Patent Publication Number: US-2005128446-A1

Title: Exposure apparatus and device manufacturing method

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to an exposure apparatus that exposes a substrate, i.e., an object to be exposed, such as a semiconductor wafer and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for an exposure apparatus that uses the ultraviolet (“UV”) or extreme ultraviolet (“EUV”) light as an exposure light. The present invention is also relates to a device manufacturing method using this exposure apparatus.  
      A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask (or a reticle) onto a wafer, etc to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology.  
      The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Therefore, recent demands for finer processing to semiconductor devices have promoted use of a shorter wavelength of the UV light from an ultra-high pressure mercury lamp (i-line with a wavelength of about 365 nm) to KrF excimer laser (with a wavelength of about 248 nm) and ArF excimer laser (with a wavelength of about 193 nm.)  
      The lithography using the UV light, however, has the limit to satisfy the rapidly progressing fine processing of semiconductor devices. Accordingly, there has been developed a reduction projection optical system using the EUV light with a wavelength, such as about 10 nm to 15 nm, shorter than that of the UV light, (which exposure apparatus is referred to as an “EUV exposure apparatus” hereinafter) for efficient transfers of very fine circuit patterns smaller than 0.1 μm.  
      The light absorption in a material remarkably increases as the wavelength of the exposure light becomes shorter, and it is difficult to use a refraction element or lens for visible light and ultraviolet light. In addition, no glass material exists in a wavelength range of the EUV light, and a reflection-type or catoptric optical system uses only a reflective element or mirror, such as Mo—Si multilayer coating mirror.  
      The mirror does not completely reflect the exposure light, but the reflectance per mirror surface is about 70%. The remaining exposure light of about 30% is absorbed in the mirror&#39;s base or mirror&#39;s primary ingredient, which usually uses glass. In order to serve as a reflective surface, a surface of the mirror&#39;s base is mirror-polished, and a reflective coating is formed on the polished base. The absorbed exposure light causes residual heat, and the temperature rise by 10 to 20° C. in the exposure light reflecting area of the mirror  120  as shown in  FIG. 12 . Then, even if the mirror base is made of a material having a very small coefficient of thermal expansion, such as low thermal expansion glass, the reflective surface deforms by 50 to 100 nm at the mirror base the mirror&#39;s periphery.  
      Since the surface shape precision required for the mirror in the exposure apparatus is between 0.1 nm to about several nanometers, it becomes difficult to guarantee the mirror&#39;s precision for the reflective surface that greatly deforms as discussed above. As a result, various problems happen in the exposure apparatus, such as deteriorated optical performance, imaging performance and light intensity, the non-uniform light intensity distribution, and the insufficient condensing performance, as well as the lowered exposure precision and throughput.  
      Accordingly, prior art proposes various mirror cooling methods for cooling a mirror. For example, Japanese Patent Application, Publication No. 05-205998 cools a mirror by providing a groove in the mirror&#39;s base and a cooling pipe that contacts the groove for circulating coolant (such as cooling water).  
      Since the cooling pipe contacts the mirror according to Japanese Patent Application, Publication No. 05-205998, the vibrations associated with circulations of the coolant in the cooling pipe transmit to the mirror. Due to the vibrating mirror (s), a pattern on an original form cannot be precisely projected onto a substrate, the exposure precision deteriorates, and the semiconductor devices manufactured from the substrate become defective.  
     BRIEF SUMMARY OF THE INVENTION  
      With the foregoing in mind, it is an exemplary object of the present invention to provide an exposure apparatus and a device manufacturing method using the exposure apparatus, which perform the temperature control of an optical element so as to improve the exposure precision.  
      An exposure apparatus according to one aspect of the present invention for projecting a pattern of an original onto a substrate using illumination light, said exposure apparatus includes a transfer system, having a channel, to transfer heat via said channel, and an optical element, upon which the illumination light enters, and in which a space, in which said channel is provided, is formed.  
      The space may include at least one of a through-hole and a concave portion. Said space and said channel may be located outside a region through which the illumination light passes. Said channel may be spaced from said optical element in said space. Said transfer system may transfer a temperature-controlled medium through said channel. Said transfer system may include a radiation plate provided in said space.  
      Said space may be located at a first surface, upon which the illumination light enters, of said optical element. Said space may be located at a second surface opposite to the first surface. Said space may be located at a surface opposite to a surface, upon which the illumination light enters, of said optical element. Said space may be located at a side surface of said optical element.  
      Said optical element may be one of a mirror and a lens. An exposure apparatus may further include a vacuum system for creating a vacuum atmosphere in which said optical element is located. An exposure apparatus may further include a light source for emitting EUV light as the illumination light. Said optical element may be an element of one of an optical system to direct the light from a light source to the original, and an optical system to direct the light from the original to the substrate.  
      Said transfer system may include a tube which passes through said space, and a circulation system to circulate temperature controlled medium via said tube. Said transfer system may include a first temperature detecting unit to detect temperature of said optical element, a second temperature detecting unit to detect temperature of the medium, and a temperature control unit to control temperature of the medium based on detection results by said first and second temperature detecting units.  
