Abstract:
A measuring apparatus for measuring optical performance of a test optics by using light includes a first member for generating a first ideal wave front, a second member for generating a second ideal wave front and a test wave front that reflects the optical performance of the test optics, and a detector for detecting an interference fringe between the test wave front and the second ideal wave front that passes the second member, wherein the first member and/or the second member include a first membrane having a first aperture for diffracting the light, and a second membrane having a second aperture for diffracting the light that has passed the first aperture, the second membrane being spaced from the first membrane so that the first and second apertures overlap each other.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to a measuring apparatus that measures the optical performance of an optical element, and more particularly to an exposure apparatus mounted with a measuring apparatus that measures a wave front aberration of a projection optical system that transfers a reticle pattern onto an object to be exposed.  
         [0002]     A projection exposure apparatus has conventionally been used to transfer a circuit pattern of a reticle (or a mask) onto an object to be exposed in manufacturing such devices as semiconductor devices, e.g., an IC and a LSI, an image pickup devices, e.g., a CCD, display devices, e.g. a liquid crystal panel, and magnetic heads, in the photolithography process. Since this exposure apparatus needs to precisely transfer a reticle pattern onto a wafer at a predetermined magnification, it is important to use a precise projection optical system having good imaging performance and reduced aberration. In general, a value of a root mean square (“RMS”) that indicates the precision of the optical system should be λ/14 or smaller in view of the Mareshal criterion, where λ is a wavelength of a light source.  
         [0003]     A catoptric optical system that includes n mirrors is used for an exposure apparatus that uses the extreme ultraviolet (“EUV”) light with a wavelength of λ of about 13.5 nm (“EUV exposure apparatus” hereinafter) and attempts to meet the recent demand for finer processing to the semiconductor device. Each mirror requires a shaping precision of λ/(28{square root}n), and a six-mirror optical system needs a surface processing precision of about 0.2 nm RMS.  
         [0004]     A conventional surface-precision measuring apparatus cannot measure such a highly precise surface shape due to its insufficient measuring precision. Accordingly, a measuring apparatus with such a high measuring precision as about 0.1 nm RMS has been proposed which utilizes a point diffraction interferometer (“PDI”) that has a pinhole for generating an ideal spherical wave, and a line diffraction interferometer (“LDI”) that has a slit for generating an ideal cylindrical or elliptical wave. See, for example, Katsuhiko Murakami, “O plus E”, New Technology Communications, Inc., 2004 January, Vol. 26, No. 1, pp. 43-47.  
         [0005]     In the measuring apparatus that uses the PDI and LDI for interference between a test wave front and a reference wave front generated from a fine aperture, such as the pinhole and slit, an error between the reference wave front and an ideal spherical or cylindrical wave, which error is referred to as a reference wave front deviation, affects a measuring error. The reference wave front deviation is caused by an optical axis offset or an offset between an optical axis of the incident light and an optical axis or center of the pinhole or slit. The optical axis offset causes the incident light to be shielded by the edge of the pinhole or slit, and disturbs the exited reference wave front. In addition, although a sufficiently small and thin pinhole or slit in a perfectly light-shielding member generates an ideal spherical or cylindrical wave, an actual pinhole or slit has a finite thickness and the generated wave front has a reference wave front deviation.  
         [0006]     Therefore, it is necessary for the reduced reference wave front deviation to make a size of the aperture as small as possible and precisely align the center of the aperture with the optical axis of the incident light. Regarding the above influence, it is reported that a measuring error problematically increases when the wave front exited from the pinhole is calculated by changing the beam shift and the wave front aberration, and used as a reference wave front. See, for example, Y. Sekine, A. Suzuki, M. Hasegawa, H. Kondo, M. Ishii, J. Kawakami, T. Oshino, K. sugisaki, Y. Zhu, K. Otaki, Z. Liu, “Wave-front errors of reference spherical waves in high-numerical aperture point diffraction interferometers” J. Vac. Sci. Technol. B22(1) 2004.  
         [0007]     The conventional interferometry uses a wavelength of the visible light, and does not require a high measuring precision, neglecting the influence of the reference wave front deviation on the measuring precision.  
         [0008]     However, the projection optical system in the EUV exposure apparatus requires a highly precise measurement of the wave front aberration, and the influence of the reference wave front deviation on the measuring precision does not become negligible. In addition, the light having a small wavelength, such as the EUV light, leaks into a member that has a fine aperture, such as a light-shielding membrane, and the light leaking into the fine aperture disturbs the reference wave front similar to offsetting the optical axis. As the member that has the fine aperture is made thicker, the influence of the leakage into the fine aperture on the EUV light reduces. However, this scheme is contrary to the demand for a thinner aperture for an ideal spherical or cylindrical wave.  
