Patent Publication Number: US-2011069290-A1

Title: Cleaning device, a lithographic apparatus and a lithographic apparatus cleaning method

Description:
This application is a continuation of co-pending U.S. patent application Ser. No. 12/081,171, filed Apr. 11, 2008, now allowed, which is a continuation-in-part application of co-pending U.S. patent application Ser. No. 11/819,954, filed Jun. 29, 2007, now allowed, which claims priority and benefit to U.S. Provisional Patent Application No. 60/924,244, filed on May 4, 2007. The contents of each of the foregoing applications is incorporated herein in its entirety by reference. 
    
    
     FIELD 
     The present invention relates to a lithographic apparatus and a method for cleaning a lithographic apparatus. 
     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 final element of 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) 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 final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in PCT patent application publication no. WO 99/49504. 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. 
     A further immersion lithography solution with a localized liquid supply system is shown in  FIG. 4 . 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). 
     In European patent application publication no. EP 1420300 and United States patent application publication no. US 2004-0136494, each of which is hereby incorporated in its 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. 
     Immersion liquid may lift debris or particles (e.g. left over from the manufacturing process) from parts of the lithographic apparatus and/or substrates or erode components so as to introduce particles. This debris may then be left behind on the substrate after imaging or may interfere with the imaging while in suspension in the liquid between the projection system and the substrate. Thus, the issue of contamination should be addressed in an immersion lithographic apparatus. 
     SUMMARY 
     It is desirable, for example, to provide a lithographic apparatus which can easily and effectively be cleaned as well as to provide a method for effectively cleaning an immersion lithographic apparatus. 
     According to an aspect of the invention, there is provided a cleaning tool to clean a surface of a liquid confinement system of an immersion lithographic apparatus, the liquid confinement system having an aperture to allow passage of a beam of radiation therethrough of an immersion lithographic apparatus, the cleaning tool comprising: 
     a sonic transducer; 
     a reservoir configured to hold liquid between the sonic transducer and the surface to be cleaned; and 
     a barrier positioned in the reservoir under the aperture to form, in use, a shield to sonic waves. 
     According to an aspect of the invention, there is provided a combination of a liquid confinement system and a cleaning tool, wherein the liquid confinement system comprises an aperture to allow passage of a beam of radiation therethrough of an immersion lithographic apparatus and the cleaning tool is configured to clean a surface of the liquid confinement system, the cleaning tool comprising: 
     a sonic transducer; 
     a reservoir configured to hold liquid between the sonic transducer and the surface to be cleaned; and 
     a barrier positioned in the reservoir under the aperture to form, in use, a shield to sonic waves. 
     According to an aspect of the invention, there is provided a lithographic projection apparatus comprising: 
     a projection system configured to project a beam of radiation; 
     a liquid confinement system configured to confine liquid in an aperture, the aperture arranged to allow passage of the beam therethrough and the aperture being open to a final element of the projection system; and 
     a cleaning tool configured to cleaning a bottom surface of the liquid confinement system, the cleaning tool comprising a sonic transducer, a liquid supply device configured to providing liquid between the sonic transducer and the bottom surface of the liquid confinement system, and a barrier configured to form a shield to prevent sonic waves from reaching the final element. 
     According to an aspect of the invention, there is provided a method of cleaning a liquid confinement system, the method comprising: 
     providing liquid between a bottom surface of the liquid confinement system and a sonic wave source; 
     generating sonic waves and directing them in the liquid towards the bottom surface; and 
     using a barrier to prevent the sonic waves from reaching an optical element positioned in an aperture in the surface of the liquid confinement system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: 
         FIG. 1  depicts a lithographic apparatus according to an embodiment of the invention; 
         FIGS. 2 and 3  depict a liquid supply system for use in a lithographic projection apparatus; 
         FIG. 4  depicts another liquid supply system for use in a lithographic projection apparatus; 
         FIG. 5  depicts, in cross-section, another liquid supply system for use in a lithographic apparatus; 
         FIG. 6  illustrates, in cross-section, an embodiment of the invention to clean a liquid confinement system; 
         FIG. 7  illustrates, in cross-section, a further embodiment of the invention to clean a liquid confinement system; and 
         FIG. 8  illustrates, in cross-section, a detail of the embodiment of  FIG. 7 . 
     
    
    
     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 B (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) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W. 
     The illumination system 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 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 can 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 can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix. 
     The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof; as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”. 
     As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable minor 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 and/or support structures may be used in parallel, or preparatory steps may be carried out on one or more tables and/or support structure while one or more other tables and/or support structures are being used for exposure. 
