Patent Publication Number: US-11378893-B2

Title: Lithographic apparatus and device manufacturing method involving a heater

Description:
This application is a continuation of U.S. patent application Ser. No. 16/376,535, filed Apr. 5, 2019, now allowed, which is a continuation of U.S. patent application Ser. No. 14/882,241, filed Oct. 13, 2015, now U.S. Pat. No. 10,254,663, which is a continuation of U.S. patent application Ser. No. 13/288,831, filed Nov. 3, 2011, now U.S. Pat. No. 9,188,880, which is a continuation of U.S. patent application Ser. No. 12/869,560, filed Aug. 26, 2010, now U.S. Pat. No. 9,268,242, which is a continuation of U.S. patent application Ser. No. 11/205,325, filed Aug. 17, 2005, now U.S. Pat. No. 7,804,575, which is a continuation-in-part of U.S. patent application Ser. No. 10/917,535, filed Aug. 13, 2004, now U.S. Pat. No. 7,304,715, the entire contents of each foregoing application is hereby incorporated by reference. 
    
    
     FIELD 
     The present invention relates to a lithographic apparatus 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. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. 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 projection system and the substrate. 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. 
     However, submersing the substrate or substrate and substrate table in a bath of liquid (see, for example, U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by reference) 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. 
     One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the 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 PCT patent application WO 99/49504, hereby incorporated in its entirety by reference. As illustrated in  FIGS. 2 and 3 , liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side.  FIG. 2  shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of  FIG. 2 , the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible. One example is illustrated in  FIG. 3  in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element. 
     In European patent application publication no. EP 1420300 and United States patent application publication no. 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 tables for supporting a substrate. Leveling measurements are carried out with a table at a first position, without immersion liquid, and exposure is carried out with a table at a second position, where immersion liquid is present. Alternatively, the apparatus has only one table. 
     Although providing improved resolution, the introduction of an immersion liquid may cause errors in the image generated on the substrate, including alignment errors between one layer and the next (i.e. overlay errors), defocus and aberrations. 
     SUMMARY 
     It is desirable to provide a system that reduces lithography errors arising from the immersion liquid. 
     According to an aspect of the invention, there is provided a lithographic apparatus, comprising: a substrate table constructed to hold a substrate; a projection system configured to project a patterned radiation beam onto a target portion of the substrate; a liquid supply system configured to at least partly fill a space between the projection system and the substrate with liquid; a barrier member arranged to substantially contain the liquid within the space between the projection system and the substrate; and a liquid evaporation controller arranged to control a net rate of evaporation of liquid supplied by the liquid supply system. 
     According to an aspect of the invention, there is provided a lithographic apparatus, comprising: a substrate table constructed to hold a substrate; a projection system configured to project a patterned radiation beam onto a target portion of the substrate; a liquid supply system configured to at least partly fill a space between the projection system and the substrate with liquid; a barrier member arranged to substantially contain the liquid within the space between the projection system and the substrate; a substrate table displacement system, arranged to move the substrate table along a predetermined path relative to the barrier member, thereby moving the target portion over the surface of the substrate; and a microwave source and microwave containment device, together configured to supply heat to liquid on the surface of the substrate. 
     According to an aspect of the invention, there is provided a lithographic apparatus, comprising: a substrate table constructed to hold a substrate; a projection system configured to project a patterned radiation beam onto a target portion of the substrate; a liquid supply system configured to at least partly fill a space between the projection system and the substrate with liquid; a barrier member arranged to substantially contain the liquid within the space between the projection system and the substrate; a substrate table displacement system, arranged to move the substrate table along a predetermined path relative to the barrier member, thereby moving the target portion over the surface of the substrate; and a substrate heater configured to heat at least a portion of the substrate according to a position of the substrate table relative to the barrier member, or a velocity of the substrate table relative to the barrier member, or an acceleration of the substrate table relative to the barrier member, or a predetermined path of the substrate table relative to the barrier member, or a local substrate temperature, or a local substrate table temperature, or any combination thereof. 
     According to an aspect of the invention, there is provided a device manufacturing method, comprising: projecting a patterned radiation beam through a liquid onto a target portion of a substrate; and controlling a net rate of evaporation of the liquid. 
     According to an aspect of the invention, there is provided a device manufacturing method, comprising: projecting a patterned radiation beam through a liquid onto a target portion of the substrate; moving a substrate table holding the substrate along a predetermined path relative to a seal member used to contain the liquid, thereby moving the target portion over a surface of the substrate; and heating liquid on the surface of the substrate using microwaves. 
     According to an aspect of the invention, there is provided a device manufacturing method, comprising: projecting a patterned radiation beam through a liquid onto a target portion of a substrate; moving a substrate table holding the substrate along a predetermined path relative to a seal member used to contain the liquid, thereby moving the target portion over a surface of the substrate; and heating at least a portion of the substrate according to a position of the substrate table relative to the seal member, or a velocity of the substrate table relative to the seal member, or an acceleration of the substrate table relative to the seal member, or a predetermined path of the substrate table relative to the seal member, or a local substrate temperature, or a local substrate table temperature, or any combination thereof. 
    
    
     
       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; 
         FIGS. 4 a  and 4 b    depict other liquid supply systems for use in a lithographic projection apparatus; 
         FIG. 5  depicts a barrier member according to an embodiment of the invention, showing interaction with a pressurized gas humidity controller, an immersion liquid temperature controller and a pressurized gas temperature controller; 
         FIG. 6  depicts a barrier member, gas-shower outlet and gas-shower outlet controller according to an embodiment of the invention; 
         FIG. 7  depicts a top view of a substrate table comprising a system of local heaters and a substrate temperature controller according to an embodiment of the invention; 
         FIG. 8  depicts a side view of the substrate table of  FIG. 7 , showing also a plurality of temperature sensors, a substrate table path determining device and substrate heaters located in the barrier member; 
         FIG. 9  depicts a top view of a substrate table showing the geometry of a substrate heater arranged to dissipate more power in the lower region of the substrate table than in the upper region; 
         FIG. 10  depicts an array of individually controllable substrate heaters according to an embodiment of the invention; 
         FIG. 11  depicts a side view of the arrangement of  FIG. 10 , showing also interaction with a heater array controller and predetermined algorithm input device; 
         FIG. 12  depicts a partial view of a barrier member according to an embodiment of the invention, showing a thermal isolation sleeve for a vacuum exhaust inlet and vacuum exhaust pipe and a barrier member heater; 
         FIG. 13  depicts interaction between a barrier member and barrier member temperature stabilizer according to an embodiment of the invention; 
         FIG. 14  depicts a barrier member comprising a network of fluid-carrying channels and a fluid supply system according to an embodiment of the invention; 
         FIG. 15  depicts a barrier member and substrate table comprising a network of fluid-carrying channels and an array of individually controllable heaters controlled by a substrate temperature controller comprising a substrate table heat-exchange fluid controller and a substrate heater controller; 
         FIG. 16  depicts a substrate table with a network of channels and circular groove according to an embodiment of the invention; 
         FIGS. 17 and 18  depict the substrate table with circular groove according to  FIG. 16 , sealed by a sealing ring according to an embodiment of the invention; 
         FIG. 19  depicts a lithographic apparatus showing positioning of temperature sensors in the substrate table and barrier member according to an embodiment of the invention; 
         FIG. 20  depicts an enlarged view of the substrate table in the region of the substrate showing an arrangement of miniature temperature control systems according to an embodiment of the invention; 
         FIG. 21  depicts a projection system controller and thermally-induced-distortion calculator according to an embodiment of the invention; 
         FIG. 22  depicts a microwave source and microwave containment cage for heating immersion liquid on the substrate surface; 
         FIG. 23  depicts an arrangement of resistive heating strips and associated electrical current flow; 
         FIG. 24  depicts a single resistive strip used as a local temperature sensor for a system of local heaters; 
         FIG. 25  depicts an arrangement for inductive heating of the substrate table WT; 
         FIG. 26  depicts apparatus for producing a flow of gas with a controlled level of humidity; 
         FIG. 27  depicts a heat exchanger for controlling the temperature of a gas flow; and 
         FIG. 28  depicts a venting system for enabling stable operation of a humidifier cabinet. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus comprises:
         an illumination system (illuminator) IL configured to condition a radiation beam PB (e.g. UV radiation or DUV radiation).   a 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 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 in accordance with certain parameters; and   a projection system (e.g. a refractive projection lens system) PL configured to project a pattern imparted to the radiation beam PB by patterning device MA onto a target portion C (e.g. comprising 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, for directing, shaping, or controlling radiation. 
     The support structure supports, i.e. bears the weight of, the patterning device. It 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 support structure may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure 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. 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 may 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 support structures). 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. The substrate W may be held directly by the substrate table WT (sometimes referred to as a mirror block) and may be held by a substrate holder (sometimes referred to as a burl plate or chuck), which is in turn held by the substrate table WT. 
     Referring to  FIG. 1 , the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source 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 comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source 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 comprise an adjuster AM for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section. 
     The radiation beam PB is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. Having traversed the patterning device, the radiation beam PB passes through the projection system PL, 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 may be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam PB. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in  FIG. 1 ) may be used to accurately position the patterning device MA with respect to the path of the radiation beam PB, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the support structure 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 support structure MT may be connected to a short-stroke actuator only, or may be fixed. Patterning device 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 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 support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam 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 support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam 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 support structure MT may be determined by the (de-) magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion. 
     3. In another mode, the support structure 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 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. 
     A further immersion lithography solution with a localized liquid supply system is shown in  FIG. 4 a   . Liquid is supplied by two groove inlets IN on either side of the projection system PL and is removed by a plurality of discrete outlets OUT arranged radially outwardly of the inlets IN. The inlets IN and OUT can be arranged in a plate with a hole in its center and through which the projection beam is projected. Liquid is supplied by one groove inlet IN on one side of the projection system PL and removed by a plurality of discrete outlets OUT on the other side of the projection system PL, causing a flow of a thin film of liquid between the projection system PL and the substrate W. The choice of which combination of inlet IN and outlets OUT to use can depend on the direction of movement of the substrate W (the other combination of inlet IN and outlets OUT being inactive). 
