Patent Publication Number: US-2011069289-A1

Title: Lithographic apparatus, coverplate and device manufacturing method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority and benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/244,367, entitled “Lithographic Apparatus, Coverplate and Device Manufacturing Method”, filed on Sep. 21, 2009. The content of that application is incorporated herein in its entirety by reference. 
    
    
     FIELD 
     The present invention relates to a lithographic apparatus, a coverplate and a method for manufacturing a device. 
     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. including 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. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. 
     It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The liquid is desirably distilled water, although other liquids can be used. An embodiment of the present invention will be described with reference to liquid. However, fluids may be suitable, particularly wetting fluids, incompressible fluids and/or fluids with higher refractive index than air, desirably a higher refractive index than water. Fluids excluding gases are particularly desired. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus. Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein, or liquids with nano-particle suspensions (e.g. particles with a maximum dimension of up to 10 nm). The suspended particles may or may not have a similar or the same refractive index as the liquid in which they are suspended. Other liquids which may be suitable are hydrocarbons, such as aromatics, fluorohydrocarbons, and aqueous solutions. 
     However, submersing the substrate or substrate and substrate table in a bath of liquid (see for example U.S. Pat. No. 4,509,852) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects. 
     Another arrangement 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 system and the substrate using a liquid confinement system (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 WO 99/49504. This type of arrangement may be referred to as a localized immersion system. 
     Another arrangement is an all wet arrangement in which the immersion liquid is unconfined as disclosed in WO2005/064405. In such a system the immersion liquid is unconfined. The whole top surface of the substrate is covered in liquid. This is beneficial because then the whole top surface of the substrate is exposed to the same conditions. This may have advantages for temperature control and processing of the substrate. In WO2005/064405, a liquid supply system provides liquid to the gap between the final element of the projection system and the substrate. That liquid is allowed to leak over the remainder of the substrate. A barrier at the edge of a substrate table prevents the liquid from escaping so that it can be removed from the top surface of the substrate table in a controlled way. 
     Although such a system improves temperature control and processing of the substrate, evaporation of the immersion liquid can still occur. One way of alleviating that problem is described in US 2006/119809 in which a member is provided which covers the substrate in all positions and which is arranged to have immersion liquid extending between it and the top surface of the substrate and/or substrate table which holds the substrate. 
     In EP-A-1,420,300 and U.S. patent application publication number US 2004-0136494, each hereby incorporated in their entirety by reference, the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two stages for supporting the substrate. Leveling measurements are carried out with a stage at a first position, without immersion liquid, and exposure is carried out with a stage at a second position, where immersion liquid is present. Alternatively, the apparatus has only one stage. 
     After exposure of a substrate in an immersion lithographic apparatus, the substrate table is moved away from its exposure position to a position in which the substrate may be removed and replaced by a different substrate. This is known as substrate swap. In a two stage lithographic apparatus, the swap of the tables may take place under the projection system. 
     In an immersion apparatus, immersion liquid is handled by a fluid handling system or apparatus. A fluid handling system may supply immersion fluid and therefore be a fluid supply system. A fluid handling system may at least partly confine fluid and thereby be a fluid confinement system. A fluid handling system may provide a barrier to fluid and thereby be a barrier member. Such a barrier member may be a fluid confinement structure. A fluid handling system may create or use a flow of fluid (such as gas), for example to help in handling liquid, e.g. in controlling the flow and/or the position of the immersion fluid. The flow of gas may form a seal to confine the immersion fluid so the fluid handling structure may be referred to as a seal member; such a seal member may be a fluid confinement structure. Immersion liquid may be used as the immersion fluid. In that case, the fluid handling system may be a liquid handling system. The fluid handling system may be located between the projection system and the substrate table. In reference to the aforementioned description, reference in this paragraph to a feature defined with respect to fluid may be understood to include a feature defined with respect to liquid. 
     In an immersion lithographic apparatus it is desirable to provide seals between objects through which gap immersion liquid may otherwise leak. Seals which contact both objects may not be suitable because this may lead to the deleterious transmission of disturbance forces between the objects. Additionally, in an all wet immersion lithographic apparatus where the whole top surface of the substrate and substrate table is immersed in immersion liquid, it is desirable that the film of liquid covering those objects does not break up into droplets. 
     SUMMARY 
     It is desirable, for example, to provide a lithographic apparatus in which means for control of immersion liquid are provided. In particular, it is desirable to provide a seal between two objects. Additionally, it is desirable to provide an all wet lithographic apparatus in which measures have been taken to avoid the film of liquid covering the substrate and substrate table from, breaking up into droplets. 
     According to an aspect, there is provided an immersion lithographic apparatus, including: first and second objects which are spaced apart with a gap therebetween and on whose top surfaces immersion liquid is provided; and a gutter positioned under the gap for collecting any immersion liquid which passes through the gap; wherein an advancing contact angle of immersion liquid with surfaces of the first and second objects defining the gap is less than 30. 
     According to an aspect, there is provided an immersion lithographic apparatus, including: a substrate table for holding a substrate; and a coverplate tiltable independently of the substrate table. 
