Abstract:
A supporting device for supporting in a lithographic projection apparatus a supported part relative to a supporting part, is presented. The supporting device includes a first part that engages the supporting part of the lithographic projection apparatus; a second part that engages the supported part of the lithographic projection apparatus; a supporting spring system disposed between the first part and the second part; and a position control system configured to control a position of the supported part. The position control system comprises at least one reference object that is movable relative to the supporting part; a reference support device that supports the reference object relative to the first part, wherein the reference object and the reference support device form a reference mass-spring system; at least one position sensor that detects at least one attribute of the position of the second part relative to at least one of the reference objects, the position sensor including a sensor output for outputting a position signal representing at least one of the attributes; and an actuator, communicatively coupled to the position sensor, that is configured to adjust the position of the second part relative to the first part, in response to the position signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Priority Information 
     This application claims priority from European Patent Application No. 03077540.7, filed Aug. 11, 2003, and European Patent Application No. 03076923.6, filed Jun. 13, 2003, herein incorporated by reference in their entirety. 
     2. Field of the Invention 
     The present invention relates to a supporting device, a lithographic apparatus, a device manufacturing method, and a position control system. 
     3. Description of the Related Art 
     Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, patterning device of the lithographic projection apparatus may generate a 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). 
     In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. More information with regard to lithographic projection apparatuses as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference. 
     In a manufacturing process using a lithographic 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. 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, chemo-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. 
     In a manufacturing process using a lithographic apparatus, the pattern has to be imaged on the substrate very accurately. Current lithographic apparatuses are commonly used to manufacture devices with typical dimensions in the micron or submicron range. Hence, the pattern has to be imaged on the substrate with a corresponding accuracy. Disturbances, such as mechanical vibrations or air pressure waves, can alter the position of the pattern with respect to the substrate and thereby affect the process. The disturbances may be caused, for example, by floor vibrations, reaction forces of a positioning device of the substrate holder, reaction forces of a further positioning device by means of which the mask holder is displaceable relative to a focusing unit or otherwise. Hence, the lithographic apparatus has to be configured such that disturbances are suppressed or circumvented. 
     As disclosed in International Patent Application WO-A-96/38766 and U.S. Pat. No. 6,226,075, lithographic apparatuses that are supported by a base via three supporting devices are known. Such supporting devices prevent, inter alia, the transmission of vibrations from the base to the frame. The three supporting devices each have a gas spring positioned between a first part, which is connected to the base,, and a second part, which is connected to the frame. The mass-spring system formed by the known supporting devices and the frame together with the components of the lithographic apparatus supported thereby has a low natural frequency to prevent transmission of vibrations from the base to the frame as much as possible. 
     SUMMARY OF THE INVENTION 
     A problem of these prior art supporting devices is that the gas spring leaks gas and, therefore, gas has to be supplied during operation in order to maintain an average gas pressure. During the supply of gas, pressure fluctuations present in the gas supply are transmitted to the pressure chamber. The pressure fluctuations in the pressure chamber cause mechanical vibrations of the second part of the supporting device, which are transmitted to the frame of the lithographic device. This results in a degraded image quality as a consequence of the vibrations. Of course, the lithographic device may be stopped while gas is being supplied to the pressure chambers of the supporting devices, to prevent such an undesirable adverse effect . But this adversely affects the production output of the lithographic device. 
     The supporting devices disclosed by U.S. Pat. No. 6,144,442, incorporate a gas spring system. The gas spring system has gas supply means for maintaining an average gas pressure during operation. The gas supply means is connected to an intermediate space which is in communication with the pressure chamber of the gas spring via a pneumatic restriction. The gas flow from the intermediate space to the pressure chamber will occur in the pneumatic restriction mentioned above when gas is supplied by the gas supply means to the intermediate space. This gas flow encounters a predetermined resistance in the pneumatic restriction. Comparatively high-frequency pressure fluctuations of the gas in the intermediate space are damped by the resistance of the restriction before being transmitted to the pressure chamber. The pneumatic restriction together with the pressure chamber thus forms a so-called pneumatic low-pass filter for the pressure fluctuations occurring in the intermediate space. The pressure fluctuations occurring in the intermediate space are limited by the use of means for controlling the gas pressure present in the intermediate space. 
     Although the prior art supporting devices reduce, to some extent, vibration-related disturbances in a lithographic apparatus compared to a lithographic apparatus having its frame directly mounted to the base, not all disturbances are suppressed. In particular, the prior art supporting devices suppress disturbances acting on the frame via the base, such as floor vibrations. Thus, disturbances acting directly on the frame—in other words, disturbances not transmitted via the base—are not suppressed. Such disturbances may, for example, be excitations caused by movements of devices, e.g. stages, mounted on the frame. For example in a lithographic projection apparatus of the scanning type, typically a scanning stage moves with a 2 Hz frequency, which may give rise to a 100 micrometer displacement of the frame. Disturbances acting directly on the frame may also be caused by, for example, air pressure variations or otherwise. Typically, air pressure variations may give rise to a disturbance in the frame position in the order of 50 micrometer. 
     Also, the gas springs in the prior art devices may themselves give rise to disturbances. Such disturbances may for example be caused by pressure variations in the gas springs due to the gas supply. Even in the prior art supporting device known from the U.S. Pat. No. 6,144,442, pressure variations due to the gas supply with a frequency below the cut-off frequency of the pneumatic low-pass filter are not suppressed. 
