Abstract:
In a projection exposure apparatus used in lithography process for transferring a predetermined pattern formed on a reflection type mask onto a photosensitive substrate, the reflection mask and the substrate are moved relative to each other by a scanning driver, and the illumination optical system located between a radiation light source and the reflection mask includes a field stop located near a position conjugate with the reflection mask. In another embodiment a scanning driver changes the positional relationship between the reflection mask and the projection optical system, or/and between the substrate and the projection optical system. In another embodiment an image of the field stop is formed on the reflection mask by a relay optical system, preferably a catoptric system, forming the predetermined pattern. The projection optical system located between the reflection mask and the substrate directs the light reflected from the mask to the substrate, and illuminance distribution of the light on the exposure area of the substrate is adjustable by changing the position of reflection optical elements located along the optical path of the radiation light in the illumination optical system.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a projection exposure apparatus and method used in the lithography process of projecting a form, such as a mask or reticle, onto a wafer for the purpose of manufacturing devices, e.g., integrated circuits, imaging devices, liquid crystal displays, or thin film magnetic recording heads. More specifically, the present invention relates to a projection exposure apparatus and method of using soft x-rays called EUVL (Extreme Ultra Violet Lithography). The present invention also relates to an illumination optical system and method suitable for the projection exposure apparatus. Finally, the present invention relates to a method of manufacturing said devices by means of said apparatus.  
           [0003]    2. Description of Related Art  
           [0004]    A projection exposure apparatus for projecting a form, such as a mask or reticle, onto a wafer by means of soft x-rays (EUV radiation) is disclosed in documents such as Sweatt, William C. High-Efficiency Condenser Design for Illuminating a Ring Field. OSA Proceedings on Soft X-Ray Projection Lithography, Vol. 18 (1993) and U.S. Pat. No. 5,315,629.  
           [0005]    In this projection exposure apparatus, a reflection type mask is used as a form, and the exposure is performed while the form is moved relative to the projection optics. In other words, a scanning exposure method is utilized for projecting the whole pattern of the form onto the shot area on the wafer (work). The exposure is performed while an exposure area, which is an image of a portion of the form, is being scanned onto the work.  
           [0006]    In the projection exposure apparatus using soft x-rays according to the prior art described above, the exposure area should be limited in order that only the specific pattern on the mask is projected.  
           [0007]    In performing the scanning exposure, in order to make the cumulative exposure equal at every point in the shot area, it is necessary to make the slit width along the scanning direction within the exposure area substantially equal at any point along the direction perpendicular to the scanning direction.  
           [0008]    In performing the scanning exposure, there are two methods for limiting the exposure area: either a slit is placed near the reflection mask, or a slit is placed near the work. In the former method, the slit must be moved relative to the reflection mask while performing the exposure. In the latter method, on the other hand, the slit must be moved relative to the work while performing the exposure. Therefore, there must be a space between the slit and the reflection mask or the work, in order to avoid direct contact between the slit and the reflection mask or the work.  
           [0009]    Instances where the distance between the slit and the reflection mask or between the slit and the work are determined are described with reference to FIG. 5. FIGS. 5A through 5C show the relation between the slit S and the light flux arriving at the reflection mask  9 . FIG. 5A shows a case where the light flux arrives at the center  9 A of the exposure area. FIGS. 5B and 5C show cases where the light flux arrives at the corners  9 B and  9 C of the exposure area. Since the light flux arriving at the center of the exposure area does not produce vignetting, as seen from FIG. 5A, the shape of the aperture of the projection optical system becomes a circle as shown in FIG. 5D. Because the light flux arriving at  9 B and  9 C, on the other hand, produce vignetting, as seen by the hatched portion in FIGS. 5B and 5C, the shape of the aperture of the projection optical system shows a portion of the circle cut out, as seen in FIGS. 5E and 5F.  
           [0010]    The resolving power of an optical system is generally represented by the following equation: 
           
         LW=k·λ/NA 
       
           [0011]    wherein the reference symbol LW represents the line width at the resolution limit, k represents a constant, λ represents the wavelength, and NA represents the numerical aperture of the projection optical system. As seen from the above equation, the line width at the resolution limit is inversely proportional to the numerical aperture of the projection optical system. It is necessary that the resolving power be constant regardless of the position in the wafer plane or the direction of the pattern (vertical, horizontal or inclined).  
