Abstract:
An exposure apparatus for exposing a pattern of a reticle onto a plate includes a projection optical system for projecting the pattern onto the plate, a switch for switching a polarization of a light for illuminating the reticle from a first polarization state to a second polarization state different from the first polarization state, and an adjuster for adjusting an aberration of the projection optical system when the switch switches the polarization of the light from the first polarization state to the second polarization state.

Description:
BACKGROUND OF THE INVENTION 
   The present invention generally relates to an exposure method and apparatus, and more particularly to an exposure apparatus and method used to manufacture various types of devices including semiconductor chips, display devices, detecting devices, image-pickup devices, and a fine pattern used for the micromechanics. 
   The photolithography technology is used to manufacture such a fine semiconductor device as a semiconductor memory and a logic circuit, and a liquid crystal display device. The conventional photolithography technology employs a reduction projection exposure apparatus that uses a projection optical system to project a circuit pattern of a reticle or mask onto a wafer, etc. to transfer the circuit pattern. 
   The minimum critical dimension (“CD”) transferable by the projection exposure apparatus or a resolution is proportionate to a wavelength of exposure light, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Accordingly, use of the exposure light having a shorter wavelength advances with the recent demands for fine processing to the semiconductor devices, from an ultra-high pressure mercury lamp (g-line with a wavelength of approximately 436 nm) and i-line with a wavelength of approximately 365 nm) to a KrF excimer laser (with a wavelength of approximately 248 nm) and an ArF excimer laser (with a wavelength of approximately 193 nm). In addition, the high NA scheme of the projection optical system is also promoted and the projection optical system having an NA of 0.9 or greater is about to reduce to practice. Moreover, an immersion lithography is proposed which fills a space between the final lens surface of the projection optical system and the wafer with a medium having a refractive index of 1.0 or greater, such as the water, increasing an apparent NA of the projection optical system up to 1.0 or greater, and improving the resolution. 
   When the high NA scheme of the projection optical system proceeds, for example, when NA becomes greater than 0.9, a polarization state of the light incident upon the projection optical system tends to significantly affect the resolving power. Two perfect coherent lights interfere with each other when their electric-field vibration directions are parallel to each other, but do not interfere at all when their electric-field vibration directions are perpendicular to each other even if they are perfectly coherent. 
     FIG. 10  shows that two lights are incident from the air and form an image on a photosensitive material or resist PR applied on a wafer WF, where a z axis denotes the optical-axis direction, an x axis denotes a direction perpendicular to the z axis, and a y axis denotes a direction perpendicular to  FIG. 10 . The y-polarized light is a polarized light that has an electric-field vibration plane on the yz plane. The x polarized light is a polarized light that has an electric-field vibration plane on the xy plane. When the polarization states of both the two lights are the y-polarized light, their vibration planes are always parallel to each other, and perfectly interfere with each other. Therefore, a repetitive image with a contrast of 100% is formed on the image plane. On the other hand, when the polarization states of both the two lights are the x-polarized light, their vibration planes are not parallel to each other and do not completely interfere with each other. Therefore, a repetitive image&#39;s contrast is not 100% on the image plane. Here, contrast C is expressed by Equation 1 below, where Imax denotes a maximum value of the light intensity, and Imin denotes a minimum value of the light intensity:
   C =( I max− I min)/( I max+ I min)  (1) 
     FIG. 11  shows that an angle between the two lights is greater than that in  FIG. 10 . Similar to the illustration in  FIG. 10 , when the polarization states of the two lights are the y-polarized light, their vibration planes are always parallel to each other. Thus, they interfere with each other, forming a repetitive image on the image plane with a contrast of 100%. On the other hand, when the polarization states of the two lights are the x-polarized light, the interference characteristic become worse due to the large angle between two lights than that in  FIG. 10 , lowering the contrast. 
   In  FIGS. 10 and 11 , the two lights are incident upon the resist applied to the wafer from the air. The air has a refractive index of 1, and the resist has a refractive index between 1.4 and 1.8. When the light is incident upon the resist from the air, the angle between the two lights decreases. In an immersion lithography that places on the resist RP a material IM having a refractive index greater than that of the air, the refractive index difference between the material IM and the resist PR is smaller than the refractive index difference between the air and the resist. Therefore, the refraction angle reduces at which the light is incident upon the material IM from the resist PR, and the angle between two lights increases. 
   As the NA of the projection optical system increase, control over the polarization state of the light incident upon the projection optical system becomes vital. Since an optimal polarization state differs according to reticle patterns, control or switch of the polarization state is needed according to the reticle patterns. The optical element and thin film, such as a antireflection coating and a reflection coating administered on the optical element, in the projection optical system have optical characteristics depending upon the polarization of the incident light. For example, the birefringent glass material has a different refractive index and a different aberration according to polarization directions of the incident light. In other words, the projection optical system causes a different aberration according to the polarization directions of the incident light. 
   Equations 2 and 3 below are met, where WA x  denotes an aberration caused by the incident x-polarized light, WA y  denotes an aberration caused by the incident y-polarized light, WA RANDOM  denotes an aberration caused by the incident non- or randomly polarized light, BWA x  denotes an aberration depending upon only the x-polarized light, and BWA y  denotes an aberration depending upon only the y-polarized light.
 
