Patent Publication Number: US-6335787-B1

Title: Projection exposure apparatus

Description:
This application is a continuation of prior application Ser. No. 09/291,300, filed Apr. 15, 1999, abandoned which is a continuation of application Ser. No. 08/813,968 filed Mar. 3, 1997 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a projection exposure apparatus used to transfer a mask pattern onto a photosensitive substrate in a photolithography process for producing, for example, semiconductor devices, image pickup devices (CCDs, etc.), liquid-crystal display devices, or thin-film magnetic heads. More specifically, the present invention relates to a scanning projection exposure apparatus, e.g. a step-and-scan projection exposure apparatus, which has a catadioptric system as a projection optical system and performs exposure by synchronously scanning a mask and a photosensitive substrate relative to the projection optical system. 
     To produce semiconductor devices, for example, a projection exposure apparatus is used to transfer a pattern formed on a reticle as a mask onto each shot area on a wafer coated with a photoresist. Hitherto, a step-and-repeat type (one-shot exposure type) reduction projection exposure apparatus (stepper) has frequently been used as a projection exposure apparatus for the pattern transfer process. On the other hand, to meet the demand that the area of a pattern to be transferred should be increased without substantially increasing the load on the projection optical system, attention has recently been paid to a step-and-scan type projection exposure apparatus wherein a reticle and a wafer are synchronously scanned relative to a projection optical system in a state where a part of a pattern on the reticle is projected as a reduced (demagnified) image on the wafer, thereby sequentially transferring the demagnified image of the reticle pattern onto each shot area on the wafer. The step-and-scan method has been developed by combining together the advantage of the transfer method of the aligner (slit scan method) that transfers a pattern on the whole surface of a reticle onto the whole surface of a wafer in the magnification ratio of 1:1 by one scanning exposure and the advantage of the transfer method of the stepper. 
     In general, it is required for resolution of projection exposure apparatuses to be increased. One approach to increase resolution is to use a light beam of a shorter wavelength as an illuminating light for exposure. Accordingly, use has recently been made of excimer laser light in the ultraviolet and far-ultraviolet regions as illuminating light for exposure, such as KrF excimer laser light (wavelength: 248 nm) or ArF excimer laser light (wavelength: 193 nm). The use of metal vapor laser light, higher harmonics of YAG laser light, etc. has also been examined. 
     When excimer laser light is used as illuminating light for exposure, for example, broadened-band laser light sources and narrowed-band laser light sources are available as excimer laser light sources. The term “narrowed-band laser light source” means a laser light source in which the spectral half-width of laser light is not more than 2 pm to 3 pm. The term “broadened-band laser light source” means a laser light source in which the spectral half-width of laser light is not less than 100 pm. When illuminating light of a short wavelength in the ultraviolet region or shorter wavelength region, such as excimer laser light, is used, vitreous materials usable for refracting lenses of projection optical systems are limited to such materials as quartz and fluorite. Therefore, as the wavelength of illuminating light used shortens as described above, it becomes more difficult to achromatize the projection optical system. Accordingly, it is desirable to use a narrowed-band laser light source in order to facilitate achromatization of the projection optical system. 
     However, the band of excimer laser light is originally broad. Therefore, in narrowed-band laser light sources, the oscillation spectral width of excimer laser light is narrowed by injection locking or the like. For this reason, the laser output of narrowed-band laser light sources is lower than that of broadened-band laser light sources. Further, narrowed-band laser light sources are inferior to broadened-band laser light sources in terms of lifetime and production cost. Therefore, in terms of the laser output, lifetime and production cost, broadened-band laser light sources are more advantageous than narrowed-band laser light sources. Accordingly, attempts have recently been made to use a broadened-band laser light source in a projection exposure apparatus having a projection optical system structured such that achromatization can be readily achieved. 
     Incidentally, projection optical systems usable in scanning exposure type projection exposure apparatuses (scanning projection exposure apparatuses) such as step-and-scan type projection exposure apparatuses include a catadioptric system that uses a concave mirror, and a refracting optical system formed from a combination of refracting lenses only, as disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. 6-132191. When such a catadioptric system is used, achromatization can be readily achieved by disposing a concave mirror in a group of refracting lenses because concave mirrors are free from chromatic aberrations. Consequently, it becomes possible to use a broadened-band laser light source, which is advantageous in terms of laser output, lifetime, etc. 
     Even in the case of using the second-mentioned refracting optical system, it is possible to use a broadened-band laser light source because the range of achromatization can be widened by increasing the proportion of fluorite in the entire refracting lens system. 
     In the above-described prior art, when the second-mentioned refracting optical system is used as a projection optical system, it is necessary to use fluorite for ten-odd lens elements in a total of twenty-odd lens elements, for example, in order to achieve achromatization over a wavelength width of the order of 100 pm to use a broadened-band laser light source. However, fluorite has the following properties: it is difficult to machine; the yield after the machining is unfavorably low; the change of refractive index with temperature is large; and the coefficient of thermal expansion is high, so that deformation occurs to a considerable extent in response to changes in temperature. Tharefore, if many fluorite lenses are used, the temperature dependence of the image-forming characteristics of the projection optical system becomes unfavorably high. 
     When the first-mentioned catadioptric system is used as a projection optical system, it is possible to achieve achromatization over a wavelength width of the order of 100 pm by disposing a concave mirror in a predetermined position in a group of a predetermined number of refracting lenses, for example, because concave mirrors are free from chromatic aberrations. However, it is necessary in a scanning projection exposure apparatus to set the demagnification ratio for a pattern transferred from a reticle to a wafer on the order of from ¼ to ⅕, for example, and if a concave mirror is merely disposed in such a group of refracting lenses, the range in which favorable image-forming characteristics can be obtained becomes an arcuate area. Since the pattern areas on reticles have a rectangular external shape, if scanning exposure is carried out with such an arcuate area, the reticle scanning distance must be set to be considerably greater than the width of the pattern area. This causes the reticle-side stage to increase in size, unfavorably. 
     Further, when such an arcuate area is used, it is also necessary to use a lens that is not axially symmetric, and it is not easy to machine a non-axially symmetric lens with the desired accuracy. 
     Moreover, when a catadioptric system is used as a projection optical system, the projection optical system becomes large in size and complicated in arrangement because of routing of the image-forming light beam. Therefore, it is desirable to form the whole projection exposure apparatus in as compact a structure as possible by taking into consideration the reticle scanning direction, etc. 
     Further, it has been pointed out that when ultraviolet light such as excimer laser light is used as illuminating light for exposure, it is necessary to circulate nitrogen (N 2 ) gas or a gas (e.g. air) having ozone removed therefrom in the projection exposure apparatus in consideration for the absorption of ultraviolet light by ozone and also the properties of photoresist. However, if all the gas in a chamber in which the projection exposure apparatus is installed is merely replaced by nitrogen gas or the like, for example, a problem arises in terms of the safety of workmen during maintenance or the like. 
     Further, it is necessary in a projection exposure apparatus to control the amount of exposure light applied to a wafer according to the sensitivity, etc. of a photoresist used. In this regard, with a method wherein the amount of ultraviolet light emitted as pulsed light, such as excimer laser light, is reduced through an ND filter plate, for example, the ND filter plate may be damaged by intense pulsed light. Further, it is desirable that the amount of ultraviolet light should be capable of being controlled continuously and accurately. However, with the method wherein the amount of ultraviolet light is controlled by using a light-reducing plate such as an ND filter, the amount of light cannot always be continuously set, depending upon the positioning accuracy of the light-reducing plate. 
     Furthermore, in the conventional step-and-scan type projection exposure apparatus, the projection optical system and the reticle-side stage are secured to a column stood on a surface plate to which a stage (wafer stage) for holding a wafer is secured. Accordingly, when the reticle and wafer are synchronously scanned during exposure, vibrations of the stages affect the projection optical system, which should essentially be stationary. This may degrade the image-forming characteristics. 
     In view of the above-described circumstances, a first object of the present invention is to provide a scanning projection exposure apparatus capable of using as a projection optical system a catadioptric system formed from a combination of refracting lenses which are all axially symmetric and a reflecting optical member, and also capable of obtaining favorable image-forming characteristics. 
     A second object of the present invention is to provide a projection exposure apparatus designed so that the whole structure of the apparatus can be made compact even when it uses a projection optical system consisting essentially of the above-described catadioptric system. 
     A third object of the present invention is to provide a projection exposure apparatus designed so that during exposure, the absorption of illuminating light for exposure is minimized, and during maintenance, the safety of workmen can be ensured. 
     A fourth object of the present invention is to provide a projection exposure apparatus using ultraviolet light as illuminating light for exposure and capable of accurately controlling the amount of ultraviolet light for exposure at all times. 
     A fifth object of the present invention is to provide a projection exposure apparatus capable of obtaining favorable image-forming characteristics independently of vibrations caused by the synchronous scanning of a reticle and a wafer. 
     SUMMARY OF THE INVENTION 
     A projection exposure apparatus according to the present invention includes a light source; a mask having a transfer pattern; a projection optical system that projects an image of a part of the transfer pattern onto a photosensitive substrate by light from the light source; and a device that transfers the transfer pattern on the mask onto the photosensitive substrate by synchronously scanning the mask and the photosensitive substrate relative to the projection optical system. The projection optical system includes a first optical system having a concave mirror and is arranged to reflect and condense a light beam from the mask such that the light beam returns toward the mask; a second optical system that directs the light beam, which is returned toward the mask by the first optical system, toward the photosensitive substrate; and a third optical system that forms an image of a part of the transfer pattern of the mask on the photosensitive substrate by the light beam from the second optical system. 
     The first and third optical systems may be disposed along the scanning direction of the mask and the photosensitive substrate, and the center of gravity of the projection optical system may lie outside the optical path of the image-forming light beam passing through the first and third optical systems. 
     The projection exposure apparatus may further include an off-axis alignment sensor for detecting the position of a mark for alignment on the photosensitive substrate. The alignment sensor may have an optical system arranged such that an optical axis of the optical system is substantially parallel to the optical axis of the third optical system and a predetermined distance away from the optical axis of the third optical system in the scanning direction of the photosensitive substrate. 
     The arrangement may be such that the optical axes of the first and third optical systems are parallel to each other, and a surface position detecting system is disposed to lie in a direction perpendicularly intersecting a plane containing the optical axes of the first and third optical systems. The surface position detecting system includes an irradiation optical system that applies a light beam obliquely to the surface of the photosensitive substrate, and a light-receiving optical system that photoelectrically converts the light beam reflected from the photosensitive substrate to obtain a photoelectrically converted signal. The surface position detecting system detects a displacement of the surface of the photosensitive substrate on the basis of the photoelectrically converted signal from the light-receiving optical system. 
     The arrangement may be such that the second optical system has a partially transmitting mirror that directs the light beam from the first optical system toward the photosensitive substrate and that transmits a part of a light beam from the photosensitive substrate, and that a photoelectric detector is provided to receive that part of the light beam from the photosensitive substrate which passes through the partially transmitting mirror. 
     The photoelectric detector may be a light-receiving element that photoelectrically converts reflected light from the photosensitive substrate. 
     The photoelectric detector may be an image pickup device that picks up an image of a fiducial mark in the vicinity of the photosensitive substrate. 
     The projection optical system may be an image-forming optical system that forms an intermediate image of a part of the transfer pattern of the mask between the mask and the photosensitive substrate. A device for correcting image-forming characteristics of the projection optical system may be provided in the vicinity of a position where the intermediate image is formed. 
     A projection exposure apparatus according to another aspect of the present invention includes an illumination optical system that illuminates a mask having a transfer pattern formed thereon by illuminating light from a light source; a projection optical system that projects an image of the pattern on the mask onto a photosensitive substrate under the illuminating light; a substrate stage that moves the photosensitive substrate; a plurality of casings that accommodate the illumination optical system, the projection optical system, and the substrate stage independently of each other; and a gas supply device that selectively supplies a plurality of different kinds of gas into at least one of the casings. 
     The plurality of different kinds of gas may be gases selected from the group consisting of nitrogen, air, and air having ozone removed therefrom. 
     The gas supply device may be provided with a device for confirming that a gas in a casing to be supplied with another gas has been substantially completely replaced by the substituting gas. 
     A projection exposure apparatus according to still another aspect of the present invention includes an illumination optical system that illuminates a mask having a transfer pattern formed thereon by illuminating light from a light source; a projection optical system that projects an image of the pattern on the mask onto a photosensitive substrate under the illuminating light; a casing having light-transmitting windows disposed in the optical path of the illuminating light between the light source and the photosensitive substrate; and a gas controller that supplies into the casing a gas in which the amount of ozone per unit volume is variable; wherein the illuminance of illuminating light emitted from the light source and applied to the photosensitive substrate is controlled by changing the amount of ozone in the gas supplied into the casing from the gas controller. 
