Abstract:
A measuring apparatus includes a first mask having a pinhole for generating a spherical wave as measuring light, a second mask provided subsequent to the first mask in a light traveling direction, the second mask having a selecting window that allows the measuring light that has passed a target optical system to transmit through the selecting window, and a two-dimensional light divider, located between the first and second masks, for two-dimensionally dividing light, wherein the measuring apparatus calculating optical performance of the target optical system from an interference fringe formed by the measuring light that has passed the selecting window.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to measuring method and apparatus, and more particularly to a measuring method and apparatus that use the shearing interferometry to measure a wave front aberration of a target optical system, such as a projection optical system that transfers a mask pattern onto an object, and an exposure method and apparatus using the measuring method and apparatus. The inventive measuring method and apparatus is suitable, for example, for a measurement of a projection optical system in an exposure apparatus that utilizes the extreme ultraviolet (“EUV”) light.  
         [0002]     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 photolithography 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 transferred pattern is more 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.  
         [0003]     A shearing interfering system is conventionally known as a method for measuring a wave front aberration of a projection optical system.  FIG. 4  shows a basic arrangement of the conventional shearing interfering system. A pinhole plate  1  is arranged at a desired object point on the object surface of a target optical system  2 . Since the pinhole plate  1  should efficiently shield the EUV light, it is made, for example, of Ta and Ni. A thickness of the pinhole plate  1  is 200 nm or larger for Ta and 150 nm or larger for Ni. A diameter of the pinhole A should be λ/2 NAi, where NAi is a numerical aperture of the target optical system  2  at the side of the illumination optical system. An image of the pinhole A is formed on a diffracted light selecting window plate  4  provided on the image surface under the influence of the aberration of the target optical system  2 . A diffraction grating plate  6  is arranged between the target optical system  2  and the diffracted light selecting window plate  4  that has diffracted light selecting windows D and E.  
         [0004]     In calculating a wave front using the shearing interferometry, wave front information obtained in two orthogonal directions, for example, the x and y directions, is synthesized. More specifically, a two-dimensional wave front restoration method using two diffraction gratings having orthogonal periodic directions is known which includes the steps of, obtaining wave front information in the x direction from x shearing wave front data obtained by offsetting or shearing a wave front in the x direction; obtaining wave front information in the y direction from y shearing wave front data obtained by offsetting or shearing a wave front in the y direction; and conducting path integrals in the x and y directions.  FIG. 4  uses a combination of the pinhole A, the diffraction grating B and the diffracted light selecting window D to measure the shearing wave front in the x direction, and a combination of the pinhole A, the diffraction grating C and the diffracted light selecting window E to measure the shearing wave front in the y direction. The pinhole A is arranged in the measurement point. In measuring the wave front in the x direction, the diffraction grating B and window D are arranged in the optical path. A stage (not shown) for holding the diffraction gratings B and C and the windows D and E is driven to exchange the diffraction grating and the window.  
         [0005]     In order to measure the wave front in the x direction, a spherical wave emitted from the pinhole A passes the target optical system  2 , is divided into plural wave fronts of plural orders diffracted lights by the diffraction grating, and enters the window D. The window size is designed so that the ±1 st order diffracted lights pass the centers of the windows D and E. In other words, a light shielding part around the window D shields unnecessary 0th and other orders diffracted lights, and a CCD  4  observes high contrast interference fringes resulting from the ±1st order lights. While the wave front is sheared by a separation interval between the ±1st order lights on the observed surface  5 , which is about {fraction (1/30)} to {fraction (1/60)} of NA. The measurement of the wave front in the y direction is similar to that in the x direction although the measurement direction rotates by 90°.  
         [0006]     A method that uses a two-dimensional diffraction grating has conventionally been proposed (see, for example, Patrik P. Naulleau and Kenneth A. Goldberg, J. Vac. Sci. Technol. B 18 (6), (2000), (simply referred to as “Patrik” hereinafter) which Fourier-transforms interference fringes including many mixed diffracted light, and extracts signal light components of the ±1st order lights through signal processing.  
