beam PB subsequently intercepts the mask MA, which is held on a mask table MT. Having traversed the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target portion C of the substrate W. With the aid of the second positioning means (and interferometric measuring means IF), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning means can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval of the mask MA from a mask library, or during a scan. In general, movement of the object tables MT, WT will be realized with the aid of a long-stroke module (course positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in  FIG. 1 . However, in the case of a wafer stepper (as opposed to a step-and-scan apparatus) the mask table MT may just be connected to a short stroke actuator, or may be fixed. 
     The depicted apparatus can be used in two different modes: 
     1. In step mode, the mask table MT is kept essentially stationary, and an entire mask image is projected at one time (i.e. a single “flash”) onto a target portion C. The substrate table WT is then shifted in the x and/or y directions so that a different target portion C can be irradiated by the beam PB; 
     2. In scan mode, essentially the same scenario applies, except that a given target portion C is not exposed in a single “flash”. Instead, the mask table MT is movable in a given direction (the so-called “scan direction”, e.g. they direction) with a speed v, so that the projection beam PB is caused to scan over a mask image; concurrently, the substrate table WT is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the lens PL (typically, M=¼ or ⅕). In this manner, a relatively large target portion C can be exposed, without having to compromise on resolution. 
       FIG. 2  shows a liquid reservoir  10  between the projection system PL and the substrate W which is positioned on the substrate stage WT. The liquid reservoir  10  is filled with a liquid  11  having a relatively high refractive index, e.g. water, provided via inlet/outlet ducts  13 . The liquid has the effect that the radiation of the projection beam is a shorter wavelength in the liquid than in air or in a vacuum, allowing smaller features to be resolved. It is well known that the resolution limit of a projection system is determined, inter alia, by the wavelength of the projection beam and the numerical aperture of the system. The presence of the liquid may also be regarded as increasing the effective numerical aperture. Furthermore, at fixed numerical aperture, the liquid is effective to increase the depth of field. 
     The reservoir  10  forms, in an embodiment, a contactless seal to the substrate W around the image field of the projection system PL so that the liquid is confined to fill the space between the substrate&#39;s primary surface, which faces the projection system PL, and the final optical element of the projection system PL. The reservoir is formed by a seal member  12  positioned below and surrounding the final element of the projection system PL. Thus, the liquid supply system provides liquid on only a localized area of the substrate. The seal member  12  forms part of the liquid supply system for filling the space between the final element of the projection system PL and the substrate with a liquid. This liquid is brought into the space below the projection system PL and within the seal member  12 . The seal member  12  in an embodiment extends a little above the bottom element of the projection system PL and the liquid rises above the final element so that a buffer of liquid is provided. The seal member  12  has an inner periphery that at the upper end closely conforms to the shape of the projection system PL or the final element thereof and may, e.g. be round. At the bottom the inner periphery closely conforms to the shape of the image field, e.g. rectangular, though this is not necessarily so. The seal member is substantially stationary in the XY plane relative to the projection system PL though there may be some relative movement in the Z direction (in the direction of the optical axis). A seal is formed between the seal member and the surface of the substrate. This seal is desired to be a contactless seal and may be a gas seal. 
     The liquid  11  is confined in the reservoir  10  by a seal device  16 . As illustrated in  FIG. 2 , the seal device is a contactless seal i.e. a gas seal. The gas seal is formed by gas, e.g. air or synthetic air, provided under pressure via inlet  15  to the gap between seal member  12  and substrate W and extracted by first outlet  14 . The over pressure on the gas inlet  15 , vacuum level or under pressure on the first outlet  14  and the geometry of the gap are arranged so that there is a high-velocity gas flow inwards towards the optical axis of the apparatus that confines the liquid  11 . As with any seal, some liquid is likely to escape, for example up the first outlet  14 . 
       FIGS. 22 and 23  also depict a liquid reservoir defined by inlet(s) IN, outlet(s) OUT, the substrate W and the final element of projection system PL. Like the liquid supply system of  FIG. 2  the liquid supply system illustrated in  FIGS. 22 and 23 , comprising inlet(s) IN and outlet(s) OUT, supplies liquid to the primary surface of the substrate in a localized area between the final element of the projection system and the substrate and can suffer from loss of liquid at the substrate edge. 
     Thus, as used herein for the embodiments, the liquid supply system can comprise that as described in relation to  FIG. 2  and  FIGS. 22 and 23 . 
     A problem with the liquid supply arrangement illustrated in  FIGS. 2, 22 and 23  occurs when imaging edge portions of the substrate W. This is because when the substrate W edge is positioned underneath the projection system PL one of the constraining walls (the substrate W) of the liquid supply system (the bottom one as illustrated) is removed thereby allowing immersion liquid to escape. However, the embodiments described herein can be used with any other type of liquid supply system. 
