Abstract:
A method of determining relative position between a reflective reticle and a substrate includes a detection step of detecting relative position between a transmissive reference mark on a reticle stage and a substrate alignment mark on a substrate stage through a reflective optical system which projects a pattern of the reflective reticle onto the substrate with extreme ultraviolet light, and a determination step of determining relative position between the reflective reticle and the substrate based on the detection result in the detection step.

Description:
This application is a divisional application of U.S. patent application Ser. No. 10/150,154, filed May 20, 2002 now U.S. Pat. No. 6,992,780. 

   FIELD OF THE INVENTION 
   This invention relates to a position detecting method and apparatus in an exposure apparatus for transferring very fine circuit patterns. 
   BACKGROUND OF THE INVENTION 
   Demagnifying projection exposure using ultraviolet light is used in lithography for manufacturing extremely fine semiconductor devices such as semiconductor memories and logic circuits. 
   The smallest dimension of a pattern that can be transferred by demagnifying projection exposure is proportional to the wavelength of light used in transfer and inversely proportional to the numerical aperture (NA) of the projection optical system. This means that it is necessary to shorten the wavelength of the light used to project extremely fine circuit patterns. For this reason, the wavelength of ultraviolet light used in pattern transfer has become increasingly shorter, e.g., 365 nm in mercury-vapor lamps, 248 nm in KrF excimer lasers and 193 nm in ArF excimer lasers. 
   As semiconductor devices are becoming smaller and smaller at an increasing rate, there is a limit on what lithography using ultraviolet light can accomplish. Accordingly, in order to burn in an extremely fine circuit pattern of less than 0.1 μm in an efficient manner, a demagnifying projection exposure apparatus using extreme ultraviolet light (EUV light) of a shorter wavelength, on the order of 10 to 15 nm, has been developed. 
   Since absorption by the material used is pronounced in the region of EUV light, a lens-based optical system that utilizes the refraction of light, as in the case of visible or ultraviolet light, is impractical. An exposure apparatus that employs EUV light, therefore, uses a reflection optical system. In such a case, use is made of a reflecting-type reticle in which the pattern to be transferred is formed on a mirror using an absorbing body. 
   A multilayer mirror and an oblique-incidence total-reflection mirror are examples of reflecting-type optical elements for constructing an exposure apparatus that relies upon EUV light. In the EUV region, the real part of the index of refraction is slightly smaller than unity and, as a result, total reflection occurs if use is made of oblique incidence in which the EUV light just barely impinges upon the mirror surface. Usually, a high reflectivity of 20 or 30% or more is obtained by oblique incidence of within several degrees measured from the surface. However, because such oblique incidence diminishes the degree of freedom in terms of optical design, it is difficult to use an oblique-incidence total-reflection mirror in a projection optical system. 
   A multilayer mirror obtained by building up alternating layers of two types of substances having different optical constants is used as a mirror for EUV light employed at an angle of incidence close to that of direct incidence. For example, molybdenum and silicon are formed in alternating layers on the surface of a glass substrate polished to have a highly precise surface shape. The layer thicknesses of the molybdenum and silicon are, e.g., 0.2 nm and 0.5 nm, respectively, and the number of layers is 20 each. The thickness of two layers of the different substances is referred to as the “film cycle”. In the above example, the film cycle is 0.2 nm+0.5 nm=0.7 nm. 
   When EUV light impinges upon such a multilayer mirror, EUV light of a specific wavelength is reflected. Only EUV light of a narrow bandwidth centered on a wavelength λ that satisfies the relationship of Bragg&#39;s equation
 
2× d ×sin θ=λ
 
where λ represents the wavelength of the EUV light and d the film cycle, will be reflected efficiently. The bandwidth in this case is 0.6 to 1 nm.
 
   The reflectivity of the reflected EUV light is 0.7 at most, and the unreflected EUV light is absorbed in the multilayer films or in the substrate. Most of this energy is given off as heat. 
   Since a multilayer mirror exhibits more loss of light than a mirror for visible light, it is necessary to hold the number of mirrors to the minimum. In order to realize a broad exposure area using a small number of mirrors, use is made of a method (scanning exposure) in which a large area is transferred by causing a reticle and a wafer to perform scanning using fine arcuate areas (ring fields) spaced apart from the optical axis at fixed distances. 
