Abstract:
An illumination optical system for illuminating a target surface using light from a light source, includes a sensor for detecting a light intensity of the light, a light splitter for splitting part of the light, and an optical element, arranged between the light source and the light splitter, for transmitting the light, and for reflecting the part of the light that has been split by the light splitter, towards the sensor.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to an illumination apparatus, and more particularly to an illumination apparatus that illuminates a patterned mask (or a reticle) in an exposure apparatus in a lithography process used to manufacture a semiconductor device, a liquid crystal device, an image-pickup device (such as a CCD), a thin film magnetic head, etc. The present invention is suitable, for example, for an illumination apparatus that illuminates a reticle at a predetermined polarization state in an exposure apparatus having a projection optical system having such a high numerical aperture (“NA”) as 0.9 or higher. 
   A projection exposure apparatus is employed and uses a projection optical system to project a circuit pattern of a reticle onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology. 
   The minimum critical dimension (“CD”) transferable by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the NA of the projection optical system. The shorter the wavelength is, the better the resolution is. 
   A demand for finer semiconductor devices promote use of a shorter wavelength of the exposure light and the higher NA of the projection optical system. Recently, the exposure light source has shifted from the ultra-high pressure mercury lamps (g-line with a wavelength of approximately 436 nm and i-line with a wavelength of approximately 365 nm) to those having shorter wavelengths, such as a KrF excimer laser (with a wavelength of approximately 248 nm) and an ArF excimer laser (with a wavelength of approximately 193 nm). A practical use of F 2  laser (with a wavelength of approximately 157 nm) is also advanced. On the other hand, for a higher NA of the projection optical system, a projection optical system having the NA of about 0.6 is dominant in the 1990s, but the development of a projection optical system having a NA higher than 0.9 is now expected. The immersion exposure technology that immerges at least part of the projection optical system and the wafer with a higher refractive index material (liquid, such as a water) than the air provides a projection optical system with a NA of 1.0 or higher. See, for example, Japanese Patent Application, Publication No. 10-303114. 
   It is necessary for the projection exposure apparatus to control the exposure dose used to expose a pattern to a predetermined one suitable for the resist or a photosensitive material, or to monitor and control the exposure dose during exposure to the wafer. Thus, the projection optical system splits the exposure light using a half mirror, and monitors the exposure dose during the exposure using a exposure dose sensor. See, for example, Japanese Patent Applications, Publication Nos. 2000-294480 and 2000-277413. 
   Problematically, the p-polarized light lowers the contrast of the interference fringe in the resist applied to the wafer, as a NA of the projection optical system increases. This is because the resist exposes due to the light intensity of the electric field component of the light: The electric field vector of the p-polarized light does not generate an interference fringe, but provides a uniform light intensity distribution irrespective of locations. 
     FIG. 7  is a view showing that the contrast of the interference fringe (or image) reduces in a high-NA projection optical system. The XYZ coordinate is set as shown in  FIG. 7 , and diffracted lights E +  and E −  interfere with each other and form the interference fringe. Each diffracted light contains s-polarized light Es having an electric field vector parallel to a substrate PL, and p-polarized light Ep orthogonal to the s-polarized light Es. The diffracted lights E +  and E −  are expressed by the following Equations 1 and 2, where ν is a frequency and λ is a wavelength. However, for simple description purposes, the s-polarized and p-polarized lights have the same phase or are linearly polarized lights in the 45° direction. 
   
     
       
         
           
             
               
                 
                   E 
                   + 
                 
                 = 
                 
                   
                     ( 
                     
                       
                         
                           
                             - 
                             Ep 
                           
                         
                         
                           cos 
                         
                         
                           θ 
                         
                       
                       
                         
                           Es 
                         
                         
                           
                               
                           
                         
                         
                           
                               
                           
                         
                       
                       
                         
                           
                             - 
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                           sin 
                         
                         
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                     ) 
                   
                   ⁢ 
                   
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                         ⅈ 
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                                 ⁢ 
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                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
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                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
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                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
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                               λ 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
             
             
               
                 [ 
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ] 
               
             
           
           
             
               
                 