     According to an aspect, there is provided an all wet immersion lithographic apparatus, including: a substrate table for holding a substrate; and an opening for the supply of liquid at the edge of a surface over which immersion liquid flows; and a controller for supplying or increasing the supply of liquid to a leading edge of the surface through the opening during movement of the surface. 
    
    
     
       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; 
         FIGS. 2 and 3  depict a liquid supply system for use in a lithographic projection apparatus; 
         FIG. 4  depicts a further liquid supply system for use in a lithographic projection apparatus; 
         FIG. 5  depicts a further liquid supply system for use in a lithographic projection apparatus; 
         FIG. 6  depicts, in plan, the substrate table, coverplate and positioner according to an embodiment; 
         FIG. 7  depicts, in cross-section, the substrate table, coverplate and positioner of  FIG. 6  along line VII-VII; 
         FIG. 8  depicts, in cross-section, the substrate table, coverplate and positioner of  FIG. 6  along line VIII-VIII; 
         FIG. 9  depicts, in cross-section, a seal between first and second objects according to an embodiment; 
         FIG. 10  depicts, in cross-section, a seal between first and second objects according to an embodiment; 
         FIG. 11  depicts, in cross-section, an edge supply according to an embodiment; and 
         FIG. 12  depicts schematically tilting of a coverplate according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or DUV radiation); a patterning device support or support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters; a substrate 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 W in accordance with certain parameters; and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. including one or more dies) of the substrate W. 
     The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, to direct, shape, or control radiation. 
     The patterning device support holds the patterning device 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 patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” 
     The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. 
     The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Patterning device (e.g. 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 mirror 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 term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”. 
     As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask). 
     The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines 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 exposure. 
     Referring to  FIG. 1 , the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and/or a beam expander. In other cases the source SO may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system. 
     The illuminator IL may include an adjuster AD to adjust 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 illuminator IL can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator IN and a condenser CO. The illuminator IL may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section. 
     The radiation beam B is incident on the patterning device MA (e.g., mask), which is held on the patterning device support MT (e.g., mask table), and is patterned by the patterning device MA. Having traversed the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (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 (which is not explicitly depicted in  FIG. 1 ) can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the patterning device support (e.g. mask table) MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the patterning device support (e.g. mask table) MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device (e.g. mask) MA and substrate W may be aligned using patterning device alignment marks M 1 , M 2  and substrate alignment marks P 1 , P 2 . Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g. mask) MA, the patterning device alignment marks may be located between the dies. 
     The depicted apparatus could be used in at least one of the following modes: 
     1. In step mode, the patterning device support (e.g. mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.
 
2. In scan mode, the patterning device support (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the patterning device support (e.g. mask table) MT may be determined by the (de-) magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion C in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion C.
 
3. In another mode, the patterning device support (e.g. mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.
 
     Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. 
     Arrangements for providing liquid between a final element of the projection system PS and the substrate can be classed into three general categories. These are the bath type arrangement, the so-called localized immersion system and the all-wet immersion system. In the bath type arrangement substantially the whole of the substrate W and optionally part of the substrate table WT is submersed in a bath of liquid. 
     The localized immersion system uses a liquid supply system in which liquid is only provided to a localized area of the substrate. The space filled by liquid is smaller in plan than the top surface of the substrate and the area filled with liquid remains stationary relative to the projection system PS whilst the substrate W moves underneath that area.  FIGS. 2-5  show different supply devices which can be used in such a system. Sealing features are present to seal liquid to the localized area. One way which has been proposed to arrange for this is disclosed in PCT patent application publication no. WO 99/49504. 
     In the all wet arrangement the liquid is unconfined. The whole top surface of the substrate and all or part of the substrate table is covered in immersion liquid. The depth of the liquid covering at least the substrate is small. The liquid may be a film, such as a thin film, of liquid on the wafer. Immersion liquid may be supplied to or in the region of a projection system and facing surface facing the projection system (such a facing surface may be the surface of a substrate and/or a substrate table). Any of the liquid supply devices of  FIGS. 2-5  can also be used in such a system. However, sealing features are not present, are not activated, are not as efficient as normal or are otherwise ineffective to seal liquid to only the localized area. 
     As illustrated in  FIGS. 2 and 3 , liquid is supplied by at least one inlet onto the substrate, preferably along the direction of movement of the substrate relative to the final element. Liquid is removed by at least one outlet after having passed under the projection system PS. 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. 2  shows the arrangement schematically in which liquid is supplied via inlet and is taken up on the other side of the element by outlet which is connected to a low pressure source. In the illustration of  FIG. 2  the liquid is supplied along the direction of movement of the substrate W 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. 3  in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element. Note that the direction of flow of the liquid is shown by arrows in  FIGS. 2 and 3 . 