     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 minors 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 AD 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 B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. Having traversed the patterning device MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in  FIG. 1 ) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the support structure MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which faun 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 PS. 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 a 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. 
     Another immersion lithography solution with a localized liquid supply system solution IH which has been proposed is to provide the liquid supply system with a barrier member that extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. Such a solution is illustrated in  FIG. 5 . 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 and may be a contactless seal such as a gas seal. 
     The barrier member  12  at least partly contains liquid in the space  11  between a final element of the projection system PS and the substrate W. A contactless seal  16  to the substrate may be formed around the image field of the projection system so that liquid is confined within the space between the substrate surface and the final element of the projection system. The space is at least partly bounded or formed by the barrier member  12  positioned below and surrounding the final element of the projection system PS. Liquid is brought into the space below the projection system and within the barrier member  12  by liquid inlet  13  and may be removed by liquid outlet  13 . The barrier member  12  may extend a little above the final element of the projection system and the liquid level rises above the final element so that a buffer of liquid is provided. The barrier member  12  has an inner periphery that at the upper end, in an embodiment, closely conforms to the shape of the projection system or the final element thereof and may, e.g., be round. At the bottom, the inner periphery closely conforms to the shape of the image field, e.g., rectangular though this need not be the case. 
     The liquid is contained in the space  11  by a gas seal  16  which, during use, is formed between the bottom of the barrier member  12  and the surface of the substrate W. The gas seal is formed by gas, e.g. air or synthetic air but, in an embodiment, N 2  or another inert gas, provided under pressure via inlet  15  to the gap between barrier member  12  and substrate and extracted via outlet  14 . The overpressure on the gas inlet  15 , vacuum level on the outlet  14  and geometry of the gap are arranged so that there is a high-velocity gas flow inwardly that confines the liquid. Those inlets/outlets may be annular grooves which surround the space  11  and the flow of gas  16  is effective to contain the liquid in the space  11 . Such a system is disclosed in United States patent application publication no. US 2004-0207824. 
     As noted above, an immersion lithographic apparatus is one in which a substrate is imaged through liquid. That is, an immersion liquid is provided between a final element of the projection system PS and the substrate. This arrangement may pose one or more particular problems. In particular, the liquid should be confined in the apparatus and also the liquid should be kept as free as possible of foreign object particles which may cause defects during imaging and/or by being left on the substrate surface after imaging and prior to, downstream processing. Sometimes the immersion liquid includes particles in suspension deliberately. 
     One way of addressing the problem of foreign object particles is by applying a cleaning liquid to the surface to be cleaned and/or introducing sonic (ultra or megasonic) waves into the liquid to clean the surface. The cleaning liquid may or may not be the same as the immersion liquid. It could, for example, be ultra pure water. 
     Compared to ultrasonic waves, megasonic waves produce cavitation bubbles (which implode or vibrate) which are very small and therefore may go very close to the surface to be cleaned. However, there is a limit to the amount of energy which may be introduced into the liquid using megasonics. Generally whereas ultrasonic energy may be introduced into a liquid anywhere and will be distributed throughout the liquid, megasonic energy is only locally high and must therefore be directed directly to the surface to be cleaned. That is, a direct path (line of sight/straight) must be present between the transducer introducing the megasonic waves and the surface to be cleaned. The whole length of that path should be filled with liquid. 
     Megasonic frequencies are generally regarded to be between 750 kHz to 3 MHz. For the present purposes frequencies of above about 750 kHz, above 1 MHz or above 1.5 MHz are used. 
     A stagnant boundary layer in the cleaning fluid near the surface of an object to be cleaned becomes thinner as the frequency of the introduced sonic energy is increased. At megasonic frequencies cleaning is partly accomplished by megasonic pulsing and acoustic streaming with high velocity pressure waves in the cleaning fluid as well as bubble vibration and, to a lesser extent cavitation and bubble bursting. 
     At megasonic frequencies, particles of less than 0.5 μm in diameter may be removed without damage to the surface being cleaned. As mentioned above, there must be a clear path from the transducer to the surface being cleaned (line of sight). In order to further increase the cleaning efficiency, gas may be dissolved into the liquid to promote cavitation (bubble formation). Suitable gases include nitrogen, carbon dioxide or oxygen and mixtures of those gases (including air) but other gases may also be suitable such as ozone, or hydrogen (containing water). The use in the liquid of, for example, a surfactant (such as a detergent, soap), an acid, an alkaline, a solvent (such as a non-polar organic solvent or a polar organic solvent), H 2 0 2  solution or any other suitable chemical for the lithographic apparatus, can further enhance cleaning efficiency. Using a detergent or a solvent in the cleaning liquid or adding H 2 0 2  solution may boost the oxidizing nature of the fluid, thereby helping to remove organic contamination. Another example is hypochlorite. 