     A projection exposure apparatus according to a further aspect of the present invention includes a light source; a mask having a transfer pattern; a projection optical system that projects an image of a part of the transfer pattern onto a photosensitive substrate by light from the light source; a device having a mask-side stage that scans the mask relative to the projection optical system, and a substrate-side stage that scan the photosensitive substrate relative to the projection optical system in synchronism with the mask-side stage, so that the transfer pattern on the mask is transferred onto the photosensitive substrate by synchronously scanning the mask and the photosensitive substrate relative to the projection optical system; an antivibration plate to which a movable part that moves synchronously with scanning exposure is secured; and another antivibration plate to which a stationary part that is not synchronous with scanning exposure is secured. 
     The projection exposure apparatus may be provided with an interferometer to measure the position of the mask-side stage or the substrate-side stage. The interferometer has a moving mirror mounted on the mask-side stage or the substrate-side stage; a reference mirror mounted on the antivibration plate to which the stationary part is secured; and an interferometer body part that detects a relative displacement between the moving mirror and the reference mirror by using a light beam. 
     A projection exposure method according to the present invention includes the step of preparing a projection optical system including at least three optical systems, and the step of projecting an image of the pattern on the above-described mask onto the above-described substrate after forming an intermediate image of the pattern in the projection optical system. 
     In addition, the present invention provides a method of controlling an environment of an exposure apparatus in which an image of a pattern on a mask is projected onto a substrate. The method includes the step of casing at least a part of constituent portions of the exposure apparatus, and the step of supplying a plurality of different kinds of gas into the cased constituent portion of the exposure apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows the arrangement of one embodiment of the scanning projection exposure apparatus according to the present invention. 
     FIG. 2 is a longitudinal sectional view showing the arrangement of a projection optical system in FIG.  1 . 
     FIGS.  3 ( a ),  3 ( b ),  3 ( c ) and  3 ( d ) show the relationship between an illumination area and an exposure area in the projection optical system shown in FIG.  2 . 
     FIG. 4 is a fragmentary sectional view showing the arrangement of an off-axis alignment sensor in FIG.  1 . 
     FIG. 5 is a fragmentary sectional view showing the arrangement of a multipoint AF sensor in FIG.  1 . 
     FIG. 6 is a plan view showing the relationship between a wafer-side laser interferometer and the projection optical system in FIG.  1 . 
     FIG.  7 ( a ) is a side view of a sample table in FIG. 6 as viewed in the direction +X, and 
     FIG.  7 ( b ) is a perspective view showing laser beams for three axes incident on a moving mirror. 
     FIG. 8 is a longitudinal sectional view showing the arrangement of a reflected light detecting system that receives reflected light from a fiducial pattern member in FIG.  1 . 
     FIG. 9 is a block diagram showing a unit-by-unit airconditioning system according to one embodiment of the present invention. 
     FIG.  10 ( a ) is a block diagram showing an essential part of a system for controlling the quantity of illuminating light according to one embodiment of the present invention, and 
     FIG.  10 ( b ) is a block diagram showing a modification of the light quantity control system. 
     FIG. 11 is a perspective view showing a structure for supporting a mechanical part of the projection exposure apparatus in FIG.  1 . 
     FIG. 12 is a partly-sectioned side view of the support structure shown in FIG. 11 as viewed in the direction −X. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     One embodiment of the projection exposure apparatus according to the present invention will be described below with reference to the accompanying. drawings. In this embodiment, the present invention is applied to a step-and-scan type projection exposure apparatus that uses an excimer laser light source as an exposure light source and also uses a catadioptric system as a projection optical system. 
     FIG. 1 schematically shows the arrangement of a projection exposure apparatus according to this embodiment. In FIG. 1, an exposure controller  1  controls the emission condition of an excimer laser light source  2 . The excimer laser light source  2  emits pulsed laser light used as illuminating light IL. The illuminating light IL is deflected by a deflection mirror  3  to reach a first illumination system  4 . In this embodiment, the excimer laser light source  2  is a broadened-band laser light source of KrF excimer laser (wavelength: 248 nm) in which the half-width of the oscillation spectrum is broadened to 100 pm or more. It should, however, be noted that a broadened-band laser light source of ArF excimer laser (wavelength: 193 nm) may also be used as a light source for exposure. It is also possible to use a metal vapor laser light source, a harmonics generator of YAG laser, or an emission-line lamp such as a mercury lamp. 
     The first illumination system  4  includes a beam expander; a light quantity varying mechanism; an illumination switching mechanism for switching the amounts of illuminating light from one to another when the coherence factor (so-called σ value) of the illumination optical system is changed; and a fly&#39;s-eye lens. The first illumination system  4  forms two-dimensionally distributed secondary light sources of illuminating light IL on an exit plane thereof. A switching revolver  5  for an illumination system aperture stop is disposed in the plane where the secondary light sources are formed. The switching revolver  5  is used to switch over various illumination conditions from one to another. The switching revolver  5  is provided in a side surface thereof with various aperture stops such as an ordinary circular aperture stop; an aperture stop for modified illumination which consists of a plurality of apertures decentered from the optical axis; an annular zone-shaped aperture stop; and an aperture stop for small σ value which consists of a relatively small circular aperture. Thus, a desired illumination system aperture stop (σ stop) can be disposed in the exit plane of the first illumination system  4  by revolving the switching revolver  5  through a switching device  6 . When illumination system aperture stops have been switched over from one to another as stated above, the illumination switching mechanism in the first illumination system  4  is synchronously switched through the switching device  6  so that the light quantity is maximized. 
     The operation of the switching device  6  is controlled by the exposure controller  1 . The operation of the exposure controller  1  is controlled by a main controller  7  that controls operations of the whole apparatus. 
     Illuminating light IL passing through an illumination system aperture stop set by the switching revolver  5  enters a beam splitter  8  having a high transmittance and a low reflectivity. Illuminating light reflected by the beam splitter  8  is received by an integrator sensor  9  consisting essentially of a photoelectric detector such as a photodiode. The integrator sensor  9  photoelectrically converts the received illuminating light to obtain a detection signal, which is supplied to the exposure controller  1 . The relationship between the detection signal and the amount of exposure light applied to the surface of a wafer has previously been measured and stored. The exposure controller  1  monitors the integral of the mount of exposure light applied to the surface of the wafer from the detection signal. The detection signal is also used to normalize output signals from various sensor systems that use the exposure illuminating light IL. 
     Illuminating light IL passing through the beam splitter  8  illuminates an illumination field stop system (reticle blind system)  11  through a second illumination system  10 . A plane where the illumination field stop system  11  is placed is conjugate with the entrance plane of the fly&#39;s-eye lens in the first illumination system  4 . The illumination field stop system  11  is illuminated by an illumination area approximately similar in sectional configuration to each lens element of the fly&#39;s-eye lens. The illumination field stop system  11  is divided into a movable blind and a fixed blind. The fixed blind is a fixed field stop having a rectangular aperture. The movable blind comprises two pairs of movable blades capable of being opened and closed and also capable of independently moving in a reticle scanning direction and a non-scanning direction, respectively. The fixed blind determines a configuration of an illumination area on a reticle, and the movable blind performs an operation of gradually opening the cover of the aperture of the fixed blind at the time of starting scanning exposure and also performs an operation of gradually closing the cover at the time of terminating the scanning exposure. Thus, areas on a wafer other than a proper shot area to be exposed are prevented from being illuminated by illuminating light. 
     The operation of the movable blind in the illumination field stop system  11  is controlled by a driver  12 . When synchronous scanning of a reticle and a wafer is carried out by a stage controller  13  as described later, the stage controller  13  synchronously drives the movable blind that is movable in the scanning direction through the driver  12 . Illuminating light IL passing through the illumination field stop system  11  passes through a third illumination system  14  and illuminates a rectangular illumination area  15  on a pattern surface (lower surface) of a reticle R with a uniform illuminance distribution. A plane where the fixed blind of the illumination field stop system  11  is placed is conjugate with the pattern surface of the reticle R. The configuration of the illumination area  15  is determined by the aperture of the fixed blind. 
     In the following description, an X-axis is taken in a direction perpendicular to the plane of FIG. 1 in a plane parallel to the pattern surface of the reticle R; a Y-axis is taken in a direction parallel to the plane of FIG. 1; and a Z-axis is taken in a direction perpendicular to the pattern surface of the reticle R. Under these conditions, the illumination area  15  on the reticle R is a rectangular area longer in the direction X. During scanning exposure, the reticle R is scanned in the direction +Y or −Y relative to the illumination area  15 . In other words, the scanning direction is set in the direction Y. 
     An image of a pattern in the illumination area  15  on the reticle R is projected onto an exposure area  16  on a photoresist-coated surface of a wafer W as a demagnified image through a projection optical system PL with a projection magnification β (β is ¼ or ⅕, for example) which is telecentric on both sides (or on the wafer side only). 
     The reticle R is held on a reticle stage  17 . The reticle stage  17  is mounted through an air bearing on a guide extending in the direction Y on a reticle support  18 . The reticle stage  17  is capable of scanning across the reticle support  18  in the direction Y at a constant speed by the operation of a linear motor, and has an adjusting mechanism capable of adjusting the position of the reticle R in the directions X and Y and also in the direction of rotation (direction O—). The position of the reticle stage  17  (reticle R) in each of the directions X and Y is constantly measured at a resolution of the order of 0.001 μm by a combination of a moving mirror  19   m  secured to an end portion of the reticle stage  17  and a laser interferometer (only the laser interferometer for the Y-axis is shown in the figure)  19  secured to a column (not shown). The angle of rotation of the reticle stage  17  is also measured by the combination of the moving mirror  19   m  and the laser interferometer  19 . The measured values are supplied to the stage controller  13 . The stage controller  13  controls the operation of the linear motor, etc. on the reticle support  18  according to the measured values supplied thereto. 
     The wafer W is held through a wafer holder  20  on a sample table  21 . The sample table  21  is mounted on a wafer stage  22 . The wafer stage  22  is mounted through an air bearing on a guide extending over a surface plate  23 . The wafer stage  22  is arranged so as to be capable of scanning at a constant speed and stepping in the direction Y and also stepping in the direction X over the surface plate  23  by the operation of a linear motor. The wafer stage  22  has a Z-stage mechanism and a tilt mechanism (leveling mechanism) incorporated therein. The Z-stage mechanism causes the sample table  21  to move in the direction Z within a predetermined range. The tilt mechanism adjusts the tilt angle of the sample table  21 . 
     The position of the sample table  21  (wafer W) in each of the directions X and Y is constantly measured at a resolution of the order of 0.001 μm by a combination of a moving mirror  24   m  secured to a side of the sample table  21  and a laser interferometer (only the laser interferometer for the Y-axis is shown in the figure)  24  secured to a column (not shown). The rotation angle and tilt angle of the sample table  21  are also measured by the combination of the moving mirror  24   m  and the laser interferometer  24 . The measured values are supplied to the stage controller  13 . The stage controller  13  controls the operation of the linear motor, etc. for driving the wafer stage  22  according to the measured values supplied thereto. 
     To carry out scanning exposure, the main controller  7  sends an exposure start command to the stage controller  13 . In response to the command, the stage controller  13  causes the reticle R to be scanned in the direction Y at a speed V R  through the reticle stage  17  and also causes the wafer W to be scanned in the direction Y at a speed V W  through the wafer stage  22  synchronously with the scanning of the reticle R. The scanning speed V W  of the wafer W is set at β. V R  by using the projection magnification β for the pattern transfer from the reticle R to the wafer W. 
     The projection optical system PL is held in a U-shaped column  25  stood on the surface plate  23 . An oblique incidence multipoint autofocus sensor (hereinafter referred to as “AF sensor”)  26  is disposed on that side of the projection optical system PL which faces in the direction X. The AF sensor  26  obliquely projects a slit image or the like onto a plurality of measuring points on the surface of the wafer W and outputs a plurality of focus signals corresponding to positions (focus positions) of the wafer surface in the direction Z at the measuring points. The focus signals from the multipoint AF sensor  26  are supplied to a focus-tilt controller  27 . The focus-tilt controller  27  obtains a focus position and tilt angle of the surface of the wafer W from the focus signals and supplies the results thus obtained to the stage controller  13 . 
     The stage controller  13  drives the Z-stage mechanism and tilt mechanism in the wafer stage  22  by a servo system such that the supplied focus position and tilt angle coincide with the focus position and tilt angle, respectively, of the image-formation plane of the projection optical system PL, which have previously been obtained. Thus, the surface of the wafer W within the exposure area  16  is controlled by autofocus and autoleveling systems such that the wafer surface coincides with the image-formation plane of the projection optical system PL even during scanning exposure. 
