Abstract:
The invention relates to a transfer apparatus for transferring an object (W). the transfer apparatus comprises a gripper ( 15 ) for at least one of gripping the object (W) at a first position and then releasing the object (W) at a second position proximate to a receiver ( 20 ) and releasing the object (W) at a first position after gripping the object at a second position proximate to the receiver ( 20 ). The transfer apparatus is further provided with a measurement device ( 22 ) arranged to measure the relative position of the gripper ( 15 ) with respect to the receiver ( 20 ) in at least one dimension. Further, a relative position error is defined with respect to a desired relative position based on the relative position measured. The relative position of the gripper ( 15 ) and the receiver ( 20 ) are adjusted for minimizing the relative position error in the second position.

Description:
[0001]    This application is a divisional of co-pending U.S. patent application Ser. No. 10/813,687, filed Mar. 31, 2004, which claims priority from European Patent Application No. 03075929.4, filed Mar. 31, 2003, and European Patent Application No. 03077320.4, filed Jul. 23, 2003, each of the foregoing applications herein incorporated in its entirety by reference. 
     
    
     FIELD 
       [0002]    The present invention relates to a lithographic apparatus and, in particular, to a lithographic support structure for supporting and moving an object. 
       BACKGROUND 
       [0003]    Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may be used to generate a desired circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). 
         [0004]    The term “patterning device” as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning device include: 
         [0005]    a mask: the concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmission mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired; 
         [0006]    a programmable mirror array: one example of such a device is a matrix-addressable surface having a visco-elastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as non-diffracted light. Using an appropriate filter, the non-diffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation mechanism. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described here above, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from United States patents U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and 
         [0007]    a programmable LCD array: an example of such a construction is given in United States patent U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required. 
         [0008]    For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth here above. 
         [0009]    In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. 
         [0010]    This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemical-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference. 
         [0011]    For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. 
         [0012]    Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference. 
         [0013]    In a lithographic apparatus, one or more actuators with grippers or clamping units, like robots, pre-aligners or substrate table pins will be used to hold and move objects from one location to another location. Those objects may be substrates, like wafers, but may also be other items. 
         [0014]    For instance, substrates are typically introduced into a lithographic apparatus and finally placed on a substrate table. During this movement, the substrate may be handed over from a robot to a stationary position (or, optionally, to another robot). During hand-over, there may be a global tilt (for instance, 3 mrad) relative to a predetermined x-axis or y-axis, defined as Rx and Ry, respectively. This global tilt between these two items involved in a take-over should be minimized to ensure the proper working of a clamping device used by the robot to clamp, e.g., the substrate. Correct functioning of the clamping device is necessary to ensure positive handling, i.e., at every point in time, it should be clear where the substrate is in the lithographic apparatus. This implies that the substrate should be actively clamped at all times. 
       SUMMARY 
       [0015]    For these and other reasons, the principles of the present invention, as embodied and broadly described herein, provide for a lithographic support system having improved compensation of tilt and/or displacement between an object and a supporting structure, such as a robot arm. In one embodiment, the lithographic support system comprises a supporting structure configured to hold and move an object, the supporting structure comprising a clamping structure, and a compliant part configured to compensate for at least one of a tilt and displacement between the object and the clamping structure. 
         [0016]    By the above-referred supporting structure, very accurate take-over is possible while no rotation or displacement of, e.g., a substrate is necessary. Forces created by a collision between, e.g., a substrate and a receiving supporting structure are absorbed by the compliant part. There will be less risk of displacement and of losing substrates in the lithographic apparatus. 
         [0017]    The invention also relates to a supporting structure where the supporting structure is a robot arm. Moreover, the invention relates to a robot for use in a lithographic projection apparatus, comprising such a robot arm. 