     It is also possible to change the cleaning fluid polarity for example by adding a liquid-miscible solvent (e.g., a water-miscible solvent) with a lower polarity than the liquid (e.g., water), examples of which, include an alcohol, a ketone, an organic acid, an amine, etc. Examples of alcohols include: diethylene glycol monobutyl ether and/or ethoxylated secondary C12-14-alcohols, e.g. aldyloxypolyethyleneoxyethanol. One potential problem with adding such an additive is the lower flash point of the resulting cleaning fluid. Thus, adding an organic-based additive with a high flash point in order to vary the cleaning fluid characteristics without causing a flammability hazard is desired. Such an additive may include N-methylpyrrolidone (NMP), glycol ether, etc. Of course other additives may be added to the liquid. For example, it may be desirable to add a chemical which has a specific chemical attack on one or more specific contaminants which are to be removed. For example, resist particles could be removed with a chemical such as methylethylkeytone, ethylacetate and/or acetone. In an embodiment, the liquid may be a mixture of water, diethylene glycol monobutyl ether, and an alcohol, C12-14-secondary, ethoxylated, such as TLDR-A001 or TLDR-A001-C4, which are manufactured by Tokyo Ohka Kogyo Co., Ltd. 
     Objects which one might want to clean in an immersion lithographic apparatus include, but are not limited to, one or more parts of a substrate table WT which supports the substrate W (its top surface), a final element of the projection system PS which is immersed in immersion liquid during imaging, and/or a liquid confinement system (for example those illustrated in  FIGS. 2-5 ) which provides liquid between the final element of the projection system PS and the substrate W during imaging. Below one or more embodiments of the invention will be described in relation to cleaning the top surface of a substrate table and a barrier member, but the invention is not limited to only cleaning a substrate table and a barrier member. 
     In an embodiment, a liquid supply system provides liquid between a sonic transducer and the surface to be cleaned. In the embodiment, the liquid supply system provides a flow of liquid so that liquid is removed as the surface is cleaned such that particles removed from the surface are carried away. One suitable liquid is water in an ultra pure form. However, other types of liquid may be suitable. Furthermore, an addition to the liquid such as a surfactant as mentioned above may also have an advantage. Other cleaning liquids include water/hydrogen peroxide, water/ethanol, water/iso-propylalcohol (IPA), water/ammonia or water/acetone mixtures, for example. Other chemicals which may be useful as an addition include tetramethylammonium hydroxide (TMAH) and SC-1 or SC-2. 
     One reason for introducing gas (or some solvents) into the liquid is that this promotes stable cavitation. This results in stable bubbles being formed in the liquid. These bubbles are then vibrated by the megasonic waves which results in cleaning which is likely to do less damage to the surface being cleaned than so called transient cavitation which is cavitation where a solvent evaporates into a bubble and then implodes or collapses. These violent implosions may lead to damage of the surface and are typically seen at ultrasonic frequencies and are less significant at megasonic frequencies where the bubbles produced tend to be smaller than those produced at ultrasonic frequencies. However, as noted above, the megasonic waves should be supplied in line of sight of the surface they are to clean. 
     A process time up to 100 seconds may lead to a particle removal efficiency of up to 100% at a frequency of about 1 MHz. If the acoustic frequency exceeds much more than 3 MHz the particle removal efficiency may be dramatically decreased compared to a frequency of just above 1 MHz. The introduction of gas into the liquid has a major effect on the particle removal efficiency. Removal of 34 nm diameter SiO 2  particles may increase from zero removal efficiency to 30% removal efficiency with the introduction of oxygen at a level of 20 ppm into the liquid. Thus, a gas concentration of above about 5 ppm can be useful. 
     Temperature may also be significant and a balance should be drawn between faster reaction time at a high temperature (e.g., 55° C.) over less gas being dissolved at a high temperature. 
     There is also an effect of the pH of the liquid. At low pH, there are many H +  ions in the liquid which results in a positive surface charge. Similarly, at high pH, the liquid contains many OH −  ions which results in a negative surface charge. Therefore ensuring that the pH of the liquid is distant from pH 7 increases the likelihood that re-deposition of particles after they have been removed does not occur. Moreover, the electrostatic repulsion between the particle and the surface when both are charged equally (either positively or negatively) assists in lifting the particle from the surface. 
