Abstract:
Holding devices are disclosed that are configured to hold an optical component of an optical system without actually contacting the optical component. An embodiment of such a holding device is operable in a vacuum environment, such as used in an Extreme UV (EUV) optical system, and effectively holds the optical component without imposing stress on the optical component. The embodiment includes a receptacle configured to receive a mounting portion of the optical component. The receptacle includes at least one gas bearing. At least one exhaust groove (or analogous feature) is situated and configured to scavenge gas discharged by the gas bearing and to exhaust the scavenged gas so as to avoid burdening the vacuum chamber with the discharged gas. Desirably, the receptacle defines multiple gas bearings each including a respective air pad, each desirably including at least one respective exhaust groove. As a result of the non-contacting gas bearings, stress and strain imposed on the holding device are not transmitted to the optical component being held by the device.

Description:
FIELD  
         [0001]    This disclosure pertains to high-precision optical systems as used in, for example, microlithography systems for transfer-exposing a pattern onto an exposure-sensitive substrate. (Microlithography is a key technique used in the fabrication of semiconductor integrated circuits, displays, and the like.) More specifically, the disclosure pertains to devices for holding an optical component (e.g., a lens, a mirror, or a light source) in a high-precision optical system, and to optical systems and microlithography systems including such devices, especially optical systems in which the optical pathway is in a subatmospheric (“vacuum”) environment.  
         BACKGROUND  
         [0002]    As performance demands imposed on optical systems have become increasingly stringent, the accuracy and precision with which components used in such systems are manufactured have had to be increased correspondingly. This trend has been especially pronounced in optical systems of microlithography systems used for transfer-exposing intricate patterns onto substrates such as semiconductor wafers. In fact, the relentless demand for maximal density and miniaturization of active circuit elements in semiconductor integrated circuits and other microelectronic devices requires that most microlithography systems (and their constituent optical systems) operate at or near their theoretical resolution limits. These demands in the evolution of microelectronic devices also have fueled the development of “next generation” lithography (NGL) technology (utilizing, for example, charged particle beams, X-ray beams, or “extreme ultraviolet” beams) offering prospects of significantly higher resolution than obtainable with conventional lithography tools that use visible or ultraviolet light.  
           [0003]    However, each of these NGL technologies poses distinctive technical challenges. For example, the optical systems used in extreme ultraviolet lithography (EUVL; performed at wavelengths ranging from 11 to 13 nm) must utilize EUV-reflective multilayer-film mirrors rather than lenses because no known materials are sufficiently refractive to EUV light of such wavelengths. To achieve adequate reflectivity and sufficiently low aberrations from the mirrors, the reflective surfaces of the mirrors must be formed and coated extremely accurately, and the mirrors must be mounted and positioned relative to each other with extremely high accuracy and stability. Also, because EUV light is attenuated by propagation through air, EUV optical systems must be contained in subatmospheric-pressure (“vacuum”) chambers.  
           [0004]    Basic concepts of EUVL are shown in FIG. 4. The system  201  of FIG. 4 performs projection-exposure of a pattern in a step-and-scan manner using EUV light having wavelength in the range of 11 to 13 nm. At the most upstream end of the system  201  is a laser light source  203  that generates laser light having a wavelength ranging from infrared to visible. For example, the laser light source  203  can be a YAG laser or excimer laser. Laser light from the laser light source  203  is converged by a condenser-lens system  205  onto a downstream a laser-plasma light source  207 .  
           [0005]    The laser-plasma light source  207  produces EUV light having a wavelength of approximately 13 nm. To such end, and as well-known in the art, the laser-plasma light source  207  is supplied, at the convergence point of the laser light from the light source  207 , with xenon gas from a discharge nozzle (not shown). As the xenon gas discharged from the nozzle is illuminated by the laser light, the gas is heated instantaneously to a temperature sufficiently high to produce a plasma from the highly energized molecules of the gas. As the energized gas molecules undergo a spontaneous transition to a lower-potential state, they emit EUV radiation. As noted above, EUV light does not pass readily through air at atmospheric pressure. Consequently, the laser-plasma light source  207  and the trajectory path of EUV light from the laser-plasma light source (and desirably also the laser light source  203 ) are enclosed in a vacuum chamber  209  evacuated to a suitable vacuum level.  
           [0006]    Disposed immediately upstream of the laser-plasma light source  207  is a collimating mirror  211  having an EUV-reflective surface profiled as a paraboloid of revolution. The collimating mirror  211  includes a surficial Mo/Si multilayer film that renders the surface of the collimating mirror  211  highly reflective to incident EUV radiation. Hence, as EUV light produced by the laser-plasma light source  207  is incident upon the collimating mirror  211 , only EUV light having a wavelength near 13 nm is reflected “downward” (in the figure) as a collimated beam of EUV light. Situated downstream of the collimating mirror  211  is a filter  213  made, e.g., of beryllium and having a thickness of 0.15 nm, for example. The filter  213  blocks transmission of visible light while allowing passage only of the desired wavelength of EUV light. Thus, of the EUV light reflected by the collimating mirror  211 , only the desired EUV wavelength passes through the filter  213 . The filter  213  and its immediate vicinity are enclosed by a chamber  215 , which typically is contiguous with the vacuum chamber  209 .  