     FIG. 5  is a schematic view illustrating a demagnifying projection exposure apparatus that employs EUV light according to an example of the prior art. This apparatus includes an EUV light source, an illuminating optical system, a reflecting-type reticle, a projection optical system, a reticle stage, a wafer stage, an alignment optical system and a vacuum system. 
   By way of example, a laser plasma light source is used as the EUV light source. In the example shown in  FIG. 5 , a target material (supplied from a target supply unit  502 ) placed in a vacuum vessel  501  is irradiated with pulsed laser light (laser light that is produced by an excitation pulse laser  503  and supplied via a condensing lens  504 ), a high-temperature plasma is produced and EUV light having a wavelength of, say, 13 nm that emanates from the plasma is utilized as the EUV light source. A thin film of metal, an inert gas or a droplet is used as the target material, which is fed into the vacuum vessel  501  by means such as a gas jet (the target supply unit  502 ). In order to increase the average intensity of the EUV light emitted, the pulsed laser should have a high repetition frequency and the apparatus should be operated at a repetition frequency of several kilohertz. 
   The illuminating optical system that introduces the light from the EUV light source to a reticle  550  comprises a plurality of multilayer mirrors or a plurality of oblique-incidence mirrors and an optical integrator, etc. A condensing mirror (first mirror  506  of the illuminating system) constituting the first stage functions to collect EUV light emitted from the laser plasma substantially isotropically. An optical integrator  507  functions to illuminate the reticle uniformly using a prescribed numerical aperture. An aperture (field-angle limiting aperture  510 ) for limiting to a circular arc the area of the reticle surface that is illuminated is provided at a conjugate point with respect to the reticle disposed in the illuminating optical system. 
   Reflected light from the optical integrator  507  is reflected by a second mirror  508  of the illuminating system, passes through the field-angle limiting aperture  510  and is reflected again by a third mirror  509  of the optical system, thereby arriving at the reticle  550 . 
   The projection optical system uses a plurality of mirrors (first through fourth mirrors  511  to  514 ). Though using a small number of mirrors allows EUV light to be utilized very efficiently, this makes it difficult to correct for aberration. The number of mirrors needed to correct for aberration is four to six. The shape of each of the reflecting surfaces of the mirrors is convex or concave spherical, or convex or concave aspherical. The numerical aperture NA is 0.1 to 0.3. 
   To fabricate the mirror, use is made of a substrate consisting of a material, such as glass, having a low coefficient of expansion or silicon carbide, that exhibits a high rigidity and hardness and a small coefficient of expansion, the substrate is formed to have a reflecting surface of a predetermined shape by grinding and polishing, and multilayer films such as molybdenum and silicon are formed on the reflecting surface. In a case wherein the angle of incidence is not constant owing to the location of the layer in the mirror surface, the wavelength of the EUV light, the reflectivity of which rises depending upon the location, shifts if use is made of multilayer films having a fixed film cycle, as is evident from Bragg&#39;s equation cited above. Accordingly, it is required that a film-cycle distribution be provided in such a manner that EUV light of the same wavelength will be reflected efficiently within the mirror surface. 
   A reticle stage  552  and a wafer stage  562  have a mechanism in which scanning is performed synchronously at a speed ratio proportional to the reducing magnification. Let X represent the scanning direction in the plane of the reticle  550  or wafer  560 , Y the direction perpendicular to the X direction, and Z the direction perpendicular to the plane of the reticle  550  or wafer  560 . 
   The reticle  550  is held by a reticle chuck  551  on the reticle stage  552 . The reticle stage  552  has a mechanism for high-speed movement in the X direction. Further, the reticle stage  552  has a fine-movement mechanism for fine movement in the X, Y and Z directions and for fine rotation about these axes, thus making it possible to position the reticle  550 . The position and attitude of the reticle stage  552  are measured by laser interferometers (not shown) and are controlled based upon the results of measurement. 