     Further, an off-axis alignment sensor  28  is secured to that side of the projection optical system PL which faces in the direction +Y. During alignment, the alignment sensor  28  detects the position of a wafer mark for alignment attached to each shot area on the wafer W, and supplies a detection signal to an alignment signal processor  29 . The alignment signal processor  29  is also supplied with the measured values from the laser interferometer  24 . The alignment signal processor  29  calculates coordinates-in a stage coordinate system (X,Y) of a wafer mark to be detected from the detection signal and the measured values from the laser interferometer  24 , and supplies the calculated coordinates to the main controller  7 . The term “stage coordinate system (X,Y)” means a coordinate system determined on the basis of the X- and Y-coordinates of the sample table  21  measured by the laser interferometer  24 . The main controller  7  obtains array coordinates in the stage coordinate system (X,Y) of each shot area on the wafer W from the supplied coordinates of the associated wafer mark, and supplies the array coordinates to the stage controller  13 . The stage controller  13  controls the position of the wafer stage  22  when scanning exposure is carried out for each shot area on the basis of the array coordinates supplied thereto. 
     A fiducial mark member FM is secured to the surface of the sample table  21 . The fiducial mark member FM is provided on the surface thereof with various fiducial marks serving as position references for the alignment sensor and a reference reflecting surface serving as a reference of the reflectivity of the wafer W. A reflected light detecting system  30  is mounted on an upper end portion of the projection optical system PL to detect a light beam reflected from the wafer W through the projection optical system PL. A detection signal from the reflected light detecting system  30  is supplied to a self-measuring device  31 . Under the control of the main controller  7 , the self-measuring device  31  performs monitoring of the amount of reflected light from the wafer W (reflectivity of the wafer surface), measurement of illuminance nonuniformity, and measurement of a spatial image, as described later. 
     Next, the arrangement of the projection optical system PL according to this embodiment, shown in FIG. 1, will be described in detail with reference to FIG.  2 . 
     FIG. 2 is a sectional view showing the projection optical system PL. In FIG. 2, the projection optical system PL mechanically consists essentially of four portions, i.e. a first objective portion  41 , an optical axis turn portion  43 , an optical axis deflecting portion  46 , and a second objective portion  52 . A concave mirror  45  is disposed in the optical axis turn portion  43 . 
     The use of broadened-band laser light as illuminating light IL in this embodiment is advantageous in that the light quantity can be increased without increasing the electric power supplied and the throughput can be increased, and that coherency is reduced and thus adverse effects of interference can be reduced. However, when illuminating light in the ultraviolet region, such as KrF excimer laser light or ArF excimer laser light, is used as in this embodiment, vitreous materials usable for refracting lenses in the projection optical system PL are limited to such materials as quartz and fluorite. Therefore, it is difficult to design the projection optical system PL using only a refracting optical system. For this reason, in this embodiment, broad-band achromatization is effected by jointly using a refracting optical system and a reflecting optical system such as a concave mirror, in which no chromatic aberration is produced. However, a general reflecting optical is a 1:1 optical system (with a magnification ratio of 1:1). Therefore, when reduction projection with a demagnification ratio of ¼ or ⅕ is performed as in this embodiment, it is necessary to devise a special scheme for the arrangement of the optical system as stated below. First, the first objective portion  41  is disposed directly below the reticle R. The first objective portion  41  includes lenses L 1 , L 2 , L 3  and L 4  in order from the reticle R side. The lenses L 1 , L 2 , L 3  and L 4  are secured in a lens barrel  42  through lens frames. Below the lens barrel  42 , a lens barrel  44  of the optical axis turn portion  43  is disposed through a lens barrel  47  of the optical axis deflecting portion  46 . The optical axis turn portion  43  includes, in order from the reticle R side, lenses L 1 , L 12 , . . . , L 20  and L 21 , and a concave mirror  45 , which are secured in the lens barrel  44  through lens frames. The first objective portion  41  and the optical axis turn portion  43  are coaxial with respect to each other. Let us assume the common axis to be an optical axis AX 1 . The optical axis AX 1  is perpendicular to the pattern surface of the reticle R. 
     A small-sized mirror  48  is disposed in the lens barrel  47  of the optical axis deflecting portion  46  between the lens barrels  42  and  44  at a position decentered in the direction +Y from the optical axis AX 1 . The small-sized mirror  48  has a reflecting surface tilted in the direction +Y at approximately 45° to the optical axis AX 1 . In the lens barrel  47 , lenses L 31  and L 32 , a correcting optical system  49 , and a beam splitter  50  are disposed in order from the small-sized mirror  48  in the direction +Y. The optical axis AX 2  of the optical axis deflecting portion  46  perpendicularly intersects the optical axis AX 1 . The beam splitter  50  has a reflecting surface tilted at approximately 45° to the optical axis AX 2  so that the reflecting surface perpendicularly intersects the reflecting surface of the small-sized mirror  48 . The beam splitter  50  has a transmittance of 5% and a high reflectivity of the order of 95%. A method of utilizing a light beam passing through the beam splitter  50  will be described later. The correcting optical system  49  includes a lens unit capable of slightly moving in a direction parallel to the optical axis AX 2 . Moreover, the tilt angle of the lens unit with respect to a plane perpendicular to the optical axis AX 2  can be finely adjusted. The position and tilt angle of the correcting optical system  49  are controlled by an image-forming characteristic correcting device  51 . The operation of the image-forming characteristic correcting device  51  is controlled by the main controller  7 , shown in FIG.  1 . The position where the correcting optical system  49  is disposed is approximately conjugate with the pattern surface of the reticle R. The correcting optical system  49  can correct mainly distortions such as a magnification error. A lens barrel  53  of the second objective portion  52  is disposed in contact with the lens barrel  47  to extend in a direction in which the optical axis AX 2  bent by the beam splitter  50  extends. In the lens barrel  53 , lenses L 41 , L 42 , L 43 , . . . , L 52  are disposed in order from the beam splitter  50  side through lens frames. The bottom surface of the lens L 52  faces the surface of the wafer W. The optical axis AX 3  of the second objective portion  52  is parallel to the common optical axis AX 1  of the first objective portion  41  and the optical axis turn portion  43  and perpendicular to the optical axis AX 2  of the optical axis deflecting portion  46 . 
     In this case, the rectangular illumination area  15  on the reticle R, illuminated by the illuminating light IL, is set at a position decentered in the direction −Y from the optical axis AX 1 . Illuminating light passing through the illumination area  15  (the light will hereinafter be referred to as “image-forming light beam”) passes through the lenses L 1 , L 2 , . . . , L 4  in the first objective portion  41  and further passes through the inside of the lens barrel  47  of the optical axis deflecting portion  46  to enter the optical axis turn portion  43 . The image-forming light beam entering the optical axis turn portion  43  passes through the lenses L 11 , L 12 , . . . , L 20  and L 21  to be incident on the concave mirror  45 . The image-forming light beam reflected and condensed by the concave mirror  45  passes through the lenses L 21 , L 20 , . . . , L 12  and L 11  and is then deflected to travel in the direction +Y by the small-sized mirror  48  in the lens barrel  47  of the optical axis deflecting portion  46 . 
     In the optical axis deflecting portion  46 , the image-forming light beam reflected by the small-sized mirror  48  passes through the lenses L 31  and L 32  and the correcting optical system  49  to enter the beam splitter  50 . At this time, an image (intermediate image) approximately equal in size to the pattern in the illumination area  15  on the reticle R is formed near the beam splitter  50  in the lens barrel  47 . Accordingly, a composite system formed from the first objective portion  41  and the optical axis turn portion  43  is referred to as “1:1 optical system”. The image-forming light beam deflected to travel in the direction −Z by the beam splitter  50  enters the second objective portion  52 , in which the image-forming light beam passes through the lenses L 41 , L 42 , . . . , L 51  and L 52  to form in the exposure area  16  on the wafer W a demagnified image of the pattern in the illumination area  15  on the reticle R. Accordingly, the second objective portion  52  will be hereinafter occasionally referred to as “reduction projection system”. 
     As stated above, the image-forming light beam passing through the illumination area  15  on the reticle R approximately in the direction −Z is turned back approximately in the direction +Z through the first objective portion  41  and the optical axis turn portion  43  in the projection optical system PL according to this embodiment. The image-forming light beam is then successively turned approximately in the directions +Y and −Z by the optical axis deflecting portion  46 . During the course of being turned, the image-forming light beam forms an intermediate image approximately equal in size to the pattern in the illumination area  15 , and thereafter, forms a demagnified image of the illumination area  15  in the exposure area  16  on the wafer W through the second objective portion  52 . By virtue of this arrangement, the projection optical system PL according to this embodiment can use axially symmetric lenses for all the lenses L 2  to L 4 , L 11  to L 21 , L 31 , L 32 , and L 41  to L 52 . Moreover, achromatization can be achieved with high accuracy over the range of about 100 μm, which is the bandwidth of the broadened-band illuminating light IL, by forming almost all the lenses from quartz and forming only three or four of the lenses from fluorite. 
     The projection optical system PL according to this embodiment is optically divided into three parts, i.e. the 1:1 optical system consisting essentially of the first objective portion  41  and the optical axis turn portion  43 ; the optical axis deflecting portion  46 ; and the reduction projection system consisting essentially of the second objective portion  52 . From the viewpoint of a mechanical structure, the small-sized mirror  48  is interposed between the lens L 4  of the first objective portion  41  and the lens L 11  of the optical axis turn portion  43 . Therefore, if the lens L 4 , the small-sized mirror  48  and the lens L 11  are incorporated into the same lens barrel, the small-sized mirror  48  and the beam splitter  50  in the optical axis deflecting portion  46  will be necessary to incorporate into respective lens barrels from the viewpoint of adjustment. However, if the small-sized mirror  48  and the beam splitter  50  are incorporated into different lens barrels, it is likely that perpendicularity between the reflecting surfaces of the two members will vary. A variation in the perpendicularity between the two reflecting surfaces causes the image-forming characteristics to be degraded. In this embodiment, therefore, the 1:1 optical system is divided into the first objective portion  41  and the optical axis turn portion  43  through the lens barrel  47  of the optical axis deflecting portion  46 , and the small-sized mirror  48  and the beam splitter  50  are secured in the lens barrel  47 . 
     To assemble the projection optical system PL, the first objective portion  41 , the optical axis turn portion  43 , the optical axis deflecting portion  46 , and the second objective portion  52  are assembled and adjusted separately in advance. Thereafter, the lower part of the lens barrel  44  of the optical axis turn portion  43  and the lower part of the lens barrel  53  of the second objective portion  52  are inserted into respective through-holes provided in a plate of the column  25 . Then, washers are put between a flange  44   a  of the lens barrel  44  and the plate of the column  25  and between a flange  53   a  of the lens barrel  53  and the plate of the column  25 , and the flanges  44   a  and  53   a  are temporarily screwed to the plate. Then, the lens barrel  47  of the optical axis deflecting portion  46  is placed on the upper ends of the lens barrels  44  and  53 . A washer is put between a flange  47   a  of the lens barrel  47  and a flange  53   b  at the upper end of the lens barrel  53 , and the flange  47   a  is temporarily screwed to the flange  53   b.    
     Then, a laser beam for adjustment is applied to the inside of the lens barrel  44  from above the lens L 11  in the lens barrel  44  to monitor a position passed by the laser beam emanating from the lowermost lens L 52  in the lens barrel  53  (i.e. a position on a plane corresponding to the surface of the wafer W), and the thicknesses of the washers under the bottoms of the flanges  44   a ,  53   a  and  47   a  are adjusted and the lens barrels  42 ,  53  and  47  are transversely moved so that the monitored position coincides with the target position. In a state where the position of the laser beam reaches the target position, the flanges  44   a ,  53   a  and  47   a  are properly screwed, thereby securing the optical axis turn portion  43 , the second objective portion  52 , and the optical axis deflecting portion  46 . Finally, the lens barrel  42  of the first objective portion  41  is moved to a position above that end of the lens barrel  47  which faces in the direction −Y. A washer is put between a flange (not shown) of the lens barrel  42  and the corresponding flange (not shown) of the lens barrel  47 , and the lens barrel  42  is placed on the lens barrel  47 . Then, a laser beam for adjustment is applied from above the lens L 1  in the lens barrel  42 , for example, to thereby perform optical axis adjustment. Thereafter, the lens barrel  42  is screwed onto the lens barrel  47 . Thus, the incorporation of the projection optical system PL into the projection exposure apparatus is completed. 
     Further, in this embodiment, the center  54  of gravity of the whole projection optical system PL is set at a position in the projection optical system PL and outside the optical path of the image-forming light beam by taking into consideration the stability of image-forming characteristics with respect to vibrations and the balance of the projection optical system PL. That is, in FIG. 2, the center  54  of gravity of the projection optical system PL is set a position (in the plate of the column  25 ) near the middle between the optical axis turn portion  43  and the second objective portion  52  and slightly lower than the flange  44   a  of the lens barrel  44  and the flange  53   a  of the lens barrel  53 . Thus, by setting the center  54 . of gravity of the projection optical system PL closer to the flanges  44   a  and  53   a , the projection optical system PL has a structure that is not readily affected by vibrations-and exhibits high rigidity. 