     For at least one of the issues identified above, the principles of the present invention, as embodied and broadly described herein, provide for an improved supporting device and, more particular, a supporting device which is less susceptible to disturbances than the known supporting devices. In one embodiment, a supporting device for supporting in a lithographic projection apparatus a supported part relative to a supporting part, the supporting device comprises a first part that engages the supporting part of the lithographic projection apparatus; a second part that engages the supported part of the lithographic projection apparatus; a supporting spring system disposed between the first part and the second part; and a position control system configured to control a position of the supported part. The position control system comprises at least one reference object that is movable relative to the supporting part; a reference support device that supports the reference object relative to the first part, wherein the reference object and the reference support device form a reference mass-spring system; at least one position sensor that detects at least one attribute of the position of the second part relative to at least one of the reference objects, the position sensor including a sensor output for outputting a position signal representing at least one of the attributes; and an actuator, communicatively coupled to the position sensor, that is configured to adjust the position of the second part relative to the first part, in response to the position signal. 
     The reference object is shielded against disturbances of the first part, such as vibrations of the gas spring, because the reference support device forms a reference mass-spring system together with the reference object. In general, a mass-spring system acts as a filter for vibrations above the natural or resonance frequency of the mass-spring system. Hence, the reference mass-spring system filters disturbances acting on the reference object via the first part. Furthermore, the reference mass-spring system is mounted on the first part and is thus isolated from the second part. Therefore, the position of the reference object is not affected by disturbances acting directly on the second part, such as moving stages, air pressure waves or otherwise. Thus, perturbation of the position of the reference object is prevented and the reference object therefore provides an inertial reference point. 
     The position sensor senses a property of the position of the second part with respect to this inertial reference point and provides a position signal which represents a property of the position. The actuator adjusts the position of the second part in response to the position signal. Hence, the actuator acts in response to a sensed property of the position relative to an inertial reference point. Therefore, the second part can be accurately positioned and disturbances acting directly on the second part suppressed. 
     In another embodiment, a lithographic apparatus is presented, which comprises a substrate holder configured to hold a substrate; a projection system configured to project a patterned beam onto a target portion of the substrate; a support structure configured to support at least the projection system; and a base that supports the support structure via a supporting device. The supporting device comprises a first part that engages the base; a second part that engages the support structure; a supporting spring system disposed between the first part and the second part; and a position control system configured to control a position of the support structure. The position control system comprises at least one reference object that is movable relative to the base; a reference support device that supports the reference object relative to the first part, wherein the reference object and the reference support device form a reference mass-spring system; at least one position sensor that detects at least one attribute of the position of the second part relative to the at least one of the reference objects, the position sensor including a sensor output for outputting a position signal representing at least one of the attributes; and an actuator, communicatively coupled to the position sensor, that is configured to adjust the position of the second part relative to the first part, in response to the position signal. 
     In another embodiment, a device manufacturing method is presented, which comprises providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a support structure that is configured to support at least a projection system; providing a base that supports the support structure via a supporting device; conditioning a beam of radiation using a radiation system; configuring the beam of radiation with a desired pattern in its cross-section; and projecting a patterned beam of radiation onto a target area of the layer of radiation-sensitive material via the projection system. The supporting device comprises first part that engages the base; a second part that engages the support structure; a supporting spring system disposed between the first part and the second part; and a position control system configured to control a position of the support structure. The position control system comprises at least one reference object that is movable relative to the base; a reference support device that supports the reference object relative to the first part, wherein the reference object and the reference support device form a reference mass-spring system; at least one position sensor that detects at least one attribute of the position of the second part relative to the at least one of the reference objects, the position sensor including a sensor output for outputting a position signal representing at least one of the attributes; and an actuator, communicatively coupled to the position sensor, that is configured to adjust the position of the second part relative to the first part, in response to the position signal. 
     Such a lithographic apparatus and device manufacturing method have an improved accuracy of positioning the patterning device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which: 
         FIG. 1  shows a perspective view of an example of a lithographic projection apparatus; 
         FIG. 2  diagrammatically depicts the lithographic projection apparatus of  FIG. 1 ; 
         FIG. 3  shows a diagrammatic cross-sectional view of an example of a supporting device in accordance with an embodiment of the present invention; and 
         FIGS. 4–9b  illustrate examples of reference mass-spring systems suitable for use, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The example of a lithographic apparatus shown in  FIGS. 1 and 2  is suitable for use in the manufacture of integrated semiconductor circuits by a lithographic process. As  FIGS. 1 ,  2  show, the lithographic apparatus, seen from the ground along a vertical direction Z, is provided with, in that order, a positioning device  1  that includes a substrate holder  3  and a focusing unit  5 , a further positioning device  7  that includes a mask holder  9 , and a radiation source  11  (see  FIG. 2 ). 
     The lithographic apparatus is an optical lithographic projection apparatus in which the radiation source  11  comprises a light source  13 , a diaphragm  15 , and mirrors  17  and  19 . The focusing unit  5  is an imaging or projection system provided with an optical lens system  21 , having an optical main axis  23  directed parallel to the Z-direction and an optical reduction factor which, for example, may be 4 or 5. However, the lithographic apparatus may likewise be of a different type comprising a different radiation source, such as for example a Deep Ultra Violet (DUV) or Extreme Ultraviolet (EUV) radiation source, an electron beam source or other suitable sources. 