      A device manufacturing method according to another aspect of the present invention includes steps of transferring a pattern of an original to a substrate using an exposure apparatus, and developing the substrate to which the pattern has been transferred.  
      Other objects and further features of the present invention will become readily apparent from the following description of the preferred embodiments with reference to accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a schematic internal structure of an exposure apparatus that uses an optical element according to the present invention.  
       FIGS. 2A and 2B  show a structure of an illumination light source in the exposure apparatus shown in  FIG. 1 , wherein  FIG. 2A  shows that a laser excites and emits the EUV light, and  FIG. 2B  is an enlarged view of the inside of the light source emitting section.  
       FIGS. 3A and 3B  show a structure of a mirror as an optical element according to a first embodiment of the present invention, wherein  FIG. 3A  is a schematic perspective view of the mirror, and  FIG. 3B  is a side view of the mirror.  
       FIG. 4  shows a schematic structure of a cooling apparatus for cooling the mirror shown in  FIG. 3 .  
       FIG. 5  is a temperature distribution map of a mirror surface cooled by the cooling apparatus shown in  FIG. 4 .  
       FIG. 6  is a flowchart for explaining a device manufacturing method using the exposure apparatus shown in  FIG. 1 .  
       FIG. 7  is a detailed flowchart of step  104  in  FIG. 6 .  
       FIGS. 8A and 83  show a structure of a mirror as an optical element according to a second embodiment of the present invention, wherein  FIG. 8R  is a schematic perspective view of the mirror, and  FIG. 8B  is a side view of the mirror.  
       FIG. 9  shows a schematic structure of a cooling apparatus for cooling the mirror shown in  FIG. 8 .  
       FIG. 10  is a perspective overview showing a structure of a mirror as an optical element according to a third embodiment of the present invention.  
       FIG. 11  shows a schematic structure of a cooling apparatus for cooling the mirror shown in  FIG. 10 .  
       FIG. 12  is a temperature distribution map on the conventional mirror&#39;s reflective surface.  
       FIGS. 13A and 13B  show a structure of a mirror as an optical element according to a fourth embodiment of the present invention, wherein  FIG. 13A  is a schematic perspective view of the mirror, and  FIG. 13B  is a side view of the mirror.  
       FIG. 14  is a perspective view of a principal part showing that the coolant circulates through and cools the mirror shown in  FIG. 13 .  
       FIGS. 15A and 15B  show a structure of a mirror as an optical element according to a fifth embodiment of the present invention, wherein  FIG. 15A  is a schematic perspective view of the mirror, and  FIG. 15B  is a side view of the mirror.  
       FIG. 16  is a perspective view of a principal part showing that the coolant circulates through and cools the mirror shown in  FIG. 15 .  
       FIGS. 17A and 173  show a structure of a mirror as an optical element according to another embodiment of the present invention, wherein  FIG. 17A  is a schematic perspective view of the mirror, and  FIG. 17B  is a side view of the mirror.  
       FIGS. 18A and 18B  show a structure of a mirror as an optical element according to another embodiment of the present invention, wherein  FIG. 18A  is a schematic perspective view of the mirror, and  FIG. 18B  is a side view of the mirror.  
       FIGS. 19A and 19B  show a structure of a mirror as an optical element according to another embodiment of the present invention, wherein  FIG. 19A  is a schematic perspective view of the mirror, and  FIG. 19B  is a side view of the mirror.  
       FIGS. 20A and 20B  show a structure of a mirror as an optical element according to still another embodiment of the present invention, wherein  FIG. 20A  is a schematic perspective view of the mirror, and  FIG. 20B  is a side view of the mirror.  
       FIGS. 21A and 21B  show a structure of a mirror as an optical element according to still another embodiment of the present invention, wherein  FIG. 21A  is a schematic perspective view of the mirror, and  FIG. 21B  is a side view of the mirror.  
       FIGS. 22A and 22B  show a structure of a mirror as an optical element according to still another embodiment of the present invention, wherein  FIG. 22A  is a schematic perspective view of the mirror, and  FIG. 223  is a side view of the mirror. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      With reference to the accompanying drawings, a description will be given of a mirror as an optical element and its cooling apparatus according one embodiment of the present invention.  FIG. 1  shows a schematic internal structure of an exposure apparatus  100  that uses this mirror. This exposure apparatus  100  exposes a pattern on a reticle  6 A as an original form onto a wafer  8 A as a substrate. The wafer  8 A is an object to be exposed, and processed (e.g., etched and cut) after the pattern is exposed. Thereby, a semiconductor device is manufactured from the wafer  8 .  
      In  FIG. 1 , reference numeral  1  denotes an excitation laser as part of an illumination light source. The excitation laser  1  excites light-source material atoms into plasma for light emissions by irradiating a laser beam onto an emitting point of the light source, at which the light-source material is in a state of gas, liquid or atomized gas. The excitation laser  1  uses a YAG solid laser etc.  
