Abstract:
A measuring apparatus for irradiating measuring light and for measuring optical performance of a target optical system includes a barrel for housing the target optical system, the barrel being rotatable around an optical axis of the target optical system, and an illumination optical system for introducing the measuring light into the barrel, the illumination optical system being movable along a direction perpendicular to the optical axis of the target optical system, wherein the measuring apparatus controls an illumination area of the measuring light in the target optical system using a polar coordinate determined by a rotational angle of the barrel and a moving amount of the illumination optical system.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to a method and apparatus for measuring performance of an optical element, and more particularly to a measuring method and apparatus for measuring a wave front of a projection optical system that transfers a pattern on a mask onto an object, etc. The present invention also relates to an exposure method and apparatus suing such a measuring method and apparatus. The inventive measuring method and apparatus are suitable, for example, for measurements that use as measuring light synchrotron radiation, such as a synchrotron ring, an undulator, etc. 
   A projection exposure apparatus is used to transfer a pattern on a mask (or a reticle) onto an object to be exposed in manufacturing semiconductor devices, etc. in the lithography process. This exposure apparatus is required to transfer the pattern on the reticle onto the object precisely at a predetermined magnification. For this purpose, it is important to use a projection optical system having good imaging performance and reduced aberration. In particular, due to the recent demands for finer processing of semiconductor devices, a transfered pattern is sensitive to the aberration of the optical system. Therefore, there is a demand to measure the wave front aberration of the projection optical system with high precision. 
     FIG. 6  shows an optical path of a conventional lens performance measuring apparatus  100 . In  FIG. 6 ,  101  denotes a target optical element or optical system, such as a projection optical system.  102  denotes an object surface of the target lens  101 .  103  denotes an image surface.  109  denotes a condenser lens, which has a final surface as a reference surface for reflecting part of incident light.  108  denotes a mirror for deflecting the measuring light.  105 ,  106  and  107  denote stages that are mounted with the condenser lens  109  and the mirror  108  and move in X, Y and Z directions, respectively.  113  denotes a spherical mirror, and its center of the radius of curvature approximately accords with the object surface  102 .  110 ,  111  and  112  denote stages that are mounted with the spherical mirror  113  and move in X, Y and Z directions, respectively.  104  denotes an interferometer body, which houses a laser light source (not shown), a lens (not shown), a collimetor lens (not shown), a beam splitter (not shown), an interferometer condenser lens (not shown), a camera (not shown), etc. 
   According to the above structure, a collimated ray emitted from the interferometer body  104  is reflected on the spherical mirror  109 &#39;s final surface and incident as interference light upon the interferometer body  104 , forming interference fringes on the camera (not shown). The wave front aberration of the target optical system  101  is calculated from the obtained interference fringes. In order to measure plural positions on the image surface  103  of the target optical system  101 , the stages  105 ,  106  and  107  that install the condenser lens  109  may move to a predetermined position, and the stages  111 ,  112  and  113  that install the spherical mirror  113  may move to a corresponding position. Such an apparatus is disclosed, for example, in Japanese Patent Application, Publication No. 9-98589. 
   The conventional measuring apparatus that uses the ultraviolet (“UV”) light as measuring light can easily reflect the light using a mirror and thus easily measure plural positions on the image surface  103 . On the other hand, due to the demand for the fine processing of the semiconductor device, the practical implementation of a reduction projection exposure apparatus that utilizes the extreme ultraviolet (EUV) light having a wavelength between 10 and 15 nm, shorter than the UV light is now promoted. It is conceivable that an interference measurement of an EUV optical system utilizes an intensifier EUV light source, such as an undulator light source inserted into an electron accumulation ring. Since the electron accumulation ring should maintain the inside ultra high vacuum (“UHV”), the optical element is provided in the UHV and free orthogonal driving of the stages  105  to  107  shown in  FIG. 6  becomes difficult. In particular, it becomes difficult to displace the measuring light in a direction perpendicular to the optical-axis direction. As a result, it becomes difficult to measure plural positions on the image surface, or a necessary area of the target optical system. 
   BRIEF SUMMARY OF THE INVENTION 
   Accordingly, it is an illustrative object of the present invention to provide a measuring method and apparatus for measuring the optical performance, such as a wave front, in a necessary area of the target optical system while the measuring method and apparatus are compatible with an undulator light source etc. as a measuring light source, an exposure method and apparatus using them, and a device manufacturing method. 
