Abstract:
A method and apparatus for measuring a wave front aberration of a projection lens with high precision and a related calibration method. The apparatus includes: either a light source and an element producing a first point source in combination with the light source or a first point source generating part; a magnifying projection optical system projecting and magnifying a point image of the first point source projected by a test object; a detector detecting the magnified point image projected and magnified by the magnifying projection optical system; a supporting member supporting the magnifying projection optical system and the detector; a calculating part calculating a wave front aberration; and either a second point source producing element or a second point source generating part.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for measuring a wave front aberration, a method for measuring a wave front aberration using the apparatus, and a method for manufacturing a projection lens by using thereof. 
     2. Description of Related Art 
     Recently, a projection lens of a projection exposure apparatus has been required to have higher resolving power in order to cope with the recent improvement of the density and the minuteness of a semiconductor device. 
     An optical performance of a projection lens has been evaluated by a wave front aberration because the wave front aberration is the sole measure capable of foreseeing resolution of any pattern. 
     There are several methods for measuring a wave front aberration. Among them, a method that measures a wave front aberration by a phase retrieve method based on point-spread functions of an object at a focused position and a defocused position and known information has been proposed in following references: J. Maeda at al. “ Retrieval of wave aberration from point - spread function or optical transfer date ” Applied Optics vol. 20, p274-279, D. L. Misell “ An examination of an iterative method for solution of the phase problem in optics and electron optics ” Test calculations Journal of physics D: Applied Physics vol. 6, p2200-2216. 
     However, when an apparatus for measuring a wave front aberration is realized based on the theory, since it is impossible to directly detect the point-spread function by a CCD on account of limited number of pixels of the CCD, it is inevitable for the apparatus f or measuring a wave front aberration to have an magnifying projection optical system for magnifying the point-spread function. 
     In this case, it is necessary to eliminate aberrations of the magnifying projection optical system. Thus the aberrations of the magnifying projection optical system have to be measured. This is fit for calibration. In the phase retrieve method from a point image, calibration method for the apparatus has not been found out. 
     Moreover, deformation of a point image may be caused not only by a wave front aberration of a test lens but also by an unnecessary element caused by a construction of the point image. When the deformation of a point image includes other elements than the wave front aberration of the test lens, the wave front aberration of the test lens cannot be measured correctly. 
     FIG. 10 is a drawing showing a construction of an ordinary point source  71 . The point source  71  is constructed by a pinhole  74  arranged on a shield film  73  formed on a glass substrate  72 , a lamp  75  for illuminating the pin hole  74  with a parallel light L 11  from the shield film  73  side, and an illumination lens  76 . 
     In the point source  71 , a light L 12  passed through the pinhole  74  is exit as a measuring light L 13  after passing through the glass substrate  72 . Then, the light L 13  is converged by a test lens (not shown), and forms a deformed point image (an image of the pinhole  74 ). 
     In this case, a light L 12  just passed through the pin hole  74  is approximately a spherical wave. However, a light incident to the test lens (not shown) is the measuring light L 13  affected with aberrations (spherical aberration and coma) caused by the glass substrate  72  while passing through the glass substrate  72 . Accordingly, the point image obtained by converging the measuring light L 13  inevitably includes deformation in accordance with aberrations caused by the glass substrate  72 . 
     Further, the parallel light L 11  illuminating the pinhole  74  has an angular divergence θ determined by the focal length of the illumination lens  76  and the aperture diameter d of the lamp  75 . Since the angular divergence θ appears directly on an angular divergence ψ of the measuring light L 13  after passing through the glass substrate  72 , the point image contains deformation in accordance with the angular divergence ψ (which is equal to the angular dispersion θ of the parallel light L 11 ) of the measuring light L 13 . 
     Furthermore, when a flare light L 14  is incident to the pinhole  74 , since an unnecessary light caused by the flare light  14  is mixed with the measuring light L 13 , deformation caused by the flare light  14  is added to the point image. 
     Thus when the wave front aberration of the test lens is measured by using the point source  71  having the construction shown in FIG. 10, deformation of the point image includes influence of unnecessary elements (aberrations in the glass substrate  72 , an angular dispersion ψ of the measuring light L 13 , a flare light L 14 ), so that the wave front aberration of the test lens cannot be measured precisely. 
     Moreover, although influence of aberrations of the glass substrate  72  can be corrected after the calculation using the phase retrieve method, it has not been easy because the thickness and the inclination of the glass substrate  72  have to be measured precisely. 
     Further, although the influence of the angular dispersion ψ of the measuring light L 13  can be relieved by making the angular dispersion θ of the parallel light L 11  smaller by using smaller aperture diameter d, it is not desirable that the light quantities of the parallel light L 11  as well as the measuring light L 13  decrease in correspondence with decrease in the aperture diameter d. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of the above-mentioned problems and has an object to provide a method and apparatus for measuring a wave front aberration of a projection lens with high precision and a calibration method of the apparatus for measuring a wave front aberration. 