   The positional relationship between the position of the reticle  550  and the optical axis of the projection optical system and the positional relationship between the position of the wafer  560  and the optical axis of the projection optical system are measured by an alignment detection mechanism that includes alignment detecting optical systems  553 ,  563 , and the positions and angles to the reticle stage  552  and wafer stage  562  are set in such a manner that the projected image of the reticle will coincide with a predetermined position on the wafer. 
   Further, the focus position along the Z axis on the wafer surface is measured by a focus-position detecting mechanism that includes a focus detecting optical system  564 , and the position and angle of the wafer stage  562  are controlled. During exposure, therefore, the surface of the wafer is always maintained at the position at which the image is formed by the projection optical system. 
   When one scanning exposure of the wafer ends, the wafer stage  562  is stepped in the X and/or Y directions to move the stage to the starting position of the next scanning exposure, then the reticle stage  552  and wafer stage  562  are again scanned synchronously in the X direction at a speed ratio that is proportional to the magnification of the projection optical system. 
   Thus, an operation for synchronously scanning the reticle and wafer in a state in which the demagnified projection image of the reticle is formed on the wafer is repeated (by a step-and-scan operation). The transfer pattern of the reticle is thus transferred to the entire surface of the wafer. 
   In the conventional EUV exposure apparatus, the following problems arise in a case where consideration is given to TTL (Through-the-Lens) alignment in which the relative positioning (referred to as alignment below) between a reticle and a wafer is carried out via a reflecting-type reticle and multilayer mirrors using non-exposing light: 
   Since the reflecting-type reticle and multilayer mirrors are optimized to obtain a high reflectivity with EUV light, sufficient reflectivity is not obtained with regard to alignment light that is non-exposing light, and it is conceivable that highly precise alignment will not be achieved. Accordingly, there is a need of an optical system in which satisfactory alignment signal light is obtained at all times in order to achieve alignment. 
   Furthermore, the alignment detecting optical system must take into consideration space-relation limitations. For example, the exposing light must not be blocked when alignment detection is not carried out. As a consequence, a limitation is imposed upon the structures of the detection system and detection optical system. 
   In the ordinary projection exposure apparatus, a method of aligning the reticle and wafer via a projection lens is referred to as TTL alignment. In an EUV exposure apparatus, however, the projection optical system is constituted not by lenses but by the multilayer-mirror optical system. It is difficult to refer to this scheme as a TTL scheme. However, for the sake of simplifying the description, an alignment system that uses the intervention of a multilayer-mirror optical system will also be defined as being a TTL alignment scheme in this specification. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to prevent a decline in the amount of light of an alignment signal and to make it possible to construct a highly precise alignment detecting optical system. 
   Another object of the present invention is to mitigate limitations relating to the structure of the detecting system and detection optical system. 
   According to one aspect of the present invention, a method of detecting relative position of a reflecting-type reticle and substrate, the reticle having an alignment mark thereon is provided. The method comprises: a holding step of holding the reticle on a reticle stage which has a reference mark; and a first detection step of detecting relative position of the reference mark of the reticle stage, and the alignment mark on the reticle. 
   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a schematic view illustrating a semiconductor exposure apparatus that includes a position detecting apparatus according to a first embodiment of the present invention; 
       FIG. 2  is a schematic view illustrating a semiconductor exposure apparatus that includes a position detecting apparatus according to the first embodiment; 
       FIG. 3  is a schematic view illustrating a semiconductor exposure apparatus that includes a position detecting apparatus according to a second embodiment of the present invention; 
       FIG. 4  is a schematic view illustrating a semiconductor exposure apparatus that includes a position detecting apparatus according to the second embodiment; 
       FIG. 5  is a schematic view illustrating a demagnifying projection exposure apparatus using EUV light; 
       FIG. 6  is a diagram useful in describing the flow of a device manufacturing process; 
       FIG. 7  is a diagram useful in describing a wafer process; 
       FIG. 8  is a diagram useful in describing detection of contrast and S/N ratio of a video signal (alignment signal); 
       FIG. 9  is a diagram illustrating the path of alignment light in a reflecting optical system; 
       FIG. 10  is a flowchart useful in describing a procedure for measuring alignment according to the first embodiment; 
       FIG. 11  is a flowchart useful in describing a procedure for measuring alignment according to the second embodiment; and 
       FIG. 12  is a flowchart useful in describing a procedure for selecting alignment procedures according to the first and second embodiments. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. 