           [0007]    An exposure chamber  233  is situated downstream of the filter  213  and connected to the chamber  215 . Inside the exposure chamber  233  is an illumination-optical system  217  that comprises multiple multilayer-film condensing mirrors as well as a fly-eye mirror (not detailed). The beam of EUV light from the filter  213  is trimmed so as to have an arc-shaped transverse profile and directed toward the left in the figure.  
           [0008]    A multilayer-film mirror  219 , reflective to incident EUV light, is disposed so as to reflect, in a convergent manner, the arc-shaped beam from the illumination-optical system  217 . The mirror  219  is disk-shaped and has a concave reflective surface  219   a . EUV light reflected from the mirror  219  is reflected by a planar bending mirror  221  disposed at an angle relative to the mirror  219 . Thus, EUV light reflected from the mirrors  219 ,  221  is incident convergently on a reflective reticle  223  horizontally disposed “above” the mirror  221 , with the reflective surface of the reticle  223  facing “downward” in the figure.  
           [0009]    Each of the mirrors  219 ,  221  is made from a respective plate of quartz or other low-thermal-expansion material. The reflective surface of each mirror is worked extremely accurately and coated with a multilayer film configured to render the reflective surface highly reflective to incident EUV light of a desired wavelength and to minimize aberrations. For example, so as to be highly reflective to an EUV wavelength of 13 nm, the multilayer film comprises alternating superposed thin layers of Mo (molybdenum) and Si (silicon). For other EUV wavelengths in the range of 10-15 nm, the multilayer film can comprise individual layers of other substances such as Ru (ruthenium), Rh (rhodium), Be (beryllium), B 4 C (carbon tetraborate), etc.  
           [0010]    Since the reticle  223  is a reflective reticle, it also has an EUV-reflective multilayer film on its surface. A desired reticle pattern to be transfer-exposed from the reticle  223  to a downstream substrate  229  is formed on the multilayer reflective film of the reticle  223 . The “upper” surface of the reticle  223  is mounted to a reticle stage  225  that is movable in at least one of the X and Y directions to allow EUV light reflected by the bending mirror  221  to irradiate selected regions of the reticle  223  in a sequential scanning manner.  
           [0011]    Downstream of the reticle  223  are a projection-optical system  227  and the substrate  229  (e.g., a semiconductor wafer coated with a suitable “resist” that is sensitive to exposure by EUV light. The projection-optical system  227  comprises multiple EUV-reflective multilayer-film mirrors that collectively project (with demagnification) a focused image of the illuminated region of the reticle  223  onto a corresponding region on the “sensitive” surface of the substrate  229 . The demagnification is according to a “demagnification ratio” such as ¼, for example. The substrate  229  is mounted to a substrate stage  231  using a suitable electrostatic chuck or the like. The substrate stage  231  usually is movable in all of the X, Y, and Z directions.  
           [0012]    A conventional holding device  240  for holding an optical component  200  in a system (e.g., the system of FIG. 4) is depicted in FIGS.  3 ( a )- 3 ( b ). The holding device  240  is used, for example, for holding a multilayer-film EUV-reflective mirror in the illumination-optical system  217  or projection-optical system  227 , relative to the exposure chamber  233 . The holding device  240  comprises a “kinematic clamp”  241  of a type conventionally used for holding a precision optical component such as a mirror or laser. The clamp  241  in FIG. 3 is called a “three-point” clamp because the clamp has three portions  241   a ,  241   b ,  241   c  situated at respective locations around the optical component  200  (FIG. 3( b )). The clamp  241  in FIG. 3 is a “kinematic three-point clamp” that holds the optical component  200  at three equally spaced places around the periphery of the optical component  200 . The clamp  241  is configured to hold a mounting ear  200   a  extending from the periphery of the optical component  200 . The mounting ear  200   a  can extend around the periphery of the optical component  200  or be provided at respective locations on the periphery at which the optical component  200  is to be clamped by a respective portion  241   a ,  241   b ,  241   c  of the three-point clamp  241 .  
           [0013]    At each clamping location the respective portion of the clamp  241  comprises opposing jaws  244   a ,  244   b  that define a receptacle  242  therebetween into which the respective mounting ear  200   a  (or portion thereof) is inserted. The jaws  244   a ,  244   b  are attached to a clamp-mounting member  245  via respective flexure springs  243   a ,  243   b . The clamp-mounting member  245  is affixed to a suitable rigid base  250 , usually by a fastener  247  that defines a respective pivot axis of the clamp portion and about which the respective clamp portion can pivot for clamping or unclamping the optical component  200  (see arc-shaped arrows in FIG. 3( b )).  
