Abstract:
Devices are disclosed that cool optical elements with which the devices are associated, most advantageously reflective optical elements such as mirrors and reflective reticles. The devices have especial utility for reducing deformation and other undesired thermal changes of the respective optical elements, such as optical elements used in extremely demanding optical systems such as used in microlithography systems, most notably EUVL systems. Many of the subject devices typically include a heat-receiving plate or analogous feature that receives heat radiated from the optical element across a gap between the optical element and the heat-receiving plate. Some devices include a plate-cooling device for removing heat from the heat-receiving plate. Other devices employ conduction of heat away from the optical element. Yet other devices employ a flowing heat-transfer medium for removing heat from the optical element. Certain devices also are configured to provide mechanical support for the respective optical elements, notably in a manner that limits deformation of the optical elements.

Description:
FIELD  
         [0001]    This disclosure pertains to devices and methods for cooling an optical element, such as a lens, mirror, mask, or reticle, especially as used in a microlithographic-exposure apparatus in which the optical behavior of the optical element could be changed unacceptably by a thermal change. The devices and methods are especially suitable for cooling an optical element so as to reduce thermal deformation of the element and accompanying degradation of optical performance, especially of the element as used in high-precision optical systems such as microlithography systems. The devices and methods can be used in a vacuum environment such as required for extreme ultraviolet lithography (EUVL) systems. The subject devices can be integrated with devices used for supporting the optical elements.  
         BACKGROUND  
         [0002]    In recent years, as the sizes of active circuit elements in semiconductor integrated circuits, displays, and the like have become progressively smaller, the need has become acute for “next generation” microlithography methods that are capable of achieving finer image resolution than currently obtainable using “optical” microlithography (e.g., microlithography performed using ultraviolet light). The resolution currently obtainable using optical microlithography (as well as any other lithographic technology using electromagnetic radiation) is limited by diffraction. But, in general, the shorter the wavelength of the radiation, the potentially finer the achievable resolution. Hence, to obtain finer resolution than obtainable using optical microlithography, much research and development has been expended on approaches that use shorter wavelengths.  
           [0003]    Current “next generation lithography” (NGL) technology relies upon use of wavelengths of exposure light that are largely in the “soft” x-ray, extreme ultraviolet (EUV), and x-ray wavelengths. For example, substantial effort has been expended in the development of extreme ultraviolet lithography (EUVL), which uses wavelengths in the range of 5 to 20 nm. Most work to date has been performed at EUV wavelengths of 11 to 13 nm.  
           [0004]    A contemporary EUVL system is shown schematically in FIG. 23, in which the subject system includes an EUV source  101  and an illumination-optical system  103  that directs EUV light  100  (λ=13.4 μm) emitted from the EUV source  101  to a reflective reticle  102 . The reticle  102  defines features of a pattern to be transferred from the reticle to a lithographic substrate  104 . Light reflected from the reticle  102  carries an aerial image of the pattern defined on the reticle and is projected by a projection-optical system  105  onto the substrate  104  (usually a resist-coated semiconductor wafer). The reticle  102  is mounted on a reticle stage  106 , and the substrate  104  is mounted on a substrate stage  107 . As the beam from the projection-optical system  105  is incident on the substrate  104 , an actual image of the illuminated portion of the reticle  102  is formed and imprinted on the substrate  104 .  
           [0005]    Essentially all known substances absorb the wavelength of EUV light  100  emitted from the EUV source  101 . Consequently, an EUV optical system, unlike the refractive optical system used in an optical microlithography system, uses a reflective optical system that comprises multiple reflective mirrors. In one example of a conventional EUVL system, the projection-optical system  105  includes six multilayer-coated EUV-reflective mirrors (not detailed) that collectively project the aerial image at a “reduction” (demagnification) ratio of 1/4 or 1/5, for example. For making exposures, the projection-optical system  105  typically has an optical field that is a portion of an annulus. Such an optical field has exemplary dimensions of 2 mm in width and 30 mm in length on the substrate  104 . The respective reflective surface of each of the mirrors of the projection-optical system  105  typically is aspherical, and includes, for exhibiting high reflectivity to EUV light of λ=13.4 nm, a Mo/Si multilayer coating. Since the optical field is too small to expose an entire pattern in one exposure shot, the pattern is exposed portion-by-portion, accompanied by corresponding scanning motions of the reticle  102  and substrate  104 , as performed by their respective stages  106 ,  107 . The scanning velocity of the substrate  104  relative to the scanning velocity of the reticle  102  reflects the demagnification ratio of the projection-optical system  105 . Such scanning exposure achieves transfer of a pattern that is substantially larger than the optical field of the projection-optical system  105 . An exemplary EUV-optical system  105  comprising two multilayer-coated reflective mirrors  111 ,  112  is shown in FIG. 24, in which the mirrors are mounted in an optical “column”  110 . The column  110  includes a main portion  110   a  and a flange portion  110   b , both desirably made of invar or a related material that exhibits very low thermal deformation.  
           [0006]    The mirror  111  is supported in the column  110  by a respective holding device  116 , with a position-adjustment mechanism  115  (e.g., piezo-electric actuator or the like) situated between the holding device  116  and the flange portion  110   b . The position-adjustment mechanism  115  is used for adjusting the position of the mirror  111  during assembly or later, e.g., during use of the EUV-optical system  105 . The mirror  112  is supported by a respective holding device  117  “below” the flange portion  110   b . Each mirror defines a respective void  11   a ,  112   a . EUV light  100  propagating from upstream (entering the top of the figure from the source or from the reticle) reaches the “upper” surface of the mirror  112  via the void  11   a  defined in the mirror  111 . The EUV light  100  propagates to the “lower” surface of the mirror  111 , from which the light is reflected “downward” through the void  112  to the reticle or substrate (not shown).  
           [0007]    In the EUV optical system of FIG. 24, each reflective surface of the mirrors  111 ,  112  has a Mo/Si multilayer coating on the respective surface. The multilayer coatings render the respective mirrors highly reflective to a particular wavelength of EUV light. Unfortunately, whereas a reflectivity of or close to 100% is desired, such high reflectivity has not been achieved even from multilayer-coated mirrors. For example, if the EUV light  100  has a wavelength of 13 nm, a reflectivity (to perpendicularly incident EUV light of this wavelength) of approximately 70% is obtained from a multilayer coating that comprises 40 to 50 pairs of alternating Mo and Si layers formed with a period of approximately half the incident wavelength. Hence, whereas 70% of the incident-light energy is reflected, the remaining 30% of incident energy is absorbed in the mirror as heat. The amount of heat absorbed under such conditions ranges from a fraction of a watt to a few watts. Nevertheless, this heat is sufficient to cause unwanted thermal deformation of the reflective surface of the mirror.  
           [0008]    A conventional EUV projection-optical system has a numerical aperture of 0.2 to 0.3, for example, and the wavefront aberration of the system should be 1 nm (RMS) or less. To achieve such a small wavefront aberration, the mirrors of an EUV optical system typically have aspherical reflective surfaces that are formed and mounted extremely accurately in the optical column.  
           [0009]    EUV light in the wavelength range of 5 to 20 nm is greatly absorbed by gas at room temperature. Consequently, the optical path of an EUV optical system must be at high vacuum in a vacuum chamber. Consequently, the mirrors and other optical elements of the EUV optical system must be situated in the vacuum chamber. As noted above, whenever EUV radiation is incident on a reflective mirror, the illuminated portion of the reflective surface experiences heating due to absorption of some of the energy of incident radiation. In a vacuum environment in which EUV optical systems typically are used, the ability of warmed mirrors to transfer heat away is poor. Consequently, the optical elements (including multilayer-coated mirrors) tend to experience an unacceptable degree of thermal deformation.  
           [0010]    To solve this problem, there is a need for devices and methods for limiting such thermal deformation, e.g., by cooling the optical elements. Unfortunately, conventional methods for cooling mirrors and the like rely upon actual contact of a cooling device with the subject optical element, which poses a substantial risk of deforming the elements due simply to the contact (which tends to apply a force to the optical element). As a result, conventionally, there is a substantial risk that the desired highly accurate profile of the reflective surface of such an optical element cannot be maintained.  
           [0011]    Also, direct-contact cooling devices tend to conduct excessive vibration directly to the optical elements. The vibration degrades the optical performance of the elements.  
           [0012]    One conventional approach to reducing thermal expansion of EUV mirrors is to form the multilayer coatings on mirror-substrate materials that exhibit low thermal expansion. For example, low-expansion glass typically is used, having a linear-expansion coefficient of approximately 10 ppb (e.g., “Zerodur” made by Schott Corporation and “ULE” made by Corning).  
           [0013]    Unfortunately, use of these low-expansion materials does not completely prevent excessive thermal expansion of the mirrors. To achieve a desired image-formation performance in EUVL, the allowable wavefront aberration in the projection-optical system is 1 to 0.5 nm or less. In a projection-optical system comprising six mirrors, the allowable profile error of each mirror is 0.2 to 0.1 nm or less. Most of this tolerance is consumed by mirror processing, assembly errors, and the like. Consequently, the residual latitude allowed for thermal deformation of the mirrors is extremely small. Indeed, the estimated thermal deformation (10 ppb) experienced even when low-expansion glass is used greatly exceeds this tolerance.  
           [0014]    Typically, whenever a mirror used in an EUV optical system experiences heating, the heating begins on the “front” (reflective) surface of the mirror and propagates by thermal conduction through the thickness dimension of the mirror to the “rear” surface of the mirror. The thermal conductivity of low-expansion glass is very low compared to metal or the like, which causes a very slow thermal conduction through the mirror and results in a large temperature gradient between the front and rear surfaces of the mirror. For the same reason, lateral propagation of heat in the mirror also is very slow, which causes a non-uniform in-plane temperature distribution of the mirror. Due to these temperature gradients, the thermal expansion of the mirror being surficially heated is anisotropic, which causes deformation of the surface profile of the mirror.  
           [0015]    Deformation of the reflective surface of the mirror also can be caused by static deformation due to the mirror having to support its own mass, fluctuations in ambient atmospheric pressure, dynamic vibration propagating upward from the floor to the mirror, reaction forces generated by stage movements, vibrations from transport systems, etc.  
           [0016]    In any event, adequate removal of heat from mirrors and other optical systems used in optical systems of the general type described above, without contributing to other deformations of the mirrors, would be especially desirable for achieving the optical-performance goals of such systems.  
         SUMMARY  
         [0017]    According to a first aspect of the invention, optical-element-cooling devices are provided. An embodiment of such a device comprises a heat-receiving plate arranged proximally to a respective optical element along a surface of the optical element at which light directed to the optical element is not incident or outgoing. Desirably, the heat-receiving plate is arranged conformably to at least a portion of the surface of the optical element at which light directed to the optical element is not incident or outgoing. In any event, the heat-receiving plate is configured to receive heat from the optical element. The device also includes a plate-cooling device that removes heat from the heat-receiving plate. The plate-cooling device desirably contacts the heat-receiving plate in a conforming manner.  
           [0018]    The optical element can be, for example, a mirror having a reflective surface. With such an optical element, the heat-receiving plate desirably is arranged along at least one surface of the mirror other than the reflective surface of the mirror, for example, along a rear surface of the mirror. Alternatively or in addition, the heat-receiving plate can be arranged along a side surface of the mirror.  