   In the description that follows, it is assumed that the exposure apparatus is an EUV exposure apparatus that employs a projection optical system using a multilayer reflecting mirror. However, the present invention is not limited to this arrangement. For example, the invention is applicable to an exposure apparatus that employs a projection optical system using a lens, and to a proximity exposure apparatus (typified by an X-ray exposure apparatus). 
   [First Embodiment] 
     FIGS. 1 and 2  are schematic views illustrating a semiconductor exposure apparatus that includes a position detecting apparatus according to a first embodiment of the invention. The alignment mechanism of the semiconductor exposure apparatus according to the first embodiment, as well as the operation of this mechanism, will be described with reference to  FIGS. 1 and 2 . 
   As shown in  FIG. 1 , an illuminating optical system  2  comprises a light source, which emits non-exposing light, and illuminating optics. The illuminating light from the illuminating optical system  2  is reflected by a half-mirror  13  and mirror  14  so as to illuminate a transmitting-type alignment mark (referred to as a reticle-stage reference mark)  3  disposed on a reticle stage  4  through a transparent portion or an aperture of the reticle stage  4 . It should be noted that a portion of the reticle-stage reference mark  3  is transmissive with respect to the wavelength of the non-exposing light from the illuminating optical system  2 . Preferably, the reticle-stage reference mark  3  is constituted by a substrate having optimum transmittance with respect to the alignment light and an opaque pattern thereon. 
   The illuminating light that has passed through the reticle-stage reference mark  3  passes through a multilayer-mirror optical system  7  and illuminates a wafer alignment mark  9  on a wafer  10 .  FIG. 9  illustrates the path of the alignment light. 
   The apparatus includes the first through fourth mirrors  511  to  514 , respectively, of the projection system described above with reference to  FIG. 5 , and an opening limiting aperture  515 . As will be understood from  FIG. 9 , the reticle-stage reference mark  3  through which the alignment light (non-exposing light) passes is provided, as a result of which the alignment light arrives at a wafer alignment mark  9  by traversing only the projection optical system. 
   The images of the reticle-stage reference mark  3  and wafer alignment mark  9  illuminated by the illuminating optical system  2  are detected by an alignment detecting optical system  1  comprising an image forming lens and an image sensing device, and relative positioning of the reticle-stage reference mark  4  and wafer alignment mark  9  is carried out by an image processing detection method. Image processing detection methods that can be mentioned are template matching, symmetrical pattern matching and centroid-position detection. 
   The relative positional relationship between the reticle-stage reference mark  3  and a reticle alignment mark  6  is detected beforehand, by a method described in  FIG. 2 , using an optical system indicated at reference numerals  16  to  20  and  30 , and the relationship is stored by a storage device  27  for storing the position of the reticle stage. Accordingly, if relative positioning of the reticle-stage reference mark  4  and wafer alignment mark  9  is performed, then positioning of the reticle  5  and wafer  10  can be carried out. It should be noted that the wafer  10  is held by a wafer chuck  11  and mounted on a wafer stage  12 . 
     FIG. 2  is a diagram illustrating a method of detecting the relative positions of the reticle-stage reference mark placed on the reticle stage  4  and the reticle alignment mark  6  placed on the reticle  5 . Image processing and detection are executed by a detecting optical system  30  comprising an image sensing unit  16  and an image forming optical system  17 , and the relative positional relationship between the reticle-stage reference mark  3  and the reticle alignment mark  6  is detected. The mechanism (the components indicated by the reference numerals  16  to  20  and  30 ), which detects the relative position of the reticle-stage reference mark and reticle alignment mark, shall be referred to as a first detection mechanism. 