 WA   x   =WA   RANDOM   +BWA   x   (2)
 
 WA   y   =WA   RANDOM   +BWA   y   (3)
 
   The aberration depending upon only the polarization has the same absolute value but inverse codes with respect to two orthogonal, polarized lights. Therefore, Equation 4 below is met:
 
 BWA   x   =−BWA   y   (4)
 
   Equations 5 and 6 are sufficient to reduce both the aberrations WA x  and WA y :
 
WA RANDOM =0  (5)
 
 BWA   x   =−BWA   y =0  (6)
 
   Equation 6 requires the projection optical system to reduce the birefringence down to 0. For this purpose, for example, various exposure apparatuses are proposed, which adjust the birefringence by a crystal orientation and an angle incorporated to the projection optical system, or which adjust the projection optical system to cancel out a birefringence effect of the antireflection coating. In this case, the optical system includes plural elements made of a crystalline glass material with a birefringence caused by the crystal structure. These exposure apparatuses are disclosed in Japanese Patent Applications, Publication Nos. 2003-050349 and 2004-172328. 
   However, due to the manufacture errors, it is difficult to correct a birefringence amount of an entire projection optical system to completely 0 even if attempted in order to correct the polarization dependent aberration in assembly and adjustment of the projection optical system. The conceivable manufacture errors include, for example, a crystal orientation of the crystalline glass material and a rotational incorporation angle, an uneven thickness of the antireflection coating, and a stress distortion applied in mounting the projection optical system on the exposure apparatus. The influence of the polarization dependent aberration simply deteriorates the contrast in the conventional non-polarized light illumination, and the influence on the imaging performance is small and negligible. However, in the polarization controlled illumination, the aberration deteriorates the contrast, causes an image shift and defocus, etc., and changes these amounts whenever a polarization state is switched. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to an exposure apparatus and method that precisely transfer a fine pattern while maintaining an imaging characteristic even when switching a polarization state of the light that illuminates an object plane having a desired pattern. 
   An exposure apparatus according to one aspect of the present invention for exposing a pattern of a reticle onto a plate includes a projection optical system for projecting the pattern onto the plate, a switch for switching a polarization of a light for illuminating the reticle from a first polarization state to a second polarization state different from the first polarization state, and an adjuster for adjusting an aberration of the projection optical system when the switch switches the polarization of the light from the first polarization state to the second polarization state. 
   An exposure apparatus according to another aspect of the present invention for exposing a pattern of a reticle onto a plate includes a projection optical system for projecting the pattern onto the plate, a detector for detecting a change of the pattern of the reticle, and an adjuster for adjusting an aberration of the projection optical system when the detector detects the change of the pattern of the reticle. 
   An exposure method according to another aspect of the present invention for exposing a pattern of a reticle onto a plate through a projection optical system includes the steps of detecting a change of the pattern of the reticle, obtaining a variation of an aberration of the projection optical system caused by the change of the pattern of the reticle, and adjusting the aberration of the projection optical system based on a result obtained by the obtaining step. 
   A device manufacturing method according to still another aspect of the present invention includes the steps of exposing a plate using the above exposure apparatus, and developing a plate 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 an exposure apparatus according to one aspect of the present invention. 
       FIGS. 2A to 2D  are schematic plane views exemplarily showing polarizers and a transmission hole on a turret of a polarizer switch shown in  FIG. 1 . 
       FIG. 3  is a schematic plane view showing a polarization state of an effective light source of the light controlled by the polarizer shown in  FIG. 2C . 
       FIGS. 4A to 4C  are schematic plane views showing illustrative reticle patterns. 
       FIG. 5A to 5F  are schematic plane views of illustrative apertures on a turret of an aperture switch shown in  FIG. 1 . 
       FIG. 6  is a view of a fast phase axis in the intrinsic birefringence in a calcium fluoride, 8.5 mm thick plane-parallel plate above a wafer in the projection optical system having a numerical aperture (“NA”) of 0.9. 
       FIG. 7  is a graph showing an aberrational difference between the x-polarized light illumination and the y-polarized light illumination in the projection optical system having the fast phase axis shown in  FIG. 6 . 
       FIG. 8  is a flowchart for explaining manufacture of devices (such as semiconductor chips such as ICs and LCDs, CCDs, and the like). 
       FIG. 9  is a detail flowchart of a wafer process as Step  4  shown in  FIG. 8 . 
       FIG. 10  is a schematic view of interference between two lights on a wafer with a low NA. 
       FIG. 11  is a schematic view of interference between two lights on a wafer with a high NA. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the accompanying drawings, a description will now be given of an exposure apparatus and method according to one aspect of the present invention. In each figure, like elements are designated by like reference numerals, and a duplicate description thereof will be omitted. Here,  FIG. 1  is a schematic sectional view of the inventive exposure apparatus  1 . 