   A measuring apparatus according to one aspect of the present invention for irradiating measuring light and for measuring optical performance of a target optical system includes a barrel for housing the target optical system, the barrel being rotatable around an optical axis of the target optical system, and an illumination optical system for introducing the measuring light into the barrel, the illumination optical system being movable along a direction perpendicular to the optical axis of the target optical system, wherein the measuring apparatus controls an illumination area of the measuring light in the target optical system using a polar coordinate determined by a rotational angle of the barrel and a moving amount of the illumination optical system. 
   The measuring light may be a synchrotron radiation from an electron accumulation ring or an undulator inserted into the electron accumulation ring. The measuring apparatus may further include at least two alignment marks fixed onto the barrel, a detector, fixed outside the barrel, for detecting the alignment mark, and an operation part for calculating an offset amount between a rotational axis of the barrel and the optical axis of the target optical system, based on a detection result by the detector. The measuring apparatus may further include a controller for controlling driving of the mirror barrel based on the offset amount. 
   A measuring method according to another aspect of the present invention includes the steps of setting, on a polar coordinate, an illumination area of measuring light on a target optical system housed in a barrel, and measuring optical performance of the target optical system by irradiating the measuring light onto the target optical system. 
   An exposure method according to still another aspect of the present invention includes the steps of calculating a wave front aberration of a target optical system using the above measuring method, adjusting the target optical system based on the calculated wave front aberration of the target optical system, and exposing an object using the adjusted target optical system. 
   An exposure apparatus according to another aspect of the present invention for exposing a pattern formed on a mask onto an object using light includes a projection optical system for projecting the pattern onto the object, and the above measuring apparatus for detecting a wave front aberration of the projection optical system as an interference fringe. The exposure light may be the EUV light having a wavelength of 20 nm or smaller. 
   A device manufacturing method according to still another aspect of the present invention includes the steps of exposing an object using the above exposure apparatus, and developing the object 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 accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are sectional and plane views showing an optical path in an exposure apparatus that includes a measuring apparatus according to one embodiment of the present invention. 
       FIG. 2  is an optical-path diagram showing an illustrative measuring principle of the measurement apparatus shown in  FIG. 1 . 
       FIGS. 3A ,  3 B and  3 C are sectional and plane views for explaining how the measuring apparatus shown in  FIG. 1  sets a measurement area. 
       FIGS. 4A and 4B  are plane views of a mask having plural pinholes of this embodiment. 
       FIGS. 5A and 5B  are plane views for explaining an alignment by the measuring apparatus shown in  FIG. 1 . 
       FIG. 6  is an optical-path diagram for explaining a structure of a conventional optical performance measuring apparatus. 
       FIG. 7  is a flowchart for explaining a method for fabricating devices (semiconductor chips such as ICs, LSIs, and the like, LCDs, CCDs, etc.). 
       FIG. 8  is a detailed flowchart for Step  4  of wafer process shown in  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1 to 3 , a description will now be given of an EUV exposure apparatus  10  having a measuring apparatus  20  according to one embodiment of the present invention. Here,  FIG. 1A  is a sectional view of the principal part of the EUV exposure apparatus  10 .  FIG. 2  is a schematic optical-path diagram showing the principal part of the measurement apparatus  20 . The measuring apparatus  20  measures a wave front of the projection optical system housed in a barrel  11  of the EUV exposure apparatus  10  using the EUV light from an undulator (not shown) as a light source. While the measuring apparatus  20  of the instant embodiment uses point diffraction interferometry (“PDI”) as an interferometry, the measuring apparatus  20  may use the lateral shearing interferometry (“LSI”) and the line diffraction interferometry (“LDI”) and other interferometries. 
   In  FIG. 1A ,  11  denotes a barrel for the projection optical system as a target optical system.  12  denotes a wafer-side mask that has a pinhole  12   a  and a window  12   b . The wafer-side mask  12  is provided near the image surface of the barrel  11 .  13  denotes a CCD camera that measures interference fringes.  14  denotes a wafer-side mask stage that supports the wafer-side mask  12  and moves it in the Y direction.  19  denotes a reticle-side mask having a pinhole  19   a .  21  denotes a reticle-side mask stage that supports a reticle-side mask  19  and moves it in the Y direction.  22  denotes a grating.  23  denotes a grating stage that supports the grating  22  and moves it in the Y direction.  30  denotes phase shift means for driving the grating stage  23  so as to shift a phase of a wave front.  17  denotes an illumination optical system for illuminating the pinhole  19   a  in the reticle-side mask  19 .  18  is an illumination system stage that supports the illumination optical system  17  and moves it in the Y direction.  16  denotes a structure that supports the barrel  11  and various stages, such as a θz stage  15 .  24  denotes a vacuum chamber that maintains the entire interferometer to be vacuum.  25  denotes a beam line for introducing the EUV light from the undulator (not shown).  27  denotes alignment-mark detecting means for detecting an alignment mark used to align the field of the barrel  11  with a measured position. The alignment-mark detecting means  27  is fixed onto the reticle-side mask stage  21 .  FIG. 1B  is a plane view of the barrel  11  viewed from the top of the barrel  11 .  26  denotes an exposure field in the barrel. 
