Abstract:
A substrate processing apparatus includes a device for partially sealing a gap between a final optical element ( 22 ) of a projection lens ( 14 ) and an immersion nozzle ( 20 ). In one embodiment, the apparatus includes a table configured to support a substrate ( 16 ); a patterning element defining a pattern ( 12 ); a projection system configured to project the pattern onto the substrate ( 16 ), the projection system having a last optical element ( 22 ); a gap between the substrate and the last optical element; an immersion element configured to maintain immersion fluid in the gap; and a first seal ( 102 ) positioned between the projection system and the immersion element. The first seal ( 104 ) is configured to substantially prevent immersion fluid from exiting the space between the projection system and the immersion element.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application is based on and claims the benefit of U.S. Provisional Patent Application No. 60/646,154, filed Jan. 21, 2005, the entire disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to a system and a method for providing fluid for immersion lithography and, more particularly, to offset partial ring seals for partial sealing of a gap between the optical element and the immersion nozzle that provides fluid delivery and recovery for immersion lithography.  
         [0003]     An exposure apparatus is one type of precision assembly that is commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that retains a reticle, an optical assembly, a wafer stage assembly that retains a semiconductor wafer, a measurement system, and a control system. The resist coated wafer is placed in the path of the radiation emanating from a patterned mask and exposed by the radiation. When the resist is developed, the mask pattern is transferred onto the wafer. In microscopy, extreme ultraviolet (EUV) radiation is transmitted through a thin specimen to a resist covered plate. When the resist is developed, a topographic shape relating to the specimen structure is left.  
         [0004]     Immersion lithography is a technique which can enhance the resolution of projection lithography by permitting exposures with numerical aperture (NA) greater than one, which is the theoretical maximum for conventional “dry” systems. By filling the space between the final optical element and the resist-coated target (i.e., wafer), immersion lithography permits exposure with light that would otherwise be totally internally reflected at an optic-air interface. Numerical apertures as high as the index of the immersion liquid (or of the resist or lens material, whichever is least) are possible. Liquid immersion also increases the wafer depth of focus, i.e., the tolerable error in the vertical position of the wafer, by the index of the immersion liquid compared to a dry system with the same numerical aperture. Immersion lithography thus has the potential to provide resolution enhancement equivalent to the shift from 248 to 193 nm. Unlike a shift in the exposure wavelength, however, the adoption of immersion would not require the development of new light sources, optical materials, or coatings, and should allow the use of the same or similar resists as conventional lithography at the same wavelength. In an immersion system where only the final optical element and its housing and the wafer (and perhaps the stage as well) are in contact with the immersion fluid, much of the technology and design developed for conventional tools in areas such as contamination control, carry over directly to immersion lithography.  
         [0005]      FIG. 1  shows an immersion lithography system  10  including a reticle stage  12  on which a reticle is supported, a projection lens  14 , and a wafer  16  supported on a wafer stage  18 . An immersion apparatus  20 , which is sometimes referred to herein as a showerhead or a nozzle, is disposed around the final optical element  22  of the projection lens  14  to provide and recover a fluid, which may be a liquid such as water or a gas, between the final optical element  22  and the wafer  16 . In specific embodiments, the immersion lithography system  10  includes the reticle and the wafer  16  that are moved synchronously in respective scanning directions during a scanning exposure. One of the challenges of immersion lithography is to design a system for delivery and recovery of a fluid, such as water, between the final optical element and the wafer, so as to provide a stable condition for immersion lithography.  
         [0006]      FIG. 2  shows a partial cross-sectional view of a portion of the immersion lithography system  10  illustrating the relationship between the final optical element  22  and the immersion apparatus  20 . To avoid transmitting vibration or other disturbances from the immersion apparatus  20  to the projection lens  14 , a nozzle gap or spacing  24  is provided between the immersion apparatus  20  and the final optical element  22 . During delivery and recovery of the fluid by the immersion apparatus  20  between the final optical element  22  and the wafer  16 , and particularly during movement of the wafer stage  18  with respect to the projection lens  14  during scanning, the immersion fluid may escape through the gap  24 .  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     Embodiments of the present invention are directed to a device for partially sealing the nozzle gap between the final optical element of the projection lens and the immersion nozzle. If the nozzle gap were completely sealed, air could be trapped in the nozzle gap and form air bubbles that flow into the exposure region during scanning exposure, which is highly undesirable. A partial sealing of the nozzle gap can impede the flow of the fluid out of the immersion fluid region via the nozzle gap while permitting air to flow out from the nozzle gap, thereby avoiding formation of trapped air bubbles.  