     In the projection optical system PL according to this embodiment, an intermediate image plane conjugate. with the pattern surface of the reticle R exists in the optical axis deflecting portion  46  and near the beam splitter  50 , and the correcting optical system  49  is disposed near the intermediate image plane. It is possible to correct image-forming characteristics such as the projection magnification and distortion of the demagnified image of the reticle, which is projected onto the wafer W, by slightly moving, for example, a lens unit serving as the correcting optical system  49  in the direction of the optical axis AX 2  or adjusting the tilt angle of the lens unit with respect to a plane perpendicular to the optical axis AX 2 . In the conventional apparatus, such an image-forming characteristic correcting mechanism has been provided approximately directly below the reticle R. In this embodiment, there is no image-forming characteristic correcting mechanism directly below the reticle R; there is no restriction on the mechanical arrangement. Therefore, the rigidity of the reticle support  18 , shown in FIG. 1, can be advantageously increased in design. If a slightly movable optical system similar to the correcting optical system  49  is provided in the optical axis turn portion  43  or the second objective portion  52 , it becomes possible to correct aberrations (e.g. astigmatism and coma) of the projected image and also possible to correct curvature of field. It is also possible to correct higher-order magnification errors by a combination of these optical systems. 
     Next, the positional relationship between the illumination area  15  on the reticle R and the exposure area  16  on the wafer W in FIG. 2 will be described with reference to FIGS.  3 ( a ) to  3 ( d ). 
     FIG.  3 ( a ) shows the illumination area  15  on the reticle R in FIG.  2 . In FIG.  3 ( a ), a rectangular illumination area  15 , which is longer in the direction X, is set in a circular effective illumination field  41   a  of the first objective portion  41  of the projection optical system PL, shown in FIG. 2, at a position slightly displaced in the direction −Y relative to the optical axis AX 1 . The direction (direction Y) of the short side of the illumination area  15  is the scanning direction of the reticle R. In the 1:1 optical system consisting essentially of the first objective portion  41  and the optical axis turn portion  43 , as shown in FIG. 2, the image-forming light beam passing through the illumination area  15  on the reticle R is turned back by the concave mirror  45  so as to be led to the small-sized mirror  48 . Therefore, the illumination area  15  must be decentered with respect to the optical axis AX 1 . 
     FIG.  3 ( b ) shows the exposure area  16  (conjugate with the illumination area  15 ) on the wafer W in FIG.  2 . In FIG.  3 ( b ), a rectangular exposure area  16 , which is longer in the direction X, is set in a circular effective exposure field  52   a  of the second objective portion  52  (reduction projection system) of the projection optical system PL, shown in FIG. 2, at a position slightly displaced in the direction +Y relative to the optical axis AX 3 . 
     FIG.  3 ( c ) shows a rectangular illumination area  15  set in a circular effective illumination field  41   a  at a position slightly displaced in the direction −Y relative to the optical axis AX 1  as in the case of FIG.  3 ( a ). FIG.  3 ( d ) shows an effective exposure field  52   a A of a second objective portion formed by modifying the second objective portion  52  in FIG.  2 . In FIG.  3 ( d ), a rectangular exposure area  16 A [conjugate with the illumination area  15  in FIG.  3 ( c )], which is longer in the direction X, is set centered on the optical axis AX 3 A of the effective exposure field  52   a A. More specifically, as shown in FIG.  3 ( d ), the exposure area  16 A on the wafer W can be set as an area centered on the optical axis of the effective exposure field  52   a A by modifying the arrangement of the second objective portion  52  (reduction projection system), which is the final stage in the projection optical system PL. The arrangements shown in FIGS.  3 ( b ) and  3 ( d ) may be selected according to the ease of designing an arrangement for removing aberrations from the projection optical system PL. The arrangement shown in FIG.  3 ( b ) is advantageous in that the design is easy. The arrangement shown in FIG.  3 ( d ) is advantageous in that the lens diameter of the second objective portion (reduction projection system) can be slightly reduced. 
     Next, the arrangement of the off-axis alignment sensor  28  shown in FIG. 1 will be described in detail with reference to FIG.  4 . 
     FIG. 4 shows an external appearance of the projection optical system PL in FIG.  2 . As shown in FIG. 4, the projection optical system PL is divided into the first objective portion  41 , the optical axis deflecting portion  46 , the optical axis turn portion  43 , and the second objective portion  52 , which are necessary to design such that these portions will not be distorted by disturbances such as vibrations and heat, which may occur therebetween. Therefore, the column  25  on which the flanges  44   a  and  53   a  are placed requires high rigidity, particularly for a portion  25   a  of the column  25  that extends between the optical axis turn portion  43  and the second objective portion  52 . In order to ensure such high rigidity, the alignment sensor  28  for detecting the position of a wafer mark WM provided on the wafer W as an alignment mark must be installed on that side of the second objective portion  52  which is remote from the portion  25   a , which requires high rigidity, that is, on the +Y side of the second objective portion  52 . A portion  25   b  of the column  25  that faces the +Y side and +X and −X sides of the second objective portion  52  has a thickness not more than ½ of that of the high-rigidity portion  25   a . The alignment sensor  28  is disposed at the bottom of the thin portion  25   b . By virtue of this arrangement, even when the reticle R and the wafer W are scanned in the direction of the arrow (direction Y) during scanning exposure, the first objective portion  41 , the optical axis deflecting portion  46 , the optical axis turn portion  43 , and the second objective portion  52  are supported as an integral structure constituting the projection optical system PL. Thus, satisfactorily high rigidity can be ensured for the projection optical system PL. 
     In the off-axis alignment sensor  28  shown in FIG. 4, when alignment is to be effected, a halogen lamp (not shown) or the like emits broad-band (white) alignment light AL to which the photoresist on the wafer W is only slightly sensitive, and the alignment light AL is led through a light guide  62  to a lens barrel  61  of the alignment sensor  28 . In the lens barrel  61 , the alignment light AL passes successively through a condenser lens  63  and a half-mirror  64 . Thereafter, the alignment light AL passes through a first objective lens  65  and is reflected by a prism-type deflection mirror  66  to illuminate an observation field over a predetermined range on the wafer W that contains the wafer mark WM. Reflected light from the wafer mark WM passes via the deflection mirror  66  and through the first objective lens  65  and is reflected by the half-mirror  64 . Thereafter, the reflected light passes through a second objective lens  67  to form an image of the wafer mark WM on an index board  68 . The index board  68  has an index mark formed thereon as a reference for the position detection of the wafer mark WM. 
     A light beam passing through the index board  68  passes through a first relay lens  69  and is reflected by a deflection mirror  70 . The reflected light beam passes through a second relay lens  71  to form an image of the wafer mark WM, together with an image of the index mark, on an image pickup device  72 , which is a two-dimensional CCD. The wafer mark WM is, for example, a grating-shaped mark for the Y-axis comprising recesses and projections alternately arranged in the direction Y at a predetermined pitch. An image signal generated from the image pickup device  72  is supplied to the alignment signal processor  29 , shown in FIG.  1 . The alignment signal processor  29  calculates an amount of positional displacement in the direction Y of the wafer mark WM relative to the index mark on the index board  68  on the basis of the image signal. The alignment signal processor  29  adds the Y-coordinate measured by the laser interferometer  24 , shown in FIG. 1, to the calculated amount of positional displacement to obtain a Y-coordinate of the wafer mark WM in the stage coordinate system (X,Y) and supplies the calculated Y-coordinate to the main controller  7 . It should be noted that the shot area concerned on the wafer W is also provided with a wafer mark for the X-axis which has a configuration obtained by rotating the wafer mark WM through 90°, and that the alignment sensor  28  also detects the X-coordinate of the wafer mark for the X-axis in the stage coordinate system (X,Y). Thus, alignment of the wafer W is effected by detecting the coordinates of the wafer marks attached to a predetermined shot area on the wafer W through the alignment sensor  28 . 
     It is desirable in order to carry out the measurement by the alignment sensor  28  with high accuracy to minimize the distance (baseline quantity) between the center of detection by the alignment sensor  28  (i.e. the center of the projected image of the index mark onto the wafer W) and the center of exposure by the projection optical system PL (i.e. the center of the exposure area  16 ). To minimize the baseline quantity, the alignment sensor  28  is disposed as close to the second objective portion  52  of the projection optical system PL as possible. 
     However, in addition to the off-axis alignment sensor  28 , the multipoint AF sensor  26  for detecting the focus position and tilt angle of the surface of the wafer W, which is shown in FIG. 1, also needs to be disposed close to the second objective portion  52 . In this embodiment, therefore, the AF sensor  26  is disposed on the +_X sides of the second objective portion  52  to extend perpendicular to the alignment sensor  28  in order to prevent mechanical interference between the alignment sensor  28  and the AF sensor  26 . 
     FIG. 5 shows the way in which the AF sensor  26  is arranged. FIG. 5 is a sectional view taken along a plane (XZ-plane) containing the optical axis AX 3  of the second objective portion  52 , shown in FIG. 4, and perpendicular to the Y-axis. For the convenience of explanation, the upper half of FIG. 5 is a side view of the reticle R and the first objective portion  41  as viewed from the left side thereof in FIG.  4 . In FIG. 5, the AF sensor  26  is divided into an irradiation optical system  26   a  and a condenser optical system  26   b . The irradiation optical system  26   a  and the condenser optical system  26   b  are disposed on the respective sides of the second objective portion  52  that face in the directions −X and +X, respectively, and at the bottom of the portion  25   b  of the column  25 , shown in FIG. 4, which is thinner than the portion  25   a  of high rigidity. 
     First, in the irradiation optical system  26   a , a halogen lamp (not shown) or the like emits approximately white illuminating light to which the photoresist is only slightly sensitive. The illuminating light is led to a side of the second objective portion  52  through a light guide  81 . Then, the illuminating light is reflected by a mirror  82  and passes through a condenser lens  83  to illuminate a multi-slit plate  84  having a plurality of slit-shaped apertures formed therein in a predetermined array pattern. Illuminating light passing through each slit-shaped aperture of the multi-slit plate  84  passes through a lens  85  and is reflected by an oscillating mirror  86 , and the reflected light passes through a lens  87  to project a plurality of slit images (one slit image  88  is representatively shown in FIG.  5 ), which are conjugate images of the plurality of slit-shaped apertures, onto the surface of the wafer W obliquely to the optical axis AX 3 . It should be noted that an area where the slit images are projected extends over from the rectangular exposure area  16  on the wafer W, shown in FIG. 1, to a pre-read area which is upstream the exposure area  16  in the scanning direction. 
     Reflected light from the plurality of slit images on the wafer W enters the condenser optical system  26   b . In the condenser optical system  26   b , the reflected light passes through a lens  89  and is reflected by a mirror  90 . Then, the reflected light passes through a lens  91  to reform the slit images ( 88  and so forth) on a multi-slit plate  92  having a plurality of slit-shaped apertures corresponding to those of the multi-slit plate  84 . A photoelectric detector  93  is disposed-on the reverse surface of the multi-slit plate  92 . The photoelectric detector  93  consists essentially of photoelectric transducers, e.g. photodiodes, that independently receive reflected light passing through the respective slit-shaped apertures of the multi-slit plate  92 . A photoelectrically converted signal (hereinafter referred to as “focus signal”) obtained from each photoelectric transducer of the photoelectric detector  93  is supplied to the focus-tilt controller  27 . 
     In this case, in response to the oscillation of the oscillating mirror  86 , the slit images formed on the multi-slit plate  92  oscillate over the corresponding slit-shaped apertures in the direction of the short side of the apertures. Moreover, if the focus position (position in the direction Z) of the surface of the wafer W changes, the center of oscillation of each slit image and the center of the corresponding slit-shaped aperture are transversely displaced relative to each other. Therefore, the focus signal, which is obtained by photoelectrically converting the reflected light passing through each slit-shaped aperture, is synchronously rectified by a driving signal of the oscillating mirror  86  in the focus-tilt controller  27 , thereby obtaining a signal corresponding to an amount of change of the focus position at the position of projection of each slit image ( 88 , etc.) on the wafer W. Further, the AF sensor  26  has previously been calibrated so that when the surface of the wafer W is coincident with the image-formation plane of the projection optical system PL, the synchronously rectified signals of the focus signals are zero. Accordingly, the focus-tilt controller  27  can obtain a mean value of the focus positions in the exposure area  16  and the pre-read area on the wafer W and also a tilt angle from the synchronously rectified signals. Information concerning the mean value of the focus positions and information concerning the tilt angle are supplied to the stage controller  13  through the main controller  7  substantially in real time. The stage controller  13  effects autofocusing and autoleveling such that the exposure area  16  on the wafer W coincides with the image-formation plate of the projection optical system PL during scanning exposure, as stated above. 