           [0014]    The holding device  240  of FIG. 3 exhibits the following problems, especially when used in an EUVL system:  
           [0015]    (1) The kinematic clamp portions  241   a ,  241   b ,  241   c  can cause deformation of the reflective surface of the optical component  200  due to stresses imposed at respective points of contact of the opposing jaws  244   a ,  244   b  with the mounting ears  200   a . I.e., although the clamp is kinematic, the stresses imposed at the three points of contact can deform the optical component  200 . In FIG. 3 the respective distribution of stress produced by each pair of jaws  244   a ,  244   b  in the respective mounting ear  200   a  is indicated by a respective dot-and-dash line “loop.” The respective forces applied by the jaws  244   a ,  244   b  to the mounting ear  200   a  are denoted by respective solid-line arrows  246   a ,  246   b.    
           [0016]    (2) No means (e.g., mirror-cooling means) is provided for suppressing temperature increases of the mirror  200  that can cause deformation of the mirror  200 .  
           [0017]    (3) Stress in and/or strain (e.g., deformation) of the base  250  can be transmitted to the mirror  200  easily via the holding device  240 .  
           [0018]    (4) If the holding device  240  is being used for holding a transparent lens, substantial stress can be created in the lens by tightening the clamp  241 . This stress can cause, for example, birefringence in the lens, which can degrade the imaging performance of the lens.  
         SUMMARY  
         [0019]    In view of the shortcomings of conventional optical-component-holding devices as summarized above, the present invention provides, inter alia, optical-component-holding devices that can be used in a vacuum environment, that provide holding-restraint of the optical component using a uniformly applied clamping force, and that provide improved suppression of temperature rises of the optical component.  
           [0020]    To such ends, and according to one aspect of the invention, devices are provided for holding an optical component relative to a mounting surface in an optical system. An embodiment of such a device comprises a mounting member configured for mounting the holding device to the mounting surface, and a receptacle configured to receive and hold a respective portion of the optical component without contacting the optical component. For example, if the optical component includes a mounting flange, then the receptacle is configured to receive and hold a respective portion of the mounting flange. The receptacle comprises at least one fluid bearing situated and configured, whenever a fluid (e.g., gas such as air) is being supplied to the fluid bearing, to support the optical component without contacting the optical component. Desirably, the receptacle defines at least one exhaust groove situated and configured, whenever fluid is being supplied to the fluid bearing, to scavenge the fluid discharged from the fluid bearing and to exhaust the scavenged fluid.  
           [0021]    In a more specific configuration, for an optical component that includes a mounting flange, the receptacle of the holding device comprises first and second facing walls defining a channel therebetween, the channel being configured to receive a respective portion of the mounting flange. The channel has respective sides defined by the first and second facing walls, and a bottom, and the holding device further comprises a respective gas bearing situated in each of the sides and bottom of the channel. Each gas bearing comprises an air pad through which air is discharged, from an air supply, toward a respective region of the mounting flange. Each gas bearing further can comprise a respective exhaust groove extending around the air pad, wherein the exhaust groove serves to scavenge and exhaust air discharged from the respective air pad. Multiple exhaust grooves can be provided for each air pad, wherein the exhaust grooves collectively provide “differential exhaust” (e.g., to atmospheric pressure, to a relatively low vacuum, and to a relatively high vacuum) for each air pad.  
           [0022]    Desirably, the receptacle comprises a position/attitude-adjustment mechanism situated and configured to adjust at least one of position and attitude of the optical component relative to the mounting member without contacting the optical component. A respective position-attitude adjustment mechanism can be associated with each fluid bearing. Each position/attitude-adjustment mechanism can comprise a non-contacting position sensor and a non-contacting positional actuator.  
           [0023]    The optical component can include a cooling passage for conducting a fluid through the optical component for cooling the optical component. In such a situation the receptacle of the holding device desirably is configured to deliver fluid to the cooling passage whenever the optical component is being held by the holding device. Thus, the fluid is allowed to flow from the receptacle through the cooling passage so as to cool the optical component. In this configuration gas discharged from the gas bearing can serve as a cooling fluid by being scavenged by the conduit and routed through the cooling passage as the optical component is being held by the holding device.  