   The exposure apparatus  1  is a projection exposure apparatus that exposes onto the plate  50  a circuit pattern of the reticle  20 , e.g., in a step-and-scan manner. The exposure apparatus can apply a step-and-repeat manner. This exposure apparatus is suitable for a sub-micron or quarter-micron lithography process, and this embodiment exemplarily describes a step-and-scan exposure apparatus (which is also called a “scanner”). 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  includes, as shown in  FIG. 1 , an illumination apparatus  10 , a reticle stage  30  for supporting the reticle  20 , a projection optical system  40 , a wafer stage  60  for supporting the plate  50 , a controller  70 , and a memory  80 . 
   The illumination apparatus  10  illuminates the reticle  20  that has a circuit pattern to be transferred, and includes a light source unit  12  and an illumination optical system  14 . 
   The light source unit  12  uses as a light source, for example, as an ArF excimer laser with a wavelength of approximately 193 nm, and a KrF excimer laser with a wavelength of approximately 248 nm. However, a type of laser is not limited to excimer laser, and the light source unit  12  may use the F 2  laser with a wavelength of approximately 153 nm. In addition, the number of laser units is not limited. For example, two independently acting solid lasers would cause no coherence between these solid lasers and reduce speckles resulting from the coherence. An optical system for reducing speckles may swing linearly or rotationally. 
   The illumination optical system  14  is an optical system that illuminates the reticle  20 , and includes a lens, a mirror, a light integrator, a stop, and the like. The illumination optical system  14  in this embodiment illuminates the reticle  20  with a light of a polarization state optimal to the pattern of the reticle  20 . The illumination optical system  14  includes, as shown in  FIG. 1 , a lens  141 , an internal reflection mirror  142 , collimator lenses  143   a  and  143   b , and a fly-eye lens  144 , a stop  145 , imaging optical systems  146   a  and  146   b , a polarizer switch  147 , and an aperture switch  148 . 
   The lens  141  introduces the light to the internal reflection mirror  142  from the light source unit  12  at a desired angle, and makes the light intensity uniform with the internal reflection mirror  142 . The internal reflection mirror  142  reflects the incident light plural times on its internal surfaces and makes uniform the light intensity distribution at its exit plane although the light intensity is non-uniform at the incident plane. 
   The collimator lenses  143   a  and  143   b  project the exit plane of the internal reflection mirror  142  onto the incident plane of the fly-eye lens while enlarging or reducing the size of the exit plane. The fly-eye lens  144  forms plural condensing points on its exit plane. The collimator lens  143   c  illuminates the stop  145  at a uniform light intensity distribution using the multiple condensing points formed by the fly-eye lens  144  as a secondary light source. 
   The stop  145  restricts the illumination area of the target surface to be illuminated or reticle  20  in this embodiment, and is illuminated at a uniform light intensity distribution by the collimator lens  143   c  using the multiple condensing points formed by the fly-eye lens  144  as a secondary light source. The imaging optical systems  146   a  and  146   b  are imaging optical systems that set a position of the stop  145  to an object plane, and a position of the reticle  20  to an image plane. The imaging optical systems  146   a  and  146   b  projects the uniform illumination distribution realized at the position of the stop  145  onto the reticle  20 , and illuminates the reticle  20  at a uniform light intensity. 
   The polarizer switch  147  defines the polarization state of the light that illuminates the reticle  20 , and includes, as shown in  FIGS. 2A to 2D , polarizers  147   a  to  147   c  and a transmission hole  147   d  on a turret. A controller  70 , which will be described later, can arbitrarily switch the turret. The coordinate system in  FIGS. 2A to 2D  is a right-hand system that sets to the Z axis an optical-axis direction from the light source unit  12  to the reticle  20 . The polarizer  147   a  controls the polarization state of the light that illuminates the reticle  20 , in the y direction. The polarizer  147   b  controls the polarization state of the light that illuminates the reticle  20 , in the x direction. The polarizer  147   c  controls the polarization state of the light that illuminates the reticle  20  so that the polarization direction of the effective light source vibrates in a tangential direction as shown in  FIG. 3 . No polarizer is set to the transmission hole  147   d , and the transmission hole  147   d  does not control the polarization state (randomly polarized light). Here,  FIGS. 2A to 2D  are schematic plane views of the illustrative polarizers  147   a ,  147   c  and the transmission hole  147   d , which the polarizer switch  147  possesses on the turret.  FIG. 3  is a schematic plane view showing the polarization state of the light controlled by the polarizer  147   c  within the effective light source. 
   The polarizer switch  147  in this embodiment changes, according to the pattern of the reticle  20 , the polarization state of the light from the light source unit  12  to the optimal one so that the diffracted lights from the reticle  20  has the parallel electric-field vibration planes on the imaging point on the plate  50 ). For example, the diffracted light spreads on the x section for a periodic pattern on the x section as shown in  FIG. 4A , and the y-polarized light illumination is suitable with the polarizer  147   a  ( FIG. 2A ) that controls a polarization state in the y direction. The diffracted light spreads on the y section for a periodic pattern on the y section as shown in  FIG. 4B , and the x-polarized light illumination is suitable with the polarizer  147   a  ( FIG. 2B ) that controls a polarization state in the x direction. The tangentially polarized light illumination with the polarizer  147   c  ( FIG. 2C ) that controls a polarization state concentrically is optimal to a pattern that extends in arbitrary directions as shown in  FIG. 4C . Here,  FIGS. 4A to 4C  are schematic plane views of illustrative patterns of the reticle  20  with a coordinate system similar to that of  FIGS. 2A to 2D . 