     Another solution which has been proposed is to provide the liquid supply system with a barrier member 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. The barrier member is substantially stationary relative to the projection system 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 barrier member and the surface of the substrate. The seal may be a contactless seal such as a gas seal. Such a system is disclosed in United States patent application publication no. US 2004-0207824 and European patent application publication no. EP 1420298, each hereby incorporated in its entirety by reference. 
       FIG. 4 b    shows an example arrangement for a barrier member  12  configured to contain an immersion liquid in the localized area  25  under the projection system PL. The barrier member  12  is provided with an extractor EX configured to extract liquid from the localized area  25  through a gauze GZ. The extractor EX may extract both liquid and gas or only liquid. A recess RE is provided radially outwardly of the extractor EX and a gas seal  27  is provided radially outwardly of the recess RE. The gas seal  27  forms a jet of gas JE which is used to dry the surface of the substrate W and/or reduce the amount of liquid that escapes from the barrier member  12 . 
     According to an aspect of the invention, overlay errors and other problems linked to the presence of an immersion liquid and barrier member  12  may be reduced by a liquid evaporation controller, which targets and controls the rate of evaporation of immersion liquid in the region of the substrate. Molecules of liquid absorb energy from the surroundings in order to evaporate and, particularly if pumped away, the cooling that results may result in significant and non-uniform variations in the temperature of critical components such as the substrate W. Thermally-induced distortions may lead to errors in the image finally written to the substrate. For example, evaporation of immersion liquid left behind on the substrate after the barrier member  12  has passed by may cause local temperature drops of up to 3K. As a result of this, over 20 nm single machine overlay error might result. 
       FIG. 5  shows an arrangement of the barrier member  12  according to an embodiment of the invention. Immersion liquid is contained within an immersion reservoir  25  located between the final element of the projection system PL and the substrate W. Immersion liquid is contained within the immersion reservoir  25  by the body of the barrier member  12  and by a gas seal  27  on its lower periphery, which limits the amount of immersion liquid that escapes from the immersion reservoir  25  through gap  22 . The gas seal  27  is connected to a pressurized gas supply system  30 , which supplies pressurized gas to the gas seal  27  via a pressurized gas outlet and a pressurized gas supply pipe  15 . Gas is pumped away via a vacuum exhaust inlet and a vacuum exhaust pipe  14 . Immersion liquid that evaporates in the region of the gas seal  27  may be pumped away via the vacuum exhaust inlet  17 . Alternatively, liquid that escapes beyond the gas seal  27  either into the region underneath the barrier member  12  in the gap  22  or beyond the outer edges of the barrier member  12 , may evaporate into the environment outside the substrate W outside of the barrier member  12 . 
     Where a substance exists both in liquid form and in gaseous form, it is normally the case that a dynamic equilibrium will exist with a rate of evaporation of liquid being balanced against a rate of condensation of vapor. The amount of cooling caused by evaporation will therefore be offset by a heating caused by condensation (where high energy gaseous molecules yield energy to their surroundings as part of the transition to the lower energy liquid state). The cooling power therefore depends on the net rate of evaporation (i.e. the difference between the number of molecules entering the gaseous state from the liquid per unit time and the number of molecules entering the liquid state from the gaseous state per unit time). Both condensation and evaporation are statistical effects and increasing the number of molecules involved will increase the rate of either process. Therefore, increasing the vapor concentration will increase the rate of condensation and lead to a reduction in the net rate of evaporation. Where the vapor consists of water molecules, this concentration is directly related to the relative humidity, defined as the amount of water vapor present as a percentage of the maximum quantity that could be present at a given temperature. 
     This understanding is exploited according to an embodiment of the invention to control the cooling caused by evaporation of immersion liquid. As illustrated in  FIG. 5 , a pressurized gas humidity controller  50 , which is provided configured to interact with the pressurized gas supply system  30  to provide pressurized gas to the gas seal  27  with a relative humidity controlled to be greater than about 10%. Increasing the relative humidity of the gas increases the rate of condensation and therefore decreases the net rate of evaporation and the cooling caused thereby. In an embodiment, the relative humidity is arranged to be within a predetermined range determined by reference to calibration measurements. For the purposes of controlling cooling, in general, the higher the relative humidity the better. However, for very high relative humidities, the barrier member  12  may leave excessive quantities of water in its wake. Furthermore, if insufficient mechanisms are provided for humid gas extraction near the outer diameter of the barrier member, humid gas may leave and interfere with the operation of position sensor IF. Therefore, the upper limit will in general be dependent on details of the barrier member construction and/or configuration. Additionally or alternatively, the predetermined range may be greater than about 40%. High relative humidities such as these may be achieved by using lower working pressures than might otherwise be selected for the purposes of achieving optimal sealing properties (6 bar may typically be used). Ideally, a working pressure should be chosen that is as close as possible to atmospheric pressure, while still providing sufficient flow rate for the gas seal  27  to perform its function. The lower the working pressure, the less the relative humidity will be reduced when the pressurized gas expands on leaving the pressurized gas supply system  30 . 
     The pressurized gas humidity controller  50  may be arranged to be responsive to changes in the temperature of the substrate W and/or substrate table WT. These temperature changes may be determined via one or more temperature sensors  60  arranged, for example, in the substrate table WT. According to an embodiment of the invention, the pressurized gas humidity controller  50  is arranged to compare the temperatures of the substrate W and/or substrate table WT and/or substrate holder, measured at one or more points by the temperature sensor(s)  60 , with a target temperature or temperatures Tt. That is to say, where a single temperature sensor  60  is present, the pressurized gas humidity controller  50  compares this one temperature reading with a single target temperature Tt. Where a plurality of temperature sensors  60  are present, the pressurized gas humidity controller  50  compares a plurality of readings with a single target temperature Tt, or with a plurality of target temperatures Tt corresponding, for example, to particular regions of the substrate W and/or corresponding regions of the substrate table/substrate holder, and therefore to particular groups of temperature sensor readings (within which an average reading may be used). The pressurized gas humidity controller  50  may then adjust the relative humidity of the pressurized gas in order to reduce a difference between the measured and target temperature(s), the efficiency of the process may be controlled by a feedback controller such as a PID system. 
     Adjusting the humidity of the gas supplied to the gas seal  27  may be most efficient for cooling caused by evaporation in the region of the gas seal  27 , particularly around the vacuum exhaust inlet  18  and vacuum exhaust pipe  14 . In an embodiment, it is desirable to have an additional mechanism to control the net evaporation of liquid beyond the gas seal  27  and outside of the barrier member  12 . Such an arrangement, according to an embodiment of the invention, is illustrated in  FIG. 6 . Here, a gas-shower outlet  70  is provided that is capable of providing a flow of gas with a relative humidity controlled to be greater than 10%. A gas-shower humidity controller  75  is provided that is capable of adjusting the relative humidity in accordance either with calibration measurements, calculations or measurements of the temperature at one or various points on the substrate W and/or substrate table/substrate holder, as provided by one or more temperature sensors  60 , and comparing them with a target temperature or temperatures Tt. A desired range of relative humidity in this case is about 40 to about 50%. In the case where the gas-shower humidity controller  75  is arranged to respond to temperature measurements, it may adjust the relative humidity of the gas in order to reduce a difference or differences between the measured and target temperature(s) Tt. That is to say, where a single temperature sensor  60  is present, the gas-shower humidity controller  75  compares this one temperature reading with a single target temperature Tt. Where a plurality of temperature sensors  60  are present, the gas-shower humidity controller  75  compares a plurality of readings with a single target temperature Tt, or with a plurality of target temperatures Tt corresponding, for example, to particular regions of the substrate W and/or corresponding regions of the substrate table/substrate holder, and therefore to particular groups of temperature sensor readings (within which an average reading may be used). A feedback controller, such as a PID system, may control the efficiency of the process. 
     The gas-shower humidity controller  75  may be arranged to interact with the pressurized gas humidity controller  50  so as to ensure that the relative humidity of the gases provided by the gas seal  27  and the gas-shower outlet  70  are matched. This feature provides a mechanism by which variations in the relative humidity outside of the gas seal  27  may be controlled and avoids disturbances to systems, such as the interferometers used to measure the position of the substrate table WT, that might otherwise occur. 
     The substrate table WT is normally arranged to be moved relative to the projection system PL and barrier member  12  by a substrate table displacement system  100  (see  FIG. 8 ), so that consecutive target regions of the substrate W may be exposed by the patterned radiation beam. This process may encourage small quantities of immersion liquid to leave the confines of the immersion reservoir  25  in spite of the operation of the gas seal  27 . Embodiments arranged to reduce component cooling caused by reducing immersion liquid evaporation have been discussed above. According to an alternative aspect of the invention, errors caused by the cooling effects of evaporating immersion liquid may be reduced by providing a substrate heater, configured to heat at least a portion of the substrate W according to the position, velocity, acceleration, or predetermined path of the substrate table WT relative to the barrier member  12 , the local substrate W, the substrate table WT temperature, or any combination thereof. The substrate heater may provide heating via a number of mechanisms. These may include: an infra-red emission source, a glow wire electrical resistance heater, a hot gas jet, or any combination thereof. Factors when deciding which kind of heater to use may include how finely and quickly the heating power needs to be adjusted, as well as how effectively the heater can be produced in miniature form. The latter factor may become more or less important depending on whether the heater needs to be embedded in or near the material whose temperature it is intended to regulate (such as a glow wire, for example, embedded in the substrate table WT), or whether the heater works to some extent at a distance (such as a radiation-emitting source or temperature-controlled gas jet source). In the case of a radiation-emitting source, the wavelength distribution of the radiation should be chosen so as not to react with resist compositions on the substrate W (infra-red radiation would be safe in this regard for most resists of interest). Selection of radiation intensity will depend on the optical properties of the resist (such as its reflectivity). This may be determined by calibration measurements during a set-up sequence of the lithography apparatus. Where there is likely to be a process stage dependency (due to variations in the reflectivity, for example), calibration may also be carried out within the production sequence as an extra measurement phase for each lot of substrates. As shall be described below, several embodiments of the invention operate on the principle that at least a subset of the substrate heaters present are actuated during a substrate exposure sequence, i.e. as the barrier member  12  passes over the substrate W. However, a system that heats the substrate W before exposure, so as to compensate for cooling that has yet to occur but is expected to occur, also falls within the scope of the invention. 