     The power of the transducer is desirably between 0.2 and 5 W/cm 2 , the irradiation distance between 5 and 20 mm and a cleaning time between 10 and 90 seconds. For the acoustic waves from the megasonic transducer to travel a direct path to the surface to be cleaned from the megasonic transducer, several designs are proposed to clean different parts of the immersion lithographic apparatus. 
     A megasonic cleaner is well suited to removing particles from a surface. 
     The issue of contamination in an immersion lithographic apparatus is, for example, addressed in U.S. patent application Ser. No. 11/437,876 filed May 22, 2006. In this application, a lithographic apparatus has a megasonic transducer configured to clean a surface easily and effectively. The megasonic transducer is well suited to removing particles from a surface. However, the particles may sometimes re-deposit and re-attach themselves to the surface being cleaned. Re-deposition readily occurs if the fluid flow is not accurately controlled which happens typically at one or more locations of zero flow velocity (stagnant zone) or at the interface of fluid and gas (e.g., air). Therefore, it is desirable to provide a flow of liquid between the megasonic transducer and the surface being cleaned. In particular, it is desirable to design the apparatus such that there are no locations of zero flow velocity (stagnant zones). 
     It is desirable to alleviate the aforementioned problem of particles re-attaching themselves by, for example, providing a lithographic apparatus in which the flow of cleaning fluid is effectively controlled so as to minimize the re-deposition of contaminant particles on a surface being cleaned. It is also desirable to provide a flow of liquid between the megasonic transducer and the surface being cleaned, so that the liquid in which the particles are suspended is quickly moved away. Thus the number of locations of zero flow velocity (stagnant zones) is reduced and re-deposition is avoided. 
     A flow of liquid across the distance between the transducer and the surface to be cleaned is desirable though not essential. 
     Another way to prevent or at least reduce re-attachment of the particles to the surface is to change one or more properties of the liquid between the transducer and the surface to ensure that the zeta potential of the particles and the zeta potential of the surface is such that the particles are not attracted to the surface, desirably such that they are repelled from the surface. 
     The zeta potential is the potential of a surface in a liquid. The zeta potential generally decreases with distance from the surface. A given type of material has a given zeta potential for a particular type of liquid. One way of varying the zeta potential of a surface is to change the concentration of electrolyte in the liquid, and an alternative or further method to change the zeta potential is to change the pH of the liquid. By careful selection of the concentration of electrolyte in the liquid (e.g. salt) and/or the pH of the liquid, (i) the zeta potential of the surface from which the particles are removed (and/or any other surface where adherence is to be avoided) and (ii) the zeta potential of the particles can be chosen. Desirably those two zeta potentials may be chosen such that they have the same polarity and thus each or one of the objects that has a respective zeta potential may repel from the other. 
     The pH of the liquid and/or concentration of the electrolyte can be chosen with a knowledge of the material of which the surface which is being cleaned is made and with a knowledge of the type of material of which the particles are likely to be made. If the materials are the same, then it should be easy to select a pH and/or electrolyte concentration at which the zeta potential is non-zero for both the surface and the particle. In that circumstance the potential would be either positive or negative for both the surface and the particle such that they would repel one another and the particles would be unlikely to re-adhere to the surface. If the materials are different, pH and/or electrolyte concentration may be harder to choose, but it is likely that there will be at least one pH and/or electrolyte concentration at which the zeta potential will have the same polarity for both materials. 
     Changing the pH of the liquid may have a negative impact on the solubility of materials, which can itself result in contamination or loss of material integrity. If this is deemed a problem it may be desirable to change the electrolyte concentration rather than the pH. If this is done by adding a well chosen salt (e.g. NaCl) then this may not greatly affect the pH of the liquid (i.e. the liquid remains neutral). 
     The use of two or more of the above techniques in combination (in particular changing pH and/or electrolyte concentration to change zeta potentials and the use of a surfactant) may be the best approach. 
     A problem encountered in cleaning a liquid confinement system LCS, such as an immersion hood (see e.g.  FIG. 5 ), is that the megasonic waves may heat up a (final) optical element (e.g., a lens) of the projection system if the cleaning takes place in-situ from below. This is because part of the projection system PS is exposed from underneath because an aperture  200  is made through the liquid confinement system LCS. The aperture  200  is, during imaging of a substrate, filled with liquid and the projection beam PB passes through the aperture  200  onto the substrate W. It is therefore desirable to clean a surface of the liquid confinement system LCS without heating the optical element, typically a WELLE lens, and without dispersing energy in the optical element. As previously shown in U.S. patent application Ser. No. 11/802,082 filed May 18, 2007, claiming priority from U.S. patent application Ser. No. 11/437,876, a gas (e.g., air) gap may be used as a shield to insulate the optical element from megasonic waves. However, liquid can, under certain conditions, enter that gap. 