   The aperture switch  148  defines an effective light source distribution, and includes, as shown in  FIGS. 5A to 5F , apertures  148   a  to  148   f  on a turret. The controller  70 , which will be described later, can arbitrarily switch the turret.  FIGS. 5A to 5F  are schematic plane views of the aperture switch  148  that has the illustrative apertures  148   a  to  148   f  on the turret with a coordinate system similar to that of  FIGS. 2A to 2D . The apertures  148   a  to  148   f  are used for a large σ illumination, a small σ illumination, a dipole illumination in the x direction, a dipole illumination in the y direction, a quadrupole illumination and an annular illumination. 
   The reticle  20  can be illuminated with a desired polarization state and an effective light source distribution by properly combining the polarizers  147   a  to  147   c  in the polarizer switch  147  and the apertures  148   a  to  148   f  in the aperture switch  148 . The effective light source distribution is variable by zooming the collimator lens  143   b  instead of the aperture switch  147 , by providing a switchable and zoomable input lens between the internal reflection mirror  142  and the collimator lens  143   a , or by using a phase-type computer generated hologram (“CGH”). 
   The reticle  20  is fed from the outside of the exposure apparatus  1  by the reticle feed system  200 , and is supported and driven by the reticle stage  30 . The reticle  20  is made, for example, of quartz, and has a circuit pattern to be transferred. The diffracted light emitted from the reticle  20  passes the projection optical system  40 , and is projected onto the plate  50 . The reticle  20  and the plate  50  are located in an optically conjugate relationship. Since the exposure apparatus  1  of this embodiment is a scanner, the reticle  20  and the plate  50  are scanned at the speed ratio of the reduction ratio, thus transferring the pattern on the reticle  20  to the plate  50 . The reticle feeding system  200  includes a reticle cassette  210  that accommodates plural reticles  20 , and a reticle feeder  220  that feeds the reticle  20 . The reticle feeding system  200  feeds the reticle  20  stored in the reticle cassette  210  to the reticle stage  30  through the reticle feeder  220 . It is preferable to optimize the polarization state and the effective light source distribution of the light for illuminating the reticle  20  in accordance with the pattern of the reticle  20  fed by the reticle feeding system  200 . 
   The reticle stage  30  supports the reticle  20  via a reticle chuck (not shown), and is connected a moving mechanism (not shown). The moving mechanism (not shown) includes a linear motor, etc., and drives the reticle stage  30  to move the reticle  20  in the XYZ directions and rotating directions around these axes. 
   The projection optical system  40  is an optical system that images the light from the object plane, such as the reticle  20 , onto an exposure area of the image plane, such as the plate  50 . The projection optical system  40  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, and a full mirror type or catoptric optical system, and so on. The projection optical system  30  has a stop  42  near the pupil to control the NA, and this embodiment sets a NA to 0.9 or greater. An aberration adjusting mechanism  46  for adjusting the aberration of the projection optical system  40  is attached to at least one of the optical elements in the projection optical system  40 . 
   The aberration adjusting mechanism  46  is, for example, a mechanism that drives a lens or lens group in the projection optical system  40  in the optical-axis direction, a mechanism that inclines one or more lens in the projection optical system  40  relative to the optical axis at an arbitrary angle, a mechanism that decenters one or more lens from the optical axis, or a mechanism that deforms a lens. The aberration adjusting mechanism  46  is controlled by the controller  70 , which will be described later, and corrects the aberration of the projection optical system  40 , which has been varied as a result of a change of the polarization state of the light for illuminating the reticle  20 . 
   The plate  50  is a wafer in this embodiment, but may broadly cover a liquid crystal plate and an object to be exposed. A photoresist is applied onto the plate  50 . 
   The wafer stage  60  supports the plate  50  via a wafer chuck (not shown). The wafer stage  60  is connected to a moving mechanism  62 , such as a linear motor. Similar to the reticle stage  30 , the moving mechanism  62  can move the plate  50  in XYZ directions and rotating directions around these axes. The moving mechanism  62  is controlled by the controller  70 , which will be described later, and moves the plate  50 , for example, to a focal point position or image plane position that varies as a result of a change of the polarization state of the light for illuminating the reticle  20 . 
   The controller  70  is electrically connected to the polarizer switch  147  and aperture switch  148  in the illumination apparatus  10 , a moving mechanism (not shown) in the reticle stage  30 , the reticle feeding system  200 , the aberration adjusting mechanism  46 , the moving mechanism  62  of the wafer stage  60  so as to controls operations of the exposure apparatus  1 . The controller  70  controls the aberration adjusting mechanism  46  and the moving mechanism  62  of the wafer stage  60  to optimize the aberration of the projection optical system  40 , which has varied when the polarization state of the light for illuminating the reticle  20  is switched and the reticle  20  is exchanged. In other words, the controller  70  corrects and optimize the aberration and image plane position of the projection optical system and the alignment error between the reticle  20  and the plate  50 , as described later in detail, via the aberration adjusting mechanism  46  and the moving mechanism  62  of the wafer stage  60 . The aberrational variation of the projection optical system  40  is detectable by detecting the aberration of the projection optical system  40  and by referring to an aberrational table stored in the memory  80 , whenever the polarization state is switched. An exchange of the reticle  20  is detectable when the reticle feeding system  200  informs the controller  70  of the exchange of the reticle  20 . The controller  70  also obtains the pattern shape of the exchanged reticle  20 , in particular the periodic direction of the pattern, and controls the polarization switch  147  and the aperture switch  148  so as to illuminate the reticle  20  with the light having the best polarization state and the effective light source distribution. 