     The substrate holder  3 , which may also be referred to as the substrate table, comprises a support surface  25  that extends perpendicular to the Z-direction and on which a semiconductor substrate  27  can be placed. The support surface  25  is displaceable relative to the focusing unit  5  in a plane defined by an X-direction and Y-direction of which both are perpendicular to each other and to the Z-direction, by means of the positioning device  1 . 
     The mask holder  9  comprises a support surface  29  that extends perpendicular to the Z-direction and on which a mask  31  can be placed. Mask holder  9  is displaceable relative to the focusing unit  5  parallel to the X-direction by means of the further positioning device  7 . 
     The semiconductor substrate  27  comprises a large number of fields  33  on which identical semiconductor circuits may be provided, while the mask  31  comprises a pattern or a sub-pattern of a single integrated semiconductor circuit. During operation, the individual fields  33  of the semiconductor substrate  27  are consecutively exposed through the mask  31 . A light beam  35  originating from the light source  13  is guided through the mask  31  via the diaphragm  15  and the mirrors  17 ,  19  during an exposure step and is focused on an individual field  33  of the semiconductor substrate  27  by the focusing unit  5 , so that the pattern present on the mask  31  is imaged on a reduced scale on said field  33  of the semiconductor substrate  27 . 
     An imaging method following the so-called “step and scan” principle is used in the shown lithographic apparatus. In this imaging method, the semiconductor substrate  27  and the mask  31  are synchronously displaced relative to the focusing unit  5  parallel to the X-direction by means of the positioning device  1  and the further positioning device  7 , respectively, during an exposure step. After an exposure of an individual field  33  a next field  33  of the semiconductor substrate  27  is brought into position relative to the focusing unit  5  in that the substrate holder  3  is displaced stepwise parallel to the X-direction and/or parallel to the Y-direction by means of the positioning device  1 . 
     The pattern present on the mask  31  is thus scanned parallel to the X-direction and imaged on the consecutive fields  33  of the semiconductor substrate  27 . This process is repeated a number of times, each time with a different mask comprising a different pattern or sub-pattern, so that integrated semiconductor circuits with complicated layered structures can be manufactured. 
     Such structures have detail dimensions which lie in the submicron range. Therefore, the patterns of sub-patterns present on the masks should be imaged on the semiconductor substrates with an accuracy which also lies in the submicron range, so that very high requirements are imposed on the accuracy with which the substrate holder and the mask holder can be positioned relative to the focusing unit by means of the positioning device and the further positioning device, respectively. 
     As  FIG. 1  illustrates, the lithographic apparatus comprises a base  37 , which can be placed on a horizontal floor. The lithographic apparatus also comprises a frame  39 , which may also be referred to as a support structure, which supports the substrate holder  3 , the focusing unit  5 , and the mask holder  9  in a vertical support direction and which extends parallel to the Z-direction. The frame  39  is provided with a triangular, comparatively stiff metal main plate  41  that extends transversely to the optical main axis  23  of the focusing unit  5  and which is provided with a central light transmission opening which is not visible in  FIG. 1 . 
     The base  37  supports the frame  39  in the vertical support direction by means of three supporting devices  53 , which are mutually arranged in a triangle and which each exert a supporting force on the main plate  41  of the frame  39  directed parallel to the support direction. The main plate  41  for this purpose has three corner portions  55  by means of which the main plate  41  rests on the three supporting devices  53 . It will be appreciated that only two of the three corner portions  55  of the main plate  41  and two of the three supporting devices  53  are visible in  FIG. 1 . 
     The substrate holder  3  is displaceably guided over a carrier  43  of the frame  39 , which extends perpendicular to the Z-direction and which is suspended from a lower side of the main plate  41  by means of three vertical suspension plates  45 . It is noted that only two of the three suspension plates  45  are partly visible in  FIG. 1 . The focusing unit  5  is fastened to the main plate  41  by means of a mounting ring  47  which is fastened to the focusing unit  5  at a lower part of the focusing unit  5 . 
     The mask holder  9  is displaceably guided over a further carrier  49  of the frame  39  which extends parallel to the X-direction. The further carrier  49  is fastened to a vertical, comparatively stiff metal column  51  of the frame  39  which is fastened on the main plate  41 . 
     As  FIGS. 1 and 2  show, the positioning device  1  comprises a first part  57  and a second part  59 , while the further positioning device  7  comprises a first part  61  and a second part  63 . The first parts  57  and  61  are fastened to the substrate holder  3  and to the mask holder  9 , respectively, while the second parts  59  and  63  are fastened to the base  37 . The second parts  59  and  63  exert driving forces on the respective first parts  57  and  61  during operation, whereupon the first parts  57  and  61  exert reaction forces on the respective second parts  59  and  63 . 
     As shown in  FIG. 1 , the second part  59  of the positioning device  1  is fastened to a comparatively stiff metal arm  65 , which is in turn fastened to the base  37 , and the second part  63  of the positioning device  7  is fastened to a further comparatively stiff metal column  67 , which is also fastened to the base  37 . 
     The reaction forces of the positioning device  1  and the further positioning device  7  are, thus, transmitted to the base  37 , which may result in reaction forces vibrations in the base  37 . Since the base  37  is placed on a floor, vibrations may also occur in the base  37 , such as, for example, from the vibrations present in the floor. 