           [0024]    An embodiment of the holding device includes a mounting means for mounting the device to the mounting surface. The embodiment also includes a non-contacting receptacle means for receiving and holding a respective portion of the optical component without contacting the optical component. The non-contacting receptacle means desirably comprises fluid-bearing means for discharging a stream of fluid (e.g., gas such as air) at a respective portion of the optical component, wherein the discharged fluid serves to support the optical component without contacting the optical component. Exhaust means can be provided for scavenging and exhausting fluid discharged from the fluid-bearing means. Position/attitude-adjustment means can be provided for adjusting at least one of position and attitude of the optical component relative to the mounting means without contacting the optical component. Optical-component cooling means can be provided for cooling the optical component being held by the holding device. Scavenging means can be provided for scavenging fluid discharged from the fluid-bearing means. The scavenged fluid can be routed to the optical-component cooling means and used for cooling the optical component.  
           [0025]    According to another aspect of the invention, optical systems are provided. An embodiment of such a system comprises at least one optical component and a device for holding the optical component and for mounting the optical component to a mounting surface in the optical system. The holding device desirably is as summarized above. By way of example, the optical component is an EUV-reflective mirror.  
           [0026]    If the optical component includes a cooling passage, the receptacle can define a conduit situated to open into the cooling passage whenever the optical component is mounted to the holding device. Thus, cooling fluid can be made to flow from the conduit through the cooling passage and thereby cool the optical component.  
           [0027]    According to another aspect of the invention, lithographic exposure systems are provided. An embodiment of such a system comprises a chamber in which lithographic exposure is performed on a substrate by an energy beam, and an optical system situated within the chamber and configured to direct the energy beam for making the lithographic exposure. The optical system comprises at least one optical component and a device for holding the optical component and for mounting the optical component to a mounting surface in the optical system. The holding device desirably is as summarized above.  
           [0028]    The optical system can be a projection-optical system situated and configured to direct the energy beam to the substrate.  
           [0029]    If the chamber is a vacuum chamber, then the mounting flange or receptacle desirably defines at least one exhaust groove situated and configured, whenever gas is being supplied to the gas bearing of the holding device, to scavenge the gas discharged from the gas bearing and to exhaust the scavenged gas.  
           [0030]    If the optical component defines an internal cooling passage for conducting a cooling fluid used for cooling the optical component, then a conduit can be situated to deliver the cooling fluid to the cooling passage.  
           [0031]    According to yet another aspect of the invention, methods are provided, in the context of an optical system comprising a mounting surface and an optical component that includes a mounting portion, mounting the optical component in the optical system. An embodiment of such a method comprises mounting the optical component to a holding device that defines a receptacle configured to receive the mounting portion and to hold the optical component via the mounting portion without contacting the mounting portion. The method also includes the step of affixing the holding device to the mounting surface of the optical system.  
           [0032]    The method further can comprises the step of cooling the optical component as the optical component is being held by the holding device.  
           [0033]    The mounting step further can include the step of providing a non-contacting gas bearing between the mounting portion and the receptacle. In this situation the step of providing a gas bearing comprises discharging air from a gas pad situated in the receptacle (the air being discharged so as to form the gas bearing) and scavenging the discharged air from the gas bearing and discharging the air.  
           [0034]    The method further can comprise the step of adjusting at least one of position and attitude of the optical component relative to the mounting surface, without contacting the optical component.  
           [0035]    With a holding device as disclosed herein, localized holding force is not applied to the optical component by the holding device. Consequently, local deformation, non-symmetrical deformation, and/or stress birefringence of the optical component are avoided.  
           [0036]    Since the holding devices are usable in a vacuum environment, holding devices that include a position/attitude-adjustment mechanism are especially advantageous because such a mechanism allows for remote adjustments of position and attitude of the optical component, including in real time. An exemplary attitude/position-adjustment mechanism includes a positional sensor (e.g., electrostatic-capacity sensor or the like) and a non-contacting-type actuator (e.g., linear motor or the like).  
           [0037]    The gas bearing avoids contact between the holding device and the optical component. Hence, the gas bearing absorbs any deformation that otherwise would be transmitted to the optical component from the mounting surface to which the holding device is attached. Also, since the discharged gas desirably is scavenged, a vacuum environment for the optical system can be maintained easily. Also, the holding device allows integration of the gas bearing with optical-component cooling, thereby suppressing thermal deformation accompanying a temperature rise of the optical component.  
           [0038]    The foregoing and additional advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    [0039]FIG. 1( a ) is an elevational section of an optical-component-holding device according to a first representative embodiment.  
         [0040]    [0040]FIG. 1( b ) is a top plan view showing three holding devices of the first representative embodiment positioned around the periphery of a mirror as a representative optical component. The mirror includes “air passages” (“cooling passages”) that route a cooling fluid (e.g., air) through the mirror independently of fluid delivered to the holding devices.  
         [0041]    [0041]FIG. 1( c ) is a top plan view showing three holding devices of the first representative embodiment positioned around the periphery of a mirror as an exemplary optical component. The mirror includes “air passages” (“cooling passages”) that route a cooling fluid (e.g., air) from the holding devices through the mirror.  