     In FIG. 1, laser interferometers and moving mirrors, which are actually two-dimensionally arranged, are represented by one laser interferometer  24  and one moving mirror  24   m , respectively. Next, one example of a specific arrangement of wafer-side laser interferometers and associated moving mirrors according to this embodiment will be described with reference to FIGS. 6,  7 ( a ) and  7 ( b ). 
     FIG. 6 is a plan view of the sample table  21  on which the wafer W is placed, as shown in FIG.  1 . For convenience of explanation, in FIG. 6, the external shape of the second objective portion  52  of the projection optical system PL, shown in FIG. 2, and an observation field  28   a  of the alignment sensor  28 , shown in FIG. 4, are shown with an accurate positional relationship to the external shapes of the first objective portion  41  and the optical axis turn portion  43  and the reticle R. Further, in FIG. 6, the system is in a state where the optical axis AX 3  of the second objective portion  52  lies in the fiducial mark member FM on the sample table  21 . 
     In this embodiment, as shown in FIG. 2, that portion of the column  25  which extends between, on the one hand, the first objective portion  41  and the optical axis turn portion  43  of the projection optical system PL and, on the other, the second objective portion  52  has a firm structure in order to enhance rigidity. Accordingly, it is difficult to dispose laser interferometers between these two sections of the projection optical system PL. Even if laser interferometers can be disposed therebetween, there will be no sufficient space for performing down-flow air conditioning along the optical axes of the laser interferometers, and the resulting structure will be such that the laser interferometers are readily affected by the fluctuation of air. 
     To avoid the influence of the air fluctuation, in this embodiment, laser interferometers are disposed, as shown in FIG. 6, at a side of the projection optical system PL opposite to the optical axis turn portion  43  with respect to the second objective portion  52 , that is, at respective positions in the directions +Y and −X relative to the second objective portion  52 . In FIG. 6, a moving mirror  24   m Y is secured to that side of the sample table  21  which faces in the direction +Y. The moving mirror  24   m Y has a reflecting surface perpendicular to the Y-axis. A moving mirror  24   m X is secured to that side of the sample table  21  which faces in the direction −X. The moving mirror  24   m X has a reflecting surface perpendicularly intersecting the reflecting surface of the moving mirror  24   m Y (i.e. perpendicular to the X-axis). A laser interferometer  24 Y for the Y-axis is disposed to face the moving mirror  24   m Y for the Y-axis. The laser interferometer  24 Y applies laser beams for three axes to the moving mirror  24   m Y in parallel to the Y-axis. Of the laser beams for three axes, laser beams LBY 1  and LBY 2  for two axes are shown in FIG.  6 . The two laser beams LBY 1  and LBY 2  are spaced apart from each other in the direction X at a predetermined interval. The two laser beams LBY 1  and LBY 2  pass through respective positions symmetric about an imaginary straight line intersecting the optical axis AX 3  of the second objective portion  52  and the center of the observation field  28   a  of the alignment sensor  28  and parallel to the Y-axis. 
     FIG.  7 ( a ) is a side view of the sample table  21  as viewed in the direction +X in FIG.  6 . As shown in FIG.  7 ( a ), a laser beam LBY 3  for a third axis is applied from the laser interferometer  24 Y to the moving mirror  24   m Y in parallel to the Y-axis at a predetermined distance in the direction Z from the two laser beams LBY 1  and LBY 2 . As shown in FIG.  7 ( b ), the laser beam LBY 3  passes through an intermediate position between the two laser beams LBY 1  and LBY 2  in the direction X. The laser interferometer  24 Y constantly detects Y-coordinates Y 1 , Y 2  and Y 3  respectively corresponding to the three laser beams LBY 1 , LBY 2  and LBY 3  at a resolution of the order of 0.0001 μm, and outputs the detected Y-coordinates Y 1 , Y 2  and Y 3  to the stage controller  13 . The stage controller  13  obtains a mean value of and a difference between the two Y-coordinates Y 1  and Y 2 , for example, as the Y-coordinate and rotation angle (yawing), respectively, of the sample table  21 . It should be noted that during the measurement of the rotation angle, the moving mirror  24   m Y is corrected for bend. 
     In FIG. 6, a laser interferometer  24 X for the X-axis is disposed to face the moving mirror  24   m X for the X-axis. The laser interferometer  24 X applies laser beams for three axes to the moving mirror  24   m X in parallel to the X-axis. Of the laser beams for three axes, laser beams LBX 1  and LBX 2  for two axes are shown in FIG.  6 . The two laser beams LBX 1  and LBX 2  are spaced apart from each other in the direction Y at a predetermined interval. The two laser beams LBX 1  and LBX 2  pass through respective optical paths. That is, the laser beam LBX 1  passes through an optical path intersecting the optical axis AX 3  of the second objective portion  52  and parallel to the X-axis, and the laser beam LBX 2  passes through an optical path intersecting the center of the observation field  28   a  of the alignment sensor  28  and parallel to the X-axis. 
     A laser beam for a third axis is applied from the laser interferometer  24 X to the moving mirror  24   m X in parallel to the X-axis at a predetermined distance in the direction Z from the two laser beams LBX 1  and LBX 2  as in the case of FIGS.  7 ( a ) and  7 ( b ). The laser interferometer  24 X constantly detects X-coordinates X 1  and X 2  respectively corresponding to the two laser beams LBX 1  and LBX 2  and an X-coordinate X 3  corresponding to the other axis at a resolution of the order of 0.0001 μm, and outputs the detected X-coordinates X 1 , X 2  and X 3  to the stage controller  13 . During exposure, the stage controller  13  uses the X-coordinate X 1  corresponding to the optical axis AX 3  as the X-coordinate of the sample table  21 . During alignment, the stage controller  13  uses the X-coordinate X 2  corresponding to the center of the observation field  28   a  as the X-coordinate of the sample table  21 . Thus, during both exposure and alignment, the so-called Abbe error due to a positional displacement between the object to be measured and the axis of measurement becomes substantially zero, and the position detection can be effected with high accuracy. It should be noted that the moving mirror  24   m X can be corrected for bend on the basis of the two X-coordinates X 1  and X 2  as in the case of the moving mirror  24   m Y for the Y-axis. 
     Thus, in this embodiment, the laser interferometers  24 Y and  24 X are, as shown in FIG. 6, disposed at respective positions in the directions +Y and −X with respect to the sample table  21 . Moreover, the projection optical system PL, which consists essentially of the optical axis turn portion  43 , the second objective portion  52 , etc., is disposed along the direction −Y with respect to the sample table  21 . Consequently, it is possible to utilize a space in the direction +X (which is in symmetric relation to the laser interferometer  24 X) with respect to the sample table  21 . In this embodiment, therefore, a wafer transfer system including a wafer loader WL, etc. for loading and unloading a wafer onto and from the sample table  21  is disposed at a position in the direction +X with respect to the sample table  21 . 
     By virtue of this arrangement, it is possible to effect down-flow air conditioning for the optical paths of laser beams from the laser interferometers  24 X and  24 Y. More specifically, down-flow air conditioning is performed, for example, by supplying air or other gas of uniform velocity distribution, which has been subjected to temperature control so as to have a uniform temperature distribution, to the floor surface, where the projection exposure apparatus is installed, from above the laser beams LBY 1 , LBY 2 , LBX 1  and LBX 2  and collecting the supplied air at the floor surface. By doing so, at the laser interferometers  24 X and  24 Y, the influence of air fluctuation in the laser beam optical paths is minimized, and the accuracy of measurement of the position and rotation angle of the sample table  21  increases, advantageously. 
     In FIG.  7 ( a ), the sample table  21  is formed from a ceramic material, and the moving mirror  24   m Y (the same is the case with  24   m X) is also formed from the same ceramic material as that of the sample table  21 . The moving mirror  24   m Y is secured to a side of the sample table  21  through fixing screws (not shown). Meanwhile, the wafer W is held through the wafer holder  20  on the sample table  21 . Therefore, the position of the wafer W is displaced in the direction Z relative to the position of the optical axes of the laser beams LBY 1  and LBY 2  from the laser interferometer  24 Y. Accordingly, if the sample table  21  pitches, a positional displacement ΔY occurs between the Y-coordinate measured by the laser interferometer  24 Y and the actual Y-coordinate of the sample table  21  (more precisely, the wafer W) owing to the so-called Abbe error. In this embodiment, therefore, the laser beam LBY 3 , which passes through a position displaced in the direction Z relative to the laser beams LBY 1  and LBY 2 , is used to calculate a tilt angle Δθ of the sample table  21  about the X-axis from a difference between a mean value of the Y-coordinates measured by the laser beams LBY 1  and LBY 2  [i.e. (Y 1 +Y 2 )/2] and the Y-coordinate Y 3  measured by the laser beam LBY 3 . Then, the mean value of the Y-coordinates measured by the laser beams LBY 1  and LBY 2  is corrected on the basis of the tilt angle Δθ, thereby correcting the Abbe error due to the difference in height between the wafer W and the laser beams LBY 1  and LBY 2 . 
     Regarding the laser interferometer  24 X for the X-axis also, the Abbe error mixed in the measured value by the laser beams LBX 1  and LBX 2  is corrected by using the measured value obtained by the laser beam for the third axis. 
     By adopting the method wherein the moving mirrors  24   m Y and  24   m X are mounted on two sides of the sample table  21  as stated above, the space above the moving mirrors  24   m Y and  24   m X can be effectively used to dispose, for example, the end portion of the wafer holder  20 , as shown in FIG.  6 . As a result, the overall size and weight of the sample table  21  can be reduced. Therefore, controllability during scanning and positioning the wafer W improves. 
     Further, complicated machining of ceramics takes a great deal of time, and the production cost becomes extremely high. In this regard, according to this embodiment, the moving mirrors  24   m Y and  24   m X and the sample table  21 , which require high surface accuracy, are produced separately despite the same material and thereafter joined together, thereby simplifying the configuration of each individual component, and thus reducing the overall production cost. In a case where the temperature control is difficult to effect, a material of low thermal expansion coefficient, e.g. Zerodure, may be used in place of ceramics, although rigidity somewhat lowers. 
     Next, the operations of the reflected light detecting system  30  and the self-measuring device  31 , shown in FIG. 1, will be described with reference to FIGS. 2 and 8. 
     In FIG. 2, illuminating light IL passing through the illumination area  15  on the reticle R is applied to the wafer W through the 1:1 optical system, which consists essentially of the first objective portion  41  and the optical axis-turn portion  43 , and further through the optical axis deflecting portion  46  and the second objective portion  52  (reduction projection system). If the fiducial mark member FM is set in place of the wafer W, reflected light from the fiducial mark member FM passes, in FIG. 2, through the second objective portion  52  to enter the beam splitter  50  in the optical axis deflecting portion  46 . The beam splitter  50  has a transmittance of 5%; therefore, a part of the reflected light passes through the beam splitter  50  and is detected by the reflected light detecting system  30 , shown in FIG.  1 . In FIG. 2, illustration of the reflected light detecting system  30  is omitted. 
     FIG. 8 is a sectional view of the projection optical system PL taken along a plane containing the optical axis AX 3  and perpendicular to the X-axis in a state where the reflected light detecting system  30 , shown in FIG. 1, is disposed above the optical axis deflecting portion  46 , shown in FIG.  2 . In FIG. 8, the reflected light detecting system  30  consists essentially of a first reflected light detecting system  30   a  and a second reflected light detecting system  30   b . The fiducial mark member FM is set in the effective exposure field of the second objective portion  52  of the projection optical system PL. The fiducial mark member FM has fiducial patterns, e.g. slits or pinholes, formed by light-transmitting apertures in a film (e.g. a metal film) of high reflectivity provided on the fiducial mark member FM. The arrangement is such that a desired fiducial pattern on the fiducial mark member FM can be moved to the vicinity of a predetermined position in the effective exposure field by driving the wafer stage  22 , shown in FIG.  1 . 
     After the desired fiducial pattern has been set in the vicinity of the predetermined position, illuminating light passing through the reticle R, shown in FIG. 2, is applied to the surface of the fiducial mark member FM through the projection optical system PL, and reflected light CL and BL are produced from respective regions on the fiducial mark member FM, which are approximately symmetric about the optical axis AX 3  to travel toward the second objective portion  52 . 
     One reflected light CL passes through the lenses L 52  to L 41  in the second objective portion  52  to enter the beam splitter  50  in the optical axis deflecting portion  46 . Reflected light CL passing through a position C 1  on the beam splitter  50  passes through an aperture in the lens barrel  47  to enter the first reflected light detecting system  30   a . In the first reflected light detecting system  30   a , the reflected light CL is reflected by a deflection mirror  101 A and passes through a first relay lens  102 A to enter a half-mirror  103 A. A light beam reflected by the half-mirror  103 A enters a light-receiving surface of a pupil position photoelectric detector  104 A consisting essentially of a photodiode or the like, and a detection signal from the pupil position photoelectric detector  104 A is supplied to the self-measuring device  31 . 