     A further immersion lithography solution with a localized liquid supply system is shown in  FIG. 4 . Liquid is supplied by two groove inlets on either side of the projection system PS and is removed by a plurality of discrete outlets arranged radially outwardly of the inlets. The inlets can be arranged in a plate with a hole in its centre and through which the projection beam is projected. Liquid is supplied by one groove inlet on one side of the projection system PS and removed by a plurality of discrete outlets on the other side of the projection system PS, causing a flow of a thin film of liquid between the projection system PS and the substrate W. The choice of which combination of inlet and outlets to use can depend on the direction of movement of the substrate W (the other combination of inlet and outlets being inactive). Note that the direction of flow of fluid and of the substrate W is shown by arrows in  FIG. 4 . 
     Another arrangement which has been proposed is to provide the liquid supply system with liquid confinement structure which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. Such an arrangement is illustrated in  FIG. 5 . 
       FIG. 5  schematically depicts a localized liquid supply system or fluid handling structure with a liquid confinement structure  12 , which extends along at least a part of a boundary of the space  11  between the final element of the projection system PS and a facing surface (e.g. the coverplate  100  or substrate W). Please note that reference in the following text to surface of the substrate W also refers in addition or in the alternative to a surface of the coverplate  100 , unless expressly stated otherwise. The liquid confinement structure  12  is substantially stationary relative to the projection system PS in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). In an embodiment, a seal is formed between the liquid confinement structure  12  and the surface of the substrate W and may be a contactless seal such as a gas seal (such as system with a gas seal is disclosed in EP-A-1,420,298) or liquid seal. 
     The liquid confinement structure  12  at least partly contains liquid in the space  11  between a final element of the projection system PS and the substrate W. A contactless seal, such as a gas seal  16 , to the substrate W may be formed around the image field of the projection system PS so that liquid is confined within the space  11  between the substrate W surface and the final element of the projection system PS. The space  11  is at least partly formed by the liquid confinement structure  12  positioned below and surrounding the final element of the projection system PS. Liquid is brought into the space  11  below the projection system PS and within the liquid confinement structure  12  by liquid inlet  13 . The liquid may be removed by liquid outlet  13 . The liquid confinement structure  12  may extend a little above the final element of the projection system PS. The liquid level rises above the final element so that a buffer of liquid is provided. In an embodiment, the liquid confinement structure  12  has an inner periphery that at the upper end closely conforms to the shape of the projection system PS 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 need not be the case. 
     The liquid may be contained in the space  11  by the gas seal  16  which, during use, is formed between the bottom of the liquid confinement structure  12  and the surface of the substrate W. The gas seal  16  is formed by gas, e.g. air or synthetic air but, in an embodiment, N 2  or another inert gas. The gas in the gas seal  16  is provided under pressure via inlet  15  to the gap between liquid confinement structure  12  and substrate W. The gas is extracted via outlet  14 . The overpressure on the gas inlet  15 , vacuum level on the outlet  14  and geometry of the gap are arranged so that there is a high-velocity gas flow inwardly that confines the liquid. The force of the gas on the liquid between the liquid confinement structure  12  and the substrate W contains the liquid in a space  11 . The inlets/outlets may be annular grooves which surround the space  11 . The annular grooves may be continuous or discontinuous. The flow of gas is effective to contain the liquid in the space  11 . Such a system is disclosed in United States patent application publication no. US 2004-0207824, which is hereby incorporated by reference in its entirety. In another embodiment, the liquid confinement structure  12  does not have a gas seal. 
     Other types of liquid confinement structure to which the present invention may be applied include the so called gas drag liquid confinement structure such as that described in U.S. Ser. No. 61/181,158 filed 25 May 2009, which is hereby incorporated by reference. US 2008/0212046 provides further details and its content is also hereby incorporated by reference in its entirety. 
     The example of  FIG. 5  may be a so called localized area arrangement in which liquid is only provided to a localized area of the top surface of the substrate W at any one time. Other arrangements are possible for example, an arrangement using a single phase extractor on the undersurface  40  of the liquid confinement structure  12  may be used. An extractor assembly including a single phase extractor with a porous member is described in United States Patent Application No. US 2006/0038968, incorporated herein in its entirety by reference. An arrangement in which such an extractor assembly is used in combination with a recess and a gas knife is disclosed in detail in United States Patent Application Publication No. US 2006/0158627 incorporated herein in its entirety by reference. An embodiment of the invention may be applied to a fluid handling structure used in all wet immersion apparatus. In the all wet embodiment, fluid is allowed to cover the whole of the top surface of the substrate and surrounding surface, for example, by allowing liquid to leak out of a confinement structure which confines liquid to between the final element of projection system and the substrate. An example of a fluid handling structure for an all wet embodiment can be found in U.S. patent application no. 61/136,380 filed on 2 Sep. 2008. 
     In all of the above liquid confinement structures, liquid is provided to a space  11  between the projection system PS and the substrate W and/or substrate table WT. In the example of  FIG. 5  this is provided through outlet  13 . 
     It is desirable to prevent immersion liquid from reaching delicate parts of the immersion apparatus. Particular examples are control circuits, electronics and actuators of the positioner PW which positions the substrate table WT and therefore substrate W under the projection system PS. One way of doing this is to provide a coverplate  100  over parts of the positioner PW which might otherwise come in contact with immersion liquid. 