         [0018]    Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle”, “wafer” or “die” in this text should be considered as being replaced by the more general terms “mask”, “substrate” and “target portion”, respectively. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]    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: 
           [0020]      FIG. 1  depicts a lithographic projection apparatus according to an embodiment of the invention; 
           [0021]      FIG. 2  schematically depicts a part of a lithographic apparatus in which the present invention can be used according to an embodiment of the invention; and 
           [0022]      FIG. 3   a  schematically shows a robot arm supporting a substrate; 
           [0023]      FIG. 3   b  shows a side view of the arrangement according to  FIG. 3   a;    
           [0024]      FIG. 4   a  shows a side view of a support frame of a robot arm with a clamp to hold a substrate according to an embodiment of the invention; 
           [0025]      FIG. 4   b  shows a side view of a support frame of a robot arm with a clamp to hold a substrate according to an embodiment of the invention, wherein the substrate is warped; 
           [0026]      FIG. 4   c  shows a cross section of a supporting structure in the form of a moveable rod with a compliant part according to an embodiment of the invention; 
           [0027]      FIG. 4   d  shows a cross section of the rod of  FIG. 4   c;    
           [0028]      FIGS. 4   e  and  4   f  show a further embodiment of a flexible supporting structure; 
           [0029]      FIG. 5  shows a side view of a support frame of a robot arm with a clamp to hold a substrate, the clamp being provided with an oxidized upper surface according to an embodiment of the invention; 
           [0030]      FIG. 6   a  shows a side view of a support frame with a clamp to hold a substrate, the clamp being a Johnson-Raybeck type clamp according to an embodiment of the invention; 
           [0031]      FIG. 6   b  shows an enlarged portion of the clamp of  FIG. 6   a;    
           [0032]      FIG. 7  shows a de-clamping voltage provided to a Johnson-Raybeck type clamp in accordance with an embodiment of the invention; 
           [0033]      FIG. 8  shows a substrate table with an actuator to lift and lower a substrate from and towards the substrate table according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0034]      FIG. 1  schematically depicts a lithographic apparatus according to a particular embodiment of the invention. The apparatus comprises: 
         [0035]    a radiation system Ex, IL: for supplying a projection beam PB of radiation (e.g. UV radiation). In this particular case, the radiation system also comprises a radiation source LA; 
         [0036]    a first object table (mask table) MT: provided with a mask holder for holding a mask MA (e.g. a reticle), and connected to first positioning mechanism PM for accurately positioning the mask with respect to item PL; 
         [0037]    a second object table (substrate table) WT: provided with a substrate holder for holding a substrate W (e.g. a resist-coated silicon wafer), and connected to second positioning mechanism PW for accurately positioning the substrate with respect to item PL; and 
         [0038]    a projection system (“lens”) PL: for imaging an irradiated portion of the mask MA onto a target portion C (e.g. comprising one or more dies) of the substrate W. 
         [0039]    As here depicted, the apparatus is of a reflective type (i.e. has a reflective mask). However, in general, it may also be of a transmissive type, for example (with a transmissive mask). Alternatively, the apparatus may employ another kind of patterning device, such as a programmable mirror array of a type as referred to above. 
         [0040]    The source LA (e.g. a mercury lamp or an excimer laser) produces a beam of radiation. This beam is fed into an illumination system (illuminator) IL, either directly or after having traversed conditioning mechanism, such as a beam expander Ex, for example. The illuminator IL may comprise adjusting mechanism AM for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the beam. In addition, it will generally comprise various other components, such as an integrator IN and a condenser CO. In this way, the beam PB impinging on the mask MA has a desired uniformity and intensity distribution in its cross-section. 
         [0041]    It should be noted with regard to  FIG. 1  that the source LA may be within the housing of the lithographic projection apparatus (as is often the case when the source LA is a mercury lamp, for example), but that it may also be remote from the lithographic projection apparatus, the radiation beam which it produces being led into the apparatus (e.g. with the aid of suitable directing mirrors); this latter scenario is often the case when the source LA is an excimer laser. The current invention and Claims encompass both of these scenarios. 
         [0042]    The beam PB subsequently intercepts the mask MA, which is held on a mask table MT. Having traversed the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target portion C of the substrate W. With the aid of the second positioning means (and interferometric measuring means IF), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning means can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval of the mask MA from a mask library, or during a scan. In general, movement of the object tables MT, WT will be realized with the aid of a long-stroke module (course positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in  FIG. 1 . However, in the case of a wafer stepper (as opposed to a step-and-scan apparatus) the mask table MT may just be connected to a short stroke actuator, or may be fixed. 