      Reference numeral  2  denotes a light-source emitting section as a part of the illumination light source, and is configured to maintain its inside to be vacuum.  FIGS. 2A and 2B  show the internal structure of the light-source emitting section  2 . An emitting point  2 A is an actual emitting point of an illumination light source. The illumination light  2   a  is, for example, the EUV light. A semispherical light source mirror  2 B is arranged inside the light-source emitting section  2 . In order to condense and reflect the illumination light  2   a  emitted from the emitting point  2 A as spherical light, towards an emitting direction, the light source mirror  2 B is arranged at a side opposite to the emitting direction (or at the side of the excitation laser  1  in the figure) so that the emitting point becomes a center of a radius of curvature.  
      Xenon (Xe)  2 C is liquefied and supplied in a form of spray or gas to the light source emitting section  2 A. Xe  2 C is used as an emitting element, and supplied to the emitting point  2 A by the nozzle  2 D.  
      Reference numeral  3  denotes a chamber for accommodating an illumination optical system  5  and a projection optical system  7  in the exposure apparatus  100 , which can maintain the vacuum state using a vacuum pump  4 .  
      Reference numeral  5  denotes an illumination optical system for introducing the illumination light  2   a  from the light-source emitting section  2  to a reticle  6 A as an original form held on the reticle stage  6 . The illumination optical system  5  includes mirrors  5   a  to  5   d , homogenizes and shapes the illumination light  2   a , and introduces the illumination light  2   a  to the reticle  6 A.  
      A reticle stage  6  holds and moves the reticle  6 A, on which a pattern is formed. Since the exposure apparatus  100  is a step-and-scan exposure apparatus, the reticle  6 A is mounted on a movable part of the reticle stage  6 , moved and scanned in synchronization with the wafer.  
      Reference numeral  7  denotes a projection optical system for introducing the illumination light  2   a  that has been irradiated onto the reticle  6 A and reflected by the reticle  6 A, onto the wafer (or the substrate)  8 A as an object to be exposed. The projection optical system  7  includes mirrors  7   a  to  7   e , and introduces the pattern on the reticle  6 A to the surface of the wafer  8 A by reflecting the pattern via the mirrors  7   a  to  7   e  in this order and reduces the pattern by a predefined reduction ratio.  
      The wafer  8 A is a Si substrate, and held on a wafer stage  8 . The wafer stage  8  positions the wafer  8 A at a predetermined exposure position, and can be driven in six axes directions, i.e., driven in XYZ directions, tilt around the XY axes, and rotated around the Z axis, so as to move the wafer  8 A in synchronization with the reticle  6 A.  
      Reference numeral  9  denotes a reticle stage support for supporting the reticle stage  6  on the installation floor of the exposure apparatus  100 . Reference numeral  10  denotes a projection optical system body for supporting the projection optical system  7  on the installation floor of the exposure apparatus  100 . Reference numeral  11  denotes a wafer stage support for supporting the wafer stage  8  on the installation floor of the exposure apparatus  100 .  
      A position measuring means (not shown) measures positions of the reticle stage  6 , the projection optical system  7 , and the wafer stage  8 , which are distinctly and independently supported by the reticle stage support  9 , and a control means (not shown) controls a relative position between the reticle stage  6  and the projection optical system  7 , and a relative position between the projection optical system  7  and a wafer stage  8  based on the position measurement results. A mount (not shown) for isolating vibrations from the installation floor of the exposure apparatus  100  is provided on the reticle stage support  9 , the projection system body  10 , and the wafer stage  11 .  
      Reference numeral  12  denotes a reticle stocker for storing the reticles  6 A in the chamber  3  of the exposure apparatus  100 . The reticle stocker  12  is an airtight container that stores plural reticle  6 A formed in accordance with different patterns and exposure conditions. Reference numeral  13  denotes a reticle changer for selecting and feeding the reticle  6 A to be used, from the reticle stocker  12 .  
      Reference numeral  14  denotes a reticle alignment unit that includes a rotatable hand that is movable in the XYZ directions and rotatable around the Z axis. The reticle alignment unit  14  receives the reticle  6 A from the reticle changer  13 , rotates it by 180°, and feeds it to the field of a reticle alignment scope  15  provided at the end of the reticle stage  6 . Then, the reticle  6 A is aligned through fine movements of the reticle  6 A in the XYZ-axes directions with respect to the alignment mark  15 A provided on the basis of the projection optical system  7 . The aligned reticle  6 A is chucked on the reticle stage  6 .  
      Reference numeral  16  denotes a wafer stocker for storing the wafer  6 A in the chamber  3  of the exposure apparatus  100 . The wafer stocker  16  stores plural wafers  8 A that have not yet been exposed. A wafer feed robot  17  selects a wafer  18 A to be exposed, from the wafer stocker  116 , and feeds it to a wafer mechanical pre-alignment temperature controller  18 .  
      The wafer mechanical pre-alignment temperature controller  18  roughly adjusts feeding of the wafer  8 A in the rotational direction, and controls the wafer temperature within predetermined controlled temperature in the exposure apparatus  100 . The inside of the chamber  3  is partitioned by the diaphragm  3   a  into an exposure space  3 A in which the illumination optical system  5  and the projection optical system  7  are installed, and a wafer space  3 B in which the wafer stocker  16 , the wafer pre-alignment temperature controller  18 , and wafer feed hand  19  are installed.  