                   E 
                   - 
                 
                 = 
                 
                   
                     ( 
                     
                       
                         
                           
                             - 
                             Ep 
                           
                         
                         
                           cos 
                         
                         
                           θ 
                         
                       
                       
                         
                           Es 
                         
                         
                           
                               
                           
                         
                         
                           
                               
                           
                         
                       
                       
                         
                           
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                           sin 
                         
                         
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                 [ 
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
                 ] 
               
             
           
         
       
     
   
   A sum of these vectors is a wave front of the interference fringe, and given by the following Equation 3: 
   
     
       
         
           
             
               
                 
                   
                     E 
                     + 
                   
                   + 
                   
                     E 
                     - 
                   
                 
                 = 
                 
                   
                     ( 
                     
                       
                         
                           
                             
                               - 
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                             ⁢ 
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                                     ⁢ 
                                     
                                         
                                     
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                             2 
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                 [ 
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
                 ] 
               
             
           
         
       
     
   
   A square of an absolute value of this wave front (Equation 3) is the light intensity of the interference fringe, and given by the following Equation 4: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
                            
                           
                             
                               E 
                               + 
                             
                             + 
                             
                               E 
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                         2 
                       
                       = 
                         
                       ⁢ 
                       
                         
                           4 
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                         + 
                       
                     
                   
                 
                 
                   
                     
                         
                       ⁢ 
                       
                         
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                         4 
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                               cos 
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                         4 
                         ⁢ 
                         
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                       ⁢ 
                       
                           
                       
                     
                   
                 
               
             
             
               
                 [ 
                 
                   EQUATION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
                 ] 
               
             
           
         
       
     
   
   A coefficient 
             cos   2     ⁡     (     2   ⁢   π   ⁢       x   ⁢           ⁢   sin   ⁢           ⁢   θ     λ       )           
in Equation 4 corresponds to the interference fringe, and provides a light intensity distribution of the line and space (“L &amp; S”) in the x direction. When a fine pattern is projected by the projection lens or optical system having a high NA, an angle θ between diffracted lights increases. For example,  FIG. 8  shows the angle θ between diffracted lights in the resist (with a refractive index of 1.7) when the L &amp; S having a period L nm is projected with a wavelength of 193 nm of the ArF excimer laser. Referring to  FIG. 8 , the angle θ between diffracted lights is 45° for a binary mask when the period of the L &amp; S becomes smaller than 160 nm, and for a Levenson phase shift mask (“PHM”) (or alternating-PSM) when the period of the L &amp; S becomes smaller than 80 nm.
 