       FIG. 6  illustrates, in plan, a substrate table WT, coverplate  100  and positioner PW according to an embodiment. The coverplate  100  is optimized for use in an all wet immersion apparatus. However, the coverplate  100  may also be used in combination with any of the above localized area liquid supply solutions described above or any other type of apparatus, in particular any type of immersion apparatus. The benefits of both types of immersion apparatus of the coverplate  100  of embodiments of the present invention are the same. That is, there is a reduction of forces transmitted between the coverplate  100  and the substrate table WT. Furthermore, the mass of the short stroke module  120  of the positioner is reduced. This allows the area of the short stroke module  120  to be reduced and the size of actuators for positioning the short stroke module  120  are also reduced because of the lower static forces on the short stroke module  120 . The Eigen frequency of the short stroke module  120  may be increased resulting in higher controller bandwidth. 
     The positioner PW is included of a long stroke module  110  which is configured to perform coarse positioning of the substrate table WT. A short stroke module  120  is supported by the long stroke module  110 . The short stroke module is configured to perform fine positioning of the substrate table WT. The short stroke module  120  is moveable relative to the long stroke module  110 . The substrate table WT is held on the short stroke module  120 , in fixed relation thereto. Sensors  140  may be held on the short stroke module  120 . Sensors  140  on the short stroke module  120  may include in a non limiting list, transmission or through image sensors (TIS), ILIAS sensors, spot sensors and encoder grids  150 . 
     The substrate table WT supports the substrate W. In one embodiment the substrate table WT is a so called pimple table. The pimple table includes a surface with a plurality of projections. An under pressure is applied between the projections and the substrate W which sits on the projections. This holds the substrate securely to the substrate table WT. Other types of substrate table WT include electrostatic clamps. 
     The coverplate  100  is mounted to the long stroke module  110 . The coverplate  100  is not in contact with the short stroke module  120  or any items mounted to the short stroke module  120 . Therefore, the coverplate  100  is substantially mechanically decoupled from the substrate table WT as well as from the short stroke module  120 . The short stroke module  120  can move relative to the coverplate  100 , as will be described below. The coverplate  100  is in a fixed position relative to the long stroke module  110 . 
     The stiffness of the long stroke module  110  is improved by the presence of the coverplate  100  and the attachment of the coverplate to the long stroke module  110 . 
     A first through hole  101  is provided in the coverplate  100 . The through hole  101  is large enough such that a substrate W positioned on the substrate table WT can be imaged by a projection beam from the projection system PS without the projection beam needing to pass through the coverplate  100 . That is, the projection beam passes through the first through hole  101  from a projection system PS onto the substrate W. As can be seen from  FIG. 7 , the first through hole  101  may also be large enough to accommodate an edge of the substrate table WT. The edge of the substrate table WT may be uncovered, in use, by the substrate W. 
     Further through holes  102  are provided in the coverplate  100  for the passage of a radiation beam, such as a projection beam from the projection system PS therethough onto a respective sensor  140 . 
     Thus, the coverplate  100  surrounds the substrate W, the substrate table WT and sensors  140 . 
     Provision of through holes  101 ,  102  in the coverplate  100  avoid errors in both imaging and measurement being introduced by the projection beam having to pass through the coverplate  100 . The provision of through holes  101 ,  102  ensures that direct contact between the coverplate  100  and the short stroke module  120  or items placed or held on the short stroke module  120  may be avoided. Thereby transmission of deleterious forces from the coverplate  100  to the short stroke module  120  is also avoided. 
     The position of the short stroke module  120  may be monitored by a position measurement system. A position measurement system may include one or more sensors  150  and one or more grid plates. One of the sensor  150  and grid plates are mounted in known relation to the projection system PS. The other of the sensor  150  and grid plate is mounted on the short stroke module  120 . 
     The sensors  150  include a transmitter and receiver. A beam of radiation passes from the transmitter to the grid plate and is reflected back to the receiver. By analyzing the signal received by the receiver the position of the sensor  150  relative to the grid plate can be calculated. In this way the position of the short stroke module  120  relative to the projection system PS may be calculated. 
     In the embodiment illustrated in  FIG. 6  sensors  150  of the position measurement system are mounted on the short stroke module  120 . In the embodiment of  FIG. 6  there are four sensors  150  mounted at the corners of the short stroke module  120 . A corresponding grid plate or grid plates are mounted above the substrate table WT in fixed relationship to the projection system PS. In an alternative embodiment, grid plates may be attached to the short stroke module  120  in place of the sensors  150  and the sensors mounted above the substrate table WT in known relation to the projection system PS. In an embodiment, the coverplate  100  surrounds the sensors  150  or grid plates. In an embodiment, the sensors  150  or grid plates may be located near or at the edge of the coverplate  100 . The coverplate  100  may be shaped to receive the sensors  150  or grid plates without surrounding them. 
     As can be seen, the coverplate  100  does not cover the sensors  150 . Therefore, any beams of radiation of the position measurement system pass directly between the sensor  150  and the grid plate without passing through the coverplate  100 . 