       FIG. 3  illustrates how the edge portion of a substrate W may be imaged without catastrophic loss of immersion liquid from the liquid supply system. This is achieved by the provision of a cover plate or edge seal member  17  on the substrate table WT. The edge seal member  17  has an upper (as illustrated) primary surface substantially co-planar with the upper primary surface of substrate W and is closely adjacent to the edge of the substrate W so that there is no sudden loss of liquid as the edge of the substrate moves under the projection system PL. Some liquid loss into the gap may still occur. Of course there are arrangements in which the whole construction illustrated in  FIGS. 2 and 3  is positioned upside down so that it is the lower surfaces of the edge seal member and the substrate which face the projection system and which are substantially co-planar. The surfaces are therefore referred to as the primary surfaces which face the projection system PL rather than upper surfaces. References herein to upper surfaces and lower surfaces may be also appropriately considered as references to lower and upper surfaces respectively in an upside-down configuration. 
     With this system, the liquid supply system (e.g. reservoir  10 ) can be positioned over the edge of the substrate W and can even be moved completely off the substrate W. This enables edge portions of the substrate W to be imaged. 
     The edge seal member  17  may form an integral part of the substrate table WT (as illustrated in  FIG. 4  as edge seal member  117 ) or may be temporarily mounted relative to the remainder of the substrate table by the use of, for example, vacuum suction or through use of electromagnetic forces. In an embodiment, the edge seal member  17  is moveable relative to the remainder of the substrate table (as illustrated in  FIGS. 5 and 6 ) such that the height above the substrate table WT of the primary surface of the edge seal member  17  may be adjusted such that it is substantially co-planar with the primary surface of the substrate W. In this way the same edge seal member  17  may be used for different thicknesses of substrate W (thickness tolerance is about 25 μm though the embodiment can account for up to about 0.2 mm variation). The positioning mechanism for the edge seal member  17  may be through use of piezoelectric elements or electromagnetism, worm gear, etc. A suitable mechanism is described in relation to the second embodiment described below. 
     The edge seal member  17  may be formed of several individual segments, each of which surrounds a portion of the edge of the substrate W. 
     Embodiment 2 
     A second embodiment is illustrated in  FIGS. 4 to 6  and is the same or similar as the first embodiment except as described below. 
     In the embodiment of  FIGS. 4 and 5  an edge liquid supply system provides liquid to a reservoir  30  via a port  40 . The liquid in the reservoir  30  is optionally the same as the immersion liquid in the liquid supply system. The reservoir  30  is positioned on the opposite side of the substrate W to the projection system PL and adjacent the edge of the substrate W and the edge of the edge seal member  17 ,  117 . In  FIG. 5 , the edge seal member  17  is comprised of an element which is separate to the substrate table WT whereas in  FIG. 4  the edge seal member  117  is provided by an integral portion of the substrate table WT. As can be seen most clearly from  FIG. 4 , the substrate W is supported on the substrate table WT by a so-called pimple table  20 . The pimple table  20  comprises a plurality of projections on which the substrate W rests. The substrate W is held in place by, e.g., a vacuum source sucking the substrate onto the top surface of the substrate table WT. With the use of the reservoir  30 , when the edge of the substrate W is being imaged, (i.e. when liquid in the liquid supply system between the projection system PL and the substrate W traverses across an edge of the substrate), liquid cannot escape from the liquid supply system into the gap between the edge seal member  17 ,  117  and the substrate W because that space is already filled with liquid. 
     The mechanism  170  shown in  FIG. 5  for moving the edge seal member  17  relative to the remainder of the substrate table WT is illustrated in detail in  FIG. 6 . The reason for moving the edge seal member  17  in this way is so that its primary surface can be made to be substantially co-planar with the primary surface of the substrate W. This allows a smooth movement of the liquid supply system over edge portions of the substrate W so that the bottom inner periphery of the liquid supply system can be moved to positions partly on the primary surface of substrate W and partly on the primary surface of the edge seal member  17 . 
     A level sensor (not illustrated) is used to detect the relative heights of the primary surfaces of the substrate W and the edge seal member  17 . Based on the results of the level sensor, control signals are sent to the actuator  171  in order to adjust the height of the primary surface of the edge seal member  17 . A closed loop actuator could also be used for this purpose. 