   More specifically, first the reticle stage  4  is moved to the detection position of the detecting optical system  30 . The position of the reticle-stage reference mark  3  is detected by the detecting optical system  30  and the position of the reticle stage at this time is stored by the storage unit  27 . It should be noted that the reticle-stage reference mark of this embodiment is obtained by placing a reticle-stage reference mark comprising an object through which alignment light does not pass on an object (membrane) through which the alignment light does pass. The reticle-stage alignment mark can be detected by the detecting optical system  30 . Next, the reticle stage  4  is moved, the position of the reticle alignment mark  6  on the reticle  5  is subjected to image processing and detection by the detecting optical system  30 , and the position of the reticle stage  4  at this time is stored by the storage unit  27 . The coordinates of the reticle stage  4  when each mark is detected by the detecting optical system  30  are thus stored in the storage unit  27 . The relative position of the reticle-stage reference mark  3  and reticle alignment mark  6  is detected from these coordinates. 
   It should be noted that the detecting optical system  30 , a light source  18 , an illuminating optical system  19  and half-mirror  20  may have moving mechanisms. When detection of alignment is not carried out, there is a possibility that the optical system constituted by the detecting optical system  30 , light source  18 , illuminating optical system  19  and half-mirror  20  will interfere with the operation of other units, as by blocking the exposing light within the exposure apparatus. Hence it is made possible to retract this optical system to a position at which it will not interference with other units. 
     FIG. 10  is a flowchart useful in describing the procedure of alignment processing executed according to the first embodiment set forth above. It should be noted that the procedure for alignment treatment shown in  FIG. 10  is implemented by a control system (not shown) provided on the above-described exposure apparatus. 
   When the reticle  5  is mounted on the reticle stage  4  (S 101 ), the reticle stage  4  is moved to a position at which relative position is detected by a first detection mechanism (S 102 ). The first detection mechanism detects the relative position of the reticle-stage reference mark  3  and reticle alignment mark  6  (S 103 ) through the procedure described above in connection with  FIG. 2 . 
   When measurement by the first detection mechanism ends, the reticle stage  4  is moved to an alignment measurement position in order to measure alignment (S 104 ). At this position the alignment light from the illuminating optical system  2  shown in  FIG. 1  passes through the reticle-stage reference mark  3  and impinges upon the multilayer-mirror optical system  7 . Meanwhile, the wafer stage  12  on which the wafer  10  has been mounted also is moved to the alignment measurement position (S 105 ). Under these conditions, the alignment light that has passed through the reticle reference mark illuminates the wafer alignment mark  9 . The alignment detecting optical system  1  detects the light reflected from the wafer alignment mark  9  and measures the relative position of the reticle-stage reference mark  3  and wafer alignment mark  9  (S 106 ). 
   From the thus obtained relative position (S 103 ) of the reticle-stage reference mark  3  and reticle alignment mark  6  and relative position (S 106 ) of the reticle-stage reference mark  3  and wafer alignment mark  9 , the relative position of the reticle alignment mark  6  and wafer alignment mark  9  is decided (S 107 ), and implemented using control system  100  as shown in each of  FIGS. 1–3 , thereby completing alignment measurement. 
   This ends one series of exposure processing steps. If there is a next wafer, then the processing from steps S 104  onward is repeated with regard to this new wafer (S 108 ). 
   When the relative positional relationship between the reticle-stage reference mark  3  and reticle alignment mark  6  is detected, the height between the two marks may be detected by detecting the defocusing characteristic of the images obtained. The detected value is used to adjust the positions of the reticle and wafer along the height direction at the time of exposure. 
   In accordance with the first embodiment, as described above, (1) the reticle-stage reference mark  3  having a portion through which non-exposing light is transmitted is provided; (2) the relative position of the reticle alignment mark  6  and reticle-stage reference mark  3  is detected by the detection system ( 16 – 19 ,  30 ) provided separately of the projection optical system; and (3) it is possible to perform alignment measurement using alignment light that has passed through the reticle-stage reference mark  3 . 
   [Second Embodiment] 
     FIGS. 3 and 4  are schematic views illustrating a semiconductor exposure apparatus that includes a position detecting apparatus according to a second embodiment of the invention. In the first embodiment, alignment is carried out by illuminating the wafer alignment mark  9  with non-exposing light that has passed through the reticle-stage reference mark  3 . In the second embodiment, alignment is carried out by illuminating a chuck mark  8 , which is provided on the wafer chuck  11 , with non-exposing light that has passed through the reticle-stage reference mark  3 . 