   When the angle θ between diffracted lights becomes 45°, a term of cos2θ becomes 0 and the p-polarized light does not contribute to the amplitude of the interference fringe, lowering the contrast of the interference fringe of sin 2 θ. The p-polarized and s-polarized lights are defined by a relationship between the diffracted light and the substrate PL. While the above description refers to the p-polarized and s-polarized lights for a pattern that extends in the x direction (“x pattern”), the diffracted light occurs in the y direction for a pattern that extends in the y direction (“y pattern) so that the s-polarized light has an electric field vector in the x-axis direction and the p-polarized light is orthogonal to the s-polarized light. In other words, the s-polarized light for the x pattern corresponds to the p-polarized light for the y pattern. Notably, the polarization state varies according to a base surface and a light incident direction. 
   Thus, the p-polarized light lowers the image contrast in an exposure apparatus having a high-NA projection optical system. In order to obtain a high-contrast image, it is necessary to reduce the p-polarized light and use the polarization controlled light, i.e., the s-polarized light for exposure. 
   One problem for exposure with the polarization controlled light is that a half mirror that splits the exposure light has different transmittances between the p-polarized and s-polarized lights. Even when an illumination optical system forms desirably polarized light, the polarization state of the light that transmitted through the half mirror cannot become a desired state, deteriorating the resolution. 
     FIG. 9  shows transmittances of the s-polarized and p-polarized lights through a non-coated surface (a surface that has no antireflection coating). A conventional exposure apparatus reflects the exposure light on a half mirror and directs the reflected light to an exposure dose sensor, while inclining the half mirror by about 40° to the optical axis so that the exposure dose sensor does not shield the exposure light. For example, the exposure light generally has an angular distribution between about ±30°, and an angle incident upon the half mirror ranges between +10° and +70°. Referring to  FIG. 9 , the light incident upon the half mirror at the angle of +10° has a transmittance of about 95% for both the p-polarized and s-polarized lights, whereas the light incident upon the half mirror at the angle of +70° has a transmittances of 96% for the p-polarized light and a transmittance of 68% for the s-polarized light. When the s-polarized light is used for exposure, the light intensity differs as large as 27% according to angles incident upon the half mirror. The angularly dependent light intensity difference is as influential as the coma to the projection lens, and deteriorates the resolution. 
   In exposing a pattern that blends the x and y patterns, suppose that the s-polarized light to the y pattern is the s-polarized light for the half mirror surface and the s-polarized light to the x pattern is the p-polarized light for the half mirror surface. As a result, the light intensity that contributes to imaging in the x direction is greater than the light intensity that contributes to imaging in the y direction. This causes a contrast difference between the x and y patterns, and a HV difference or a difference of the resolved CD between the x and y patterns. This error problematically causes inaccurate pattern transfers. 
   BRIEF SUMMRY OF THE INVENTION 
   The present invention is directed to an illumination optical system that provides an illumination at a predetermined polarization state by reducing a transmittance difference caused by an angle and polarization state of the light incident upon the half mirror that is arranged on the optical path. 
   An illumination optical system according to one aspect of the present invention for illuminating a target surface using light from a light source includes a sensor for detecting a light intensity of the light, a light splitter for splitting part of the light, and an optical element, arranged between the light source and the light splitter, for transmitting the light and for reflecting the part of the light that has been split by the light splitter, towards the sensor. 
   An illumination optical system according to another aspect of the present invention for illuminating a target surface using light from a light source includes a light splitter for splitting part of the light, and an optical element arranged between the light source and the light splitter, the optical element having a reflective surface that has a reflectance of 2% or higher and reflects the part of the light that has been split by the light splitter, towards the sensor. 
   An exposure apparatus according to another aspect of the present invention includes the above illumination optical system for illuminating a reticle, and a projection optical system for projecting light that has transmitted the reticle onto an object. 
   An exposure apparatus according to another aspect of the present invention includes a projection optical system for projecting light from a light source that passes a pattern onto an object to be exposed, a light splitter arranged between the light source and the pattern, the light splitter splitting part of the light that reaches the object, a sensor for detecting the part of the light that is split by the light splitter, and for monitoring an exposure dose on the object, and an optical element arranged between the light source and the light splitter, the optical element reflecting part of the light that has been split by the light splitter, towards the sensor. 