         [0043]    The depicted apparatus can be used in a variety of different modes: 
         [0044]    step mode: the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the projection beam is projected onto a target portion C in one go (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; 
         [0045]    scan mode: the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the projection 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 mask table MT is 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; and 
         [0046]    other mode: the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the projection 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. 
         [0047]    Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. 
         [0048]      FIG. 2  schematically depicts several parts of the lithographic apparatus through which, in operation, substrates (and possibly other objects) may be moved.  FIG. 2  shows a load lock LL, a handling chamber HC and a projection chamber PC. The load lock LL comprises two doors  2 ,  4 . The first door  2  faces the inside of the lithographic projection apparatus  1 , comprising the handling chamber HC and the projection chamber PC, in which, in an embodiment, vacuum conditions, having a pressure P vac , are maintained. The second door  4  faces, e.g., atmospheric conditions, having a pressure that is, for instance, equal to atmospheric pressure P atm . However, the invention can also advantageously be applied for other pressure values. 
         [0049]    The load lock LL comprises a wall that forms an inner space. The load lock LL is further provided with supporting means (not shown) for supporting one or more objects, such as a substrate (wafer) W, as will be known to a person skilled in the art. 
         [0050]    As can be seen in  FIG. 2 , the load lock LL is also provided with a gas inlet  3  and a gas outlet  5 . The gas outlet  5  can be provided with a pump  6  to pump down the load lock LL to vacuum conditions of, for instance, 10 −3 -10 −5  Pa, a pressure substantially equal to or lower than P vac . The movement of, for instance, a substrate W from the atmospheric environment to the vacuum via the load lock LL, usually comprises the following steps: 
         [0051]    opening the second door  4  facing the atmospheric conditions P atm , 
         [0052]    transferring the substrate W from atmospheric conditions P atm  into the load lock LL, 
         [0053]    closing the second door  4 , 
         [0054]    depressurizing the load lock LL to a pressure substantially equal to or less than vacuum conditions P vac  through gas outlet  5  using, for example, pump  6 , 
         [0055]    opening the first door  4  facing the vacuum conditions P vac , and 
         [0056]    transferring the substrate W to the vacuum conditions P vac  from the load lock LL. 
         [0057]    The gas inlet  3  can be used to vent the load lock in order to raise the pressure in the load lock from P vac  to P atm . Movement of a substrate W from the vacuum to the atmospheric environment via the load lock LL, usually comprises the following steps: 
         [0058]    depressurizing the load lock LL to a pressure substantially equal to or less than the vacuum conditions P vac , 
         [0059]    opening the first door  2  facing the vacuum conditions P vac , 
         [0060]    transferring the substrate W from the vacuum conditions P vac  into the load lock LL, 
         [0061]    closing the first door  2 , 
         [0062]    venting the load lock LL to a pressure substantially equal to or more than atmospheric conditions P atm  via the gas inlet  3 , 
         [0063]    opening the second door  4  facing the atmospheric conditions P atm , and 
         [0064]    delivering the substrate W to the atmospheric conditions P atm . 
         [0065]    These actions as listed above refer to atmospheric conditions outside the first door  2 . However, as is known to persons skilled in the art there may be other pressure conditions outside the load lock. The example given above is by no means intended to restrict the use of the load lock LL. Hazardous particles and contaminating molecules, such as oxygen, hydrocarbons and/or H 2 O that may interfere with processes in the lithographic projection apparatus, are limited from entering the load lock LL by filling the load lock LL with a specially chosen gas that doesn&#39;t comprise these particles or molecules. Gasses, such as N 2  gas, Ar gas or synthetic air, but of course also other suitable gasses, can be used, as will be understood by a person skilled in the art. 
         [0066]    When the load lock LL is brought back from P vac  to P atm  by supplying gas through gas inlet  3  to the load lock LL a special gas is typically used instead of normal environmental air.  FIG. 2  shows a gas supply  7  comprising N 2  gas. Gas supply  7  may be a high pressure tank. 
