Abstract:
A measuring apparatus for measuring optical performance of a target optical system to be measured includes an optical unit for splitting light from a light source into measuring light and reference light so that the measuring light can be introduced into the target optical system, a reflection unit for reflecting the measuring light from the target optical system toward the target optical system via a fluid, and a detector for detecting an interference fringe generated between interference between the measuring light that has emitted from the target optical system after being reflected by the reflection unit and the reference light that does not pass the target optical system.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to a measuring apparatus and an exposure apparatus having the same, which measures the optical performance of an optical element, such as a wavefront aberration and a Zernike coefficient. More specifically, the present invention relates to a measuring apparatus and an immersion exposure apparatus having the same, which measures the optical performance of a projection optical system in an immersion exposure apparatus.  
         [0002]     An interferometer as an optical-performance measuring apparatus measures the optical performance of the projection optical system, without filling the fluid or liquid in the space between the projection optical system and the wafer, in the conventional immersion exposure apparatus. Rather, the space is filled with air and a large spherical aberration occurs due to a refractive index difference between the air and the fluid. In this case, the interference fringes are too dense to measure or to precisely measure. Therefore, the wavefront aberration is measured after a spherical aberration, a non-axial coma, and the like are minimized by adjusting the height of the object plane of the projection optical system or reticle surface, and the intervals among the optical elements, such as a lens and a mirror, in the projection optical system. Prior art include Japanese Patent Publications, Application Nos. 2001-074605 and 2002-071513, and 2002-250678.  
         [0003]     Referring now to  FIG. 7 , a description will be given of details of a conventional optical-performance measuring apparatus. A target optical system  115  to be measured guides the light from a light source  101  having a good coherency and an oscillation wavelength close to a usable wavelength of the target optical system  15 , such a laser light source, to an interferometer unit  102 . The light from the light source  101  is split into measuring light and reference light in the middle of the optical path. As to the optical path of the measuring light, a condenser lens  103  condenses the light onto a spatial filter  104  in the interferometer unit  102 . A diameter of a spatial filter  104  is set to about half an airy disc diameter determined by a numerical aperture (“NA”) of a collimeter lens  106 . Thereby, the exited light from the spatial filter  104  becomes an ideal spherical wave, passes a half-mirror  105 , is converted by the collimeter lens  106  into collimated light, and exits from the interferometer unit  102 .  
         [0004]     Then, the measuring light is guided to a top of the object plane of the target optical system  105  via a deflective optical system  110 , and incident upon TS-XYZ stages  122 - 124 . A mirror  111  fixed on the stage base  121  reflects the light in the Y direction, a Y moving mirror  112  on the TS-Y stage  122  reflects the light in the X direction, and a X-moving mirror  113  on a TS-X stage  123  reflects the light in the Z direction. A TS lens  114  on the TS-Z stage  121  condenses the light upon the object plane of the target optical system  115 , and the light re-images on the image plane (wafer surface) via the target optical system  115 .  
         [0005]     The object plane shifts along an optical-axis direction to the reticle surface in the exposure apparatus. Since the fluid is not filled in the space, the spherical aberration occurs on the normal object plane position and a measurement of the precise wavefront aberration becomes difficult. The position of the object plane is set so that the spherical aberration is minimized.  
         [0006]     Thereafter, an RS mirror  132  on RS-XYZ stages  125 - 127  reflects the light, and the light goes back to the interferometer unit  102  via the target optical system  115 , the TS lens  114 , the mirrors  113 - 111  and the deflective optical system  110 .  
         [0007]     The measuring light is incident upon the interferometer unit  102  and the collimeter lens  106 , reflected by the half-mirror  105 , and condensed upon the spatial filter  107 . The spatial filter  107  shields the stray light and a steep slope wavefront. The light passes the spatial filter  107 , and is incident upon the CCD camera  109  via the imaging lens  108 .  
         [0008]     On the other hand, the reference light is incident upon the TS lens  114  from the X moving mirror  113 , and the part of the light is reflected on the TS lens  114 . The surface reflection light from a Fizeau surface as a final surface of the TS lens  114  goes back along the same optical path, and is incident as the reference light upon the CCD camera  109 . A superposition of the reference light and measuring light forms interference fringes.  
         [0009]     The TS-XYZ stages  122 - 124  and RS-XYZ stages  125 - 127  move to an arbitrary image-point position of the target optical system  115  based on a command from the host computer  131  via a controller  130 , a TS-XYZ stage driver  128 , and an RS-XYZ stage driver  129 .  