           [0019]    Desirably, the heat-receiving plate does not contact the optical element. Nevertheless, by absorbing heat radiating from the optical element, the heat-receiving plate maintains the optical element at a desired temperature. This temperature maintenance is especially enhanced by cooling the heat-receiving plate. Thus, thermal deformation of the optical element is prevented while also preventing deformation of the optical element that otherwise could be caused by transfer of mechanical stresses to the optical element from the heat-receiving plate.  
           [0020]    The heat-receiving plate desirably is made of a material selected from a group consisting of metals and ceramics, and the plate-cooling device desirably comprises at least one heat pipe affixed to a surface of the heat-receiving plate facing the optical element. This configuration further can include a liquid-cooled body to which the at least one heat pipe is connected. The liquid-cooled body removes heat conducted thereto by the at least one heat pipe from the heat-receiving plate.  
           [0021]    A surface of the heat-receiving plate facing the optical element desirably is processed so as to increase a heat-absorption efficiency of the surface compared to an otherwise similar non-processed surface. For example, the surface can be processed to provide a ceramic, oxide-, carbide-, or nitride-containing coating on the surface. As another example, the surface can be processed to provide an increased surficial roughness or surficial irregularity resulting in an increased heat-absorption area of the surface. The surface can be processed in a spatially distributed manner to provide the increased heat-absorption efficiency to pre-determined locations on the surface.  
           [0022]    In addition or alternatively, a surface of the optical element facing the heat-receiving plate can be processed so as to increase a heat-radiation efficiency of the surface compared to an otherwise similar non-processed surface. For example, the surface can be processed to provide an increased surficial roughness or surficial irregularity resulting in an increased heat-radiation area of the surface. The surface can be processed in a spatially distributed manner to provide the increased heat-radiation efficiency to pre-determined locations on the surface.  
           [0023]    The optical-element-cooling device further can comprise a heat-proofing device situated relative to the heat-receiving plate and plate-cooling device so as to block thermal radiation selected from: (a) from the heat-receiving plate to another optical element, and (b) from another optical element to the optical element being cooled by the heat-receiving plate. The heat-proofing device desirably comprises a second heat-receiving plate and an associated heating device that collectively offset heat-sink effects of the heat-receiving plate and plate-cooling device on a neighboring heat-sensitive component. Further desirably, the plate-cooling device conforms to the heat-receiving plate, and the heat-proofing device conforms to the first heat-receiving plate.  
           [0024]    As noted above, the heat-receiving plate desirably is separated from the optical element by a gap. Alternatively, the heat-receiving plate can be in actual contact with the surface of the optical element so as to conduct heat from the surface. If a gap is provided, however, the gap can be constant or variable. A variable gap provides predetermined respective greater or lesser heat-transfer rates from selected corresponding locations on the surface of the optical element.  
           [0025]    According to another aspect of the invention, methods are provided for cooling an optical element. In an embodiment of such a method, a heat-receiving plate is situated in proximity to an optical element along a surface of the optical element at which light is not incident to or outgoing from the optical element. Thus, the heat-receiving plate receives and absorbs heat from the optical element. The heat-receiving plate is cooled to remove absorbed heat from the heat-receiving plate. The heat-receiving plate desirably is situated so as to absorb heat radiated from the optical element.  
           [0026]    Cooling the heat-receiving plate desirably is performed by conducting heat from the heat-receiving plate using at least one heat pipe coupled to a liquid-cooled body that removes heat from the heat pipe, wherein the heat pipe conducts away heat from the heat-receiving plate.  
           [0027]    The method further can comprise the step of blocking radiation of heat from the heat-receiving plate to another optical element. Similarly, the method further can comprise the step of blocking any heat-sink effect, resulting from cooling the heat-receiving plate, of the heat-receiving plate on neighboring components.  
           [0028]    The optical element can be cooled during actual use of the element or during periods in which the optical element is not being used.  
           [0029]    According to another aspect of the invention, optical systems are provided. An embodiment of an optical system comprises an optical element and an optical-element-cooling device, as summarized above, situated relative to the optical element so as to cool the optical element. Such an optical system can be configured for use in a microlithography system, such as a microlithography system utilizing reflective optics (e.g., an EUVL system). In an EUVL system the subject optical element can be, for example, a mirror or reflective reticle. An EUV-reflective mirror typically has an EUV-reflective multilayer-coated surface. The subject optical systems can be, for example, a projection-optical system as used in an EUVL system.  
           [0030]    Another embodiment of an optical-element-cooling device is especially suitable for use in a vacuum environment. This embodiment comprises at least one electronic cooling element (e.g., Peltier element) having a hot side and a cold side. A respective spring member is arranged between the optical element and the cold side of the electronic cooling element. At least one heat pipe is connected to the hot side of each electronic cooling element and configured for transferring heat away from the hot side and thus from each respective electronic cooling element. A heat-pipe-cooling device is connected to the at least one heat pipe and configured for removing heat from each heat pipe. By connecting the electronic cooling element to the optical element using a spring member, optical-element deformations that otherwise could be transmitted to the optical element are blocked by the spring member. The spring member desirably has low rigidity (e.g., a flat spring), desirably is made of a material having high thermal conductivity, desirably exhibits little to no outgassing in a vacuum environment, and desirably has a low coefficient of thermal expansion.  
           [0031]    The optical element can be, for example, a reflective optical element having a reflective surface and a non-reflective surface. In this instance multiple spring members desirably are attached to the non-reflective surface and extend from the non-reflective surface to the at least one heat pipe. The non-reflective surface desirably is configured to define multiple voids therein, wherein each void has an associated respective electronic cooling element extending into it. At least one respective spring member is associated with each void so as to connect a portion of the non-reflective surface in the void to a cold side of the respective electronic cooling element. This configuration also desirably includes a controller connected to each of the electronic cooling elements, wherein the controller is configured to operate each of the electronic cooling elements in a controllable manner to provide a respective desired amount of cooling to the respective portion of the non-reflective surface. Reflective optical elements typically absorb a portion of the radiation incident on them, which causes heating of the elements. Since the intensity of incident light is usually not uniform, the distribution of temperature of the element is correspondingly not uniform. By separately controlling operation of the multiple electronic cooling elements, cooling of the optical element can be tailored for the particular thermal distribution of the optical elements. This control of the electronic cooling elements can be facilitated by feedback from temperature sensors strategically placed relative to the optical element.  
           [0032]    This embodiment also can include an optical-element-holding device situated and configured to mount the optical element in an optical column. The optical-element-holding device can be configured with multiple holding “cells” that are connected to respective locations on the optical element by a respective spring member. Each holding cell desirably is connected to a heat pipe by a respective spring member. Further desirably, a respective unit of thermally insulative material is situated between each holding cell and its respective spring member connecting the holding cell to the heat pipe.  
           [0033]    Another embodiment of an optical-element-cooling device, particularly suitable for use in a vacuum environment, comprises at least one electronic cooling element (e.g., Peltier element) as summarized above, wherein the cold side is adjacent the optical element but separated from the optical element by a gap. The cold side of the electronic cooling element desirably extends into a respective void in the surface of the optical element. A heat-transfer element is situated relative to the hot side of each electronic cooling element and configured to conduct heat away from the hot side. A cooling mechanism is provided for removing heat from the heat-transfer element. The device desirably further comprises a gas-delivery device situated and configured to direct a flow of a gas into the gap, wherein the gas serves to conduct heat from the optical element to the at least one cooling element. The device also desirably includes a gas-evacuation device situated and configured to remove the gas from the gap. By introducing the gas into the gap, thermal transfer from the optical element to the electronic cooling elements is facilitated.  
           [0034]    Extending the electronic cooling elements into respective voids allows the gap to be very small, resulting in more efficient radiation of heat from the optical element to the electronic cooling elements. Optical elements configured as mirrors typically are fabricated from glass or other material having a low thermal conductivity, but high precision and stability of the optical element requires that the glass be thick, which works against more efficient thermal transfer. Hence, thinner elements are preferred. The voids help provide adequate rigidity of thinner optical elements. The voids also increase the thermal-radiation area of the optical elements, which increases the efficiency of thermal transfer from the optical elements to the electronic cooling elements. By way of example, the voids can provide a honeycomb profile to the surface of the optical element.  
           [0035]    The gap can be configured to define a substantially closed space between the optical element and the electronic cooling element(s), wherein the gas-evacuation device removes gas from the substantially closed space.  
           [0036]    The device can include multiple electronic cooling elements situated adjacent respective locations on a surface of the optical element and separated from the respective location by a respective gap. This configuration desirably further includes a controller connected to each of the electronic cooling elements and configured to control independently each of the cooling elements to provide a respective desired amount of cooling to a respective portion of the optical element.  
           [0037]    The device further can comprise at least one spring member that connects the cold side of each electronic cooling element to a respective portion of the non-reflective surface in each void. In this configuration the heat-transfer element desirably is a heat pipe, and each spring member desirably is thermally conductive. The spring member(s) prevent transmission of mechanical stresses from the optical element to the electronic cooling element(s) or from the electronic cooling element(s) to the optical element, which providing good thermal transfer from the optical element to the electronic cooling element(s).  
           [0038]    This device embodiment further can comprise an optical-element-holding device situated and configured to provide a physical mounting for the optical element in an optical column as the optical-element-cooling device cools the optical element. The optical-element-holding device desirably comprises multiple holding cells connected to the heat-transfer member via a respective spring member, as summarized above. Further desirably, a respective unit of thermally insulative material is situated between each holding cell and its respective spring member connecting the holding cell to the heat-transfer member. These spring members and insulative material provide desirable isolation from transmission of thermal and physical stresses.  
           [0039]    Another embodiment of an optical system is especially suitable for use in a microlithography system. The optical system comprises at least one reflective optical element having a reflective surface that receives incident radiation and reflects at least a portion of the incident radiation. The optical system also includes a respective optical-element-cooling device associated with the optical element. The optical-element-cooling device comprises: (a) at least one electronic cooling element having a hot side and a cold side, wherein each cooling element is situated relative to the optical element such that the cold side is adjacent the optical element but separated from the optical element by a gap, (b) a first heat-transfer element situated relative to the hot side of each electronic cooling element and configured to conduct heat away from the hot side, and (c) a first cooling mechanism for removing heat from the first heat-transfer element. The optical element can be, for example, a mirror or a reflective reticle. The optical system can be configured as an illumination-optical system for directing a flux of light to a reticle or as a projection-optical system for directing a flux of light from the reticle to a lithographic substrate.  
           [0040]    The optical system further can comprise a vacuum chamber containing the at least one reflective optical element and associated respective optical-element-cooling device. In this configuration, a second heat-transfer element desirably is arranged so as to extend through a wall of the vacuum chamber. A spring member connects the first heat-transfer element to the second heat-transfer element. The system also includes a second cooling mechanism for cooling the second heat-transfer element. The system further can include a flexible member situated between the second heat-transfer element and the wall of the vacuum chamber. In addition, a unit of thermally insulative material can be situated between the second heat-transfer element and the wall of the vacuum chamber. These various spring members and units of thermally insulative material provide desirable isolation from transmission of thermal and physical stresses.  