       FIGS. 7 and 8  show an arrangement according to an embodiment of the invention, comprising a system of heaters  85 , 86  embedded either in the substrate table as “local heaters”  85 , or in the barrier member  12  as “remote heaters”  86 , or both. The local heaters  85  are each arranged to heat predominantly a particular region of the substrate W and together may be used to control the temperature profile of at least a portion of the substrate W. The remote heaters  86  will heat a different portion of the substrate W depending on the position of the barrier member  12  relative to the substrate W. 
     According to a first mode of operation involving the local heaters  85 , the heating power and relative timing of each heater may be adjusted to set up a starting temperature profile for the substrate W at a known period of time before the substrate exposure cycle is started. By reference to calibration measurements and/or analysis of test patterns generated by the lithographic apparatus, a starting temperature profile may be selected that substantially compensates for cooling that will occur due to evaporation of immersion liquid during the exposure cycle. 
     According to a second mode of operation involving the local heaters  85 , each of these heaters  85  may be arranged to be switched to a heat-emitting state only when the barrier member  12  passes over the region that they are positioned to heat. For example, in the case where the barrier member  12  (and thus the target area) moves relative to the substrate W as shown in  FIG. 7 , along a path  150  between an initial die (or target area)  160  to a final die  170 , the local heaters  85  would also be switched on in a progressive manner substantially along the same path  150 . This may be achieved by programming a substrate temperature controller  110  to provide a series of time-delayed actuation signals that for each local heater  85  closely lags behind the intended path of the barrier member  12  relative to the substrate table WT. The intended path may be stored in a substrate table path determining device  90 . As an alternative or additional approach, the actuation sequence of the local heaters  85  may be derived from further functions of the substrate table path determining device  90 . For example, the substrate table path determining device  90  may comprise a device (based on interferometry, for example) to measure the position, velocity and/or acceleration of the substrate table WT and to feed this information to the substrate temperature controller  110 , which may be configured to calculate at this point when to activate each local heater  85 . For example, the path determining device  90  may be configured to send an actuation signal to a given heater when it detects that the barrier member  12  is moving away from, or moved past, that particular heater. The power supplied by each local heater  85  may be arranged to be constant or time-varying and be the same as or different to the other local heaters  85 . The optimal arrangement to use for each heater is that which best compensates the power lost due to evaporation for the region concerned. In the case where the rate of loss of liquid from the barrier member  12  is constant, the power to be supplied by each heater  85  once activated may be substantially the same (because once the barrier member  12  has passed by, the amount of liquid left behind on the substrate W to evaporate may be found to be roughly constant). Alternatively, more heating power may be required in certain regions, such as where the barrier member  12  changes direction relative to the substrate table WT. Calibration measurements may be carried out to determine the most effective way to operate the heater powers as a function of the particular substrate table path and rate required. 
     The remote heaters  86  in the barrier member  12  may, in an embodiment, be positioned around the periphery of the barrier member  12 , as shown in  FIG. 8 . This arrangement allows the heaters to operate in close proximity to the region where the evaporation processes may be extracting the most heat. Placement near the outer diameter may be chosen as a compromise to avoid the region immediately around the gas seal  27 , which in practice is already heavily occupied by holes, pipes and ducts. Because they operate at a distance from the substrate W, heater mechanisms such as those based on radiation or hot gas jets may be appropriate. Creating a hot surface in the base of the barrier member  12  is one method by which a radiation source may be implemented. Thermal isolation of such a device from the rest of the barrier member  12  may improve the performance of such a feature. Alternatively or additionally, infra-red bulbs may be used. 
     As was arranged for the local heaters  85  above, the power of the remote heaters  86  may be controlled depending on the direction of motion of the substrate table. For example, it may be arranged to provide more heat from one side of the barrier member  12  than from the other. As one aspect of the cooling is related to evaporation of liquid escaping from the barrier member  12 , the remote heaters  86  on the trailing edge of the barrier member  12  (where immersion liquid is likely to be escaping) may be configured to emit a higher heating power than those located on the front edge of the barrier member  12  (where the substrate W is still dry). The effectiveness of the remote heaters  86  may be varied on demand by varying the power and/or width of the heaters  86  around the circumference of the barrier member  12 . This latter parameter may be varied, for example, by progressively activating different segments of a segmented heater  86 , or one heater  86  of a plurality of heaters  86 . 
     Although shown embedded in the substrate table WT or barrier member  12 , it is to be understood that the heaters  85 , 86  may be positioned anywhere where they can influence the temperature of the substrate W. Radiation-emitting heaters, for example, may be positioned in a separate body distinct from the substrate table WT and barrier member  12 . Where the substrate W is heated prior to exposure, this may occur in a region away from that used for exposure, so as to implement more easily the remote heaters  86 . 
     The lithographic apparatus may also comprise local temperature sensors  60 , which in the example illustrated in  FIG. 8  are embedded in the substrate table WT. According to an embodiment of the invention, these temperature sensors  60  are arranged to measure the temperature of each region of the substrate W and/or corresponding region of the substrate table/substrate holder affected by each local heater  85 . This information is fed to the substrate temperature controller  110 , which may then calculate how to control the output of the local heaters  85  and/or remote heaters  86  so as to reduce a difference between a target temperature or temperatures Tt and the temperatures measured by the local temperature sensors  60 . In this embodiment, it may be desirable to arrange for the heaters  85  and/or  86  to have a variable rather than fixed output. In either case, a feedback controller (such as PID) may be used to optimize the efficiency of the convergence process. 
     Modulating the temperature of the liquid supplied by the liquid supply system  130  may also control the temperature of the substrate W and/or substrate table/substrate holder. For example, the immersion liquid may be heated to a controlled temperature greater than 295 K.  FIG. 5  shows an embodiment of the invention comprising an immersion liquid temperature controller  120  arranged to carry out this function in cooperation with the liquid supply system  130 . The temperature control of the immersion liquid may be effected with reference to calibration measurements or with reference to readings from one or more temperature sensors  60  in order to select an immersion liquid temperature that will compensate effectively for evaporation heat loss. In the latter case, the output of the immersion liquid temperature controller  120  may be controlled so as to minimize a difference or differences between a target temperature or temperatures Tt and the temperature(s) provided by the temperature sensor(s)  60 , the convergence process being controlled by a feedback controller, such as a PID controller. That is to say, where a single temperature sensor  60  is present, the immersion liquid temperature controller  120  compares this one temperature reading with a single target temperature Tt. Where a plurality of temperature sensors  60  are present, the immersion liquid temperature controller  120  compares a plurality of readings with a single target temperature Tt, or with a plurality of target temperatures Tt corresponding, for example, to particular regions of the substrate W and/or corresponding regions of the substrate table/substrate holder, and therefore to particular groups of temperature sensor readings (within which an average reading may be used). 
     Modulating the temperature of the gas supplied by the pressurized gas supply system  30  may also control the temperature of the substrate W and/or substrate table/substrate holder. For example, the pressurized gas may be heated to a controlled temperature greater than about 300 K. The lower temperature limit here is higher than that required for the immersion liquid temperature controller  120  described above due to the lower heat capacity of the gas relative to the liquid. According to one embodiment of the invention, pressurized gas is provided at temperatures in the range of about 300 to about 320 K.  FIG. 5  shows an embodiment of the invention comprising a pressurized gas temperature controller  140  arranged to carry out the temperature control function in cooperation with the pressurized gas supply system  30 . The temperature control of the pressurized gas may be effected with reference to calibration measurements or with reference to readings from one or more temperature sensors  60 . In the latter case, the output of the pressurized gas temperature controller  140  may be controlled so as to minimize a difference or differences between a target temperature or temperatures Tt and the temperature(s) provided by the temperature sensors  60 , the convergence process being controlled by a feedback controller, such as a PID controller. That is to say, where a single temperature sensor  60  is present, the pressurized gas temperature controller  140  compares this one temperature reading with a single target temperature Tt. Where a plurality of temperature sensors  60  are present, the pressurized gas temperature controller  140  compares a plurality of readings with a single target temperature Tt, or with a plurality of target temperatures Tt corresponding, for example, to particular regions of the substrate W and/or corresponding regions of the substrate table/substrate holder, and therefore to particular groups of temperature sensor readings (within which an average reading may be used). 
     As has already been discussed, the substrate heating requirements have a positional dependence that may be at least in part determined by the path of the barrier member  12  over the substrate W. At least two processes have been identified as contributing to the cooling process: evaporation of liquid in the gap  22  between the substrate W and the barrier member  12 , and evaporation of residual liquid left on the substrate W after exposure, if the exposed area is left wet. The cooling power of the barrier member  12  (i.e. cooling from the first process) is constant in time although it depends, inter alia, on the velocity of the barrier member  12  relative to the substrate W. The cooling power of the second process depends, inter alia, on the amount of liquid left on the substrate W. The amount of cooling that needs to be compensated is in general a complex function of both processes, leading to a cooling power with a complex positional dependence. Thermal conduction within the substrate W will also be a factor, meaning that unexposed areas of the substrate W may start to cool even before the barrier member  12  reaches them due to cooling in exposed portions of the substrate W. Taking one process at a time, however, it is possible to make some estimations. For example, considering only the direct cooling from evaporation of residual immersion liquid on the substrate W, a substrate exposure configured to take about 30 seconds, with a time between a last exposure and substrate unloading of about 5 seconds, and an exposure sequence such as that labeled  150  in  FIG. 7 , about 20% to about 30% more heat may be expected to be extracted by this mechanism at the location  160  of the first exposure than at location  170  of the last exposure. In certain embodiments described above comprising substrate heaters  85 , 86 , this effect was taken into account by delaying the action of individual heaters along the path of the barrier member  12 . A similar effect may also be achieved by configuring the substrate heater to provide a higher heating power at target regions on the substrate W where the projection system is configured to project the patterned radiation beam at a first time and provide progressively lower heating powers at target regions on the substrate W where the projection system PL is configured to project the patterned radiation beam at later times. This arrangement may be varied to give a more complex position-dependent heating depending on the cooling characteristics of a particular setup that have to be compensated. 