         [0010]     This configuration enables the wavefront aberration to be continuously measured at arbitrary image points in the exposure area.  
         [0011]     However, the above conventional optical-performance measuring apparatus has difficulties in canceling all the axial and non-axial aberrations, such as low and high orders spherical aberrations and non-axial coma, resulting in the increased aberrational residues as the target optical system has a high NA. As a result, the fringe interval in the interference fringes are denser than the spatial resolution of the interferometer and the wavefront aberration cannot be measured. The measuring accuracy generally lowers with a magnitude of the target wavefront, and the wavefront can be measured but not precisely.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     Accordingly, the present invention is directed to a measuring apparatus and an exposure apparatus having the same, which precisely measures the optical performance (such as a wavefront aberration and a Zernike coefficient) of a target optical system (such as a projection optical system in an immersion exposure apparatus). More specifically, the present invention is directed to a measuring apparatus and an exposure apparatus having the same, which fills the fluid in the fluid material in an image space of the target optical system, reproduces the operating condition of the immersion exposure apparatus, and precisely measures the optical performance.  
         [0013]     A measuring apparatus according to one aspect of the present invention for measuring optical performance of a target optical system to be measured includes an optical unit for splitting light from a light source into measuring light and reference light so that the measuring light can be introduced into the target optical system, a reflection unit for reflecting the measuring light from the target optical system toward the target optical system via a fluid, and a detector for detecting an interference fringe generated between interference between the measuring light that has emitted from the target optical system after being reflected by the reflection unit and the reference light that does not pass the target optical system.  
         [0014]     An exposure apparatus according to another aspect of the present invention includes an illumination optical system for illuminating a reticle, a projection optical system for projecting a pattern of the reticle onto a substrate, and the above measuring apparatus for measuring optical performance of the projection optical system as a target optical system.  
         [0015]     A device manufacturing method according to still another aspect of the present invention includes the steps of exposing a substrate using the above exposure apparatus, and developing the object that has been exposed.  
         [0016]     Other objects and further features of the present invention will become readily apparent from the following description of the preferred embodiments with reference to accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a schematic block diagram of a structure of an optical-performance measuring apparatus according to a first embodiment of the present invention.  
         [0018]      FIG. 2  is an enlarged view of principal part near a reflecting element in the optical-performance measuring apparatus shown in  FIG. 1 .  
         [0019]      FIG. 3  is an enlarged view of principal part near a reflecting element in an optical-performance measuring apparatus according to a second embodiment of the present invention.  
         [0020]      FIG. 4  is a schematic block diagram of a structure of an exposure apparatus according to a third embodiment of the present invention.  
         [0021]      FIG. 5  is a flowchart of a device manufacturing method using the exposure apparatus shown in  FIG. 4 .  
         [0022]      FIG. 6  is a detailed flowchart for Step  104  of wafer process shown in  FIG. 5 .  
         [0023]      FIG. 7  is a schematic block diagram of a structure of a conventional optical-performance measuring apparatus. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0024]     Referring now to the accompanying drawings, a description will be given of an optical-performance measuring apparatus or interferometer according to a first embodiment of the present invention.  FIG. 1  is a schematic block diagram of a structure of the optical-performance measuring apparatus S according to the first embodiment of the present invention. The optical-performance measuring apparatus S introduces the light that has a good coherency and an oscillation wavelength close to the usable wavelength of the target optical system  15 , from a light source, such as a laser light source, to an interferometer unit  2 . The light from the light source  1  is split in the middle of the optical path into measuring light and reference light. As to the optical path of the measuring light, a condenser lens  3  condenses a light upon the spatial filter  4  in the interferometer unit  2 . The spatial filter  4  has a diameter about half an airy disc diameter determined by the NA of a collimeter lens  6 , whereby the exited light from the spatial filter  4  becomes an ideal spherical wave, passes the half-mirror  5 , is converted by the collimeter lens  6  into a collimated light, and exits from the interferometer unit  2 .  
         [0025]     Next, a deflective optical system  10  guides the measuring light to the top of the object plane of a target optical system  15 , and the light then enters TS-XYZ stages  22 - 24 . A mirror  11  fixed on a stage base  21  reflects the light to the Y direction, a Y moving mirror  12  fixed on a TS-Y stage  22  reflects the light in the X direction, and a X moving mirror  13  on a TS-X stage  23  reflects the light in the Z direction. The light is condensed on the object plane of the target optical system  14  by a TS lens  14  on the TS-Z stage  21 , and re-images on the image plane (wafer plane, see  FIG. 2 ) via the target optical system  15 .  