           [0041]    Yet another embodiment of an optical system comprises a vacuum chamber and multiple reflective optical elements situated relative to each other inside the vacuum chamber. Each optical element has a respective reflective surface that receives incident radiation and reflects at least a portion of the incident radiation. A respective optical-element-cooling device is associated with at least one of the optical elements. The optical-element-cooling device comprises: (a) a cooling element situated inside the vacuum chamber and configured to remove heat from the respective optical element, (b) a first heat-transfer element situated inside the vacuum chamber and configured to remove heat from the cooling element, (c) a second heat-transfer element situated inside the vacuum chamber, (d) a third heat-transfer element situated outside the vacuum chamber, (e) a first thermally conductive spring member connecting the first heat-transfer element to the second heat-transfer element so as to provide a heat-conduction pathway from the first heat-transfer element to the second heat-transfer element, and (f) a second thermally conductive spring member connecting the second heat-transfer element to the third heat-transfer element so as to provide a heat-conduction pathway from the second heat-transfer element to the third heat-transfer element.  
           [0042]    The second spring member desirably comprises a respective pair of spring-shaped members conjoined at the wall of the vacuum chamber.  
           [0043]    According to another aspect of the invention, EUVL systems are provided. An embodiment of such a system comprises an illumination-optical system that guides EUV light to a reflective reticle, and a projection-optical system that guides EUV light from the reflective reticle to a lithographic substrate while transferring an image of a pattern, defined on the reticle, to the substrate. At least one of the illumination-optical system and projection-optical system comprises a mirror. The system also includes a mirror-cooling device for cooling the mirror. The mirror-cooling device comprises a delivery device situated and configured to deliver a cooling medium (e.g., gas or liquid) to at least one of the reflective surface, rear surface, and side surface of the mirror. The delivery device can comprise a nozzle or the like connected to a supply of the cooling medium. The delivered cooling medium contacts the mirror and cools the mirror by thermal conduction and heat of vaporization.  
           [0044]    The system desirably further comprises a controller configured to control a timing by which the mirror is cooled using the mirror-cooling device. The controller desirably causes the cooling medium to be delivered whenever the lithographic-exposure system is not being used for making a lithographic exposure.  
           [0045]    The system further can comprise a cooling-medium-evacuation device situated and configured to recover cooling medium delivered by the delivery device.  
           [0046]    Another embodiment of a lithographic-exposure system comprises an illumination-optical system that guides EUV light from a source to a reflective surface of a pattern-defining reflective reticle. A projection-optical system guides EUV light from the reflective surface to a lithographic substrate, thereby transferring the pattern to the substrate. The system includes a reticle-cooling device for cooling the reflective reticle. The reticle-cooling device comprises a delivery device situated relative to the reticle and configured to deliver a cooling medium so as to pass over the reflective surface of the reflective reticle. The system desirably further comprises a controller configured to control cooling of the reflective reticle by the reticle-cooling device, in which controlled cooling the delivery device is situated at a position that will not block EUV light incident on the reflective surface of the reticle, at which position the cooling medium is delivered by the delivery device.  
           [0047]    The system further can comprise an evacuation device situated relative to the reticle and delivery device and configured so as to evacuate the cooling medium delivered by the delivery device.  
           [0048]    The cooling medium can be a gas or a liquid that cools the reticle by thermal conduction and heat of vaporization. Exemplary gases are helium and nitrogen. Exemplary liquids are liquid nitrogen, liquid helium, and ethanol.  
           [0049]    Another method embodiment is directed to cooling an EUV-reflective mirror having a front surface and a rear surface. The method comprises delivering a cooling medium into contact with at least one of the front surface or the rear surface of the mirror. The method desirably further includes the step of evacuating the cooling medium after the cooling medium has contacted the surface.  
           [0050]    Yet another method embodiment is directed to cooling a reflective reticle having a reflective surface that defines a pattern to be transferred to a lithographic substrate by a beam of EUV light. The method comprises delivering a cooling medium at the reticle so as to cause the cooling medium to contact the reflective surface of the reticle. The method further can comprise the step of evacuating the cooling medium after the cooling medium has contacted the reflective surface.  
           [0051]    Another reticle-cooling embodiment is set forth in the context of a microlithography method in which a pattern, defined on a reflective reticle, is transferred to a lithographic substrate by guiding a beam of EUV light through an illumination-optical system to the reflective reticle. A beam of EUV light also is guided through a projection-optical system from the reticle to the substrate. In the reticle-cooling method a delivery device is placed relative to the reticle so as not to block EUV light incident on or reflected from the reticle. While lithographic exposure is being performed using the reticle, the reticle is cooled by releasing a cooling medium from the delivery device at a surface of the reticle.  
           [0052]    Yet another embodiment of an optical-element-cooling device is a device that cools the optical element as the optical element is being supported in an optical column. The optical element has an optical-function surface, a non-optical-function surface, and a peripheral-side surface. The device comprises multiple (desirably at least three) optical-element-support members mounted to respective locations (desirably equi-angularly distributed) on a surface (desirably a peripheral surface) of the optical element other than the optical-function surface. The device also comprises a cooling frame to which the optical-element-support members are attached so as to support the optical element relative to the cooling frame. The device also comprises a heat-conductive plate disposed along the non-optical-function surface of the optical element. The heat-conductive plate has a periphery that is connected to the cooling frame in a manner allowing the cooling frame to remove heat from the heat-conductive plate. The cooling frame defines a conduit through which a cooling medium is conducted so as to remove heat from the cooling frame and hence from the heat-conductive plate as the optical element is being supported by the cooling frame.  
           [0053]    Each optical-element-support member desirably is made of a material having a low coefficient of thermal expansion and that is flexibly deformable in directions that are normal to the respective location on the peripheral-side surface. The optical-element-support members desirably are attached to respective locations on the peripheral-side surface.  
           [0054]    The heat-conductive plate desirably is separated from the non-optical-function surface by a defined gap that can be constant or variable.  
           [0055]    The heat-conductive plate desirably conforms to the shape of the non-optical-function surface, and can be configured to have a convex inverted-bowl shape extending toward the non-optical-function surface.  
           [0056]    The gap can have a respective dimension at each of multiple locations so as to achieve a respective desired individual rate of thermal transfer at each location from the non-optical-function surface to the heat-conductive plate. The individual rates desirably are sufficient to achieve a substantially uniform distribution of temperature at the optical-function surface of the optical element. The individual rates can be sufficient to achieve a pre-determined distribution of temperature at the optical-function surface of the optical element.  
           [0057]    Each optical-element-support member can comprise a linking member including a pair of flat springs providing the flexible deformability of the optical-support member.  
           [0058]    The device can further comprises a base and multiple cooling-frame-support members affixed to the base and extending between the base and respective locations on the cooling frame so as to support the cooling frame relative to the base. For example, the cooling-frame-support members can be configured to support the cooling frame peripherally relative to the base. Each cooling-frame-support member can comprise at least one respective flat spring that is flexibly deformable in a respective direction that is normal to the respective location on the cooling frame. Each cooling-frame-support member further can comprise: (a) a respective cooling-frame actuator that is movable, when energized, in a direction parallel to an optical axis of the optical element, thereby moving the cooling frame in the direction relative to the base, and (b) a first sensor situated and configured to detect a distance between the base and the non-optical-function surface of the optical element. The first sensor can comprise a laser interferometer that directs a beam of laser light through the heat-conductive plate to the optical element. The first sensor is especially suitable for providing feedback positional data to the respective cooling-frame actuator.  
           [0059]    The base typically is mounted to the optical column. In such a configuration, the device further can comprise: (a) at least one base actuator that is movable in at least one orthogonal direction perpendicular to the optical axis of the optical element, thereby moving the base and cooling frame in the orthogonal direction relative to the optical column, and (b) a second sensor situated and configured to detect a displacement of the base relative to the optical column. The second sensor can be configured to provide feedback position data to the respective base actuator.  
           [0060]    Another embodiment of an optical system, configured for use in a microlithography system, comprises an optical column, a reflective optical element having an optical-function surface that receives radiation and reflects at least a portion of the incident radiation, and an optical-element-cooling device as summarized above. The optical-element-cooling device is especially suitable for cooling the optical element as the optical element is being supported in the optical column. The system further can comprises a vacuum chamber enclosing the optical column.  
           [0061]    Another embodiment of a method for cooling an EUV-reflective optical element, as the optical element is being supported in an optical column, comprises the step of attaching the optical element to a cooling frame by multiple optical-element support members extending from the cooling frame to respective locations on the non-optical-function surface. The cooling frame defines a fluid conduit. A heat-conductive member, connected to the cooling frame, is placed relative to the non-optical-function surface and separated from the non-optical-function surface by a gap so as to allow the heat-conductive member to absorb heat radiating across the gap from the optical element. A cooling fluid is circulated through the fluid conduit while conducting heat from the heat-conductive member to the cooling frame, thereby transferring heat from the heat-conductive member to the cooling frame and to the cooling fluid.  
           [0062]    The method further can comprise mounting the cooling frame to a base that is connected to the optical column, detecting a position of the cooling frame (and hence of the optical element) relative to the base, and, based on the detected position of the cooling frame, adjusting the position of the cooling frame relative to the base. The method further can comprise the steps of detecting a position of the base relative to the optical column, and, based on the detected position of the base, adjusting the position of the base relative to the optical column.  
           [0063]    The foregoing and additional features and 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  
       [0064]    [0064]FIG. 1 is a schematic diagram showing certain general features of an extreme ultraviolet lithography (EUVL) system that can be provided with at least one cooled optical element.  
         [0065]    [0065]FIG. 2 is a schematic diagram showing certain general features of another EUVL system that can include at least one cooled optical element.  
         [0066]    [0066]FIG. 3 is a side elevational view schematically depicting a mirror (as an exemplary optical element) and optical-element-cooling device according to the first representative embodiment.  
         [0067]    [0067]FIG. 4 is a plan view of the embodiment of FIG. 3, detailing the arrangement of heat pipes that conduct heat away from the mirror.  
         [0068]    FIGS.  5 (A)- 5 (B) are oblique elevational views of two respective configurations of protrusions formed on the first heat-receiving plate of the embodiment of FIG. 3, wherein the protrusions facilitate radiative thermal transfer from the mirror to the first heat-receiving plate.  
         [0069]    [0069]FIG. 5(C) is a plan view showing a thermal-transfer-enhancing coating applied to a selected region on the surface of the first heat-receiving plate of the embodiment of FIG. 3.  
         [0070]    [0070]FIG. 6(A) is a side elevational view schematically showing a mirror having a convex rear surface and conforming first heat-receiving plate situated adjacent the rear surface without contacting the rear surface, as described in the first representative embodiment.  
         [0071]    [0071]FIG. 6(B) is a side elevational view showing detail of interdigitated protrusions on the rear surface of a mirror and on the conforming surface of the first heat-receiving plate, as described in the first representative embodiment, for enhancing radiative thermal transfer from the mirror to the first heat-receiving plate.  
