Abstract:
An optical element for an exposure apparatus is disclosed, where the exposure apparatus has a projection optical system configured to project a pattern of an original plate illuminated with extreme ultraviolet light from a light source onto a substrate and exposes the substrate to the light via the original plate and the projection optical system. The optical element is to be placed in one of a first path of the light located in a side of the light source with respect to the original plate and a second path of the light located in a side of the substrate with respect to the original plate. The element includes a film configured to transmit the extreme ultraviolet light, and a shield placed on the film and configured to shield part of said film from the extreme ultraviolet light.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an optical element suitable for measurement using extreme ultraviolet light, an exposure apparatus incorporating the optical element, and a device manufacturing method using the exposure apparatus.  
       BACKGROUND OF THE INVENTION  
       [0002]     Generally, manufacturing processes of semiconductor devices composed of super minute patterns such as VLSI circuits employ a reduced projection exposure apparatus which burns circuit patterns drawn on a mask upon a substrate coated with a photosensitive material by projecting the circuit patterns onto the substrate on a reduced scale. Increases in packaging density of semiconductor devices demand further miniaturization of pattern line widths. As a result, there has been continuous improvement in resolution to accommodate improved resist processes and miniaturized exposure apparatus. Thus, light sources for exposure apparatus have been using increasingly shorter wavelengths, such as from KrF excimer laser (with a wavelength of 248 nm) to ArF excimer laser (with a wavelength of 193 nm) and to F 2  laser (with a wavelength of 157 nm). To burn finer patterns efficiently, reduced projection exposure apparatus which use extreme ultraviolet light (EUV light) with shorter wavelengths than ultraviolet light, i.e., with wavelengths on the order of 10 to 15 nm, are currently under development.  
         [0003]     On the other hand, along with the improvement of resolution and miniaturization of semiconductor devices, increasingly tighter tolerances are required for alignment between mask patterns and patterns on photosensitive substrates. Description will be given below of alignment between a mask pattern and a pattern on a photosensitive substrate on an ordinary reduced projection exposure apparatus which does not use EUV light.  
         [0004]     Conventionally, the following two methods are available to observe an alignment mark on a wafer, i.e., a photosensitive substrate, to obtain positional information about the wafer.  
         [0005]     1. Off-axis method which measures the alignment mark on a wafer without using a projection optical system.  
         [0006]     2. TTR (Through The Reticle) method which detects relative positional relationship between a wafer and reticle by observing them simultaneously using a projection optical system.  
         [0007]     Of the two methods, the off-axis method is used for wafer-by-wafer position measurement because the TTR method requires longer measurement time. The off-axis method employs a wafer inspection microscope (hereinafter referred to as an off-axis microscope). Position measurement under an off-axis microscope, which does not use a projection optical system, not only allows the use of any wavelength, but also allows the use of a light source with a wide wavelength band. Advantages of using light with a wide band of wavelengths for position measurement include that thin-film interference effects on the photosensitive material (resist) applied to a wafer can be removed.  
         [0008]     However, when aligning a wafer and reticle using an off-axis microscope, it is not possible to measure observation position and exposure position directly. Thus, it is necessary to determine a so-called base line amount in advance, where the base line amount is the distance between the center of measurement under the off-axis microscope and center of a projected pattern image on a reticle (center of exposure). When performing the alignment, a deviation of an alignment mark in a shot on the wafer from the center of measurement is measured under the off-axis microscope. After that, the wafer is moved from the off-axis microscope position by a distance equal to the sum of the base line amount and the deviation, thereby aligning the center of the shot area with the center of exposure accurately. However, the base line amount can change gradually with time during the use of the exposure apparatus. Such changes in the base line will make it impossible to feed the center of a shot on a wafer to the center of a projected pattern image on a reticle, resulting in reduced alignment accuracy (superimposition accuracy). Thus, it is necessary to take base line measurements (calibration measurements) to accurately measure the distance between the center of measurement under the off-axis microscope and center of a projected pattern image on a reticle on a regular basis. The TTR measurement system must be used for the base line measurements.  
         [0009]      FIG. 8  is a diagram schematically showing the principle of base line measurement on a projection exposure apparatus. A dummy reticle R 2  bears a slit-shaped mark M 1  within an exposure area of a projection optical system  2 . As shown in  FIG. 8 , the dummy reticle R 2  is held on a reticle drive stage  1 , which moves in such a way that the center of the dummy reticle R 2  will coincide with an optical axis AX of the projection optical system  2 . On a wafer drive stage  3 , a dummy wafer W 2  which bears a mark M 2  equivalent to M 1  is mounted at an image-forming position in the projection optical system  2 , keeping clear of a wafer W 1 . The mark M 2  on the dummy wafer W 2  is a slit-shaped light-transmitting area created by placing a light shielding member on an exposure light-transmitting member. A light quantity sensor S 1  is placed under the mark M 2 . The wafer drive stage  3  is positioned using a laser interferometer (not shown) in such a way as to bring the mark M 2  into position in a projection area of the projection optical system  2  and exposure light L 1  is directed into the projection optical system  2  via an illumination optical system  5  by operating an exposure light source laser  4 . While monitoring the light quantity from the light quantity sensor S 1  placed under the mark M 2 , position which maximizes the light quantity is found by moving the wafer stage  3  slightly in X, Y, and Z directions. The position which maximizes the light quantity makes the mark M 1  on the dummy reticle and mark M 2  on the dummy wafer W 2  coincide in relative position.  