   As shown in  FIG. 3 , the illuminating optical system  2  comprises a light source, which emits non-exposing light, and illuminating optics. The illuminating light from the illuminating optical system  2  is reflected by the half-mirror  13  and mirror  14  so as to illuminate the reticle-stage reference mark  3  disposed on the reticle stage  4  through the transparent portion or the aperture of the reticle stage  4 . The illuminating light that has passed through the reticle-stage reference mark  3  passes through the multilayer-mirror optical system  7  and illuminates the chuck mark  8  disposed on the wafer chuck  11 . 
   The images of the reticle-stage reference mark  3  and chuck mark  8  illuminated by the illuminating optical system  2  are detected by the alignment detecting optical system  1  comprising an image forming lens and an image sensing device, and relative positioning of the reticle-stage reference mark  4  and chuck mark  8  is carried out by an image processing detection method. The relative positional relationship between the reticle-stage reference mark  3  and reticle alignment mark  6 , as well as the relative positional relationship between the chuck mark  8  and wafer alignment mark  9 , is detected beforehand, by a method described later, whereby the relative position of the reticle-stage reference mark  3  and wafer alignment mark  9  is detected so that the reticle  5  and wafer  10  can be positioned relative to each other. 
   Described next will be a method of detecting the relative positional relationship between the reticle-stage reference mark  3  and reticle alignment mark  6  and the relative positional relationship between the chuck mark  8  and wafer alignment mark  9 . 
   Through a method similar to that described above in connection with  FIG. 2 , the relative positional relationship between the reticle-stage reference mark  3  and reticle alignment mark  6  is found by executing image processing and detection using the detection mechanism comprising the detecting optical system  30 , light source  18 , illuminating optical system  19  and half-mirror  20 , and storing the coordinates of the positions of the marks in the storage unit  27  for storing the position of the reticle stage. 
   The method of detecting the relative positional relationship between the chuck mark  8  and wafer alignment mark  9  will be described next with reference to  FIG. 4 . 
   This method is disclosed in Japanese Patent Application Laid-Open No. 61-263127 filed by the present applicant. According to this method, the relative positional relationship between the chuck mark  8  and wafer alignment mark  9  is detected before the wafer chuck  11  is mounted on the wafer stage  12 , which is for being exposed by the exposure apparatus. As shown in  FIG. 4 , image processing and detection are executed by a detecting optical system  31 , which comprises an image sensing device  24  and an image forming optical system  25 , disposed externally of the exposure apparatus, and the relative positional relationship between the chuck mark  8  and wafer alignment mark  9  is detected. This detection system (the components indicated by the reference numerals  21  to  25  and  31 ) shall be referred to as a second detection mechanism. 
   The detection method includes first using the detecting optical system  31  to detect the position of the chuck mark  8  disposed on a moving stage  26  and storing the position of the moving stage  26  at this time in a storage unit  29  for storing stage position. Next, the moving stage  26  is moved, the position of the wafer alignment mark  9  on the wafer  10  is subjected to image processing and detection by the detecting optical system  31 , and the position of the moving stage  26  at this time is stored by the storage unit  29 . The relative position of the chuck mark  8  and wafer alignment mark  9  is detected from the coordinates, thus stored in the storage unit  29 , of the moving stage  26  when each of the marks is detected by the detecting optical system  31 . 
   Next, with the wafer  10  being held by the wafer chuck  11  carrying the chuck mark  8  (the condition in which measurement was carried out by the method shown in  FIG. 4 ), as shown in  FIG. 3 , the chuck and wafer are transported and placed on the wafer stage, which is for being exposed by the exposure apparatus. The relative positional relationship between the reticle-stage reference mark  3  and chuck mark  8  is then detected, as described in connection with  FIG. 3 , and the wafer stage  12  is driven based upon the information concerning the relative positional relationship between the chuck mark  8  and wafer alignment mark  9 , thereby performing exposure. 
   The processing for the above operation will now be described with reference to the flowchart of  FIG. 11 . The processing described below is implemented by a control system (not shown) provided on the above-described exposure apparatus. 