     According to an aspect of the present invention, an apparatus for measuring a wave front aberration includes: either a light source and an element that produces a first point source in combination with the light source and that is removably or movably arranged or a first point source generating part; a holding mechanism that holds a test object; an magnifying projection optical system that projects and enlarges a point image of the first point source projected by the test object; a detector that detects the magnified point image projected and magnified by the magnifying projection optical system; a supporting member that supports the magnifying projection optical system and the detector and that can be moved along the optical axis and in a plane perpendicular to the optical axis; a calculating part that calculates a wave front aberration by means of a phase retrieve algorithm based on a point-spread function detected by the detector and known information input in advance; and either a second point source producing element that is removably or movably arranged and that produces a second point source on the image plane of the test object by means of the test object in combination with any one of the light source, the light source and the element, and the first point source generating part, or a second point source generating part that produces the second point source on the image plane of the test object and that is removably or movably arranged. 
     The point image of the first point source is projected by the test object in the image plane of the test object. 
     The image is deformed by the aberration of the test object relative to an ideal point image. 
     A point image which is further projected and magnified image by the magnifying projection optical system is further deformed by an aberration of the magnifying projection optical system. 
     Accordingly, the point image formed on the CCD includes the aberration of the test object superimposed by the aberration of the magnifying projection optical system. 
     Therefore, in order to measure the wave front aberration of only the test object, it is necessary that the aberration of only the magnifying projection optical system is measured first, and, then, the aberration of the test object and that of the magnifying projection optical system is subtracted by that of only the magnifying projection optical system. 
     At first, the second point source is generated in the image plane of the test object and, then, the aberration of only the magnifying projection optical system can be calculated by the phase retrieve algorithm based on the point-spread function formed by the detector via the magnifying projection optical system and known information. 
     In other words, since an ideal light from a point source is projected on the detector via the magnifying projection optical system, the point-spread function of the image contains the aberration of only the magnifying projection optical system. 
     According to another aspect of the present invention, an apparatus for measuring a wave front aberration includes: a point source that has a shield member and an illuminating member for illuminating a pinhole part made on the shield member from one side, and that emanates a measuring light from the other side of the pinhole part; a holding mechanism that holds a test lens; and a detector that detects the intensity distribution of the point image on the other side projected by the test lens; wherein the diameter of the pinhole part made on the shield member of the point source, facing to the one side illuminated by the illuminating member is larger than that on the other side emanating the measuring light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing an outline of an apparatus for measuring a wave front aberration according to a first embodiment of the present invention. 
     FIG. 2 is a diagram showing an outline of an apparatus for measuring a wave front aberration according to a second embodiment of the present invention. 
     FIG. 3 is a diagram showing an outline of an apparatus for measuring a wave front aberration according to a third embodiment of the present invention. 
     FIG. 4 is a sectional view showing a construction of a mask  43 . 
     FIG.  5 (A) is a top plan view of the mask  43 . 
     FIG.  5 (B) is a bottom plan view of the mask  43 . 
     FIG. 6 is a sectional view explaining behavior of a light passing through a pinhole part  47  of the mask  43 . 
     FIG. 7 is a diagram explaining behavior of the apparatus for measuring a wave front aberration  30 . 
     FIGS.  8 (A) and (B) are drawings explaining a method for calibrating the apparatus for measuring a wave front aberration  30 . 
     FIG. 9 is a drawing explaining another method for calibrating the apparatus for measuring a wave front aberration  30 . 
     FIG. 10 is a drawing explaining a construction of a point source  71  according to prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An apparatus for measuring a wave front aberration according to embodiments of the present invention will be explained below with reference to the attached drawings. 
     &lt;First Embodiment&gt; 
     FIG. 1 is a diagram showing an outline of an apparatus for measuring a wave front aberration according to a first embodiment of the present invention. 
     In FIG. 1, reference number  1  denotes a light source,  2  denotes an illumination optical system,  3  denotes a mask on which a pinhole is formed,  4  denotes a test object,  6  denotes an magnifying projection optical system,  7  denotes a CCD,  8  denotes a supporting member,  9  denotes a computer,  11  denotes a memory,  12  denotes a mask on which a pinhole is formed and having a moving mechanism (in x, y, and z direction) which is not shown, and  15  denotes a display. 
     In the apparatus for measuring a wave front aberration according to the first embodiment of the present invention, a light emanated from the light source  1  illuminates the mask  3  on which the pinhole is formed via the illumination optical system  2 . This part as a whole plays a part for generating a point source (a first point source). 
     The diameter of a pinhole formed on the mask  3  is preferably smaller than λ 1 /NA T  where NA T  denotes the numerical aperture of the test object  4  to the mask  3  side and λ 1  denotes a wavelength of the light source. When the diameter of the pinhole exceeds λ 1 /NA T , the pupil of the test object  4  cannot be illuminated uniformly, and, moreover, angular distribution of the incident light to the illumination optical system affects to the illumination condition. 
     However, a means for generating the first point source is not limited to this, a first point source generating part other than this can be preferably used for this purpose. 