         [0009]     On the other hand, as disclosed in the above second reference, the influence on the measuring precision is non-negligible for the reference wave front emitted from the conventional fine aperture, since the beam shift etc. increase the wave front deviation. Therefore, a finer aperture that generates a reference wave front closer to the ideal spherical or cylindrical wave is preferable for the improved measuring precision.  
         [0010]     Thus, the conventional measuring apparatus cannot precisely measure the aberration suitable for a highly precise optical system. In other words, the conventional measuring apparatus does not meet the measuring precision required for the highly precise optical system.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention is directed to a measuring apparatus, and an exposure apparatus having the same, wherein the measuring apparatus reduces an error between the ideal wave front and the reference wave front generated by the fine aperture, and precisely measures the optical performance, such as a wave front aberration, of an optical system.  
         [0012]     A measuring apparatus according to one aspect of the present invention for measuring optical performance of a test optics by using light includes a first member for generating a first ideal wave front, a second member for generating a second ideal wave front and a test wave front that reflects the optical performance of the test optics, and a detector for detecting an interference fringe between the test wave front and the second ideal wave front that passes the second member, wherein the first member and/or the second member include a first membrane having a first aperture for diffracting the light, and a second membrane having a second aperture for diffracting the light that has passed the first aperture, the second membrane being spaced from the first membrane so that the first and second apertures overlap each other.  
         [0013]     A measuring apparatus according to another aspect of the present invention for measuring optical performance of a test optics by using light includes a first member for generating a first ideal wave front, a second member for generating a second ideal wave front and a test wave front that reflects the optical performance of the test optics, and a detector for detecting an interference fringe between the test wave front and the second ideal wave front that passes the second member, wherein the first member and/or the second member include a first membrane having a first aperture for diffracting the light, a second membrane having a second aperture for diffracting the light that has passed the first aperture, and a third membrane for introducing the light that has passed the first aperture to the second aperture, the third membrane being arranged between the first and second membranes, and connecting the first and second apertures so that the first and second apertures overlap each other, wherein k 1 &gt;k 3  and k 2 &gt;k 3  are met, where k 1  is an extinction coefficient of the first membrane, k 2  is an extinction coefficient of the second membrane, and k 3  is an extinction coefficient of the third membrane.  
         [0014]     A measuring apparatus according to still another aspect of the present invention for measuring optical performance of a test optics by using light includes a member that includes a first membrane having a first aperture for generating an ideal wave front from the light, and a second membrane having a second aperture for generating an ideal wave front from the light that has passed the first aperture, the second membrane being spaced from the first membrane so that the first and second apertures overlap each other, and a detector for detecting an interference fringe between the ideal wave front that passes the second aperture and the test wave front that reflects the optical performance of the test optics.  
         [0015]     A measuring apparatus according to another aspect of the present invention for measuring optical performance of a test optics by using light includes a member that includes a first membrane having a first aperture for generating an ideal wave front from the light, a second membrane having a second aperture for generating an ideal wave front from the light that has passed the first aperture, and a third membrane for introducing the light that has passed the first aperture to the second aperture, the third membrane being arranged between the first and second membranes, and connecting the first and second apertures so that the first and second apertures overlap each other, and a detector for detecting an interference fringe between the ideal wave front that passes the second aperture and the test wave front that reflects the optical performance of the test optics, wherein k 1 &gt;k 3  and k 2 &gt;k 3  are met, where k 1  is an extinction coefficient of the first membrane, k 2  is an extinction coefficient of the second membrane, and k 3  is an extinction coefficient of the third membrane.  
         [0016]     An exposure apparatus according to another aspect of the present invention for exposing a pattern of a reticle onto an object includes a projection optical system for projecting the pattern onto the object, optical performance of the projection optical system which has been measured by the above measuring apparatus having a predetermined value.  
         [0017]     An exposure apparatus according to another aspect of the present invention for exposing a pattern of a reticle onto an object by using light from a light source includes a projection optical system for projecting the pattern onto the object, and the above measuring apparatus for measuring optical performance of the projection optical system using the light.  
         [0018]     An exposure method according to another aspect of the present invention includes the steps of calculating optical performance of a projection optical system using the above measuring apparatus, adjusting the projection optical system based on the optical performance of the projection optical system, which is calculated by the calculating step, and exposing an object using an exposure apparatus that includes the projection optical system adjusted by the adjusting step.  
         [0019]     A device manufacturing method according to another aspect of the present invention includes the steps of exposing an object using the above exposure apparatus, and developing the object that has been exposed.  