   A device manufacturing method according to another aspect of the present invention includes the steps of exposing an object using the above exposure apparatus, and developing the object that has been exposed. 
   Other objects and further features of the present invention will become readily apparent from the following description of the preferred embodiments with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic sectional view of a structure of an inventive exposure apparatus. 
       FIG. 2  is a schematic plane view showing a relationship between an effective light source shape and a polarization state on a pupil surface in an illumination optical system shown in  FIG. 1 . 
       FIG. 3  is an enlarged sectional view showing a positional relationship among a condenser lens, a half mirror, and a sensor in a conventional exposure apparatus. 
       FIG. 4  is an enlarged sectional view showing a positional relationship among a condenser lens, a half mirror, and a sensor in the exposure apparatus shown in  FIG. 1 . 
       FIG. 5  is a flowchart for explaining a method for fabricating devices (semiconductor chips such as ICs, LSIs, and the like, LCDs, CCDs, etc.). 
       FIG. 6  is a detailed flowchart for Step  4  of wafer process shown in  FIG. 5 . 
       FIG. 7  is a view for explaining that the contrast of the interference fringe (image) reduces in a high-NA projection optical system. 
       FIG. 8  is a graph showing an angle θ between diffracted lights in a resist when a line and space is projected with a wavelength of an ArF excimer laser. 
       FIG. 9  is a graph showing transmittances of s-polarized and p-polarized lights through a non-coated surface. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A description will now be given of an exposure apparatus  1  as one aspect of the present invention, with reference to the accompanying drawings. In each figure, the same reference numeral denotes the same element. Therefore, duplicate descriptions will be omitted.  FIG. 1  is schematic sectional view of a structure of the inventive exposure apparatus  1 . 
   The exposure apparatus  1  is a projection exposure apparatus that exposes a circuit pattern of a reticle  200  onto an object  400 , e.g., in a step-and-repeat or a step-and-scan manner. 
   Such an exposure apparatus is suitably applicable to a submicron or quarter-micron lithography process, and a description will be given below of this embodiment taking a step-and-scan exposure apparatus (which is also called “a scanner”) as an example. The step-and-scan manner, as used herein, is an exposure method that exposes a mask pattern onto a wafer by continuously scanning the wafer relative to the mask, and by moving, after a shot of exposure, the wafer stepwise to the next exposure area to be shot. The step-and-repeat manner is another mode of exposure method that moves a wafer stepwise to an exposure area for the next shot every shot of cell projection onto the wafer. 
   The exposure apparatus  1 , as shown in  FIG. 1 , includes an illumination apparatus  100 , a reticle stage (not shown) mounted with the reticle  200 , a projection optical system  300 , a wafer stage  500  mounted with an object to be exposed  400 , and a controller  600 . 
   The illumination apparatus  100  illuminates the reticle  200  that has a circuit pattern to be transferred, and includes a light source unit  110  and an illumination optical system  120 . 
   The light source unit  110  uses as a light source, for example, an ArF excimer laser with a wavelength of approximately 193 nm, a KrF excimer laser with a wavelength of approximately 248 nm, but the a type of laser is not limited to excimer laser and, for example, a F 2  laser with a wavelength of approximately 157 nm and an extremely ultraviolet light having a wavelength of 20 nm may be used. Similarly, the number of laser units is not limited. For example, two independently acting solid lasers would cause no coherence between these solid lasers and significantly reduces speckles resulting from the coherence. An optical system for reducing speckles may swing linearly or rotationally. A light source applicable to the light source unit  110  is not limited to a laser, and may use one or more lamps such as a mercury lamp and a xenon lamp. 
   The illumination optical system  120  is an optical system that illuminates a target surface, such as a reticle  200  having a desired pattern, using the light emitted from the light source unit  110 . The illumination optical system  120  in this embodiment includes a random phase plate  121 , an attenuating or ND filter  122 , a micro lens array (“MLA”)  123 , an internal surface reflector  124 , optical elements  125   a  and  125   b , a condenser lens  126 , a zoom relay optical system  127 , a polarized light splitting film  128 , a fly-eye lens  129 , a condenser lens  130 , a half mirror  131 , a sensor  132 , and a relay optical system  133 . 
   The random phase plate  121  turns the approximately linearly polarized light emitted from the light source unit  110  into the random polarized light. The ND filter  122  serves to adjust the light intensity of the target surface, and multistage ND filters are arranged switchably. 