         [0067]    It will be appreciated by persons skilled in the art that for supporting and moving a substrate W, and possibly other objects in, towards, and from the lithographic apparatus  1  (like in a track, the load lock, a pre-aligner and the projection chamber), use is made of a supporting structure driven by an actuator to move the substrate W in either one, two or three dimensions, such as a robot  8  with an arm provided with a suitable gripper or clamp. Another example of a supporting structure is a combination of pins to displace, lift or lower a substrate in a direction, which are, optionally, provided with a clamp to hold the substrate. In the embodiments described below, reference will be made to substrates, but the invention is equally applicable to other objects. 
       EMBODIMENT 1 
       [0068]      FIG. 3   a  shows a robot arm  10  of robot  8  in more detail. The robot arm  10  comprises a rod  12  connected to a support frame  18  by means of a compliant part  14  and an intermediate part  16 . 
         [0069]    As shown, the support frame  18  is provided with a clamping structure, indicated by three clamps  20 ,  22 , and  24 . The support frame  18  supports a substrate W. The substrate W can be held on the support frame  18  by means of the clamps  20 ,  22  and  24 . 
         [0070]    The support frame  18  defines a plane in which the substrate W lies when it is clamped by the clamps  20 ,  22 , and  24 . That plane is defined by an x-axis and an y-axis. The x-axis is in the longitudinal direction of the rod  12 , whereas the y-axis is perpendicular to the x-axis. A z-axis is defined perpendicular to both the x-axis and the y-axis. The z-axis is not shown in  FIG. 3   a  but is shown in  FIG. 3   b . The x-, y- and z-axes have an origin  19  that is defined to be the location where the center of the substrate W is to be substantially located during movement of the substrate W through the apparatus shown in  FIG. 2 . 
         [0071]      FIG. 3   b  shows a side view of the arrangement shown in  FIG. 3   a .  FIG. 3   b , additionally, shows the substrate table WT (which is not shown in  FIG. 3   a ). Moreover,  FIG. 3   b  shows that support frame  18  may be provided with a compliant portion (or flexible portion)  26 . The compliant portion  26  may be located behind the “fingers” of support frame  18  but may also be located within one or both of the “fingers”. 
         [0072]    As shown in  FIG. 3   b , the compliant portion  14  may comprise two or more compliant rods  14 , rotatable at their ends. Thus, the intermediate part  16  is able to rotate slightly about both the x-axis and the y-axis. Moreover, it can be lifted slightly in the z-direction. Of course, the compliant portion  14  can be made of any other structure or of any material, having a flexible or compliant feature. 
         [0073]    As shown in  FIG. 3   b , the compliant portion  26  of the support frame  18  is produced by providing the support frame  18  with a notch, such that the front side of the support frame designed to hold the substrate W is allowed to rotate about both the x-axis and the y-axis, and to lift slightly in the z-direction, without any movement of the rod  12 . 
         [0074]    Preferably, the compliant portion  14  is designed such that it has rotation poles in a predetermined center coinciding with the center of the coordinate system, as defined by the x-, y- and z-axes. This point of origin is also the location where the center of the substrate W is to be located during gripping/clamping by the robot arm. Thus, any rotation errors in a positive and negative direction of the substrate W about the x- and/or y-axis (generally a tilt) can be adapted best. 
         [0075]    By providing the robot arm  10  with at least one of the compliant parts  14 ,  26 , a tilt and/or a displacement in z between the substrate W and another part in the lithographic apparatus can be compensated. This is, for instance, significant during substrate hand-over from a substrate handler robot to the substrate table WT and from a pre-aligner to a substrate handler robot. However, this also holds for other hand-overs in the apparatus. It is especially important for the hand-over to a device provided with clamps to hold, e.g., the substrate W. If the tilt and/or a displacement in z between the substrate W and the part to receive the substrate W is too large, the hand-over cannot take place because the clamps may not function. By providing the compliant parts  14  or  26 , small tolerances are introduced enabling, in this case, the substrate W to adapt its tilt and/or a z-displacement to the other receiving part under a small force. 
         [0076]    European Patent Application 03075929.4 of the present applicant, from which the priority is claimed and which is incorporated herein in its entirety, discloses that a compliant mechanism is also provided between the gripper and the armset (as described in EP 03075929.4), to decouple the movements of the gripper with respect to the armset. As a result of the mechanical link, the accuracy of the position of the gripper, and thus the substrate, with respect to the receiver is increased. Also, vibrations, and in particular the relative vibrations of the gripper and the receiver are reduced and the mutual accuracy of the substrate and the receiver can be increased. Of course, the mechanical docking solution can also be used for the reverse procedure, i.e. picking up objects, as for instance a substrate, from the receiver. 