     Because very stringent requirements are imposed on the accuracy with which the substrate holder  3  and the mask holder  9  can be positioned relative to the focusing unit  5  by the positioning device  1  and the further positioning device  7 , respectively, the transmission of vibrations from the base  37  to the frame  39  is undesirable. In particular, the frame  39  supports the substrate holder  3 , the mask holder  9 , and the focusing unit  5  parallel to the vertical support direction, so that movements of the frame due to vibration may cause a loss in accuracy of positioning the parts of the lithographic projection apparatus relative to each other. In order to prevent a transmission of vibrations present in the base  37  through the supporting devices  53  into the frame  39  during operation, the supporting devices  53  are provided with a system, to be described in more detail further below, that prevents the transmission of vibrations from the base  37  into the frame  39 . 
     As depicted in  FIG. 3 , the supporting device  53  used in the example of a lithographic apparatus of  FIG. 1  comprises a first part  69 , which can be fastened to the base  37  of the lithographic projection apparatus, a second part  71 , which can be fastened to the frame  39  of the lithographic projection apparatus, and a gas spring  73  for supporting the second part  71  relative to the first part  69  by means of a supporting force which is directed parallel to the vertical support direction. As will be described below in greater detail, the supporting device  53  also comprises a position control system that controls the position of the second part  71 , in accordance with an embodiment of the present invention. 
     The gas spring  73  comprises a pressure chamber  75  in which a comparatively high gas pressure is present during operation. The pressure chamber  75  is bounded by a cylindrical inner wall  77  of a beaker-shaped intermediate part  79  of the supporting device  53  and by a piston  81  which is displaceable in the intermediate part  79  parallel to the support direction. 
     The piston  81  comprises a sleeve  83  which is supported relative to the intermediate part  79  perpendicular to the support direction via a static gas bearing  85 , which is present between the cylindrical inner wall  77  of the intermediate part  79  and a cylindrical outer wall  87  of the sleeve  83 . 
     The static gas bearing  85  may comprise a conical gap bearing, which is usual and known in the art, and is provided with a gas supply line  89 , which is provided in the sleeve  83  and is in communication with the pressure chamber  75 . 
     Adjacent to a lower side  91  of the sleeve  83 , a sealing gap device  93  is positioned between the inner wall  77  of the intermediate part  79  and the outer wall  87  of the sleeve  83 , to prevent a leakage of gas from the pressure chamber  75  along the piston  81 . A trapping groove  95  for gas leaking along the sealing gap  93  and gas flowing from the static gas bearing  85  is provided between the sealing gap  93  and the static gas bearing  85  in the outer wall  87  of the sleeve  83 . 
     The trapping groove  95  is in communication with a number of exhaust channels  97  provided in the intermediate part  79  for the removal of the gas present in the trapping groove  95  to the surroundings. The trapping groove  95  and the exhaust channels  97  prevent the operation of the static gas bearing  85  from being influenced by gas which leaks along the sealing gap  93 . The supporting force supplied by the supporting device  53  in the vertical support direction is, therefore, a gas pressure force exerted by the gas in the pressure chamber  75  on the annular lower side  91  of the sleeve  83  and on an inner wall  99  of the piston  81  extending transversely to the support direction. 
     The piston  81  is fastened to the second part  71  via a connection member  101 , while the beaker-shaped intermediate part  79  is supported relative to the first part  69  in the vertical support direction via a further static gas bearing  103 . The further static gas bearing  103  may also comprise a conical gap bearing and is positioned between a support surface  105  of the first part  69  extending perpendicularly to the vertical support direction and a bottom wall  107  of the intermediate part  79 . The bottom wall  107  bounds the pressure chamber  75  and also extends perpendicularly to the vertical support direction. The beaker-shaped intermediate part  79  is guided in a substantially frictionless manner over the support surface  105  of the first part  69  through the use of the further static gas bearing  103  and is, thus, displaceable in a substantially frictionless manner relative to the first part  69  along directions perpendicular to the vertical support direction. 
     A passage  109  of the gas spring  73  is provided centrally in the bottom wall  107  of the intermediate part  79 . The passage  109  is present in line with a further passage  111  of the gas spring  73 . The further passage  111  is provided in the support surface  105  of the first part  69  and in communication with a main chamber  113  of the gas spring  73  arranged in the first part  69 . The pressure chamber  75  of the gas spring  73  provided in the intermediate part  79  is in communication with the main chamber  113  of the gas spring  73  provided in the first part  69  via the passage  109  and the further passage  111 . 
     As depicted in  FIG. 3 , the conical bearing gap of the further static gas bearing  103  merges directly into the passage  109  provided in the bottom wall  107  of the intermediate part  79 , so that the passage  109  at the same time forms a gas supply line in communication with the pressure chamber  75  for the further static gas bearing  103 . Since the passage  109  in the bottom wall  107  of the intermediate part  79  has a dual function, a particularly simple and practical construction of the further static gas bearing  103  and the gas supply used therein is obtained. 
     The gas springs  73  of the supporting devices used in the lithographic apparatus together with the frame  39  and the components of the lithographic apparatus supported by the frame  39  constitute a mass-spring system in which the frame  39  is displaceable relative to the base  37  in the vertical support direction and perpendicularly to the vertical support direction. Additionally, the frame is rotatable relative to the base  37  about a vertical axis of rotation and is pivotable about two mutually perpendicular pivot axes, which are perpendicular to the vertical support direction. 