         [0008]     An aspect of the present invention is directed to an apparatus, comprising a table configured to support a substrate; a patterning element defining a pattern; a projection system configured to project the pattern onto the substrate, the projection system having a last optical element; a gap between the substrate and the last optical element; an immersion element configured to maintain immersion fluid in the gap; and a first seal positioned between the projection system and the immersion element. The first seal is configured to substantially prevent immersion fluid from exiting the space between the projection system and the immersion element.  
         [0009]     In some embodiments, the first seal contains an aperture to allow air to exit the gap. The first seal may be made from a hydrophobic material. The first seal may be circular in shape. A second seal may be positioned between the projection system and the immersion element, the first seal and the second seal being configured to substantially prevent immersion fluid from exiting the space between the projection system and the immersion element. The second seal may have an aperture to allow air to exit the gap. The aperture of the first seal and the aperture of the second seal are offset circumferentially. The aperture of the first seal and the aperture of the second seal may be generally aligned with a scan direction of the substrate. The first seal may be flexible. The first seal may comprise a spiral shaped seal extending more than one revolution around the gap.  
         [0010]     In specific embodiments, the immersion element comprises an inner part including a lens opening to accommodate a portion of the lens and position the lens apart from the substrate separated by the space to receive a fluid in the space between the lens and the substrate; and an outer part disposed around the inner part, the outer part including a porous member fluidicly coupled with the space and with a fluid recovery outlet to draw fluid from the space via the porous member to the fluid recovery outlet. The inner part is spaced from the outer part by an intermediate spacing. The inner part includes an inner cavity forming a part of the spacing between the lens and the substrate, and the inner part includes apertures disposed above the inner cavity for at least one of introducing fluid into and drawing fluid from the inner cavity. The inner part includes apertures disposed on opposite sides of the lens opening for introducing fluid into the inner cavity. The inner part includes a pair of buffer slots disposed on opposite sides of the lens opening. The inner part includes purge holes and wherein each of the pair of buffer slots is fluidicly coupled to at least one of the purge holes.  
         [0011]     In some embodiments, the immersion element comprises a seal element disposed between the last optical element and the substrate, the seal element including a gas seal formed between the seal element and the substrate. The gas seal is disposed in a gas seal region that surrounds a portion of the substrate exposed to the immersion fluid to maintain immersion fluid in the gap. The immersion element comprises a gas inlet to direct a gas into the gas seal region and a gas outlet to provide suction from the gas seal region so as to form the gas seal. The immersion element comprises a gas inlet to direct a gas into the gas seal region and two gas outlets to provide suction from the gas seal region so as to form the gas seal, the two gas outlets being disposed on opposite sides of the gas inlet. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a simplified elevational view schematically illustrating an immersion lithography system.  
         [0013]      FIG. 2  is a simplified partial cross-sectional view of a portion of the immersion lithography system of  FIG. 1 .  
         [0014]      FIG. 3  is a perspective view of a nozzle for fluid delivery and recovery for immersion lithography according to one embodiment of the present invention.  
         [0015]      FIG. 4  is a simplified cross-sectional view of the nozzle of  FIG. 3 .  
         [0016]      FIG. 5  is a cross-sectional view of the inner part of the nozzle of  FIG. 3 .  
         [0017]      FIG. 6  is a cross-sectional view of a pair of partial ring seals for partially sealing the circumferential gap between an optical element of a projection lens and an immersion nozzle according to an embodiment of the present invention.  
         [0018]      FIG. 7  is a plan view of the pair of partial ring seals of  FIG. 6 .  
         [0019]      FIG. 8  shows a spiral shaped seal which extends more than one revolution around the circumferential gap according to another embodiment of the present invention.  
         [0020]      FIG. 9  is a schematic view of a liquid reservoir for an immersion element according to another embodiment of the invention.  