         [0042]    [0042]FIG. 1( d ) is an elevational section of the optical-component-holding device of the first representative embodiment, detailing an exemplary path of fluid (e.g., air) from the air pads to the air passages in the mirror.  
         [0043]    [0043]FIG. 1( e ) is an elevational section of the optical-component-holding device of the first representative embodiment, detailing three exhaust grooves situated around an air pad.  
         [0044]    [0044]FIG. 1( f ) is a view, along the line  1 ( f )- 1 ( f ) of FIG. 1( e ), showing the configuration of the exhaust grooves.  
         [0045]    [0045]FIG. 1( g ) is an example configuration of a device used for radially moving and positioning three optical-component-holding devices of the first representative embodiment relative to an optical component. The depicted device is configured in the manner of a chuck such as used on a machine lathe.  
         [0046]    [0046]FIG. 1( h ) is an elevational section of an alternative configuration of the first representative embodiment, in which exhaust grooves are defined in the mounting flange of the mirror or other optical component held by the optical-component-holding device.  
         [0047]    [0047]FIG. 2( a ) is an elevational section showing certain details (namely the air pads, exhaust grooves, and actuators) of an optical-component-holding device according to a second representative embodiment.  
         [0048]    [0048]FIG. 2( b ) is an elevational section showing certain other details (namely of exemplary actuators and proximity sensors) of the second representative embodiment.  
         [0049]    [0049]FIG. 3( a ) is an elevational section of a conventional optical-component-holding device that includes a kinematic three-point clamp.  
         [0050]    [0050]FIG. 3( b ) is a plan view of the device of FIG. 3( a ).  
         [0051]    [0051]FIG. 4 is a schematic optical diagram of certain components and systems of a conventional extreme ultraviolet lithography (EUVL) system. 
     
    
     DETAILED DESCRIPTION  
       [0052]    The invention is described below in the context of representative embodiments that are not intended to be limiting in any way.  
         [0053]    A first representative embodiment of an optical-component-holding device  10  is shown in FIG. 1( a ). The device  10  of FIG. 1( a ) is depicted holding a mirror  1  as an exemplary optical component. The device  10  includes a mounting member  19  that is mounted to a suitable mounting surface  30  of the optical system. The device also includes a receptacle  18  configured to receive a respective portion of a mounting flange  3  (as an exemplary “mounting portion”) of the mirror  1 .  
         [0054]    The mirror  1  can be configured for use as an EUV-reflective multilayer-film mirror for use in a projection-optical system of an EUVL system. As such, the mirror  1  typically is disk-shaped, with a concave reflective surface  1   a . Extending around the periphery of the mirror  1  and outward from the periphery is the mounting flange  3  used for mounting the mirror  1  to the holding device  10 . The mounting flange  3  can extend completely around the mirror  1  or extend locally from the mirror in the manner of mounting “ears” each configured as a separate extension from a respective location on the edge of the mirror  1 .  
         [0055]    The mirror  1  desirably defines an interior passage  5  that, in the embodiment of FIG. 1( a ), comprises constituent “air passages” (“cooling passages”)  5 A- 5 E. The passage  5  is used for conducting a flow of air or other fluid suitable for cooling the mirror  1 . In this embodiment the air passage  5 A extends axially at the center of the mirror  1 , from the rear surface  1   b  toward the reflective surface  1   a . (See also FIGS.  1 ( b )- 1 ( d ).) Further regarding the embodiment of FIG. 1( a ), the air passage SB connects at a right angle to the air passage  5 A and extends “horizontally” (in the figure) in a radial manner below the reflective surface  1   a . The air passage  5 C connects at a right angle to the air passage  5 B and extends parallel to the axis AX from the air passage  5 B toward the rear surface  1   b  of the mirror. The air passage  5 D connects at a right angle to the air passage  5 C and extends “horizontally” (in the figure) in a radial manner toward the mounting flange  3 . The air passage  5 E extends “vertically” (in the figure) from the air passage  5 D parallel to the axis AX.  
         [0056]    The passages  5  desirably extend with radial symmetry relative to the center of the mirror  1  (i.e., the axis AX). For example, in FIGS.  1 ( b ) and  1 ( c ), three or four, respectively, passages  5  are shown extending radially outward from the axis. The passages  5  can be formed by bonding together two mirror members (i.e., a “front” member and a “rear” member) having mating surfaces in which corresponding grooves have been cut. The distal end of the air passage SA is connected to a flexible conduit  31  (made, e.g., of flexible polymer) for conducting fluid to or from the passages  5 .  