   The MLA  123  includes plural lens elements that are arranged in an array, and integrated with a substrate. The MLA  123  forms a plurality of secondary light sources using the light emitted from the light source unit  110 . The MLA  123  may be micro cylindrical lens array. The laser (or light source unit) and an exposure unit may be placed separately, for example, at different floors. Floor vibrations occur asynchronously and independently between the light source unit and the exposure unit, always causing an axial offset and inclination between these units. The MLA  123  serves as a field type of a fly-eye lens, and emits the light of a predetermined angular distribution around the optical axis of the MLA  123 , even when the optical axis of the light incident upon the MLA  123  inclines. Thereby, even with the axial inclinations due to the floor vibrations, the exposure unit is supplied with the light having a constant angular distribution. 
   The internal surface reflector  124  reflects the light emitted from the MLA  123 , and generates the light having an approximately uniform distribution on an exit surface of the internal surface reflector  124 . The light having a uniform distribution, exited from the internal surface reflector  124 , maintains the uniformity, even when an optical path offsets between the light source unit and the exposure unit. 
   A combination between the MLA  123  and the internal surface reflector  124  provides the light with a uniform and constant angular distribution relative to the optical axis of the exposure unit, on the exit surface of the internal surface reflector  124 , even when an optical axis offsets and inclines between the light source unit and the exposure unit. 
   The optical elements  125   a  and  125   b  are diffraction optics, such as a computer generated hologram (“CGH”) and refractive optical elements such as a MLA. The optical elements  125   a  and  125   b  arrange plural CGHs and/or plural MLAs switchably. The MLA uses a field type hexagonal micro lenses (hexagonal MLA), or disc-shaped micro lenses (circular MLA). The CGH can generate an effective light source having an arbitrary shape, such as an annular shape, a quadrupole shape, and a dipole shape. The optical elements  125   a  and  125   b  serve as effective light source generating means that determine the effective light source shape on the pupil surface. 
   The condenser lens  126  forms a Fourier transformation image of the CGH or MLA on a position A. The condenser lens  126  forms a hexagonal light intensity distribution at the position A when the optical elements  125   a  and  125   b  are the hexagonal MLAs, and an approximately circular light intensity distribution at the position A when the optical elements  125   a  and  125   b  are the circular MLAs. 
   The zoom relay optical system  127  projects a light intensity distribution formed at the position A onto an incident surface of the fly-eye lens  129 . 
   The polarized light splitting film  128  is removably arranged on and inclined to the optical path. The polarized light splitting film  128  is coated on a plane-parallel plate. The polarized light splitting film  128  generally transmits the p-polarized light and reflects the s-polarized light for the plane-parallel plate. The plane-parallel plate that is coated with the polarized light splitting film  128  and inclines in the x direction and the plane-parallel plate that is coated with the polarized light splitting film  128  and inclines in the y direction are removably inserted or adapted to change the inclination direction. This configuration realizes a polarization illumination suitable for a x repetitive pattern and a polarization illumination suitable for a y repetitive pattern. In other words, the polarized light splitting film  128  serves as a polarization control means for controlling the polarization state of the incident light to a predetermined polarization state. Thus, the optical elements  125   a  and  125   b  and the polarized light splitting film  128  can provide an illumination having an effective light source and polarization state suitable for the pattern of the reticle  200 . 
   The effective light source and polarization state realized on the pupil surface by the optical elements  125   a  and  125   b  and the polarized light splitting film  128  include a polarization illumination suitable for a binary mask having a repetitive pattern in the x direction using the s-polarized light having a dipole effective light source shown in  FIG. 2A , a polarization illumination suitable for a binary mask having a repetitive pattern in the y direction using the s-polarized light having a dipole effective light source shown in  FIG. 2B , a polarization illumination suitable for a Levenson PSM having a repetitive pattern in the x direction using the s-polarized light having a small a effective light source shown in  FIG. 2C , a polarization illumination suitable for a Levenson PSM having a repetitive pattern in the y direction using the s-polarized light having a small a effective light source shown in FIG.  2 D, and a tangential polarization illumination suitable for a repetitive pattern in the x and y direction using the tangentially polarized light having an annular effective light source shown in  FIG. 2E . The present invention does not limit the effective light source shape and the polarization state to those shown in  FIGS. 2A to 2E , and may combine various effective light source shapes and polarization states. Here,  FIGS. 2A to 2E  are schematic plane views showing a relationship between the effective light source shape and the polarization state on the pupil surface of the illumination optical system  120 . 