     As for the first point source generating part, a single mode optical fiber connected to a laser light source can be mentioned. A laser light exit from a single mode optical fiber does not include any wave front aberration. 
     In order to prevent a substrate from generating aberrations, a shielding film with a pinhole, for example, metallic film such as chromium, or the like is formed on a surface of a transparent substrate as a mask  3 . The mask  3  is arranged such that a transparent substrate is facing to incident side. 
     A light diffracted by the pinhole formed on the mask  3  forms a point image on the image plane  5  of the magnifying projection optical system  6  after passing through the test object (projection lens)  4 . 
     The image plane  5  is optically conjugate with the mask  3 . 
     Moreover, the point image is formed an magnified point image on the CCD  7  by the magnifying projection optical system  6  and, then, the intensity distribution of the magnified point image is detected by the CCD  7 . 
     The magnifying projection optical system  6  and the CCD  7  are supported by the supporting member  8  constructed in a body and a point image in a defocusing state can be formed on the CCD  7  by moving the supporting member  8  in the Z direction. 
     The information of the point-spread function detected by the CCD  7  is transmitted to the computer  9 . 
     Information such as a wavelength (λ 1 ) of the light source  1 , a numerical aperture (NA T ) of the test object (projection lens)  4 , a pupil shape of the test object  4 , a wavelength of a second point source generating part, which is explained later, (hereinafter, called known information), and the well-known phase retrieve algorithm is input in the computer  9  in advance. 
     As for the well-known phase retrieve algorithm, we can use one mentioned in a literature such as J. Maeda et al. “ Retrieval of wave aberration from point - spread function or optical transfer date ” Applied Optics vol. 20, p274-279, D. L. Misell “ An examination of an iterative method for solution of the phase problem in optics and electron optics ” Test calculations Journal of physics D: Applied Physics vol. 6, p2200-2216. 
     Accordingly, a wave front aberration is calculated in the computer  9  based on information of the point-spread function in the state of in focus and a plurality of out of focus states by using the phase retrieve algorithm. 
     Moreover, on performing calibration of the apparatus for measuring a wave front aberration, the mask  12  (not shown) is inserted by the moving mechanism. This is an element for generating a point source. The aforementioned light from the first point source passes through the test object  4 , converges in the vicinity of the pinhole of the mask  12 , and, as a result, a role of a point source is preformed as a whole (a second point source). 
     By the way, although when the second point source is to be generated, the mask  3  is not an indispensable element, a light from the light source  1  passes through the test object  4 , converges in the vicinity of the pinhole of the mask  12 , and, as a result, a role of a point source is performed as a whole. 
     The diameter of the pinhole formed on the mask  12  is preferably smaller than λ 2 /NA E  where NA E  denotes the numerical aperture of the magnifying projection optical system  6  to the mask  12  side and λ 2  denotes a wavelength of the light source. When the diameter of the pinhole exceeds λ 2 /NA E , the pupil of the magnifying projection optical system  6  cannot be illuminated uniformly, and, moreover, angular distribution of the incident light to the illumination optical system affects to the illumination condition. 
     A method for measuring a wave front aberration according to the first embodiment will be described below. 
     A wave front aberration of the test object is calculated by subtracting a second wave front aberration from a first wave front aberration described later. 
     First, the first wave front aberration is explained. 
     The test object  4  is arranged in a test-object holder  16  of an apparatus for measuring a wave front aberration according to the first embodiment, and a focusing position is arranged to the state of in focus by moving the supporting member  8  in the Z direction. The test object holder  16  is shown in block form and may be any known type of object holder. 
     The point-spread function in the state of in focus is measured by the CCD  7 . 
     The information of the point-spread function is transmitted from the CCD  7  to the computer  9 . 
     The state of in focus is changed to the state of out of focus by moving the supporting member  8  in the Z direction. 
     The point-spread function in the state of out of focus is measured by the CCD  7 . 
     The information of the point-spread function is transmitted from the CCD  7  to the computer  9 . 
     In the same way, the point-spread function in the other state of out of focus is measured a plurality of times. 
     A wave front aberration is calculated based on the information of the point-spread functions in the state of in focus and a plurality of times of out of focus by repeating calculation using the well-known phase retrieve algorithm (the value is the first wave front aberration). 
     The measurement of the point-spread function is preferably performed more than three positions on the optical axis including in the state of in focus and out of focus. 
     Furthermore, in order to measure wave front aberrations on the important positions within the angle of view of the test object  4 , the mask  3  is moved in a stepping manner by a predetermined amount in the direction perpendicular to the optical axis, and the wave front aberration in each position is measured. 
     Since the position of the image moves in accordance with this, the magnifying projection optical system  6  and the detector  7  is to be moved by moving the supporting member  8  in the X and Y directions. 
     In the projection exposure apparatus, this is a necessary step for the wave front aberration of the illumination area to keep within a standard value in order to illuminate a predetermined area of the photomask and to form a predetermined pattern on the wafer with high precision by the projection lens. 