      Reference numeral  19  denotes a wafer feed hand. The wafer feed hand  19  feeds the wafer  8 A that has been aligned and temperature-controlled by the wafer mechanical pre-alignment temperature controller  18  to the wafer stage  8 .  
       20 ,  21  and  22  are gate valves. The gate valves  20  and  21  are provided on a wall surface of the chamber  3 , and serve as opening/closing mechanisms for supplying the reticle  6 A and wafer  8 A from the outside of the chamber  3  to the inside of the chamber  3 . The gate valve  22  is provided on the diaphragm  3   a , and serves as an opening/closing mechanism for opening and closing a gate of the diaphragm  3   a  when the wafer  8 A is fed by the wafer feed hand  19  from the wafer pre-alignment temperature controller  18  to the aligned and temperature-controlled wafer stage  8 . Thus, the separation using the diaphragm  3   a  between the exposure space  3 A and the wafer-use space  3 B, and opening and closing using the gate  22  can minimize a capacity to be temporarily released to the air, and form a vacuum equilibrium state.  
      The projection optical system  7  uses a Mo—Si multilayer coating formed on a reflective surface of each of the mirrors  7   a  to  7   e  by vacuum evaporation or sputtering. When the illumination light  2   a  is reflected on each mirror&#39;s reflective surface, about 70% of the light is reflected but the remaining about 30% of the light is absorbed in the mirror&#39;s base and converted into heat. Without cooling of the mirror, the temperature rises by about 10 to 20° C. in the area that reflects the illumination light  2   a  (“illumination area”), and the reflective surface deforms by about 50 to 100 nm around the mirror peripheral even when the mirror uses a material having an extremely small coefficient of thermal expansion. As a result, this configuration cannot maintain extremely strict mirror surface shape precisions, e.g., between 0.1 nm to several nanometers, necessary for the projection optical system  7 &#39;s mirrors, the illumination optical system  5 &#39;s mirrors, and the light source  2 B&#39;s mirrors.  
      In the projection optical system  7 , the lowered mirror surface precision deteriorates the imaging performance to the wafer  8 A and lowers light intensity. In the illumination optical system  5 , the lowered mirror surface precision deteriorates the light intensity to the mask  6 A and the uniformity of the light intensity distribution. The light source mirror  2 B deteriorates the light intensity due to the bad condensing performance of the illumination light  2   a.    
      The instant embodiment cools the mirror as follows, in order to solve the problems of the heating mirror. Since mirrors&#39; shapes are different depending upon positions, this embodiment describes a cylindrical concave mirror as a representative example. While the instant embodiment regards all the optical elements as mirrors, the sprit of the present invention is applicable to another optical element, such as a lens.  
     First Embodiment  
      Referring to FIGS.  3  to  5 , a description will be given of a mirror and its cooling method according to a first embodiment of the present invention. This mirror  50  is applicable to the light source mirror  2 B, the mirrors  5   a  to  5   d  of the illumination optical system  5 , and the mirrors  7   a  to  7   e  of the projection optical system  7 . In the first embodiment, the perforation hole  52  is formed in the side surface  50 . In other words, when a side surface  50   c  is defined as a cylindrical peripheral surface that is held between the reflective surface  50   a  that serves as incident and exit surfaces and reflects the illumination light  2   a  and the rear surface  50   b  at the rear side of the reflective surface  50   a , an entrance of the perforation hole  52  is formed in the side surface  50   c.    
      The perforation hole  52  is formed so as not to shield the optical path of the illumination light  2   a . For example, in the first embodiment, the perforation hole  52  avoids a reflecting point  50   d  of the illumination light  2   a  on the reflective surface  50   a , and the entrance is formed in the side surface  50   c . Therefore, the perforation hole  52  does not affect the optical path of the illumination light  2   a.    
      The cooling pipe  53  perforates the perforation hole  52 . Coolant  54  for cooling the mirror  50  circulates through this cooling pipe  53 . The coolant  54  may be, for example, cooling water or solution or gas. The cooling pipe  53  does not contact the mirror  50 , as shown in  FIG. 3B . Therefore, the mirror  50  is not affected by vibrations when the coolant  54  circulates in the cooling pipe  53 , and other problems.  
       FIG. 4  is a block diagram of a schematic structure of a cooling apparatus for cooling this mirror  50 . This cooling apparatus  60  includes the cooling pipe  53 , a circulator  61 , a mirror thermometer  62  as a first temperature detector, a coolant thermometer  63  as a second temperature detector, and a temperature regulator  64  for controlling the temperature of the coolant  54 . The controller  101  of the exposure apparatus  100  is connected to the temperature regulator  64  so that the temperature regulator  64  can receive the exposure control information and the light-intensity control information of the exposure apparatus  100 . The circulator  61  and the temperature regulator  64  are connected to the cooling pipe  53 .  