         [0021]      FIG. 10  is an enlarged view of apart of the liquid reservoir of the immersion element of  FIG. 9 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0000]     Immersion Nozzle  
         [0022]      FIGS. 3 and 4  show an example of an apparatus or nozzle  20  for delivery and recovery of the fluid between the final optical element  22  and the wafer  16  for immersion lithography.  FIG. 3  shows the bottom perspective view of the nozzle  20 , which includes an outer part  30  and an inner part  32 . The inner part  32  defines an inner cavity  34  to receive the fluid between the final optical element  22  and the wafer  16 . The inner part  32  includes apertures  38  for fluid flow into and out of the inner cavity  34 . As seen in  FIG. 3 , there are apertures  38  disposed on both sides of the final optical element  22 . The inner part  32  has a flat portion  33  surrounding the inner cavity  34 . The flat portion  33  is substantially parallel to the wafer  16 . The distance D 1  between the end surface of the final optical element  22  and the wafer  16  is greater than the distance D 2  between the flat portion  33  and the wafer  16 . The distance D 1  could be 1.0-5.0 mm, and the distance D 2  could be 0.5-2.0 mm. In another embodiment, the distance D 1  is substantially equal to the distance D 2 . The inner part  32  further includes a pair of buffers or buffer slots  40  with purge holes  42 . The buffers  40  are arranged at or near the flat portion  33 . The buffers  40  are disposed on opposite sides of the final optical element  22 . A cross-sectional view of the inner part  32  in the direction of scan  44  is illustrated in  FIG. 5 .  
         [0023]     The outer part  30  is spaced from the inner part  32  by an intermediate spacing or groove  48 , which may be referred to as an atmospheric groove. The outer part  30  includes one or more fluid recovery openings  50  disposed on opposite sides of the final optical element  22 . A porous member  51  is disposed in a slot  53  which extends around the inner part  32  and fluidicly communicates with the pair of fluid recovery openings  50 . The porous member  51  may be a mesh or may be formed of a porous material having holes typically in the size range of about 50-200 microns. For example, the porous member  51  may be a wire mesh including woven pieces or layers of material made of metal, plastic, or the like, a porous metal, a porous glass, a porous plastic, a porous ceramic, or a sheet of material having chemically etched holes (e.g., by photo-etching). The outer part  30  further includes a fluid buffer outlet  56  and a fluid recovery outlet  58 . In the embodiment shown in  FIG. 4 , the inner part  32  does not contact the final optical element  22 , but is spaced from the final optical element  22  by a nozzle gap  24 . The gap prevents nozzle vibrations from being transmitted from the nozzle  20  to the final optical element  22 .  
         [0024]     One feature of the nozzle  20  is that it is made in two pieces, namely, the outer part  30  and the inner part  32 . The inner part  32  keeps the fluid between the lens and the wafer surface, and the outer part  30  is mainly provided for fluid recovery. Vibration might be introduced during fluid recovery from the outer part  30  through the porous member  51  to the other components of the lithography system, including the inner part  32  which may be used to direct an autofocus beam to the wafer  16 . A damping material can be mounted between the outer part  30  and the mounting piece to which the outer part  30  is mounted to minimize the transmission of vibration from the outer part  30 . In addition, the outer part  30  that includes the porous member may be prone to contamination and thus needs to be replaced for maintenance. Making the outer part  30  as a separate part facilitates easier maintenance. It can also minimize readjustment and recalibration time after replacement of the outer part as opposed to replacing the entire nozzle  20 . Manufacturability of the nozzle  20  can also be improved if the nozzle  20  is made in two separate parts. It is understood that the nozzle  20  may be made of a single piece in alternative embodiments.  
         [0025]     Another feature of the nozzle  20  is the atmospheric groove  48  between the inner part  32  and the outer part  30 . This atmospheric groove  48  functions as a breaking edge to prevent fluid in the inner part  30  from being drawn out by the porous member  51  on the outer part  30  if the fluid recovery rate is faster than the fluid supply rate. In the situation when there is no breaking edge, a balance between the fluid recovery rate and the fluid supply rate has to be maintained so that fluid can be kept within the inner part  32  at all times during scanning. Having the atmospheric groove  48  allows the recovery rate to be set at a maximum to minimize fluid leakage out of the outer part  30  during scanning. The atmospheric groove  48  also acts as a buffer for fluid to go in and out during scanning, minimizing water supply and recovery requirements.  
         [0026]     In the process of immersion lithography, a fluid is to be filled between the projection lens  14  and the wafer  16  from a dry state and, at other times, the fluid is to be recovered. For example, in the beginning of exposure of a new wafer, the fluid is to completely fill the inner cavity  34  of the inner part  32  before starting exposure. During this process, ideally no air bubbles can exist between the projection lens  14  and wafer  16  or other optical paths such as the auto focus beam. The fluid supply in the inner cavity of the inner part  32  is designed to be at the highest point in the cavity (via apertures  38 ) so that the fluid is filled from top down, allowing air bubbles to be pushed out of the inner cavity during the filling process. The fluid desirably is initially supplied from one side in this embodiment (the set of apertures  38  on one side), so that the fluid is filled from one side to the other, again allowing air bubbles to be pushed out to avoid trapping air therein. Other arrangements are also possible, as long as the fluid is being filled from the inside out.  