         [0020]     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  
       [0021]      FIG. 1  is a schematic block diagram showing a structure of a measuring apparatus as one aspect according to the present invention.  
         [0022]      FIGS. 2A  to  2 C are schematic plane views of components in the measuring apparatus shown in  FIG. 1 .  
         [0023]      FIGS. 3A  to  3 C are schematic plane views of components of the measuring apparatus that utilizes an LDI.  
         [0024]      FIG. 4  is a schematic sectional view of a first mask shown in  FIG. 1 .  
         [0025]      FIG. 5  is a schematic sectional view of a second mask shown in  FIG. 1 .  
         [0026]      FIG. 6  is a detailed sectional view of a first mask shown in  FIG. 4 .  
         [0027]      FIG. 7  is a detailed sectional view of a second mask shown in  FIG. 5 .  
         [0028]      FIG. 8  is a schematic sectional view of a section mask having a spacer.  
         [0029]      FIG. 9  is a schematic sectional view of a second mask having a third membrane between first and second membranes.  
         [0030]      FIG. 10  is a graph showing a relationship between an optical axis offset and a reference wave front deviation in the second mask shown in  FIG. 9  and a conventional mask.  
         [0031]      FIG. 11  is a graph showing a relationship between an optical axis offset and a reference wave front deviation in the second mask shown in  FIG. 9  and a conventional mask.  
         [0032]      FIG. 12  is a graph showing a relationship between the astigmatism of the irradiated light and a reference wave front deviation in the second mask shown in  FIG. 9  and a conventional mask.  
         [0033]      FIG. 13  is a graph of the light intensity distribution in a direction parallel to the aperture surface when Equation 1 is met and the light propagates by 2 nm from a first aperture.  
         [0034]      FIG. 14  is a graph of the light intensity distribution in a direction parallel to the aperture surface when Equation 2 is met and the light propagates by 47 nm from a first aperture.  
         [0035]      FIG. 15  is a graph showing a relationship between the thickness of the third membrane and the reference wave front deviation generated from the second mask.  
         [0036]      FIG. 16  is a graph showing a relationship between the thickness of the first membrane and the reference wave front deviation generated from the second mask.  
         [0037]      FIG. 17  is a graph showing a relationship between the fist aperture in the first membrane and a reference wave front deviation in the inventive second mask.  
         [0038]      FIG. 18  is a schematic block diagram showing a structure of an exposure apparatus as one aspect of the present invention.  
         [0039]      FIG. 19  is a flowchart for explaining a method for fabricating devices (semiconductor chips such as ICs, LSIs, and the like, LCDs, CCDs, etc.).  
         [0040]      FIG. 20  is a detailed flowchart for Step  4  of wafer process shown in  FIG. 19 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]     A description will now be given of a measuring apparatus  1  as one aspect of the present invention, with reference to the accompanying drawings. In each figure, the same reference numeral denotes the same element, and a duplicate description will be omitted.  FIG. 1  is schematic block diagram showing a structure of the measuring apparatus  1 .  FIGS. 2A-2C  are schematic planes view of components of the measuring apparatus  1 . More specifically,  FIG. 2A  is a plane view of a first mask  20 ,  FIG. 2B  is a plane view of a grating  30 , and  FIG. 2C  is a plane view of a second mask  40 .  
         [0042]     The measuring apparatus  1  measures the optical performance of a test optics MOS by detecting an interference fringe. The measuring apparatus  1  measures the wave front aberration of the test optics MOS using the PDI.  
         [0043]     The measuring apparatus  1  includes, as shown in  FIG. 1 , an illumination unit  110  and a light receiving unit  120 . MOS is a test optics. The illumination unit  110  includes an illumination optical system  10  that emits the illumination light LL, a first mask  20  as a first patterned member, and a grating  30  that diffracts the illumination light LL. The light source is omitted. The light receiving unit  120  includes a second mask  40  as a second patterned member, and a detector  50  that includes a light receiving element, such as a CCD. DL 1  is one diffracted light from the grating  30 , and transmits through a transmission window  44  in the second mask  40  after transmitting the test optics MOS. DL 2  is one diffracted light from the grating  30 , and has a diffraction order different from the diffracted light DL 1 . The diffracted light DL 2  is irradiated onto the second pinhole  42  in the second mask  40 , and generated from a second pinhole  42  in the second mask  40 .  
         [0044]     The illumination optical system  10  includes an optical element, and is likely to have a wave front aberration larger than that of the test optics MOS in this embodiment. Accordingly, as shown in  FIG. 2 , the illumination light LL is irradiated onto a first pinhole  22  in the first mask  20 , and generates an ideal spherical wave that has a reduced aberration by using the first pinhole  22 . Plural orders of diffracted lights are generated as a result of the spherical wave transmitting through the grating  30 .  