     Although it may be technically possible to position a large number of local heaters  85  at many different locations on the substrate W, it is, in practice, almost as effective and significantly less expensive to provide a more limited number of heaters and position them to substantially follow the path of the barrier member  12 . An arrangement of this type is depicted in  FIG. 7 . Here, elongate substrate heaters  85 , such as glow wires, are arranged so that one individually controllable element is aligned with one of the principal scanning or stepping axes  181 - 187  (each corresponding to a row of dies) of the barrier member  12  over the substrate W. In the example shown, each glow wire  85  is arranged to emit a constant heating power per unit length and is configured so that the glow wire aligned with the principal scanning or stepping axis  187  has the highest heating power, that aligned with  186  the next highest heating power and so on, progressively decreasing until the final glow wire, corresponding to principal scanning or stepping axis  181 , is reached, which is assigned the lowest heating power. 
     Where a large number of local heaters are provided at different locations (for example, between 100 and 700 per substrate), it is desirable to position the heaters as close as possible to the surface of the substrate W. In the arrangement shown in  FIGS. 7 and 8 , however, where fewer heaters are provided, it is desirable to position the heaters substantially further away so that each heater may have effective control over a larger portion of the substrate W. 
       FIG. 9  shows an arrangement wherein a continuous glow wire heater  85  is provided to heat the substrate W. In the example shown, the glow wire heater  85  is arranged to follow the path of the barrier member  12  to some extent in that it has longer sections  195  substantially parallel to the principal scanning axes  181 - 187  (i.e. perpendicular to the scanning direction) of the barrier member  12  (as shown in  FIG. 7 ). However, the pitch  191 - 193  between these longer sections is arranged to become shorter and shorter towards the lower end of the substrate W as depicted, which corresponds to the region within which the substrate W will be first exposed (i.e. pitch  191 &gt;pitch  192 &gt;pitch  193 ). This means that the glow wire heater  85  may be given the simplest and most robust construction (that where the heating power is constant per unit length, which in practice may correspond to an elongate resistive element of constant cross-section) and yet still provide a heating power that is larger towards the region within which the substrate W will be first exposed, which is the region of the substrate W that will require the largest correction for cooling effects. As an alternative and/or additional arrangement, the glow wire heater  85  may be arranged to provide a heating power per unit length that varies along its length (for example, increasing towards the lower end of the substrate W in the orientation depicted). In the case where the glow wire operates by means of electrical dissipation associated with an electric current passed along its length, variable heating power may be achieved either by varying the cross-section (for example, providing a glow wire that becomes thinner where more power is needed) or by varying the material used. In the latter arrangement, care should be taken to avoid points of high resistance where junctions are made between materials of different composition. 
       FIGS. 10 and 11  show an arrangement wherein the substrate heater comprises a system of individually controllable heaters  85 . In the embodiment shown in  FIG. 10 , the individually controllable heaters  85  are arranged as elongate members substantially parallel to the principal scanning axes  181 - 187  (i.e. perpendicular to the scanning direction) and confined to heat within the geometrical limits of the substrate W. However, alternative arrangements of heaters would also be compatible with this embodiment of the invention as long as they may be controlled individually. A heater array controller  180  controls each individually controllable heater  85  via an address bus. The heater array controller  180  receives in turn input from a predetermined algorithm  190 , which describes how the heating power of each individual heater should be controlled as a function of time (and therefore as a function of the position of the barrier member  12  relative to the individual heater in question). The appropriate algorithm to use may be derived from calibration measurements and/or calculation (based, for example, on the amount of time an expected amount of liquid is expected to remain on the substrate W). This approach has the advantage of not requiring temperature sensors, which may greatly simplify construction. 
     Evaporation of immersion liquid may also lead to cooling of the barrier member  12  itself. This effect may lead in turn to cooling of the substrate W, for example, by cooling the immersion liquid and/or pressurized gas by convection and/or by radiative effects. According to an aspect of the invention, a barrier member temperature stabilizer is provided to reduce cooling of the substrate W by this mechanism. 
     A region of particular concern is around the vacuum exhaust inlet  17  and in the vacuum exhaust pipe  14 , as shown in  FIG. 12 . Where immersion liquid is present in these regions, net evaporation may be particularly pronounced, as the concentration of vapor may be kept low by the vacuum system (evaporated liquid will immediately be pumped away). One way in which overall cooling of the barrier member  12  due to this mechanism may be controlled is illustrated in  FIG. 12 , where the barrier member temperature stabilizer is effected by means of a thermal isolation sleeve  210  arranged around the vacuum exhaust pipe  14 . In an embodiment, the thermal isolation sleeve  210  should be formed from a material having very low thermal conductivity at the expected operating temperature of the lithographic apparatus. General purpose plastics, PTFE, etc. may be appropriate materials for the thermal isolation sleeve  210 . Alternatively or additionally, the barrier member itself may be constructed wholly or partially of a thermally isolating material. This approach may be more effective and more easily implemented than having only a thermal isolation sleeve  210 , although the choice of materials with suitable mechanical characteristics may be restricted. 
     An additional and/or alternative approach is to provide a dedicated barrier member heater  220 , which is arranged to provide a compensating heating power to those areas of the barrier member  12  cooled by evaporation of the immersion liquid. Although directed on the one hand to heating the barrier member  12  itself, and thus, indirectly, the substrate W, the barrier member heater  220  may be arranged to heat the substrate W directly. This may be achieved through the use of a radiation-emitting heater such as an infra-red heater, which has been described above in the context of possible substrate heaters  85 , 86 . In the arrangement shown in  FIG. 12 , the barrier member heaters  220  are arranged around the vacuum exhaust inlet  17  and may follow the geometry of the vacuum exhaust inlet in a plane perpendicular to the axis of the barrier member  12  (into the page in the orientation depicted). 
     The heating power of the barrier member heater  220  may be controlled by the barrier member temperature stabilizer according to input from one or more of a number of possible sources. For example, the barrier member heater power may be adjusted in response to the flow rate in the vacuum exhaust pipe  14 , which may be provided by the pressurized gas supply system  30 . Here, it is expected that a higher heating power may be required for higher flow rates. 
     The barrier member heater  220  may also be controlled by reference to the temperature of the substrate W and/or substrate table/substrate holder, which may be measured at one or more positions by one or more temperature sensors  60 . As with previous embodiments, a feedback controller may be employed to reduce a difference between the measured substrate temperature(s) and one or more predefined target temperatures Tt. 
     The barrier member heater  220  may also be controlled in response to the relative humidity of the gas supplied by the pressurized gas outlet  18 . This information may be provided by humidity sensors, which may be arranged either in the barrier member  12  or as part of the pressurized gas supply system  30  (the latter case being illustrated in  FIG. 13 ). 
     Finally, the barrier member temperature stabilizer  200  may control the output of the barrier member heater  220  by reference to a calibration table  230  of required corrections, constructed from measurements of the barrier member temperature as a function of: substrate temperature, pressurized gas flow rate, pressurized gas flow temperature, vacuum exhaust flow rate, vacuum exhaust temperature, pressurized gas relative humidity, immersion liquid temperature, or any combination thereof. Although calibration measurements should be undertaken, this approach greatly reduces the need to incorporate additional functional components in the final lithographic apparatus to be shipped to the customer. 
     When considering the problem of substrate cooling linked to a cooled barrier member  12 , an important region of the barrier member  12  is that closest to and/or facing the substrate W. According to an embodiment of the invention depicted in  FIG. 14 , the barrier member  12  is constructed with a network of channels distributed in a layer  400  in the portion of the barrier member  12  closest the substrate W. The barrier member temperature stabilizer  200  is configured to control a heat-exchange fluid supply system  410  that provides a heat-exchange fluid to the network at a controlled temperature and/or flow rate. As in previous embodiments, a feedback controller may be provided to help control the substrate temperature in an efficient manner. In this case, the temperature and/or flow rate of the heat-exchange fluid provided by the fluid supply system  400  may be adjusted so as to reduce a difference between one or more substrate temperatures and/or substrate table temperatures, as measured by a system of local temperature sensors  60 , and a target temperature or temperatures Tt. That is to say, where a single temperature sensor  60  is present, the fluid supply system  400  compares this one temperature reading with a single target temperature Tt Where a plurality of temperature sensors  60  are present, the fluid supply system  400  compares a plurality of readings with a single target temperature Tt, or with a plurality of target temperatures Tt corresponding, for example, to particular regions of the substrate W and/or corresponding regions of the substrate table/substrate holder, and therefore to particular groups of temperature sensor readings (within which an average reading may be used). The temperature and/or flow rate of the fluid may also be controlled by reference to a calibration table  230  of required corrections, constructed from measurements of the barrier member temperature as a function of: substrate temperature, pressurized gas flow rate, pressurized gas flow temperature, vacuum exhaust flow rate, vacuum exhaust temperature, pressurized gas relative humidity, immersion liquid temperature, or any combination thereof. Although calibration measurements should be undertaken, this approach greatly reduces the need to incorporate additional functional components in the final lithographic apparatus to be shipped to the customer. 
     A general advantage of those embodiments described above that depend on mechanisms located in the barrier member  12  is that they may be implemented without influencing the dynamic performance of the substrate table WT (this may be true both for fluid-based and electrical systems). Barrier member temperature conditioning also improves not only short-term (die to die) temperature variations in the substrate W, but also long-term temperature variations from one substrate W to the next. More generally, the development costs (and development time) associated with barrier member improvements are likely to be significantly lower than those involving the substrate table WT. Apart from the problems associated with controlling the dynamics of the substrate table WT, a further factor in favor of working on the barrier member  12  rather than the substrate table WT relates to the flatness requirements, which are approximately 100 times more relaxed for the barrier member  12 . This may be important, for example, where channels are machined into the barrier member  12 . Introducing holes near the surface (where they are most effective) tends to introduce surface irregularities (bulges) as might variations in the pressure of the heat-exchange fluid (due to the reduced stiffness of the thin width of material left between the outer surface of the barrier member and the internal channel edge. 