     Displacements of the frame  39  relative to the base  37  along the vertical support direction are possible by virtue of the pistons  81  of the supporting devices  53  are displaceable in the intermediate parts  79  parallel to the support direction, while displacements of the frame  39  relative to the base  37  perpendicular to the support direction are possible in that the intermediate parts  79  of the supporting devices  53  are displaceable relative to the first parts  69  perpendicularly to the support direction. 
     Rotations of the frame  39  about a vertical axis of rotation directed parallel to the support direction are possible in that the intermediate parts  79  of the supporting devices  53  are rotatable relative to the first parts  69  about an axis of rotation parallel to the support direction through the use of the further static gas bearings  103 . Pivoting movements of the frame  39  about the pivot axes which are perpendicular to the support direction are possible in that the pistons  81  of the supporting devices  53  are fastened to the second parts  71  via the connection members  101  mentioned above. 
     As illustrated in  FIG. 3 , each of connection members  101  comprise a rubber ring  115 , which functions to clamp the pistons to the bottom plate  69  when there is no pressure in the pistons, such as, for example, during the transport of the support system. A pin  116  is present inside the rubber ring  115  so that, in use, i.e. when the gas spring is pressurized, the rubber ring will be compressed, until the pin makes contact with the second part  71  and decouples the rotation of the second part  71  with respect to the gas spring. 
     To prevent the transmission of vibrations from the base  37  to the frame  39  of the lithographic apparatus, as well as from the first parts  69  to the second parts  71  of the supporting devices  53 , the mass-spring system should be designed to have natural frequencies that are as low as possible parallel to the support direction, perpendicular to the support direction, around the axis of rotation which is parallel to the support direction, and about said pivot axes which are perpendicular to the support direction. As such, the supporting devices  53  have a stiffness, which is as low as possible in the support direction, perpendicular to the support direction, around the axis of rotation parallel to the support direction, and around the pivot axes perpendicular to the support direction. 
     As is known from the laws of physics, the natural frequency of a mass-spring system is proportional to the ratio of the spring constant and the mass. The spring constant is a factor that indicates the amount of force required to displace the ends of a spring 1 meter. Thus, a spring constant, and thus stiffness, which is as low as possible results in a commensurately low natural frequency. 
     The lowest possible stiffness of the supporting device  53  parallel to the support direction is achieved by virtue of the gas spring  73  having a given volume that is as large as possible. The gas spring  73  provided with the main chamber  113  described above in addition to the pressure chamber  75 , in which the main chamber  113  is in communication with the pressure chamber  75 . The use of the main chamber  113  makes it possible to reduce the volume of the pressure chamber  75 , so that the dimensions and weight of the displaceable intermediate part  79  can remain within suitable limits. 
     Furthermore, a gas supply channel (not shown in the figures) of the gas spring  73 , by means of which constant average gas pressure in the gas spring  73  is maintained, can be connected to the main chamber  113 . Since the pressure chamber  75  is bounded by the piston  81 , which is displaceably guided in the intermediate part  79  via the static gas bearing  85 , the stiffness of the supporting device  53  in the support direction is to a significant degree determined by the stiffness of the gas spring  73 . The static gas bearing  85  and the sealing gap  93  exert substantially no frictional forces on the piston  81  as seen in the support direction. Neither is the stiffness of the supporting device  53  in the support direction substantially influenced by the presence of the connection member  101 , which is substantially undeformable parallel to the support direction, or by the presence of the further static gas bearing  103 , which also has a very high stiffness seen in the support direction. 
     Since the stiffness of the supporting device  53  parallel to the support direction is substantially entirely determined by the stiffness of the gas spring  73 , a suitable design of the pressure chamber  75  and the main chamber  113  will achieve a sufficiently low stiffness of the gas spring  73 , so that the transmission of vibrations directed in the support direction from the first part  69  to the second part  71  of the supporting device  53  is prevented as much as possible. 
     The fact that the intermediate part  79  of the supporting device  53  is displaceable in a substantially frictionless manner over the support surface  105  of the first part  69  via the further static gas bearing  103  implies that the supporting device  53  has a stiffness which is substantially zero in directions perpendicular to the support direction, while the mass-spring system of the lithographic projection apparatus mentioned above has a natural frequency which is also substantially zero perpendicular to the support direction. Transmission of vibrations directed perpendicular to the support direction from the base  37  and the first part  69  to the frame  39  and the second part  71  is substantially fully prevented thereby. 
     The supporting device  53  has a stiffness which, as noted above, is substantially zero around an axis of rotation extending parallel to the support direction because the intermediate part  79  of the supporting device  53  is substantially frictionless rotatable over the support surface  105  of the first part  69  owing to the use of the further static gas bearing  103 , so that the transmission of rotational vibrations of the first part  69  about an axis of rotation extending parallel to the support direction to the second part  71  of the supporting device  53  is also substantially fully prevented. 
     As discussed above, the second part  71  of the supporting device  53  is displaceable relative to the first part  69  in directions parallel to the support direction and perpendicular to the support direction, is rotatable about an axis of rotation extending in the support direction, and is pivotable about two mutually perpendicular pivot axes which are perpendicular to the support direction. It is noted that the supporting device  53  may be provided with means for preventing or restricting such displacements, rotations, and pivoting movements. For example, a system of motion dampers may be provided between the first part  69  and the second part  71 . Such a system of motion dampers, may for example, be a system of Lorentz force actuators, usual and known per se, between the first part  69  and the second part  71 , as is shown in  FIG. 3  with reference numeral  203 . 