      The circulator  61  serves to circulate the coolant in the cooling pipe  53 , and includes, for example, a circulation pump. This circulator  61  may be integrated with the temperature regulator  64 , which will be described later. The circulator  61  sequentially supplies the coolant  54  that is temperature-controlled and cooled by the temperature regulator  64 , to the cooling pipe  52  in the perforation hole  52  in the mirror  50 . The coolant  54  heated by the mirror  50 &#39;s heat is sequentially fed to the temperature regulator  64 . Thereby, the temperature of the mirror  50  can be controlled within a certain range.  
      The mirror thermometer  62  serves to measure the temperature of the mirror  50 . The mirror thermometer  62  may be a contact type or non-contact type. The coolant thermometer  63  serves to measure the temperature of the coolant  54 . These thermometers can use any known thermometers, and a detailed description will be omitted.  
      The temperature regulator  64  serves to regulate the temperature of the coolant  54 . In other words, the mirror&#39;s temperature measured by the mirror thermometer  62  is compared with the coolant&#39;s temperature measured by the coolant thermometer  63 , and it is determined whether the mirror&#39;s temperature is within a predetermined temperature range, and the temperature of the coolant  54  is regulated in accordance with the determination result. The desired temperature range as a control target is determined by the exposure control information and light intensity control information from the controller  101 .  
       FIG. 5  shows a temperature distribution on the reflective surface  50   a  when the mirror is cooled by the coolant  54  that is circulated by the circulator  61  in the cooling pipe  53  using the thus structured mirror  50  and cooling apparatus  60 . The reflecting point  50   d  that has the highest exothermic heat in the area illuminated by the illumination light  2   a  causes the temperature rise by about 10 to 20° C. if there is no cooling apparatus  60 . However, when this cooling apparatus  60  is applied to the mirror  50  to cool the mirror  50 , the temperature rise at the reflective point  50   d  is reduced down to about 1 to 4° C., as shown in  FIG. 5 . Therefore, the deformation amount on the mirror&#39;s reflective surface  50   a  is reduced below 2 nm due to the temperature rise at the reflecting point  50   d.    
      As shown in  FIG. 3B , it is preferable that the perforation hole  52  is formed below and near the reflecting point  50   d  in the mirror  50  without negatively affecting the reflective surface  50   a . In other words, radiation cooling of the mirror  50  from the cooling pipe  53  improves its cooling efficiency when a distance from the cooling pipe  53  to the reflecting point  50   d  is as small as possible. Since the optical element is the mirror  50 In the first embodiment, the perforation hole  52  formed below and near the reflecting point  50   d  does not shield the optical path of the illumination light  2   a . However, when the optical element is a lens, the illumination light  2   a  transmits through the lens and thus the perforation hole should be formed to avoid an incident area on an incident surface of the illumination light  2   a , a light transmission area of the illumination light  2   a  that transmits the lens&#39; base, and an exit area on an exit surface of the illumination light  2   a . As long as the perforation hole is thus formed, the present invention is applicable to the lens.  
      The first embodiment uses the cooling pipe  53  for cooling of the mirror  50 , but may use a radiation plate (not shown) instead of the cooling plate. In this case, the perforation hole  52  is formed in the radiation plate to be cooled by the coolant  54 . The coolant  54  may be pass the perforation hole  52  with the radiation plate or may contact and cool the radiation plate at part other than the perforation hole  52 . Even when the coolant  54  does not pass the perforation hole, the radiation cools the radiation plate in the perforation hole  52  and consequently cools the mirror  50 .  
      Referring to  FIGS. 6 and 7 , a description will now be given of an embodiment of a device fabricating method using the above mentioned exposure apparatus  100 .  FIG. 6  is a flowchart for explaining a fabrication of devices (i.e., semiconductor chips such as IC and LSI, LCDs, CCDs, etc.). Here, a description will be given of a fabrication of a semiconductor chip as an example. Step  101  (circuit design) designs a semiconductor device circuit. Step  102  (mask fabrication) forms a mask having a designed circuit pattern. Step  103  (wafer making) manufactures a wafer using materials such as silicon. Step  104  (wafer process), which is referred to as a pretreatment, forms actual circuitry on the wafer through photolithography using the mask and wafer. Step  105  (assembly), which is also referred to as a post-treatment, forms into a semiconductor chip the wafer formed in Step  104  and includes an assembly step (e.g., dicing, bonding), a packaging step (chip sealing), and the like. Step  106  (inspection) performs various tests for the semiconductor device made in Step  5 , such as a validity test and a durability test. Through these steps, a semiconductor device is finished and shipped (Step  107 ).  
       FIG. 7  is a detailed flowchart of the wafer process in Step  104  shown in  FIG. 6 . Step  111  (oxidation) oxidizes the wafer&#39;s surface. Step  112  (CVD) forms an insulating film on the wafer&#39;s surface. Step  113  (electrode formation) forms electrodes on the wafer by vapor disposition and the like. Step  114  (ion implantation) implants ion into the wafer. Step  115  (resist process) applies a photosensitive material onto the wafer. Step  116  (exposure) uses the exposure apparatus  100  to expose a circuit pattern on the mask onto the wafer. Step  117  (development) develops the exposed wafer. Step  18  (etching) etches parts other than a developed resist image. Step  119  (resist stripping) removes disused resist after etching. These steps are repeated, and multilayer circuit patterns are formed on the wafer. The device fabrication method of this embodiment may manufacture higher quality devices than the conventional one.  