         [0057]    The passages  5  can be supplied with air or other fluid in any of various ways. For example, in FIG. 1( b ), each air passage  5  opens onto the mounting flange  3  and is connected to a respective flexible conduit  32   a - 32   d . The conduits  32   a - 32   d  in FIG. 1( b ) can be either fluid-supply conduits (with the conduit  31  being an exhaust conduit), or fluid-exhaust conduits, and are used solely for delivering or exhausting air (or other fluid) from the passages  5 . In the embodiment of FIG. 1( c ), each passage  5  opens onto a respective mounting-flange portion  3 . To supply fluid to the passages  5  in the configuration shown in FIG. 1( c ), respective flexible conduits  33  connect the passages  5  to respective conduits  14   a  that connect to respective exhaust grooves (see below). Thus, air (or other fluid) discharged from the air pads is routed to the passages  5  and used as a cooling fluid for the mirror  1 .  
         [0058]    In the embodiment of FIG. 1( a ) the distal ends of the air passage  5 E are aligned with corresponding air passages  5 F defined in facing walls  19   a ,  19   b  of the mounting member  19 , described later below. Fluid (e.g., air) supplied to the air passage  5 F flows from the air passage  5 F into and through the air passages  5 E,  5 D,  5 C,  5 B,  5 A so as to conduct heat away from the mirror  1 . Thus, thermal deformation of the mirror  1  that otherwise would accompany a temperature increase of the mirror is decreased.  
         [0059]    The air passages  5 A- 5 E of the mirror  1  also can be used for conducting away fluid (e.g., air) discharged from air pads  15   a ,  15   b ,  16 , as noted above and described later below. Thus, at least some of the fluid supplied to the air pads  15   a ,  15   b ,  16  also may be used for cooling the mirror  1 . Alternatively, fluid may be supplied to the air pads  15   a ,  15   b ,  16  from the air passages  5 F.  
         [0060]    As shown in FIGS.  1 ( b ) and  1 ( c ), at least three (desirably three) devices  10   a ,  10   b ,  10   c  are disposed at respective locations around the periphery of the mirror  1 . Desirably, the devices  10   a - 10   c  are disposed equi-angularly relative to each other. The devices  10   a - 10   c  desirably are movable (as indicative of exemplary movability, see radial arrows in FIG. 1( b )) relative to the mirror  1  by means of respective movement mechanisms (not shown). An exemplary movement mechanism is shown in FIG. 1( g ), in which the devices  10   a - 10   c  are mounted on a chuck mechanism  40  similar to that used for holding a workpiece on a machine lathe. A suitable tool is inserted (arrow  44 ) into an adjustment receptacle  42  and used for rotating the receptacle  42 . Rotating the receptacle  42  causes simultaneous radial movement (arrows  46 ) of the devices  10   a - 10   c  relative to each other. By moving the devices  10   a - 10   c  in this manner, attachment and removal of the mirror  1  can be performed easily.  
         [0061]    Returning to FIG. 1( a ), the receptacle  18  includes respective facing walls  18   a ,  18   b  that define a channel  11  therebetween for receiving the respective portion of the mounting flange  3 . The channel  11  includes an air pad  15   a ,  15   b  in each of the facing walls  18   a ,  18   b , respectively, and desirably also an air pad  16  in the “bottom” (left-facing edge in the figure) of the channel  11 . The air pads  15   a ,  15   b ,  16 , charged with air or other suitable fluid via respective conduits  34   a ,  34   b ,  34   c  (see FIG. 1( d )), allow the mounting flange  3  to be held by the receptacle  18  (i.e., held in the channel  11 ) without actually contacting the facing walls  18   a ,  18   b  or any other part of the receptacle  18 . Thus, the holding device  10  of this embodiment is a “non-contacting” type holding device that holds the mirror by fluid flotation.  
         [0062]    Each facing wall  18   a ,  18   b  in the configuration of FIG. 1( a ) defines at least one respective exhaust groove  13   a ,  13   b . Although only one pair of opposing exhaust grooves  13   a ,  13   b  is shown in FIG. 1( a ), multiple exhaust grooves desirably are provided in each facing wall  18   a ,  118   b , wherein the grooves collectively perform “differential” exhaust. For example, as shown in FIGS.  1 ( e )- 1 ( f ) and with respect to a facing wall  18 , one exhaust groove  23  is connected to an atmospheric-pressure-release conduit  26  (such as the conduits  14   a ,  14   b  shown connected to the grooves  13   a ,  13   b , respectively) providing exhaust to atmosphere. Another exhaust groove  24  is connected to a low-vacuum-exhaust conduit  27  providing exhaust to a relatively low vacuum, and yet another exhaust groove  25  is connected to a high-vacuum-exhaust conduit  28  providing exhaust to a relatively high vacuum. Desirably, a set of exhaust grooves (one atmospheric  23 , one low-vacuum  24 , one high-vacuum  25 ) is provided for each air pad  15   a ,  15   b ,  16 . As shown in FIG. 1( f ), each set of exhaust grooves  23 ,  24 ,  25  desirably extends around the circumference of the respective air pad.  