     Then, a calibration method of the apparatus for measuring a wave front aberration will be explained (a second method for measuring a wave front aberration). 
     In this case, the position of the pinhole is made agree with focal point of the test object on the optical axis. 
     Accordingly, the second point source generating part is formed. 
     A focus position is arranged to the state of in focus by moving the supporting member  8  in the Z direction. 
     An image of the second point source is formed on the image plane of the magnifying projection optical system  6 , which is the CCD  7 . 
     The point-spread function in the state of in focus is measured by the CCD  7 . 
     The information of the point-spread function is transmitted from the CCD  7  to the computer  9 . 
     The state of in focus is changed to the state of out of focus by moving the supporting member  8  in the Z direction. 
     The point-spread function in the state of out of focus is measured by the CCD  7 . 
     The information of the point-spread function is transmitted from the CCD  7  to the computer  9 . 
     In the same way, the point-spread function in the other state of out of focus is measured a plurality of times. 
     A wave front aberration is calculated based on the information of the point-spread functions in the state of in focus and a plurality of times of out of focus by repeating calculation using the well-known phase retrieve algorithm (the value is the second wave front aberration). 
     &lt;Second Embodiment&gt; 
     FIG. 2 is a diagram showing an outline of an apparatus for measuring a wave front aberration according to a second embodiment of the present invention. 
     The apparatus for measuring a wave front aberration according to a second embodiment has a construction having the moving mechanism according to the first embodiment, and having a single mode optical fiber  14  equipped with a laser light source  13  instead of the mask  12  on which a pinhole is formed. The wavelength of the laser light source  13  is necessary to be approximately equal to that of the light source  1 . 
     On performing calibration, the light source  1  is put off and the laser light source  13  is put on. 
     The exit laser beam from the single mode optical fiber  14  does not include any wave front aberration. 
     The single mode optical fiber  14  equipped with a laser light source  13  becomes the second point source generating part. 
     Moreover, the exit surface of the optical fiber  14  is arranged on the image plane  5  of the test object  4 . The spot size of the laser beam is selected to be smaller than λ 2 /NA E  where NA E  denotes the numerical aperture of the magnifying projection optical system  6  to the image plane  5  (image plane of the test object) side and λ 2  denotes a wavelength of the laser light source  13 . 
     The reason why the spot size is made to be smaller than λ 2 /N E  is to uniformly illuminate the pupil of the magnifying projection optical system  6 . 
     When the spot size exceeds λ 2 /NA E , the pupil of the magnifying projection optical system  6  cannot be illuminated uniformly. 
     A condenser optical system for converging the light from the laser light source  13  onto the image plane  5  can be arranged instead of the single mode optical fiber  4 . 
     In this case, the real wave front aberration of the magnifying projection optical system is the value that the wave front aberration of the magnifying projection optical system measured while performing calibration is subtracted by that of the condenser optical system corrected its numerical aperture. 
     Furthermore, when the wave front aberration of the condenser optical system is not known, a mask on which a pinhole is formed may be arranged at the focal point of the condenser optical system. 
     The method for manufacturing a projection lens is explained below. 
     A wave front aberration of a projection lens is measured by the apparatus for measuring a wave front aberration according to the first or second embodiment of the present invention. A projection lens whose wave front aberration is not satisfied with the predetermined value is readjusted, and measured its wave front aberration. This process is repeated until the wave front aberration has been satisfied with the predetermined value. Thus, a projection lens with high precision can be manufactured. 
     &lt;Third Embodiment&gt; 
     FIG. 3 is a diagram showing an outline of an apparatus for measuring a wave front aberration according to a third embodiment of the present invention. 
     An apparatus for measuring a wave front aberration  30  according to the third embodiment shown in FIG. 3 is constructed by a point source part  31  and an imaging part  32 . A test lens  33  (for example, a projection lens in an exposure apparatus) is arranged between the point source part  31  and the imaging part  32  with aligning the optical axis  33 A of the test lens with the axis  30 A of the apparatus  30 . 
     The point source part  31  of the apparatus for measuring a wave front aberration  30  is constructed by a lamp  41 , an illumination lens  42 , and a mask  43 . 
     The exit surface of the lamp  41  is located on the focal plane of the illumination lens  42 . The optical axis of the illumination lens  42  is located on the axis  30 A. These lamp  41  and illumination lens  42  are corresponding to “an illuminating member” in attached claim. 
     FIG. 4 is a sectional view showing a construction of a mask  43 . The mask  43  (shield member) is constructed by a glass substrate  44  (transparent substrate) coated on both surfaces with thin metallic films  45 ,  46  (shield film with a thickness from 0.2 μm to 0.3 μm). One side of the metallic film  45  is formed on the illumination lens  42  side surface  44   a  of the glass substrate  44 , and the other metallic film  46  is formed on the test lens  33  side surface  44   b . The surfaces  44   a  and  44   b  on which the metallic films  45  and  46  are formed are perpendicular to the axis  30 A. 