     In an embodiment, the actuator  171  is a rotating motor which rotates a shaft  176 . The shaft  176  is connected to a circular disc at the end distal to the motor  171 . The shaft  176  is connected away from the centre of the disc. The disc is located in a circular recess in a wedge portion  172 . Ball bearings may be used to reduce the amount of friction between the circular disc and the sides of the recess in the wedge portion  172 . The motor  171  is held in place by leaf springs  177 . On actuation of the motor the wedge portion is driven to the left and right as illustrated (i.e. in the direction of the slope of the wedge portion) because of the excentre position of the shaft  176  in the disc. The motor is prevented from moving in the same direction as the direction of movement of the wedge portion  172  by the springs  177 . 
     As the wedge portion  172  moves left and right as illustrated in  FIG. 6 , its top surface  175  (which is the surface of the wedge which is sloped in relation to the primary surface of the edge seal member  17 ) contacts the bottom sloped surface of a further wedge member  173  which is fixed to the bottom of the edge seal member  17 . The edge seal member  17  is prevented from moving in the direction of movement of the wedge member  172  so that when the wedge member  172  moves left and right the edge seal member  17  is lowered and raised respectively. Some biasing of the edge seal member  17  towards the substrate table WT may be necessary. 
     Obviously the further wedge member  173  could be replaced by an alternative shape, for example a rod positioned perpendicularly to the direction of movement of the wedge  172 . If the coefficient of friction between the wedge member  172  and the further wedge member  173  is greater than the tangent of the wedge angle then the actuator  170  is self-braking meaning that no force is required on the wedge member  172  to hold it in place. This is advantageous as the system will then be stable when the actuator  171  is not actuated. The accuracy of the mechanism  170  is of the order of a few μm. 
     Especially in the case of the edge seal member  117  being an integral part of the substrate table WT, a mechanism may be provided to adjust the height of the substrate W or the member supporting the substrate W so that the primary surfaces of the edge seal member  17 ,  117  and the substrate can be made substantially co-planar. 
     Embodiment 3 
     A third embodiment is illustrated in  FIG. 7  and is the same or similar as the first embodiment except as described below. 
     This embodiment is described in relation to an edge seal member  117  which is an integral part of the substrate table WT. However, this embodiment is equally applicable to an edge seal member  17  which is movable relative to the substrate table WT. In this embodiment it is not vital however that the edge seal member  17  has an upper surface co-planar with the primary surface of the substrate, but this is desired. A vacuum port  46  connected to a vacuum source is provided underneath and adjacent edge portions of the edge seal member  117  and the substrate W on the opposite side of the substrate W to the projection system PL. In an embodiment, the port  46  is annular and formed by a continuous groove but may be discontinuous i.e. a discrete number of openings arranged in a circular pattern. In its simplest form the embodiment may work only with that vacuum supply via port  46 . However, the basic idea can be improved by the provision of a substrate table WT as illustrated in detail in  FIG. 7 a    which illustrates a first version of the third embodiment. 
     A portion  48  of the substrate table WT extends from the edge of the edge seal portion  117  radially inwardly so that it is positioned below the substrate table W on the other side of the substrate W to the projection system PL. Any immersion liquid which leaks through the gap between the portion  48  and the substrate W is attracted towards the vacuum source via port  46 . A channel  42  is provided radially inwardly of the vacuum source also under the substrate W and is connected to a gas source. This may be a gas at a pressure greater than atmospheric pressure or it may be that the channel  42  is simply open to the atmosphere. This creates a flow of gas radially outwardly below the substrate W between the portion  48  of substrate table WT below the substrate W and the pimple table  20 . (The pimple table  20  has its own vacuum source to hold the substrate in place.) With this flow of gas any liquid escaping between edge seal member  117  and the substrate W is pulled towards an annular compartment  44  (roughly 3×3 mm in cross section) in fluid connection with the vacuum source. The compartment  44  is positioned between an annular port  47  open to the gap and the port  46  connected to the vacuum source. The compartment helps in establishing uniform flow around the periphery. The channel  42  is connected to a continuous annular groove (shown as a widening of the duct). The compartment  44 , port  47 , and/or the groove of channel  42  need not be annular and can be other appropriate shapes or configurations. 
     In one working embodiment, the gap between the portion  48  of substrate table WT and the substrate W is of the order of up to 100 μm (though the gap may not exist i.e. is zero), which prevents a high flow rate of liquid through the gap due to capillary action. The height of the portion  45  of the substrate table WT between the groove connected to channel  42  and compartment  44  is such that the distance between the bottom of the substrate W and the top of that portion  45  (indicated as distance D 1  in  FIG. 7 a   ) is typically of the order of 100 μm and is chosen such that a uniform gas flow of in the region of at least 1 m/s is achievable with a pressure loss of less than 0.5 bar. Such an arrangement ensures that only very little, if any, liquid passes through the gap D 1  and interferes with the pimple table  20 . Other values will also work. 