   The memory  80  stores an aberrational table that correlates the polarization state of the light for illuminating the reticle  20  with the arbitration of the projection optical system  40 , for example, for each lens unit in the projection optical system  40 . The aberrational table may include, for example, data of a spherical aberrational variation amount, a coma variation amount, a astigmatism variation amount, a distortion variation, a defocus amount, and a pattern&#39;s positional shift variation amount, when the x-polarized light is switched to the y-polarized light, the coma variation. Alternatively, the aberrational table may be a Zernike coefficient in the Zernike polynomial. 
   A description will be given of an exposure method using the exposure apparatus  1 . The inventive exposure method is characterized in always correcting the aberration of the projection optical system  40  to the optimal state for exposure. When a change of the pattern of the reticle  20  is detected, the polarizer switch  147  switches to the polarization state optimal to the pattern of the reticle  20 . 
   When the polarization state is switched, the aberration of the projection optical system  40  varies and thus the aberration adjusting mechanism  46  adjusts so as to optimize the aberrational amount of the projection optical system  40 . The aberrational variation amount of the projection optical system  40  to be corrected is obtained by previously measuring the aberrational variation amount at each polarization state of the light for illuminating the reticle  20 , or by measuring the aberration through a measuring mechanism, such as a shearing interferometer and a point diffraction interferometer, provided in the exposure apparatus  1 , which measures the aberration of the projection optical system  40  whenever the polarization state is switched and calculating the aberrational variation amount based on the measurement result. 
   When the polarization state of the light for illuminating the reticle  20  is switched, the image plane position of the projection optical system  40  changes. Accordingly, the wafer stage  60  moves the plate  50  to the best image plane position along the optical-axis direction. The image-plane position may be previously measured at each polarization state of the light for illuminating the reticle  20 , and the moving amount of the wafer stage  60  in the optical-axis direction may be calculated. Alternatively, the image-plane position may be measured through the TTL (“through-the-lens”) autofocus mechanism in the exposure apparatus  1  whenever the polarization state is switched, and the moving amount of the wafer stage  60  in the optical-axis direction may be calculated based on the measurement result. 
   When the polarization state of the light for illuminating the reticle  20  is switched, an alignment shifts between the reticle  20  and the plate  50 . Accordingly, the wafer stage  60  is moved on the xy plane to correct the alignment shift. An alignment shift amount between the reticle  20  and the plate  50  may be previously measured at each polarization state of the light for illuminating the reticle  20 , and the moving amount of the wafer stage  60  on the xy plane may be calculated. Alternatively, an alignment shift amount may be measured through the alignment measuring mechanism (not shown) in the exposure apparatus  1  whenever the polarization state is switched, and the moving amount the wafer state  60  on the xy plane may be calculated based on the measuring result. 
   A concrete description of the correction to the aberration of the projection optical system  40  will be given in an illustration where the projection optical system  40  has an NA of 0.9, and a 5-mm thick plane-parallel plate is placed above the plate  50  and made of calcium fluoride (“CaF 2 ”) of isometric crystal. CaF 2  has an intrinsic birefringence depending upon an incident angle of the light, and it is 3.4 [nm/cm] at maximum to the wavelength of the ArF excimer laser. Since the projection optical system  40  is telecentric at both sides of the reticle  20  and the plate  50 , the incident angle distribution of the light upon the plane-parallel plate above the plate  50  is uniquely determined once the NA and image point are determined. The plane-parallel plate is set so that the [1 1 1] axis relating to the crystal orientation of CaF 2  orientates in the optical-axis direction, and the projection direction of the [1 0 0] axis of the crystal orientation upon the xy plane accords with the y axis.  FIG. 6  shows a fast phase axis of the plane-parallel plate. Although the actual exposure apparatus is subject to the influence of the birefringence due to the residue stress in the glass material manufacturing time, the mechanical stress, and antireflection coating and reflection coating, this embodiment assumes that such influence is negligible to simplify the description. 
   When the polarization state of the light for illuminating the reticle  20  is switched from the y-polarized light to the x-polarized light, the projection optical system  40  that is assumed to have a fast phase axis shown in  FIG. 6  exhibits the aberrational variation shown in  FIG. 7 .  FIG. 7  is a graph of the aberrational variation amount on the pupil plane in the projection optical system  40 , fitted by the Zernike polynomial, where the abscissa axis denotes the Zernike coefficient, and the ordinate axis denotes the aberrational variance amount when the polarization state is switched. The Zernike polynomial uses a polar coordinate (R, θ) as a coordinate system, and the Zernike cylindrical function as the orthogonal function system. The aberrational variance amount W (R, θ) is expressed by Equation 7 below, where Ci is a coefficient of each term in the Zernike polynomial, and Zi is a Zernike cylindrical function as indicated in Table 1 below:
 