     The supporting devices  53  in the lithographic apparatus of  FIGS. 1–2  may each be provided, for example, with one or more Lorentz force actuators, in which case the Lorentz force actuators of the supporting devices  53  in combination serve to prevent or restrict said displacements, rotations, and pivoting movements of the frame  39  relative to the base  37 . In  FIG. 3 , reference numeral  119  denotes an vibration or motion sensor which is to cooperate with the Lorentz force actuators  203  for measuring vibrations of the second part  71 . 
       FIG. 3  additionally illustrates a position control system comprising a reference object  200 , which is movable with respect to the first part  69  and supported by the first part  69  via a reference support structure  201 . The position control system also has a position sensor  202 , which is mounted on the second part  71 . The position sensor  202  can detect properties or attributes of the position of the reference object  200  relative to the position sensor  202  (and thus relative to the second part  71 ). The attributes of the position may, for example, include the distance between the reference object  200  and the position sensor  202  or changes, such as, the velocity or acceleration therein. The position sensor outputs a position signal, which represents one or more of the detected attributes of the position, such as a change in distance between the reference object  200  and the position sensor  202 . 
     The position sensor  202  is communicatively coupled to an actuator  203 . The actuator  203  can adjust the position of the second part  71  relative to the first part  69  of the supporting device  53  in response to a position signal provided by the position sensor  202 . Thus, the position control system controls the position of the second part  71  relative to the reference object  200 . In the illustrated embodiment, the actuator  203  comprises, although not limited to, a system of Lorentz force actuators, between the first part  69  and the second part  71 . Lorentz force actuators do not transmit any vibrations from the first part  69  to the second part  71  since Lorentz force actuators are contactless. 
     In the embodiment of  FIG. 3 , the reference support structure  201  comprises a reference spring on which the reference object is suspended. The reference object  200  and the reference support  201  together form a reference mass-spring system. The reference support structure  201  is mounted between the first part  69  and the reference object  200 . The reference mass-spring system has a certain stiffness and mass and, therefore, a certain natural frequency (and like all mass-spring systems, a certain inherent damping; however, a separate damper may also be provided to the reference mass-spring system). 
     Disturbing vibrations acting on the first part  69  with a frequency above the natural frequency of the reference mass-spring system are suppressed by the reference mass-spring system. The reference object is, therefore, shielded from disturbances acting on the first part  69 , such as, for example, vibrations in the floor on which the first part  69  is mounted or reaction forces, as explained above with reference to the base of the lithographic apparatus of  FIGS. 1 and 2 . Moreover, the reference mass-spring system is positioned on the first part. Disturbances acting directly on the second part, such as air pressure changes or acoustics, do not affect the position of the reference object  200 . Thus, control of the position of the second part  71  relative to the reference object  200  results in an increased accuracy of the position of the second part  71 . 
     In the embodiment of  FIG. 3 , the accuracy of the position control of the second part  71  by the position control system is further improved because the reference spring system is a separate spring system and not a part of the gas spring  73 . Because the reference spring system is separate from the gas spring  73 , the position of the reference object  200  is not influenced by disturbances caused by the gas spring  73  of the supporting device  53 , such as pressure variations due to the supply of gas to the gas spring  73 . 
     The accuracy of the positioning of the second part can be further improved by making the resonance frequency of the reference mass-spring system lower than natural frequency of the support system (which typically is 0.5 Hz), because the reference mass is constant and may be relatively low (such as 1 kg or less if so desired) whereas the resonance frequency of the support system is typically 0.5 Hz, and may be difficult to make lower due to the high load on the support system. Thus, the position of the reference object can be shielded more accurately with respect to disturbances acting via the base than the position of the supported part. 
     Preferably, the natural frequency of the reference mass-spring system and the gas spring system  73  are as low as possible, e.g. almost zero. The natural frequencies of both spring systems may be similar or different. A suitable value for the natural frequency of the reference mass-spring system, which suppresses most of the occurring disturbances and which can for example be used in the known lithographic apparatuses manufactured by the assignee is between 0.3 and 0.6 Hz, e.g., 0.5 Hz. Suitable values of the mass and spring stiffness in a 0.5 Hz system are 1 kg and 10 N/m. However, a higher or lower mass and/or spring stiffness may likewise be used, depending on the specific implementation. 
     In the art, lithographic apparatuses are known which comprise a control loop with a vibration or motion sensor which senses the position (or changes therein) of the second part  71  relative to the first part  69  and operates an actuator, such as the Lorentz force actuator in  FIG. 3 , in response to the measured position or change. However, in these prior art apparatuses, the actuator may only actuate with a frequency at or below the natural frequency of the support system. If the actuator frequency exceeds the natural frequency of the support system, the vibration isolation of the support system may deteriorate as the disturbances filtered by the support system are still transferred from the first part  69  to the second part  71  via the control loop. 