     A first version of the third embodiment illustrated in  FIG. 7 a    may suffer from deflection of the outer 10 mm or so of the substrate W. As can be seen from  FIG. 7 a    this area is unsupported even though, as said above, portion  45  can be extended to underneath the substrate W where it supports the substrate W. However, at the very outer radius both the weight of the substrate W and the capillary force of liquid between the substrate W and portion  48  of the substrate table WT can still deflect the edge of the substrate W. This may be deleterious. Solutions to this problem are illustrated in  FIGS. 7 b - d    which illustrate second through fourth versions of the third embodiment. 
     In the second version illustrated in  FIG. 7 b   , the portion  48  has at least one set of burls  348  positioned around and near to the edge of the periphery of the substrate W (typically in a circular pattern). As the burls  348  are discrete, immersion liquid can still seep between the portion  48  and the substrate W but the weight of the substrate W is supported by the at least one set of burls  348 . In an embodiment, the burls  348  have a smaller height than the burls of the pimple table  20  which compensates for the difference in the force downwards on the substrate W caused by the vacuum  22  of the pimple table  20  compared to the force on the substrate W at the edge in the vicinity of burls  348 . The calculation must take the stiffness of the burls into account and if the burls are manufactured from a low expansion material such as Zerodur, they should be about 80 nm less high than the burls of the pimple table  20 . The gap between the portion  48  and the bottom of the substrate W is in an embodiment about 20 μm. 
     In the version of  FIG. 7 b   , portion  45  is similar in shape to that of the first version. However, an alternative has a ring or circular pattern of burls  345  positioned above portion  45 . The discrete nature of the burls  345  allows gas from channel  42  to be sucked into the compartment  44 . These burls  345  are also about 80 nm less high than the burls of the pimple table  20 . In an embodiment, gap D 1  in between the burls  345  is about 50 μm. The burls  345  may be formed by the pimple table  20  and need not necessarily be part of the substrate table WT. 
     From the above two versions of the third embodiment it will be clear that the architecture of the gas seal formed by passages  42  and  47  can be formed either completely by the substrate table WT, completely by the pimple table  20  or by a combination of both.  FIGS. 7 c  and 7 d    illustrate two further versions of the third embodiment.  FIG. 7 c    illustrates a third version of the third embodiment in which the gas seal is formed by members of the pimple table  20 . The portion  45  of the first and second versions is formed by a (annular) portion of the pimple table  2045  and portion  48  of the first and second versions is formed by (annular) portion  2048  of the pimple table  20 . Passages  2042 ,  2047  equivalent to  42  and  47  are formed between the portions  20 ,  2045  and  2048 . However, only a part of the gas flow passes through the two passages  2042 ,  2047 ; as illustrated, some gas flows under the pimple table  20  which is effective to block further ingression of immersion liquid which seeps under the outer edge of the pimple table  20 . This arrangement has an advantage that all of the accurate dimensions are made in the pimple table  20  and the substrate table WT does not contain any complex grooves. 
     In a fourth version of the third embodiment illustrated in  FIG. 7 d   , no inlet channel  42  is provided and gas flows from the pimple table  20  into (annular) port  47 . This version has an advantage that a more stable pressure is experienced between the substrate W and the pimple table  20  because the pimple table  20  does not need its own vacuum source. Furthermore, extra passage  2047  which is provided in the third version is no longer necessary and only passage  2042  is used. Thus, a single vacuum source is effective both to clear away leaking immersion fluid as well as holding the substrate in place. A gas source may be required under the pimple table  20  (perhaps the more usual vacuum port in the substrate table under the pimple table can be used for this purpose) so that a flow of gas outwards can be established. 
     It will be clear that various features of each of the versions of the third embodiment can be combined so long as a flow of gas radially outwardly from the centre of the pimple table towards the vacuum  46  is achieved. 
     Embodiment 4 
     A fourth embodiment is illustrated in  FIGS. 8 and 9  and is the same or similar as the first embodiment except as described below. 
     This embodiment is described in relation to an edge seal member  117  which is an integral part of the substrate table WT. However, this embodiment is equally applicable to an edge seal member  17  which is movable relative to the substrate table WT. 
     In a first version of this embodiment as illustrated in  FIG. 8 a   , a further edge seal member  500  is used to bridge the gap between the edge seal member  117  and the substrate W. The further edge seal member is affixed to the edge seal member  117 . The further edge seal member  500  is removably attachable against the surface of the substrate W opposite the primary surface. In this embodiment the further edge seal member  500  can be a flexible edge seal member which is actuatable to contact the under surface of the substrate W. When the flexible edge seal member  500  is deactivated it falls away from the substrate under gravity. The way this may be achieved is illustrated in  FIG. 9  and is described below. 