   When a new reticle is mounted, the relative position of the reticle-stage reference mark  3  and reticle alignment mark  6  is detected (S 209 ) by executing the steps S 101  to S 103  in  FIG. 10 . 
   In a case wherein a new wafer is mounted, on the other hand, first, the wafer chuck  11  is mounted on the moving stage  26  and the wafer  10  is mounted on the wafer chuck  11  (S 201 ). The wafer  10  is then moved to the position at which detection is performed by the second detection mechanism (S 202 ), and the relative position of the chuck mark  8  and wafer alignment mark  9  is detected by the second detection mechanism (S 203 ). The wafer chuck  11  holding the wafer  10  is then moved onto the wafer stage  12  (S 204 ) and alignment measurement is carried out. 
   In alignment measurement, the wafer stage  12  and reticle stage  4  are moved to the position for alignment measurement (S 205 ), and the relative position of the reticle-stage reference mark  3  and chuck mark  8  is detected by the illuminating optical system  2  and alignment detecting optical system  1  (S 206 ). Next, the relative position of the reticle alignment mark  6  and wafer alignment mark  9 , namely the relative position of the reticle and wafer, is decided (S 207 ) based upon the relative position of the reticle-stage reference mark  3  and reticle alignment mark  6  detected at step S 109 , the relative position of the wafer alignment mark  9  and chuck mark  8  detected at step S 203 , and the relative position of the reticle-stage reference mark  3  and chuck mark  8  measured at step S 206 . 
   This ends one series of exposure processing steps. If there is a next wafer, then the processing from steps S 201  onward is repeated with regard to this new wafer (S 208 ). 
   Thus, according to the first and second embodiments, the relative position of the reticle-stage reference mark  3  and chuck mark  8  is detected by an on-axis TTL alignment optical system. According to this method, alignment measurement and exposure are, broadly speaking, carried out in parallel and the structural restrictions on the alignment detection system (restrictions imposed by the projection optical system in many cases) are eliminated. With regard to designing the alignment detection system, therefore, it is unnecessary to take space into consideration (within certain limits, of course), the detection mechanism and the like can be arranged with the highest priority being given to detection precision, and it becomes possible to construct various alignment detection systems. Further, stabilized detection rate and high precision can be achieved with a high throughput for a variety of wafer processes. Further, since the detection system for measuring the relationship between the reticle-stage reference mark and chuck mark in the exposure apparatus is an on-axis TTL alignment optical system, there is also no base line. (A base line is generated when off-axis alignment is performed. Hence there is a discrepancy between the position of the wafer stage when alignment is performed by an off-axis alignment scope and the position of the wafer stage when exposure is performed. This discrepancy is eliminated.) This makes it possible to eliminate a cause of poor measurement precision. 
   Further, when the relative positional relationship between the chuck mark  8  and wafer alignment mark  9  is detected, the height between the two marks can also be detected by detecting the defocusing characteristics of the images obtained. 
   When positioning the wafer  10  relative to the reticle  5  in the second embodiment, whether to use the chuck mark  8  or the wafer alignment mark  9  as the alignment mark on the wafer side may be decided by detecting the image of the wafer alignment mark  9  using a send-a-head wafer, etc. According to measuring using a send-a-head wafer, when a wafer is exposed by the exposure apparatus, measurement of various parameters (e.g., verification of alignment signal level) for the exposure apparatus, which are necessary when actual exposure is carried out, is performed before the actual exposure operation using a wafer employed in actual exposure or a wafer equivalent thereto. If the S/N ratio or contrast of the image signal from the wafer alignment mark  9  is a level that satisfies the specifications for aligmnent measurement, the detection method illustrated in the first embodiment is adopted. If the S/N ratio or contrast of this image signal is a level that does not satisfy the specifications, then measurement should be performed using the detection method described in the second embodiment ( FIG. 11 ). 
   The contrast of the image signal (alignment signal) can be found from (b−a)/(a+b) in  FIG. 8 , for example, and the S/N ratio can be found from (b−a)/a. 
   The above processing will be described with reference to the flowchart of  FIG. 12 . 