         [0063]    In the embodiment of FIG. 1( a ), the air pads  15   a ,  15   b ,  16  are located “deeper” (further to the right) in the channel  11  than the exhaust grooves  13   a ,  13   b . Whenever the mirror  1  is mounted in the holding device  10 , the air pads  15   a ,  15   b  are situated opposite respective faces of the mounting flange  3 , and the air pad  16  is situated opposite the distal edge of the mounting flange  3 .  
         [0064]    Each of the air pads  15   a ,  15   b ,  16  comprises a respective porous member through which gas (as an exemplary fluid, supplied via the conduits  6   a ,  6   b ,  17 , respectively) is discharged toward the respective opposing surface of the mounting flange  3 . Whenever the mounting flange  3  is seated fully in the channel  11  as shown in FIG. 1( a ), and gas is being discharged from the respective air pads, the respective gap between each air pad and the respective opposing surface of the mounting flange  3  is approximately 5 μm (see gap  29  in FIG. 1( e ), for example). Thus, the air pads  15   a ,  15   b ,  16  (and any associated exhaust grooves) collectively comprise a fluid-flotation bearing for the mirror  1 . This fluid-flotation bearing can be used in a vacuum environment such as the subatmospheric pressure inside the chamber  233  shown in FIG. 4.  
         [0065]    By optimally specifying the static rigidity of the air pads  15   a ,  15   b ,  16 , the resonant frequency of the mirror  1  can be made sufficiently high, and air-pad vibration can be minimized sufficiently to avoid interfering with proper functioning of the mirror. Multiple sets of air pads  15   a ,  15   b ,  16  (each set desirably being in a respective device  10   a - 10   c ) are situated desirably equi-angularly around the circumference of the mirror  1 .  
         [0066]    In the embodiment of FIG. 1( a ) air (as an exemplary fluid) is supplied to the air pads  15   a ,  15   b ,  16  from passages  6   a ,  6   b ,  17  inside the mounting member  19  and via supply conduits  34   a ,  34   b ,  34   c . Air discharged from the air pads  15   a ,  15   b ,  16  is directed to the opposing surface of the mounting flange  3  of the mirror  1 . Nearly all this discharged air is scavenged by the exhaust grooves  13   a ,  13   b  through respective conduits  14   a ,  14   b . Air discharged from the air pads  15   a ,  15   b ,  16  also can be routed through the air passages  5 A- 5 E inside the mirror  1  (see FIG. 1( d )).  
         [0067]    Alternatively to defining exhaust grooves in the facing walls, exhaust grooves can be defined in the mounting flange  3  of the mirror  1 . For example, as shown in FIG. 1( h ), exhaust grooves  53   a  are associated with the air pad  15   a , and exhaust grooves  53   b  are associated with the air pad  15   b . The exhaust grooves  53   a ,  53   b  are connected to the air passage  5  via conduits  54   a ,  54   b , respectively, defined in the mounting flange  3 . Thus, air discharged from the air pads  15   a ,  15   b  is scavenged by the exhaust grooves  53   a ,  53   b  and used directly for cooling the mirror  1 .  
         [0068]    As discussed above, multiple exhaust grooves  13   a ,  13   b  can be provided so as to optimize the number of differential exhaust levels according to the prevailing vacuum level. For example, a sufficient number of exhaust grooves can be provided to provide two-level or three-level differential exhaust (e.g., respective grooves providing exhaust to atmosphere, exhaust to low vacuum, and exhaust to high vacuum). See FIGS.  1 ( e ) and  1 ( f ), discussed above, for an example of three exhaust grooves. Further by way of example, whenever the mirror  1  must be enclosed in a high-vacuum environment during use, at least one exhaust groove in each facing wall  19   a ,  19   b  provides high-vacuum exhaust. In the depicted embodiments each high-vacuum exhaust groove is located in the respective facing wall  18   a ,  18   b  closest, of all the exhaust grooves, to the high-vacuum environment of the mirror, which provides the best avoidance of substantial leaks of air into the high-vacuum environment. Otherwise, such leaks could deteriorate the high-vacuum condition substantially. The same principle desirably is applied whenever the exhaust grooves are located in the mounting flange  3  (FIG. 1( h )).  
         [0069]    The embodiment of FIG. 1( a ) and other embodiments discussed above effectively prevent deformation of the mounting member  19  and receptacle  18  by stress imparted thereto from the mounting surface  30 , and prevents mounting stress from being transmitted to the mirror  1 . This stress isolation is provided by the non-contacting fluid bearings established by the air pads  15   a ,  15   b ,  16  and exhaust grooves (e.g., grooves  13   a ,  13   b ) of the receptacle  18 . Thus, localized holding force is not applied to the mirror  1 , which prevents local deformation and nonsymmetrical deformation of the mirror  1 , thereby ensuring the accuracy and precision of mirror performance.  