     It is likely that the further edge seal member  500  will not prevent all of the immersion liquid from the liquid supply system from entering the space under the substrate W and for this reason a port  46  connected to a low pressure source may be provided under the substrate W adjacent edges of the edge seal member  117  and the substrate Win some or all of the versions of this embodiment. Of course the design of the area under the substrate could be the same as that of the third embodiment. 
     The same system can be used for sensors such as a transmission image sensor (TIS) on the substrate table as opposed for the substrate W. In the case of sensors, as the sensors do not move, the edge seal member  500  can be permanently attached to the sensor, for example using glue. 
     Furthermore, the edge seal member  500  can be arranged to engage with the top surface of the object (that surface closest to the projection system PL) rather than the bottom surface. Also, the further edge seal member  500  may be provided attached to or near the top surface of the edge seal member  117  as opposed to under the edge seal member  117  as is illustrated in  FIG. 8   a.    
     A second version of this embodiment is illustrated in  FIG. 8 b   . Two further edge seal members  500   a ,  500   b  are used. The first of these edge seal members  500   a  is the same as in the first version. The second of these edge seal members  500   b  is affixed to the substrate table  20  i.e. underneath the substrate W and extends with its free end radially outwardly from its attachment point. The second further edge seal member  500   b  clamps the first further edge seal member  500   a  against the substrate W. Compressed gas can be used to deform or move the second further edge seal member  500   b.    
     A third version of this embodiment is shown in  FIG. 8 c   . The third version is the same as the second version except the first further edge seal member  500   c  clamps the second further edge seal member  500   d  to the substrate W. This avoids, for example, the need for the compressed gas of the second version. 
     It will be appreciated that the embodiment will also work with only the second further edge seal member  500   b ,  500   d  with or without connection to vacuum. 
     Various ways of deforming the further edge seal members  500 ,  500   a ,  500   b ,  500   c ,  500   d  will now be described in relation to the first version of the embodiment. 
     As can be seen from  FIG. 9 , a channel  510  is formed in the elongate direction of a flexible further edge seal member  500  (which in an embodiment is an annular ring) and (a) discrete port(s) are provided in an upper surface of the flexible further edge seal member which faces the projection system PL and the underside of the substrate W. By connecting a vacuum source  515  to the duct  510  the flexible further edge seal member can be made to abut the substrate W by suction. When the vacuum source  515  is disconnected or switched off, the flexible further edge seal member  500  drops under gravity and/or pressure from port  46  to assume the position shown in dotted lines in  FIG. 9 . 
     In an alternative embodiment a flexible further edge seal member  500  is formed with a mechanical pre-load such that it contacts the substrate W when the substrate is placed on the pimple table  20  and the flexible further edge seal member  500  deforms elastically so that it applies a force upwards on the substrate W to thereby make a seal. 
     In a further alternative, a flexible further edge seal member  500  may be forced against the substrate W by an overpressure generated by pressurised gas on port  46 . 
     A flexible further edge seal member  500  may be fashioned from any flexible, radiation and immersion liquid resistant, non-contaminating material, for example, steel, glass e.g. Al 2 O 3 , ceramic material e.g. SiC, silicon, Teflon, low expansion glasses (e.g. Zerodur™ or ULE™), carbon fibre epoxy or quartz and is typically between 10 and 500 μm thick, in an embodiment between 30 and 200 μm or 50 to 150 μm in the case of glass. With a flexible further edge seal member  500  of this material and these dimensions, the typical pressure to be applied to the duct  510  is approximately 0.1 to 0.6 bar. 
     Embodiment 5 
     A fifth embodiment is illustrated in  FIG. 10  and is the same or similar as the first embodiment except as described below. 
     This embodiment is described in relation to an edge seal member  117  which is an integral part of the substrate table WT. However, this embodiment is equally applicable to an edge seal member  17  which is movable relative to the substrate table WT. 
     In the fifth embodiment, the gap between the edge seal member  117  and the substrate W is filled with a further edge seal member  50 . The further edge seal member is a flexible further edge seal member  50  which has a top surface which is substantially co-planar with the primary surfaces of the substrate W and the edge seal member  117 . The flexible further edge seal member  50  is made of a compliant material so that minor variations in the diameter/width of substrate W and in the thickness of the substrate W can be accommodated by deflections of the flexible further edge seal member  50 . When liquid in the liquid supply system under the projection system PL passes over the edge of the substrate, the liquid cannot escape between the substrate W, flexible further edge seal member  50  and edge seal member  117  because the edges of those elements are tight against one another. Furthermore, because the primary surfaces of the substrate W and the edge seal member  117  and the top surface of the flexible further edge seal member  50  are substantially co-planar, the liquid supply system operation is not upset when it passes over the edge of the substrate W so that disturbance forces are not generated in the liquid supply system. 