     Second Embodiment  
      Referring to  FIGS. 8 and 9 , a description will be given of a mirror as an optical element and its cooling method according to a second embodiment of the present invention. Those elements which are corresponding elements in the first embodiment are designated by the same reference numerals, and a description will be omitted.  
      Similar to the mirror  50 , the mirror  70  is applicable to the light source mirror  2 B, the mirrors  5   a  to  5   d  of the illumination optical system  5 , and the mirrors  7   a  to  7   e  of the projection optical system  7 . Plural perforation holes  72 ,  73  and  74  are formed in a base  71  of this mirror  70 . In this second embodiment, the perforation holes  72  to  74  are formed in a side surface  70   c . In other words, when a side surface  70   c  is defined as a cylindrical peripheral surface that is held between the reflective surface  70   a  that serves as incident and exit surfaces and reflects the illumination light  2   a  and the rear surface  50   b  at the rear side of the reflective surface  50   a , entrances of the perforation holes  72  to  74  are formed in the side surface  70   c.    
      The perforation holes  72  to  74  are formed so as not to shield the optical path of the illumination light  2   a . For example, in the second embodiment, the perforation hole  72  avoids a reflecting point  70   d  of the illumination light  2   a  on the reflective surface  70   a , and its entrance is formed in the side surface  70   c . Therefore, the perforation holes  72  to  74  do not affect the optical path of the illumination light  2   a  The perforation hole  730  is formed, as shown in  FIG. 8B , below and near the reflecting point  70   d  in a range that does not negatively affect the reflective surface  70   a . The perforation holes  72  and  74  are formed near and adjacent to the perforation hole  73  at left and right sides of the perforation holes  73 . While the second embodiment exemplarily discusses three perforation holes  72  to  74 , the number of perforation holes may increase or decrease if necessary or based on various parameters and cooling and other necessary performances.  
      Cooling pipes  72   a  to  74   a  perforate the perforation holes  72  to  74 . Coolant  54  for cooling the mirror  70  circulates through each of these cooling pipes  72   a  to  74   a . The coolant  54  may be, for example, cooling water or solution or gas. The cooling pipes  72   a  to  74   a  do not contact the mirror  70 , as shown in  FIG. 8B . Therefore, the mirror  70  is not affected by vibrations when the coolant  54  circulates in these cooling pipes  72   a  to  74   a , and other problems.  
       FIG. 9  is a block diagram of a schematic structure of a cooling apparatus for cooling this mirror  70 . This cooling apparatus  60  includes the cooling pipes  72   a  to  74   a , a circulator  61 , a mirror thermometer  62  as a first temperature detector, a coolant thermometer  63  as a second temperature detector, and a temperature regulator  64  for controlling the temperature of the coolant  54 . The controller  101  of the exposure apparatus  100  is connected to the temperature regulator  64  so that the temperature regulator  64  can receive the exposure control information and the light-intensity control information of the exposure apparatus  100 . The circulator  61  and the temperature regulator  64  are connected to plural cooling pipes  72   a  to  74   a , and the coolant thermometer  63  measures the temperature of the coolant  54  that circulates in these cooling pipes  72   a  to  74   a . Other structures and functions are similar to those in the first embodiment.  
      When the mirror  70  is cooled by the coolant  54  that is circulated by the circulator  61  in the cooling pipes  72   a  to  74   a  using the thus structured mirror  70  and cooling apparatus  60 , the mirror  70  is more efficiently cooled than the first embodiment. For example, the temperature rise at the reflective point  70   d  is reduced down to about 1 to 2° C.  
      In the second embodiment, radiation cooling of the mirror  70  from the cooling pipes  72   a  to  74   a  improves its cooling efficiency when a distance from the cooling pipe  53  to the reflecting point  70   d  is as small as possible.  
     Third Embodiment  
      Referring to  FIGS. 10 and 11 , a description will be given of a mirror as an optical element and its cooling method according to a third embodiment of the present invention. Those elements which are corresponding elements in the first embodiment are designated by the same reference numerals, and a description will be omitted.  
      Similar to the mirror  50 , the mirror  80  is applicable to the light source mirror  2 B, the mirrors  5   a  to  5   d  of the illumination optical system  5 , and the mirrors  7   a  to  7   e  of the projection optical system  7 . Plural perforation holes  82 ,  83 ,  84  and  85  are formed in a base  81  of this mirror  80 . In this third embodiment, the perforation holes  82  to  85  perforate this mirror  80  from the reflective surface  80   a  to the rear surface  80   b , and entrances of the perforation holes  82  to  85  are formed in the reflective surface  80   a  and the rear surface  80   b.    