         [0077]      FIGS. 4   a  and  4   b  show that the provision of compliant parts within a clamp may also help correcting for tilt and displacements when clamping a substrate W (or any other object) to a clamp on a support frame  18  of a robot arm. 
         [0078]      FIG. 4   a  shows the support frame  18  provided with the clamp  20 , approaching a substrate W to be clamped. There is a tilt angle a between the top of the clamp  20  and the substrate W. The substrate W has a diameter D 1  that may be 200 mm. The clamp  20  has a diameter D 2  that may be 40 mm. In an example, for a proper clamp functioning, the tilt angle a requirements can be so low that the distance indicated by D 3  in  FIG. 4   a  needs to be below 8 micrometers. The clamping device may be a vacuum pad or an electrostatic clamp, like a Johnson-Raybeck effect clamp. As is known to persons skilled in the art, tilt requirements are more stringent for the electrostatic clamps. 
         [0079]      FIG. 4   b  shows that the substrate W may not be perfectly flat in a plane but may, e.g., be undulated when viewed in cross-section. The undulation amplitude D 4  may be as large as 500 micrometer. This undulation may be the result of all kinds of process steps during the lithography process, as is known to persons skilled in the art. Due to the undulated cross-section of the substrate W, the stringent tilt requirements may not be met at all locations of the substrate W. 
         [0080]    To compensate then for a tilt and/or displacement between the substrate and the clamp, the clamp  20  may be provided with a compliant portion  28  as shown in  FIG. 4   b . This compliant portion  28  may be made of a metal flexure or any other flexible material or structure suitable for a lithographic projection apparatus. The compliant portion  28  provides the clamp with Rx, Ry, and z compliance. Either the weight of the substrate W or a small positive force in the z-direction used during hand-over of the substrate W may, then, ensure that the surface of the clamp will align with the rear surface of the substrate W. When aligned, the clamp  20  will hold the rear surface of the substrate W. The clamp force will then be sufficient to keep the substrate W clamped during movement through the apparatus. The clamp compliance can be made such that the substrate&#39;s x, y and Rz position are not changed. This maintains positional accuracy of the substrate, as is required for other processes in the lithographic projection apparatus. 
         [0081]    Preferably, the weight of the substrate W is sufficient to align the clamp since the clamp should also work after losing clamp force due to, e.g., cut off of electrical power, after an emergency stop of the apparatus, etc. As may be evident to a person skilled in the art, the compliant portion  28  can also correct for any curvature of the support frame  18 . 
         [0082]      FIGS. 4   c  and  4   d  show the provision of compliant portions with supporting structures that are allowed to move in one direction only. The supporting structure shown comprises a rod like a substrate table pin  38  having a clamp  20  mounted on an end. The pin  38  comprises compliant portion  28 . The compliant portion  28  is made by providing the pin with two notches  29  that provide the pin  38  with a flexibility in a first direction perpendicular to its direction of movement. There may be provided two other notches  31 , as indicated with dashed lines, in the pin  38  perpendicular to the notches  29  to provide the pin  38  with a flexibility in a second direction perpendicular to the first direction. 
         [0083]      FIG. 4   d  shows a cross section of the pin of  FIG. 4   c  in the direction of IVd-IVd, as shown in  FIG. 4   c  to better show the notches  29  and optional notches  31 . 
         [0084]    Of course, other compliant parts than the one shown in  FIGS. 4   c  and  4   d  can be used. The rod  38  may a cross section different from the circular one shown. 
         [0085]    A further embodiment of a flexible supporting structure to be used with, e.g., an electrostatic clamp for clamping, e.g., a wafer or a reticle, is shown in  FIGS. 4   e  and  4   f.    