     As can be seen from  FIG. 10 , the flexible further edge seal member  50  is in contact with a surface of the substrate W opposite the primary surface of the substrate W, at an edge portion. This contact has two functions. First the fluid seal between the flexible further edge seal member  50  and the substrate W may be improved. Second, the flexible further edge seal member  50  applies a force on the substrate W in a direction away from the pimple table  20 . When the substrate W is held on the substrate table WT by, e.g. vacuum suction, the substrate can be held securely on the substrate table. However, when the vacuum source is switched off or disconnected, the force produced by the flexible further edge seal member  50  on the substrate W is effective to push the substrate W off the substrate table WT thereby aiding loading and unloading of substrates W. 
     The flexible further edge seal member  50  is made of a radiation and immersion liquid resistant material such as PTFE. 
     Embodiment 6 
       FIG. 11  illustrates a sixth embodiment which is the same or similar as the first embodiment except as described below. 
     This embodiment is described in relation to an edge seal member  117  which is an integral part of the substrate table WT. However, this embodiment is equally applicable to an edge seal member  17  which is movable relative to the substrate table WT. 
     The sixth embodiment illustrates how the pimple table  20  can be decoupled from the liquid supply system between the substrate W and the edge seal member  117 . This is done by positioning an opening exposed to the atmosphere  65  between the edge of the substrate W and the vacuum holding the substrate W on the substrate table WT and associated with the pimple table  20 . 
     A layer  60 , positioned on the opposite side of the substrate W to the projection system PL and under the substrate at its edge leaving a gap between the substrate W and the layer  60  of about 10 μm, comprises any material which is hydrophobic such as Teflon™, silicon rubber, or other plastics material. Inorganic materials are desired because they have better radiation resistance. In this way, liquid which finds its way into the gap between the substrate W and the edge seal member  117  when the liquid supply system is positioned over the edge of the substrate W is repelled such that an effective seal is formed and liquid does not find its way to the pimple table  20 . In an embodiment, the immersion liquid has a contact angle of at least 90° with the hydrophobic layer  60 . 
     Embodiment 7 
     A seventh embodiment will be described with reference to  FIGS. 12 to 15 . The seventh embodiment is the same or similar as the first embodiment except as described below. 
     In the seventh embodiment, as is illustrated in  FIG. 12 , the edge seal member  17  is annular with a central hole larger than the diameter of the circular substrate W. The shapes of the substrate W and edge seal member  17  may be different than annular so long as the central hole of the edge seal member  17  is larger than the outer diameter/width of the substrate W. In this way, the edge seal member  17  may accommodate variations in the substrate W diameter/width. 
     The edge seal member  17  is movable on the substrate table WT such that when the liquid supply system moves towards an edge portion of the substrate W in order to expose it, the edge seal member  17  can be moved closely to abut that edge portion of the substrate W which is to be exposed. This is best illustrated in  FIG. 13  where the left hand side of the substrate W is about to be exposed. 
     As is clearly illustrated in  FIG. 14 , the edge seal member  17  is movable both in the plane of the primary surface of the substrate W and in an embodiment also in the Z direction (i.e. in the direction of the optical axis of the apparatus). In this way, the edge seal member  17  can be moved to the edge of the substrate W when required and can have the height of its top (primary) surface adjusted so that its primary surface is closely co-planar with the primary surface of the substrate W. This allows the liquid supply system to effectively contain the immersion liquid in its reservoir even when the edge of the substrate W is being imaged. 
     Also illustrated in  FIG. 14  is a projection  175  which has a top surface which is co-planar with the primary surface of the edge seal member  17 , i.e. the primary surface of the edge seal member  17  overhangs on an edge adjacent the substrate W so that the projection extends towards the optical axis of the apparatus. As can be seen from  FIG. 14 , this allows the gap between the primary surfaces of the substrate W and edge seal member  17  to be minimised even when the edge of the substrate W is slightly curved (i.e. the edge of the substrate W is not perpendicular to the primary surface). 
     Another way of improving or reducing the gap between the edge seal member  17  and the substrate W is to provide a further (flexible) edge seal member  177  between the edge of the edge seal member  17  closest to the substrate W and the substrate W. This is illustrated in  FIG. 15 . This may be done with or without a projection  175 . A further flexible edge seal member  177  can deform around the edge of the substrate W so as to form a tight seal with the substrate W. The further flexible edge seal member  177  is attached to the edge seal member  17 . The further flexible edge seal member  177  has an upper surface which is substantially co-planar with the primary surfaces of the substrate W and the edge seal member  17 . 