         [0010]     An off-axis microscope  6  is placed outside the projection optical system  2  (outside the exposure area). On the side of a projected image, the optical axis of the off-axis microscope  6  is parallel to the optical axis AX of the projection optical system  2 . An index mark M 3  for use as a reference for position measurement of the mark on the wafer W 1  or mark M 2  on the dummy wafer W 2  is provided in the off-axis microscope  6 . The index mark M 3  is located at a position conjugate to a projected image plane (a surface of the wafer W 1  or surface of the dummy wafer W 2 ).  
         [0011]     The position of the wafer drive stage  3  when the mark M 1  on the dummy reticle R 2  and mark M 2  on the dummy wafer W 2  are aligned with each other is measured by the laser interferometer (not shown). The value obtained is denoted by X 1 . Also, the position at which the wafer stage  3  is located when the index mark M 3  in the off-axis microscope  6  is aligned with the mark M 1  is measured by the laser interferometer. The value obtained is denoted by X 2 . In this case, a base line amount BL is determined by calculating the difference (X 1 −X 2 ). The base line amount BL is used later as a reference quantity when measuring an alignment mark on the wafer W 1  by the off-axis microscope  6  and sending it to under the projection optical system  2 . Let XP denote the distance between the center of a shot (exposure field) on the wafer W 1  and the alignment mark and let X 3  denote the position of the wafer drive stage  3  when the alignment mark on the wafer W 1  coincides with the index mark M 3  in the off-axis microscope  6 .  
         [0012]     To make the center of the shot coincide with a reticle center C, the wafer drive stage  3  can be moved to the position determined by “X 3 −BL−XP.” 
         [0013]     To perform the alignment in this way, first, the position of the alignment mark on the wafer W 1  is measured using the off-axis microscope  6 . Subsequently, by simply feeding the wafer drive stage  3  by a predetermined amount in relation to the base line amount BL, it is possible to superimpose a reticle R 1  pattern accurately upon the shot area on the wafer W 1  for exposure. However, it is necessary to measure distance between a reticle set mark M 4  and the mark M 1  on the dummy reticle R 2  by some other means and align the reticle R 1  with the reticle set mark M 4  in advance.  
         [0014]     Incidentally, although a one-dimensional direction has been taken into consideration in the above description, actually two-dimensional directions should be taken into consideration.  
         [0015]     In this way, to align a reticle with a wafer via a projection optical system, a system is used which measures light quantity using a sensor placed under a mark formed by placing a light shielding member on a dummy wafer made of an exposure light-transmitting member.  
         [0016]     Calibration measurement using the TTR method has been described above. A similar setup is used for aberration measurement in a projection optical system.  
         [0017]      FIG. 9  is a schematic diagram of typical aberration measurement system for an exposure apparatus.  
         [0018]     Components having the same functions as those in  FIG. 8  are denoted by the same reference numerals/characters as the corresponding components in  FIG. 8 , and description thereof will be omitted.  
         [0019]     As shown in  FIG. 9 , an aberration measurement system has a diffraction grating  7  in front of a reticle surface to diffract light. On the downstream side of the diffraction grating  7 , a mark M 5  with a slit-shaped or pinhole-shaped transparent part is placed near the reticle surface. The mark M 5  cancels out the aberration caused by the illumination optical system  5  or diffraction grating  7  and an illuminating beam enters the projection optical system  2 . A mark M 6  formed by placing a light shielding member on an exposure light-transmitting member such as quartz is provided on a wafer-side image plane. The mark M 6  has a slit-shaped or pinhole-shaped transparent part (M 6 - c  in  FIG. 10 ) similar to that of the mark M 5  and a window-shaped large transparent part (M 6 - d  in  FIG. 10 ) on the upstream surface of the exposure light-transmitting member. A CCD camera S 2  is placed under the mark M 6  on the wafer image plane.  