     Gaps exist between the substrate table WT, the substrate W, sensor  140  or short stroke module  120  and the coverplate  100  or long stroke module  110 . Such gaps are large enough to accommodate the stroke of the short stroke module  120 . It may be desirable to keep such gaps as small as possible so that the gap varies in size from being virtually non existent to being the size of the maximum stroke of the short stroke module  120 . It is deleterious that immersion liquid passes through such a gap in an uncontrolled way. In one embodiment stickers may be provided to cover the gap. In this way, immersion liquid is prevented from entering the gap. The stickers are desirably flexible and/or of low stiffness (low E-modulus) so as to avoid (disturbance) force being transferred from the coverplate  100  (and long stroke module  110 ) to the short stroke module  120 . Alternatively or additionally one or more of the gaps may be treated with one of the seals illustrated in  FIGS. 9 and 10  and described below. A higher maximum elastic strain is desirable in order to achieve a sufficient lifetime. For a given range of dynamic gap size, a material with a higher maximum elastic strain allows a smaller sticker to be used. 
     In both the all wet type of immersion apparatus and the localized area type of immersion apparatus liquid may find its way over a gap such as between the substrate table WT, the substrate W, sensor  140  or short stroke module  120  and the coverplate  100  or long stroke module  110 . For example, when imaging the edge of a substrate Win the localized area immersion apparatus the liquid confinement structure may be partly over the substrate W and partly over the coverplate  100 . Additionally, when the assembly illustrated in  FIG. 6  is moved under the projection system PS such that a sensor  140  is under the projection system PS, the liquid confinement structure may move over the gap between the substrate table WT and the coverplate  100  and between the coverplate  100  and the sensor  140 . In an all wet immersion apparatus liquid will desirably cover the whole of the coverplate  100  and be collected in a gutter  180  around an edge of the coverplate  100  (illustrated in more detail in  FIGS. 7 and 8 ). The gutter  180  may surround the coverplate  100 . 
       FIG. 7  illustrates a cross section of the assembly of  FIG. 6  through line VII-VII. As can be seen, the coverplate  100  is attached at its end to the long stroke module  110 . There is no contact between the short stroke module  120  and the coverplate  100 . A through hole  101  exists in which the substrate table WT is positioned. Desirably the plane of the top surface of the cover plate  100  is within +/−40 μm of the plane of the top surface of the substrate W. This magnitude of difference in height can be dealt with if the bottom surface of the liquid supply system is 100-300 μm above the plane of the top surface of the substrate W. If this is not possible, the height of the top surface of the coverplate  100  is lower than the top surface of the substrate table WT and the substrate W (as illustrated). That is, there is a difference in the distance of the plane of the top surface of the coverplate  100  from the projection system PS to the plane of the top surface of the substrate W from the projection system PS. This difference in distance may be up to 100 or even up to 300 μm. This helps in ensuring the desired direction of fluid flow. The top surface of the sensor  140  may be in the same plane as the top surface of the substrate W. 
     A positioner is located between the long stroke module  110  and short stroke module  120  which effectively decouples the short stroke module  120  from the long stroke module  110 . The gap between the edge of the coverplate  100  defining the through hole  101  and the substrate table WT is large enough to accommodate the stroke of the short stroke module  120 . The gap may be of the order of 1 mm which is a typical stroke of a short stroke module  120 . Also illustrated in  FIG. 7  is a liquid supply system  12 . As can be seen, the liquid supply system is of the all wet type. Therefore, the liquid is unconfined by the liquid supply system  12 . A film of liquid  210  covers the top surface of the substrate W and the coverplate  100  irrespective of whether or not they are positioned under the projection system PS and/or liquid supply system  12 . 
     When the film of liquid  210  reaches an edge of the coverplate  100  it is collected by a gutter  180 . The edge of the coverplate  100  is curved (for example in accordance with what is described in U.S. 60/996,737 and U.S. 61/176,802. The edge of the coverplate  100  is curved downwards away from projection system PS. This helps in encouraging liquid to flow off the edge of the coverplate as well as maintaining the edge of the coverplate wet. The gutter may be continuous or non-continuous around the outer edge of the coverplate  100 . As illustrated, the coverplate  100  may include a projection  182  radially outwardly of the gutter  180  with respect to the substrate table WT thereby to prevent any liquid spilling out of the gutter  180  before being removed from the gutter. The curve of the edge of the coverplate  100  extends all the way into the gutter  180 , from where the immersion liquid is removed. Extending the curve of the edge of the coverplate  100  into the gutter  180  helps to ensure that liquid flows efficiently to the gutter  180  from the coverplate  100 . For example that no dripping or splashing of immersion liquid occurs as it moves into the gutter  180 . Immersion liquid may be removed from the bottom of the gutter  180  through one or more openings. For example, immersion liquid may be sucked through the openings which are attached to an underpressure source. 