   This embodiment forms a random polarization state prior to the polarized light splitting film  128 , and thus does not have to control the polarization state prior to the polarized light splitting film  128  or consider differences in reflectance and transmittance due to the glass material&#39;s birefringence and film&#39;s polarization state. 
   The fly-eye lens  129  forms plural secondary light sources at a pupil position of the illumination optical system  120 . The fly-eye lens  129  may be replaced with a MLA and a micro cylindrical lens array. 
   The condenser lens  130  forms an approximately uniform light intensity distribution by superimposing the light from the secondary light source formed by the fly-eye lens  129 , at a position B where a variable stop (not shown) is arranged for controlling an illumination area of the target surface. The condenser lens  130  also serves to reflect part of the light reflected by the half mirror  131 , towards the sensor  132  as described later. 
   The half mirror  131  splits or reflects the part of the exposure light emitted from the light source unit  110 . This embodiment arranges the half mirror  131  perpendicular to the optical axis, reduces the transmittance difference caused by the light polarization state, and illuminates the reticle  200  in the desired polarization state. 
   More specifically, the conventional exposure apparatus reflects the light on the half mirror directly to the sensor as shown in  FIG. 3 , and inclines the half mirror remarkably to a surface perpendicular to the optical axis in order to arrange the sensor outside the exposure light&#39;s optical path. On the other hand, this embodiment reflects the exposure light on the half mirror  131  and then reflects the resultant light on the exit surface  130   a  of the condenser lens  130  or a lens surface closer to the light source unit  110  than the half mirror  131 , towards the sensor  132 , arranging the half mirror  131  approximately perpendicular to the optical axis. Here,  FIG. 3  is an enlarged sectional view showing a positional relationship among the condenser lens, the half mirror and the sensor in the conventional exposure apparatus.  FIG. 4  is an enlarged sectional view showing a positional relationship among the condenser lens  130 , the half mirror  131  and the sensor  132  in the exposure apparatus  1 . 
   When the light reflected on the half mirror  131  is reflected on the lens surface (or the condenser lens  130 &#39;s exit surface  130   a ) at the light source unit  110  side, an inclined angle of the half mirror  131  can be reduced. Splitting of the light is facilitated, since both the condensed light upon the sensor  132  and the exposure light from the condenser lens  130  are condensed toward the reticle  200  surface. In addition, the sensor  132  can be arranged closer to the optical axis than the prior art, without shielding the exposure light. This configuration can arrange the half mirror at an inclination angle of about 15° relative to the optical axis, whereas the prior art configuration arranges the half mirror at an inclination angle of about 40° relative to the optical axis. Therefore, the light is incident at inclination angles between −15° and +45° upon the half mirror  131  in this embodiment. Since the light incident at an inclination angle of −15° to the half mirror  131  is equivalent to the light at an inclination angle of +15° to the half mirror  131 , the incident angle of the light upon the half mirror  131  ranges between 0° and 45°. 
   Referring to  FIG. 9 , the transmittance of the light incident at an inclination angle of 0° upon the half mirror  131  is about 95% for both the p-polarized light and the s-polarized light. The transmittance of the light incident at an inclination angle of 45° is about 98% for the p-polarized light and about 89% for the s-polarized light. Therefore, the s-polarized light incident upon the half mirror  131  generates a light intensity difference of about 6% depending upon angles during exposure. Since the prior art causes a light intensity difference of about 27% as discussed above, the present invention remarkably reduces the light intensity difference and greatly improve the resolving power. 
   While  FIG. 9  shows the transmittance on a non-coated surface, use of the coating in accordance with the appropriate coating design can reduce the transmittance difference due to the incident angles. Therefore, use of the half mirror  131  that has a coating with a small transmittance difference is preferable. While this embodiment reflects the light on the exit surface  130   a  of the condenser lens  130  that is closest to the half mirror  131  on the side of the light source unit  110 , the light may be reflected on the condenser lens  130  that is closest to the light source unit  110 . In other words, the light may be reflected on any lens surface of the condenser lens  130  or plural surfaces and then introduced to the sensor  132 . Since the light that is reflected on a surface other than the reflective lens surface and enters the sensor  132  causes noises, the reflective lens surface should have a higher reflectance than other lens surfaces. Usually, the lens surface having an antireflection coating has a reflectance of 0.2% or smaller. The reflective lens surface has a reflectance of 2% or greater, ten times as high as that of other lens surfaces. The reflectance of the non-coated surface that has no antireflection coating is about 4%, and the reflective lens surface preferably has no antireflection coating. On the other hand, as the reflectance of the reflective surface reduces, the light intensity of the target surface and the throughput increase, since the reflective surface reflects the exposure light that is expected to reach the target surface or the light incident upon the reflective surface before the light enters the half mirror  131 . Therefore, it is preferable that the reflective coating having a reflectance of 2% or greater is formed on the reflective lens surface. 