         [0020]     Wavefront aberration is removed from an exposure light L 1  by the slit-shaped or pinhole-shaped mark M 5  placed on the reticle surface. Thus, the exposure light which enters and exits the projection optical system  2  contains only aberration produced in the projection optical system  2 .  FIG. 10  is an enlarged schematic view of the wafer-side mark M 6  on the dummy wafer W 2  and part around the CCD camera S 2 . In  FIG. 10 , reference character M 6 - a  denotes an excimer laser-transparent member, such as quartz, which is used as a parent material on which a light shielding member M 6 - b  such as chromium is placed to produce a desired mark shape. The exposure light reaches a surface of the CCD camera S 2  after passing through the slit or pinhole M 6 - c  in  FIG. 10  which cancels out the aberration contained in the exposure light. On the other hand, the exposure light passing through the window M 6 - d  on the mark M 6  reaches the surface of the CCD camera S 2 , still containing the wavefront aberration produced in the projection optical system  2 . The former is referred to as a reference beam and the latter is referred to as a sample beam. The difference in wavefront between the two beams is the wavefront aberration produced in the projection optical system  2 . The wavefront aberration is observed as interference fringes when taken into the CCD camera S 2  as an image. Image processing of the interference fringes makes it possible to measure wavefront aberration quantitatively up to the 36th term of Zernike polynomials.  
         [0021]     Besides, a configuration in which a pinhole is provided in the wafer image plane to transmit the exposure light and a CCD is placed under it is also used to measure pupil-fill intensity distribution in an illumination optical system. In this way, systems widely used currently involve creating a desired mark by placing a light shielding member on an exposure light-transmitting member, installing a sensor under the mark, and observing light quantity or an image passing through the mark. Incidentally, a technique for measuring the base line using non-exposure light on an EUV exposure apparatus is disclosed in Japanese Patent Laid-Open No. 2002-353088.  
         [0022]     As described above, alignment measurement, aberration measurement, pupil-fill intensity distribution measurement, and the like involve creating a desired mark by placing a light shielding member on an exposure light-transmitting member which serves as a parent material, installing a sensor under the mark, and observing the light quantity or image of exposure light passing through the mark. However, exposure apparatus which use EUV light as a light source cannot use typical measurement systems such as those described above.  
         [0023]     Unlike excimer laser exposure apparatus, EUV exposure apparatus use almost no exposure light-transmitting member. This is because typical transparent members cause great loss of exposure light with a very short wavelength such as EUV light, making it difficult to use a transmissive optical system. Thus, both EUV-based illumination optical system and projection optical system are constituted of mirror-based reflection optical systems. Regarding reticles, reflective reticles are conceived. Reflective reticles consist of absorption bands formed on a reflective multilayer film formed on zero-expansion glass and produce reflected light of desired patterns.  
         [0024]     To perform alignment by the TTR method on an EUV exposure apparatus configured as described above, it is necessary to:  
         [0025]     (1) install a dummy retile by placing a light shielding member in such a way as to provide a slit-shaped reflector; and  
         [0026]     (2) put a mark with a slit-shaped transparent part whose size is reduced by a reduction ratio of the projection optical system on the dummy wafer and install a light quantity sensor under the mark to measure the quantity of transmitted light.  
         [0027]     A problem is that the EUV light may not be able to pass through quartz M 6 - a  such as shown in  FIG. 10 . An exposure apparatus whose light source has a wavelength longer than F2 laser can use a dummy wafer as shown in  FIG. 10 . That is, the exposure apparatus can have a configuration in which light transmitted through a mark enters a sensor under the mark, where the mark is formed by placing a light shielding member on a glass member a few millimeters thick. However, EUV light cannot pass through quartz and other glass members. Consequently, the transmitted light does not enter the sensor installed under the dummy wafer. This makes it impossible to take measurements.  
         [0028]     In this way, EUV-based exposure apparatus do not allow marks to be created on glass. To deal with this problem, it is conceivable to create a mark on the dummy wafer by attaching an absorption band on a reflecting surface as is the case with the reticle. However, this configuration makes it necessary to provide space for a sensor which detects the light reflected from the mark on the dummy wafer.  
       SUMMARY OF THE INVENTION  
       [0029]     The present invention has been made in view of the above difficulty found by the inventor&#39;s dedicated study and has as its exemplary object to provide an optical element suitable for measurement using extreme ultraviolet light for which it is difficult to use a transmissive optical element, an exposure apparatus incorporating the optical element, and a device manufacturing method using the exposure apparatus.  
         [0030]     According to one aspect of the present invention, there is provided an optical element for an exposure apparatus which has a projection optical system configured to project a pattern of an original plate illuminated with extreme ultraviolet light from a light source onto a substrate and exposes the substrate to the light via the original plate and the projection optical system, the optical element being to be placed in one of a first path of the light located in a side of the light source with respect to the original plate and a second path of the light located in a side of the substrate with respect to the original plate, the element comprising: a film configured to transmit the extreme ultraviolet light; and a shield placed on the film and configured to shield part of the film from the extreme ultraviolet light.  
         [0031]     Also, according to another aspect of the present invention, there is provided an exposure apparatus which has a projection optical system configured to project a patterns of an original plate illuminated with extreme ultraviolet light from a light source onto a substrate and exposes the substrate to the light via the original plate and the projection optical system, the apparatus comprising: an optical element which as defined above, configured to be placed in one of a first path of the light located in a side of the light source with respect to the original plate and a second path of the light located in a side of the substrate with respect to the original plate.  