       FIG. 8  is a cross-section of the assembly illustrated in  FIG. 6  along lines VIII-VIII. The sensors  150  are enclosed in a through hole in the coverplate  100  in the same way as the substrate table WT and sensors  140  mounted on the short stroke module  120 . It may be necessary to include a groove or gutter in the coverplate surrounding the sensors  150  so as to avoid liquid flowing over or splashing onto the sensors  150 . In an alternative embodiment the coverplate  100  is shortened at the corners so as not to enclose the sensors  150 . The gutter  180  runs in front of the sensors  150 . 
     So that the coverplate  100  is mechanically decoupled from the short stroke module  120 , the substrate table WT, the substrate W and any sensors  140  there are physical gaps between the coverplate  100  and those objects. The gaps allow the short stroke module  120  to move relative to the coverplate  100  in the plane of the coverplate  100 . As described above, those gaps can, in some instances, be covered with stickers. In other instances that may not be acceptable. In those instances a seal such as that illustrated in  FIG. 9  or  10  could be employed. Additionally, a seal such as that illustrated in  FIGS. 9 and 10  can be employed in other circumstances where liquid could seep into a gap between a first object and a second object. For example, in the case where the immersion liquid runs over the top surface of the short stroke module  120  or a coverplate  100  attached to the short stroke module  120 , any gutters for collecting immersion liquid from an edge of the short stroke module  120  or coverplate  100  may be carried by the long stroke module  110 . In this case a gap will exist between the short stroke module  120  or coverplate  100  and the surfaces forming the gutter. 
     As illustrated in  FIGS. 9 and 10  which are cross-sectional views of two embodiments of seal, a gap  250  exists between a surface of the long stroke module  110  and the short stroke module  120 . Thus, a gap  250  exists between a first object and a second object on whose top surfaces immersion liquid is provided (in the form of a film  210 ). The gap  250  could alternatively be any of the gaps illustrated in  FIGS. 7 and 8  or described above or any other gap. 
     In the embodiment of  FIG. 9  immersion liquid is deliberately allowed to flow through the gap  250 . However, the flow is controlled positionally so that any liquid which does pass through the gap  250  can be collected in a sub-gutter  252  positioned under the gap  250 . A surface  255  of the long stroke module  110  (including the coverplate  100  or gutter  180 ) and a surface  256  of the short stroke module  120  define the gap. These surfaces  255 ,  256  face each other and the immersion liquid has an advancing contact angle with them of less than 30°, desirably less than 25° and more desirably less than 20°. That is, the surfaces defining the gap  250  are lyophilic to the immersion liquid. 
     One of the surfaces  255 ,  256  defining the gap  250  extends in the form of an extending surface  257  to the sub-gutter  252 . There is no abrupt change of angle between the surface  255  defining the gap and the extending surface  257 . That is, the surface is smooth. The surface  257  which extends to the sub-gutter  252  desirably has the same characteristics as the surfaces  255 ,  256  defining the gap  250 . That is, immersion liquid has an advancing contact angle with the extending surface  257  extending to the sub-gutter  252  of less than 30°, desirably less than 25° and more desirably less than 20°. In this way liquid which passes through the gap  250  runs down the extending surface  257  which extends to the sub-gutter  252  and into the sub-gutter  252  where the liquid may be removed or may be passed to the main gutter  180 . The sub-gutter  252  is desirably mounted on the long stroke module  110 , but this is not necessarily the case. 
     The surface  256  which defines the gap  250  which is opposite to the surface  255  which extends via extending surface  257  to the sub-gutter  252  has an edge  258  defined in it at the end of the gap  250 . That is, the surface  256  has an abrupt change of angle. For example, the plane of the surface  256  defining the gap is at an angle of less than 90° relative to the plane of an adjacent surface  259  on the other side of the edge  258 . That angle is illustrated as being 90° in  FIG. 9  but may be less, desirably as low as possible, for example less than 10°, less than 50°, less than 60°, less than 70° or less than 80°; i.e. the angle may be in the range of less than 90°, preferably less than 50° or in the range of 10° to 90°. 
     In one embodiment the immersion liquid has an advancing contact angle of greater than 90°, desirably greater than 100° and more desirably greater than 110° with the surface  259  on the other side of the edge  258  and adjacent to the surface  256  defining the gap. This is beneficial because the surface  259  is thereby lyophobic and encourages the liquid to detach from the short stroke module  120  at the end of the gap  250 . 
     The seal of  FIG. 10  is the same as that of  FIG. 9  except as described below. Because the width of the gap is of the order of 1 mm (e.g. the inaccuracy of the long stroke module  110  relative to the short stroke module  120 ) it is possible to generate a large surface tension force on liquid in the gap acting on the liquid such that it remains in the gap. This is possible if surfaces  255 ,  256  defining the gap  250  both have an edge  258  and/or a surface  259  with which the immersion liquid has a high advancing contact angle, of for example greater than 90°, desirably greater than 100°, more desirably greater than 130° and most desirably greater than 150°. Immersion liquid in such a seal can withstand a pressure of about 1 mBar. That is, the surface tension of the meniscus bridging the edges  258  withstands this pressure which is equal to about 10 mm height of liquid (water). As shown in  FIG. 10  pressure may be generated in the seal, and the seal may prevent liquid from flowing through the gap  250 . However, it may be that forces are generated on the immersion liquid  210  which overcome the seal (for example in the case of a localized liquid supply system a gas knife directed downwards passing over the gap  250 ) and so a sub-gutter  252  is provided to catch any drops. As in the embodiment of  FIG. 9 , the sub-gutter  252  is connected to the long stroke module  110  thereby avoiding introducing disturbance forces into the short stroke module  120 . 