       FIGS. 15 to 18  depict an arrangement also comprising a network of fluid-carrying channels. In this arrangement, the channels are located in the substrate table WT in close proximity to the substrate W. This arrangement of channels is configured to control the temperature of the substrate W, which may be adversely affected by evaporation of immersion liquid from its top surface. 
     In the present embodiment, a substrate table heat-exchange fluid controller  510  is provided for controlling the temperature and flow rate of a heat-exchange fluid arranged to flow through the network of channels  500 . 
     As in previous embodiments, a feedback controller may be provided to help control the substrate temperature in an efficient manner. In this case, the temperature and/or flow rate of the substrate table heat-exchange fluid may be adjusted so as to reduce a difference between one or more substrate temperatures and/or substrate table/substrate holder temperatures, as measured by a system of local temperature sensors  60 , and a target temperature or temperatures Tt. 
     The arrangement may work particularly effectively if local substrate heaters, such as glow wires, are also included so as to implement a “push-pull” principle of temperature control. According to this embodiment, a substrate temperature controller  520  controls the operation of a substrate heater controller  430  and the substrate table heat-exchange fluid controller  510 . A feedback controller may be included as part of the substrate temperature controller  520 , arranged to minimize a difference or differences between the substrate temperature, as measured at one or more locations on the substrate W and/or substrate table/substrate holder by local temperature sensors  60 , and a target temperature or temperatures Tt. That is to say, where a single temperature sensor  60  is present, the substrate temperature controller  520  compares this one temperature reading with a single target temperature Tt. Where a plurality of temperature sensors  60  are present, the substrate temperature controller  520  compares a plurality of readings with a single target temperature Tt, or with a plurality of target temperatures Tt corresponding, for example, to particular regions of the substrate W and/or corresponding regions of the substrate table/substrate holder, and therefore to particular groups of temperature sensor readings (within which an average reading may be used). Alternatively, a feedforward loop may be used if heat flows can be calculated as a function of the velocity and position of the barrier member  12  relative to the substrate W. According to the “push-pull” principle, the heat-exchange fluid controller  510  may be arranged to provide fluid at a temperature below the target temperature Tt, effectively acting to cool the substrate W. The local substrate heaters, which may be electrical resistance heaters (glow wires) as mentioned above, may respond much more quickly to sudden increases in the evaporation rate than can the heat-exchange fluid controller. Their response speed is additionally improved by being set against the cooling action of the heat-exchange fluid controller. Furthermore, should overshoot of the substrate temperature occur, the provision of the cooling heat-exchange fluid may allow a more rapid return to equilibrium than would be the case if no additional cooling mechanism was provided. 
     For ease of machining (among other reasons), the network of channels  500  comprise an array of substantially straight holes (which may be drilled) oriented in the plane of the substrate table, as depicted in  FIG. 16 . The ends of these straight holes should be connected and closed watertight. This may be done with plugs glued in the holes. However, in a typical configuration comprising about 4 mm holes on an about 8 mm pitch, more than 80 plugs may be required. In addition to the problem that many individual elements need to be constructed, the possibility exists in such an arrangement that dead ends occur in which either no fluid arrives at all, or fluid is not circulated. According to an embodiment of the invention, these problems may be overcome by providing a circular groove  420  (shown in  FIGS. 16 to 18 ), at the edge of the substrate table WT into which all through holes can be connected in a neat fashion without dead ends. This arrangement may have a further advantage in that fluid may be made to circulate closer to the edge of the substrate table WT. The circular groove  420  may be sealed using a much smaller number of components. In the embodiment shown, a sealing ring  410  is used, which may be split into two components for ease of assembly and attached to the groove by glue or some other standard bonding technique. The improved fluid distribution provides a more even and controlled cooling for the substrate table WT, allowing more effective thermal management and improved overlay. 
     In the above-described embodiments, local substrate temperature sensors  60 , where included, have been shown embedded in the substrate table WT close to the substrate W. These sensors may operate on a variety of principles based generally on the measurement of a calibrated and reproducible temperature-dependent property (such as electrical resistance). Although shown embedded in the substrate, the local sensors may also be positioned in the barrier member  12  as shown in  FIG. 19 . As the thermal connection is relatively poor across the gap  22  (unlike that between the substrate W and sensors when embedded in the substrate table WT, where a high thermal conductance can more easily be arranged), it is desirable that sensors  60  located in the barrier member  12  operate by analyzing radiation emitted from the substrate W. According to an embodiment of the invention, sensors  60  of this type are provided that comprise a radiation capture and analysis device that is capable of determining an intensity spectrum of captured radiation over a range of wavelengths. In general, the temperature may be determined most accurately if a wide range of wavelengths are chosen. However, for the temperatures of interest in the present application, it is cost-efficient to choose a limited wavelength range encompassing and/or centered on the infra-red radiation band. 
       FIG. 20  shows an embodiment of the invention, in which local miniature temperature control systems  600  are built into the substrate table WT. In the example shown, these control systems  600  are positioned near the tips of raised portions of the substrate table WT (burl tops  640 ), which are in turn in contact with the substrate W. Each miniature control system  600  comprises a miniature temperature sensor  610 , which may be realized as a micro-power integrated circuit temperature sensor, and a miniature heater  620 , which may be realized as an integrated circuit heater (dissipating heat resistively). The miniature control systems  600  are arranged so that the heater component  620  is activated to emit heat when the local temperature of the substrate, as measured by the miniature temperature sensor  610 , falls below a predetermined threshold value. Once the temperature is increased such that it passes again through the threshold value, the miniature control system  600  is configured to switch the miniature heater off. This arrangement may have the advantages of being able to provide highly localized temperature control due to the miniature size of the control systems  600  and also of not needing a separate external control system to control the heaters  620 . Only two wires (connection  630 ) will be required to supply voltage to all of the miniature control systems  600  in the substrate table WT. Construction of the miniature temperature sensors  600  in the burl tops  640  may be carried out by forming the substrate table WT from a silicon wafer. Microfabrication techniques, such as MEMS (Micro-Electro-Mechanical Systems), and CMOS (Complementary Metal Oxide Semiconductor) technologies may be used to provide an exact reproduction of the standard substrate table WT construction, while also adding the integrated-circuit temperature sensor/heater  610 / 620  on each burl top  640  and providing the means for connecting them electrically to the external world (connection  630 ). 
       FIG. 21  depicts an embodiment of the invention comprising a projection system controller  710 , configured to adjust the properties of the patterned radiation beam in response to measurements of the substrate and/or substrate table temperatures made by temperature sensors  60 . In the embodiment shown, a plurality of temperature sensors  60  are embedded in the substrate table WT. However, it would also be within the scope of the invention to provide temperature sensors elsewhere, such as in the barrier member  12 , and/or for only a single temperature sensor to be provided. 
     As discussed above, the evaporation of immersion liquid on the substrate W may lead to substrate cooling, the resulting distortions possibly leading to overlay errors, defocus, aberration, etc. According to the present embodiment, the projection system controller  710  is capable of adjusting parameters of the patterned projection beam, such as its overall scaling, positional offset etc., so as to compensate for the thermally-induced distortions of the substrate W. As a simple example, if the projection system controller  710  receives input from the temperature sensors  60  indicated that the substrate W is, to a first approximation, uniformly below a target temperature, it may be configured to scale the patterned projection beam by a factor so as to reduce the size of the image generated on the cooled substrate W. Where the temperature of the substrate W and/or substrate table WT is measured by a plurality of temperature sensors  60 , such that a temperature profile is obtained, more complex corrections may be implemented by the projection system controller  710  in order to reduce errors such as overlay errors, defocus and aberration. This approach may provide a rapid way for responding to sudden changes in temperature without having to incorporate heating elements in either the barrier member  12  or the substrate table WT, which may be expensive to implement and/or interfere with the dynamic performance of the substrate table WT. This form of compensation may have the added advantage of not being dependent on the particular cooling mechanism at work and may be applicable to situations in which at least a contribution to the change in temperature of the substrate W occurs due to processes other than evaporation of the immersion liquid. 
     In the embodiment shown in  FIG. 21 , a thermally-induced-distortion calculator  700  is also provided to translate the readings taken by the temperature sensors  60  to an estimated distortion of the substrate W. This is achieved by first deriving a temperature profile of the substrate W and then using the known thermal properties of the substrate W, such as the thermal expansion coefficient of the substrate material, to calculate the thermally-induced distortion. To a first approximation, the relative distortion of a portion of the substrate W will be proportional to a temperature difference between that of the portion and a reference operating temperature (corresponding to zero relative distortion). In the embodiment shown, the temperature sensors  60  are embedded in the substrate table WT so that additional calculations need to be carried out to derive the substrate temperature profile from the temperature sensor readings. How this may be achieved is described below in relation to this and other embodiments of the invention. 
     A measure of the temperature of the substrate W may be determined according to several embodiments discussed above by temperature sensors  60  positioned in the substrate table WT. This arrangement may have constructional advantages as there is relatively more space to position the sensors, they may be robustly and accurately positioned, and they may be more easily serviced by whatever electrical connections are required. As discussed earlier, positioning sensors at a distance from the substrate W in the substrate table WT may also provide an effective way to sample a larger area of substrate W per sensor  60 . However, it should be understood that while the temperature of the material immediately surrounding the temperature sensor  60  may give an approximate indication of the temperature of the substrate W, it is possible to obtain a more accurate picture of the substrate temperature profile by further analysis, as described below. This analysis may be implemented as part of any of the embodiments described above comprising temperature sensors  60  positioned in the substrate table WT. 