     In this embodiment, under the reflected light CL, an intermediate image of the fiducial pattern is formed in the vicinity of the beam splitter  50 , and a Fourier transform pattern of the intermediate image is produced by the first relay lens  102 A and formed on the light-receiving surface of the pupil position photoelectric detector  104 A. That is, the light-receiving surface of the pupil position photoelectric detector  104 A is a Fourier transform plane (pupil plane) with respect to the surface of the fiducial mark member FM. The self-measuring device  31  detects a positional relationship between the pattern on the reticle R, shown in FIG. 2, and the fiducial pattern on the fiducial mark member FM by utilizing the fact that the detection signal from the pupil position photoelectric detector  104 A changes as the reticle R and the fiducial mark member FM are moved relative to each other. 
     Meanwhile, a light beam passing through the half-mirror  103 A passes through a second relay lens  105 A to form an image of the fiducial pattern on an imaging surface of an image pickup device  106 A consisting essentially of a two-dimensional CCD or the like. That is, the imaging surface of the image pickup device  106 A is conjugate with the surface of the fiducial mark member FM. An image signal from the image pickup device  106 A is supplied to the self-measuring device  31 . The self-measuring device  31  detects a positional relationship between the pattern on the reticle R and the fiducial pattern on the basis of the image signal in a state where the reticle R and the fiducial mark member FM are stationary. The arrangement is such that both the first relay lens  102 A and the deflection mirror  101 A are movable to a position above an arbitrary measuring point in the area (illumination field) corresponding to the effective exposure field of the second objective portion  52 , and thus it is possible to capture a detection signal from the pupil position photoelectric detector  104 A and an image signal from the image pickup device  106 A at the arbitrary measuring point. 
     The second reflected light detecting system  30   b  is disposed above a position C 2  on the beam splitter  50  of the optical axis deflecting portion  46  in symmetric relation to the first reflected light detecting system  30   a . The second reflected light detecting system  30   b  includes a deflection mirror  101 B, a first relay lens  102 B, a half-mirror  103 B, a pupil position photoelectric detector  104 B, a second relay lens  105 B, and an image pickup device  106 B. The second reflected light detecting system  30   b  is arranged to be capable of receiving the reflected light BL generated from the surface of the fiducial mark member FM in approximately symmetric relation to the reflected light CL. In actuality, the first relay lens  102 B and the deflection mirror  101 B in the second reflected light detecting system  30   b  are also arranged to be movable independently of the first reflected light detecting system  30   a . Thus, it is possible to carry out measurement at two arbitrary points in the illumination field corresponding to the effective exposure field of the second objective portion  52 . 
     In actual measurement, if the reticle R is of the total transmission type, only reflected light from the periphery of a fiducial pattern (aperture pattern) on the fiducial mark member FM is detected in the reflected light detecting systems  30   a  and  30   b  through the second objective portion  52  and the beam splitter  50 . Therefore, it is possible to check a light intensity distribution over the fiducial pattern. 
     In a case where the reticle R has a predetermined pattern formed thereon, an amount of light returning to the reflected light detecting systems  30   a  and  30   b  is determined by superposition of the projected image of the reticle pattern onto the fiducial mark member FM and the fiducial pattern. Accordingly, a positional relationship between the pattern on the reticle R and the fiducial pattern on the fiducial mark member FM can be checked by receiving the reflected light passing through the beam splitter  50  with the reflected light detecting systems  30   a  and  30   b.    
     The self-measuring device  31  processes the detection signals and image signals from the reflected light detecting systems  30   a  and  30   b , thereby effecting various operations, i.e. aligning the reticle R and the fiducial mark member FM; checking the image-forming characteristics of the projection optical system-PL on the fiducial mark member FM; monitoring the reflectivity of the wafer W based on the amount of light reflected by the fiducial mark member FM; and detecting illuminance nonuniformity in the exposure area  16  on the wafer W. In the conventional apparatus, an illumination system and a light-receiving system are disposed in the sample table  21  in FIG. 1, for example, to effect measurement of image-forming characteristics, etc. Therefore, it is possible according to this embodiment to simplify the internal and top structures of the sample table  21 , to reduce the weight of the sample table  21  and to prevent generation of heat due to irradiation with illuminating light. Further, the conventional apparatus also uses a detection system that detects the reflectivity of a wafer by receiving reflected light from the wafer at the reticle R. In this regard, this embodiment also makes it possible to simplify the mechanism associated with the reticle R. 
     In the conventional apparatus, detecting light for measuring image-forming characteristics, etc. and illuminating light for exposure are different from each other; therefore, the measured image-forming characteristics may differ from image-forming characteristics under the illuminating light for exposure owing to a mismatch between the numerical aperture (σ value as a coherence factor) of the detecting light and the numerical aperture (σ value) of the illuminating light for exposure. The conventional apparatus further suffers from the problem of the insufficient numerical aperture of a light-receiving system for measuring image-forming characteristics, etc. According to this embodiment, illuminating light IL for exposure is also used as detecting light as it is, and the reflected light detecting systems  30   a  and  30   b  can be disposed in a space sufficiently wide to increase the numerical aperture. Therefore, image-forming characteristics, etc. can be measured with high accuracy. 
     Although the off-axis alignment sensor  28  according to the foregoing embodiment is of the field image alignment (FIA) type sensor, it should be noted that it is also possible to use a laser step alignment (LSA) type sensor a in which a slit-shaped laser beam and a dot train-shaped wafer mark are scanned relative to each other, or a laser interferometric alignment (LIA) type sensor in which a diffraction grating-shaped wafer mark is illuminated by two coherent light beams, and position detection is performed on the basis of the phase of light resulting from an interference between a pair of diffracted light generated in the same direction from the wafer mark. Although in the foregoing embodiment the image-forming characteristics are corrected by driving an optical system in the projection optical system PL, it is also possible to use a mechanism for varying the pressure or temperature of a gas between a predetermined pair of lenses in the projection optical system PL to correct the image-forming characteristics. 
     Although in the foregoing embodiment the illumination area  15  on the reticle R is rectangular, it may be arcuate. However, in a case where the shape of the illumination area  15  is a rectangle substantially inscribed in the effective illumination field as in this embodiment, the amount of scanning of the reticle R can be reduced advantageously because the pattern area on the reticle R is also rectangular. 
     According to the above-described projection exposure apparatus, the projection optical system has a first optical system having a concave mirror to return a light beam toward a mask; a second optical system that deflects the returned light beam toward a photosensitive substrate; and a third optical system that forms on the photosensitive substrate an image of a part of a transfer pattern formed on the mask by the light beam from the second optical system. Accordingly, it is possible to use as the projection optical system a catadioptric system formed from a combination of refracting lenses which are all axially symmetric and a reflecting optical member (e.g. a concave mirror). These refracting lenses can be readily machined with high accuracy and can be achromatized over a relatively wide wavelength width by jointly using a reflecting optical member. Thus, favorable image-forming characteristics can be obtained. 
     Further, because it is possible to use a broadened-band laser light source of large output, for example, as a light source for exposure, the lifetime of the exposure light source increases, and the throughput can be increased. 
     It is also possible to shorten the scanning distance of the mask because a range in which favorable image-forming characteristics are available can be readily formed into an approximately rectangular area. 
     The arrangement may be such that the first and third optical systems are disposed along the scanning direction of the mask and the photosensitive substrate, and the center of gravity of the projection optical system lies outside the optical path of an image-forming light beam passing through the first and third optical systems. In this case, the projection optical system can be stably supported, and thus the image-forming characteristics are stabilized. Further, it is possible to enhance the rigidity of the projection optical system and to improve the vibration characteristics. 
     The projection exposure apparatus may further include an off-axis alignment sensor for detecting the position of an alignment mark on the photosensitive substrate. The alignment sensor may be arranged such that the optical axis of its optical system is substantially parallel to the optical axis of the third optical system and a predetermined distance away from the optical axis of the third optical system in the scanning direction of the photosensitive substrate. In this case, if a projection optical system consisting essentially of a catadioptric system as described above is used, the overall structure of the projection exposure apparatus can be made compact. 
     The arrangement may be such that the optical axes of the first and third optical systems are parallel to each other, and a surface position detecting system is disposed to lie in a direction perpendicularly intersecting a plane containing the optical axes of the first and third optical systems. The surface position detecting system includes an irradiation optical system that applies a light beam obliquely to the surface of the photosensitive substrate, and a light-receiving optical system that photoelectrically converts the light beam reflected from the photosensitive substrate to obtain a photoelectrically converted signal. The surface position detecting system detects a displacement of the surface of the photosensitive substrate on the basis of the photoelectrically converted signal from the light-receiving optical system. In this case, the constituent elements of the projection exposure apparatus can be arranged in a compact form without a mechanical interference between the surface position detecting system and other sensors. 
     The arrangement may be such that the second optical system has a partially transmitting mirror that directs the light beam from the first optical system toward the photosensitive substrate and that transmits a part of a light beam from the photosensitive substrate, and that a photoelectric detector is provided to receive that part of the light beam from the photosensitive substrate which passes through the partially transmitting mirror. In this case, it is possible to measure image-forming characteristics, etc. of the projection optical system without providing a detecting system in the vicinity of the mask or on the photosensitive substrate-side stage or so. Accordingly, it is possible to simplify the structures of the mask- and photosensitive substrate-side stages and reduce the weights thereof. Thus, controllability during scanning exposure can be improved. 
     If the photoelectric detector is a light-receiving element that photoelectrically converts reflected light from the photosensitive substrate, it is possible to monitor the reflectivity, etc. of the photosensitive substrate. 
     The photoelectric detector may be an image pickup device that picks up an image of a fiducial mark in the vicinity of the photosensitive substrate. In this case, it is possible to monitor the image-forming characteristics, etc. of the projection optical system. 
     The projection optical system may be an image-forming optical system that forms an intermediate image of a part of the transfer pattern of the mask between the mask and the photosensitive substrate, and a device for correcting image-forming characteristics of the projection optical system may be provided in the vicinity of a position where the intermediate image is formed. In this case, distortion, etc. can be readily corrected because the position of the correcting device is close to the intermediate image plane. Moreover, the rigidity of the mask-side stage can be enhanced because it becomes unnecessary to provide an image-forming characteristic correcting device directly under the mask. 
     Next, an air conditioning system of the projection exposure apparatus according to the present invention will be described with reference to FIG.  9 . The projection exposure apparatus according to this embodiment is installed in a given chamber as a whole. The projection exposure apparatus is divided into a plurality of units, and air conditioning is independently carried out for each unit. Such an air conditioning system will hereinafter be referred to as “unit-by-unit air conditioning system”. 
     FIG. 9 shows a unit-by-unit air conditioning system according to this embodiment. In FIG. 9, the projection exposure apparatus shown in FIG. 1 is roughly divided into an illumination system unit  111 , a reticle stage system unit  112 , a projection optical system unit  113 , a wafer stage system unit  114 , and a wafer transfer system unit  115 . More specifically, the illumination system unit  111  has a box-shaped casing accommodating the illumination optical system, shown in FIG. 1, which includes the excimer laser light source  2 , the deflection mirror  3 , the first illumination system  4 , the switching revolver  5 , the beam splitter  8 , the integrator sensor  9 , the second illumination system  10 , the illumination field stop system  11 , and the third illumination system  14 . The reticle stage system unit  112  has a box-shaped casing accommodating the reticle support  18 , the reticle stage  17  (including the moving mirror  19   m ), the reticle R, the optical path between the reticle R and the third illumination system  14 , and the optical path between the reticle R and the projection optical system PL, which are shown in FIG.  1 . 
     The projection optical system unit  113  is the projection optical system PL itself, which is shown in FIG.  2 . An integral structure comprising the lens barrels  42 ,  44 ,  47  and  53  of the projection optical system PL is regarded as a casing, and the projection optical system unit  113  is arranged to be capable of controlling the flow of a gas between the lens units in this casing. The wafer stage system unit  114  has a box-shaped casing installed on the surface plate  23 , shown in FIG. 1, to accommodate the wafer stage  22 , the sample table  21  (including the moving mirror  24   m  and the fiducial mark member FM), the wafer holder  20 , the wafer W, and the space portion between the projection optical system PL and the wafer W. The wafer transfer system unit  115  has a box-shaped casing accommodating the wafer transfer system, which is not shown in FIG.  1 . The unit-by-unit air conditioning system according to this embodiment is arranged to be capable of supplying and discharging a predetermined gas for each of the illumination system unit  111 , the reticle stage system unit  112 , the projection optical system unit  113 , the wafer stage system unit  114 , and the wafer transfer system unit  115  independently of each other. 