         [0086]    The supporting structure shown comprises an outer ring  46 , an interface ring  48  and a central part  50 . The outer ring  46  will be fixed in a gripper (not shown) or a stage (not shown). On top of the central part  50 , an electrostatic clamp (not shown) will be attached, e.g., by gluing. In the outer ring  46 , the interface ring  48  is fixed with two leaf springs  52   a  and  52   b  releasing the y, Rx and Rz d.o.f.&#39;s (=degrees of freedom). The central part  50  on its turn is fixed to the interface ring  48  by two leaf springs  54   a ,  54   b  and two rods  56   a ,  56   b . The leaf springs  54   a ,  54   b  are oriented to block the Rx d.o.f. Thus, together with the rods  56   a ,  56   b  the arrangement is such that the interface ring with respect to the central part  50  releases only one rotation, i.e., Ry. Two remaining translations are fixed with the flexible supporting structure of  FIGS. 4   e  and  4   f , i.e., x and z d.o.f.&#39;s. 
       EMBODIMENT 2 
       [0087]      FIG. 5  shows a further embodiment of the invention, in which the clamp  20  on the support frame  18  is a Johnson-Raybeck effect type clamp. As known to persons skilled in the art, a Johnson-Raybeck effect type clamp is provided with a dielectric material made from a very low conductive material. Due to the very low conductivity (of electrical resistance) of the dielectric material, electrical clamping charges migrate through the dielectric material towards the clamping surface. In use, contact with substrate W is only made at some tiny spots. Between those tiny spots there are “open” spaces defining a distance between the clamping surface and the rear side of substrate W. Charges opposite the “open” spaces in both the clamping surface and the substrate W create a clamp with a very small distance between the charges, only the remaining gap, and hence create very large clamping forces. 
         [0088]    In a Johnson-Raybeck clamp a small electrical current is flowing through the dielectric layer (furthermore in this example a small current flows in between the wafer and the dielectric material), which leads to a relatively small heat load of the combined clamp substrate system. For most applications this is not a problem because of the absence of lithography requirements like overlay and critical dimensions. For a clamp to be used in a lithography tool this can be a problem. 
         [0089]    A solution to that problem according to an embodiment of the invention is to provide the upper surface of the clamp  20  a high electrical resistance coating (for example a thin dielectric layer of several (1-20) microns) such as an oxidized layer  30  (for example SiO 2 ), as shown in  FIG. 5 . This oxidized layer  30  may be provided to the clamp  20  in any way known to persons skilled in the art. By providing the clamp  20  with the oxidized layer  30 , only a very small electrical current will flow in the contact area between the clamp and the substrate and into the substrate W itself, thus having a negligible impact on the temperature of the substrate W. 
       EMBODIMENT 3 
       [0090]      FIG. 6   a  shows the support frame  18  provided with the clamp  20 , the clamp  20  being a Johnson-Raybeck effect clamp. The clamp  20  holds the substrate W. The clamping effect is generated by a controller, shown as a series connection of a DC voltage source and an AC voltage source. As will be appreciated by a person skilled in the art, the controller may be based on any suitable electronic circuit or may be implemented by a computer. 
         [0091]      FIG. 6   b  shows a portion of  FIG. 6   a  on an enlarged scale.  FIG. 6   b  shows that the Johnson-Raybeck effect type clamp  20  is provided with a conductive part  34  and a electrically leaky insulator  32 . Due to the leaky insulator  32 , an electrical current will flow between the conductor part  34  and the substrate W resulting in trapped charges in the leaky insulator  32 . 
         [0092]    For clamping the substrate W to the clamp  20 , only the DC source is switched on and the AC source is switched off. To de-clamp the substrate W from the clamp  20 , one can simply switch off the DC source. However, due to the trapped charges in the leaky insulator  32 , it will take a while before the attracting force between the clamp  20  and the substrate W is small enough to remove the substrate W. This is due to the trapped charges in the leaky insulator  32  only leaking away from the leaky insulator  32  very slowly. 
         [0093]    It is known to solve this problem by applying one or more reverse-polarity voltage steps, opposite of the clamping potential provided by the DC source. 