     Assuming that the heat transport from the substrate surface to the level in the substrate table WT where the temperature sensors  60  are positioned may be described as:
 
 T   ∞   chuck   −T   current   check   =f (Δ T   substrate ),  (1)
 
where T ∞   chuck  is the initial temperature of the substrate W, T current   chuck  is the current temperature of a region of the substrate measured by a sensor  60  embedded in the substrate table WT, and ΔT substrate  is the temperature difference at substrate level for the region in question, the temperature of the substrate region, and therefore a temperature profile for the substrate as a whole (if required), may be obtained based on this relationship. For example, the following model may be used:
 
 T   ∞   chuck   −T   current   chuck   =k·e   −ΔT     substrate     /τ ,  (2)
 
from which it follows that
 
Δ T   substrate =−τ ln(( T   ∞   chuck   −T   current   chuck )/ k ),  (3)
 
which provides an expression for the temperature difference at substrate level based only on parameters T and k, which in turn may be estimated from test data.
 
     Similar analyses may be used to derive a better measure of the substrate temperature from infra-red temperature sensor signals. Here, a problem is that silicon (which is frequently used as a substrate material) is significantly transparent to infra-red, so that infra-red sensors positioned in the barrier member  12  “looking down” on the substrate W may receive a mixture of radiation emitted from both the substrate W and the substrate table WT immediately below it. 
     As mentioned above, as the barrier member  12  moves relative to the substrate W, a thin film of liquid may be left behind on the top surface of the substrate W in the wake of the barrier member  12 . If no effective counter measures are taken, evaporation of this liquid may extract heat from the substrate W and/or substrate table WT. The resulting decrease in the temperature of the substrate W and/or substrate table WT may lead to shrinking which may in turn lead to overlay error, general loss of performance/resolution and/or yield loss of the device to be manufactured. Several solutions to this issue have been described above, including providing a net of substrate heating channels and/or an array of independently controlled electrical heaters. However, it may be difficult to coordinate the operation of these heating mechanisms in such a way that heat is only generated where evaporation actually occurs. Therefore, it may be difficult to ensure that temperature gradients within the substrate W are reduced or minimized. 
     According to an embodiment of the invention, the lithographic apparatus is provided with a device to heat immersion liquid left in the wake of the barrier member  12  using microwave radiation. The frequency of the microwave radiation can be tuned so as predominantly to heat the immersion liquid directly, and not to couple to the surrounding apparatus elements (such as the substrate table WT, substrate W, barrier member  12 , etc.). Heating power may therefore be directed precisely to where it is required and temperature gradients may thereby be reduced or minimized. In principle, the heat needed to evaporate the liquid may be completely provided by the microwave source so that no heat is extracted from the substrate W. 
       FIG. 22  shows an example arrangement, comprising a microwave source  800 , configured to provide microwave radiation suitable to heat the immersion liquid being used, and a microwave containment cage  810 , designed to contain the microwave radiation within a region of interest (and to protect regions, such as the immersion liquid reservoir  25 , where heating is not desirable). In the embodiment shown, the region of interest covers a substantially peripheral region of the substrate W surrounding the barrier member  12 . The size of the region covered by the microwave containment cage  810  may be chosen to be large enough that the microwave radiation can completely evaporate liquid left behind in the wake of the barrier member  12  before the substrate table WT has moved far enough relative to the barrier member  12  that the liquid would leave the region exposed to the microwaves. The size of the microwave containment cage  810  should therefore be a function of the intensity of the microwave radiation to be maintained in the containment cage  810 , the speed at which the substrate table WT moves under the barrier member  12 , and the quantity of liquid that is expected to be left behind in the wake of the barrier member  12 . 
     The microwave containment cage  810  may be formed from a metallic material, with openings of a size suitable to ensure substantially complete reflection of the microwaves. Propagation of the microwave radiation within the microwave containment cage  810  is shown schematically via arrows  830 . The power of the microwave source  800 , which determines the rate at which liquid left behind on the substrate W will be heated, may be selected on the basis of calibration measurements. For example, test measurements may be carried out for a number of different microwave source powers in order to determine which power leads to a minimum overlay error, for example. Alternatively, one or more temperature sensors  60  may be provided and incorporated into a feedback loop controlled by a microwave source controller  820  via data connection  850 , the microwave source controller  820  configured to control the microwave source  800  via data connection  840 . This arrangement may be advantageous where the speed of the substrate table WT varies with time and/or when the amount of immersion liquid escaping from the barrier member  12  varies. The feedback mechanism may also be useful when the microwave heating arrangement is to be used in combination with other temperature compensation methods, which may vary in efficiency with time. As in the feedback arrangements discussed above, the feedback loop here may involve adjusting the power of the microwave source  800  in order that the temperatures measured by the temperature sensor(s)  60  converge towards one or more target temperatures. It is envisaged that the microwave source  800  will emit microwave wavelength radiation. However, if the wavelength of radiation that couples most effectively to the immersion liquid being used happens to be outside of the conventional range of wavelengths normally associated with microwaves, it is to be understood that the source  800  could be adapted to emit the appropriate wavelengths of radiation (for example, in the infrared or visible spectra). 
       FIG. 23  shows an arrangement of substrate heaters that can adjust the spatial variation in heat output according to the spatial variation in substrate temperature without the need for a complex arrangement of temperature sensors and/or external control systems. This is achieved by forming one or more electrically conductive strips  900  near the surface of the substrate table WT, in good thermal contact with the substrate W. For example, the electrically conductive strip(s)  900  may be formed by coating an electrically conductive material on a top surface of the substrate table WT. In the embodiment shown, an external current source  920  (not shown) is provided that passes a constant current  910  through each of the one or more electrically conductive strips  900 . According to a first variation, a single current source  920  may be provided that passes the same current through each of the one or more electrically conductive strips  900 . Alternatively, a plurality of current sources  920  may be provided that are configured to pass different currents through the one or more electrically conductive strips  900 . In either case, the current that is passed through each electrically conductive strip should be held constant so that the rate of heat produced by resistive heating per unit length depends only on the local resistivity of the material forming the electrically conductive strips  900 . According to the present embodiment, the material is chosen to have a negative temperature dependence (i.e. so that an increase in temperature leads to a decrease in resistivity), desirably a strongly negative temperature dependence, so that significantly more heat is generated in cooler regions (which have higher resistivity) than in relatively warmer regions of each of the electrically conductive strips  900 . In this way, more heating power is naturally directed to those regions which need to be heated the most, thus reducing temperature gradients. In particular, the current  910  may be varied until the difference in heating power between the cooler regions and the warmer regions is such as to substantially compensate the heat extracted by evaporation of liquid on the surface of the substrate W (which, as discussed above, may be a major contributor to substrate temperature inhomogeneity). The heat generated in any given segment of one of the electrically conductive strips is expected to be proportional to the square of the current multiplied by the resistance of the segment. 
     In the above embodiment, the temperature dependent resistivity of the electrically conductive strip(s)  900  was used directly to provide temperature dependent substrate heating by using the one or more electrically conductive strips themselves as heaters. According to an alternative embodiment of the invention, the one or more electrically conductive strips  900  can be used as one or more temperature sensors, possibly in combination with their function as one or more heating elements.  FIG. 24  shows an arrangement according to this embodiment. Again, a current  910  is passed through an electrically conductive strip  900 , which is arranged to have a temperature dependent resistivity. In an embodiment, as before, the temperature dependent resistivity is strongly negative, but a weaker temperature dependence and/or a positive temperature dependence may still be tolerable. According to this embodiment, one or more separate local substrate heaters  930  are provided which are each powered by a localized power supply/amplifier  950 . The power supplied to each substrate heater  930  is controlled by reference to a measurement of the local resistivity in a segment of an electrically conductive strip  900  that is closest to the substrate heater  930  in question. This may be achieved, for example, by measuring the potential difference between the nearest pair of electrodes  940 , as shown in  FIG. 24 . As before, the local resistivity of the electrically conductive strip  900  is a function of the local temperature. 
     Calibration measurements may be used to establish the relationship between the resistivity and the local temperature of the substrate W and the power supply/amplifier  950  may be configured to adjust the power of the substrate heater  930  based on the difference between the measured resistivity and the resistivity corresponding to a desired temperature. 
     The above arrangement may have an advantage of not being restricted by the strength of the temperature dependence of the resistivity of the electrically conductive strip(s)  900  and can, in principle, provide a more strongly spatially dependent heating power to the substrate W. Providing a large number of pairs of power supplies/amplifiers  950  and substrate heaters  930  allows a high spatial resolution. In addition, because the heating power to be provided to the substrate heater  930  is determined via a simple measurement of the resistivity of a nearby segment of an electrically conductive strip  900 , there is no need for complex and bulky electronics at the substrate table level, or for complex control electronics provided externally. The amplification factor (or amplification function: the function describing how the local heater power should vary as the temperature offset from the desired temperature varies) provided by the one or more power supplies/amplifiers  950  may be determined beforehand by reference to calibration measurements and may be provided in hardware. 
       FIG. 25  shows an embodiment of the invention in which the substrate W is heated by induction heaters. This approach may have an advantage that heating is provided predominantly where it is required (e.g. in the proximity of the barrier member  12 ). An induction source  960  acts to provide the induction heating power by coupling to an induction element  970  formed within the substrate table WT and, in an embodiment, in a position so as to be in good thermal contact with the substrate W. The power output of the induction source  960  is controlled in turn by an induction controller  980 . The induction controller  980  may vary the power of the induction source  960  according to a pre-set routine (for example, so as to heat predominantly the region of the substrate W which recently passed the barrier member  12 ). A plurality of spatially separated induction sources and/or elements may be provided, each configurable, for example, to provide the same or a different heating power, in order that heating may be provided to the substrate W in such a way that temperature gradients may be reduced. Alternatively, the induction controller  980  may use a feedback model. This may be configured to vary the output power of the one or more induction sources  960  so that the temperature measured by one or more temperature sensors  60  (connected to the induction controller  980 ) converge on one or more corresponding target temperatures. The use of induction heating to heat the substrate table WT may have a further advantage in that only minor alterations may need to be made to the substrate table WT (such as adding the induction element  970 ). Therefore, the mechanical operation of the substrate table WT may not be significantly interfered with. The fact that the induction source  960  is mechanically separated from the induction element  970  is advantageous also from the point of view of upgradeability: each component may be adapted to a large extent independently of the other. 