         [0045]     The diffracted lights generated from the grating transmit through or are reflected by the test optics MOS. One diffracted light transmits through the transmission window  44  in the second mask  40 , and another diffracted light enters the second pinhole  42  in the second mask  44 .  
         [0046]     The light DL 1  that transmits through the transmission window  44  in the second mask  40  has a wave front aberration caused by the surface precision error and the adjustment error of the test optics MOS. On the other hand, the light DL 2  generated from the pinhole  42  in the second mask  40  has a wave front close to the ideal spherical wave after diffracted by the second pinhole  42 . The detector  50  detects an interference fringe (intensity pattern) formed by the interference between the light DL 2  as the reference light and the light DL 1  having the test wave front. The surface precision error of the test optics MOS is calculated through an analysis of the detected interference fringe.  
         [0047]     Here, the first mask  20  and the second mask  40  are used to reduce the wave front aberration of the illumination optical system  10  and the test optics MOS, and generate a wave front close to the ideal spherical wave. In other words, the first mask  20  and the second mask  40  serve to generate a reference wave front.  
         [0048]     The measuring apparatus  1  when using the LDI applies first masks  20 A and  20 B shown in  FIG. 3A  having first slits  22   a  and  22   b  instead of the first mask  20 , and gratings  30 A and  30 B shown in  FIG. 3B  instead of the grating  30 , and second masks  40 A and  40 B shown in  FIG. 3C  having second slits  42   a ,  42   b  and transmission windows  44   a ,  44   b . Here,  FIGS. 3A  to  3 C are schematic plane view of components in the measuring apparatus  1  that utilizes the LDI. More specifically,  FIG. 3A  is a plane view of the first mask  20 A,  FIG. 3B  is a plane view of a grating  30 A, and  FIG. 3C  is a plane view of the second masks  40 A and  40 B.  
         [0049]     In order to make the reference wave front generated from the first pinhole  22  and/or the second pinhole  42  close to the ideal spherical wave, the first mask  20  and the second mask  40  need to shield the light in an area GPA outside the pinhole, and thus the first pinhole  22  and the second pinhole  42  (or the first mask  20  and the second mask  40 ) should have the finite thickness. In the LDI, the area GPA outside the slit needs to shield the light similarly. Therefore, the first slits  22   a  and  22   b , and the second slits  42   a  and  42   b  (or the first masks  20 A and  20 B, and the second masks  40 A and  40 B) should have the finite thicknesses.  
         [0050]      FIG. 4  is a schematic sectional view of the first mask  20 , and BIP 22  is a beam intensity profile irradiated onto the first pinhole  22  in the first mask  20 .  FIG. 5  is a schematic sectional view of the second mask  40 , and BIP 42  and BIP 44  are beam intensity profiles irradiated onto the second pinhole  42  and the transmission window  44  in the second mask  40 . The beam intensity profiles of the first masks  20 A and  20 B, and the second masks  40 A and  40 B are the same as those shown in  FIGS. 4 and 5  in the measuring apparatus that utilizes the LDI. The following description refers to a pinhole and slit as an aperture and ideal spherical and cylindrical waves as an ideal wave front.  
         [0051]     The first mask  20  and/or the second mask  40  that have apertures and possess a finite thickness cause a wave front shape generated from the aperture to include an error (or reference wave front deviation) from the ideal spherical or cylindrical wave. The reference wave front deviation increases when the aperture center shifts from the optical axis of the irradiated light, and when the irradiated light includes the wave front aberration. The instant inventor has discovered that the increase of the reference wave front deviation is restrained when the first mask  20  and the second mask  40  are made of two-layer membranes, for example, and by according the aperture centers of each of two-layer membranes.  
         [0052]      FIG. 6  shows the first mask  20  made of two-layer membranes in order to restrain the increase of the reference wave front deviation. Similar to  FIG. 4 , BIP 22  is a beam intensity profile irradiated onto the first pinhole  22 , which transmits through or is reflected by the illumination optical system  10 .  
         [0053]     Referring to  FIG. 6 , the first mask  20  includes a first membrane  27  having a first aperture  27   a  that diffracts the illuminated light LL, and a second membrane  28  having a second aperture  28   a  that diffracts the light from the first aperture  27   a . The first membrane  27  and the second membrane  28  are arranged so that the center of the first aperture  27   a  accords with the center of the second aperture  28   a  via a space SP that is less likely to absorb the illuminated light LL.  