     In the embodiment of  FIG. 3 , the position of the second part relative to the reference object is measured. As such, the bandwidth of the position control system, in other words, the actuator frequency relative to the reference object, may be much higher than the natural frequency of the support mass-spring system (e.g., up to 20 Hz or higher) without affecting the vibration isolation of the support system. Thereby, the position control system can also correct for disturbances above the natural frequency of the support system, such as, for example, 2 Hz stage movements. However, a control system in which the position of the second part  71  is measured relative to the first part  69 , such as the known control loops, may also be provided, as is shown in the example of  FIG. 3  with the vibration or motion sensor  119 . 
     In the embodiment of  FIG. 3 , the reference support structure  201  comprises a spring of a different type than the gas spring in the support device. That is, reference support structure  201  comprises a non-gas spring, such as, for example, a mechanical spring, an electrostatic spring, a magnetic spring or any other suitable spring mechanism. Hence, the position of the reference object is not subject to the type of disturbances caused by a gas spring system, such as vibrations due to a gas supply and the position of the reference object is more stable. However, the reference support device may likewise comprise a spring of a similar type as the support mass-spring system, e.g. a gas spring. 
     In  FIGS. 4–9   b,  examples of reference mass-springs systems are shown suitable for use in the position control system of the example of  FIG. 3 . However, other types of mass-spring systems may likewise be used and the invention is not limited to a specific mass-spring system. The example of  FIG. 4  depicts an electrostatic spring system. The electrostatic spring system has electrostatic chargeable members  213 , 214 . The electrostatic chargeable members  213 , 214  each have a plate-shaped chargeable surface and are positioned parallel to each other with the chargeable surfaces facing each other. A first electrostatic chargeable member  214  is positioned near the first part  69  and a second electrostatic chargeable member  213  is positioned at a distance from the first electrostatic chargeable member  214  and the first part  69 . The first chargeable member  214  is fixated in position. The second electrostatic chargeable member  213  is movable along guiding means  215 , 216  with respect to the first electrostatic chargeable member in a transverse direction with respect to the chargeable surface. In the present example, the guiding means  215 , 216  are elongated poles placed in an upright position on the first part  69  of a supporting device according to the invention. The elongated poles extend through holes in the electrostatic chargeable members  213 , 214 . 
     The chargeable surfaces of the electrostatic chargeable members  213 , 214  are chargeable with an electrostatic charge of similar sign by means of a voltage source  217  connected with its positive output contact to the electrostatic chargeable members  213 , 214 . The chargeable surfaces of electrostatic chargeable members  213 , 214  are charged with a positive electrostatic charge, however the chargeable surfaces may likewise be charged with a negative electrostatic charge. Because of the electrostatic charge, a repulsive force is exerted on each the electrostatic chargeable members  213 , 214 . The repulsive force is proportional to the distance between the electrostatic chargeable members  213 , 214  and hence the plates act as a spring with a stiffness which is dependent on the electrostatic charge at the electrostatic chargeable members  213 , 214 . The stiffness of the electrostatic spring system is adjustable via the charge on the chargeable plates provided the source  217 . A reference object  200  is positioned at a side of the second chargeable plate  213  facing away from the second chargeable plate  214 . Thus, a gravitational force caused by the mass of the reference object  200  acts on the second electrostatic chargeable member  213  in a direction opposite to the electrostatic force. The electrostatic chargeable members  213 , 214  and the reference object together form an electrostatic mass-spring system. 
       FIG. 5  depicts a reference object  200  supported by a reference support  201  that comprises a magnetic spring system. The magnetic spring system has an air core coil  211  placed in an upright position on the first part  69 . The air core of the air core coil  211  forms a passage with an opening at the top of the air core coil  211 . A magnet  212  is positioned within the air core coil  211 . The magnet  212  extends partially through the opening outside the passage. A reference object  200  is positioned on top of the magnet  212 . The air core coil  211  is connected to a, not shown, current source which provides a current flowing through the coil such that in the space enclosed by the coil, e.g. in the air core of the coil, a magnetic field exists which exerts a magnetic force on the magnet  202  in a direction from the first part  69  towards the opening, i.e. in a direction opposite to the gravitational force exerted on the magnet  212  by the reference object  200 . The magnetic spring has a stiffness which can be adjusted by changing the magnetic field inside the air core coil, i.e. by adjusting the current flowing through the coil. 
       FIG. 6  illustrates a reference object  200  supported by a reference support  201  that comprises a mechanical spring  210 . The mechanical spring  210  is placed in an upright position on the first part  69 . The reference object  200  lies on top of the mechanical spring  210 . In the examples of  FIGS. 4–6 , the reference object  200  is supported by a single spring in a vertical direction with respect to the second part. However it is likewise possible to use a reference support structure which supports the reference object  200  in other directions as well. 
       FIG. 7  depicts a number of mechanical springs  210  that support the reference object  200 . One of the springs  210  supports the reference object  200  in a vertical direction, while the two other springs support the reference object in horizontal directions. As such, the reference object is not only shielded with respect to vertical vibrations above the natural frequency of the vertical spring, but vibrations in a horizontal direction are suppressed too. In this example, only mechanical springs are used in the reference mass-spring system, however a combination of spring types may likewise be used, such as for example an electrostatic spring supporting the reference object in a vertical direction and mechanical springs supporting the reference object in horizontal directions or otherwise. 