     Embodiment 8 
       FIG. 16  illustrates an eighth embodiment which is the same or similar as the first embodiment except as described below. 
     This embodiment is described in relation to an edge seal member  117  which is an integral part of the substrate table WT. However, this embodiment is equally applicable to an edge seal member  17  which is movable relative to the substrate table WT. 
     As can be seen from  FIG. 16 , the eighth embodiment includes a further edge seal member  100  for bridging the gap between the edge seal member  117  and the substrate W. In this case the further edge seal member  100  is a gap seal member which is positioned on the primary surfaces of the substrate W and the edge seal member  117  to span the gap between the substrate W and edge seal member  117 . Thus, if the substrate W is circular, the gap seal member  100  will also be circular (annular). 
     The gap seal member  100  may be held in place by the application of a vacuum  105  to its underside (that is a vacuum source exposed through a vacuum port on the primary surface of the edge seal member  117 ). The liquid supply system can pass over the edge of the substrate W without the loss of liquid because the gap between the substrate W and the edge seal member  117  is covered over by the gap seal member  100 . The gap seal member  100  can be put in place and removed by the substrate handler so that standard substrates and substrate handling can be used. Alternatively, the gap seal member  100  can be kept at the projection system PL and put in place and removed by appropriate mechanisms (e.g. a substrate handling robot). The gap seal member  100  should be stiff enough to avoid deformation by the vacuum source. Advantageously the gap seal member  100  is less than 50, in an embodiment 30 or 20 or even 10 μm thick to avoid contact with the liquid supply system, but should be made as thin as possible. 
     The gap seal member  100  is advantageously provided with tapered edges  110  in which the thickness of the gap seal member  100  decreases towards the edges. This gradual transition to the full thickness of the gap seal member ensures that disturbance of the liquid supply system is reduced when it passes on top of the gap seal member  100 . 
     The same way of sealing may be used for other objects such as sensors, for example transmission image sensors. In this case, as the object is not required to move, the gap seal member  100  can be glued in place (at either end) with a glue which does not dissolve in the immersion liquid. The glue can alternatively be positioned at the junction of the edge seal member  117 , the object and the gap seal member  100 . 
     Furthermore, the gap seal member  100  can be positioned underneath the object and an overhang of the edge seal member  117 . The object may be shaped with an overhang also, if necessary. 
     The gap seal member  100 , whether above or below the object, can have a passage provided through it, from one opening in a surface in contact with the edge seal member  117  to another opening in a surface in contact with the object. By positioning one opening in fluid communication with vacuum  105 , the gap seal member  100  can then be kept tightly in place. 
     Embodiment 9 
     A ninth embodiment will be described with reference to  FIG. 17 . The solution shown in  FIG. 17  bypasses some of the problems associated with imaging edge portions of the substrate W as well as allows a transmission image sensor (TIS)  220  (or other sensor) to be illuminated by the projection system PL under the same conditions as the substrate W. 
     The ninth embodiment uses the liquid supply system described with respect to the first embodiment. However, rather than confining the immersion liquid in the liquid supply system under the projection system PL on its lower side with the substrate W, the liquid is confined by an intermediary plate  210  which is positioned between the liquid supply system and the substrate W. The spaces  222 ,  215  between the intermediary plate  210  and the TIS  220  and the substrate W are also filled with liquid  111 . This may either be done by two separate space liquid supply systems via respective ports  230 ,  240  as illustrated or by the same space liquid supply system via ports  230 ,  240 . Thus the space  215  between the substrate W and the intermediary plate  210  and the space  222  between the transmission image sensor  220  and the intermediary plate  210  are both filled with liquid and both the substrate W and the transmission image sensor can be illuminated under the same conditions. Portions  200  provide a support surface or surfaces for the intermediary plate  210  which may be held in place by vacuum sources. 
     The intermediary plate  210  is made of such a size that it covers all of the substrate W as well as the transmission image sensor  220 . Therefore, no edges need to be traversed by the liquid supply system even when the edge of the substrate W is imaged or when the transmission image sensor is positioned under the projection system PL. The top surface of the transmission image sensor  220  and the substrate W are substantially co-planar. 
     The intermediate plate  210  can be removable. It can, for example, be put in place and removed by a substrate handling robot or other appropriate mechanism. 
     Embodiment 10 
       FIG. 18  shows a modification of the liquid supply system which is applicable to any other embodiment described herein which is effective to increase the size of gap the immersion liquid can span before sudden liquid loss. 