           [0012]    If the slit S is placed near the reflection mask or the work, vignetting is produced in a portion of the light flux at the exposure area near the edge of the slit S. Since resolving power varies according to the direction of the pattern, it is not possible to obtain an accurate image. In the scanning exposure method, because the scanning exposure is performed also in areas where an inaccurate image is produced, the difficulty is that the image in the entire illumination area becomes less accurate.  
           [0013]    If a change in the magnitude or shape of the exposure area or illumination area is desired, it is necessary for the slit itself to be movable. In this case, it is impossible to project an accurate image because of problems such as those described below. In order to prevent interference between the movable mechanism of the slit S and the reflection mask or the work, the distance between the slit S and the reflection mask or the work increases. Also, dust is produced by the movable mechanism of the slit S.  
         SUMMARY OF THE INVENTION  
         [0014]    Accordingly, it is an object of the present invention to provide an accurate image even in cases where a reflection mask is used.  
           [0015]    In order to achieve the object described above, a projection exposure apparatus according to an embodiment of the present invention comprises:  
           [0016]    a light source providing radiation light;  
           [0017]    an illumination optical system directing said radiation light to a reflection mask on which a predetermined pattern is formed;  
           [0018]    a projection optical system directing said radiation light reflected by said reflection mask to an exposed substrate (work) for forming an image of a predetermined pattern on said exposed substrate; and  
           [0019]    a scanning driver relatively moving at least one of said reflection mask and said exposed substrate;  
           [0020]    wherein said illumination optical system includes a field stop located near a position substantially conjugate with said reflection mask.  
           [0021]    In order to achieve the object described above, a projection exposure apparatus according to another embodiment of the present invention is used for directing radiation light to a reflection mask on which a predetermined pattern is formed, and for projecting an image of said predetermined pattern onto a work by said radiation light reflected by said reflection mask, the projection exposure apparatus comprising:  
           [0022]    a radiation light source;  
           [0023]    an illumination optical system located between said radiation light source and said reflection mask, and directing said radiation light from said radiation light source to said reflection mask;  
           [0024]    a projection optical system located between said reflection mask and said work, directing said radiation light reflected by said reflection mask to an exposed substrate, and forming the image of said predetermined pattern on said exposed substrate;  
           [0025]    a scanning driver relatively changing the positional relationship between at least one of said reflection mask and said work, and said projection optical system; and  
           [0026]    a field stop located near a position conjugate with said reflection mask.  
           [0027]    In order to achieve the object described above, an illumination apparatus according to another embodiment of the present invention is designed for use in a photolithography apparatus forming an image of a reflection mask on which a predetermined pattern is formed on a work, the illumination apparatus comprising:  
           [0028]    a radiation light source; and  
           [0029]    an illumination optical system located between said radiation light source and said reflection mask, to direct said radiation light provided from said radiation light source to a predetermined illumination area on said reflection mask;  
           [0030]    wherein said illumination optical system includes a field stop and a relay optical system forming an image of said field stop on said reflection mask as said predetermined illumination area.  
           [0031]    In the embodiments described above, it is desirable for said relay optical system to be a catoptric system. In any embodiment described above, it is desirable for said radiation source to provide radiation light having a wavelength shorter than 50 nm.  
           [0032]    In order to achieve the object described above, a projection exposure apparatus according to another embodiment of the present invention is designed for directing radiation light to a reflection mask on which a predetermined pattern is formed, and for projecting an image of said predetermined pattern on a work by said radiation light reflected by said reflection mask, the projection exposure apparatus comprising:  
           [0033]    a radiation light source;  
           [0034]    an illumination optical system located between said radiation light source and said reflection mask, for directing said radiation light provided from said radiation light source to said reflection mask;  
           [0035]    a projection optical system located between said reflection mask and said work, directing said radiation light reflected by said reflection mask to an exposed substrate, and forming the image of said predetermined pattern on said exposed substrate;  
           [0036]    wherein a position of at least one of the reflection optical elements located along the optical path of said radiation light in said illumination optical system is adjustable in order to adjust illuminance distribution of said radiation light on said exposure area.  