     For this purpose, the unit-by-unit air conditioning system is provided with a first air conditioner  116  incorporating a dust-removing filer and an ozone-removing filter, and a second air conditioner  117  for circulating nitrogen (N 2 ) supplied from a nitrogen gas cylinder (not shown). The first air conditioner  116  removes, through the dust-removing filter, dust, etc. from air taken in from the outside of the chamber and air returned through a piping  118 B, controls the temperature and flow rate of air having ozone removed therefrom through the ozone-removing filter, and supplies the ozone-free air, which has been subjected to the temperature and flow rate control, to a gas changeover device  120 A through a piping  118 A. The second air conditioner  117  controls the temperature and flow rate of a high-purity portion of nitrogen gas returned through pipings  119 B and  133 B, and circulates the high-purity portion of nitrogen gas through pipings  119 A and  133 A. Moreover, the second air conditioner  117  releases the low-purity portion of nitrogen gas through a piping  136  into the atmospheric air outside a clean room in which the chamber is installed. Further, the second air conditioner  117  supplements a deficiency of nitrogen gas with nitrogen gas from the nitrogen gas cylinder, that is, carries out purging for the deficiency. 
     The gas changeover device  120 A supplies one of two different kinds of gas (i.e. ozone-free air prepared by the removal of ozone, and nitrogen gas) to a gas volume controller  122 A through a piping  121 A. The gas volume controller  122 A supplies the gas to the inside of the illumination. system unit  111  through a piping  123 A and also supplies the gas to a gas volume controller  125 A through a piping  124 A. The gas volume controllers  122 A and  125 A (the same is the case with other gas volume controllers) each have the function of supplying the gas supplied thereto after controlling the temperature and flow rate (volume) thereof. The gas volume controller  125 A supplies the gas to the inside of the reticle stage system unit  112  and a gas volume controller  128 A through respective pipings  126 A and  127 A. The gas volume controller  128 A supplies the gas to the inside of the wafer transfer system unit  115  through a piping  129 A and also supplies the gas to the inside of the wafer stage system unit  114  through a piping  130 A, a gas volume controller  131 A and a piping  132 A. 
     The gas circulating through the wafer transfer system unit  115  is discharged to a gas volume controller  128 B through a piping  129 B. The gas circulating through the wafer stage system unit  114  is discharged to the gas volume controller  128 B through a piping  132 B, a gas volume controller  131 B and a piping  130 B. The gas from the gas volume controller  128 B and the gas circulating through the reticle stage system unit  112  are discharged to a gas volume controller  125 B through respective pipings  127 B and  126 B. Similarly, the gas from the gas volume controller  125 B and the gas circulating through the illumination system unit  111  are discharged to a gas volume controller  122 B through respective pipings  124 B and  123 B. The gas discharged from the gas volume controller  122 B is supplied to a gas changeover device  120 B through a piping  121 B. When the supplied gas is air, the gas changeover device  120 B returns it to the first air conditioner  116  through the piping  118 B, whereas, when the supplied gas is nitrogen gas, the gas changeover device  120 B returns it to the second air conditioner  117  through the piping  119 B. Accordingly, either the ozone-free air or nitrogen gas is selectively supplied in common to the illumination system unit  111 , the reticle stage system unit  112 , the wafer stage system unit  114 , and the wafer transfer system unit  115 . 
     The second air conditioner  117  supplies nitrogen gas, whose temperature and flow rate have been controlled, to the projection optical system unit  113  through the piping  133 A, a gas volume controller  134 A and a piping  135 A. The nitrogen gas circulating through the projection optical system unit  113  is returned to the second air conditioner  117  through a piping  135 B, a gas volume controller  134 B and the piping  133 B. Accordingly, unlike the other units, the projection optical system unit  113  is constantly supplied with nitrogen gas only. The reason for this is that the projection optical system PL does not particularly need maintenance. That is, the projection optical system unit  113  is arranged to maintain high hermeticity with respect to the outside, and constantly supplied with nitrogen gas. 
     In each of the gas volume controllers  134 A and  134 B, a temperature sensor and a purity sensor for measuring a degree of purity of nitrogen gas are installed. When the measured purity becomes below a predetermined allowable level, the second air conditioner  117  discharges the low-purity portion of nitrogen gas to the outside through the piping  136 , and the deficiency of nitrogen gas is supplemented from the nitrogen gas cylinder. 
     In this embodiment, the excimer laser light source  2 , e.g. KrF excimer laser or ArF excimer laser, is used as an exposure light source in FIG.  1 . Regarding the absorptances of gases with respect to ArF excimer laser light, for example, ozone (O 3 ) shows the highest absorptance among the ordinary air components, and oxygen (O 2 ) as it changes into ozone shows the next highest absorptance. The absorption of ArF excimer laser light by nitrogen gas is so small as to be substantially negligible. Therefore, if a gas needs to be supplied through the. optical path of illuminating light IL for exposure, nitrogen gas should preferably be supplied. By doing so, exposure for the wafer W can be performed most efficiently (with the highest transmittance). 
     Therefore, when an ordinary exposure sequence is to be carried out, the gas changeover device  120 A in FIG. 9 supplies nitrogen gas from the second air conditioner  117  to the piping  121 A. Consequently, nitrogen gas is supplied in common to the illumination system unit  111 , the reticle stage system unit  112 , the wafer stage system unit  114 , and the wafer transfer system unit  115 . Thus, transfer exposure is carried out for the wafer at a high illumination efficiency. 
     During maintenance or experimental exposure, the casing of a unit other than the projection optical system unit  113  may be opened by workmen; therefore, nitrogen gas cannot be supplied to any unit other than the projection optical system unit  113  from a safety standpoint. For this reasons, during maintenance or the like, the gas changeover device  120 A in FIG. 9 supplies the ozone-free air from the first air conditioner  116  to the piping  121 A. Consequently, the ozone-free air is supplied in common to the illumination system unit  111 , the reticle stage system unit  112 , the wafer stage system unit  114 , and the wafer transfer system unit  115 . Thus, workmen can safely perform a given operation. Moreover, what is supplied to the units other than the projection optical system unit  113  is air from which ozone has already been removed, and the ozone-free air shows a low absorptance with respect to the illuminating light IL for exposure. Therefore, there is only a slight reduction in the illumination efficiency. 
     In FIG. 9, concentration sensors  137 A to  137 D for nitrogen gas are installed in the illumination system unit  111 , the reticle stage system unit  112 , the wafer stage system unit  114  and the wafer transfer system unit  115  near the respective exhaust openings. Results of detection by the concentration sensors  137 A to  137 D are supplied to the main controller  7 , shown in FIG.  1 . When a gas to be supplied to these units has been switched to ozone-free air to perform maintenance or the like, the main controller  7  stops indicating permission to start an operation or locks the cover of the chamber in a closed state until the nitrogen concentration detected by the concentration sensors  137 A to  137 D reaches the level of nitrogen concentration in ordinary air. Thus, the operation can be safely conducted. 
     The arrangement may be such that a timer is connected to the main controller  7 , and when a gas to be supplied to those units has been switched to ozone-free air to perform maintenance or the like, the main controller  7  gives the indication of permission to start an operation after a predetermined time has elapsed on the timer. 
     Since the absorptance varies according to the kind of gas in the optical path of illuminating light for exposure, the illuminance on the wafer surface for each gas has previously been stored in the main controller  7  as parameters, and when gases are switched from one to another, the parameters are changed correspondingly. 
     Although in the example shown in FIG. 9, a gas is supplied in parallel to the illumination system unit  111 , the reticle stage system unit  112 , the wafer stage system unit  114 , and the wafer transfer system unit  115 , the arrangement may be such that all or some of these units are connected in series by piping, and a selected gas is supplied in series to the connected units. With this arrangement, the piping arrangement is simplified. 
     Although ozone-free air can be obtained simply by passing air taken in from the outside (outside air) through the ozone-removing filter, nitrogen gas requires replacing the nitrogen gas cylinder when it is continuously used. Moreover, to perform maintenance after an exposure operation, nitrogen gas in the apparatus needs to be partially replaced by air. Therefore, the arrangement may be such that when the photoresist coated on the wafer requires a relatively large amount of exposure (i.e. the sensitivity is low), nitrogen gas is supplied to all the units, including the illumination system unit  111  through the wafer transfer system unit  115 , to lower the rate of absorption of illuminating light with a view to increasing the throughput, whereas, when the photoresist requires a relatively small amount of exposure (i.e. the sensitivity is high), ozone-free air is supplied because a reduction in the light quantity due to absorption has substantially no effect on the throughput. Thus, the throughput and the running cost can be optimized as a whole by selecting a gas to be used according to the sensitivity conditions. 
     When the photoresist requires a relatively small amount of exposure as in the latter case, air taken in from the outside (outside air) may be used as it is in place of ozone-free air. It is also possible to use, in place of nitrogen gas, another gas (e.g. an inert gas such as helium) showing a low absorptance with respect to KrF excimer laser light or ArF excimer laser light. 
     In this embodiment, the amount of illuminating light is controlled by utilizing the absorption of illuminating light by a gas. The light quantity control method according to this embodiment will be described below with reference to FIGS. 10 a  and  10   b.    
     FIG. 10 a  shows the excimer laser light source  2  and a part of the first illumination system  4  (illustration of the deflection mirror  3  is omitted), which are shown in FIG.  1 . In FIG.  10 ( a ), illuminating light IL from the excimer laser light source  2  passes through a container  141  which has light-transmitting windows provided in both sides thereof and which is supplied therein with a predetermined gas. In this embodiment, the container  141  is supplied with a gas (e.g. air) containing ozone (O 3 ) at a predetermined concentration through a piping (not shown). The container  141  is connected with a bellows mechanism  142 . The amount of extension and contraction of the bellows mechanism  142  is controlled by the exposure controller  1  through the switching device  6 , which are shown in FIG. 1, thereby enabling the pressure of the gas in the container  141 , that is, the absorptance with respect to the illuminating light IL, to be continuously controlled in a predetermined range. More specifically, when it is desired to reduce the amount of illuminating light IL, the pressure of the gas in the container  141  is increased to raise the absorptance of the gas. When the light quantity is desired to increase, the pressure of the gas in the container  141  is reduced as far as a level close to a vacuum. Consequently, the transmittance increases, and thus a high power can be obtained. This method enables the light quantity to be continuously varied and minimizes damage to the control mechanism in contrast to a case where an ND filter is used. 
     It should be noted that the gas in the container  141  and the bellows mechanism  142  may be replaced in a predetermined proportion at a time because it is likely that the absorption of illuminating light by ozone will be saturated. 
     FIG.  10 ( b ) shows an example of changing the concentration of ozone in the gas. In FIG.  10 ( b ), illuminating light IL emitted from the excimer laser light source  2  passes through the container  141 . In this example, the container  141  is supplied therein with a gas (e.g. air) wherein the ozone concentration is controlled in the range of from 0 to 100% through a concentration varying mechanism  143 . To increase the amount of illuminating light IL, the ozone concentration is reduced toward 0%, whereas to reduce the amount of illuminating light IL, the ozone concentration is increased toward 100%, thereby enabling the amount of illuminating light IL to be continuously controlled. When this system is used, the container  141  must be cooled because of heat generation by absorption of illuminating light. However, the system shown in FIG.  10 ( b ) is advantageous, as in the case of FIG.  10 ( a ), in that the light quantity can be continuously varied, and that damage to the control mechanism is minimized in contrast to a case where an ND filter is used. 
     It should be noted that in the systems shown in FIGS.  10 ( a ) and  10 ( b ), a gas that absorbs excimer laser light, e.g. oxygen (O 2 ), may be used in place of ozone. 
     Next, other support mechanisms of the projection exposure apparatus according to the present invention, together with the structure of a laser interferometer for measuring stage coordinates, will be described with reference to FIGS. 11 and 12. 
     FIG. 11 schematically shows the arrangement of a mechanical part of the projection exposure apparatus according to this embodiment. In FIG. 11, a surface plate  223  for wafer-stage is mounted over a large rectangular surface plate  232  through four vibration isolating mechanisms  233 A to  233 D (only  233 A,  233 B and  233 C are shown in FIG.  11 ). A four-legged first column  234  is stood on the surface plate  223 . 
     The wafer stage  22 , the sample table  21 , etc. (see FIG. 1) are mounted on the surface plate  223 , and the reticle stage  17  and the reticle R are mounted through the reticle support  18  on an upper plate  234   a  of the first column  234 . An L-shaped support member  234   b  projects from the upper surface of the upper plate  234   a  of the column  234 , and the movable blind of the illumination field stop system  11  in the illumination optical system, shown in FIG. 1, is secured to the distal end portion of the support member  234   b . That is, in this embodiment, all portions that move synchronously during scanning exposure are mounted directly or indirectly through the column  234  on the surface plate  223  supported by the vibration isolating mechanisms  233 A to  233 D. 