     In an all wet immersion apparatus de-wetting of the surface surrounding the substrate W is deleterious. It has been found that de-wetting of a coverplate  100  is most likely to occur at its edges where the immersion liquid leaves the coverplate  100 . In particular, a leading edge of the coverplate  100  has been found to be prone to de-wetting. This may be due to forces being applied to the immersion liquid by movement of the coverplate  100 . The inertia of the immersion liquid means that it tends to be left behind when the coverplate  100  moves. The film of liquid at the leading edge becomes thinner and the film breaks up into droplets at a thickness of under about 30 μm.  FIGS. 11 and 12  show two measures which can be taken to address this issue. This is done in one embodiment by preventing the film thickness at the leading edge from falling below about 30 μm, for example. 
     In  FIG. 11 , an opening  300  is provided adjacent an edge of the surface over which immersion liquid flows (which may be lyophilic to the immersion liquid). As illustrated in  FIG. 11  this is a surface of the long stroke module  110  close to the gutter  180 . However, the opening  300  may also be provided adjacent an edge of a coverplate  100 , or between the surface of the long stroke module  110  (or coverplate  100 ) and the edge of the short stroke module  120  surface adjacent the areas in which the sensors  140  are positioned. The opening  300  may be a single slit or may be a plurality of discrete openings such as holes or slits. 
     The opening  300  is attached to a liquid source under the control of a controller  310 . 
     Liquid may be provided continually through opening  300  to the surface over which immersion liquid flows. The controller  300  controls supply of fluid through the opening  300  to start supplying or to increase the supply of liquid through opening  300  at points around the outer edge of the surface over which immersion liquid flows which are a leading edge during movement of the surface. Conversely, the controller  310  may reduce supply or prevent supply of liquid through the opening  300  to a trailing edge of the outer edge of the surface over which immersion liquid flows. Therefore, it can be seen that liquid may be provided through the opening  300  in varying amounts around the circumference of the surface over which immersion liquid flows (e.g. a coverplate  100 ). In this way, liquid is provided to the surface at the area where de-wetting is most likely to occur and at a time when de-wetting is most likely to occur. The provision of extra fluid can help to ensure that de-wetting does not occur. 
       FIG. 12  shows a further embodiment which may be used in conjunction with the embodiment of  FIGS. 6 ,  7  and  8 . In this embodiment, the coverplate  100  moves independently of the short stroke module  120 . Therefore, the short stroke module  120  is moved under control of controller  310  in the normal way (for example such that the top surface of the substrate W is substantially perpendicular to the projection beam PB or optical axis of the projection system PS). However, the angle of the coverplate  100  may be tilted from parallel to the top surface of the substrate to encourage flow of liquid in a particular direction thereby avoiding de-wetting. Thus, the coverplate, in particular by being attached to the long stroke module  110  is tiltable independently of the substrate table WT which is mounted on the short stroke module  120 . The controller  310  can control the long stroke module  110  thereby to tilt the coverplate  100  relative to the optical axis of the projection system PS during movement of the substrate table WT and coverplate  100 . If the coverplate  100  is tilted such that its leading edge is lower than its trailing edge liquid may flow towards the leading edge thereby reducing the risk of de-wetting at the leading edge. It is at the leading edge where de-wetting is most likely to occur. 
     Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers. 
     The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm). The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive and reflective optical components. 
     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 embodiments of the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein. Further, the machine readable instruction may be embodied in two or more computer programs. The two or more computer programs may be stored on one or more different memories and/or data storage media. 
     The controllers described herein may each or in combination be operable when the one or more computer programs are read by one or more computer processors located within at least one component of the lithographic apparatus. The controllers may each or in combination have any suitable configuration for receiving, processing, and sending signals. One or more processors are configured to communicate with the at least one of the controllers. For example, each controller may include one or more processors for executing the computer programs that include machine-readable instructions for the methods described above. The controllers may include data storage medium for storing such computer programs, and/or hardware to receive such medium. So the controller(s) may operate according the machine readable instructions of one or more computer programs. 
     One or more embodiments of the invention may be applied to any immersion lithography apparatus, in particular, but not exclusively, those types mentioned above and whether the immersion liquid is provided in the form of a bath, only on a localized surface area of the substrate, or is unconfined. In an unconfined arrangement, the immersion liquid may flow over the surface of the substrate and/or substrate table so that substantially the entire uncovered surface of the substrate table and/or substrate is wetted. In such an unconfined immersion system, the liquid supply system may not confine the immersion fluid or it may provide a proportion of immersion liquid confinement, but not substantially complete confinement of the immersion liquid. 