 W ( R , θ)=Σ Ci·Zi   (7)
 
   
     
       
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
           
           
             
                 
               Z1 
               1 
             
             
                 
               Z2 
               R cos θ 
             
             
                 
               Z3 
               R sin θ 
             
             
                 
               Z4 
               2R 2  − 1 
             
             
                 
               Z5 
               R 2 cos2 θ 
             
             
                 
               Z6 
               R 2 sin2 θ 
             
             
                 
               Z7 
               (3R 3  − 2R)cos θ 
             
             
                 
               Z8 
               (3R 3  − 2R)sin θ 
             
             
                 
               Z9 
               (6R 4  − 6R 2  + 1) 
             
             
                 
               Z10 
               R 3 cos3 θ 
             
             
                 
               Z11 
               R 3 sin3 θ 
             
             
                 
               Z12 
               (4R 4  − 3R 2 )cos2 θ 
             
             
                 
               Z13 
               (4R 4  − 3R 2 )sin2 θ 
             
             
                 
               Z14 
               (10R 5  − 12R 3  + 3R)cos θ 
             
             
                 
               Z15 
               (10R 5  − 12R 3  + 3R)sin θ 
             
             
                 
               Z16 
               (20R 6  − 30R 4  + 12R 2  − 1) 
             
             
                 
                 
             
           
        
       
     