           [0037]    In order to achieve the above described object, a method according to another embodiment of the present invention is a method of projecting and exposing a predetermined pattern formed on a reflection mask onto a work comprising the steps of:  
           [0038]    providing a radiation light source;  
           [0039]    illuminating a reflection mask on which a predetermined pattern is formed with said radiation light source by means of an illumination optical system;  
           [0040]    projecting said pattern of said reflection mask onto a work based on reflected light from said reflection mask; and  
           [0041]    moving at least one of said reflection mask and said work relative to the other; wherein said illuminating step further comprises a step of forming an image of a field stop placed in said illumination optical system on said reflection mask.  
           [0042]    In order to achieve the object described above, a projection exposure method according to another embodiment of the present invention is a method of projecting and exposing a predetermined pattern formed on a reflection mask onto a work comprising the steps of:  
           [0043]    providing a radiation light source;  
           [0044]    illuminating a reflection mask on which a predetermined pattern is formed with said radiation light by means of an illumination optical system;  
           [0045]    projecting said pattern of said reflection mask onto a work based on reflected light from said reflection mask;  
           [0046]    adjusting the position of at least one optical element located along the optical path where said radiation light is directed to said field stop in order to change the illuminance distribution in said exposure area on said work.  
           [0047]    In the present invention, radiation light means electromagnetic waves including the deep ultraviolet, vacuum ultraviolet, soft x-ray, and hard x-ray areas. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0048]    [0048]FIG. 1 represents the schematic diagram showing the optical design of the projection exposure apparatus according to the present invention.  
         [0049]    [0049]FIG. 2 represents the schematic diagram showing the state of the scanning exposure.  
         [0050]    [0050]FIG. 3 represents the schematic diagram showing the mechanism of the field stop according to the modification of the present invention; FIG. 3A shows its sectional view, and FIG. 3B shows its plan view.  
         [0051]    [0051]FIG. 4 represents the schematic diagram showing the movement of the field stop shown in FIG. 3. FIGS. 4A through 4E represent the plan view showing the relative position between the fixed field stop and the movable field stop. FIGS. 4F through 4J represent the plan view showing the relative position between the shot area and the exposure area on the wafer.  
         [0052]    [0052]FIG. 5 represents the schematic diagram exhibiting the problems in a projection exposure apparatus according to the prior art. FIGS. 5A through 5C represent the plan views showing the vignetting of the light flux caused by a field stop. FIGS. 5D through 5F represent the schematic diagrams showing the shape of the aperture produced by the vignetting of the light flux. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0053]    The present invention will be described in detail below in accordance with the embodiment shown in FIG. 1. A laser source  100  generates laser light having a wavelength from the infrared to the visible light regions. A YAG laser, for example, may be used for the laser source  100 . Laser light generated by the laser source  100  is converged by a collecting optics  101  focusing at a focal point  3 . An object (EUV light-generating object) dripped from an object source  2  is illuminated with high illuminance laser light at the focal point  3 . The central portion of the object thereby becomes plasma, and a soft x-ray is then generated from this portion. The following objects may be used for the EUV light-generating object (target): water drops, ice particles, drops of any water solution, Xe gas, Kr gas, or tape or wire shaped Cu or Ta (Tantalum).  
         [0054]    For the wavelength of the soft x-ray, it is desirable to use a wavelength smaller than SO nm, e.g. a radiation light source of 13 nm. Since the transmittance of soft x-rays relative to air is rather low, the whole apparatus is placed in a vacuum chamber  1 .  
         [0055]    The soft x-ray generated at focal point  3  is directed to a field stop (aperture)  7 , having an opening of a predetermined area, by the optics  4 ,  5 , and  6 , which is a combination of plane mirrors and concave mirrors. In this embodiment, a reflection-increasing film is applied to the reflecting surface of the optics  4 ,  5 , and  6 , increasing the reflectance in the soft x-ray region. For the reflection-increasing film, such combinations of materials as Mo and Si or Mo and Be, which are alternately applied in a number of layers, may be used.  
         [0056]    For the field stop  7 , materials blocking soft x-rays are suitable.  
         [0057]    In this embodiment, although a laser plasma x-ray source is used for the light source, an undulator equipped with a synchrotron orbital radiation (SOR) may be used for the light source. In this embodiment, although a wavelength of 13 nm generated from a SOR is used, wavelengths such as 26 nm or 39 nm generated from a SOR as well as 1 nm of hard x-rays may also be used.  