         [0027]     On occasion, the fluid has to be fully recovered from the inner cavity of the inner part  32 . In  FIG. 5 , there are small holes  42  in each of the buffers  40  in the inner cavity. These holes  42  are provided for fast fluid recovery or fluid purge when the fluid has to be fully recovered. Sucking the fluid out from these holes  42  using high vacuum with the combination of some movement in the wafer stage  18  allows all the fluid to be recovered within a reasonable time.  
         [0028]     The inner part  32  has two groups or rows of holes  38  for supplying or recovering the fluid. Each row can be independently controlled to either supply or recover the fluid. In the case where both rows are chosen for fluid supply, all the fluid is recovered through the porous member  51  in the outer part  30 . Since both rows are supplying fluid, a pressure can build up in the inner cavity causing deformation of the final optical element  22  of the projection lens  14  or the wafer  16  or both. The fluid flow across the final optical element  22  may also be limited, and thus the temperature of the fluid between the final optical element  22  and the wafer  16  may eventually rise, causing adverse effect. On the other hand, if one row is chosen for supply and the other for recovery, a fluid flow will be driven across the final optical element  22 , minimizing temperature rise. It can also reduce the pressure otherwise created by supplying fluid from both rows. In this case, less fluid needs to be recovered through the porous member  51 , lowering the fluid recovery requirement in the porous member. In other nozzle configurations, multiple fluid supplies and recoveries may be provided so as to optimize the performance.  
         [0029]     During scanning motion of the wafer stage  18  (in the direction of scan  44  in  FIG. 3 ), the fluid may be dragged in and out of the inner cavity of the inner part  32 . When the fluid is dragged out, it is recovered through the porous member  51  in the outer part  30 . When the wafer stage  18  is moved in the opposite direction, air may be dragged into the inner cavity of the inner part  32 . During this time, the fluid in the buffers  40 , as well as the fluid supplied from within the inner cavity, helps to refill the fluid that is dragged along the scanning direction, preventing air from getting into the inner cavity. The buffers  40  and the porous member  51  work together to minimize fluid leaking out from the outer part  30 , and air dragging into the inner cavity of the inner part  32  during scanning motion of the wafer stage  18 .  
         [0000]     Partial Ring Seal  
         [0030]      FIG. 6  shows a cross-sectional view of a pair of partial ring seals  102 ,  104  for partially sealing the circumferential gap  106  between an optical element  108  of a projection lens and an immersion nozzle  110  for providing a fluid to an immersion region  112  between the optical element  108  and a wafer  114  for immersion lithography. Although the circumferential gap  106  in  FIG. 6  is conical due to the slanted walls of the optical element  108  and nozzle  110 , the shape of the circumferential gap may be different in other embodiments.  
         [0031]     As seen in  FIG. 7 , the first partial ring seal  102  is disposed in the circumferential gap  106  to be in contact with the immersion nozzle  110  and the optical element  108 . The first partial ring seal  102  extends generally circumferentially to cover a first sealed portion of the circumferential gap  106  and leaves a first seal opening  122  between the immersion nozzle  110  and the optical element  108 . The first seal opening  122  is smaller in area than the first sealed portion of the circumferential gap  106 . The second partial ring seal  104  is disposed in the circumferential gap  106  to be in contact with the immersion nozzle  110  and the optical element  108 . The second partial ring seal  104  is spaced from the first partial ring seal  102  generally in the axial direction perpendicular to the surface of the wafer  114 . The second partial ring seal  104  extends generally circumferentially to cover a second sealed portion of the circumferential gap  106  and leaves a second seal opening  124  between the immersion nozzle  110  and the optical element  108 . The second seal opening  124  is smaller in area than the second sealed portion of the circumferential gap  106 . Preferably, the first seal opening  122  is substantially smaller in area than the first sealed portion of the circumferential gap  106 , and the second seal opening  124  is substantially smaller in area than the second sealed portion of the circumferential gap  106  (for example, less than about 10% in area). The first seal opening  122  and the second seal opening  124  are offset circumferentially with respect to each other. In  FIG. 7 , they are offset circumferentially with respect to each other by about 180° (which is the maximum offset). In that case, the first seal opening  122  and the second seal opening  124  may be generally aligned with a scan direction  200  of the wafer  114  during lithography. In other embodiments, the offset angle may vary (e.g., 150°, 90°, 60°, etc.), but is preferably greater than about 30°.  