   
   This embodiment contemplates a switch to a differently polarized light illumination from a polarized light illumination that combines the polarizer  147   a  shown in  FIG. 2A  with the aperture  148   c  shown in  FIG. 5C  for the pattern shown in  FIG. 4A  having a period only in the x direction. The differently polarized light illumination state combines the polarizer  147   b  shown in  FIG. 2B  with the aperture  148   c  shown in  FIG. 5D , and exchanges the pattern to the pattern shown in  FIG. 4B  that has a period only in the y direction. Then, the aberrational amount of the projection optical system  40  varies, as shown in  FIG. 7 , and thus should be corrected to the optimal one. 
   For example, the in-slit uniform Zernike coefficient C 5  shown in  FIG. 7  is an astigmatism component that changes the optimal imaging position of a periodic pattern in the y direction and a periodic pattern in the x direction. Accordingly, the in-slit uniform astigmatism component is corrected by inclining two plane-parallel plates under the reticle  20  in opposite directions with respect to the optical axis in the projection optical system  40  or by adding a predetermined amount of a shape change of the Zernike cylindrical function Z 5  to the lens near the pupil in the projection optical system  40 . 
   A periodic pattern on the reticle  20  is often limited for an illumination of a linearly polarized light. When the pattern&#39;s periodic structure is limited to a specific direction, only the aberration on the specific section through which the diffracted light passes may be reduced. Therefore, instead of correcting the astigmatism component of the pupil, the plate  50  is moved to the best imaging position to the pattern in the specific direction by moving the wafer stage  60  in the z direction. 
   The in-slit uniform Zernike coefficient C 3  generates a positional shift between the reticle  20  and the plate  50 , and deteriorates the alignment accuracy. Accordingly, when the polarization state of the light for illuminating the reticle  20  is switched, the wafer stage  60  is moved on the xy plane to correct the positional shift between the reticle  20  and the plate  50 . 
   In order to correct the in-slit uniform Zernike coefficient C 8 , the projection optical system  40 &#39;s lens is adjusted or decentered in parallel. Alternatively, a predetermined amount of a shape change of the Zernike cylindrical function Z 8  may be added to the mirror near the pupil in the projection optical system  40 . 
   The aberrational variation of the in-slit Zernike coefficient C 10  occurs when the polarized light illumination that combines the polarizer shown in  FIG. 2C  with the aperture  148   f  shown in  FIG. 5F . This is correctible by adding a predetermined amount of a shape change of the Zernike cylindrical function Z 10  to the lens and mirror near the pupil in the projection optical system  40 . 
   This embodiment addresses the intrinsic birefringence of CaF 2 . Indeed, the aberration varies in the projection optical system  40  when the polarization state of the light for illuminating the reticle  20  is switched, due to the residue stress in the glass material manufacturing time, the mechanical stress, and antireflection coating and reflection coating. Then, various aberrations occur, such as a spherical aberration, a coma, astigmatism, a magnification, and a distortion. This embodiment is not limited to the above correcting method of the aberration of the projection optical system  40  but may cover driving, decentering or parallel decentering in the optical-axis direction, of one or more arbitrary lenses, as long as the aberrational amount is optimal. A provision of an arbitrary shape deformation to a lens or mirror of the projection optical system  40  is also viable to correct the aberration. The image plane position and the positional shift between the reticle  20  and the plate  50  can be corrected as described above. 
   It is unnecessary to correct the entire pupil in order to correct the aberration of the projection optical system  40 . For example, when the apertures  148   c  and  148   d  shown in  FIGS. 5C and 5D  are used, used part in the pupil in the projection optical system  40  is limited to a specific section that allows the diffracted light to pass and defines the effective light source. Therefore, only the used part may be corrected on the pupil section in the projection optical system  40 . 
   In addition, when the post-exchange reticle pattern has a periodic structure in the same direction as that of the pre-exchange reticle pattern, the polarization state of the reticle illuminating light does not have be switched. If the post-exchange reticle pattern pitch differs from the pre-exchange reticle pattern pitch even though the pattern&#39;s periodic direction is the same, the diffracted light distribution changed on the pupil plane in the projection optical system. Then, it is necessary to correct the specific part on which the diffracted light distributes on the pupil plane in the projection optical system. 