         [0026]     Thereafter, a reflection element or optical system  16  on the RS-XYZ stages  25 - 27  reflects the light, and the light goes back to the interferometer unit  2  via the target optical system  15 , the TS lens  14 , the mirrors  13 - 11  and the deflective optical system  10 .  
         [0027]     The measuring light is incident upon the interferometer unit  2  and the collimeter lens  6 , reflected by the half-mirror  5 , and condensed upon the spatial filter  7 . The spatial filter  7  shields the stray light and a steep slope wavefront. The light passes the spatial filter  107 , and is incident upon the CCD camera (or detector)  9  via an imaging lens  8 .  
         [0028]     On the other hand, the reference light is incident upon the TS lens  14  from the X moving mirror  13 , and the part of the light is reflected on the TS lens  14 . The surface reflection light from a Fizeau surface as a final surface of the TS lens  14  goes back along the same optical path, and is incident as the reference light upon the CCD camera  9 . A superposition of the reference light and measuring light forms interference fringes.  
         [0029]     The TS-XYZ stages  22 - 24  and RS-XYZ stages  25 - 27  move to an arbitrary image-point position of the target optical system  15  based on a command from a host computer  31  via a controller  130 , a TS-XYZ stage driver  28 , and an RS-XYZ stage driver  29 .  
         [0030]     As a result, the optical-performance measuring apparatus S can continuously measure the wavefront aberration at an arbitrary image point in the exposure area.  
         [0031]     While the optical-performance measuring apparatus S is similarly configured to the conventional one, the optical-performance measuring apparatus S further includes the reflection element  16 , a plane-parallel plate  17 , a fluid supply system  19 , and a fluid recovery system  20  near the wafer side or the image plane  201  side of the target optical system  15 . A description will now be given of their structures with reference to  FIG. 2 .  
         [0032]      FIG. 2  is an enlarged view of principal part of the optical-performance measuring apparatus S near the reflection element  16 . The reflection element  16  reflects the light from the target optical system  15  so as to redirect the light to the target optical system  15 , and is made of a transparent material. The reflection element  16  has a plane surface  202  opposing to the target optical system  15 , and a planoconvex reflection element having a convex surface opposing to the plane surface  202 . The light from the target optical system  15  is incident upon the inside of the reflection element  16  through its plane surface  202 , and exits from the plane surface  202  after being reflected on the convex surface. Thus, the convex surface of this reflection element  16  internally reflects the measuring light, and serves as a condenser optical system for the measuring light. In this case, a reflective coating is formed at the convex surface side to avoid a loss of the measuring light intensity.  
         [0033]     The reflection element  16  is adhered or welded to the transparent plane-parallel plate  17  made of glass or metal. A material having a large contact angle with the (immersion) fluid or liquid  18  is coated or arranged on the plane-parallel plate  17 , thereby forming a fluid film even when the reflection element  16  has a small contact angle to the fluid  18 . When the reflection element is made of SiO 2 , the contact angle is so small that a fluid film formation becomes difficult whereas the surrounding Teflon coating enables the fluid film to be formed.  
         [0034]     The image space between the reflection element  16  and the target optical system  15  is filled with the fluid  18 , enabling the optical-performance measuring apparatus S to reproduce the operating condition of the target optical system  15  in the immersion exposure apparatus.  
         [0035]     The fluid  18  is supplied from the fluid supply system  19 , and recovered by the fluid recovery system  20 . As shown in  FIG. 2 , in order to prevent the fluid  18  from flowing out of a stage part  40  of the plane-parallel plate  17  to the outside, the plane-parallel plate  17  has a low wall material  17   a . The plane-parallel plate  17  and the reflection element  16  are adhered or welded to each other as discussed above so that no fluid leaks out between them. The plane-parallel plate  17  and the reflection element  16  are arranged on the RS-XYZ stages  25 - 27 , which move them to an arbitrary image point of the target optical system  15 .  