     In an embodiment, a shield, desirably comprising a plate, may be used to shield the final element of the projection system from megasonic waves and/or liquid. Desirably the shield is a solid barrier  310  between the transducer  20  and the part of the projection system PS which is exposed through the aperture  200 . In an embodiment, the shield is a physical presence (bather) in all straight paths from the transducer  20  to the aperture  200 . The bather and/or gas gap  75  (which the bather helps to maintain) are effective to reflect sonic vibrations from the transducer  20  away from the aperture  200 . In an embodiment, the barrier  310  also seals the aperture  200  so that no liquid enters the aperture  200 . This helps ensure the integrity of the gas gap and that no liquid contacts the surface of the optical element. 
       FIGS. 6 and 7  show a cleaning tool which can be secured to the underside of the liquid confinement system LCS. The embodiment of  FIG. 6  will be described first. Note that the apparatus is, in plan, generally circular but that an embodiment of the invention may work with any type of liquid confinement system, even those which are not circular. The tool is secured to the liquid confinement system LCS by an outer sealing  400  that seals a cleaning tool body  300  to the undersurface of the liquid confinement system LCS. The outer sealing  400  is typically an O-ring. The outer sealing defines the maximum surface of the underside of the liquid confinement system LCS to be cleaned. Within the tool body  300  is a megasonic transducer  20  which is arranged to direct megasonic waves at the undersurface of the liquid confinement system LCS. Cleaning fluid fills the space between the tool body  300  and the undersurface of the liquid confinement system LCS, and so forms a reservoir. 
     The cleaning apparatus has a shield that in addition to a gas gap  75  between the top surface of the reservoir and the optical element comprises a barrier  310  that forms a solid separator between the transducer  20  and the gap  75 . The bather  310  improves the effectiveness of the shield by reflecting megasonic waves away from the optical element. The barrier  310  is in the form to act functionally as an umbrella to the optical element, i.e. to shield the optical element from liquid. The umbrella covers an aperture  200  defined by an inner rim  318  formed in the liquid confinement system LCS around the aperture  200  and is secured to the liquid confinement system LCS by an inner sealing  500 , typically an O-ring, between the barrier  310  (umbrella) and the liquid confinement system LCS. The inner sealing  500  prevents cleaning liquid from entering the gap  75  to compromise the performance of the gap  75 . 
     The cleaning tool also comprises a tube  320  which is attached to the bather  310 . The tube passes through the tool body  300 . The tube has an inlet  330  to supply cleaning liquid into the reservoir. The inlet is located near or adjacent the top of the cleaning liquid reservoir, near the inner rim  318  formed in the liquid confinement system LCS. The inlet  330  is provided as a single slit or as a plurality of discrete holes around the outer edge of the bather  310  such that a flow of liquid is provided in all radial directions. Formed in the undersurface of the liquid confinement system LCS is an outlet  10  connected to an under pressure source (e.g., a wet vacuum) to remove cleaning fluid. The outlet  10  adjoins, is, adjacent to, or is next to, the surface of the liquid confinement system LCS to be cleaned and is a feature already present on conventional liquid confinement system (e.g., outlet OUT in any of  FIGS. 2-4  and/or outlet  14  in  FIG. 5 ). So, in use, cleaning liquid flows between the tool body and the liquid confinement system LCS, toward the outlet. Thus, this embodiment also provides a flow of liquid past the surface being cleaned. The flow of liquid is radial but in this embodiment the flow is generally outwardly. However, because the transducer  20  does not move relative to the surface to be cleaned, that is not so important in this embodiment. 
     The cleaning tool may be implemented an in-line or off-line arrangement. If implemented in an off-line arrangement, the liquid confinement system LCS may be removed from the lithographic apparatus for fitment to the cleaning tool. 
     The embodiment shown in  FIG. 6  may encounter one or more problems. A source of potential instability is the under pressure in the cleaning tool, i.e. the connection of the outlet  10  to the under pressure. The suction may vary and is difficult to control because of bubble formation and pressure variation. If the under pressure is too large, bubbles form within the cleaning bath. The bubbles compromise the performance of the megasonic transducer  20 . If a surfactant and/or detergent is used in the liquid, foaming may occur—see below. If the under pressure is too small, pressure builds up within the apparatus, causing cleaning liquid to flow through existing passages in the liquid confinement system LCS and so flood the gas gap  75 . 