     A liquid supply system as contemplated herein should be broadly construed. In certain embodiments, it may be a mechanism or combination of structures that provides a liquid to a space between the projection system and the substrate and/or substrate table. It may include a combination of one or more structures, one or more fluid openings including one or more liquid openings, one or more gas openings or one or more openings for two phase flow. The openings may each be an inlet into the immersion space (or an outlet from a fluid handling structure) or an outlet out of the immersion space (or an inlet into the fluid handling structure). In an embodiment, a surface of the space may be a portion of the substrate and/or substrate table, or a surface of the space may completely cover a surface of the substrate and/or substrate table, or the space may envelop the substrate and/or substrate table. The liquid supply system may optionally further include one or more elements to control the position, quantity, quality, shape, flow rate or any other features of the liquid. 
     In an embodiment there is provided an immersion lithographic apparatus comprising first and second objects which are spaced apart with a gap therebetween and on whose top surfaces immersion liquid is provided. The lithographic apparatus is further provided with a gutter positioned under the gap and configured, in use, to collect any immersion liquid which passes through the gap. An advancing contact angle of immersion liquid with surfaces of the first and second objects defining the gap is less than 30°. 
     One of the surfaces of the first and second objects defining the gap may have an edge at the end of the gap. A plane of the one surface defining the gap may be at an angle of 90° or less relative to the plane of an adjacent surface on the other side of the edge. 
     The immersion liquid may have an advancing contact angle greater than 90° with the surface on the other side of the edge and adjacent to the surface defining the gap. 
     The other of the surfaces of the first and second objects defining the gap may have an edge at the end of the gap. The plane of the other surface defining the gap may be at an angle of 90° or less relative to the plane of an adjacent surface on the other side of the edge. The immersion liquid may have an advancing contact angle greater than 90° with the surface on the other side of the edge and adjacent to the other surface defining the gap. 
     One of the surfaces of the first and second objects defining the gap may extend to the gutter. The surface which may extend to the gutter may be smooth and the immersion liquid may have an advancing contact angle with it of less than 30°. 
     The first object may comprise a coverplate. The coverplate may be attached to a long stroke module of a positioner configured to move a substrate table relative to a projection system. The positioner may further comprise a short stroke module configured to perform fine positioning movements and the substrate table may be held on the short stroke module, wherein the short stroke module is positioned on the long stroke module which is configured to perform coarse positioning movements. 
     The coverplate may be mechanically decoupled from the substrate table and/or the short stroke module. 
     The second object may comprise the substrate table, the short stroke module, the substrate, or a sensor. 
     The immersion liquid may have an advancing contact angle of less than 25° with the surfaces of the first and second object defining the gap. 
     In an embodiment, there is provided an immersion lithographic apparatus, comprising a substrate table and a coverplate. The substrate table is configured to hold a substrate. The coverplate is tiltable independently of the substrate table. 
     The apparatus may further comprise a controller configured to control the tilt of the coverplate relative to an optical axis of a projection system during movement of the substrate table and coverplate. 
     The controller may be adapted to tilt the coverplate so that a leading edge of the coverplate is lower than a trailing edge of the coverplate as it moves. 
     The controller may be configured to control the tilt of the coverplate independently of the tilt relative to the optical axis of the substrate table and/or substrate. 
     The substrate table may be held on a short stroke module configured to perform fine positioning movements. The short stroke module may be positioned on a long stroke module configured to perform coarse positioning movements. The coverplate may be mounted to the long stroke module. Independent movement of the coverplate relative to the substrate table may be achieved by movement of the short stroke module relative to the long stroke module. 
     The apparatus may be an all wet immersion lithographic apparatus. 
     In an embodiment, there is provided an immersion lithographic apparatus comprising a substrate table, an opening and a controller. The substrate table is configured to hold a substrate. The opening is for the supply of liquid at the edge of a surface over which immersion liquid flows. The controller is configured to supply or increase the supply of liquid to a leading edge of the surface through the opening during movement of the surface. 
     The controller may be configured to reduce supply or prevent supply of liquid through the opening to a trailing edge of the surface. 
     In an embodiment there is provided a device manufacturing method comprising holding a substrate on a substrate table, and tilting a coverplate independently of the substrate table. 
     The method may further comprise controlling the tilt of the coverplate relative to an optical axis of a projection system during movement of the substrate table and coverplate. 
     The method may further comprise tilting the coverplate so that a leading edge of the coverplate is lower than a trailing edge of the coverplate as it moves. 
     The method may further comprise controlling the tilt of the coverplate independently of the tilt relative to the optical axis of the substrate table and/or substrate. 
     The method may further comprise positioning the substrate table relative to a projection system using a short stroke module for fine positioning, the short stroke module holding the substrate table. 
     The method may further comprise supporting the short stroke module on a long stroke module for coarse positioning movements. 
     The method may further comprise covering at least a part of a top surface of the short stroke module using the coverplate mounted to the long stroke module. 
     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.