         [0032]     In  FIG. 7 , the first partial ring seal  102  includes two ends which are separated and spaced from one another to form the first seal opening  122 , and the second partial ring seal  104  includes two ends which are separated and spaced from one another to form the second seal opening  124 . For example, the two ends of the first partial ring seal  102  are separated from one another by about 0.5-3 mm, and the two ends of the second partial ring seal  104  are separated from one another by about 0.5-3 mm. In alternative embodiments, the two ends of each partial ring seal may be connected by a thin portion and still leave a seal opening for fluid to pass therethrough.  
         [0033]     In the embodiment of  FIGS. 6 and 7 , the first partial ring seal  102  and the second partial ring seal  104  are generally circular and configured to be spaced from each other generally in the axial direction. The spacing between the two partial ring seals  102 ,  104  may be uniform or nonuniform.  
         [0034]     The first partial ring seal  102  and the second partial ring seal  104  are desirably flexible to form deformable sealing members. The first partial ring seal  102  and the second partial ring seal  104  may each comprise a foam material or a soft plastic material. A typical soft plastic material has a durometer of about 40 or lower. The first partial ring seal  102  and the second partial ring seal  104  each desirably include a surface or a coating material which is hydrophobic to a fluid to be provided to the immersion region  112  between the optical element  108  and the wafer  114  for immersion lithography. The fluid tends to bead up in the presence of the hydrophobic coating, having a contact angle of greater than about 90°. For water as the immersion fluid or other fluids having comparable characteristics, PolyTetraFluoroEthylene (PTFE) is a suitable hydrophobic coating material. The material of the sealing device should be clean room compatible.  
         [0035]     In the embodiment shown in  FIGS. 6 and 7 , the partial sealing device comprising the two partial ring seals  102 ,  104  provides a partially blocked flow path in the circumferential gap  106  between the immersion region  112  and an external region which is external to the immersion region  112  and the circumferential gap  106 . The partially blocked flow path is substantially longer than an unblocked flow path between the immersion region  112  and the external region without the partial sealing device. The flow path is longest when the first seal opening  122  and the second seal opening  124  are circumferentially offset by about 180°. Of course, additional partial ring seals can be added to further lengthen the flow path.  
         [0036]     Other partial sealing devices may also produce a substantially longer flow path between the immersion region  112  and the external region. For example,  FIG. 8  shows a spiral shaped seal  130  which extends more than one revolution around the circumferential gap  106 . The higher the number of revolutions the spiral shaped seal extends in the circumferential gap  106 , the longer the flow path. More than one spiral shaped seal  130  may be used.  
       ANOTHER EMBODIMENT  
       [0037]      FIG. 9  shows another embodiment of the immersion lithography apparatus employing an air curtain or gas seal. A liquid reservoir is disposed between the projection system PL and the substrate W. The liquid reservoir is filled with a liquid  211 . The reservoir  10  forms a contactless seal to the substrate around the image field of the projection system so that liquid is confined to fill a space between the substrate surface and a final element of the projection system. The reservoir is formed by a seal member  212  positioned below and surrounding the final element of the projection system PL. Liquid is brought into the space below the projection system and within the seal member  212 . The seal member  212  preferably extends a little above the final element of the projection system and the liquid level rises above the final element so that a buffer of liquid is provided. The seal member  212  has an inner periphery that at the upper end closely conforms to the step of the projection system or the final element thereof and may be round, for example. At the bottom, the inner periphery closely conforms to the shape of the image field, for example, rectangular, or other shapes.  
         [0038]     The liquid is confined in the reservoir by a gas seal  216  between the bottom of the seal member  212  and the surface of the substrate W. The gas seal is formed by gas provided under pressure via an inlet  215  to the gap between the seal member  212  and the substrate W and extracted via a first outlet  214 . The overpressure on the gas inlet  215 , vacuum level on the first outlet  214 , and geometry of the gap are arranged so that there is a high-velocity gas flow inwards that confines the liquid.  
         [0039]     As seen in  FIG. 10 , the gas seal is formed by two annular grooves  218 ,  219  which are connected to the first inlet  215  and the first outlet  214 , respectively, by a series of small conduits spaced around the grooves. A large annular hollow in the seal member  212  may be provided in each of the inlet  215  and the outlet  214  to form a manifold. The gas seal  216  may also be effective to support the seal member  212  by behaving as a gas bearing.  