      As shown in  FIG. 10 , the entrances of the perforation holes  82  to  85  are formed so as not to shield the optical path of the illumination light  2   a . The perforation hole  82  avoids a reflecting point  80   d  of the illumination light  2   a  on the reflective surface  80   a , and is formed near and around the reflective surface  80   d . Therefore, formations of the perforation holes  82  to  84  do not affect the optical path of the illumination light  2   a . While the third embodiment exemplarily discusses four perforation holes  82  to  85 , the number of perforation holes may increase or decrease if necessary or based on various parameters and cooling and other necessary performances.  
      Cooling pipes  82   a  to  85   a  perforate the perforation holes  82  to  85 . Coolant  54  for cooling the mirror  80  circulates through each of these cooling pipes  82   a  to  85   a . The coolant  54  may be, for example, cooling water or solution or gas. The cooling pipes  82   a  to  85   a  do not contact the mirror  80 , and thus the mirror  80  is not affected by vibrations when the coolant  54  circulates in these cooling pipes  82   a  to  85   a , and other problems.  
       FIG. 11  is a block diagram of a schematic structure of a cooling apparatus for cooling this mirror  80 . This cooling apparatus  60  includes the cooling pipes  82   a  to  85   a , a circulator  61 , a mirror thermometer  62  as a first temperature detector, a coolant thermometer  63  as a second temperature detector, and a temperature regulator  64  for controlling the temperature of the coolant  54 . The controller  101  of the exposure apparatus  100  is connected to the temperature regulator  64  so that the temperature regulator  64  can receive the exposure control information and the light-intensity control information of the exposure apparatus  100 . The circulator  61  and the temperature regulator  64  are connected to plural cooling pipes  82   a  to  85   a , and the coolant thermometer  63  measures the temperature of the coolant  54  that circulates in these cooling pipes  82   a  to  85   a . Other structures and functions are similar to those in the first embodiment.  
      When the mirror  80  is cooled by the coolant  54  that is circulated by the circulator  61  in the cooling pipes  82   a  to  85   a  using the thus structured mirror  80  and cooling apparatus  60 , the mirror  80  is more efficiently cooled than the first embodiment. For example, the temperature rise at the reflective point  80   d  is reduced down to about 1 to 2° C.  
      In the third embodiment, radiation cooling of the mirror  80  from the cooling pipes  82   a  to  85   a  improves its cooling efficiency when a distance from the cooling pipe  53  to the reflecting point  80   d  is as small as possible.  
     Fourth Embodiment  
      Referring to  FIGS. 13 and 14 , a description will be given of the mirror  50  as an optical element and its cooling method according to a fourth embodiment of the present invention. In this fourth embodiment, a groove-shaped notch  26  is provided as a non-perforation hole or a convexo-concave groove, as shown in  FIG. 13 , on a mirror&#39;s base near the reflecting portion of the illumination light  2   a  in the mirrors  7   a  to  7   e  of the projection optical system  7  and the mirrors  5   a  to  5   d  of the illumination optical system  5 . This notch  26  is formed from a side surface to side surface of the mirror  50 . The cooling pipe  53  for circulating the coolant  54  is provided on the groove portion of this notch  26  so that the cooling pipe  53  does not contact the mirror&#39;s base.  
      Similar to the first embodiment, the temperature distribution on the mirror surface is as shown in  FIG. 5  when the coolant  54  flows in the cooling pipe  53 . The reflecting point  50   d  that has the highest exothermic heat in the area illuminated by the illumination light  2   a  causes the temperature rise by about 10 to 20° C. if there is no cooling apparatus  60 . However, when the mirror  50  is cooled according to the fourth embodiment, the temperature rise at the reflective point  50   d  is reduced down to about 1 to 4° C., as shown in  FIG. 5 . Therefore, the deformation amount on the mirror&#39;s reflective surface  50   a  is reduced below 2 nm due to the temperature rise at the reflecting point  50   d.    
      The temperature control of the coolant  54  in the fourth embodiment is similar to a method of the first embodiment. In other words, the cooling apparatus  60  shown in  FIG. 4  is applied to the mirror  50  shown in  FIG. 14 , and the coolant  54  is circulated in the cooling pipe  53 . The coolant  54  that is temperature-controlled to a target temperature by the temperature regulator  64  circulates the cooling pipe  53  and passes the groove portion on the notch  26  provided on the base of the mirror  50 . Since the cooling pipe  53  does not contact the base of the mirror  50 , the mirror  50  is radiation-cooled by a temperature difference between them.  
     Fifth Embodiment  
      Referring to  FIGS. 15 and 16 , a description will be given of the mirror  50  as an optical element and its cooling method according to a fifth embodiment of the present invention. In this fifth embodiment, a partially groove-shaped notch  27  is provided, as shown in  FIG. 15 , on a mirror&#39;s base near the reflecting portion of the illumination light  2   a  in the mirrors  7   a  to  7   e  of the projection optical system  7  and the mirrors  5   a  to  5   d  of the illumination optical system  5 . This notch  27  is partially formed on a bottom surface below and near the reflection portion of the illumination light  2   a  in the mirror  50 . The cooling pipe  53  for circulating the coolant  54  is provided on the groove portion of this notch  27  so that the cooling pipe  53  does not contact the mirror&#39;s base.  