         [0032]     Further, according to another aspect of the present invention, there is provided an exposure apparatus which exposes a substrate to extreme ultraviolet light via an original plate, the apparatus comprising: a projection optical system configured to project a pattern of the original plate onto the substrate: a substrate stage configured to hold the substrate and to move; an optical element as defined above, placed on the substrate stage; and a detector configured to detect extreme ultraviolet light emitted from the projection optical system and transmitted through the optical element.  
         [0033]     Furthermore, according to another aspect of the present invention, there is provided a method of manufacturing a device, the method comprising steps of: exposing a substrate to light via an original plate using an exposure apparatus as defined above, developing the exposed substrate; and processing the developed substrate to manufacture the device.  
         [0034]     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  
       [0035]     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.  
         [0036]      FIG. 1  is a schematic diagram showing a schematic configuration of an exposure apparatus according to a first embodiment;  
         [0037]      FIG. 2  is a flowchart illustrating procedures for base line measurement according to the first embodiment;  
         [0038]      FIG. 3  is an enlarged schematic view of a mark on a dummy wafer and part around a light quantity detection sensor according to the first embodiment;  
         [0039]      FIG. 4  is an enlarged schematic view showing a variation to the mark on the dummy wafer and part around the light quantity detection sensor according to the first embodiment;  
         [0040]      FIG. 5  is a schematic diagram showing a schematic configuration of an exposure apparatus according to a second embodiment;  
         [0041]      FIG. 6  is a flowchart illustrating procedures for wavefront aberration measurement according to the second embodiment;  
         [0042]      FIG. 7  is an enlarged schematic view of a mark on a dummy wafer and part around a light quantity detection sensor according to the second embodiment;  
         [0043]      FIG. 8  is a schematic diagram illustrating a base line measurement method on a typical exposure apparatus;  
         [0044]      FIG. 9  is a schematic diagram showing an aberration measurement method in a projection optical system of a typical exposure apparatus;  
         [0045]      FIG. 10  is a schematic diagram showing a region near a wafer image plane of a typical exposure apparatus;  
         [0046]      FIG. 11  is a diagram illustrating a flow of a device manufacturing process; and  
         [0047]      FIG. 12  is a diagram illustrating a wafer process. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.  
         [0049]     In the following embodiments, description will be given of suitable construction of the following parent materials used on an exposure apparatus which employs EUV light as a light source:  
         [0050]     (1) parent material for a mark provided on a light quantity sensor on a wafer stage and used for TTR measurement;  
         [0051]     (2) parent material for a mark provided on a CCD camera on the wafer stage and used for measurement of aberration in a projection optical system; and  
         [0052]     (3) parent material for diffraction grating, pinhole, and the like.  
         [0053]     These parent materials are thin films of particular materials not exceeding a certain thickness. They are sufficiently transparent to EUV light. Thus, by placing a light shielding member on the parent material, it is possible to create a desired optical element such as a desired mark or diffraction grating.  
       First Embodiment  
       [0054]      FIG. 1  is a schematic diagram illustrating a schematic configuration of an exposure apparatus according to a first embodiment. The same components/functions as those in  FIG. 8  are denoted by the same reference numerals/characters as the corresponding components/functions in  FIG. 8 . In the exposure apparatus according to this embodiment, an EUV light source  8  (hereinafter referred to as the light source  8 ) outputs EUV light. An EUV illumination optical system  9  (hereinafter referred to as the illumination optical system  9 ) forms EUV light L 2  emitted from the light source  8  into luminous flux of a predetermined shape. A reflective projection optical system  10  focuses the EUV light L 2  on a wafer W 1  which is a photosensitive substrate after it is formed into a predetermined shape by the illumination optical system  9  and reflected by a reflective reticle R 3  for EUV (hereinafter referred to as the reticle R 3 ). The reticle R 3  and wafer W 1  are mounted on a reticle drive stage  11  and wafer drive stage  3 , respectively. Scanning exposure is enabled by driving the two stages ( 11  and  3 ) in synchronization by changing a feed ratio according to a magnification of the projection optical system  10 . Position measurement of the two stages ( 11  and  3 ) are performed by a laser interferometer (not shown).  
         [0055]     A reflective dummy reticle R 4  (hereinafter referred to as the dummy reticle R 4 ) is mounted on the stage  11  and a mark M 7  equipped with a slit-shaped reflector is mounted on the reflective dummy reticle R 4 . On the other hand, a dummy wafer W 3  is mounted on the stage  3  and a mark M 8  equipped with a slit-shaped transparent part is mounted on the dummy wafer W 3 . The dummy reticle R 4  and dummy wafer W 3  are used for base line measurement. Incidentally, parent material of the dummy wafer W 3  is a thin film of Si, SiC, SiNx, diamond, or diamond-like carbon 2 μm or less in thickness. The diamond-like carbon is an amorphous hard carbon film created by mainly carbon and hydrogen and is also known as amorphous carbon. The mark M 8  equipped with a slit-shaped reflector of tantalum, tungsten, or other metal is placed on the thin film. Besides, a EUV light quantity detection sensor S 3  (hereinafter referred to as the light quantity detection sensor S 3 ) is installed under the mark M 8 .  