         [0054]     The illuminated light LL irradiated onto the first membrane  27  is diffracted by the first aperture  27   a  in the first membrane  27 , and generates the light that reduces a wave front aberration caused by the illumination optical system  10  and the optical axis offset. The diffracted light is irradiated onto the second membrane  28  after propagating the space SP.  
         [0055]     The light irradiated onto the second membrane  28  is diffracted by the second aperture  28   a  in the second membrane  28 , and generates the light that further reduces a wave front aberration caused by the illumination optical system  10  and the optical axis offset. Therefore, the first mask  20  reduce the reference wave front aberration, and forms a reference wave front closer to the ideal wave front than the conventional pinhole mask, improving the measuring precision of the measuring apparatus  1 . While the first mask  20  is made of the two-layer membranes, it may be made of three or more layers of membranes. While this embodiment arranges the first membrane  27  and the second membrane  28  so that the center of the first aperture  27   a  accords with the center of the second aperture  28   a , the similar effect is obtained by arranging the first membrane  27  and the second membrane  28  so that the first aperture  27   a  overlaps the second aperture  28   a.    
         [0056]      FIG. 7  shows a second mask  40  that is made of two-layer membranes, similar to the first mask  20 , so as to restrain the increase of the reference wave front deviation. Similar to  FIG. 5 , BIP 42  is a beam intensity profile irradiated onto the second pinhole  42  in the second mask  40 , which has transmitted or been reflected by the test optics MOS.  
         [0057]     Referring to  FIG. 7 , the second mask  40  includes a first membrane  47  having a first aperture  47   a  that diffracts the light from the test optics MOS, and a second aperture  48  having a second aperture  48   a  that diffracts the light from the first aperture  47   a . The first membrane  47  and the second membrane  48  are arranged so that the center of the first aperture  47   a  accords with the center of the second aperture  48   a  via a space SP that is less likely to absorb the light. The second mask  40  can reduce the reference wave front deviation of the light generated from the second pinhole  42 . Since the reference wave front becomes closer to the ideal wave front, the measuring apparatus  1  can improve the measuring precision. While the first mask  20  is made of the two-layer membranes, it may be made of three or more layers of membranes. While this embodiment arranges the first membrane  47  and the second membrane  48  so that the center of the first aperture  47   a  accords with the center of the second aperture  48   a , the similar effect is obtained by arranging the first membrane  47  and the second membrane  48  so that the first aperture  47   a  overlaps the second aperture  48   a.    
         [0058]     The second mask  40  may arrange a spacer  45  in the space SP as shown in  FIG. 8 . The spacer  45  is connected to the first membrane  47  and the second membrane  48 , and maintains the space SP at a certain distance between the first membrane  47  and the second membrane  48 . The spacer  45  reduces thermal and gravity deformations of the first membrane  47  and the second membrane  48 , and precisely accords the center of the first aperture  47   a  with the center of the second aperture  48   a . Of course, this spacer is applicable to the first mask  20 . Here,  FIG. 8  is a schematic sectional view of the second mask  40  having the spacer  45 .  
         [0059]      FIG. 9  is a schematic sectional view of the second mask  40  having a third membrane  49  in the space SP between the first membrane  47  and the second membrane  48 . The third membrane  49  has a third aperture  49   a  that introduces the light from the first aperture  47   a  to the second aperture  48   a , and connects the first membrane  47  to the second membrane  48  so that the center of the first aperture  47   a  accords with the center of the second aperture  48   a . When the third membrane  49  is unlikely to absorb the light, the third aperture  49   a  may be omitted. As discussed above, the third membrane  49  may connect the first membrane  47  to the second membrane  48  so that the first aperture  47   a  overlaps the second aperture  48   a . The second mask  40  shown in  FIG. 9  is applicable to the first mask  20 .  
         [0060]     The first aperture  47   a  generates the spreading diffracted light when k 1  is set greater than k 3  in the second mask  40 , where k 1  is an extinction coefficient of the first membrane  47 , k 2  is an extinction coefficient of the second membrane  48 , and k 3  is an extinction coefficient of the third membrane  49 . In addition, when k 2  is set greater than k 3 , the second aperture  48   a  generates the light that is less affected by the wave front aberration of the irradiated light and the positional offset between the optical axis of the irradiated light and the center of the second aperture  48   a . The extinction coefficient is an imaginary part of complex index of refraction.  
         [0061]     As shown in  FIG. 9 , the third membrane  49  improves the rigidity of the second mask  40 , and reduces the influence of the deformation. The second mask  40  for the visual light can be easily produced when the third membrane  49  is made of an approximately transparent quartz substrate, a membrane has a large extinction coefficient, such as chrome, on front and rear surfaces of the substrate, and an aperture is formed by using the photolithography, etching, electron beam, and ion beam.  