         [0036]     When the target optical system  15  is a projection optical system for the ArF excimer laser, water is used for the fluid  18  and calcium fluoride (CaF 2 ) or synthetic quartz (SiO 2 ) is used for the reflection element  16 . When the reflection element  16  uses calcium fluoride, an antireflection coating is formed on calcium fluoride at contact portion to prevent its dissolution into water. When the reflection element  16  uses synthetic quartz, compaction and rarefaction measures are needed and it is necessary to prevent the light from condensing inside the reflection element  16 . The reflection element  16  should oppose to the target optical system  15  on its plane surface  202  side, and the image surface  201  of the target optical system  15  should be arranged between the plane surface  202  of the reflection element  16  and the target optical system  15  or above the plane surface  202  of the reflection element  16 . As a result, when the convex surface of the reflection element is spherical, the maximum thickness of the reflection element  16  should be made smaller than the radius of curvature of the convex surface of the reflection element  16 . When the convex surface of the reflection element is made aspheric in order to reduce the aberration, the thickness of the reflection element  16  should be made smaller than a distance between the convex surface and the condensing point or a double focal length. As discussed above, the defocus amount between the reflection element  16  and the condensing point is important to not only the compaction measures but also the ghost measures in the interference measurement. A diameter of the ghost fringe should preferably be about 1% or smaller of the pupil diameter, and the defocus amount should be greater than 0.6 mm when the NA is 1.2, for example. Conversely, this defocus amount satisfies the compaction condition (of 0.1 mm or smaller).  
         [0037]     The condensing point is formed in the fluid  18 . Therefore, the fluid  18 &#39;s refractive index varies due to its temperature rise resulting from the light energy absorptions or its temperature fluctuation resulting from the heat conduction from the structure around the fluid  18 , and the measuring errors occur in measuring the wavefront aberration. Accordingly, the velocity of the fluid  18  is controlled through the fluid supply and recovery systems  19  and  20  so that the fluid  18  moves at a constant speed in the image space in the wavefront aberration measurement. Preferably, the velocity V of the fluid  18  satisfies the following Equation (1), 
 
 V&gt;D×X   (1) 
 
         [0038]     D is a maximum width (mm) of the measuring light in the fluid  18 , and X is a detecting frame rate (Hz) of the detector or CCD camera  9 .  
         [0039]     This configuration averages the influence of the fluctuating refractive index distribution of the fluid  18  that results from the temperature rise particularly near the condensing point, in the frame rate of the CCD camera  9 , and reduces the measuring error caused by the refractive index fluctuation.  
         [0040]     Since a refractive index difference between the reflection element  16  and the fluid  18  is known, the spherical aberration caused by a difference of refractive indexes is correctable, for example, by turning a reflective spherical surface into an aspheric convex surface, or a reflective plane into a spherical or aspheric surface. Alternatively, the low order aberration is correctable by adjusting an object surface in an optical-axis direction so that the spherical aberration becomes minimum. These configurations can minimize the aberration at the wavefront measuring time, and provide excellent aberrational measurement.  
         [0041]     A correction to a position of the object plane or reticle surface along the optical-axis direction to minimize the spherical aberration maintains the measuring reciprocity and thus the good measurement irrespective of the disturbance.  
         [0042]     A distance between the center of curvature of the convex surface and the plane surface of the reflection element  16  is preferably 0.6 mm or greater, because the reflected light from the plane surface of the reflection element  16  becomes ghost and deteriorates the measuring accuracy. The distance of 0.6 mm or greater maintains the wavefront-accuracy deteriorating area caused by the ghost influence to be 1% or smaller of the pupil radius with no substantial problem.  
       Second Embodiment  
       [0043]     Referring to  FIG. 3 , a description will be given of an optical-performance measuring apparatus S 2  according to a second embodiment of the present invention. While the first embodiment uses a planoconvex lens for the reflection element  16  that has a plane surface  202  opposing to the target optical system  15  and a convex surface opposite to the plane surface  202 , this embodiment uses a convex reflector  16   a  for the reflection element  16  having a reflective convex surface opposing to the target optical system  15 . The first embodiment arranges the plane surface  202  of the reflection element  16  at a defocus position by a predetermined distance from the image plane  201  position of the target optical system  15  to reduce the temperature rise near the condensing point or to avoid the compaction of the reflection element  16 . This configuration elongates the optical path in the fluid  18 , and may increase the influence of the fluctuation. However, this embodiment arranges the convex reflector  16   a  above the image plane  201  position of the target optical system  15  as shown in  FIG. 3  or so that the measuring light is reflected by the convex surface of the convex reflector  16   a  before reaching the image plane  201  position. This configuration reduces the fluctuation influence associated with the temperature rise and elongated optical path in the fluid  18 . For example, when the target optical system  15  is a projection optical system for the ArF excimer laser, the back focus is about 1 mm. Therefore, the radius of curvature of the convex surface of the convex reflector  16   a  should be made smaller than 1 mm.  
         [0044]     The reflection element  16  may have a so-called spherical concave surface RS used for a normal interference measurement, and fills the fluid.  