     The seal  500  between the tool and the liquid confinement system LCS may deteriorate, allowing liquid to leak into the gap  75 . Once cleaning liquid contacts the optical element, the optical element may absorb megasonic waves and may heat-up. Mechanical damage by the megasonic energy may be an issue. For these reasons the performance of the gas gap as an insulating shield of temperature and of megasonic waves may be compromised. 
     A further or alternative problem with the inner seal  500  is that the design of the cleaning tool is defined by the size of the barrier  310  (umbrella) required for the inner seal  500  to function. The cleaning tool may not be generic and so different designs of cleaning tool may be required for different designs of liquid, confinement system LCS. Also, there may be one or more apertures in a liquid confinement system LCS open to the underside of the liquid confinement system LCS and also in fluid communication with the aperture  200 . It is possible for liquid to migrate through these one or more apertures through the liquid confinement system LCS into the gap  75 . Once there, there may be no way in which to remove that liquid in the embodiment of  FIG. 6 . 
     Although the addition of an additive (e.g. a surfactant and/or detergent) to the cleaning liquid, such as ultra-pure water, may improve the cleaning, the additive may cause a problem. In a wet vacuum system, the cleaning liquid is agitated which may cause the cleaning liquid to foam. Foaming is a problem because certain components of the lithographic apparatus, such as sensors, can be damaged on contact with foam. The foam may cause the lithographic apparatus to malfunction or cause further problems for the fabrication environment. The problem may be enhanced by the use of a re-cycling loop in the wet vacuum generator. If a lower pressure is used than that provided by the wet vacuum, this allows pressure within the reservoir to build-up such that the leaking of liquid through one or more apertures into the gap  75  mentioned above may be more likely to occur. 
     Shown in  FIGS. 7 and 8  is a further embodiment of the cleaning tool fitted to the undersurface of a liquid confinement system LCS. It has some of the same features as the tool shown in  FIG. 6  with the following modifications. The liquid  30  flows in the opposite direction through the reservoir and the bather  310 , so the open aperture in the barrier  310  serves as an outlet  10 . The rim  317  of the open aperture of the barrier  310  adjoins, is adjacent to, or is next to, the undersurface of the liquid confinement system LCS to be cleaned. There is sufficient clearance between the inner rim  318  of liquid confinement system LCS and the barrier  310  for the cleaning liquid  30  to flow from the cleaning reservoir into the outlet  10  defined by the open aperture of the shield  310 . 
     The bather  310  serves as an umbrella and is funnel-shaped (or in other words a truncated cone shape in the reservoir). This shape together with the other features of this cleaning tool may improve the effectiveness of the insulating shield in that, in use, megasonic waves are reflected away from the optical element at a different angle to the angle at which they impinge on the shield. The liquid  30  flows over the rim  317  of the barrier  310  and down, into the outlet  10  and into an outflow tube  320 , under the influence of gravity. Thus, the barrier  310  provides an outlet  10  through which liquid exits the reservoir under gravity. Thus, the system can be seen as being an open system (compared with the embodiment of  FIG. 6  which may be seen as a closed system). Furthermore, the funnel shape of the outlet  10  and tube  320  results in a gentle flow of liquid out of the reservoir. This is particularly because of the funnel shape of the aperture and tube  320  so that liquid runs down the inclined surface of the aperture to the start of the tube part of the outlet. In this way little force is applied to the liquid to reduce the chance of foaming. Extracting liquid from the center of the reservoir results in a radial flow which in this case is radially inwardly, as opposed to radially outwardly in the embodiment of  FIG. 6 . A liquid inlet is provided and this can be provided in the tool body  300  and/or in a surface of the liquid confinement system LCS. 
     The rim  317  can be provided with a cross-sectional variation or shape that complements inner rim  318  of the liquid confinement system LCS, so that the flow path of cleaning liquid  30  out of the reservoir is curved or otherwise changes direction (i.e. is not straight) as it passes through the space that separates the rim  317  from the inner rim  318 . See  FIG. 8 . This may prevent megasonic waves from entering the aperture  200 . Alternatively or in addition, the cross-sectional variation or shaping can be provided on the inner rim  318  of the liquid confinement system LCS, so as to compliment the shape of the rim  317 . The height of the top of the rim  317  along with the hydrostatic pressure in the reservoir determines the size of the gap  75 . 
     The flow of liquid may be assisted by a ‘slow’ pump  325  (connected to the outlet  10 ) that provides a small under-pressure to avoid the generation of bubbles in the liquid. A typical flow rate is between 0.5 and 3 liters/minute. In an embodiment, the liquid flow does not pass through a wet vacuum. Between the funnel-shaped barrier  310  and the slow pump may be a restriction  328  (i.e. a flow restrictor), desirably, in the outflow tube  320 . The restriction helps minimize the formation of foam. 