      Similar to the first embodiment, the temperature distribution on the mirror surface is as shown in  FIG. 5  when the coolant  54  flows in the cooling pipe  53 . The reflecting point  50   d  that has the highest exothermic heat in the area illuminated by the illumination light  2   a  causes the temperature rise by about 10 to 20° C. if there is no cooling apparatus  60 . However, when the mirror  50  is cooled according to the fifth embodiment, the temperature rise at the reflective point  50   d  is reduced down to about 1 to 4° C., as shown in  FIG. 5 . Therefore, the deformation amount on the mirror&#39;s reflective surface  50   a  is reduced below 2 nm due to the temperature rise at the reflecting point  50   d.    
      The temperature control of the coolant  54  in the fifth embodiment is similar to a method of the first embodiment. In other words, the cooling apparatus  60  shown in  FIG. 4  is applied to the mirror  50  shown in  FIG. 16 , and the coolant  54  is circulated in the cooling pipe  53 . The coolant  54  that is temperature-controlled to a target temperature by the temperature regulator  64  circulates the cooling pipe  53  and passes the groove portion on the notch  26  provided on the base of the mirror  50 . Since the cooling pipe  53  does not contact the base of the mirror  50 , the mirror  50  is radiation-cooled by a temperature difference between them.  
     Another Embodiment  
      Referring to FIGS.  17  to  19 , a description will be given of another embodiment according to the present invention. In  FIG. 17 , the perforation hole  52  similar to that shown in  FIG. 3  is formed in the mirror  50 . In  FIG. 18 , the groove-shaped notch  26  similar to that shown in  FIG. 13  is formed on the mirror  50 . In  FIG. 19 , the partially groove-shaped notch  27  similar to that shown in  FIG. 15  is formed on the mirror  50 .  
      In FIGS.  17  to  19 , the cooling pipe  53  passes and contacts the mirror  50 &#39;s base on a contact surface  42  for the perforation hole  52  and the notches  26  and  27 . Thereby, the cooling efficiency improves higher than that of  FIGS. 3, 13  and  15  in which the cooling pipe  53  does not contact the mirror  50 &#39;s base. Therefore, the embodiment shown in FIGS.  17  to  19  can maintain the temperature rise by about 0 to 0.5° C. near the reflecting point of the illumination light  2   a  on the reflective surface, which corresponds to a high temperature portion in an exposure light reflecting area in  FIG. 5 .  
      The temperature control of the coolant  54  in this embodiment is similar to a method of the first embodiment. In mounting the cooling pipe  53  of the contact state, the distortion of the cooling pipe  53  more easily affects the mirror  50  than the non-contact state. Therefore, an elastic member (not shown) etc. is preferably to be used especially to support the cooling pipe  53  so that no deformation transmits the mirror  50 .  
     Still Another Embodiment  
      Referring to FIGS.  20  to  22 , a description will be given of still another embodiment according to the present invention. In  FIG. 20 , the perforation hole  52  similar to that shown in  FIG. 3  is formed in the mirror  50 . In  FIG. 18 , the groove-shaped notch  26  similar to that shown in  FIG. 13  is formed on the mirror  50 . In  FIG. 19 , the partially groove-shaped notch  27  similar to that shown in  FIG. 15  is formed on the mirror  50 .  
      This embodiment directly circulates the coolant  54  in the perforation hole  52  and the groove portions of the notches  26  and  27 , instead of providing the cooling pipe in the perforation hole or notch in the mirror&#39;s base. Therefore, the coolant  54  directly contacts the base of the mirror  50 . The notches  26  and  27  shown in  FIGS. 21 and 22  seal the bottom surface of the mirror  50  using the covers  44  and  46  so that the coolant  54  does not leak.  
      Thereby, the cooling efficiency improves higher than the embodiment shown in  FIGS. 3, 13  and  15  in which the cooling pipe  53  does not contact the mirror  50 &#39;s base and the above other embodiment. Therefore, the embodiment shown in FIGS.  20  to  22  can maintain the temperature rise by about 0 to 0.1° C. near the reflecting point of the illumination light  2   a  on the reflective surface, which corresponds to a high temperature portion in an exposure light reflecting area in  FIG. 5 . The temperature control of the coolant  54  in this embodiment is similar to a method of the first embodiment.  
      The present invention efficiently and definitely cools an optical element used for an exposure apparatus without lowering the exposure precision, such as vibrations associated with coolant circulations Thereby, a surface precision of the optical element improves, and the exposure precision and throughput also improve by maintaining the light intensity, the light intensity uniformity, and condensing performance. Ultimately, the present invention improves the qualities of the objects exposed by this exposure apparatus and devices manufactured from this exposed objects.  
      Further, the present invention is not limited to these preferred embodiments, and various variations and modifications may be made without departing from the scope of the present invention.  
      This application claims a foreign priority based on Japanese Patent Application No. 2003-412776, filed Dec. 11, 2003, which is hereby incorporated by reference herein.