   The sensor  132  is an exposure dose sensor that monitors the exposure dose upon the object  400  so as to monitor the exposure dose while the object  400  is being exposed. An illumination photometer  510  is provided on the wafer stage  500 , which will be described later, and measures the light intensity on the object  400 . Since a relationship between the exposure dose detected by the sensor  132  and the light intensity on the object  400  changes according to the transmittance changes of the projection optical system  300  etc., the illumination photometer  510  is provided on the optical path on a regular basis, the light intensity detected by the illumination photometer  510  is correlated with the exposure dose detected by the sensor  132 , and the controller  600  stores the result. 
   The relay optical system  133  projects a light intensity distribution at the position B onto a pattern surface of the reticle  200 . 
   While this embodiment illustratively forms a random polarization for the light from the light source unit  110  by using the random phase plate  121  and then converts the polarization state to the desired one at the polarized light splitting film  128 , the present invention is effective irrespective of the control method of the polarization state. Other control method of the polarization state includes, for example, a method for maintaining the polarization state of the laser emitted from the light source unit  110 , and for inserting a λ/2 plate and a λ/4 plate instead of the polarized light splitting film  128 , and a method for arranging a polarization controller at a position other than a position just prior to the fly-eye lens  129 . The present invention is effective to any method. The present invention is also effective to an illumination optical system that does not provide a polarization illumination, since the incident angle distribution on the target surface becomes more uniform than the conventional. 
   The illumination optical system  120  can reduce the transmittance difference from the half mirror  131  inserted into the exposure optical path, which transmittance difference is caused by the polarization state of the light. Therefore, the illumination optical system  120  can provide a desired polarization illumination, and realize transferring at a sufficiently high resolution in the exposure apparatus having a high-NA projection optical system. 
   The mask  200  is made for example, of quartz, and has a circuit pattern (or an image) to be transferred. It is supported and driven by the reticle stage (not shown). The diffracted light from the reticle  200  passes the projection optical system  300 , and then is projected onto the object  400 . The reticle  200  and the object  400  are located in an optically conjugate relationship. Since the exposure apparatus  1  of the instant embodiment is a scanner, the reticle  200  and the object  400  are scanned at a speed ratio of the reduction ratio. Thus, the pattern of the reticle  200  is transferred to the object  400 . If it is a step-and-repeat exposure apparatus (referred to as a “stepper”), the reticle  200  and the object  400  remain still in exposing the mask pattern. 
   The projection optical system  300  projects the light that reflects a pattern of the reticle  200  onto the object  400 , and has a NA of 0.9 or higher. The projection optical system  300  projects a circuit pattern of the reticle  200  onto the object  400  at a reduction ratio of 1/4. The projection optical system  300  may use a (dioptric) optical system solely including a plurality of lens elements, a (catadioptric) optical system including a plurality of lens elements and at least one mirror, an optical system including a plurality of lens elements and at least one diffraction optical element such as a kinoform, and a (catoptric) optical system of a full mirror type, and so on. Any necessary correction of the chromatic aberrations may use a plurality of lens units made from glass materials having different dispersion values (Abbe values) or can arrange diffraction optics such that it disperses in a direction opposite to that of the lens unit. The projection optical system  300  may be an immersion type projection optical system that immerses a space between its final lens at the object  400  side and the object  400  with water and highly refractive index solution, or an optical system having a NA of 1.0 or higher, such as a solid immersion type projection optical system. 
   The object  400  is a wafer in this embodiment, but may broadly covers a LCD and another object to be exposed. A photoresist is applied to the object  400 . 