         [0072]    [0072]FIG. 7 is a schematic diagram of a projection-optical system of an exemplary EUVL system that includes one or more multiple optical-element-cooling devices, according to the second representative embodiment.  
         [0073]    [0073]FIG. 8 is a side elevational view schematically depicting a mirror (as an exemplary optical element) and mirror-cooling device, according to the third representative embodiment.  
         [0074]    [0074]FIG. 9 is a plan view of a portion of the mirror shown in FIG. 8, showing details of the configuration of the mirror-cooling device.  
         [0075]    [0075]FIG. 10 is a side elevational view schematically depicting a mirror (as an exemplary optical element) and mirror-cooling device, according to the fourth representative embodiment.  
         [0076]    [0076]FIG. 11 is a side elevational view schematically depicting a mirror (as an exemplary optical element) and mirror-cooling device, according to the fifth representative embodiment.  
         [0077]    [0077]FIG. 12 is a side elevational view schematically depicting certain features of an EUVL system including multiple optical elements and at least one optical-element-cooling device, according to the sixth representative embodiment.  
         [0078]    [0078]FIG. 13 is a side elevational view showing certain details of the arrangement of heat pipes and spring members in the embodiment of FIG. 12.  
         [0079]    [0079]FIG. 14 is a side elevational view of a mirror (as an exemplary optical element) and mirror-cooling device, according to the seventh representative embodiment.  
         [0080]    [0080]FIG. 15 is a side elevational view of the mirror of FIG. 14 and a modification of the mirror-cooling device shown in FIG. 14.  
         [0081]    [0081]FIG. 16 is a side elevational view of a mirror (as an exemplary optical element) and mirror-cooling device, according to the eighth representative embodiment.  
         [0082]    [0082]FIG. 17 is a side elevational view of a mirror (as an exemplary optical element) and mirror-cooling device, according to the ninth representative embodiment.  
         [0083]    [0083]FIG. 18 is an elevational schematic diagram of an EUVL system including at least one optical-element-cooling device, according to the tenth representative embodiment.  
         [0084]    FIGS.  19 (A)- 19 (B) depict an optical-element-cooling device according to the eleventh representative embodiment, wherein FIG. 19(A) is an elevational section, and FIG. 19(B) is a plan view.  
         [0085]    [0085]FIG. 20 is an elevational section showing certain details of the embodiment shown in FIG. 19(A).  
         [0086]    [0086]FIG. 21 is an oblique view of the bowl face of the heat-conductive plate of the cooling device shown in FIGS.  19 (A)- 19 (B).  
         [0087]    [0087]FIG. 22(A) is a plan view of a support member used in the embodiment of FIGS.  19 (A)- 19 (B).  
         [0088]    [0088]FIG. 22(B) is a plan view of the cooling-frame-support member used in the embodiment of FIGS.  19 (A)- 19 (B).  
         [0089]    [0089]FIG. 23 is a block diagram of a conventional EUVL system.  
         [0090]    [0090]FIG. 24 is an elevational section of a portion of an optical column as used in a conventional EUVL system. 
     
    
     DETAILED DESCRIPTION  
       [0091]    The invention is described below in the context of multiple representative embodiments, which are not intended to be limiting in any way. Although the embodiments are described in the context of extreme ultraviolet (EUV) microlithography systems, it will be understood that the general principles described herein are applicable with equal facility to other types of optical systems and to optical systems intended for use with wavelengths other than EUV wavelengths.  
         [0092]    General Considerations  
         [0093]    [0093]FIG. 1 depicts certain general features of the optical system  201  of an EUV microlithography system, particularly such a system that operates in a step-and-scan manner. The depicted optical system  201  includes a laser-light source  203 , which is situated at the extreme upstream end of the system. The laser-light source  203  emits laser light at a wavelength ranging from infrared to visible. For example, the laser-light source  203  can be a YAG laser or excimer laser. Laser light emitted from the laser-light source  203  is converged by a focusing-optical system  205  on a laser-plasma EUV source  207  situated downstream.  
         [0094]    The laser-plasma EUV source  207  generates EUV light (λ=13 nm) with good efficiency in the following manner. A nozzle (not shown) delivers xenon gas to the laser-plasma EUV source  207 . The xenon gas released from the nozzle receives pulses of extremely intense laser light from the laser-light source  203 , which causes the xenon gas to experience heating to an extremely high temperature sufficient to form a plasma. As the molecules of xenon transition back to a lower energy state, EUV radiation is produced. Since the EUV radiation has low transmission through air, the components  203 ,  205 ,  207  are contained inside a vacuum chamber  209  maintained at high vacuum.  
         [0095]    Situated just upstream of the laser-plasma light source  207  is a rotating parabolic reflective mirror  211  that has a surficial Mo/Si multilayer coating. EUV radiation radiating from the laser-plasma EUV source  207  is incident to the parabolic mirror  211 , and only EUV light having a wavelength in the vicinity of 13 nm is reflected therefrom as a collimated (parallel) beam. The collimated beam propagates downstream to a visible-light cutoff filter  213  that comprises a 0.15-nm thick layer of beryllium. Hence, of the EUV light reflected by the mirror  211 , only the desired EUV light passes through and propagates downstream of the cutoff filter  213 . The vicinity of the cutoff filter  213  is enclosed by a vacuum chamber  215 .  
         [0096]    An exposure chamber  233  is situated downstream of the cutoff filter  213 , and contains an illumination-optical system  217 . The illumination-optical system  217  typically comprises a condenser mirror, a fly-eye mirror, and other mirror(s) as required to form the EUV beam from the cutoff filter  213  into a beam having an arc-shaped transverse section. The EUV beam propagates to the left (in the figure) from the illumination-optical system  217  to an EUV-reflective mirror  219 . The mirror  219  is disc-shaped and has a concave reflective surface  219   a . EUV light reflected from the mirror  219  is reflected from a mirror  221 , which is oriented at an angle relative to the mirror  219 . EUV light from the mirror  221  propagates to a reflective reticle  223  (also called a “mask”) oriented horizontally “above” the mirror  221  such that the reflective surface of the reticle  223  is oriented “downward” in the figure. Thus, the EUV radiation is incident on a selected region of the reflective surface of the reticle  223 .  
         [0097]    Each of the mirrors  219 ,  221  is made from a respective high-precision quartz substrate and has a reflective surface including a Mo/Si multilayer coating. Thus, each of the mirrors  219 ,  221  exhibits a high (about 70%) reflectivity for EUV light having a wavelength of 13 nm. Note that, for other EUV wavelengths in the range of 10 to 15 nm, the respective multilayer coating can be made of layers of Ru (ruthenium) or Rh (rhodium) alternating with layers of Si, Be (beryllium), or B 4 C (carbon tetraboride), to ensure maximal reflectivity to the particular EUV wavelength.  
         [0098]    The reflective surface of the reticle  223  also comprises a multilayer coating tailored for the particular wavelength of incident EUV light. On this multilayer coating are formed elements of the pattern defined on the reticle and to be transferred lithographically. The reticle  223  is mounted on a reticle stage  225 , which is movable in at least one direction (X or Y direction). EUV light reflected by the mirror  221  is sequentially irradiated onto successive pattern-defining regions on the reticle  223 .  
         [0099]    Situated downstream of the reticle  223  are a projection-optical system  227  and a lithographic substrate  229 . The projection-optical system  227  comprises multiple multilayer-coated mirrors, and is configured such that, as the pattern on the reticle  223  is transferred to the substrate  229 , the image of the pattern is “reduced” (demagnified) according to a pre-set demagnification ratio (e.g., 1/4) and resolved onto the surface of the substrate  229 . The substrate  229  (e.g., semiconductor wafer) is secured onto the surface of a substrate stage  231  that typically is movable in the X, Y, and Z directions.  
         [0100]    Another configuration of an EUV microlithography system  250  is depicted in FIG. 2, in which the depicted system  250  comprises an illumination system IL including an EUV light source (not detailed). The EUV light source produces a beam of EUV light (in general, λ=5 to 20 nm, desirably λ=11 or 13 nm) that is shaped and directed as required by the illumination system IL. The EUV beam from the illumination system IL is directed by a mirror  251  to be incident on a reflective reticle  252 . The reticle  252  is mounted on a reticle stage  253 , which has a large movement range of 100 mm or more in the scanning direction (Y-axis direction), a smaller movement range in the X-axis direction, and an even smaller movement range in the optical-axis direction (Z-axis direction). Note that movements in the X-axis and Y-axis directions are within a plane denoted as the X-Y plane. The position of the reticle  252  in the X-Y plane is monitored with high precision using a laser interferometer (not shown, but well understood in the art). The position of the reticle  252  in the Z-axis direction is monitored by a reticle-focus sensor including a light-transmission system  254  and a light-receiving system  255 .  
         [0101]    EUV light reflected by the reticle  252  includes an aerial image of the pattern portion (on the reflective surface of the reticle  252 ) illuminated by the beam from the illumination system IL. Thus, as described above, the reticle  252  includes a multilayer coating (e.g., Mo/Si or Mo/Be alternating layers) that is reflective to incident EUV light. Pattern features are defined by respective portions and voids in an absorption layer (e.g., Ni or Al) formed on the surface of the multilayer coating. EUV light from the reticle enters an optical column  264  of the projection-optical system. In the optical column  264 , the light is reflected by a first mirror  256  and sequentially reflected by a second mirror  257 , a third mirror  258 , and a fourth mirror  259  so as to be perpendicularly incident on the surface of a lithographic substrate  260  (e.g., semiconductor wafer). Hence, the mirrors  256 - 259  constitute the projection-optical system, which has a demagnification ratio of, for example, 1/4 or 1/5. Although this configuration includes four mirrors  256 - 259  in the projection-optical system, six or eight mirrors, for example, alternatively could be used (more mirrors would increase the numerical aperture). The optical column  264  also includes an off-axis microscope (not detailed but well understood in the art) used for aligning the reticle  252  and substrate  260 .  
         [0102]    For exposure the substrate  260  is mounted on a substrate stage  261 . The substrate stage  261  is movable freely within a plane (an X-Y plane) that extends perpendicularly to the optical axis of the projection-optical system. The range of motion of the substrate stage typically is 300-400 mm in each of the X-axis and Y-axis directions. The substrate stage  261  also can be raised or lowered in the Z-axis direction (optical-axis direction). The position of the substrate in the Z-axis direction is monitored by an autofocus detector comprising a light-transmission system  262  and a light-receiving system  263 . The position of the substrate in the X-Y plane (defined by the X-axis and Y-axis directions) is monitored with high precision using a laser interferometer (not shown, but well understood in the art). During lithographic exposure the reticle stage  253  and substrate stage  261  are moved in a simultaneously scanning manner at respective velocities according to the demagnification ratio.  
         [0103]    First Representative Embodiment  
         [0104]    An optical-element-cooling device according to this embodiment is depicted in FIGS. 3, 4,  5 (A)- 5 (C), and  6 (A)- 6 (B). The optical element shown and referred to in this embodiment as a reflective mirror, by way of example, but this is not intended to be limiting in any way.  
         [0105]    Turning first to FIG. 3, the depicted mirror includes an aspherical convex reflective surface  1   a  that includes a Mo/Si multilayer coating for reflecting a prescribed wavelength of incident EUV light (λ=13.4 nm). The reverse (“rear”) surface  1   b  of the mirror  1  is planar, and represents a surface at which no EUV light is incident and from which substantially no EUV reflects.  