   Referring now to  FIG. 2 , a description will be given of the measurement procedure. The illumination optical system  7  introduces the EUV light from the undulator (not shown) into the pinhole  19   a  in the reticle-side mask  19  at a predetermined NA. The EUV light emitted from the pinhole  19   a  becomes a spherical wave, is split into two rays by the grating  22 , and enters the barrel  11 . These two rays enter the pinhole  12   a  and the window  12   b  in the wafer-side mask  12 . The light incident upon the pinhole  12   a  becomes a spherical wave. The light incident upon the window  12   b  passes through the window  12   b  while maintaining the wave front information of the barrel  11 . The interference light between these two rays enters the CCD camera  13 , and forms interfere fringes on the image pickup surface. Thus, one point in the field  26  in the barrel  11  can be measured. 
   Referring to  FIG. 3 , a description will be given of measurements of plural points in the field  26 . Here,  FIG. 3A  is a partial sectional view for showing how the measuring apparatus  20  controls the illumination area of the measuring light in the target optical system.  FIG. 3B  is a plane view of the barrel  11  viewed from the top of the barrel  11 , showing changes of the irradiated area (or the field  26 ) as the illumination optical system  17  moves.  FIG. 3C  is a plane view of the barrel  11  viewed from the top of the barrel  11 , showing changes of the field  26  as the barrel  11  rotates. 
   It is understood from  FIG. 3B  that as the illumination optical system stage  18  moves the illumination optical system  17  in the Y direction, a measurement position (or a spot of the illumination light) of the field  26  moves in the Y direction. Depending upon this amount, the reticle-side mask stage  19  and the grating stage  23  move in the Y direction. The wafer-side mask stage  14  moves in the Y direction by an amount of the reticle-side stage  19  times the magnification of the barrel  11 . Thus, plural points in the field  26  can be measured in the Y direction. When the synchrotron radiation is used, a position in the radiation direction (or Y direction) is relatively easily changeable by moving the entire illumination system in the Y direction without an additional mirror. On the other hand, it is understood from  FIG. 3C  that as the θz stage  15  rotates the barrel  11 , a measurement position of the field (or a spot of the illumination light) rotates. Driving of this polar coordinate system provides measurements of the entire surface of the field  26 . While the field  26  has an arc shape around the optical axis, it may have a rectangular shape. 
   As discussed, by combining the Y stage of the illumination optical system  17  with the θz stage  15  of the barrel  11 , the wave front measuring apparatus that uses the EUV light from the undulator as a light source easily measures an arbitrary position on the entire surface in the barrel  11 &#39;s field  26 . One pinhole  19   a  in the mask  19  for measuring the entire surface in the field  26  would be less expensive than plural types of masks  19 . Alternatively, the mask  19  may have plural pinholes having different shapes as shown in  FIG. 4A , or a preliminary pinhole as shown in  FIG. 4B .  FIG. 4A  arranges differently sized pinholes in the Y direction.  FIG. 4B  provides a preliminary pinhole in the X direction, although  FIG. 4B  requires an addition of an X-axis stage to the mask stage. 
   Referring now to  FIG. 5 , a description will be given of an alignment between the field  26  of the barrel  11  and the reticle-side pinhole  19   a  and a correction of an offset error between the rotational center of the θz stage  15  and the optical axis. In  FIG. 5 ,  28  denotes alignment marks, fixed onto the barrel  11 , for indicating a center of the field  26 . In order to detect an offset of the rotational center when the barrel  11  rotates by an angle θz, at least two alignment marks are arranged an equal distance apart from the optical-axis center.  27  denotes alignment mark detector means fixed onto the reticle-side mask stage  21 . A positional relationship between the alignment mark detector means  27  and the reticle-side pinhole  19   a  has been previously measured as discussed above. 