     A four-legged second column  225  is stood through four vibration isolating mechanisms  235 A to  235 D (only  235 A and  235 C are shown in FIG. 11) on the surface plate  232 . The vibration isolating mechanisms  235 A to  235 D are provided outside the vibration isolating mechanisms  233 A to  233 D. An intermediate plate  225   c  of the column  225  extends through the space between the upper plate  234   a  of the column  234  and the upper surface of the surface plate  223 . An upper plate  225   d  of the column  225  extends over the upper plate  234   a  of the column  234 . The vibration isolating mechanisms  233 A to  233 D and  235 A to  235 D are each active-type vibration isolating mechanism including an air pad and an electromagnetic damper for vibration damping. The projection optical system PL is secured in the intermediate plate  225   c  of the second column  225  (see FIG.  2 ). A casing  236  of the illumination optical system is mounted on the upper plate  225   d  of the column  225 . The illumination optical system including the excimer laser light source  2  through the third illumination system  14 , shown in FIG. 1, is secured on the upper plate  225   d  within the casing  236 . It should, however, be noted that only the movable blind of the illumination field stop system  11  in the illumination optical system is secured to the distal end portion of the support member  234   b  of the column  234 , which is inserted into the casing  236  through a window  236   a . In other words, portions that are stationary during scanning exposure are mounted on the column  225  supported by the vibration isolating mechanisms  235 A to  235 D. 
     In this embodiment, the vibration isolating mechanisms  235 A to  235 D, which support the stationary members, mainly effects control for damping vibrations from the floor. The vibration isolating mechanisms  233 A to  233 D, which support the members movable during scanning exposure, effects control for damping vibrations from the floor and for absorbing reaction of the reticle stage  17  and the wafer stage  22  during scanning exposure. In other words, the projection optical system PL and the illumination optical system in the casing  236  have no heavyweight member driven at a high acceleration. Therefore, the vibration isolating mechanisms  235 A to  235 D are each formed by combining together an air pad for eliminating vibrations due to high-frequency disturbance from the floor and an electromagnetic damper for eliminating low-frequency vibrations resulting therefrom. 
     The stage system, which includes the wafer stage  22  and the reticle stage  17 , includes heavyweight members driven at a high acceleration. Therefore, the vibration isolating mechanisms  233 A to  233 D are each provided with an electromagnetic damper having a sufficiently large power to prevent reaction of the heavyweight members. The wafer stage  22  and the reticle stage  17  are each supported with respect to the surface plate  223  directly or indirectly through an air guide and driven by a linear motor. Therefore, although the surface plate  223  itself vibrates in response to vibrations due to high-frequency disturbance from the floor, it is unlikely that the high-frequency disturbance will be transmitted to the stages  17  and  22 . Accordingly, the air pads in the vibration isolating mechanisms  233 A to  233 D need not be of particularly high accuracy. Thus, according to this embodiment, portions that move synchronously and portions that are stationary during scanning exposure are supported by different vibration isolating mechanisms. Therefore, the vibration isolating mechanisms can be optimized according to use application, and a pattern on a reticle can be transferred onto a wafer with high overlay accuracy. 
     In FIG. 1, the wafer-side laser interferometer  24  represents laser interferometers for a plurality of axes in each of the directions X and Y. Similarly, the reticle-side laser interferometer  19  represents laser interferometers for a plurality of axes in each of the directions X and Y. The moving mirrors  19   m  and  24   m  also each represent moving mirrors for the X- and Y-axes. FIG. 11 shows laser interferometers  19 Y 1  and  19 Y 2  for two axes in the Y-axis direction (scanning direction) in the reticle-side laser interferometer  19 , and a laser interferometer  24 Y 1  for one axis in the Y-axis direction in the wafer-side laser interferometer  24 , together with a reticle-side moving mirror  19   m Y for the Y-axis. 
     In FIG. 11, the wafer-side laser interferometer  24 Y 1  for the Y-axis is secured to one leg portion of the column  225 . The reticle-side laser interferometers  19 Y 1  and  19 Y 2  for the Y-axis are secured between the upper plate  225   d  and leg portions of the column  225 . Similarly, the other laser interferometers for the Y-axis are also secured to the column  225 . Thus, all the laser interferometers for the Y-axis, which is the scanning direction in this embodiment, are mounted on the column  225 , which supports stationary members. In FIG. 11, it seems difficult to mount the laser interferometers for the X-axis on the column  25 . The laser interferometers for the X-axis may be mounted on a frame or the like (not shown) provided on a side of the column  225 . 
     FIG. 12 is a partly-sectioned side view of the support structure shown in FIG. 11 as viewed in the direction −X. In FIG. 12, the wafer-side laser interferometer  24 Y 1  is secured in a leg portion of the column  225 . An external laser light source  145  emits a laser beam containing components which are different in frequency and whose polarization directions perpendicularly intersect each other. The laser beam is reflected successively by mirrors  146  and  147  to enter a polarization beam splitter (hereinafter referred to as “PBS”)  148  in the laser interferometer  24 Y 1 . At this time, a laser beam LB 1  of p-polarized light component passes through the PBS  148  and then passes through a quarter-wave plate  149  to travel toward the moving mirror  24   m Y for the Y-axis, which is mounted on a side of the sample table  21 . The laser beam LB 1  reflected by the moving mirror  24   m Y passes through the quarter-wave plate  149  and is reflected by the PBS  148  as s-polarized light. 
     Meanwhile, a laser beam LB 2  of s-polarized light component in the laser beam entering the PBS  148  passes through a quarter-wave plate  150  and is reflected by a reflecting surface  151   a  of a prism-type mirror  151 . Thereafter, the reflected laser beam LB 2  passes through the quarter-wave plate  150  and then passes through the PBS  148  as p-polarized light. The laser beams LB 1  and LB 2  having different frequencies are coaxially combined together by the PBS  148  and reflected by a prism-type mirror  152  to emanate from the laser interferometer  24 Y 1  to the outside thereof. The emanating laser beams LB 1  and LB 2  are reflected successively by mirrors  153  and  154  and received by a receiver  155 , e.g. a photodiode. In this case, the mirror  151  operates as a reference mirror. A photoelectrically converted signal from the receiver  155  is a beat signal having a predetermined frequency. As the sample table  21  (the moving mirror  24   m Y) moves in the direction Y, the phase or frequency of the beat signal changes. By utilizing this fact, the Y-coordinate of the sample table  21  is measured with respect to the reference mirror (i.e. the mirror  151 ). 
     Similarly, the reticle-side laser interferometer  19 Y 1  for the Y-axis is secured between the upper plate  225   d  and leg portion of the column  225 . The laser interferometer  19 Y 1  includes a PBS  148 A, quarter-wave plates  149 A and  150 A, and mirrors  151 A and  152 A as in the case of the laser interferometer  24 Y 1 . The mirror  151 A having a prism-type reflecting surface operates as a reference mirror. A laser beam from an external laser light source  145 A is led to the laser interferometer  19 Y 1  via a mirror  147 A, etc. The laser beam from the laser interferometer  19 Y 1  is received by a receiver  155 A via a mirror  153 A, etc. The position of the reticle stage  17  (i.e. the moving mirror  19   m Y) is measured on the basis of a beat signal from the receiver  155 A with respect to the reference mirror. 
     Thus, according to this embodiment, the laser interferometers  24 Y 1  and  19 Y 1 , together with the mirrors  151  and  151 A serving as reference mirrors, are mounted on the column  225 , which supports stationary members such as the projection optical system PL. This makes it possible to realize an interferometer monitor system that is not affected by vibrations from the floor. In this regard; it is also conceivable to provide the laser interferometers  24 Y 1  and  19 Y 1  on the column  234 , which supports movable portions. With this method, it is easy to synchronize the reticle stage  17  and the wafer stage  22  with each other on the basis of measured values by the laser interferometers. However, since the column  234  itself is distorted by acceleration due to the movement of the stage system, the confidence as to whether the reticle R and the wafer W are accurately aligned with each other is not always high. 
     In this embodiment, the laser interferometers  24 Y 1  and  19 Y 1 , the other laser interferometers, and the projection optical system PL are secured to the column  225  different from the column  234 , which may be distorted by the acceleration of the stage system. Accordingly, the position of each of the reticle and wafer stages  17  and  22  can be measured with high accuracy, and consequently, the accuracy of alignment of the reticle R and the wafer W increases. 
     In the foregoing embodiment, the mirrors  151  and  151 A, which operate as reference mirrors of the laser interferometers, are provided on the column  225 . However, the column  225  may expand or contract with changes in temperature, etc. In such,a case, mirrors serving as reference mirrors may be provided on a side of the projection optical system PL. In the foregoing embodiment, the projection optical system PL and the stage system are provided on the independent columns  225  and  234 , respectively. Therefore, it is possible to combine the constituent portions of the projection exposure apparatus after each of them has been adjusted with high accuracy, Thus, the period of time required to produce the apparatus is advantageously shortened. In this case, however, it is necessary to assemble the components by properly detaching and attaching a part of the stage-side column  234 , which does not require such rigorous accuracy as in the column  225 . 
     In the foregoing embodiment, the projection exposure apparatus has a system in which the projection exposure apparatus is divided into a plurality of units, and gases are replaced independently for each unit as shown in FIG. 9, and a system in which the intensity of illuminating light for exposure is controlled according to the absorptance of a gas as shown in FIGS.  10 ( a ) and  10 ( b ). These systems are applicable not only to scanning projection exposure apparatuses but also to static exposure type (one-shot exposure type) projection exposure apparatuses such as steppers. 
     According to the projection exposure apparatus having the above-described structure, there is provided a gas supply device for selectively supplying a gas into each of a plurality casings accommodating an illumination optical system, a projection optical system and a substrate stage of the projection exposure apparatus separately from each other. Therefore, during ordinary exposure, for example, a gas showing a low absorptance with respect to illuminating light for exposure is supplied. To perform experimental exposure in a state where workmen are present near the apparatus during maintenance or the like, for example, a gas showing a low absorptance with respect to illuminating light for exposure is supplied into the projection optical system, whereas a gas (e.g. air) that is harmless to workmen is supplied to the other portions. Thus, during exposure, the absorption of illuminating light for exposure is minimized, and during maintenance or the like, the safety of workers can be ensured. 
     In a case where the plurality of different kinds of gas used in the apparatus are those selected from the group consisting of nitrogen, air, and air having ozone removed therefrom, nitrogen or air having ozone removed therefrom is used during ordinary exposure. By doing so, it is possible to minimize the absorption of illuminating light for exposure particularly when the illuminating light is ultraviolet light. During maintenance or the like, air or air having ozone removed therefrom is used to thereby ensure the safety of workmen. 
     The gas supply device may be provided with a device for confirming that a gas in a casing to be supplied with another gas has been substantially completely replaced by the substituting gas. In this case, when the gas in the casing is changed over from a gas that only slightly absorbs illuminating light to a gas harmless to workmen, for example, it is possible to confirm that the gas in the casing has been substantially completely replaced by the harmless gas. Accordingly, the safety of workmen improves advantageously. Examples of the confirming device include a gas concentration sensor, a timer, etc. When a concentration sensor is used, for example, there is no useless waiting time, and the safety is improved in comparison to a case where a timer is used. 
     According to another aspect of the present invention, the projection exposure apparatus uses a laser light source that oscillates at a wavelength in the ultraviolet region or shorter wavelength region as an exposure light source. The apparatus has a casing with light-transmitting windows disposed in the optical path of the illuminating light between the exposure light source and the photosensitive substrate. The apparatus further has a gas controller that supplies into the casing a gas in which the amount of ozone per unit volume is variable. The illuminance of illuminating light on the photosensitive substrate, which is emitted from the light source and applied to the photosensitive substrate, is controlled by changing the amount of ozone in the gas supplied into the casing from the gas controller. 
     Accordingly, the illuminance of illuminating light for exposure is continuously controlled without damaging any optical member by changing the amount of ozone per unit volume in the gas in a predetermined casing. Since ozone absorbs ultraviolet light, when ultraviolet light is used as illuminating light for exposure, the amount of ultraviolet light can be accurately controlled at all times. 
     According to the above-described projection exposure apparatus, a movable part that moves synchronously with scanning exposure and a stationary part that is not synchronous with scanning exposure are secured to different antivibration plates. Therefore, the projection optical system is not affected by vibrations due to synchronous scanning of a mask (reticle) and a photosensitive substrate (wafer), and thus favorable image-forming characteristics can be obtained. 
     In this case, the projection exposure apparatus may be provided with an interferometer to measure the position of the mask-side stage or the substrate-side stage. The interferometer has a moving mirror mounted on the. mask-side stage or the substrate-side stage; a reference mirror mounted on the antivibration plate to which the stationary part is secured; and an interferometer body part that detects a relative displacement between the moving mirror and the reference mirror by using a light beam. With this arrangement, since the reference mirror is not affected by vibrations of the stage system, it is possible to improve controllability control accuracy, etc.) for the position and speed of the stage system during scanning exposure. 
     It should be noted that the present invention is not necessarily limited to the above-described embodiments, but may adopt various arrangements without departing from the gist of the present invention. 
     The entire disclosure of Japanese Patent Application Nos. 045795 and 045796 each filed on Mar. 4, 1996 including specification, claims, drawings and summary is incorporated herein by reference in their entireties.