         [0106]    A mirror-cooling device  10  is situated adjacent the rear surface  1   b  of the mirror  1 . The mirror-cooling device  10  of this embodiment comprises a first heat-receiving plate  3 , a plate-cooling device  5  (comprising a heat pipe  4  and liquid-cooled body  6 , the latter being an example of a fluid-cooled body), and a heat-proofing device  8  (heating device  7  and second heat-receiving plate  9 ). The first heat-receiving plate  3  is adjacent the rear surface  1   b  and side-circumferential surface (ie., surfaces other than the reflective surface  1   a ) of the mirror  1  without contacting the respective surfaces. The first heat-receiving plate  3  is situated and configured so as not to interfere mechanically with either the mirror  1  or the mirror-holding device (not shown). Whenever the mirror  1  has experienced heating from absorption of a portion of the energy of incident EUV light, the heat radiates to the first heat-receiving plate  3 .  
         [0107]    The first heat-receiving plate  3  is formed from a thin plate made of a suitable metal or ceramic material (an example of the latter is Al 2 O 3 ) that exhibits an appropriately high heat-radiation rate and appropriately high thermal conductivity. The thin-plate configuration of the first heat-receiving plate  3  facilitates forming the plate so as to conform to at least a portion of the rear surface  1   b  of the mirror  1 . This allows the mirror  1  to be made thicker and thus provided with a desirable high rigidity for resistance to mirror-deformation caused by gravity.  
         [0108]    Heat transfer (notably heat radiation) from the mirror  1  to the first heat-receiving plate  3  is enhanced by certain processing performed on the mirror-facing surface  3   s  (or a portion of the surface) of the first heat-receiving plate  3 . Exemplary processing in this regard includes forming on the surface  3   s  a coating of a ceramic or metal-oxide (e.g., copper oxide, aluminum oxide, nickel oxide, etc.), of a carbide (e.g., silicon carbide, molybdenum carbide, etc.), or of a nitride (e.g., silicon nitride, tantalum nitride, etc.). These coatings can be produced by sputtering or other suitable technique. Alternatively, a suitable coating can be formed by oxidation or other chemical treatment of the base material of the first heat-receiving plate  3 . Another alternative processing involves forming a chemical membrane on the surface  3   s.    
         [0109]    Yet another alternative processing involves increasing the “roughness” of the surface  3   s  so as to increase the heat-absorption area of the surface  3   s . For example, the surface  3   s  can be provided with multiple fin-shaped protrusions  3   a  such as shown in FIG. 5(A) or multiple pin-shaped protrusions  3   b  such as shown in FIG. 5(B). These protrusions can be configured to interdigitate with other protrusions provided on the rear surface  1   b  of the mirror  1 , as discussed later below.  
         [0110]    The flux of EUV light incident to the mirror  1  is not necessarily uniform. In many instances, the intensity of EUV light incident to the mirror  1  at a first region on the reflective surface  1   a  is greater than the intensity at another region on the surface  1   a . The more intensely irradiated region will exhibit more heating (and thus a greater temperature increase) than the less intensely irradiated region. In response to this phenomenon, certain corresponding regions of the surface  3   s  of the first heat-receiving plate  3  can be processed (compared to other regions being not processed) or more extensively processed to increase heat absorption from correspondingly more intensely irradiated regions of the mirror. In other words, the result of processing of the first heat-receiving plate  3  can have a spatial distribution tailored to the degree of local heating of the mirror  1  at various locations on the reflective surface  1   a . For example, as shown in FIG. 5(C), increased surface roughness can be provided to the region  3   c  of the surface  3   s , wherein the region  3   c  corresponds to a region of the mirror that receives more intense radiation than other regions. Thus, cooling of the mirror  1  is tailored to the distribution of local heating of the mirror during use.  
         [0111]    As an alternative or in addition to performing processing on the surface of the first heat-receiving plate, the rear surface  1   b  of the mirror  1  can be processed to increase its efficiency of thermal radiation to the first heat-receiving plate  3 , as discussed later below.  
         [0112]    An exemplary configuration of the plate-cooling device  5  attached to the first heat-receiving plate  3  is shown in FIG. 4, which depicts heat pipes  4  affixed to the mirror-facing surface  3   c  of the first heat-receiving plate  3 . The respective distal termini  4   b  of the heat pipes  4  are connected to a liquid-cooled (e.g., water-cooled) body  6  or analogous heat sink. In the example shown in FIG. 4, the plate-cooling device  5  comprises eight individual heat pipes  4  attached to the surface  3   s  of the first heat-receiving plate  3 . The respective proximal ends  4   a  of the heat pipes  4  are secured at the center of the surface  3   c , and the distal termini  4   b  extend equi-angularly in a radial manner. The distal ends  4   b  are attached to the liquid-cooled body  6 . Each heat pipe  4  can be configured as having a cross-sectional diameter (or width) of several mm or less so as to conserve space. Heat pipes  4  are usable in a vacuum environment and facilitate efficient thermal transfer from the heat-receiving plate  3 . I.e., heat from the mirror  1  is absorbed by the first heat-receiving plate  3 , and the resulting heat in the corresponding region of the first heat-receiving plate  3  is absorbed by the respective proximal end(s)  4   a  of the heat pipes  4 . Heat thus absorbed into the heat pipes  4  is conducted rapidly to the respective distal ends  4   b , from which heat is removed by the liquid-cooled body  6 .  
         [0113]    In FIG. 3 the first heat-receiving plate  3  is shaped conformably to the rear surface  1   b  and the side-circumferential surface of the mirror  1 . Alternatively, the first heat-receiving plate  3  can have a shape that simply conforms to the rear surface  1   b , such as a simple planar circular configuration (e.g., disc-shaped) or other suitable configuration (e.g., semicircular). In these alternative configurations, the heat pipes  4  can be concentrated at locations subjected to greater heating and generally connected at respective locations to facilitate attainment of the desired temperature regulation at the respective locations. Further alternatively, the first heat-receiving plate  3  may be segmented into multiple plate segments situated adjacent respective regions of the surface  1   b  requiring cooling, with one or more respective heat pipes  4  connected to each plate segment.  
         [0114]    The heat-proofing device  8 , as noted above, comprises a heating device  7  and second heat-receiving plate  9 . The heat-proofing device  8  is situated outboard of the first heat-receiving plate  3  and plate-cooling device  5 , and serves to reduce a possible thermal influence of the first heat-receiving plate  3  and plate-cooling device  5  on the wafer  2  (or reticle or other heat-sensitive component). The heating device  7  can be configured structurally and/or operationally similarly to the heat pipe  4  discussed above. The heating device  7  blocks (offsets) heat radiated outward from the first heat-receiving plate  3  and blocks heat-sink effects of the plate-cooling device  5  on other nearby optical elements (not shown). The second heat-receiving plate  9  desirably has a plate-like configuration and is situated adjacent the side of the heating device  7  facing away from the cooling device  5 . The second heat-receiving plate  9  makes uniform the heat produced by the heating device  7  over the heat-proofing device  8 .  
         [0115]    Any one or more of the following increases the efficiency of thermal transfer from the mirror  1  to the mirror-cooling device  10 : (1) increasing surface roughness or irregularity, either globally or locally, of the rear surface  1   b  of the mirror or providing a ceramic or other high-thermal-transfer coating to the surface  1   b  either locally or globally; (2) as shown in FIG. 6(A), forming the rear surface  11   b  of the mirror  11  as a curved surface, such as a convex spherical surface, rather than planar, thereby increasing the surface area of the surface  11   b  (accompanied by arranging the first heat-receiving plate  13  and plate-cooling device  15  in proximate conformity to the surface  11   b ); and (3) as shown in FIG. 6(B), forming respective protrusions  1 X,  3 X on the rear surface  1   b  of the mirror  1  and on the mirror-facing surface of the first heat-receiving plate  3 , respectively, wherein the protrusions  1 X,  3 X are arranged so as to interdigitate with each other. Note that, despite the conformance of the first heat-receiving plate  3  with the rear surface  1   b , the respective protrusions  1 X,  3 X do not contact each other. The protrusions  1 X,  3 X substantially increase the opposing surface areas of the surface  1   b  and first heat-receiving plate  3 . Combining (2) and (3) is especially effective in achieving good thermal transfer.  
         [0116]    Since the first heat-receiving plate  3 ,  13  is cooled by the respective plate-cooling device  5 ,  15 , the first heat-receiving plate  3 ,  13  is maintained at a desired temperature. Thus, thermal deformation of the mirror  1 ,  11  is effectively prevented.  
         [0117]    Second Representative Embodiment  
         [0118]    This embodiment is directed to a projection-optical system, of an EUVL system, that includes an optical-element-cooling device as described, for example, in the first representative embodiment. The subject projection-optical system is shown in FIG. 7 in association with a reflective-type reticle  14  (located upstream of the projection-optical system) and a substrate  2  (e.g., semiconductor wafer) located downstream of the projection-optical system. The depicted projection-optical system comprises six EUV-reflective mirrors M 1 -M 6  each including a respective mirror-cooling device C 1 -C 6  arranged in proximity thereto. The projection-optical system also includes two “heaters” H 2 , H 3  associated with the mirror-cooling devices C 2 , C 3 , respectively.  
         [0119]    Each of the mirror-cooling devices C 1 -C 6  includes a respective heat-receiving plate with attached respective plate-cooling device (not detailed, but see first representative embodiment, e.g., heat-receiving plates  3 , heat pipes  4 , and liquid-cooled jackets  6 ). In this example, each of the first heat-receiving plates is a copper plate having a thickness of 2 mm. The surface of each copper plate facing the respective mirror has a copper thermal oxide film (exemplary of processing for increasing the efficiency with which the copper plate absorbs heat from the respective mirror). Connected to each copper plate are multiple respective heat pipes each having an exemplary diameter of 2 mm. The gap between the rear surface of each mirror M 1 -M 6  and the respective first heat-receiving plate is, for example, 0.5 mm. Desirably, each gap is 3 mm or less, but such a gap need not be uniform from every location on the rear surface (it is sufficient if the closest distance in the gap is 3 mm or less). Each of the first heat-receiving plates C 1 -C 6  has a shape that generally conforms to the shape of the respective mirror M 1 -M 6  so as essentially to “cover” the rear surfaces of the respective mirrors.  
         [0120]    Each of the heaters H 2 , H 3  is configured substantially as a respective heat-proofing device  8  (comprising a respective heating device  7  and second heat-receiving plate  9 ), as discussed in the first representative embodiment.  
         [0121]    By way of example, the mirror-cooling devices C 1 -C 6  of the projection-optical system shown in FIG. 7 are set to maintain the following average temperatures:  
         [0122]    Cooling device C 1 : 18.6° C.  
         [0123]    Cooling device C 2 : 0.4° C.  
         [0124]    Cooling device C 3 : 6.7° C.  
         [0125]    Cooling device C 4 : 0.9° C.  
         [0126]    Cooling device C 5 : −5.0° C.  
         [0127]    Cooling device C 6 : 14.6° C.  