     Moreover, a plurality of pinhole parts  47  are formed on the mask  43  two-dimensionally within the effective visual field of the test lens  33 . The plurality of pinhole parts are arranged, for example, in latticed or circular shape in accordance with an image point to be measured. 
     Although only three pinhole parts  47  are shown in FIG. 4, the practical number of pinhole part is from 50 to 100. 
     FIGS.  5 (A) and (B) are top and bottom plan views of the mask  43 , respectively. In each pinhole part  47 , the illumination lens  42  side is an aperture  48  shown in FIG.  5 (A) formed on the metallic film  45  and the test lens  33  side is a pinhole  49  shown in FIG.  5 (B) formed on the metallic film  46 . 
     The aperture  48  and the pinhole  49  of the pinhole part  47  are arranged such that an axis  47 A pausing through the center of the aperture  48  and that of the pinhole  49  is parallel to the axis  30 A (which is the optical axis  33 A of the test lens  33 ) of the apparatus for measuring a wave front aberration  30 . The diameter D 1  of the aperture  48  in FIG.  5 (A) is larger than that D 2  of the pinhole  49 . By the way, the diameter D 2  of the pinhole  49  is about 0.5 μm to 1.0 μm. 
     In FIG. 3, the imaging part  32  of the apparatus for measuring a wave front aberration  30  is composed of an magnifying lens  52  and a CCD imaging device  53  (detector). The magnification of the magnifying lens  52  is about 40 to 600 times. The CCD imaging device  53  is an area sensor having a plurality of pixels arranged two-dimensionally. 
     These magnifying lens  52  and CCD imaging device are supported by a supporting member  54  (driving member) in common, and can be moved in directions parallel (Z direction) and perpendicular (XY direction) to the axis  30 A. 
     Further, a computer  55  (calculating member) is connected to the CCD imaging device  53  of the imaging part  32 , a memory  56 , an input member  57 , and a display  58 . 
     In the apparatus for measuring a wave front aberration  30  of the above-described construction shown in FIG. 3, a light beam L 1  emitted from the lamp  41  illuminates the mask  43  with approximately parallel light (illumination light L 2 ) after passing through the illumination lens  42 . In this case, each pinhole part  47  of the mask  43  is illuminated by the illumination light L 2  from the aperture  48  side as shown in FIG.  4 . By the way, in FIGS. 3 and 4, only a light path exit from the center (on the axis  30 A) of the lamp  41  is shown. 
     The illumination light L 2  passed through each aperture  48  is led from the surface  44   a  of the glass substrate  44  to the inside, and reaches the other side surface  44   b  after passing through the glass substrate  44 . Then, the light L 3  arrived at the other side surface  44   b  illuminates the pinhole  49 , and a light passed through each pinhole  49  becomes an exit light as a measuring light L 4 . 
     The measuring light L 4  exit from each pinhole  49  is converged by the test lens  33  as shown in FIG.  3 . At this moment, a plurality of images (point images) of the pinhole  49  are formed on the image plane  51  of the test lens  33 . The positions of the plurality of the point images (images of the pinhole  49 ) in the image plane  51  are corresponding to the positions of the plurality of the pinholes  49  of the mask  43 . 
     Incidentally, in FIG. 3, the light path (dotted line) of the measuring light L 4  exit from the pinhole  49  located at the center P 1  (on the axis  30 A) of the mask  43  and the light path (solid line) of the measuring light L 4  exit from the pinhole  49  located at P 2  of the mask  43 , which is off the center P 1 . The image of the pinhole  49  located at the center P 1  of the mask  43  is formed at the center Q 1  of the image plane  51 . The image of the pinhole  49  located at P 2  of the mask  43  is formed at Q 2  away from the center Q 1  of the image plane  51 . 
     Next, each point image (image of the pinhole  49 ) formed on the image plane  51  of the test lens  33  is being considered. As described above, since each point image is obtained from the measuring light L 4  formed by the test lens  33 , each point image is inevitably deformed from ideal point image in consequence of a wave front aberration of the test lens  33 . Moreover, the deformation of each point image is liable to be affected not only by a wave front aberration of the test lens  33  but also by an unnecessary element caused by the construction of the point source part  31 . 
     In the point source part  31 , since an exit surface of the lamp  41  has a finite dimension, the illumination light. L 2  illuminating the mask  43  has a well-known angular divergence θ defined by a diameter of the aperture of the lamp  41  and the focal length of the illumination lens  42  as shown in FIG.  6 . 
     The illumination light L 2  having the angular divergence θ is incident to the pinhole part  47  of the mask  43  from the aperture  48  side, and exit from the pinhole  49  side as the measuring light L 4  after passing through the glass substrate  44 . 
     The aperture  48  of the mask  43  has a larger diameter than the pinhole  49  (see FIGS.  4  and  5 ), and functions as an aperture stop, so that the angular divergence φ of the measuring light L 4  exit from the pinhole  49  side of the pinhole part  47  is defined to smaller angular divergence than that θ of the illumination light L 2 . The angular divergence φ of the measuring light L 4  is defined by the diameter D 1  of the aperture  48 , the diameter D 2  of the pinhole  49 , and the thickness D 3  of the glass substrate  44  shown in FIGS. 4 and 5. Conditional expression is as follows: 
      φ=λ 1   /NA   T   +D   1   /D   3   
     where λ 1  denotes the wavelength of the measuring light L 4 , NA T  denotes a numerical aperture to the mask  43  side of the test lens  33 . 