         [0056]     An exemplary method for making a mark on a thin film of an optical element is as follows. That is, optical lithography technology used for semiconductor device manufacture can be used for optical elements. For example, a light shielding body is vapor-deposited on a thin film, a photoresist which is a photosensitive material is applied to it, and a pattern corresponding to the mark to be formed is transferred to the photoresist by an electron beam exposure apparatus. Subsequently, the photoresist is developed (an area corresponding to the mark is removed from the resist) and the light shielding body is removed by etching using the developed photoresist as a mask, thereby creating a blank area (extreme ultraviolet light-transmitting part) as the mark.  
         [0057]     A position detection mark M 9  is mounted on the reticle R 3 . The distance between the mark M 9  and the mark M 7  on the dummy reticle is measured by a reticle microscope and/or interferometer (both not shown). An off-axis microscope  6  is installed on the side of the wafer to measure position of a wafer alignment mark.  
         [0058]     The off-axis microscope  6  incorporates an index mark M 3  for use as a reference for position measurement of the mark on the wafer W 1  or mark M 8  on the dummy wafer W 3 . Incidentally, in the above configuration, all areas transparent to EUV light are enclosed in a vacuum chamber  12  which is maintained under vacuum.  
         [0059]     Next, description will be given of an alignment process performed on the exposure apparatus described above.  FIG. 2  is a flowchart illustrating procedures for base line measurement according to this embodiment. Incidentally, the procedures shown in  FIG. 2  are carried out as a controller (not shown) of the exposure apparatus according to the present embodiment controls various parts of the exposure apparatus. The controller has a storage which stores a computer program corresponding to the procedures and CPU which executes the computer program stored in the storage.  
         [0060]     When scan-exposing the pattern on the reticle R 3  held on the reticle drive stage  11  by superimposing it on the wafer, it is necessary to drive the two stages ( 11  and  3 ) in tight synchronization as well as make the reticle R 3  and wafer W 1  coincide with each other in absolute position via the projection optical system  10 . This makes it necessary to carry out base line measurements by the TTR method. Base line measurement operation will be described below.  
         [0061]     The dummy reticle R 4  is fed by the reticle drive stage  11  into an exposure range (base line measurement position) of the projection optical system  10  (Step S 101 ). The dummy reticle R 4  uses a lamination of exposure light reflecting films as its parent material. The mark M 7  is formed by arranging absorbing members on the parent material in such a way that reflected light which is slit-shaped in the X and Y directions will enter the projection optical system  10 .  
         [0062]     Next, the wafer drive stage  3  is moved in such a way as to position the dummy wafer W 3  under the projection optical system  10  (at the base line measurement position) (Step S 102 ). The mark M 8  is placed on the dummy wafer W 3 . The mark M 8  which is a slit-shaped transparent part of the same size as the mark M 7  (but scaled down by the projection optical system) is formed by placing a light shielding member on an exposure light-transmitting member. An enlarged schematic view of the mark on the dummy wafer W 3  and part around the light quantity detection sensor S 3  are shown in  FIG. 3 . A thin film M 8 - a  in  FIG. 3  is made of Si, SiC, SiNx, diamond, or diamond-like carbon and is 2 μm or less in thickness. The mark M 8  with a transparent area M 8 - c  of the same shape as a reflecting area of the mark M 7  on the dummy reticle R 4  which is a projected body is formed on the thin film M 8 - a . The mark M 8  is made of a tantalum, tungsten, or other light shielding member M 8 - b . Thus, the mark M 8  has a slit shape in the X and Y directions. Also, the mark M 8  is located almost in the same plane as the exposure surface of the wafer W 1  and right above the light quantity detection sensor S 3  for alignment measurement. Incidentally, the patterns on the marks M 7  and M 8  are not limited to slit shape. The pattern on the mark M 7  (the pattern of the reflector) and pattern on the mark M 8  (the pattern in the transparent area) may have any shape as long as they are similar to each other and the pattern on the mark M 7  is reduced at a reduction ratio of the projection optical system  10 .  
         [0063]     Next, the exposure light L 2  is admitted (Step S 103 ) and the wafer stage  3  is moved slightly in the X and Y directions by monitoring light quantity using the light quantity detection sensor S 3  installed under the dummy wafer W 3 . By finding the position which maximizes the light quantities in the X and Y directions on the wafer stage  3 , it is possible to align the reticle and wafer via the projection optical system (Steps S 104  and S 105 ). The position at which the light quantities are maximized corresponds to the position at which slit-shaped EUV light reflected by the mark M 7  on the dummy-reticle R 4  passes through the slit-shaped transparent part (M 8 - c ) of the mark M 8  on the dummy wafer W 3  efficiently. This is the position at which the marks (M 7  and M 8 ) on the reticle and wafer are superimposed when viewed through the projection optical system  10 .  