         [0128]    Based on experiments, maintaining the cooling devices C 1 -C 6  at the respective temperatures noted above allows the average temperature of each of the respective mirrors M 1 -M 6  to be maintained at 20° C.  
         [0129]    In FIG. 7, because the area of the rear surface of the second mirror M 5  from the reticle  14  is relatively small, the temperature of this mirror M 5  is set notably low. The size of the mirror M 5  is approximately ten times the actual effective area of the mirror so as to allow the area of the first heat-receiving plate of the respective cooling device C 5  to be correspondingly larger. The cooling device C 5  is set to approximately −5° C., which is a temperature that is easy to achieve, to cool the mirror M 5  appropriately.  
         [0130]    Thus, cooling of the mirrors M 1 -M 6  is performed in a manner that prevents undesirable temperature increases of the mirrors and hence reduces thermal deformation and positional changes of the mirrors during use. The projection-optical system is able to operate without significant deterioration in optical performance (e.g., wavefront aberration) of the constituent mirrors.  
         [0131]    Third Representative Embodiment  
         [0132]    An optical-element-cooling device  30  according to this embodiment is shown in FIGS. 8 and 9. FIG. 8 depicts a single optical element  31  exemplified by a mirror. The mirror  31  has a reflective surface  31   a  that is aspherically concave, and includes a multilayer coating for reflecting incident EUV light. The mirror  31  can be used, for example, in a projection-optical system of an EUVL system. The mirror  31  also has a rear surface  31   b  in which multiple voids  31   c  are defined. Each void  31   c  has a hexagonal shape in this embodiment (see FIG. 9); the resulting “honeycomb” structure provides substantial rigidity to the mirror  31 . The mirror  31  also has a side surface  31   d  that is secured by means of “cells”  32  (respective mirror-holding devices) via spring members  33 . Each of the cells  32  is secured to an optical column (not shown). In FIG. 8 the voids  31   c  are defined only on the rear surface  31   b  of the mirror  31 , but it will be understood that voids  1   c  also or alternatively can be defined on the side surface  31   d  wherever a cell  32  is not attached.  
         [0133]    The optical-element-cooling device  30  also includes multiple electronic cooling elements  34  (desirably Peltier elements, which is the term used generally herein) positioned relative to correspond to respective voids  31   c . As shown in FIG. 9, each Peltier element  34  is independently electrically connected to a controller  50 . Note that the Peltier elements  34  can be arranged not only adjacent the mirror rear surface  31   b  but also or alternatively adjacent the side surface  31   d  and front surface  31   a . If the Peltier elements  34  are arranged adjacent the front surface  31   a , they desirably are positioned so as not to block incident or reflected light flux. Respective heat pipes  36  are connected to the respective “hot” sides of one or more Peltier elements  34 . Various individual heat pipes  36  are brought together into a trunk heat pipe through which heat is conducted to a cooling device  37 . For example, multiple individual heat pipes  36  may be brought together by joining them to a circular trunk heat pipe (not shown) via respective spring members.  
         [0134]    In the voids  31   c , the “cold” sides of respective Peltier elements  34  are connected dynamically to the mirror  31  via one or more respective spring members  35  each configured with one or more flat and/or coil springs, for example. Each spring member  35  is made of a material exhibiting a low coefficient of thermal expansion, high thermal conductivity, and low outgassing in a vacuum environment. An exemplary material in this regard is “super invar.” Each spring member  35  also desirably has low rigidity so as to prevent force from being transmitted through them from the Peltier elements  34  to the mirror  31 , which prevents the mirror from being deformed during assembly of the mirror into the optical column or during actual use.  
         [0135]    As noted above, the “hot” sides of the Peltier elements  34  are secured to the heat pipes  36 . The heat pipes  36  also desirably are connected to the cells  32 , by respective units of a thermally insulating material  39  and by respective spring members  40  such as flat springs. The insulating material  39  desirably has low thermal conductivity. An exemplary thermal conductivity is 100 J·m −2 ·K −1   or less, which provides good insulating performance. In addition, the insulating material  39  desirably does not adversely affect obtaining or maintaining a desired vacuum level in which the optical system is used.  
         [0136]    According to the criteria above, exemplary insulating materials are as follows: A generally desirable material is a metal alloy of Fe and Ni. More desirable is a ternary alloy such as Ni—Cr—Fe or Fe—Ni—Co. Specific examples are a ternary alloy having a composition ratio of 72% Ni, 15% Cr, and 6% Fe (Inconel 600) or a ternary alloy having a composition ratio of 52% Fe, 29% Ni, and 17% Co (Kovar). Other candidate insulating materials are ceramic materials, metal oxides, metal carbides, metal nitrides, or silicon oxides. Exemplary materials in this regard include Al 2 O 3 , TiC, SiC, ZrC, HfC, TaC, BN, TiN, AlN and SiO 2  (quartz). Exemplary silicon-oxide ceramics include MgO.SiO 2  (steatite), 3Al 2 O 3 .2SiO 2  (mullite), and ZrO 2 .2SiO 2  (zircon).  
         [0137]    The insulating material desirably has a coefficient of thermal expansion that is as close as possible to the thermal expansion coefficient of the material of which the optical column is made. For example, if the optical column is made of invar, the insulating material desirably is low-expansion quartz or low-expansion glass.  
         [0138]    The insulating material  39  prevents heat from the heat pipes  36  from being transmitted to the cells  32  (thereby preventing thermal deformation of the cells  32 ). Also, the spring members  40  desirably are configured to prevent deformation and/or vibration of the heat pipes  36  from being transmitted to the cells  32 . If the spring members  40  are made of a material having sufficiently low thermal conductivity, the respective units of insulating material  39  can be omitted.  
         [0139]    As an alternative to the heat pipes  36  and the Peltier elements  34  being secured to the cells  32 , the heat pipes  36  and Peltier elements  34  can be secured to the optical column (not shown) of which the mirror  31  is a part. The heat pipes  36  are secured to the cooling device  37  via a spring member  38 . The cooling device  37  can be configured, for example, to circulate water to cool the heat pipes  36  connected to it. The cooling device  37  also is connected to a controller  50  (not shown in FIG. 8, but see FIG. 9). The controller  50  controllably delivers electrical commands to the respective Peltier elements  34  and electrical command signals to the cooling device  37 , according to the temperature distribution of the mirror  31  obtained in advance or measured in real time. Thus, the thermal distribution of the mirror  31  is controlled actively so as to be uniform or have some other desired distribution. Heat produced in the mirror  31  is exhausted outside the optical column via the Peltier elements  34 , heat pipes  36 , and cooling device  37 .  
         [0140]    Desirably, all materials used inside the optical column are materials that exhibit an acceptably low level of outgassing in a vacuum environment (if the optical elements in the column normally must be operated in a vacuum environment). Similarly, the materials desirably do not produce contaminants that would compromise performance of the optical elements and other components inside the optical column. To such end, the materials can be coated with a material such as TiN or NiP.  
         [0141]    Fourth Representative Embodiment  
         [0142]    This embodiment is shown in FIG. 10, in which components that are similar to respective components discussed in the third representative embodiment have the same respective reference numerals. This embodiment differs from the third representative embodiment in that, in the instant embodiment, the spring members  35  are eliminated. Otherwise, this embodiment is similar to the third representative embodiment.  
         [0143]    Heat from the mirror  31  is transmitted to the Peltier elements  34  by radiation across gaps  31   g  between the voids  31   c  and the cold sides  34   c  of respective electronic cooling elements (Peltier elements)  34 . Corresponding heat from the hot sides  34   h  of the Peltier elements  34  is conducted to outside the optical column via heat pipes  36 . In this embodiment, omission of the spring members  35  results in the Peltier elements  34  not physically contacting the mirror  31 . Thus, any forces that otherwise would be applied to the mirror  31  from the Peltier elements  34  are isolated from the mirror, which avoids any possible deformation of the mirror by those forces.  
         [0144]    In this embodiment as in the third representative embodiment, the Peltier elements  34  have been shown as being associated only with the respective voids  31   c  in the rear surface  31   b  of the mirror. However, it will be understood that the Peltier elements can be associated with the side surface  1   d  and/or front surface  1   a  as practical or indicated. It also will be understood that, even though multiple Peltier elements  34  are shown (which is advantageous for independent temperature control of respective regions of the mirror), it is possible to use only one Peltier element  34 .  
         [0145]    Fifth Representative Embodiment  
         [0146]    This embodiment is shown in FIG. 11, which differs from the embodiment of FIG. 10 in that, in the instant embodiment, gas is supplied to the gap between the mirror and Peltier elements  34 . Otherwise, this embodiment is similar to the embodiment of FIG. 10.  
         [0147]    Specifically, referring to FIG. 11, a gas is released (arrow  40   a ) from a gas nozzle  43  into the voids  31   c  in the rear surface  31   b  of the mirror  31 . The gas is routed to the nozzle  43  from a gas supply  41  via a gas conduit  42 . The gas passing through the voids  31   c  is evacuated (arrow  40   b ) by a suction nozzle  44  or the like connected to a pump  46  via a gas conduit  45 . The gas can be, for example, helium, and serves to conduct heat from the mirror  31  to the Peltier elements  34 . Note that the evacuated gas can be reused after being brought to a desired temperature.  
         [0148]    This embodiment functions best if there is a substantially closed space between the mirror  31  and the Peltier elements  34  and heat pipes  36 . If it is difficult to form such a closed space, a wall or guard ring can be placed around the circumference of the mirror  31  to prevent leakage of gas generally to the interior of the vacuum chamber enclosing the optical column.  
         [0149]    In FIG. 11 gas is delivered and evacuated from respective sides of the mirror  31 . But, this configuration is not intended to be limiting. Gas alternatively may be delivered and evacuated at any suitable location relative to the mirror  31 , Peltier elements  34 , and heat pipes  36 . Also, the number of gas nozzles  43  and suction nozzles  44  is not limited to one each. Multiple nozzles of either or both types can be used.  
         [0150]    Sixth Representative Embodiment  
         [0151]    This embodiment is depicted in FIGS. 12 and 13, of which FIG. 12 is a side elevational view schematically depicting an optical system  48  as used in, e.g., a lithography system. The optical system  48  includes optical-element-cooling devices as described in the third and fourth representative embodiments, and includes a base  51  secured to a floor  49  or the like. The base  51  passes through walls of a vacuum chamber  53  so as to support an optical column  52  contained within the vacuum chamber  53 . The column  52  is supported relative to the base  51  by spring members  61 . The vacuum chamber  53  is secured to the floor  49  by a member (not shown), and the base  51  is connected to the vacuum chamber  53  via flanges  62  and bellows  63 . Thus, the vacuum chamber  53  is physically isolated from the base  51 . Consequently, any deformation of the vacuum chamber  53  experienced while evacuating the interior of the vacuum chamber to high vacuum is not transmitted to the base  51 . Attached to the optical column  52  are multiple optical elements (e.g., mirrors)  31 .  