         [0007]     The interferometer shown in  FIG. 4  uses two orthogonal diffraction gratings, obtains the wave front information in the x direction from the x shearing wave front data and the wave front information in the y direction from the y shearing wave front data, conducts a path integral in the x and y directions, and restores the two-dimensional wave front. However, this interferometer has a problem shown in  FIG. 5 . Here,  FIG. 5  is an optical-path diagram for explaining the problem of the system shown in  FIG. 4 . The wave front measurements in the y direction follow the wave front measurements in the x direction. Therefore, the above problem occurs when a position in the optical-axis direction (or z direction) offsets from F to G in  FIG. 5  due to stage&#39;s driving errors, influence of the vibration, etc., during a replacement of the diffraction grating from B to C.  
         [0008]     The diffraction grating located at the position F causes the wave front to diffract at H and to image at a position J. On the other hand, the diffraction grating located at a position G causes the wave front to diffract at I and to image at a position K. Since a segment HJ is parallel to a segment IK, ΔJ=ΔF tang is met, where ΔJ is a shift amount of the position J at which the 1st order light condenses when a position z of the diffraction grating varies by ΔF from F to G, and θ is a diffraction angle of the first order light. Since this shift similarly happens to the −1st order light, an interval between the ±1st order lights on the imaging surface varies by 2ΔF·tanθ. This offset appears as a tilt fringe in the wave front component. Since the shearing interferometer directly observes the differentiated wave front, the tile component is observed as a defocus component as a result of integration in the shearing direction. In this case, the focus component of the wave front data in the xy components include an offset due to 2ΔF·tanθ, and this offset is finally measured as astigmatism. One design example needs to maintain ΔF to be about 10 nm in order to reduce the astigmatism error down to 0.1 nm RMS or smaller, and it is extremely difficult to control two physically different grating surfaces in such a range.  
         [0009]     Patrik can avoid this problem, but causes new problems of inevitable optical contrast deteriorations in the signal component and extremely complex signal processing.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     Accordingly, it is an illustrative object of the present invention to provide a measuring method and apparatus which utilize the shearing interferometry and provide higher precision and easier signal processing than the conventional method, an exposure method and apparatus using them, and a device manufacturing method.  
         [0011]     A measuring apparatus according to one aspect of the present invention includes a first mask having a pinhole for generating a spherical wave as measuring light, a second mask provided subsequent to the first mask in a light traveling direction, the second mask having a selecting window that allows the measuring light that has passed a target optical system to transmit through the selecting window; and a two-dimensional light divider, located between the first and second masks, for two-dimensionally dividing light, wherein the measuring apparatus calculating optical performance of the target optical system from an interference fringe formed by the measuring light that has passed the selecting window. The optical performance may be a wave front aberration. The measuring apparatus may calculate the optical performance from wave front aberration of the target optical system with respect to two orthogonal directions, wherein the selecting window in the second mask allows ±1st order diffracted lights of the measuring light in one or both of the two orthogonal directions to simultaneously pass through the selecting window.  
         [0012]     A measuring method according to another aspect of the present invention includes the steps of dividing measuring light using a two-dimensional divider, obtaining interference information with respect to two orthogonal directions, through a shearing interference between predetermined orders of the measuring lights that have passed a target optical system, a position of the two-dimensional divider being fixed during the obtaining step, the obtaining step using a selecting window plate that has at least two windows aligned with one direction among the two orthogonal directions, and calculating optical performance of the target optical system by integrating the interference information, and by using the interference information of the measuring light that has passed the selecting window plate.  
         [0013]     A measuring method according to still another aspect of the present invention includes the steps of dividing measuring light using a two-dimensional divider, obtaining interference information with respect to two orthogonal directions, through an interference between predetermined orders of the measuring lights that have passed a target optical system, a position of the two-dimensional divider being fixed during the obtaining step, the obtaining step using a selecting window plate that has two pairs of windows aligned with the two orthogonal directions, and calculating optical performance of the target optical system by Fourier-transforming the interference information, by performing spatial frequency filtering for a component of an interference fringe generated by a combination of predetermined openings so as to selectively extract the component, and by using the interference information of the measuring light that has passed the selecting window plate.  