         [0062]     When the measuring apparatus  1  measures the optical performance of the test optics MOS by using the LDI, a circular shape of the first pinhole  22  in the first mask  20  and the second pinhole  42  in the second mask  42  is replaced with a slit shape. The slit width (in the short direction of the slit) generates the diffracted light that reduces the wave front aberration of the irradiated light when it is shorter than the width of the beam intensity profile of the irradiated light. When the light from the rectangular aperture, such as a slit is condensed, the spot width is λ/NA, where λ is a wavelength of the illuminated light LL and NA is a numerical aperture of the condenser optical system (or the test optics MOS). Thus, when the slit width is smaller than λ/NA, the diffracted light reduces the wave front aberration of the irradiated light.  
         [0063]     A description will now be given of a measuring apparatus that utilizes an LDI to measure the optical performance of the projection optical system having the NA of 0.20 in an EUV exposure apparatus (with the wavelength of 13.5 nm). The reference wave front deviation of the wave front generated from the inventive second mask  40  and the conventional mask are calculated using the strict electromagnetic-field numerical calculation. The conventional mask is made of Ni and has the thickness of 150 nm, whereas the inventive second mask  40  includes the first membrane  47  that is made of Ni and has the thickness of 50 nm, the second membrane  48  that is made of Ni and has the thickness of 150 nm, and the third membrane  49  that is made of SiN and has the thickness of 150 nm. Both the inventive second mask  40  and the conventional mask have the slit width of 50 nm.  
         [0064]      FIG. 10  is a graph of the error from the ideal cylindrical wave of the wave front generated from the slit or the reference wave front deviation from the inventive second membrane  40  (Ni 50 nm/SiN 150 nm/Ni 150 nm) and the conventional mask (Ni 150 nm) when the optical axis offset or positional offset occurs between the optical axis of the irradiated light and the center of the slit. In  FIG. 10 , the abscissa axis is the optical axis offset, and the ordinate axis is the reference wave front deviation.  
         [0065]     Referring to  FIG. 10 , the reference wave front deviation increases greatly in the conventional mask as the optical axis offset increases, because the wave front deviation of the wave front generated from the slit is 2.5 mλ or greater for the optical axis offset of 5 nm and  5  mλ or greater for the optical axis offset of 10 nm. On the other hand, it is understood that the inventive second mask  40  maintains the reference wave front deviation of 2 mλ or smaller even when the optical axis offset of 20 nm occurs.  
         [0066]     The measuring precision for the projection optical system in the EUV exposure apparatus is required to have about 0.1 nm RMS, which corresponds to 0.1 nm/13.5 nm=7.4 mλ RMS when converted into the wavelength unit. The measuring error in the interferometer roughly includes a system error, a reference wave front deviation generated from the first mask, a reference wave front deviation generated from the second mask, and the interference fringe analysis error. Among them, the reference wave front deviation in the second mask has a permissible range of 7.4 mλ/4=1.85 mλ.  
         [0067]     Referring to  FIG. 10 , the optical axis offset that provides the reference wave front deviation of 1.85 λRMS or smaller is 2.5 nm or smaller in the conventional mask, and it is difficult to control positions of the optical axis and the mask with that precision. On the other hand, the inventive second mask  40  allows the optical axis offset of about 20 nm, improving the measuring precision of the LDI. Therefore, use of the measuring apparatus is facilitated since no highly precise positioning is necessary.  
         [0068]     The measuring apparatus  1  that utilizes the PDI generates the light having a wave front close to the ideal spherical wave when an approximately circular shape is used for the first and second pinholes  22  and  42  in the first and second masks  20  and  40 . When the light from the circular aperture, such as a pinhole, is condensed, the spot diameter becomes 1.22×λ/NA. Thus, when the pinhole diameter is 1.22×λ/NA or smaller, the light reduces the wave front aberration of the irradiated light.  
         [0069]     Similar to the measuring apparatus that utilizes the LDI, the reference wave front deviation of the wave front from the inventive second mask  40  and the conventional mask is calculated by using the strict electromagnetic-field numerical calculation. The conventional mask is made of Ni and has the thickness of 150 nm, whereas the inventive second mask  40  includes the first membrane  47  that is made of Ni and has the thickness of 50 nm, the second membrane  48  that is made of Ni and has the thickness of 150 nm, and the third membrane  49  that is made of SiN and has the thickness of 150 nm. Both the inventive second mask  40  and the conventional mask have the pinhole diameter of 50 nm.  