         [0064]     Furthermore, the position which maximizes the light quantity is found by moving the wafer stage  3  slightly in the Z direction, and thereby the best focus plane of the reticle pattern is detected (Steps S 106  and S 107 ). At the best focus, blurring due to defocusing of the slit-shaped EUV light reflected by the mark M 7  on the dummy reticle R 4  is minimized. This reduces stray caused by the light shielding part (M 8 - b ) of the mark M 8  on the dummy wafer W 3 , and consequently allows the EUV light to reach the light quantity detection sensor S 3  efficiently. This makes it possible to detect the best focus plane at a maximum light quantity level. Consequently, X-Y position of the mark M 7  on the dummy reticle R 4 , X-Y position of the mark M 8  on the dummy wafer W 3 , and focus position are put in correspondence. In this state, the positions of the reticle drive stage  11  and wafer drive stage  3  are measured by the laser interferometer and stored (Step S 108 ).  
         [0065]     Next, the mark M 8  on the dummy wafer W 3  is moved to below the off-axis microscope  6  based on the measured values from the laser interferometer (hereinafter referred to as the interferometer basis). The position of the mark M 8  is measured with reference to the index mark M 3  by using the off-axis microscope  6 . As described above, an offset of the optical axis of the off-axis microscope  6  (origin of the measurement coordinate system) in relation to the optical axis of the projection optical system  10 , i.e., a base line BL, can be measured based on these measured values as well as on the measured values from the wafer stage laser interferometer (Step S 109 ).  
         [0066]     When the base line is determined, the center of measurement (origin) of the off-axis microscope  6  is aligned with the mark (center of shot) on the wafer W 1  and the wafer drive stage is driven by the amount equivalent to the base line. This makes it possible to feed the center of shot on the wafer W 1  to a position on the optical axis of the projection optical system  10 . On the side of the reticle, the distance between the mark M 7  on the dummy reticle R 4  and mark M 9  on the dummy reticle R 3  has been measured by the interferometer and reticle microscope. If this distance is denoted by RL, by moving the reticle drive stage by RL from the position it was located during the base line measurement, it is possible to bring the center of shot on the wafer W 1  into coincidence with the center of reticle image in the projection optical system  10 .  
         [0067]     The base line measurement is performed in this way. Subsequently, during an exposure operation, scan exposure is repeated by feeding the reticle R 3  into place on the interferometer basis using the reticle drive stage measuring the position of the mark on the wafer W 1  using the off-axis microscope  6 , and moving the wafer drive stage  3  by the amount equal to the base line. By performing the base line measurements and focus calibrations at certain intervals, it is possible to cancel out changes in the base line caused by various factors such as thermal deformation of various parts.  
         [0068]     Incidentally, although in this embodiment, the mark M 8  on the dummy wafer W 3  is created by placing a metal as a light shielding member on an EUV-transparent parent material, there is no need to place a light shielding member on the transparent member as long as processing accuracy is achieved. For example, as shown in  FIG. 4 a  light shielding member M 10 - a  with slits M 10 - b  cut through it may be used. In this case, although the light shielding member M 10 - a  is preferably as thin as possible, since the EUV-transparent part is provided as through-holes, there is no need to consider transmittance of the member, and thus the upper thickness limit of 2 μm is lifted. Also, since the light shielding member M 10 - a  does not need to transmit EUV light, any metal or material suitable for machining can be freely selected as well as Si, SiC, SiNx, or diamond.  
         [0069]     Thus, the first embodiment makes it possible to conduct TTR-based calibration measurement (base line measurement, image plane position measurement in the projection optical system, and the like) on an exposure apparatus which uses exposure light with a very short wavelength such as EUV light which is difficult to handle in a transmissive optical system.  
       Second Embodiment  
       [0070]     TTR measurement used mainly for base line correction has been described in the first embodiment, but the present invention is not limited to base line correction. For example, the present invention is useful for measurement of deviations in reticle stage and wafer state travels as well as for all measurement techniques which involve observing transmitted light by placing a mark in front of a sensor. As examples of such measurement, measurement of wavefront aberration in a projection optical system and measurement of pupil-fill intensity distribution (effective light source) will be described in a second embodiment.  
         [0071]      FIG. 5  is a schematic diagram showing a schematic configuration of an exposure apparatus according to the second embodiment.  FIG. 5  shows a measurement system which is equipped with functions different from those of the first embodiment and which is used for an EUV exposure apparatus similar to that of the first embodiment in  FIG. 1 . The same components/functions as those in  FIG. 1  are denoted by the same reference numerals/characters as the corresponding components/functions in  FIG. 1 . Whereas the configuration of the first embodiment implements base line measurement and the like (measurement of relative position between a reticle stage and wafer stage by the TTR method), the second embodiment has a configuration which has a function to measure aberration in a projection optical system.  