     Thus, in the point source part  31  of this embodiment, since the angular divergence φ of the measuring light L 4  can be small, the deformation of the point image (image of the pinhole  49 ) in accordance with the angular divergence φ of the measuring light L 4  can be minimized. By the way, in the point source part  31 , the angular divergence φ of the measuring light L 4  can be small regardless of the angular divergence θ of the illumination light L 2  Accordingly, the diameter of the aperture of the lamp  41  needs not to be small in order to make the angular divergence φ of the measuring light L 4  smaller. Therefore, the light quantity of the measuring light can be secured. 
     Furthermore, even if a flare light L 5  is incident to the aperture  48  of the pinhole part  47  from the side,f the unnecessary light caused by the flare light L 5  can be blocked from mingling with the measuring light L 4 . Accordingly, the flare light LB does absolutely not affect to the deformation of the point image (image of the pinhole  49 ) that is converged image of the measuring light L 4 . 
     Moreover, since the light (roughly spherical wave) passed through the pinhole  49  of the pinhole part  47  is directly incident to the test lens  33  as a measuring light L 4 , the aberration caused by the glass substrate  44  does absolutely not affect to the deformation of the point image (image of the pinhole  49 ) as has been doing currently. 
     Thus, in the deformation of each point image(image of the pinhole  49 ) formed on the image plane  51  of the test lens  33 , although the influence of the angular divergence φ of the measuring light L 4  is slightly included, the influence of the aberration of the glass substrate  44  or the flare light L 5  is absolutely not included. Therefore, it is understood that the deformation of each point image (image of the pinhole  49 ) is caused only by the influence of the aberration of the test lens  33 . 
     For the reason mentioned above, a plurality of point images (image of the pinhole  49 ) deformed in response to the wave front aberration of the test lens  33  is formed on the image plane  51  of the test lens  33  in an arrangement (for example, Q 1  or Q 2 ) according to the position (for example, P 1  or P 2 ) of the pinhole  49  of the mask  43 . 
     In this case, the deformation of each point image (image of the pinhole  49 ) formed at different position (for example, Q 1  or Q 2 ) on the image plane  51  is usually not same. The reason is that the deformation of the point image formed at the center Q 1  of the image plane  51  corresponds to the wave front aberration of the test lens  33  in response to the image formation from the center P 1  of the mask  43  to the center Q 1  of the image plane  51 . On the other hand, the deformation of the point image formed at Q 2  of the image plane  51  corresponds to the wave front aberration of the test lens  33  in response to the image formation from the position P 2  of the mask  43  to the position Q 2  of the image plane  51 . 
     Then, the method for obtaining the wave front aberration of the test lens  33  in response to the image formation from the center P 1  of the mask  43  to the center Q 1  of the image plane  51  is being explained. 
     In this case, the magnifying lens  52  and the CCD imaging device  53  are positioned at the XY position where the optical axis  52 A of the magnifying lens  52  and the axis  30 A overlap each other by the supporting member  54  as shown in FIG.  3 . At this moment, the center Q 1  of the image plane  51  is located at the center of the visual field of the magnifying lens  52 . In the Z direction, the magnifying lens  52  and the CCD imaging device  53  are positioned at the in focus position where the object plane of the magnifying lens  52  and the image plane  51  overlap each other or at the out of focus position where the object plane of the magnifying lens  52  slightly slips from the image plane  51 . 
     By this positioning, the point image (image of the pinhole  49 ) is magnified by the magnifying lens  52  and projected on the imaging plane of the CCD imaging device  53 . 
     Then, the CCD imaging device  53  detects the light intensity distribution of the point image (image of the pinhole  49 ) formed at the center Q 1  of the image plane  51 . This detection of the light intensity distribution is performed. The of the light intensity distribution performed at the in focus position and the out of focus position are transmitted from the CCD imaging device  53  to the computer  55 . 
     In computer  55 , a repeating calculation using the phase retrieve method is performed based on the light intensity distribution (results of detection at the in focus position and the out of focus position) of the point image (image of the pinhole  49 ) formed at the center Q 1  of the image plane  51  and the known information (the wavelength of the measuring light L 4 , numerical aperture of the test lens  33 , shape of pupil, and the like). 
     As a result, the wave front aberration of the test lens  33  in response to the image formation from the center P 1  of the mask  43  to the center Q 1  of the image plane  51  is calculated. Thus, the obtained wave front aberration is input to the memory  56  and displayed on the display  58 . 