         [0070]      FIG. 11  is a graph of the error from the ideal spherical wave of the wave front generated from the pinhole or the reference wave front deviation generated from the inventive second membrane  40  (Ni 50 nm/SiN 150 nm/Ni 150 nm) and the conventional mask (Ni 150 nm) when the optical axis offset or positional offset occurs between the optical axis of the irradiated light and the center of the slit. In  FIG. 11 , the abscissa axis is the optical axis offset, and the ordinate axis is the reference wave front deviation.  
         [0071]     Referring to  FIG. 11 , the optical axis offset that provides the reference wave front deviation of 1.85 λRMS or smaller is 11 nm or smaller in the conventional mask, but the inventive second mask  40  enlarges the optical axis offset up to about 25 nm or greater, improving the measuring precision of the PDI. Use of the measuring apparatus is facilitated, since no highly precise positioning is necessary.  
         [0072]     The illumination optical system  10  and the test optics MOS are not stigmatic but have a predetermined aberration. The reference wave front deviation should be maintained small even for the aberrational incident light.  
         [0073]      FIG. 12  is a graph of the error from the ideal spherical wave of the wave front generated from the pinhole or the reference wave front deviation generated from the inventive second membrane  40  (Ni 50 nm/SiN 150 nm/Ni 150 nm) and the conventional mask (Ni 150 nm) when the irradiated light has astigmatism. In  FIG. 12 , the abscissa axis is a low order astigmatism coefficient in the Fringe Zernike polynomial, and the ordinate axis is the reference wave front deviation.  
         [0074]     Referring to  FIG. 12 , the reference wave front deviation is 1.85 λRMS in the conventional mask when the astigmatism coefficient is in a range between −0.05 λ and 0.1λ, whereas the reference wave front deviation is 1.85 λRMS in the inventive second mask  40  when the astigmatism coefficient is in a broader range between −0.5λ and 0.25 λ, improving the measuring precision of the PDI. Use of the measuring apparatus is facilitated, since no highly precise positioning is necessary. When the irradiated light is a linearly polarized light, the aperture shape (or the circular shape) of the second mask  40  can further reduce the reference wave front deviation by converting the light into the elliptical shape according to the polarization direction.  
         [0075]     The diffracted light from the first aperture  47   a  in the first membrane  47  that receives the irradiated light first is similar to the Fresnel diffraction in the region close to the first aperture  47   a , and then becomes similar to the Fraunhofer diffraction as a result of a further propagation. Equation 1 below provides a region that generates the diffracted light similar to the Fresnel diffraction, where d is a propagation distance in the perpendicular direction from the exit of the first aperture  47   a , λ is the wavelength of the light, n is the refractive index of the propagating material, and a is the maximum width of the first aperture  47   a:  
 
 d&lt;n×a   2 /(4λ)  [EQUATION 1]
 
         [0076]     The spread of the diffracted light is greater than the maximum width a of the first aperture  47   a  in the area expressed by Equation 2 below: 
 
 d&gt;n×a   2 /(4λ)  [EQUATION 2]
 
         [0077]     The reference wave front deviation from the second aperture  48   a  reduces when the spread of the diffracted light is greater than the maximum width of the second aperture  48   a  before the light reaches the second aperture  48   a  in the second membrane  48 .  
         [0078]     For example, the light intensity distribution of the diffracted light from the first aperture  47   a  having the aperture width of 50 nm is calculated on the assumption that the wavelength is 13.5 nm and the first membrane  47  is an infinitely thin and perfect light shielding member.  FIG. 13  is a graph of the light intensity distribution in the direction parallel to the aperture surface when the light propagates by 2 nm from the first aperture  47   a . In  FIG. 13 , the abscissa axis is the position of the first aperture  47   a , and the ordinate axis is the light intensity. Referring to  FIG. 13 , it is understood that the light intensity distribution within the aperture width 50 nm does not have a peaked intensity along the center axis and the light intensity abruptly decreases outside the aperture width 50 nm.  
         [0079]      FIG. 14  is a graph showing the light intensity distribution in the direction parallel to the aperture surface when the light propagates by 47 nm from the first aperture  47   a  while the condition of Equation 2 is met. In  FIG. 14 , the abscissa axis is a position of the first aperture  47   a , and the ordinate axis is the light intensity. Referring to  FIG. 14 , it is understood that the light spreads over the area outside the aperture width of 50 nm.  