         [0014]     An exposure method according to one aspect of the present invention includes the steps of calculating optical performance of a target optical system using the above measuring method, adjusting the target optical system based on the optical performance of the target optical system, which is calculated by the calculating step, and exposing an object using the target optical system adjusted by the adjusting step.  
         [0015]     An exposure apparatus according to another aspect of the present invention for exposing a pattern 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. The light may have a wavelength of 20 nm or smaller.  
         [0016]     A device manufacturing method according to another aspect of the present invention includes the steps of exposing an object to be exposed using the above exposure apparatus, and developing the object exposed. Claims for a device fabricating method for performing operations similar to that of the above exposure apparatus cover devices as intermediate and final products. Such devices include semiconductor chips like an LSI and VLSI, CCDs, LCDs, magnetic sensors, thin film magnetic heads, and the like.  
         [0017]     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  
       [0018]      FIG. 1  is an optical-path diagram of a measuring apparatus according to one embodiment of the present invention.  
         [0019]      FIGS. 2A and 2B  are perspective views of the optical path for explaining operations of selecting windows in the measuring apparatus shown in  FIG. 1 .  
         [0020]      FIG. 3  is a perspective view of the optical path for explaining a variation of a selecting window in the measuring apparatus shown in  FIG. 1 .  
         [0021]      FIG. 4  is an optical-path diagram of a conventional measuring apparatus.  
         [0022]      FIG. 5  is an optical-path diagram for explaining a problem of the measuring apparatus shown in  FIG. 4 .  
         [0023]      FIG. 6  is an optical-path diagram for explaining an exposure apparatus according to one embodiment of the present invention.  
         [0024]      FIG. 7  is a flowchart for explaining a method for fabricating devices (semiconductor chips such as ICs, LSIs, and the like, LCDs, CCDs, etc.).  
         [0025]      FIG. 8  is a detailed flowchart for Step  4  of wafer process shown in  FIG. 7 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]      FIG. 1  shows a basic arrangement of the measuring apparatus  100  according to one embodiment of the present invention. The measuring apparatus  100  includes a pinhole plate  10 , a target optical system  12 , a diffraction grating  14 , a stage  16  for the diffraction grating  14 , a selecting window plate  20 , a stage  26  for the selecting window plate  20 , a detector  28 , and a controller  30 . While  FIG. 1  shows the target optical system  12  as a lens, it is a reflection optical system when the exposure apparatus uses the EUV light as a light source.  
         [0027]     The pinhole plate  10  has a pinhole  10   a  that generates a spherical wave that serves as the measurement light, and is arranged at a desired measurement point on the object surface of the target optical system  12 . In an EUV exposure optical system having an NA of 0.25, the NA at the illumination optical system side is about 0.0625, and a diameter of the pinhole  10   a  is 13.5/(2×0.0625)=108 nm to cover this range of the diffraction angle. Therefore, the pinhole  10   a  having about 100 nm is used. The pinhole plate  10  is illuminated by the illumination optical system (not shown). A high-intensity and high-coherence light source is necessary for the pinhole  10   a . Ideally, an undulator radiation X-ray that narrows a band of the X-ray generated from the synchrotron radiation is preferably used. The pinhole plate  10  should efficiently shield the EUV light and is made, for example, of Ta and Ni. A thickness of the pinhole plate  1  is 200 nm or larger for Ta and 150 nm or larger for Ni.  
         [0028]     The illumination light emitted as a spherical wave from the pinhole  10   a  passes the target optical system  12  and images on the diffracted light selecting window plate  20 . The diffracted light selecting window plate  20  has a pair of identically shaped windows  22  aligned with the x direction and another pair of identically shaped windows  24  aligned with the y direction. The windows  22  are selected for shearing measurements in the x direction, and positioned so that an image is formed at the center between two windows  22 .  