     In an embodiment, the pressure used to cause liquid to flow through the cleaning tool is always the same. The pressure is determined by the difference in height between the rim  317  of the funnel-shaped barrier  310  and an inlet to supply cleaning liquid  30  into the reservoir. In an embodiment, the cleaning apparatus is a stable system because the pressure required to operate the cleaning apparatus is automatically controlled. Thus, the risk of flooding the gap  75  is reduced so that the gap  75  is more stable. The tool may provide an effective insulating shield for the optical element from temperature variation and/or megasonic waves. 
     A feature that provides improved safety and further control is a tube  340  that can be provided parallel to the funnel-shaped barrier  310  and in fluid communication with the reservoir which can be used as an indicator. The level of liquid in the tube  340  is the same as the level of liquid in the reservoir and therefore a judgment can be made as to whether the gas gap  75  is large enough. The indicator can show if there is a blockage in the tube in the barrier  310  that could cause the gas gap  75  to flood with cleaning fluid  30 . 
     An advantage of the consistent pressure in the tool may be to reduce the quantity of bubbles formed within the reservoir. The effectiveness of the megasonic transducer may increase as a result. 
     As the cleaning liquid flows smoothly through the cleaning tool, there may be less gas/cleaning liquid contact than in known wet vacuum systems. Therefore, foaming within the cleaning tool may be reduced. This enables an additive (surfactant and/or detergent) to be added to the cleaning fluid that would normally foam excessively and so may usually be prohibited from use. Such an additive may be a surfactant such as soap. The additive may be a formulation, such as a soap formulation, which includes an agent to control the foaming characteristics of the additive. If the formulation does not include such an agent, an anti-foaming additive may be added to the liquid to reduce and control foaming. An example of an anti foaming agent includes, but is not limited to, a compound based on paraffine, polyglycol, or silicone such as polydimethylsiloxane. 
     Note that, in an embodiment, the system is open instead of closed. Therefore, there is no need for an inner seal (there is still an outer seal). This may provide an advantage. In the  FIG. 6  embodiment, the umbrella  310  was dimensioned to fit the aperture  200  formed in the liquid confinement system LCS. Therefore slightly different designs of cleaning tool, and completely different process settings, may be required for different types of liquid confinement system LCS. The cleaning apparatus of  FIGS. 7 and 8  may be used with several types of liquid confinement system LCS, using fixed process settings. 
     The cleaning tool shown in  FIGS. 6 to 8  may be modified so that the cleaning fluid  30  exhibits radial flow. 
     Furthermore, it will be appreciated that the idea of draining cleaning liquid away from a reservoir in a cleaning tool under gravity in order to avoid foaming can be used in any type of cleaning tool and not just the type illustrated in  FIGS. 7 and 8 . 
     Attaching the megasonic cleaning tool to the liquid confinement system LCS and activating the transducer  20  will induce vibrations (e.g., in the MHz range) to the portion to be cleaned. Vibrations in this range may cause bubble cavitation, megasonic pulsing, and acoustic streaming, which together can be the basis for megasonic cleaning. Notably, the supply of cleaning liquid  30  is provided in such a way that the optical element is protected against megasonic energy and the cleaning fluid is introduced in such a way that negative effects (like bubbles, particle re-deposition, and/or foaming) in the apparatus are avoided. 
     The exemplary fluid supply configurations may improve product yield, for example, by improving the cleanliness of the lithographic apparatus which, in turn, contributes to reduction in defectivity levels. 
     All embodiments may be implemented off-line. There may be in-line implementations of the embodiments. 
     In an embodiment, there is provided a cleaning tool to clean a surface of a liquid confinement system of an immersion lithographic apparatus, the liquid confinement system having an aperture to allow passage of a beam of radiation therethrough of an immersion lithographic apparatus, the cleaning tool comprising: a sonic transducer; a reservoir configured to hold liquid between the sonic transducer and the surface to be cleaned; and a barrier positioned in the reservoir under the aperture to form, in use, a shield to sonic waves. 