         [0152]    Multiple respective primary heat pipes  36  transfer heat from each mirror  31 . The primary heat pipes  36  are secured to the optical column  52  in a manner that prevents transmission of vibration, deformation, and heat from the primary heat pipes  36  to the column  52 . To such end, the primary heat pipes  36  are connected to a secondary heat pipe  54  via respective spring members  55 . The secondary heat pipe  54  is secured to the base  51  via a support member  60  and an insulating member  59  (or by a support member that also serves as an insulating member, which eliminates the need for a separate insulating member  59 ). Although not shown, the connection between the secondary heat pipe  54  and the base  51  also desirably involves a spring member that blocks transmission of vibration and positional changes. (The support member  60 , for example, can be configured with spring-like properties, thereby eliminating the need for a separate spring member.)  
         [0153]    Also, the secondary heat pipe  54  is connected to the vacuum chamber  53  via a flange  56 , insulating members  57 , and bellows  58 . Thus, the secondary heat pipe  54  is isolated from the vacuum chamber  53  thermally and with respect to vibration and deformation. By way of example, the vacuum chamber  53 , flange  56 , and bellows  58  may be made of a material such as stainless steel or a titanium alloy. Heat transmitted via the secondary heat pipe  54  is transmitted to a cooling device  37  via a spring member  38 .  
         [0154]    Although not shown in FIG. 12, in order to maintain optimal thermal performance, it is desirable that the heat pipes  36 ,  54  be covered with an insulating material in regions other than locations that are in thermal contact with other members (e.g., locations connected to spring members). Also, compared with the primary heat pipes  36 , the secondary heat pipe  54  has a relatively small thermal and physical influence on the mirror  31 . Hence, phenomena such as vibration from and between the mirror  31  and secondary heat pipe  54  are alleviated. The secondary heat pipe  54  can be liquid-cooled.  
         [0155]    [0155]FIG. 13 schematically depicts an exemplary deformation of the system shown in FIG. 12. In FIG. 13, two heat pipes  71 ,  72  are used instead of the secondary heat pipe  54  used in FIG. 12. The heat pipe  71  is situated inside the vacuum chamber  53  and is connected to a flange  75  of the vacuum chamber  53  by a spring member  74 . The heat pipe  72  is situated outside the vacuum chamber  53  and is connected to the flange  75  by a spring member  73 . The flange  75  is connected to the wall of the vacuum chamber  53  via a unit of insulating material  58 . Heat is transmitted from the respective mirror  31  via the heat pipe  71 , the spring member  74 , the flange  75 , the spring member  73 , and the heat pipe  72 . Hence, the flange  75  desirably is made of a material having a high thermal conductivity; e.g., stainless steel. Copper, as an exemplary material exhibiting even higher thermal conductivity than stainless steel, can be embedded in the flange if desired or indicated, wherein the heat pipes are connected to the copper.  
         [0156]    Seventh Representative Embodiment  
         [0157]    This embodiment is depicted in FIGS. 14 and 15, which depict a mirror  21  (as an exemplary optical element). Referring first to FIG. 14, a light flux  16  (e.g, EUV light) from an upstream illumination-optical system (not shown) is incident on a mirror  21 . Some of this incident light is absorbed by the mirror  21 , which results in localized heating of the mirror  21  in the irradiated region A of the surface of the mirror  21 .  
         [0158]    In this embodiment, heat in the mirror  21  is removed by directing a stream of gas onto the surface of the mirror. This gas delivery desirably is performed whenever the mirror  21  is not being used to make an actual exposure, e.g., while performing substrate alignment, while replacing the substrate with a new substrate to be exposed, or during a time dedicated to mirror cooling. Turning to FIG. 15, the mirror  21  is cooled by positioning a gas-delivery nozzle  17  over the reflective surface  21   a  of the mirror  21 , including over the irradiation region A. The gas released from the nozzle  17  desirably is an inert gas such as He or N 2 . As the gas is incident on the surface  21   a , the gas conducts away heat from the mirror, thereby achieving cooling of the mirror  21  to a desired temperature. The temperature of the surface  21   a  can be ascertained by directing an infrared camera  20  or the like onto the surface  21   a . Gas release can be halted whenever the mirror  21  is at the desired temperature. Whenever the gas is not being delivered to the mirror  21 , the nozzle  17  can be withdrawn from the mirror as shown in FIG. 14, allowing the mirror  21  to be used in its intended manner (e.g., for making lithographic exposures). Control of gas release desirably is achieved using a controller (not shown) that processes data from other systems as well as data from the camera  20 .  
         [0159]    Desirably, the gas released from the nozzle  17  is evacuated after use. This evacuation desirably is performed in the vicinity of the surface  21  as the gas is being delivered. Evacuation can be performed by any of various intake ports  18  situated and configured for efficiently evacuating the gas. More than one intake port  18  can be used, and the intake port(s) desirably are situated in the vicinity of the surface  21   a  being exposed to the gas. The intake port  18  desirably is connected to a vacuum pump  19  or the like that aspirates the gas. Further desirably, the gas is recycled for repeated use.  
         [0160]    If the mirror  21  is part of the optical system of a lithography system, the mirror  21  can be cooled in the manner described above between exposures of successive substrates, at strategic moments during such exposure, or after several substrates have been exposed. The goal of mirror cooling is to bring the wavefront aberration of the optical system including the mirror  21  (resulting from a temperature increase of the mirror  21 ) back to within an acceptable operational tolerance. If a threshold mirror temperature is known that will result, when exceeded, in an unacceptable wavefront aberration, cooling can be performed while measuring the temperature of the surface  21   a  of the mirror  21  using the infrared camera  20 . The camera  20  also is useful for obtaining measurements of a temperature distribution on the surface  21   a , allowing gas delivery to be directed only to those regions of the surface  21   a  exhibiting an excessive temperature increase. I.e., mirror cooling can be performed according to the temperature distribution of the surface  21   a , and gas delivery can be applied selectively (or more intensively applied) to regions at which the temperature increase is excessive. Other regions exhibiting a lower temperature increases either receive no gas or a reduced amount of gas.  
         [0161]    By releasing gas at the reflective surface  21   a  of the mirror  21 , heat present on the surface is removed by thermal conduction (from the mirror to the gas) and by heat of vaporization. As a result, deformation of the mirror by thermal expansion is reduced substantially so as to allow the mirror to operate within established tolerances of optical performance. Another advantage of this embodiment is that temperature differences between the front and rear surfaces of the mirror are reduced, thereby establishing a more uniform temperature distribution through the thickness dimension of the mirror  21 . Also, since the force applied by the stream of gas to the mirror  21  is very weak, the mirror experiences no physical deformation from being cooled.  
         [0162]    Whereas FIGS. 14 and 15 depict delivering the gas at the front (reflective) surface  21   a  of the mirror  21 , this is not intended to be limiting. In an alternative embodiment, gas can be delivered only at the rear surface of the mirror or at both the front and rear surfaces.  
         [0163]    In addition, mirror cooling according to this embodiment is not limited to optical elements of a projection-optical system, but rather can be applied to an optical element in any optical system, such as an illumination-optical system. Furthermore, the embodiment is not limited to use with respect to mirrors; it can be applied to any of various optical components (including reticles).  
         [0164]    Eighth Representative Embodiment  
         [0165]    This embodiment is depicted in FIG. 16, in which components that are similar to corresponding components discussed in the seventh representative embodiment have the same respective reference numerals and are not described further. In this embodiment, the mirror  21  is cooled by directing a “spray”  23  of liquid coolant from a nozzle  28  to the surface  21   a  of the mirror  21 . Desirably, the coolant is released from the nozzle  28  as a fine mist. Desirable coolants in this regard include liquid nitrogen, liquid helium, or liquid ethanol. Liquid nitrogen and liquid helium vaporize at room temperature, which prevents the liquids from remaining on the surface  21   a  (where residual liquid could become a source of contamination). Ethanol is useful in a vacuum environment because it vaporizes instantly in a vacuum and hence does not remain on the surface  21   a . Ethanol also is advantageous because it prevents carbon contaminants such as hydrocarbons from adhering to the mirror  21 .  
         [0166]    Ninth Representative Embodiment  
         [0167]    This embodiment is depicted in FIG. 17, in which components that are similar to corresponding components shown in FIG. 16 have the same reference numerals and are not described further. In this embodiment, the mirror  21  is cooled by directing a liquid  24 , released from a nozzle  29 , to a selected region A on the surface  21   a  of the mirror  21 . The liquid can be, for example, liquid helium, liquid nitrogen, or liquid ethanol.  
         [0168]    Tenth Representative Embodiment  
         [0169]    This embodiment is depicted in FIG. 18, and is directed to implementation of the mirror-cooling devices of, for example, the seventh, eighth, or ninth representative embodiments in an optical system, for example a projection-optical system as used in an EUVL system. FIG. 18 is similar in many respects to FIG. 2, and components that are the same in both figures have the same respective reference numerals and are not described further.  
         [0170]    EUV light emitted from the illumination system IL is directed by a mirror  251  to a selected region on the surface of a reflective reticle  252 , which tends to cause local heating of the reticle  252 . To reduce heating of the reticle  252 , the reticle is provided with a cooling device that comprises a spray nozzle  22  situated and configured to direct a spray of gas or liquid coolant at the reflective surface of the reticle  252 . The spray nozzle  22  is situated at a location at which incident or reflected light is not blocked. By directing the spray  25  at a selected location on the surface of the reticle  252 , the reticle  252  is locally cooled, which offsets the irradiation-heating of the reticle  252  and cools the reticle to a desired operating temperature, even during use of the reticle for making exposures. Desirably, the reflective surface of the reticle is observed using an infrared camera or the like (not shown, but see any of FIGS. 15, 16, and  17 ). Delivery of coolant can be discontinued whenever the surface of the reticle  252  is at a desired temperature at which the reticle exhibits an acceptable level of wavefront aberration. To such end, the infrared camera and spray nozzle  22  can be controlled operationally in a feedback manner by a controller (not shown) as described above.  
         [0171]    It also is desirable that the released coolant be actively evacuated after use, as discussed above in connection with FIG. 15.  
         [0172]    Cooling of the reticle  252  can be performed at any opportune time, including during use of the reticle for exposure. Exemplary cooling periods can be during periods in which new substrates are positioned for exposure. Cooling of the reticle  252  desirably is continued until the reticle exhibits a profile error, resulting from thermal deformation, that is within an acceptable tolerance. For this purpose, use of an infrared camera can be especially advantageous, as mentioned above, for providing real-time feedback of the cooling process. Use of the camera is advantageous in applying coolant (or more coolant) only to regions of the reticle  252  that actually require cooling, i.e., that are exhibiting a temperature rise in excess of specifications.  
         [0173]    Eleventh Representative Embodiment  
         [0174]    This embodiment is depicted in FIGS.  19 (A)- 19 (B),  20 ,  21 , and  22 (A)- 22 (B). Turning first to FIGS.  19 (A)- 19 (B), a mirror  301  (as an exemplary optical element) is situated inside an optical column (not shown) of an EUV optical system (as a representative optical system). The mirror  301  in this embodiment has a plan profile as shown in FIG. 19(B) and has a thickness of approximately 1-2 cm. (Alternatively, the mirror  301  can have a round or fan-shaped profile, for example.) The mirror  301  has a reflective surface  301 A (also termed the “optical-function face”), a rear surface  301 B, and a side surface  301 C (these other surfaces are also termed “non-optical-function faces”). The mirror  301  defines support cutouts  303  in the side surface  301 C. In this embodiment three support cutouts  303  are provided that are roughly equi-angularly displaced relative to each other around the circumference of the mirror  301 . The mirror  301  is supported on a ring-like cooling frame  320  via a respective support member  310  provided at each support cutout  303 . That is, the mirror  301  is supported at three points on the cooling frame  320  collectively by the support members  310 .  