     As described above, one approach to reducing cooling caused by immersion liquid evaporating from the surface of the substrate W, is to supply humidified gas to the gas seal  27  (“humidified gas” being understood in a broad sense to mean gas that contains a significant proportion of the immersion liquid in vapor form). Where the atmosphere above the substrate W contains a higher proportion of immersion liquid vapor, the dynamic equilibrium that is established between evaporation of the liquid from the substrate W and condensation of the liquid vapor onto the substrate W is such that the net evaporation rate is lower than if the atmosphere above the substrate W was dry (i.e. not containing any significant quantity of immersion liquid vapor). In order for this mechanism to work in a reproducible and consistent manner, a reliable mechanism to humidify the gas supplied to the gas seal  27  should be provided. According to an embodiment, which is illustrated schematically in  FIG. 26 , a humidifying section  1000  is provided for this purpose. Gas is input to the humidifying section  1000  from a clean gas source (for example) via conduit  1005  to evaporation vessel  1010 . The evaporation vessel  1010  comprises one or more internal heating elements that heat one or more baths of immersion liquid in order to produce immersion liquid vapor. The immersion liquid vapor thus produced is mixed with the clean gas supplied via conduit  1005  and output from the evaporation vessel  1010  via conduit  1015 . The partially saturated gas is then input to condensation vessel  1020  (also referred to as a “cooling vessel”), in which it is cooled to such an extent that the mixture of gas and immersion liquid vapor becomes over saturated and immersion liquid condenses out of the mixture. What remains in gaseous form is very close to or exactly 100% saturated with immersion liquid vapor at the working temperature of the condensation vessel  1020 . This 100% saturated gas supply is then input via conduit  1025  to mixing chamber  1040  where it may be mixed with a source of dry gas input at a controlled rate via conduit  1035  from dry gas source  1030  in order to enable the output of gas, via conduit  1045 , at a controlled temperature and/or controlled level of saturation, which may then be supplied to the gas seal  27 , for example. 
     An alternative system for humidifying gas is to pass it through a so-called bubbler, which is a porous device immersed in a vessel containing liquid and liquid vapor. The gas becomes increasingly saturated with liquid as it passes through. However, it may be difficult in this arrangement to control the level of saturation or humidity of the gas that is produced. Variations in flow, in vessel temperature, or in liquid level may all influence the amount of liquid vapor held in the gas that leaves the system. In particular, it may be difficult to achieve 100% saturation using this method. Optimizing the performance of such a system may require the design of a relatively complex apparatus, for example to ensure adequate and reproducible contact between the liquid and gas. 
     As described above, evaporation of immersion liquid from the substrate W may have negative effects on the performance of the lithographic apparatus. Contamination in the liquid may cause particle contamination (also known as water stains) on the substrate W. Evaporation may also negatively effect overlay performance, focus and optical performance because of cooling effects. Humidified gas in the gas seal  27  may be used to minimize evaporation. According to one approach, 100% saturated gas is desirable so as to achieve zero net evaporation from the surface of the substrate W. An embodiment has been described above that is designed to produce 100% saturated gas in a controlled manner. However, the relative humidity of the gas inevitably drops as it is output from the gas seal  27  due to expansion of the gas. In practice, this may mean that at the point of use (i.e. at the substrate surface) the maximum achievable humidity may be substantially lower than 100%, for example around 60%, when the gas is delivered at the working temperature of the substrate W (for example 22° C.). If an atmosphere of less than 100% relative humidity is maintained above the substrate surface, some net evaporation will occur. 
     According to an embodiment, which is illustrated schematically in  FIG. 27 , the humidity of the gas after it has left the gas seal  27  and has expanded, is controlled (increased) by increasing the temperature of the gas supplied to the gas seal. 
     In this scenario, the hot gas leaving the gas seal  27  is suddenly exposed to an environment at a lower temperature (i.e. the normal working temperature of the lithographic apparatus) and cools. Cooling tends to increase the level of saturation or relative humidity. The overall temperature drop may be controlled to compensate closely for the expansion of the gas and the associated reduction in saturation. 
     For a typical system maintained at a working temperature of 22° C. with gas supplied to the gas seal  27  close to saturation (for example 90-100% relative humidity), and a typical barrier member pressure drop of 0.4 bars, a temperature offset of between 1 and 5 K may be adequate for maintaining near 100% relative humidity in the gas left above the substrate W outside of the gas seal  27 . Careful design of the system may be required to prevent condensation of the highly saturated gas before it leaves the gas seal  27 . For example, the walls of the conduit leading to the gas seal  27  through the barrier member  12  should be thermally insulated so as to isolate the hot gas from the cool barrier member  12  and prevent condensation on the walls of the conduit. 
       FIG. 27 , mentioned above, shows an example arrangement for controlling the temperature of the gas to be supplied to the gas seal  27 , which may be positioned, for example, between the humidifying section  1000  and the gas seal  27 . Relatively cool saturated gas is supplied by conduit  1045  to heat exchanger  1100 , which heats the saturated gas to a target temperature by exchanging heat with a heat exchange fluid provided by heater  1110 . The heater  1110  provides heat exchange fluid via output line  1120  at temperature T 1  and receives heat exchange fluid via input line  1130  at temperature T 2 , where T 1  is greater than T 2 . The heater  1110  may heat the heat exchange fluid by means of a Peltier heater, for example. According to an example arrangement, a Peltier heater is provided that works in the 500 watt to 1500 watt range to generate temperature controlled liquid (e.g., water) to an accuracy of ±0.01° C. with a set point of 27° C. 
     According to an embodiment of the invention illustrated schematically in  FIG. 28 , a highly purified stream of humidified gas is created using a humidifier cabinet  1200 , in which several evaporating units  1220  in parallel act to evaporate liquid. The temperature of the humidified gas produced is controlled by supplying to each of the evaporators  1220  a stream of temperature controlled heat exchange fluid by conduit  1205 . The heat exchange fluid may be provided by heat exchange fluid source  1110 , which may also be used to control the temperature of the saturated gas just before it is fed to the gas seal  27  as described above. Alternatively, a separate heat exchange fluid source may be provided. The humidified and temperature controlled gas is passed via hydrophobic filters  1210  to output valve  1250  before being passed via heat exchanger  1100  to the gas seal  27 . 
     Changing or even stopping the humidified gas flow from the humidifier cabinet  1200  changes the equilibrium and may require a long stabilization time before humidified gas may again be supplied to the gas seal  27  with a well controlled temperature and degree of saturation. However, due to the dynamic nature of the task performed by the barrier member  12  and gas seal  27 , the rate of gas required by the gas seal  27  may vary considerably with time: for example, there may be short periods where the gas seal  27  does not function. Rather than adapt the system so that it can stabilize more quickly, which might require substantial and complex additional apparatus, the present embodiment includes a variable vent system  1240  which allows gas to be vented at a controlled rate to an external reservoir or to an exhaust. The vent system  1240  can be configured such that the flow from the humidifier cabinet  1200  remains constant. This may be achieved in practice by ensuring that the total flow through main valve  1250  and vent system  1240  is constant. This may be implemented by configuring the vent system  1240  to have a flow impedance that is responsive to the reading of pressure gauge  1230 , which corresponds to the backpressure “felt” by the humidifier cabinet  1200 . In particular, this pressure should be kept constant. This arrangement provides not only for better stability but also higher throughput because the need for a stabilization time between different operational phases of the gas seal  27  may be avoided. 
     All of the above features can be combined in any combination and applied where relevant to all types of liquid supply system including those mentioned in the background section above. 
     In an embodiment, there is provided a lithographic apparatus, comprising: a substrate table constructed to hold a substrate; a projection system configured to project a patterned radiation beam onto a target portion of the substrate; a liquid supply system configured to at least partly fill a space between the projection system and the substrate with liquid; a barrier member arranged to substantially contain the liquid within the space between the projection system and the substrate; and a liquid evaporation controller arranged to control a net rate of evaporation of liquid supplied by the liquid supply system. 
     In an embodiment, the apparatus further comprises a gas seal connected to a gas source, configured to control the amount of liquid that escapes from the barrier member through a gap delimited on one side by a boundary of the barrier member and on a second side by the substrate, wherein the liquid evaporation controller comprises a gas humidity controller configured to interact with the gas source to provide gas with a controlled relative humidity of greater than about 10%. In an embodiment, the gas humidity controller is configured to produce a stream of humidity-controlled gas at a constant flow rate and comprises a gas seal flow-rate controller configured to receive the constant stream from the humidity controller and vary the rate of flow of gas to the gas seal by selectively venting a portion of the constant stream supplied by the humidity controller to an external reservoir. In an embodiment, the humidity controller comprises a humidifying section configured to humidify a stream of gas to a controlled extent, the humidifying section comprising: an evaporation vessel configured to receive a stream of relatively dry gas and at least partially humidify the stream of gas with liquid vapor evaporated from a bath; and a cooling vessel configured to be held at a temperature substantially lower than that of the evaporation vessel and to receive and cool the at least partially humidified gas stream in order to obtain a stream of completely saturated gas. In an embodiment, a saturated gas output of the cooling vessel is connectable to a dry gas source, and wherein the gas humidity controller is configured to adjust the rate at which dry gas is mixed with the stream of saturated gas output from the cooling vessel in order to obtain a stream of gas with a controlled relative humidity level. In an embodiment, the apparatus further comprises a gas temperature controller arranged to interact with the gas source to control the temperature of the gas supplied to the gas seal, wherein the temperature of the gas before entering the gas seal is arranged to be higher than the average temperature of the substrate. In an embodiment, the temperature of the gas before entering the gas seal is arranged to be between 1 and 5 K higher than the average temperature of the substrate. In an embodiment, the temperature of the humidified gas supplied to the gas seal is such as to achieve a desired level of humidity after expansion of the gas in the gas seal. In an embodiment, the gas humidity controller is configured to provide gas with a relative humidity greater than 40%. In an embodiment, the apparatus further comprises a temperature sensor configured to measure the temperature of at least a portion of the substrate, or at least a portion of the substrate table, or at least a portion of a substrate holder, or any combination thereof, and wherein the humidity controller is capable of adjusting the relative humidity of the gas supplied by the gas source so as to reduce a difference between the temperature measured by the temperature sensor and a target temperature. In an embodiment, the liquid evaporation controller comprises a gas-shower outlet configured to supply gas with a controlled relative humidity of greater than about 10% to the region outside of the barrier member between the substrate and the projection system. In an embodiment, the gas-shower outlet is configured to supply gas with a relative humidity in the range of about 40% to about 50%. In an embodiment, the apparatus further comprises: a temperature sensor configured to measure the temperature of at least a portion of the substrate, or at least a portion of the substrate table, or at least a portion of a substrate holder, or any combination thereof; and a gas-shower outlet controller capable of adjusting the relative humidity of the gas supplied by the gas-shower outlet so as to reduce a difference between the temperature measured by the temperature sensor and a target temperature. In an embodiment, the apparatus further comprises: a gas seal connected to a gas source, configured to control the amount of liquid that escapes from the barrier member through a gap, delimited on one side by a boundary of the barrier member and on a second side by the substrate, wherein the gas source provides gas with a controlled relative humidity of greater than 10%; and a gas-shower outlet configured to supply gas with a controlled relative humidity, substantially equal to that of the gas supplied by the gas source, to the region outside of the barrier member between the substrate and the projection system. 