     FIG. 5A  shows a state where the barrel  11  is driven by the angle θz (see the right side in  FIG. 5A ) from a state where the optical axis of the barrel  11  accords with the rotational center of the θz stage  15  (see the left side in  FIG. 5A ). As shown in  FIG. 5A , as the barrel  11  rotates by the angle θz, the second alignment mark  28  moves to a position just below the alignment mark detector means  27 . On the other hand,  FIG. 5B  shows an offset between the optical axis of the barrel  11  and the rotational center of the θz stage  15 . 
   First, positions of the θz stage  15  and the reticle-side mask stage  21  are adjusted so that one of the alignment marks  28  moves to a position just below the alignment mark detector means  27 . When the θz stage  15  is rotated in this state, the barrel  11  rotates eccentrically because of the offset between these centers. As shown in  FIG. 5B , a position of the second alignment mark offsets from the center of the alignment mark detector means  27 . This offset amount ΔB=(δx, δy) is measured by driving the alignment mark  28  moves to a position just below the alignment mark detector means  27 . An offset of the rotational center at the setting time ΔA=(ΔX, ΔY) is calculated using the following equations and obtained ΔB, where R is a matrix of rotation, “A” is a coordinate of the first alignment mark, and “B” is a coordinate of the second alignment mark. The capital indicates a matrix:
 
 B=R×A   (1)
 
 B′=R× ( A+ΔA )  (2)
 
Δ B=B′−B   (3)
 
 ΔA=R   −1   ×ΔB   (4)
 
   Based on obtained values (ΔX, ΔY), driving amounts of the stage are corrected to measure predetermined positions in the field. 
   Thus, an arbitrary position in the field can be precisely measured by providing two alignment marks in the barrel, measuring an offset between the center of the barrel and the rotational center of the θz stage, and correcting the driving amount of the stage. 
   As discussed, according to the instant embodiment, even when the electron ring etc. are used as a light source, the entire surface of the field of the target optical system can be easily measured. Since the alignment of the target optical system is conducted by at least two points, an offset between the optical axis of the target optical system and the center of the driving means around the optical axis can be calculated, providing a more precise field alignment for the target optical system by correcting stage&#39;s driving based on the calculated offset amount. 
   The exposure apparatus  10  includes an illumination apparatus (not shown) different from the measuring apparatus has a mask, on which a circuit pattern of a semiconductor device (such as a semiconductor chip, e.g., an IC and an LSI, a liquid crystal panel and a CCD) is formed, and a plate, and uses the EUV light to expose a circuit pattern on the mark onto the plate, for example, by a step-and-scan manner or step-and-repeat manner. A laser plasma light source that is known in the art may be used as a EUV light source rather than the measuring electron accumulation ring. Of course, the inventive exposure apparatus is not limited to one that uses the EUV light. This embodiment can easily add an aberration measuring function to the projection exposure apparatus by using a reflection mask pattern. 
   A description will now be given of an aberration correction method according to one embodiment of the present invention. The exposure apparatus  10  allows plural optical elements (not shown) in the projection optical system to move in the optical-axis direction and/or a direction orthogonal to the optical-axis direction. By driving one or more optical elements using the driving system (not shown) for aberrational adjustments based on aberrational information obtained from the instant embodiment, it is possible to correct or optimize one or more aberrations of the projection optical system, in particular Seidel&#39;s classification of aberrations. The means for adjusting the aberration of the projection optical system can use various known system, such as a movable lens, a movable mirror (when the projection optical system is a catadioptric optical system or full-mirror optical system), an inclinable parallel plate, a pressure-controllable space, and a surface correction using an actuator. 
   A description will now be given of an embodiment of a device manufacturing method using the exposure apparatus  10 .  FIG. 7  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 making) 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. 8  is a detailed flowchart of the wafer process in Step  4  shown in  FIG. 7 . 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  200  to expose a circuit pattern on the mask 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. The manufacturing method of the present invention can manufacture semiconductor devices which have been difficult to manufacture, because the wave front aberration has been corrected with high precision. 
   The present invention thus uses a polar coordinate system rather than a Cartesian coordinate system, and provide a measuring method and apparatus for measuring the optical performance, such as a wave front, in a necessary area of the target optical system while the measuring method and apparatus are compatible with an undulator light source etc. as a measuring light source, an exposure method and apparatus using them, and a device manufacturing method This application claims a foreign priority based on Japanese Patent Application No. 2003-399487, filed Nov. 28, 2003, which is hereby incorporated by reference herein.