     Moreover, when the wave front aberration of the test lens  33  in response to the image formation from the position P 2  of the mask  43  to the position Q 2  of the image plane  51  is calculated, the magnifying lens  52  and the CCD imagine device  53  are moved in the XY direction by the supporting member  54 , and positioned at the predetermined position as shown in FIG.  7 . At this moment, the position Q 2  of the image plane  51  is located at the center of the visual field of the magnifying lens  52 . In the Z direction, the magnifying lens  52  and the CCD imagine device  53  are moved at the in focus position and the out of focus position as described above. 
     When positioned as shown in FIG. 7, the point image (image of the pinhole  49 ) formed at the position Q 2  of the image plane  51  is magnified by the magnifying lens  52  and projected on the imaging plane of the CCD imaging device  53 . 
     Then, the CCD imaging device  53  detects the light intensity distribution of the point image (image of the pinhole  49 ) formed at the position Q 2  of the image plane  51 . These detection of the light intensity distribution are performed at the in focus position and the out of focus position, and the results of the detection are transmitted from the CCD imaging device  53  to the computer  55 . 
     In computer  55 , a repeating calculation using the phase retrieve method is performed based on the light intensity distribution (results of detection at the in focus position and the out of focus position) of the point image (image of the pinhole  49 ) formed at the position Q 2  of the image plane  51  and the known information (the wavelength of the measuring light L 4 , numerical aperture of the test lens  33 , shape of pupil, and the like). 
     As a result, the wave front aberration of the test lens  33  in response to the image formation from the position P 2  of the mask  43  to the position Q 2  of the image plane  51  is calculated. Thus, the obtained wave front aberration is input to the memory  56  and displayed on the display  58 . 
     In order to measure the wave front aberration of the test lens  33  in response to the image formation from the other positions of the mask  43  to the image plane  51 , the above-mentioned procedure may be repeated. If the position to be measured is in the area where the pinhole part  47  can be located on the mask  43  and where the point image (image of the pinhole  49 ) can be formed on the image plane  51 , the wave front aberration can be obtained by the similar procedure described above. 
     Thus, according to the apparatus for measuring a wave front aberration  30  of this embodiment, it can be considered that the deformation of each point image (image of the pinhole  49 ) formed on the image plane  51  is caused almost only by the influence of the wave front aberration of the test lens  33 , so that the wave front aberration of the test lens  33  can be precisely obtained by the repeating calculation using the phase retrieve method. 
     Moreover, since a plurality of pinhole parts  47  are arranged on the mask  43 , a plurality of point images (image of the pinhole  49 ) can be formed in the image plane  51  at a time, and wave front aberrations of the test lens  33  corresponding to a plurality of points can be easily measured by moving the imaging part  32  in the XY direction even if the test lens  33  has a wide effective visual field. As a result, the efficiency of the measurement can be enhanced. 
     Furthermore, since the wave front aberration of the test lens  33  can be measured with high speed, if the test lens  33  is a projection lens, an adjustment after assembly can be performed with high precision. As a result, a projection lens having very few remaining aberration with a high optical quality can be obtained. 
     By the way, although the diameter D 2  of the pinhole  49  of the pinhole part  47  is made to be about 0.5 μm to 1.0 μm, the diameter D 2  of the pinhole  49  is preferably smaller than λ 1 /NA T , where λ 1  denotes the wavelength of the measuring light L 4  and NA T  denotes numerical aperture of the teat lens  33  to the mask  43  side. When the diameter D 2  satisfies this condition, the whole area of the pupil of the teat lens  33  can be uniformly illuminated by the measuring light L 4  exit from the pinhole  49 . 
     Further, in the embodiment described above, when the wave front aberration of the test lens  33  is measured, the wave front aberration of the test lens  33  is calculated based on the light intensity distribution of the point image (image of the pinhole  49 ) magnified and projected on the imaging plane of the CCD imaging device  53  by the magnifying lens  52  without considering the wave front aberration of the magnifying lens  52 . Strictly speaking, the wave front aberration of the test lens  33  and the wave front aberration of the magnifying lens  52  are superimposed on the point image (image of the pinhole  49 ) formed on the CCD imaging device  53 . 
     In order to obtain only the wave front aberration of the test lens  33 , the wave front aberration of the magnifying lens  52  is measured first, and, then, the wave front aberration of the magnifying lens  52  may be subtracted from the measured result (that corresponds to the wave front aberration measured by the aforementioned embodiment) superimposed with both wave front aberrations. 
     Here, the measurement of the wave front aberration of the magnifying lens  52  is explained. In order to measure the wave front aberration of the magnifying lens  52  by using the apparatus for measuring a wave front aberration according to the aforementioned embodiment, a mask  61  (auxiliary shielding member) similar to the mask  43  is arranged on the image plane  51  of the test lens  33  as shown in FIG.  8 ( a ). 
     The mask  61  is constructed by a glass substrate  64  (transparent substrate) coated on both surfaces with thin metallic films  65 ,  66  (shield film with a thickness from 0.2 μm to 0.3 μm) as shown in FIG.  8 ( b ). One side of the metallic film  65  is formed on the test lens  33  side surface of the glass substrate  64 , and the other metallic film  66  is formed on the magnifying lens  52  side surface. The surfaces on which the metallic films  65  and  66  are formed are perpendicular to the axis  30 A. 