         [0094]    However, an alternative solution is for the AC source to be is arranged to provide a decaying AC voltage from the time that de-clamping starts. This is further explained in  FIG. 7 .  FIG. 7  shows that the clamping voltage V clamp  has a predetermined value V 0  at the time of clamping. At time t 1 , the substrate W is to be de-clamped from the clamp  20 . Then, at time t 1 , the DC source is switched off and the AC source is switched on such that an AC voltage with decaying amplitude is provided to the clamp  20  until the amplitude of the AC voltage is substantially zero at time t 2 . Preferably, the frequency of the AC source is a radio frequency. At time t 2 , the clamping force of the clamp  20  is so small that the substrate W can be safely removed from the clamp  20 . 
       EMBODIMENT 4 
       [0095]      FIG. 8  shows the substrate table WT with a substrate W. The lithographic apparatus is provided with an actuator  42  provided with pins  38 . In use, the actuator  42  is arranged to displace the substrate W from and towards the substrate table WT. The substrate table WT is provided with a substrate support  36  (such as pimples) for supporting the substrate W. By providing the substrate table WT with the substrate support  36 , the actual surface of contact between the substrate table WT and the substrate W is reduced while still providing a proper support. 
         [0096]    The substrate table WT is provided with a clamp  40 , e.g., an electrostatic clamp or vacuum clamp. However, other types of clamps may be used as well, as is known to persons skilled in the art. 
         [0097]    A processor  44  is provided that is connected to both the clamp  40  and the actuator  42 , in order to provide them with suitable control signals. 
         [0098]    The robot  8  with its robot arm  10  is shown again, together with its connection to the processor  44 , such that the processor  44  can also control the operation of the robot  8 . 
         [0099]    Sometimes, substrate cycling is part of a substrate table cleaning procedure. After a chemical and/or mechanical cleaning part of this procedure, typically about 8 substrates are cycled successively onto the substrate table to remove remaining contaminants from the substrate support  36  and to reach a substrate rear side contamination specification again. 
         [0100]    However, there are at least two problems related to this current cleaning procedure: 
         [0101]    1. availability and cost of the required number (8) of clean substrates at customer locations for substrate cycling; 
         [0102]    2. downtime of the apparatus because of cycling cleaning substrates. 
         [0103]    In accordance with an embodiment of the invention, the number of substrates needed for substrate cycling is drastically reduced to one single substrate. To that end, the processor  44  is arranged to control the clamp  40 , the actuator  42  and the robot  8  in the following way: 
         [0104]    introducing the substrate W in the lithographic projection apparatus by means of the robot  8 ; 
         [0105]    clamping the substrate W to the substrate table WT by means of the clamp  40 , such that the substrate W contacts the substrate table WT at a first location; 
         [0106]    de-clamping the substrate W from the substrate table WT by providing a de-clamping voltage to the clamp  40 ; 
         [0107]    repeating the clamping and de-clamping a predetermined number of times, such that the substrate contacts the substrate table WT repeatedly, either at a first location or at other locations than the first location. 
         [0108]    In order to be sure that successive other locations of contacts with the substrate are arranged by which contamination will be removed from the substrate support  36 , preferably, the actuator  42  is arranged to displace the substrate W from the substrate support  36  after de-clamping, to have the robot  8  rotate or shift the substrate W, to displace the substrate W to the substrate support  36  to clamp the substrate W by the clamp  40  again, etc. This may be repeated, e.g., 8 times. 
         [0109]    By programming the processor  44  for this proposed method, the cleaning procedure may be much faster than in the past. Moreover, only one substrate is necessary for such a cleaning procedure. Thus, several portions of the rear surface of the substrate W are used to remove contamination on the substrate support  36 ; thus, saving the number of substrates necessary to perform the cleaning procedure. 
         [0110]    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 instance, it will be understood that the number of robots may be more than the single robot  8  shown in the figures. Moreover, there may be more load locks than the single load lock shown. Moreover, although a single processor  44  is shown ( FIG. 8 ), there may be more than one processor to control different components of the apparatus. These processors may be connected to one another and arranged to co-operate, e.g., in a master slave arrangement. 
         [0111]    Thus, the description is not intended to limit the invention and the configuration, operation, and behavior of the present invention has been described with the understanding that modifications and variations of the embodiments are possible, given the level of detail present herein. Accordingly, the preceding detailed description is not meant or intended to, in any way, limit the invention—rather the scope of the invention is defined by the appended claims.