   First, through the procedure described at steps S 101  to S 107  in  FIG. 10  and S 201  to S 204  in  FIG. 11 , i.e., by the method using the first and second detection mechanisms, the relative position of the wafer alignment mark and reticle alignment mark in the send-a-head wafer scheme is measured (S 301 ). The S/N ratio and contrast are detected with regard to the image signal representing the wafer alignment mark obtained by this measurement (S 302 ). 
   In a case wherein the S/N ratio and contrast exceed respective ones of predetermined values that have been set for them (“YES” at step S 303 ), the wafer alignment mark can be used directly when alignment measurement is performed. Control, therefore, proceeds to step S 304 , wherein the apparatus is set for carrying out alignment measurement described in the first embodiment. On the other hand, if at least one of the S/N ratio and contrast falls below the predetermined value that has been set for it (“NO” at step S 303 ), then it is preferred that alignment measurement be carried out using the chuck mark  8 . Thus, the apparatus is set (S 305 ) in such a manner that alignment measurement described in the second embodiment is executed. 
   A process for manufacturing a semiconductor device utilizing the exposure apparatus set forth above will now be described.  FIG. 6  illustrates the overall flow of a process for manufacturing semiconductor devices. The circuit for the device is designed at step  1  (circuit design). A mask on which the designed circuit pattern has been formed is fabricated at step  2  (mask fabrication). Meanwhile, a wafer is manufactured using a material such as silicon or glass at step  3  (wafer manufacture). The actual circuit is formed on the wafer by lithography, using the mask and wafer that have been prepared, at step  4  (wafer process), which is also referred to as “pre-treatment”. A semiconductor chip is obtained, using the wafer fabricated at step  4 , at step  5  (assembly), which is also referred to as “post-treatment”. This step includes steps such as actual assembly (dicing and bonding) and packaging (chip encapsulation). The semiconductor device fabricated at step  5  is subjected to inspections such as an operation verification test and a durability test at step  6  (inspection). The semiconductor device is completed through these steps and then is shipped (step  7 ). The pre- and post-treatments are performed at separate special-purpose plants. Maintenance is carried out on a per-plant basis by the above-described remote maintenance system. Further, information for production management and equipment maintenance is communicated by data communication between the pre- and post-treatment plants via the Internet or leased-line network. 
     FIG. 7  is a flowchart illustrating the detailed flow of the wafer process mentioned above. The surface of the wafer is oxidized at step  11  (oxidation). An insulating film is formed on the wafer surface at step  12  (CVD), electrodes are formed on the wafer by vapor deposition at step  13  (electrode formation), and ions are implanted in the wafer at step  14  (ion implantation). The wafer is coated with a photoresist at step  15  (resist treatment), the wafer is exposed to the circuit pattern of the mask to print the pattern onto the wafer by the above-described exposure apparatus at step  16  (exposure), and the exposed wafer is developed at step  17  (development). Portions other than the developed photoresist are etched away at step  18  (etching), and unnecessary resist left after etching is performed is removed at step  19  (resist removal). Multiple circuit patterns are formed on the wafer by implementing these steps repeatedly. Since the manufacturing equipment used at each step is maintained by the remote maintenance system described above, malfunctions can be prevented and quick recovery is possible if a malfunction should happen to occur. As a result, the productivity of semiconductor device manufacture can be improved over the prior art. 
   Thus, in accordance with the embodiments as described above, in an EUV exposure apparatus, for example, transmittance or reflectivity for alignment light can be optimized for relative positioning of a reflecting-type reticle and wafer, and it is possible to obtain an alignment signal of excellent S/N ratio or contrast by using a reticle-stage reference mark, which has been disposed on a reticle stage, as an alignment mark. This makes highly precise alignment feasible. Especially good effects are obtained when position detection is performed by a TTL scheme. Furthermore, by adopting a reticle-stage reference mark that transmits light, an alignment detecting optical system can be situated above the reticle stage, as shown in  FIG. 1 . As a result, the exposing light is no longer blocked, there is a greater degree of freedom in terms of laying out the alignment detecting optical system, and measurement can be carried out without interfering with the exposing light. 
   Thus, in accordance with the invention as described above, it is possible to prevent a decline in the amount of light of an alignment signal and to construct a highly precise alignment detecting optical system. 
   As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.