         [0175]    For enhanced cooling effect, the gap can be filled with a gas exhibiting a suitably high rater of thermal transfer. To prevent gas leaking from the gap in amounts that could adversely affect the intensity of the EUV light propagating in the EUV optical system, the pressure of the gas should be less than several tens Torr. An exemplary gas for this purpose is helium.  
         [0176]    As shown in FIG. 22(A), each support member  310  comprises a fixed part  311 , flat springs (or analogous flexible members)  313 A,  313 B, and link parts  315 A,  315 B. The support member  310  desirably is formed from a material exhibiting low thermal conduction, such as ZrO 2  or the like, having a thermal conductivity of a few W/m·° K or less. The fixed part  311  is connected to the cooling frame  320  via a bolt b 1  inserted through a bolt hole  311   a  (FIG. 19(A)). The flat springs  313 A,  313 B extend laterally in respective opposite directions from one end of the fixed part  311  (FIG. 22(A)). Each flat spring  313 A,  313 B has a thickness of approximately a few mm in this example. The flat springs are flanked by cutouts  313 ′ that improve the flexibility of the flat springs.  
         [0177]    The link parts  315 A,  315 B are formed integrally and distally of the respective flat springs  313 A,  313 B. The link parts  315 A,  315 B are attached to the respective support cutout  303  on the side surface  301 C of the mirror  301  using respective linking bolts b 3  inserted through respective bolt holes  315   a ,  315   b  in the link parts  315 A,  315 B (see FIG. 19(B)).  
         [0178]    The flat springs  313 A,  313 B in the support members  310  are configured to undergo at least lateral flexure to reduce strains associated with any of various stresses on the mirror, for example: (a) heating of the mirror  301  due to incident EUV light irradiation causes thermal expansion of the mirror, (b) static changes in shape of the mirror  301  due to the mirror having to support its own mass or due to changes in ambient pressure, and (c) dynamic changes in shape of the mirror  301  due to vibrations from the floor supporting the optical system, from movements of a stage associated with the optical system, or from neighboring transport systems (e.g., robots) associated with the optical system. These strains are absorbed by corresponding flexure of the flat springs  313 A,  313 B. This absorption prevents or substantially reduces strain in the mirror  301  that otherwise would degrade the accuracy of the reflective surface  301 A.  
         [0179]    The fixed part  311  of each support member  310  is affixed to the cooling frame  320  by a respective bolt b 1  (see FIG. 19(A)), as described above. As further shown in FIG. 19(A), the inner circumference of the cooling frame  320  is larger than the outside dimensions of the periphery of the mirror  301 . The cooling frame  320  defines a channel  321  in which a cooling medium such as water or fluorinated cooling liquid is conducted. To supply and remove cooling liquid from the channel  321 , as shown on the left in FIG. 19(B), an inlet port  323  and outlet port  324 , respectively, are provided on the side surface of the cooling frame  320 . A coolant-circulation pump (not shown but well understood) is connected to the ports  323  and  324  for continuously circulating the cooling liquid through the channel  321 .  
         [0180]    Also shown in FIGS.  19 (A) and  20 , a heat-conductive plate  330 , forming a heat pipe having a thickness of approximately 1 mm in this example, is situated adjacent the rear surface  301 B of the mirror  301 . Turning now to FIG. 21, the heat-conductive plate  330  in this embodiment has a bowl-like elliptical profile of which the bowl “face”  331  is vertically inverted, by which is meant that the bowl face  331  projects “upward” from a rim-like peripheral edge  333  toward the rear surface  301 B. Turning now to FIG. 20, the bowl face  331  is situated and configured so as to define a predetermined gap “t” (approximately 20 μm in this embodiment) between the bowl face  331  and the rear surface  301 B. Desirably, a through-hole  335  is defined in the bowl face  331  for passage of a beam of laser light from a laser interferometer  355 , described later below. Note that the gap t is shown exaggerated in FIG. 20 for clarity.  
         [0181]    The peripheral edge  333  of the heat-conductive plate  330  is inserted into a circumferential slit  326  defined in the inside circumferential surface of the cooling frame  320  (FIG. 20). Thus, the peripheral edge  333  of the heat-conductive plate  330  is contacted by the cooling medium being circulated inside the channel  321 . For mechanical and hydraulic integrity, the peripheral edge  333  is soldered or brazed, for example, to the inside circumferential surface of the cooling frame  320 .  
         [0182]    Whenever heat is generated in the mirror  301  due to incident EUV irradiation, the heat first is radiated across the gap t from the rear surface  301 B of the mirror  301  to the bowl face  331  of the heat-conductive plate  330 . This transferred heat is conducted via the peripheral edge  333  of the bowl face  331  to the cooling medium circulating inside the channel  321 . By conducting this heat away using the cooling medium, thermal deformation of the mirror is reduced. Also, since there is no physical contact between the heat-conductive plate  330  and the mirror  301 , no physical forces are applied to the mirror  301  from the plate  330 .  
         [0183]    Alternatively, instead of having the rear surface  301 B separated from the bowl face  331  by a small gap “t”, the bowl face  331  can be configured actually to contact the rear surface  301 B (i.e., t=0). In such a configuration, heat is transmitted mainly by conduction from the mirror  301  to the heat-conductive plate  330 .  
         [0184]    The gap t between the rear surface  301 B and the heat-conductive plate  330  need not be constant; this gap can be adjustably different at different respective regions of the mirror  301  so as to provide a more tailored temperature control of the reflective surface  301 A of the mirror  301 . The gap t can be adjusted by configuring the heat-conductive plate  330  to a desired shape, or by attaching respective actuators (e.g., piezoelectric actuators) at predetermined locations on the heat-conductive plate  330 . By controlling the temperature distribution of the reflective surface  301 A in this manner, the optical performance of the mirror  301  (wavefront aberrations or the like) is improved correspondingly.  
         [0185]    Whereas the channel  321  is shown as having a rectangular cross-section (see FIGS.  19 (A) and  20 ), the channel  321  alternatively can have rounded transverse corners for effective suppression of fluid vibrations caused by flow of cooling medium in the channel  321 . The flow of the cooling medium desirably is laminar (Reynolds number approximately 1000 or less). To increase heat-removal efficiency, the heat-conductive plate  330  desirably has a large area contacting the cooling medium. To such end, a large portion of the peripheral edge  333  that is inserted into the channel  321  is in contact with the cooling medium, and the transverse dimensions of the channel  321  desirably are large.  
         [0186]    Returning to FIGS.  19 (A)- 19 (B), the cooling frame  20  is supported on a mirror base  350 , having a plate-like configuration, by three cooling-frame-support members  340 . The cooling-frame-support members  340  desirably are disposed, in a circumferential sense, intermediate the positions of the support members  310  holding the mirror  301 . A respective insulative spacer  352  (see FIG. 19(A)) desirably is interposed between each cooling-frame-support member  340  and the mirror base  350 .  
         [0187]    Turning now to FIG. 22(B), each cooling-frame-support member  340  comprises a fixed part  341 , an actuator  342 , flat springs  343 A,  343 B, and link parts  345 A,  345 B. In the same manner as described previously, the cooling-frame-support members  340  are made of a material exhibiting low thermal conductivity, such as ZrO 2 , for example. The fixed parts  341  are attached to the mirror base  350  via respective fixing bolts b 2  inserted into respective bolt holes  341   a  (see FIG. 19(A)). The actuators  342  are integral with their respective fixed parts  341 . Each actuator  342  comprises a piezoelectric actuator, for example, and is used for finely adjusting the position and orientation of the cooling frame  320  (and mirror  301  mounted thereto) in the Z, θ x , and θ y  directions, based on detection results obtained by the laser interferometer  355 , as described later below.  
         [0188]    The flat springs  343 A,  343 B extend laterally from one end of the fixed part  341 . The flat springs  343 A,  343 B each have a thickness of approximately a few mm in this embodiment. Each flat spring  343 A,  343 B is flanked by respective cutouts  343 ′ to increase flexibility of the flat spring.  
         [0189]    The link parts  345 A,  345 B are formed integrally and distally relative to the flat springs  343 A,  343 B, respectively. The link parts  345 A,  345 B are used for mounting the cooling-frame-support members  340  to the side surface of the cooling frame  320 . For mounting, linking bolts b 4  are inserted into respective bolt holes  345   a ,  345   b  (see FIG. 19(B)).  
         [0190]    In this embodiment three laser interferometers  355  ( 355   a ,  355   b ,  355   c ; as exemplary distance sensors) are installed on the “upper” face of the mirror base  350 , “below” the mirror  301 . As shown in FIG. 20, each interferometer  355   a ,  355   b ,  355   c  produces a respective laser beam that passes through the through-hole  335  defined in the heat-conductive plate  330  and impinges on the rear surface  301 B of the mirror  301 . Deformations, tilts, and other displacements of the mirror  301  are detected by receiving respective reflected laser beams. Based on data obtained by the interferometers  355   a ,  355   b ,  355   c , the actuators  342  in the cooling-frame-support members  340  are energized to achieve fine adjustment as required of the position and orientation of the cooling frame  320  (and of the mirror  301  mounted thereon) in the Z, θ x , and θ y  directions.  
         [0191]    The mirror base  350  defines an integral extension  351  (see lower part of FIG. 19(B)) having a side surface  351 ′. Two actuators  356 ,  357  are attached to the side surface  350 ′ of the mirror base  350 , and a single actuator  358  is attached to the side surface  351 ′ of the extension  351 . Each actuator  356 ,  357 ,  358  comprises a respective piezoelectric actuator, for example, and is used for adjusting the position of the mirror base  350  in the X, Y, and θ z  directions. Laser interferometers (position sensors)  359   a ,  359   b ,  359   c  are installed outboard of the mirror base  350 , and correspond to respective actuators  356 ,  357 ,  358 . Based on data obtained by the interferometers  359   a ,  359   b ,  359   c , the actuators  356 ,  357 ,  358  finely adjust the position and orientation of the mirror base  350  in the X, Y, and θ z  directions. Thus, by finely adjusting the mirror base  350 , corresponding fine adjustments are made of the mirror  301  in the X, Y, and θ z  directions.  
         [0192]    Thus, this embodiment achieves a six-axis adjustment of the mirror  301  (namely, in the X, Y, Z, θ x , θ y , θ z  directions). The actuator  342  incorporated into the support member  340  achieves fine-adjustment movements in the Z, θ x , and θ y  directions, and the actuators  356 ,  357 ,  358  (attached to the mirror base  350  and extension  351 ) achieve fine-adjustment movements in the X, Y, and θ z  directions. As a result of these adjustments, the position and orientation of the mirror  301  can be finely adjusted so as to correct positional errors, while performing mirror cooling.  
         [0193]    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.