         [0072]      FIG. 6  is a flowchart showing procedures for aberration measurement according to this embodiment. Incidentally, the procedures shown in  FIG. 6  are carried out as a controller (not shown) of the exposure apparatus according to this embodiment controls various parts of the exposure apparatus. The controller has a storage which stores a computer program corresponding to the procedures and CPU which executes the computer program stored in the storage. The second embodiment will be described in detail below with reference to  FIGS. 5 and 6 .  
         [0073]     First, a reflective dummy reticle R 5  (hereinafter referred to as the dummy reticle R 5 ) is moved to a measurement position (Step S 201 ) as shown in  FIG. 5 . A diffraction grating  13  is placed in an exposure path in front of or behind (in front of, in the case of  FIG. 5 ) a reflecting surface of the reticle to diffract light as shown in  FIG. 5 . The diffraction grating  13  may be provided as lattice-like perforations produced mechanically in a metal or other member which shields EUV. Alternatively, the diffraction grating  13  may be produced by attaching light shielding bands of tungsten, tantalum, or the like to a thin film of SiC, Si, SiNx, diamond, or diamond-like carbon 2 μm or less in thickness. Incidentally, when attaching the light shielding bands to a thin film, the diffraction grating can be produced by a method similar to that of the optical element. Next, a dummy wafer W 4  is moved to a measurement position, such as shown in  FIG. 5  (Step S 203 ). After that, exposure light (EUV light) is directed into the illumination optical system  9  from the light source  8 .  
         [0074]     Incidentally, absorbing members are arranged on a reflecting surface of a mark M 11  on the dummy reticle R 5  so that part which reflects the diffracted light from the diffraction grating  13  will have a very fine slit or pinhole. The reflecting surface with the slit or pinhole cancels out the aberration caused by the illumination optical system  9  and makes illuminating EUV light with an ideal wavefront enter the projection optical system  10 .  
         [0075]     The dummy wafer W 4  placed near a wafer-side image plane has a parent material transparent to EUV light similar to that of the first embodiment, and a mark M 12  with a light shielding member arranged in a predetermined pattern is formed on the parent material. The parent material is a thin film of Si, SiC, SiNx, diamond, or diamond-like carbon 2 μm or less in thickness. The light shielding member is made of tungsten, tantalum, or the like. The mark M 12  is formed by placing a light shielding member on the parent material in such a way as to form a slit-shaped or pinhole-shaped transparent region and a window-shaped transparent region with a large transparent area.  
         [0076]     A CCD camera S 4  for EUV is placed under the mark M 12  on the wafer image plane.  FIG. 7  is an enlarged schematic view of a region near the mark M 12  of the dummy wafer W 4 . The mark M 12  with a slit-shaped or pinhole-shaped transparent part M 12 - c  and a window-shaped transparent part M 12 - d  is formed by placing a light shielding member M 12 - b  on a thin film M 12 - a  which, being made of one of the above-described materials and having a thickness within the above-described range, transmits EUV light.  
         [0077]     As described above, the mark M 11  placed on the dummy reticle R 5  and equipped with the slit-shaped or pinhole-shaped reflector allows EUV light with an aberration-free ideal wavefront to enter the projection optical system  10 . Consequently, the EUV light coming out of the projection optical system  10  only contains aberration attributable to the projection optical system. As the EUV light passes through the slit-shaped or pinhole-shaped transparent part M 12 - c  in  FIG. 7 , the aberration contained in the EUV light is cancelled out, and consequently the EUV light reaches a plane of the CCD camera S 4  as an ideal wavefront. Preferably, the CCD camera S 4  consists of a light-sensitive element which is sensitive to EUV light.  
         [0078]     However, a CCD camera for visible radiation may also be used if it is configured to detect fluorescence which is produced through scintillation by a scintillator placed in front of the CCD camera and which is guided to the CCD camera via a fiber-optic plate.  
         [0079]     On the other hand, the EUV light passing through the window-shaped transparent part M 12 - d  in  FIG. 7  reaches the plane of the CCD camera S 4 , still containing the aberration caused by the projection optical system  10 . The EUV light passing through the transparent part M 12 - c  is used as a reference beam and the EUV light passing through the window-shaped transparent part M 12 - d  is used as a sample beam. The difference between the wavefronts of the two lights is wavefront aberration caused by the projection optical system  10 . When taken into the CCD camera S 4 , the wavefront aberration is observed as interference fringes caused by the reference beam and sample beam. If the interference fringes are subjected to image processing, image processing of the interference fringes by means of an electronic moire technique makes it possible to measure wavefront aberration quantitatively up to the 36th term of Zernike polynomials (Step S 206 ).  
         [0080]     An example of how to produce interference fringes on the CCD camera plane has been described in the second embodiment, but this is not restrictive. For example, a conceivable technique involves directing EUV light in which aberration is cancelled out by a slit-shaped or pinhole-shaped reflector on a reticle surface into the projection optical system, producing interference fringes using difference in wavefront aberration between the 0th-order light and first-order light by placing a diffraction grating on the wafer image plane, observing the interference fringes with a CCD camera, and measuring the wavefront aberration through integrating and image processing. In this case, the configuration consisting of a light shielding member placed on the EUV-transparent member (M 12 - a ) may be used as the diffraction grating installed near the wafer-side image plane (approximately the same plane as the exposed surface of the wafer).  