     A plurality of capillaries  600  are provided between the liquid supply system (e.g. seal member  12 ) and the projection system PL. These capillaries extend generally upwardly, i.e. away from the substrate W. If the capillaries have a radius r, the liquid film thickness h, which can be supported by the capillary, is given by the formula: 
     
       
         
           
             h 
             = 
             
               
                 2 
                 ⁢ 
                 σ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 cos 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 θ 
               
               
                 rg 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 ρ 
               
             
           
         
       
     
     where σ is the interfacial tension, θ the contact angle between the liquid and the capillaries W and ρ the liquid density. Thus by making cos θ positive (i.e. making the inner surface of the capillaries hydrophobic, for example by a coating) the capillaries can support a portion of liquid with height h above the gap so that a larger gap can be spanned. 
     By applying a voltage between the hydrophobic coated capillaries and the liquid, cos θ can be reduced to around zero and this allows free flow of liquid through the capillaries  600  (according to equation 1 above) so that liquid can be removed from the liquid supply system under the projection system PL in little time by keeping the length of the capillaries low. This is advantageous for keeping the liquid clean. When the edge of the substrate W is imaged, the voltage can be removed so that the gap can be spanned. In order to lift the liquid film from the substrate W, it is proposed to coat the substrate W edges with a hydrophobic material (or the resist on the substrate W edges can be removed as the substrate material itself is hydrophobic). 
     The capillaries  600  may be provided by substantially straight ducts with a substantially circular cross-section or by other shaped ducts. For example, the capillaries may be made up of voids in a porous material. 
     Embodiment 11 
       FIG. 19  shows an eleventh embodiment which is the same as the first embodiment except as described below. 
     In the eleventh embodiment the object on the substrate table WT is a sensor  220  such as a transmission image sensor (TIS). In order to prevent immersion liquid seeping underneath the sensor  220 , a bead of glue  700  which is undissolvable and unreactable with the immersion fluid is positioned between the edge seal member  117  and the sensor  220 . The glue is covered by immersion fluid in use. 
     Embodiment 12 
     A twelfth embodiment is described with reference to  FIGS. 20 and 21 . In the twelfth embodiment it is a sensor  220  which is being sealed to the substrate table WT. In both versions illustrated in  FIGS. 20 and 21 , a vacuum  46  is provided adjacent the gap with an opening passage  47  and a chamber  44  for taking away any immersion liquid which should find its way through the gap between the edge seal member  117  and the edge of the sensor  220 . 
     In the  FIG. 20  version, the vacuum  46  is provided in the substrate table WT under an overhang portion of the object  220 . The passage  47  is provided in an overhanging inwardly protruding portion of the substrate table WT. Optionally a bead of glue  700  is positioned at the inner most edge of the protruding portion between the substrate table WT and the object  220 . If no bead of glue  700  is provided, a flow of gas from underneath the object  220  helps seal the gap between the sensor  220  and the substrate table WT. 
     In the version of  FIG. 21 , the vacuum  46 , compartment  44  and passage  47  are provided in the object  220  itself under an inwardly protruding edge seal member  117 . Again there is the option of providing a bead of glue between the object  220  and the substrate table WT radially outwardly of the passage  47 . 
     All of the above described embodiments may be used to seal around the edge of the substrate W. Other objects on the substrate table WT may also need to be sealed in a similar way, such as sensors including sensors and/or marks which are illuminated with the projection beam through the liquid such as a transmission image sensor, an integrated lens interferometer and scanner (wavefront sensor) and spot sensor plates. Such objects may also include sensors and/or marks which are illuminated with non-projection radiation beams such as levelling and alignment sensors and/or marks. The liquid supply system may supply liquid to cover all of the object in such a case. Any of the above embodiments may be used for this purpose. In some instances, the object will not need to be removed from the substrate table WT as, in contrast to the substrate W, the sensors do not need to be removed from the substrate table WT. In such a case the above embodiments may be modified as appropriate (e.g. the seals may not need to be moveable). 
     Each of the embodiments may be combined with one or more of the other embodiments as appropriate. Further, each of the embodiments (and any appropriate combination of embodiments) can be applied simply to the liquid supply system of  FIG. 2  and  FIGS. 19 and 20  without the edge seal member  17 ,  117  as feasible and/or appropriate. 
     The shape of the edge seal member  117  and the top outer most edge of the sensor  220  can be varied. For example, it may be advantageous to provide an overhanging edge seal member  117  or indeed an outer edge of the sensor  220  which is overhanging. Alternatively, an outer upper corner of the sensor  220  may be useful. 
     While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. In particular, the invention is also applicable to other types of liquid supply systems, especially localized liquid area systems. If the seal member solution is used, it may be one in which a seal other than a gas seal is used. The description is not intended to limit the invention.