       Third Embodiment  
       [0045]     Referring now to  FIG. 4 , a description will be given of the exposure apparatus S 3  according to a third embodiment of the present invention. The exposure apparatus S 3  has a structure similar to that disclosed in U.S. Patent application, Publication No. 2005-0099635-A1, and includes the optical-performance measuring apparatus S 1  according to the first embodiment. In  FIG. 4 , reference numeral  401  denotes an exposure light source,  402  and  406  denote deflective optical systems,  403  denotes an optical-path switching mirror,  404  denotes an incoherent turning unit,  405  denotes an illumination optical system,  407  denotes a condenser lens, and  408  denotes a spatial filter.  409  and  412  denote collimeter lenses,  410  denotes a half-mirror,  411  denotes a mirror,  413  denotes an XYZ stage,  414  denotes a collimeter lens unit,  415  denotes a reticle surface. Those elements, which are corresponding elements in the first embodiment, are designated by the same reference numerals of the first embodiment.  
         [0046]     The plane-parallel plate  17  and the reflection element  16  adhered or welded to the plane-parallel plate  17 , which are referred to as an immersion reflection unit, are arranged on a wafer stage  418  of the exposure apparatus S 3 . The optical-path switching mirror  403  is switched to the side of the optical-performance measuring optical path so that the light from the exposure light source  401  enters the deflective optical system  406 . The light is guided to the reticle surface  415  via the deflective optical system  406 , and the projection or target optical system  15  re-images the light on the wafer surface  201 . The wafer stage  418  moves the reflection element  16  so that the condensing point (or center of curvature) of the reflection element  16  accords with the wafer surface  201 . Next, the measuring light is reflected by the convex surface of the reflection element  16 , goes back to the projection optical system  15  and then the inside of the interference optical system  421 . A detecting means, such as a CCD camera, provided in the interference optical system  421  detects the interference fringes, and measures the wavefront aberration and Zernike coefficient of the projection optical system  15 .  
         [0047]     The projection optical system  15  may use a dioptric optical system that includes only plural lenses, a catadioptric optical system that includes one mirror and plural lenses, and a catoptric optical system that includes only plural mirrors.  
         [0048]     While the above embodiment bypasses the light from the light source to the deflective optical system  406 , the light irradiated onto the reticle surface may be used to measure the optical performance via the illumination optical system  405  in the exposure apparatus as long as the necessary light intensity is secured to measure the optical performance on the CCD camera.  
       Fourth Embodiment  
       [0049]     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 S 3 .  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  101  (circuit design) designs a semiconductor device circuit. Step  102  (mask fabrication) forms a mask having a designed circuit pattern. Step  103  (wafer preparation) manufactures a wafer using materials such as silicon. Step  104  (wafer process), which is referred to as a pretreatment, forms actual circuitry on the wafer through photolithography using the mask and wafer. Step  105  (assembly), which is also referred to as a post-treatment, forms into a semiconductor chip the wafer formed in Step  104  and includes an assembly step (e.g., dicing, bonding), a packaging step (chip sealing), and the like. Step  106  (inspection) performs various tests for the semiconductor device made in Step  105 , such as a validity test and a durability test. Through these steps, a semiconductor device is finished and shipped (Step  107 ).  
         [0050]      FIG. 6  is a detailed flowchart of the wafer process in Step  104 . Step  111  (oxidation) oxidizes the wafer&#39;s surface. Step  112  (CVD) forms an insulating film on the wafer&#39;s surface. Step  113  (electrode formation) forms electrodes on the wafer by vapor disposition and the like. Step  114  (ion implantation) implants ions into the wafer. Step  115  (resist process) applies a photosensitive material onto the wafer. Step  116  (exposure) uses the exposure apparatus S 3  to expose a mask pattern onto the wafer. Step  117  (development) develops the exposed wafer  47 . Step  118  (etching) etches parts other than a developed resist image. Step  119  (resist stripping) removes disused resist after etching. These steps are repeated, and multilayer circuit patterns are formed on the wafer. The manufacturing method of this embodiment can quickly and easily obtain the imaging performance of the projection optical system without lowering the exposure throughput, and utilize the projection optical system whose wave front aberration has been highly precisely corrected. The projection optical system whose wave front aberration has been highly precisely corrected can provide an alignment for the wafer stage with high precision. Therefore, the manufacture method of this embodiment can manufacture higher-quality devices than the conventional ones.  
         [0051]     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.  
         [0052]     This application claims a foreign priority based on Japanese Patent application No. 2004-322997, filed Nov. 5, 2004, which is hereby incorporated by reference herein.