     Moreover, one pinhole part  67  is arranged on the mask  61 . In the pinhole part  67 , an aperture  68  is arranged to the test lens  33  side on the metallic film  65 , and a pinhole  69  is arranged to the magnifying lens  52  side on the metallic film  66 . The aperture  68  and the pinhole  69  of the pinhole part  67  are arranged such that the axis passing through the center of the aperture  68  and that of the pinhole  69  coincides with the axis  30 A (axis of the magnifying lens  52 ). The diameter of the aperture  68  is larger than that of the pinhole  69  (about 0.1 μm to 0.2 μm). 
     The mask  61  having this construction is arranged such that the pinhole side surface  61   a  coincides with the image plane  51  of the test lens  33 . Therefore, the wave front aberration of only the magnifying lens  52  can be measured without being affected by the wave front aberration of the test lens  33 . 
     The pinhole part  67  of the mask  61  is illuminated by a light L 6  passed through the aperture  68  from the aperture  68  side. A light L 7  passed through the aperture  68  passes through the inside of the glass substrate  64  and illuminates the pinhole  69  side. Then, a light passed through the pinhole  69  becomes the measuring light L 8  (approximately spherical wave) and is exit from the pinhole  69 . The light L 8  passed through the pinhole  69  is converged on the imaging plane of the CCD imaging device  53  by the magnifying lens  52 . At this time, images (point images) of the pinhole  69  are formed on the imaging plane of the CCD imaging device  53 . 
     The point image (image of the pinhole  69 ) formed on the imaging plane of the CCD imaging device  53  are naturally deformed from an ideal point image by the influence of the wave front aberration of the magnifying lens  52 . There is some possibility that the influence of an unnecessary element caused by the construction of the mask  61  is included into the deformation of the point image. However, it can be considered that the deformation of the point image (image of the pinhole  69 ) is mostly generated only by the influence of the wave front aberration of the magnifying lens  52  similar to the aforementioned result of the consideration regarding the mask  43 . 
     The computer  55  performs the repeating calculation based on the light intensity distribution (detected result at in focus plane and out of focus plane) of the point image (image of the pinhole  69 ) input from the CCD imaging device  53  and the known information (wavelength of the measuring light L 8 , numerical aperture of the magnifying lens  52 , shape of the pupil, and the like) by using the phase retrieve method. As a result, the wave front aberration of the center of the visual field of the magnifying lens is calculated. The wave front aberration of the magnifying lens  52  is also stored in the memory  56 . 
     Accordingly, in the computer  55 , the wave front aberration of only the test lens  33  is obtained by subtracting the wave front aberration of the magnifying lens  52  from the measured result (corresponding to the wave front aberration of the test lens  33  measured by the aforementioned embodiment) superposed by both wave front aberrations stored in the memory  56 . 
     By the way, the diameter of the pinhole  69  of the pinhole part  67  is preferably smaller than λ 2 /NA E , where λ 2  denotes the wavelength of the measuring light L 8  and NA E  denotes numerical aperture of the magnifying lens  52  to the mask  61  side. When the diameter of the pinhole  69  satisfies the condition, the whole pupil of the magnifying lens  52  can be uniformly illuminated by the measuring light L 8  exit from the pinhole  69 . 
     Thus, the calibration of the apparatus for measuring a wave front aberration  30  can be easily performed by the method for inserting the mask  61  into the image plane  51  of the test lens  33 . The wave front aberration of the test lens  33  can be measured with high precision even if the magnifying lens  52  attached to the apparatus for measuring a wave front aberration  30  has an intrinsic aberration. 
     There is another method for calibrating the apparatus for measuring a wave front aberration  30 . It is the method that a laser spot is used as the point source instead of the aforementioned mask  61  as shown in FIG.  9 . In this case, the lamp  41  is put off. Then, a light (approximately same wavelength as the lamp  41 ) from a laser light source  62  is led to a single mode optical fiber  63 . An exit surface of the single mode optical fiber  63  is arranged on the image plane  51  of the test lens  33 . The diameter of the laser spot is preferably smaller than λ 2 /NA E , where λ 2  denotes the wavelength of the measuring light L 8  and NA E  denotes numerical aperture of the magnifying lens  52  to the exit surface side. In this case, the whole pupil of the magnifying lens  52  can be uniformly illuminated by the measuring light L 8  exit from the exit surface of the optical fiber  63 . 
     In order to converge the light from the laser light source  62 , an ordinary optical system with an already-known wave front aberration can be used instead of the aforementioned optical fiber  63 . In this case, the wave front aberration of the magnifying lens  52  can be obtained by subtracting that of the laser light converging optical system corrected its numerical aperture from that of the magnifying lens  52  measured while calibration. When the laser light is converged by an optical system whose wave front aberration is unknown, the mask  61  may be used at the converging point. 
     Although a few preferred embodiments of the present invention has been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.