     In the embodiment of  FIG. 3 , the position control system has a single position sensor  202  which measures the distance between the reference object  200  and the position sensor  202 . In an alternative embodiment, more than one position sensor, e.g. three position sensors  202 , may be used. For example, as shown in  FIG. 8 , three position sensors  202  are placed on a second part  71  and a corresponding number of reference mass-spring systems are provided on a first part  69 . The position sensors  202  are connected to a processor  204  which is connected to one or more actuators (which are not shown in  FIG. 8 ) for adjusting the position of the second part. 
     Each of the position sensors  202  of the embodiment illustrated in  FIG. 8 , measures the position of the second part  71  with respect to a reference object  200  in one direction, as is indicated with the lines X, Y, and Z. Thus, the position sensor  202  connected via the line Z, measures the position of the second part  71  with respect to the respective reference object  200  in a vertical direction. Similarly, the position sensors  202  connected via with the lines X and Y, measure the position of the second part with respect to the respective reference object in the X or Y direction, respectively. Thus, the position sensors  202  measure the position of the second part in three degrees of freedom. Depending on the specific implementation, the position control system may likewise measure the position of the second part in another number of degrees of freedom. For example, the frame has six degrees of freedom if the frame can be moved translationally in the X, Y and Z direction as well as rotated around the X, Y and Z direction. To provide an accurate control, the position control system may then measure and control the position of the frame in all six degrees of freedom. 
     In the embodiment of  FIG. 8 , the position sensors  202  measure the position of the second part  71  in the respective direction and output a position signal which is proportional to a property of the measured position, such as, the absolute value of the distance between the respective reference object  200  and the second part  71 , a change in the distance between the respective reference object  200  and the second part  71 , or other suitable attributes. The position signals from the position sensors  202  are fed into the processor  204  which derives from the position signals how the actuator  203  has to adjust the position of the second part  71 . 
     In the embodiment shown in  FIG. 8 , the processor  204  may also be omitted and each of the position sensors  202  may be connected to a separate actuator which can adjust the position of the second part  72  in the respective degree of freedom, e.g. translate or rotate the second part in the X, Y or Z direction only. In such case, each reference mass-spring system, position sensor and actuator combination forms a position control system for a specific degree of freedom. 
     In the embodiment shown in  FIG. 9a , the mass spring system comprises a magnet support system and a mechanical spring. The magnetic support system comprises three magnets, two inner cylindrical magnets  306 ,  307  with their magnetization in vertical direction and a cylindrical outer magnet  310  magnetized in radial direction. The magnetic support system is characterised by a low stiffness in both vertical and horizontal direction. A mass  308  may be added to the inner magnet assembly to compensate for the force generated by the magnetic support system. In this way, the selection or design of a mechanical spring with a low stiffness becomes more easy since the spring has little weight to compensate. The mechanical spring may further serve to maintain the inner magnet assembly in the appropriate horizontal position relative to the outer magnet. The outer magnet may be attached to a small vacuum chamber  304  that is mounted on the second part  69  of the lithographic apparatus. By doing so, the inner magnet assembly is not disturbed by air vibrations. 
     The inner magnet system is connected to the vacuum chamber by a mechanical spring  300 . The combined stiffness of the spring and the magnetic support system provides a mass-spring system with a very low natural frequency (&lt;1 Hz). The inner magnet assembly may further comprise an optical element  302  such as a mirror for measuring the position of the inner magnet system relative to a position sensor mounted on the second part (not shown) of the lithographic device. 
     A further reduction of the tilt stiffness of the mass-spring system described in  FIG. 9   a  can be obtained by adding an inverted pendulum to the inner magnet assembly, as shown in  FIG. 9   b . The negative tilt stiffness of the inverted pendulum  312  can compensate for the positive tilt stiffness of the inner magnet assembly relative to the outer magnet. It will be appreciated by a person skilled in the art that the embodiment as described in  FIGS. 9   a  and  9   b  may also be applied without the enclosing vacuum chamber. Also, the inner magnet assembly may be mounted on a mechanical spring instead of being suspended from a spring. 
     While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. As such, the description is not intended to limit the invention. 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. 
     For instance, in the example of  FIGS. 1–2 , the frame  39  supports not only the focusing unit  5  but also the substrate holder  3  and the mask holder  9 . However, it is apparent that the substrate holder and the mask holder may likewise be supported by other supporting parts of the lithographic projection apparatus. Furthermore, a supporting device according to the present invention may be used not only in a lithographic projection apparatus but also, for example, in finishing machines, machine tools, and other machines or devices in which the transmission of vibrations to certain components thereof supported by the supporting device is to be prevented or suppressed as much as possible. Furthermore, for example the reference mass-spring system may be positioned in a housing to further shield the reference mass-spring system from disturbances caused, for example, by air pressure waves, e.g. sound or otherwise. Also, the example of  FIGS. 1 and 2  is used for an imaging method following the so-called “step and scan” principle. However, the invention is not limited thereto and the invention may likewise be used in, for example, an embodiment of a lithographic projection apparatus using an imaging method following the “step and repeat” principle, wherein the mask and the semiconductor substrate are held in constant positions relative to the focusing unit during the exposure of the semiconductor substrate. 
     Also, although specific reference may be made in this text to the use of the lithographic projection apparatus 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. 
     Furthermore, the invention is not limited to lithographic projection apparatus using optical radiation, but may likewise be applied for other types of radiation, such as for example electromagnetic radiation, including ultraviolet (UV) radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range 5–20 nm), as well as particle beams, such as ion beams or electron beams or otherwise. 
     As such, 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.