         [0040]     A gap G 1 , on the outer side of the gas inlet  215 , may be small and long so as to provide resistance to air flow outwards. A gap G 2 , at the radius of the inlet  215 , is a little larger to ensure a sufficient distribution of gas around the seal member  212 , the inlet  215  being formed by a number of small holes around the seal member  212 . A gap G 3  is chosen to control the gas flow through the seal member  212 . A gap G 4  is larger to provide a good distribution of vacuum, the outlet  214  being formed of a number of small holes in the same manner as the inlet  215 . A gap G 5  is small to prevent gas/oxygen diffusion into the liquid in the space, to prevent a large volume of liquid entering and disturbing the vacuum and to ensure that capillary action will always fill it with liquid. The gas seal  216  is thus a balance between the capillary forces pulling liquid into the gap and the airflow pushing liquid out. As the gap widens from G 5  to G 4 , the capillary forces decrease and the airflow increases so that the liquid boundary will lie in this region and be stable even as the substrate moves under the projection system PL.  
         [0041]     The pressure difference between the inlet  215  at G 2  and the outlet  214  at G 4 , as well as the size and geometry of gap G 3 , determine the gas flow through the gas seal  216  and will be determined according to the specific embodiment. However, if the length of gap G 3  is short and absolute pressure at G 2  is twice that at G 4 , the gas velocity will be the speed of sound in the gas and cannot rise any higher. A stable gas flow will therefore be achieved.  
         [0042]     The gas outlet system can also be used to completely remove the liquid from the system by reducing the gas inlet pressure and allowing the liquid to enter gap G 4  and be sucked out by a vacuum system, which can easily be arranged to handle the liquid, as well as the gas used to form the seal. Control of the pressure in the gas seal can also be used to ensure a flow of liquid through gap G 5  so that liquid in this gap that is heated by friction as the substrate moves does not disturb the temperature of the liquid in the space below the projection system.  
         [0043]     The shape of the seal member  212  around the gas inlet  215  and outlet  214  should be chosen to provide laminar flow as far as possible so as to reduce turbulence and vibration. Also, the gas flow should be arranged so that the change in flow direction at the liquid interface is as large as possible to provide maximum force confining the liquid. The liquid supply system circulates liquid in the reservoir so that fresh liquid is provided to the reservoir.  
         [0044]     The gas seal  216  can produce a force large enough to support the seal member  212 . Indeed, it may be possible to bias the seal member  212  towards the substrate to make the effective weight supported by the seal member  212  higher. The seal member  212  will in any case be held in the XY plane (perpendicular to the optical axis) in a substantially stationary position relative to and under the projection system but decoupled from the projection system. The seal member  212  is free to move in the Z direction and can therefore move to accommodate changes in the surface height of the substrate.  
         [0045]     When the substrate W is being moved, shearing forces will try to move the penetration level of the liquid in the gap between the liquid supply system and the substrate either to the outside or to the inside (left or right as illustrated). Both are unwanted, to the outside may lead to leakage, and to the inside may lead to air bubbles in the liquid. This can also happen as the height of the liquid supply system varies. One way to keep the liquid meniscus in a constant position is to monitor and actively control the position of liquid under the liquid supply system. The control may be implemented by locally increasing and decreasing the air and vacuum pressures in the gas seal  216 .  
         [0046]     As seen in  FIG. 9 , a second gas outlet  316  is provided on the opposite side of the gas inlet  215  to the first gas outlet  214 . In this way any gas escaping from the gas inlet  215  outwards away from the optical axis of the apparatus is sucked up by second gas outlet  316  which is connected to a vacuum source. In this way gas is prevented from escaping from the gas seal so that it cannot interfere, for example, with interferometer readings or with a vacuum in which the projection system and/or substrate are housed.  
         [0047]     One or more partial ring seals may be used to partially seal the circumferential gap between the final optical element of the projection lens PL and the sealing member  212 .  FIG. 9  shows two partial ring seals  302 ,  304 . The structure and orientation of the partial ring seals  302 ,  304  may be similar to those of the partial ring seals  102 ,  104  in  FIGS. 6 and 7 . In addition, one or more spiral shaped seals similar to the spiral shaped seal  130  in  FIG. 8  may be used. Because these similar seals have been described in connection with  FIGS. 6-8  above, the description is not repeated here.  
         [0048]     It is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention, therefore, is not limited to the above description.