         [0058]    The radiation light passing through the opening of the field stop  7  is then directed to a reflection mask  9  through a relay optical system  8  consisting of reflection optics. A pattern consisting of a portion reflecting, and a portion not reflecting, soft x-rays is formed on the surface of the reflection mask  9 . The optical systems  4 ,  5 ,  6 , and  8  compose an illumination optical system illuminating the reflection mask  9 .  
         [0059]    The field stop  7  is optically conjugate with the reflection mask  9  with respect to the relay optical system  8 , and an image of the field stop  7  is projected on the reflection mask  9 . The image of the opening of the field stop  7  coincides with the illuminated area on the reflection mask  9 .  
         [0060]    The radiation light selectively reflected by the reflection mask  9  is directed to a wafer  11  as an exposed substrate (work), placed on a substrate stage ST 2  by a projection optical system  10  having a predetermined reduction magnification. The pattern in the illuminated area on the reflection mask  9  is then projected on the wafer  11 . As shown in FIG. 2, an exposure area IE is formed on the wafer  11 . The exposure area IE has a shape similar to the illuminated area on the reflection mask  9  which is also similar to the opening of the field stop  7 . The image of the pattern in the illuminated area on the reflection mask  9  is formed in the exposure area IE.  
         [0061]    Returning to FIG. 1, a mask stage ST 1  and the substrate stage ST 2  are connected with drivers MT 1  and MT 2  respectively. These drivers MT 1  and MT 2  are connected with a main controller MCU.  
         [0062]    While performing the exposure, the reflection mask  9  and  20  wafer (exposed substrate)  11  are moved relative to the projection optical system  10  in the direction indicated by the arrow shown in FIG. 1 by means of the drivers MT 1  and MT 2 . In this embodiment, since there is a vacuum inside the chamber  1 , it is desirable to use a magnetic levitation type linear actuator or a differential exhaust type pneumatic bearing in combination with an actuator such as a linear motor for these drivers MT 1  and MT 2 . It is also desirable to use an electrostatic chuck as a chucking mechanism for the mask stage ST 1  and the substrate stage ST 2 .  
         [0063]    As shown in FIG. 2, the exposure area IE sweeps along the scanning direction, indicated by the arrow in the figure, on the wafer  11  as a work (exposed substrate). The exposure area on the wafer  11  consequently corresponds with the locus of the sweep of the exposure area IE, and the exposure area corresponds with the single shot area SA on the wafer  11 .  
         [0064]    In this embodiment, the field stop  7  is optically conjugate with the reflection mask  9  with respect to the relay optical system  8 . The field stop  7  is optically conjugate with the wafer  11  with respect to the relay optical system  8 , the reflection mask  9  and the projection optical system  10 . Since it is optically equivalent to placing the field stop  7  on the reflection mask  9 , the illuminated area (or the exposure area) may be limited. In this configuration, because no field stop  7  exists in the vicinity of the reflection mask  9 , there is no vignetting produced by the field stop  7  at any point in the exposure area, and good resolution may be obtained in the whole exposure area.  
         [0065]    If an intermediate image of the reflection mask  9  is formed in the projection optical system  10 , the field stop  7  may be placed at the intermediate image position.  
         [0066]    It is desirable to compose the projection optical system  10  with a reflection optical system having less than six reflection surfaces.  
         [0067]    Although only one field stop  7  is used in the embodiment shown in FIG. 1, the field stop is not limited to one. It is possible to combine plural elements such as blades limiting the width in the direction perpendicular to the sweeping direction together with blades limiting the width in the sweeping direction.  
         [0068]    A modified example of the field stop is described with reference to FIG. 3. FIG. 3A represents a sectional view showing a field stop  70  consisting of a fixed field stop  71  and variable field stops  72 A and  72 B. FIG. 3B represents a plan view showing the relative position of the fixed field stop  71  and the variable field stops  72 A and  72 B.  
         [0069]    In FIGS. 3A and 3B, the field stop has the fixed field stop  71  and the movable field stop  72 A and  72 B, wherein the field stop has an arc shaped opening  71 A and the movable field stop has two blades  72 A,  72 B defining the width in the sweeping direction of the illumination area (or exposure area) on the reflection mask  9  (or on the wafer  11 ). The fixed field stop  71  shown in FIGS. 3A, 3B is equivalent to the field stop  7  in FIG. 1.  