     In an embodiment, the barrier, in use, forms the shield by maintaining a gas gap in the aperture and/or by the physical presence of the barrier. In an embodiment, the cleaning tool further comprises outer sealing to seal the tool to the surface of the liquid confinement system around the surface to be cleaned. In an embodiment, the barrier is a physical presence in all straight paths from the sonic transducer to the aperture. In an embodiment, the cleaning tool further comprises an inner sealing to prevent liquid from entering the aperture by sealing between the bather and an inner rim of the liquid confinement system which defines the aperture. In an embodiment, the barrier comprises a liquid inlet to provide liquid into the reservoir. In an embodiment, the inlet is located near a top of the reservoir. In an embodiment, the inlet is provided as a continuous slit or as a plurality of discrete inlets around an outer edge of the bather. In an embodiment, the barrier comprises an outlet configured to allow flow of liquid therethrough out of the reservoir. In an embodiment, the outlet is in a side of the barrier facing away from the sonic transducer. In an embodiment, the cleaning tool is arranged to allow liquid from the reservoir to flow between the aperture and the bather. In an embodiment, a flow path for liquid between the aperture and the bather is not straight. In an embodiment, the barrier and outlet are shaped such that liquid which flows between the aperture and the barrier enters the outlet under gravity. In an embodiment, the outlet is substantially funnel shaped. In an embodiment, the outlet is connected to a pipe, the pipe having a flow restriction in it. In an embodiment, the outlet is connected to a pipe, the pipe being connected to an under pressure source of a magnitude such that, in use, liquid flows through the pipe at a rate of between 0.5 and 3 liters per minute. In an embodiment, the barrier has surfaces which are, in use, at an angle to sonic waves from the sonic transducer such that a portion of the barrier is substantially a truncated cone shape with the widest end furthest from the sonic transducer. In an embodiment, in use, supply or collection of liquid through an inlet or an outlet, respectively, positioned on the barrier causes a radially outwardly or a radially inwardly flow of liquid in the reservoir next to the surface to be cleaned. In an embodiment, the cleaning tool is constructed and arranged to provide a soap to the liquid for between the sonic transducer and the surface to be cleaned. In an embodiment, the cleaning tool is constructed and arranged to provide an anti-foaming agent to the liquid for between the sonic transducer and the surface to be cleaned. 
     In an embodiment, there is provided a combination of a liquid confinement system and a cleaning tool, wherein the liquid confinement system comprises an aperture to allow passage of a beam of radiation therethrough of an immersion lithographic apparatus and the cleaning tool is configured to clean a surface of the liquid confinement system, the cleaning tool comprising: a sonic transducer; a reservoir configured to hold liquid between the sonic transducer and the surface to be cleaned; and a barrier positioned in the reservoir under the aperture to form, in use, a shield to sonic waves. 
     In an embodiment, there is provided a lithographic projection apparatus comprising: a projection system configured to project a beam of radiation; a liquid confinement system configured to confine liquid in an aperture, the aperture arranged to allow passage of the beam therethrough and the aperture being open to a final element of the projection system; and a cleaning tool configured to cleaning a bottom surface of the liquid confinement system, the cleaning tool comprising a sonic transducer, a liquid supply device configured to providing liquid between the sonic transducer and the bottom surface of the liquid confinement system, and a barrier configured to form a shield to prevent sonic waves from reaching the final element. 
     In an embodiment, the bather, in use, forms a shield by its physical presence and/or by maintaining a gas gap in the aperture between the bather and the final element. In an embodiment, the apparatus further comprises a releasable seal configured to seal between the aperture and the barrier. In an embodiment, the barrier, in cross-section, is funnel shaped. In an embodiment, the barrier comprises an outlet in its upper surface. In an embodiment, the barrier has an outer rim and any liquid which flows over the rim flows into the outlet under gravity. In an embodiment, a flow path for liquid between the barrier and the liquid confinement system from the reservoir into the aperture is not straight. 
     In an embodiment, there is provided a method of cleaning a liquid confinement system, the method comprising: providing liquid between a bottom surface of the liquid confinement system and a sonic wave source; generating sonic waves and directing them in the liquid towards the bottom surface; and using a barrier to prevent the sonic waves from reaching an optical element positioned in an aperture in the surface of the liquid confinement system. In an embodiment, the barrier prevents the sonic waves from reaching the optical element by maintaining a gas gap in the aperture below the optical element and/or by blocking the sonic waves by its physical presence. 
     Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications; any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers. 
     The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm). 
     The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive and reflective optical components. Thus, a lens is an optical element and in an embodiment, the optical element is a lens. 
     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. The invention can be performed under the control of a controller, perhaps programmed by the above mentioned computer program. 
     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 immersion liquid used in the apparatus may have different compositions, according to the desired properties and the wavelength of exposure radiation used. For an exposure wavelength of 193 nm, ultra pure water or water-based compositions may be used and for this reason the immersion liquid is sometimes referred to as water and water-related terms such as hydrophilic, hydrophobic, humidity, etc. may be used. 
     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.