     In an embodiment, there is provided a lithographic apparatus, comprising: a substrate table constructed to hold a substrate; a projection system configured to project a patterned radiation beam onto a target portion of the substrate; a liquid supply system configured to at least partly fill a space between the projection system and the substrate with liquid; a barrier member arranged to substantially contain the liquid within the space between the projection system and the substrate; a substrate table displacement system, arranged to move the substrate table along a predetermined path relative to the barrier member, thereby moving the target portion over the surface of the substrate; and a microwave source and microwave containment device, together configured to supply heat to liquid on the surface of the substrate. 
     In an embodiment, the microwave containment device comprises a metallic cage and defines a volume within which microwave radiation produced by the microwave source can propagate. In an embodiment, the microwave containment device is fixed relative to the barrier member and the volume extends to cover only a sub-region of the surface of the substrate around the barrier member, at any one time. 
     In an embodiment, there is provided a lithographic apparatus, comprising: a substrate table constructed to hold a substrate; a projection system configured to project a patterned radiation beam onto a target portion of the substrate; a liquid supply system configured to at least partly fill a space between the projection system and the substrate with liquid; a barrier member arranged to substantially contain the liquid within the space between the projection system and the substrate; a substrate table displacement system, arranged to move the substrate table along a predetermined path relative to the barrier member, thereby moving the target portion over the surface of the substrate; and a substrate heater configured to heat at least a portion of the substrate according to a position of the substrate table relative to the barrier member, or a velocity of the substrate table relative to the barrier member, or an acceleration of the substrate table relative to the barrier member, or a predetermined path of the substrate table relative to the barrier member, or a local substrate temperature, or a local substrate table temperature, or any combination thereof. 
     In an embodiment, the substrate heater comprises an induction heater, or a visible-light source, or an infrared emission source, or a glow wire electrical resistance heater, or a temperature-controlled gas jet, or any combination thereof. In an embodiment, the induction heater is configured to heat the substrate via an induction plate associated with the substrate table and formed of a material suitable for induction heating. In an embodiment, the substrate heater comprises a plurality of local heaters, each capable of heating substantially separate portions of the substrate, wherein the local heaters are arranged to be switched to a heat emitting state when positioned to heat a region of the substrate over which the barrier member has already passed, and to be switched to a non-heat emitting state when positioned to heat a region of the substrate over which the barrier member has yet to pass. In an embodiment, the apparatus further comprises a substrate table path determining device, configured to determine the position of the substrate table, or the velocity of the substrate table, or the acceleration of the substrate table, or the predetermined path of the substrate table, or any combination thereof. In an embodiment, the substrate heater comprises a plurality of remote heaters positioned around the periphery of the barrier member. In an embodiment, the power output of the remote heaters is controlled depending on the direction of motion of the substrate table relative to the barrier member, as determined by a substrate table path determining device. In an embodiment, remote heaters at positions closest to a leading edge of the barrier member are configured to provide a lower power output than remote heaters positioned closest to a trailing edge of the barrier member. In an embodiment, the apparatus further comprises: a temperature sensor configured to measure the temperature of at least a portion of the substrate, or of at least a portion of the substrate table, or of at least a portion of a substrate holder, or any combination thereof; and a substrate temperature controller, arranged to control the output of each of the plurality of remote heaters so as to reduce a difference between the temperature measured by the temperature sensor and a target temperature. In an embodiment, the temperature sensor comprises a radiation capture and analysis device capable of determining an intensity spectrum of captured radiation over a range of wavelengths including infrared. In an embodiment, the substrate heater comprises a local heater arranged to heat predominantly a distinct portion of the substrate and the apparatus further comprises: a temperature sensor configured to measure a temperature of at least a portion of the substrate, or of at least a portion of the substrate table, or of at least a portion of a substrate holder, or any combination thereof; and a substrate temperature controller, arranged to control the output of the local heater so as to reduce a difference between the temperature measured by the temperature sensor and a target temperature. In an embodiment, the local heater is arranged to follow substantially a predetermined path of the substrate table relative to the barrier member. In an embodiment, the substrate heater comprises a liquid temperature controller arranged to interact with the liquid supply system to control the temperature of the liquid to be filled in the space between the projection system to a temperature greater than about 295 K. In an embodiment, the apparatus further comprises a gas seal configured to control the amount of liquid that escapes from the barrier member through a gap delimited on one side by a boundary of the barrier member and on a second side by the substrate, the gas seal being supplied with pressurized gas by a pressurized gas supply system, and wherein the substrate heater comprises a gas temperature controller arranged to interact with the pressurized gas supply system to control the temperature of the pressurized gas supplied to the gas seal to a temperature greater than about 300 K. In an embodiment, the substrate heater comprises elongated elements arranged in substantially parallel strips, the strips being oriented substantially perpendicular to a scanning direction of the substrate table relative to the barrier member, the separation of the strips being arranged to increase progressively from a first strip, corresponding to a region of a substrate in which the projection system is configured to project the radiation beam during a first time period, to a final strip, corresponding to a region of the same substrate in which the projection system is configured to project the radiation beam during a time period later than the first time period. In an embodiment, each substantially parallel strip provides a uniform power per unit length along its length. In an embodiment, the substrate heater comprises elongated elements arranged in substantially parallel strips, the strips being oriented substantially perpendicular to a scanning direction of the substrate table relative to the barrier member, and wherein the strips are arranged to provide a progressively decreasing power per unit length of strip from a first strip, corresponding to a region of a substrate in which the projection system is configured to project the radiation beam during a first time period, to a final strip, corresponding to a region of the same substrate in which the projection system is configured to project the radiation beam during a time period later than the first time period. In an embodiment, the substrate heater is arranged to provide a higher heating power at target regions on a substrate where the projection system is configured to project the patterned radiation beam at a first time and progressively lower heating powers at target regions on the same substrate where the projection system is configured to project the patterned radiation beam at later times. In an embodiment, the substrate heater comprises an array of individually addressable local heaters and a heater array controller adapted to control actuation of the individually addressable local heaters according to a predetermined algorithm, the predetermined algorithm controlling actuation with respect to heater position, or timing, or amount of heat generated, or a rate of heat generated, or any combination thereof. In an embodiment, the substrate heater comprises: an electrically conductive strip arranged to be in good thermal contact with a portion of the substrate, the resistivity of the electrically conductive strip selected so that resistive heating from electrical current is higher in relatively cooler regions of the substrate than it is in relatively warmer regions of the substrate; and a current source configured to pass a current of controlled magnitude through the electrically conductive strip. In an embodiment, the magnitude of the current is such as to minimize a temperature gradient caused by evaporation of liquid from the surface of the substrate. In an embodiment, the conductive strip is formed from a material with an electrical resistance that decreases with increasing temperature. In an embodiment, the apparatus further comprises a plurality of local substrate heaters, each configured to supply heat to a localized sub-region of the substrate; a local substrate heater controller configured to control the output power of at least a subset of the plurality of local substrate heaters; and a plurality of resistivity measuring devices, each configured to measure the resistivity of at least a portion of the electrically conductive strip, wherein the output power of each of the local substrate heaters is controlled according to the resistivity of a portion of the electrically conductive strip nearest to the localized sub-region to be heated and measured by one of the resistivity measuring devices. In an embodiment, the apparatus further comprises a constant current source configured to pass a current of controlled magnitude through the electrically conductive strip and wherein one of the resistivity measuring devices operates by measuring a potential difference arising between at least two points on the electrically conductive strip. 
     In an embodiment, there is provided a device manufacturing method, comprising: projecting a patterned radiation beam through a liquid onto a target portion of a substrate; and controlling a net rate of evaporation of the liquid. 
     In an embodiment, there is provided a device manufacturing method, comprising: projecting a patterned radiation beam through a liquid onto a target portion of the substrate; moving a substrate table holding the substrate along a predetermined path relative to a seal member used to contain the liquid, thereby moving the target portion over a surface of the substrate; and heating liquid on the surface of the substrate using microwaves. 
     In an embodiment, there is provided a device manufacturing method, comprising: projecting a patterned radiation beam through a liquid onto a target portion of a substrate; moving a substrate table holding the substrate along a predetermined path relative to a seal member used to contain the liquid, thereby moving the target portion over a surface of the substrate; and heating at least a portion of the substrate according to a position of the substrate table relative to the seal member, or a velocity of the substrate table relative to the seal member, or an acceleration of the substrate table relative to the seal member, or a predetermined path of the substrate table relative to the seal member, or a local substrate temperature, or a local substrate table temperature, or any combination thereof. 
     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. 
     Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured. 
     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) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams. 
     The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic 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 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. 
     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 or only on a localized surface area of the substrate. 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 comprise a combination of one or more structures, one or more liquid inlets, one or more gas inlets, one or more gas outlets, and/or one or more liquid outlets that provide liquid to the space. 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. 
     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.