         [0070]    Each blade  72 A,  72 B defining the width in the sweeping direction of the movable field stop is movable such that each blade  72 A,  72 B has a driving unit  73 A,  73 B driving each blade in the sweeping direction independently. Here, the sweeping direction corresponds to the direction in which the projected image of the reflection mask  9  (or the wafer  11 ) on the field stop  7  by the relay optical system  8  (or by the relay optical system  8  in combination with the projection optical system  10 ) is swept. In the modified embodiment shown in FIG. 3, the radiation light illuminates the predetermined area defined by the movable field stop  72 A,  72 B, which is within the scope of the arc shaped illumination area on the reflection mask  9  (or the wafer  11 ) defined by the fixed field stop  71 .  
         [0071]    Said main controller MCU controls the movable blade-driving unit  73 A,  73 B. An example of a control system will be described below with reference to FIGS. 3 and 4. FIGS. 4A through  4 E represent plan views showing the relative position between the opening of the fixed field stop  71  and the blades  72 A,  72 B of the movable field stop. FIGS. 4F through 4J represent plan views showing the position of the exposure area IE relative to the single shot area on the wafer  11 . FIG. 4F shows the relative position between the exposure area IE and the shot area SA where the position of the movable field stop is shown in FIG. 4A. In the same way, FIG. 4G corresponds to FIG. 4B, FIG. 4H to FIG. 4C, FIG. 4I to FIG. 4D, and FIG. 4J to FIG. 4E. In FIGS. 4F through 4J showing the relative position between the shot area SA and exposure area IE, the hatched area shows where the radiation light actually illuminates on the wafer  11 .  
         [0072]    The main controller MCU shown in FIG. 3 drives the mask stage ST 1  and the substrate stage ST 2  by means of the drivers MT 1 , MT 2 , and places the exposure area IE at the left side of the shot area SA shown in FIG. 4F. At this time, since the blades  72 A and  72 B of the movable field stop are closed as shown in FIG. 4A, radiation light does not reach the exposure area IE (the exposure area IE is not formed).  
         [0073]    The main controller MCU then drives the mask stage ST 1  and the substrate stage ST 2  along the sweeping direction with velocities determined by the magnification of the projection optical system by means of the driving units MT 1  and MT 2 . At the same time, the main controller MCU drives the blade  72 B along the sweeping direction by means of the movable blade-driving unit  73 B. At this time, in order that the edge image of the blade  72 B substantially coincides with the edge of the shot area SA, the blade  72 B is driven in accordance with the stages ST 1  and ST 2 .  
         [0074]    When the blades  72 A and  72 B are opened as shown in FIG. 4C, radiation light passes through the whole area of the opening of the fixed field stop  71 . As a result, the exposure area IE corresponding to the image of the opening of the fixed field stop  71  is formed in the shot area SA on the wafer  11  as shown in FIG. 44. In this instance, since the stages ST 1  and ST 2  are still being driven along the sweeping direction, the exposure area IE sweeps the shot area SA. When the exposure area IE approaches the edge of the shot area SA as shown in FIG. 4I, the main controller MCU drives the blade  72 A along the sweeping direction by means of the movable blade driving unit  73 A, as shown in FIG. 4D. Therefore, radiation light overflowing from the shot area SA, which is directed to the exposure area IE, is blocked by the blade  72 A. At this time, the blade  72 A is driven in order that the image of the edge of the blade  72 A substantially coincides with the edge of the shot area SA in accordance with the stages ST 1  and ST 2 .  
         [0075]    When the exposure area IE goes beyond the shot area SA as shown in FIG. 4J, the blades  72 A and  72 B are then closed, and radiation light is no longer directed to the exposure area IE (the exposure area IE is no longer formed).  
         [0076]    From the series of movements described above, the pattern on the reflection mask  9  is projected in a single shot area on the wafer  11 . The main controller MCU then drives the substrate stage ST 2  by means of the driving unit MT 2 , and the exposure area IE is moved to another shot area (typically next to the former shot area SA). Then, the series of movements described above is repeated.  