         [0029]     Then, a diffraction grating plate  14  having a two-dimensional diffraction grating  15  is inserted by the stage  16  so that the ±1st order lights can pass two windows ( FIG. 2A ) . While a shearing ratio between the ±1st order lights is determined by a window interval, a distance between the diffraction grating  14  and the diffracted light selecting window plate  20 , a spatial frequency necessary for the wave front recovery, and the contrast necessary for the interference fringes, the preferable shearing ratio is 1/30.  
         [0030]     For measurements in the y direction, the selecting window plate  20  is driven to position the 0th order light at the center between the two selecting windows  24 . Since the windows  22  and  24  are located on the same mask, an interval between the patterns is adjustable using the electron beam imaging speed, such as about 50 nm, for manufacturing the mask. Moreover, the stage  26  for driving the selecting window plate  20  has the driving precision of about 0.1 μm even if employing a normal pulsed motor, and the windows  22  and  24  of the selecting window plate  20  have a width of about 1.3 μm. Therefore, the driving of the stage  26  can provide the sufficient precision for positioning.  
         [0031]     For x and y shearing measurements, the conventional structure shown in  FIG. 4  has difficulties in spatially according the diffraction gratings B and C with the same surface when the windows D and E are exchanged and the diffraction grating B for the x measurements is replaced with the diffraction grating C for the y measurements. As a result, the measurements errors occur as described above with reference to  FIG. 5 . On the other hand, the instant embodiment does not replace the diffraction grating plate  14  although exchanging the selecting window  20 , and thus maintains the highly precise measurements in varying the measurement direction.  
         [0032]     Thus, the optical arrangement of the instant embodiment shown in  FIG. 1  is distinguished from Patrik&#39;s one shown in  FIG. 4 , in that the instant embodiment changes the diffraction grating from the one-dimensional diffraction gratings B and C to the two-dimensional diffraction grating  15 , and includes the selecting window plate  20 .  
         [0033]     The detector  28  is a detector or camera that serves as interference fringe observer means, such as a backlight type CCD. The controller  30  controls the stages  16  and  26 , obtains the shearing interference information detected by the detector  28 , conducts the wave front analysis, and calculates the wave front aberration of the target optical system  12 .  
         [0034]     The wave front can be restored by introducing the same order diffracted lights to the CCD  28  from the selecting windows  22  and  24  in the similar manner. More specifically, the CCD  28  photographs the independently sheared wave fronts in the x and y components. The controller  28  integrates the wave fronts in the shearing direction, and restores the two-dimensional wave front by the path integral. For the improved precision, the interference fringe image is obtained by scanning the grating in the x direction by ¼ pitch during the x shearing time, and the phase information (or wave front) is calculated by the 5 or 9 bucket method. Since the phase information is discrete information every 360°, unwrapping for smooth phase connections provides the highly precise wave front. These are obtained from the differentiated wave front, and the integral operation restores the original wave front.  
         [0035]     The above method can remarkably reduce measurement errors, such as a positional error of the diffraction grating shown in  FIG. 5 , but the diffraction grating itself moves or drifts in the optical-axis direction when x and y components are time-sequentially taken in. The drift during the measurement causes slight errors due to the similar principle. One effective solution for this problem is to use the configuration shown in  FIG. 3  and to simultaneously measure the x and y components.  FIG. 3  provides a selecting window plate  20 A with two pairs of windows  22 A and  24 A in the x and y directions. The windows  22 A correspond to the windows  22 , and the windows  24 A correspond to the windows  24 . These two pairs of windows  22 A and  24 A are arranged at vertexes of a square in  FIG. 3 .  