         [0070]    A second representative embodiment of an optical-component-holding device  20  is shown in FIGS.  2 ( a )- 2 ( b ). The device  20  is depicted holding a mirror  1  as an exemplary optical component. The device  20  includes a receptacle  28  and a mounting member  29 , the latter being mounted to a suitable mounting surface  30  of the optical system.  
         [0071]    The device  20  of FIG. 2( a ) is similar to the device  10  of FIG. 1, but additionally comprises one or more position/attitude-adjustment mechanisms  21   a ,  21   b ,  21   c  situated in the receptacle  28 . Each position/attitude-adjustment mechanism  21   a ,  21   b ,  21   c  desirably comprises a respective non-contacting position sensor and a respective non-contacting actuator. The position/attitude-adjustment mechanisms  21   a ,  21   b ,  21   c  desirably are situated inboard of the air pads  15   a ,  15   b ,  16 . Without intending to be limiting, an exemplary non-contacting position sensor is an electrostatic-capacitance sensor (see below), and an exemplary non-contacting actuator is a linear motor. The position sensors and actuators are connected to and driven by respective drivers (not shown), and achieve active adjustment of the mirror  1  to the correct position and attitude in real time.  
         [0072]    Similar to the embodiment of FIG. 1( a ), the receptacle  28  of the embodiment of FIGS.  2 ( a )- 2 ( b ) includes facing walls  28   a ,  28   b  that include the air passages  5 F, the air pads  15   a ,  15   b ,  16 , conduits  14   a ,  14   b , and exhaust grooves  13   a ,  13   b . The receptacle  28  defines a channel  11 , situated between the facing walls  28   a ,  28   b , that receives a respective portion of a mounting flange  3  (as a representative “mounting portion”) of the mirror  1 .  
         [0073]    A specific example is shown in FIG. 2( b ), in which certain detail shown in FIG. 2( a ) is omitted for clarity. Shown in FIG. 2( b ) are the facing walls  28   a ,  28   b  of the receptacle  28  that defines the channel  11 . The channel  11  receives a respective portion of the mounting flange  3  of the mirror  1 . The actuators in this configuration comprise respective permanent magnets  60   a ,  60   b  embedded in respective surfaces of the mounting flange  3 . Opposing each permanent magnet  60   a ,  60   b  is a respective electromagnet  61   a ,  61   b  embedded in a respective facing wall  28   a ,  28   b . Appropriate energization of the electromagnets  61   a ,  61   b  causes attraction or repulsion relative to the respective permanent magnet  60   a ,  60   b . By controlling the electrical current delivered to each electromagnet  61   a ,  61   b , the respective force between the receptacle  11  and mounting flange  3  can be adjusted. Desirably, the applied forces are in response to positional data provided by the electrostatic capacitance sensors  62   a ,  63   a  and  62   b ,  63   b . As shown, for each sensor, one electrode  62   a ,  62   b  is embedded in the respective facing wall  28   a ,  28   b , and the other electrode  63   a ,  63   b  is embedded in the respective opposing surface of the mounting flange  3 . These sensors serve as proximity sensors that are connected to a controller (not shown), wherein the controller processes the proximity data to produce data regarding the gap between the mounting flange  3  and the walls  28   a ,  28   b . The controller desirably also is configured to route appropriate commands as required to the actuators (magnets  62 ,  63 ) and, if necessary, to a fluid supply providing fluid to the air pads  15   a ,  15   b.    
         [0074]    An optical-component-holding device according to any of the various embodiments described above can be used for holding an optical component (laser, mirror, filter, etc.) of an EUVL system such as the system shown in FIG. 4. Thus, the optical component is held with high accuracy and precision, thereby facilitating the obtaining of high-accuracy lithographic transfers performed using the system. The optical-component-holding devices are especially effective in holding optical components in a vacuum environment without deforming the optical components and while allowing small positional adjustments of the optical components to be made. The optical-component-holding devices are not limited to use in EUVL optical systems; the devices also can be employed in any of various other ultra-high-precision optical systems. The optical-component-holding devices also are effective in suppressing the occurrence of local deformation, non-symmetrical deformation, and stress-related birefringence in optical components held by the devices. Achievement of these ends is facilitated by the air pads that define respective non-contacting fluid bearings between the holding device and the optical component. Also, if the optical component includes an internal conduit for circulating a cooling fluid, then the optical component can be cooled (while being held by the holding device) by the circulation of fluid through the conduits, thereby suppressing thermal deformation of the optical component. The fluid used for cooling can be the same fluid as used in the non-contacting fluid bearings.  
         [0075]    Whereas the invention has been described in connection with multiple representative embodiments, the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.