         [0081]     Also, measurement of wavefront aberration has been described in the second embodiment, but an effective-light-source measurement system for measurement of pupil-fill intensity distribution in the illumination optical system can be obtained by removing the diffraction grating  13  and making pinholes in both reticle-side image plane (object plane) and wafer-side image plane. In this case, the pinhole in the wafer-* side image plane can be produced by placing a light shielding member made of tungsten, tantalum, or the like on an EUV-transparent member of Si, SiC, SiNx, diamond, or diamond-like carbon 2 μm or less in thickness in such a way as to leave a pinhole-shaped transparent part.  
         [0082]     Thus, by using a light-transmitting member of a material (Si, SiC, SiNx, diamond, or diamond-like carbon) and thickness (2 μm or less) cited in the above embodiments, it is possible to obtain an optical element which can be used for various measurements of EUV light passing through a predetermined pattern (slit, pinhole, diffraction grating, or the like). Incidentally, various patterns for optical elements are conceivable in addition to those cited in the first and second embodiments. For example, the above technique is also available for use to form a diffraction pattern leading to a predetermined pupil-fill intensity distribution in the illumination optical system, as an optical element with a function similar to that of the diffraction grating. In this embodiment, as is the case with the first embodiment, a transparent part may be punctured in a light shielding member instead of placing a light shielding member on a transparent member as long as processing accuracy is achieved.  
         [0083]     As described above, the first and second embodiments make it possible to conduct various measurements using EUV light on the EUV-based exposure apparatus, including TTR-based measurement of relative positional relationship between a wafer and reticle (measurement of relative positional relationship in directions perpendicular and/or parallel to the optical axis of a projection optical system), measurement of aberration in the projection optical system, or measurement of pupil-fill intensity distribution of a illumination optical system. Thus, it is possible to conduct various measurements using exposure light passing through a predetermined pattern on an EUV exposure apparatus or other exposure apparatus which uses exposure light with a very short wavelength which is difficult to handle in a transmissive optical system. This in turn makes it possible, for example, to always perform high accuracy focusing and alignment or optimize performance of the optical system, and thus transfer fine circuit patterns in a stable manner.  
       Third Embodiment  
       [0084]     Now, a device manufacturing method which used the above-described exposure apparatus will be described with reference to  FIGS. 11 and 12 .  
         [0085]      FIG. 11  is a flowchart illustrating manufacture of a device (IC, LSI, or other semiconductor chip, LCD, CCD, or the like). This embodiment will be described, taking manufacture of a semiconductor chip as an example. In Step S 1  (circuit design), a circuit of the device is designed. In Step S 2  (mask fabrication), a mask (also known as a reticle) is fabricated using the designed circuit pattern. In Step S 3  (wafer fabrication), wafer is fabricated using silicon or other material. In Step S 4  (wafer process), which is called a front-end process, actual circuits are formed on the wafer by lithography technology using the mask and wafer. In Step S 5  (assembly), which is called a back-end process, semiconductor chips are produced from the wafer fabricated in Step S 4 . This step includes an assembly process (dicing and bonding), packaging process (chip encapsulation), and other processes. In Step S 6  (inspection), inspections are performed, including an operation checking test and durability test of the semiconductor device fabricated in Step S 5 . The semiconductor device is completed through these processes, and shipped out subsequently (Step S 7 ).  
         [0086]      FIG. 12  is a detailed flowchart of the wafer process in Step S 4 . In Step S 11  (oxidation), a surface of the wafer is oxidized. In Step. S 12  (CVD) an insulating film is formed on the wafer surface. In Step S 13  (electrode formation), electrodes are formed on the wafer by vapor deposition. In Step S 14  (ion implantation), ions are implanted into the wafer. In Step S 15  (resist process), a photosensitive material is applied to the wafer. In Step S 16  (exposure), the wafer is exposed through the mask by the exposure apparatus. In Step S 17  (development), the exposed wafer is developed. In Step S 18  (etching), part other than the developed resist image is etched. In Step S 19  (resist removal), any unnecessary resist remaining after the etching is removed. As these steps are repeated, a multiple layers of circuit patterns are formed on the wafer. The device manufacturing method according to this embodiment can manufacture devices of higher quality than conventional ones. Thus, the device manufacturing method using the exposure apparatus as well as resulting devices also constitute an aspect of the present invention.  
         [0087]     These embodiments provide an optical element suitable for extreme ultraviolet light-based measurement for which it is difficult to use a transmissive optical element, exposure apparatus incorporating the optical element, and device manufacturing method using the exposure apparatus.  
         [0088]     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.  
         [0089]     This application claims the benefit of Japanese Application No. 2005-099416, filed on Mar. 30, 2005, which is hereby incorporated by reference herein in its entirety.