         [0036]     The wave front restoration procedure may apply almost the similar approach to Patrik&#39;s scheme. Patrik&#39;s scheme two-dimensionally Fourier-transforms the interference fringes and filters the result so that the interference spectra of the 0th and 1st order lights remain, because the interference contrast between the 0th and 1st order lights is maximum. Since the instant embodiment uses the ±1st order lights, the filtering leaves twofold spatial frequencies, conducts an inverse Fourier transformation to the result, and extracts the signal component. The interference fringe contrast formed by the ±1st order lights becomes higher by about 10% than the contrast formed by the 0th and 1st order lights. In addition, the Patrik&#39;s scheme causes the interference fringe between other diffracted orders to deteriorate the signal light contract. Therefore, the present invention can provide more highly precise measurements than Patrik&#39;s scheme because the present invention can obtain the higher contrast interference fringes.  
         [0037]     Thus, according to the measuring method and apparatus of the instant embodiment, the two-dimensional diffraction grating  15  is fixed when the shearing interference information is obtained in two orthogonal directions, and therefore the measurement error caused by a replacement of the diffraction grating does not occur. In addition, unlike Patrik&#39;s scheme, the instant embodiment calculates the wave front aberration of the target optical system  12  from the shearing interference information directed to the measuring light that has passed through the selecting windows, and facilitates the operation processing because the operation amount is remarkably lower than that in Patrik&#39;s scheme.  
         [0038]     Referring not to  FIG. 6 , a description will be given of an exposure apparatus  40  according to another embodiment of the present invention. Here,  FIG. 6  is a schematic block diagram of the exposure apparatus  40  that utilizes the EUV light as the exposure light, although the inventive exposure apparatus is not limited to the EUV light.  
         [0039]     In  FIG. 6, 41  denotes an illumination optical system including the light source,  42  denotes a reticle stage, and  44  denotes a reticle. The reticle  44  may be the first mask  12  or a reticle that has a circuit pattern of a semiconductor device (a semiconductor chip, such as IC and LSI, a liquid crystal panel, and a CCD).  12 A denotes a projection optical system as a target optical system.  45  denotes a wafer stage.  14 A denotes a diffraction grating plate (or light dividing means) . The diffraction grating plate  14 A is located at the wafer stage  45  side in  FIG. 5 , but may be located at the reticle stage  42  side. The diffraction grating plate  14 A has a similar structure to the diffraction grating plate  14  shown in  FIG. 1 .  46  denotes a pattern surface, on which the windows  22  and  24  are arranged.  28  denotes a detector.  47  denotes an object to be exposed, which is a wafer in the instant embodiment. The pattern surface  46  and the detector  28  are integrated with each other and arranged on the wafer stage  45 .  
         [0040]     In this configuration, similar to  FIG. 1 , the illumination optical system  41  illuminates the mask  44 , and the diffraction grating  14 A divides the wave front that is emitted from the pinhole  10   a  and spherical in one direction. The projection optical system  12 A shields the 0th order light, and allows the ±1st order diffracted lights to enter the windows  22  (or  22 A) and  24  (or  24 A), and the detector  28  obtains interference fringes. Since the interference fringes correspond to a differentiation of the original wave front, and the controller obtains the original wave front information by integrating the interference fringes obtained by the detector  28 . For measurements of the aberrational characteristics of the projection optical system  12 A in the view angle, the phase shift means  16  moves the diffraction grating  14 A and aberrations are similarly measured at several points in the view angle of the projection optical system  12 A. This embodiment can easily add an aberration measuring function to the projection exposure apparatus by using a reflection mask pattern.  
         [0041]     A description will now be given of an aberration correction method according to one embodiment of the present invention. The exposure apparatus  40  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 a 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  12 A 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.  
         [0042]     A description will now be given of an embodiment of a device manufacturing method using the projection exposure apparatus  40 .  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 ).  
         [0043]      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  40  to expose a circuit pattern on the mask  42  onto the wafer  47 . Step  17  (development) develops the exposed wafer  47 . 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  47 . 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.  
         [0044]     The present invention thus can provide a measuring method and apparatus which utilize the shearing interferometry and provide higher precision and easier signal processing than the conventional method, an exposure method and apparatus using them, and a device manufacturing method.  
         [0045]     This application claims a foreign priority based on Japanese Patent Application No. 2003-398722, filed Nov. 28, 2003, which is hereby incorporated by reference herein.