         [0077]    Although the pattern is projected in a single shot area with a one stroke scanning exposure movement as described in FIGS. 3 and 4, a so called “scan and stitch exposure” may be used so that the pattern is projected in a single shot with a multi stroke scanning exposure movement. When the scan and stitch exposure is used, since a wide exposure area (in scanning direction) is not required for the projection optical system  10 , it becomes simple to manufacture the projection optical system, and it has the merit of improving the imaging quality of the projection optical system. Suzuki discloses this type of scan and stitch exposure apparatus in U.S. patent application No. 08/654,747 filed on May 29, 1996.  
         [0078]    In the embodiment shown in FIG. 1, it is possible to use an illumination optical system disclosed in U.S. patent application No. 09/359,137 filed on Feb. 26, 1999 by Komatsuda, a coinventor of the present invention.  
         [0079]    It should be briefly explained that U.S. patent application No. 09/359,137 is applied to the present invention. In the illumination optical system disclosed in patent application No. 09/359,137, radiation light provided from light source  54  is directed to a reflection type optical integrator, forms a second source having a predetermined shape, and illuminates mask M through a condenser optical system  64 . It is possible to replace the optical system  4 ,  5 , and  6  shown in FIG. 1 of the present invention with the illumination optical system ( 54 ,  56 , and  64 ) of patent application No. 09/359,137, so that the illumination optical system ( 54 ,  56 , and  64 ) is used for illuminating the field stop  7 , and the image of the field stop  7  is projected to the reflection mask  9  by the relay optical system  8  shown in FIG. 1.  
         [0080]    The illuminance distribution on the reflection mask  9  or on the wafer  11 , may be varied by adjusting the position of at least one of the optical elements comprising the illumination optical system, or by inserting a reflecting element having a spatial distribution of reflectance into the illumination optical system. By means of varying the illuminance distribution on the reflection mask  9 , or on the wafer  11  (exposed substrate as a work), it is possible to obtain a uniform (or other desired) illuminance distribution. In this case, it is desirable that the position of the optical element performing the adjustment be located in the optical path between the field stop  7  and the radiation source.  
         [0081]    In the illumination optical system shown in FIG. 1, it is possible to change distortion (isotropic or anisotropic) in the optical elements illuminating the field stop  7  by such procedures as changing the inclination of at least one of the plane mirrors  5 , by changing the position of the concave mirror  6  along or perpendicular to the optical axis, or by changing the slight inclination of the concave mirror  6 . As a result, the illuminance distribution on the field stop  7 , on the reflection mask  9 , or on the wafer  11 , may be varied.  
         [0082]    It is likely that while moving one of the said optical elements, optical properties other than distortion (e.g., telecentricity) will change. In this case, moving the position of at least one other optical element may produce an adjustment.  
         [0083]    At this point, it is not desirable that an optical element between the field stop  7  and the reflection mask  9 , which is an optical element consisting of the relay optical system  8 , be moved to adjust the position. The reason is that it not only changes the luminance distribution of the radiation light but also changes the shape of the opening of the field stop  7 . On the other hand, there is no limitation of position where a reflecting element having a spatial distribution of reflectance is inserted.  
         [0084]    In the embodiment described above, an arc shaped illumination area is formed on the reflection mask  9 . However, the present invention is not limited to the arc shape. If the projection optical system is made for forming a rectangular shaped exposure area, the illumination optical system according to the present invention may easily be modified to illuminate the rectangular shaped area. This is accomplished by modifying the shape of the opening of the field stop from an arc shape to a rectangular shape.  
         [0085]    As for the projection optical system, a system having unit magnification and expansion magnification as well as reduction magnification may be used. The present invention may also be applied to a proximity type scanning exposure apparatus, such as an x-ray exposure apparatus where a mask and a wafer are moved in a body relative to the arc shaped illumination area illuminated with x-ray radiation.  
         [0086]    The present invention may be applied not only to a projection exposure apparatus for manufacturing semiconductor devices but also to a projection exposure apparatus for manufacturing display devices, including liquid crystal displays, by projecting device patterns onto a glass plate. It may also be applied to projection exposure apparatuses for manufacturing thin film magnetic heads by projecting a device pattern onto a ceramic wafer, for manufacturing image detectors (such as CCDs), or for manufacturing a reticle or mask by projecting a device pattern onto a glass substrate or silicon wafer.  
         [0087]    The foregoing description of the embodiment has been presented for the purpose of clarifying the technical content of the present invention. It is not intended to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.