   Thus, the inventive exposure apparatus and method can maintain the aberration of the projection optical system optimal even when the polarization state of the reticle illuminating light is switched, and allows a fine pattern to be precisely transferred without deteriorating the imaging performance. While this embodiment illustrates an exposure apparatus having a projection optical system that applies isometric crystal to a plane-parallel plate, the present invention is not limited to this embodiment and covers an exposure apparatus having a projection optical system that uses isometric crystal for a lens having a power, and an exposure apparatus having a projection optical system that does not contain isometric crystal. 
   When a polarizer is used to turn the reticle illuminating light to a specific polarization state, the illumination efficiency deteriorates and the throughput lowers. On the other hand, a large angle between interfering lights or a high NA of the projection optical system enhances an effect to illuminate the reticle with the specific polarization state. In other words, a low NA of the projection optical system has little effect on the polarization controlled illumination. Therefore, an NA of 0.90 or greater that has a remarkable effect of the polarization controlled illumination. For example, an immersion lithography apparatus is preferable which fills a space between a final lens surface of the projection optical system (closest to the optical system) and the plate with a material (liquid) having a refractive index greater than that of the air. Suppose a correction of the Petzval sum necessary to correct the image plane in the immersion lithography apparatus that has a projection optical system with an NA greater than 1.2 that has a more conspicuous effect of the polarization controlled illumination. The dioptric optical system causes a very large diameter. Therefore, a catadioptric optical system is preferable which includes at least one mirror and enables a correction of the Petzval sum without enlarging the diameter. 
   In exposure, light emitted from the light source unit  12 , e.g., Koehler-illuminates the reticle  20  after the illumination optical system  14  turns the light into the optimal polarization state and effective light source distribution. The light that passes the reticle  20  and reflects the reticle pattern is imaged on the plate  50  by the projection optical system  40 . Since the aberration of the projection optical system  40  used for the exposure apparatus  1  is always maintained optimal, the exposure apparatus  1  exhibits an excellent imaging performance, and provides devices (such as semiconductor devices, LCD devices, image pickup devices (such as CCDs, etc.), thin film magnetic heads, and the like) at a high throughput and economical efficiency. 
   Referring now to  FIGS. 8 and 9 , a description will be given of an embodiment of a device manufacturing method using the above exposure apparatus  1 .  FIG. 8  is a flowchart for explaining how to fabricate devices (i.e., semiconductor chips such as IC and LSI, LCDs, CCDs, and the like). Here, a description will be given of the fabrication of a semiconductor chip as an example. Step  1  (circuit design) designs a semiconductor device circuit. Step  2  (reticle fabrication) forms a reticle having a designed circuit pattern. Step  3  (wafer preparation) manufactures a wafer using materials such as silicon. Step  4  (wafer process), which is also referred to as a pretreatment, forms actual circuitry on the wafer through lithography using the mask and wafer. Step  5  (assembly), which is also referred to as a posttreatment, 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. 9  is a detailed flowchart of the wafer process in Step  4 . 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 circuit pattern of the reticle onto the wafer. Step  17  (development) develops the exposed wafer. Step  18  (etching) etches parts other than a developed resist image. Step  19  (resist separation) removes disused resist after etching. These steps are repeated, and multi-layer circuit patterns are formed on the wafer. Use of the fabrication method in this embodiment helps fabricate higher-quality devices than ever. The device manufacturing method that uses the exposure apparatus  1  and resultant devices constitute one aspect of the present invention. 
   Further, 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. For example, the present invention is applicable to a step-and-repeat exposure apparatus (also referred to as a “stepper”), which exposes the wafer while maintaining stationary the reticle and the wafer. 
   This application claims a foreign priority benefit based on Japanese Patent Application No. 2004-362668 filed on Dec. 15, 2004, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.