   The wafer stage  500  supports the object  400 . The wafer stage  500  may use any structures known in the art, and a detailed description of its structure and operation is omitted. The wafer stage  500  may use, for example, a linear motor to move the object  400  in the XYZ directions. The reticle  200  and object  400  are, for example, scanned synchronously. The positions of the wafer stage  500  and reticle stage (not shown) are monitored, for example, by a laser interferometer so that both are driven at a constant speed ratio. The wafer stage  500  is provided, for example, on a stage stool that is supported on a floor etc. via dampers, and the reticle stage and the projection optical system  300  are placed, for example, on a barrel stool (not shown) that is supported on a base frame that is provided on the floor etc. via dampers. A scanning direction on a plane that includes the reticle  200  or the object  400  is defined as x, a direction perpendicular to the x direction is defined as y, and a direction perpendicular to the plane of the reticle  200  or the object  400  is defined as z. 
   The controller  600  has a CPU and a memory (not shown), and controls operations of the exposure apparatus  1 . The controller  600  is electrically connected to the illumination apparatus  100 , the reticle stage (not shown), and the wafer stage  500 . The CPU includes a processor irrespective of its name, and controls operation of each component. The memory includes a ROM and a RAM, and stores the firmware to operate the exposure apparatus  1 . The controller  600  in this embodiment calculates the exposure dose onto the object  400  surface based on a relationship between the exposure dose and the light intensity input from the sensor  132  and the illumination photometer  510 , and controls the exposure dose based on the calculation result. 
   In exposure, the light emitted from the light source unit  110 , for example, Koehler-illuminates the reticle  200  via the illumination optical system  120 . The light that passes the reticle  200  and reflects the reticle pattern is imaged onto the object  400  by the projection optical system  300 . The illumination optical system  120  reduces a transmittance difference caused by the polarization state of the half mirror  131 , and forms the effective light source shape and the polarization state suitable for the reticle pattern, providing a high-quality exposure to the object  400  with desired resolution. The high-NA projection optical system with a desired polarization state of the light can provide devices, such as a semiconductor device, a LCD device, an image-pickup device (such as a CCD), and a thin film magnetic head, at a high throughput and economical efficiency. 
   Referring now to  FIGS. 5 and 6 , a description will now be given of an embodiment of a device manufacturing method using the above exposure apparatus  1 .  FIG. 5  is a flowchart for explaining a fabrication of devices (i.e., semiconductor chips such as IC and LSI, LCDs, CCDs, etc.). Here, a description will be given of a fabrication of a semiconductor chip as an example. Step  1  (circuit design) designs a semiconductor device circuit. Step  2  (mask fabrication) forms a mask having a designed circuit pattern. Step  3  (wafer preparation) manufactures a wafer using materials such as silicon. Step  4  (wafer process), which is referred to as a pretreatment, forms actual circuitry on the wafer through photolithography using the mask and wafer. Step  5  (assembly), which is also referred to as a post-treatment, forms into a semiconductor chip the wafer formed in Step  4  and includes an assembly step (e.g., dicing, bonding), a packaging step (chip sealing), and the like. Step  6  (inspection) performs various tests for the semiconductor device made in Step  5 , such as a validity test and a durability test. Through these steps, a semiconductor device is finished and shipped (Step  7 ). 
     FIG. 6  is a detailed flowchart of the wafer process in Step  4  shown in  FIG. 5 . Step  11  (oxidation) oxidizes the wafer&#39;s surface. Step  12  (CVD) forms an insulating film on the wafer&#39;s surface. Step  13  (electrode formation) forms electrodes on the wafer by vapor disposition and the like. Step  14  (ion implantation) implants ions into the wafer. Step  15  (resist process) applies a photosensitive material onto the wafer. Step  16  (exposure) uses the exposure apparatus  1  to expose a reticle pattern onto the wafer. Step  17  (development) develops the exposed wafer. Step  18  (etching) etches parts other than a developed resist image. Step  19  (resist stripping) removes disused resist after etching. These steps are repeated, and multilayer circuit patterns are formed on the wafer. This embodiment can provide higher-quality semiconductor devices than the prior art. Thus, the device manufacturing method that uses the exposure apparatus  1 , and its resultant (intermediate and final) products also constitute one aspect of the present invention. 
   Thus, the inventive illumination optical system can provide an illumination at a predetermined polarization state by reducing a transmittance difference caused by an angle and polarization state of the light incident upon the half mirror that is arranged on the optical path. 
   Furthermore, the present invention is not limited to these preferred embodiments and various variations and modifications may be made without departing from the scope of the present invention. 
   This application claims a foreign priority benefit based on Japanese Patent Applications No. 2004-110338, filed on Apr. 2, 2004, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.