         [0080]     Suppose that the first membrane  47  has the finite thickness and absorbs the light. A description will be given of the reference wave front aberration of the second mask  40  that illustratively includes the first membrane  47  that is made of Ni and has the thickness of 50 nm, and the second membrane  48  that is made of Ni and has the thickness of 150 nm.  FIG. 15  is a graph showing the reference wave front deviation generated from the second mask  40  when the thickness of the third membrane  49  changes. In  FIG. 15 , the abscissa axis is the thickness of the third membrane  49  and the ordinate axis is the reference wave front deviation.  
         [0081]     Referring to  FIG. 15 , the reference wave front deviation with no optical axis offset is compared with that with an optical axis offset of 10 nm. When d&gt;n×a 2 /(4λ) or d&gt;45.6 nm where d is the thickness of the third membrane  49 , the reference wave front deviation of the second mask that uses SiN having a small light absorption for the third membrane  49  is smaller than that of the second mask that uses Ni having a large light absorption for the third membrane  49 . Therefore, the reference wave front deviation from the second mask  40  reduces when the thickness of the third membrane  49  in the second mask  40  is made equal to or greater than n×a 2 /(4λ). The similar effect is available even when the first aperture  47   a  in the first membrane  47  has a different shape from the second aperture  48   a  in the second membrane  48 .  
         [0082]     The first membrane  47  that receives the irradiated light first not only diffracts the light but also shields the light. When the optical axis offset occurs in the first membrane  47 , the optical axis of the incident light or the position having the maximum incident light intensity should move to an area other than the first aperture  47   a . In order for the first aperture  47   a  to generate the diffracted light, the attenuating thickness z′ should be equal to or smaller than the light intensity at the spot diameter edge of 1/e 2  where e is a natural logarithm.  
         [0083]     When the plane wave having a field intensity I 0  enters the first membrane  47 , the field intensity I of the plane wave that transmits the dielectric by a distance z is expressed by Equation 3 below: 
 
I=I 0  exp(−αz)  [EQUATION 3]
 
         [0084]     Here, α is an absorption coefficient and is defined by Equation 4 below where k is an extinction coefficient, and λ is a wavelength: 
 
α=4 πk/λ   [EQUATION 4]
 
         [0085]     The distance z′ that attenuates the incident light peak intensity down to 1/e 2  is defined by the following Equation 5: 
 
 z′ =λ/(2 πk )  [EQUATION 5]
 
         [0086]     The extinction coefficient k=0.0727, where the wavelength λ is 13.5 nm and the Ni is used as a material.  
         [0087]      FIG. 16  is a graph of the reference wave front deviation generated when the thickness changes while the aperture is a pinhole having a diameter of 50 nm, the third membrane  49  is made of SiN and has the thickness of 150 nm, the second membrane  48  is made of Ni and has the thickness of 150 nm, and the incident light has the wavelength of 13.5 nm, the second aperture  48  having a diameter of 50 nm in the inventive second mask  40  (Ni 50 nm/SiN 150 nm/Ni 150 nm). In  FIG. 16 , the abscissa axis is the thickness of the first membrane  47 , and the ordinate axis is the reference wave front deviation.  
         [0088]     Referring to  FIG. 16 , it is understood that when the thickness of the first membrane  47  is λ/(2πk) or greater, or 30 nm, the increase of the reference wave front deviation is restrained even when the optical axis shifts by 10 nm. Therefore, the thickness of the first membrane  47  of λ/(2πk) or greater would reduce the reference wave front deviation and improve the measuring precision of the measuring apparatus  1 .  
         [0089]     The second membrane  48  that finally generates the reference wave front needs to have higher light shielding performance than the first membrane  47 . In this case, when the incident light intensity that is attenuated down to about 0.1% reduces the interference between the light generated from the second aperture  48   a  and the light that transmits through part other than the second aperture  48   a . When the thickness z 2 ′ of the second membrane  48  greater than a value defined in Equation 6 would reduce the incident light intensity down to 0.1%. 
 
 z   2 ′=1.727λ/(π k )  [EQUATION 6]
 
         [0090]      FIG. 17  is a graph of the reference wave front deviation generated when the first aperture  47   a  in the first membrane  47  changes while the second membrane  48  has the second aperture  48  having a diameter of 50 nm in the inventive second mask  40  (Ni 50 nm/SiN 150 nm/Ni 150 nm). In  FIG. 17 , the abscissa axis is a diameter of the first aperture  47   a , and the ordinate axis is the reference wave front deviation.  
         [0091]     Referring to  FIG. 17 , it is understood that the reference wave front reduces when the diameter of the first aperture  47   a  maintains smaller than that of the second aperture  48   a . One conventional problem is that a small aperture diameter increases the light intensity irradiated onto part other than the aperture, the light intensity that cannot be blocked by the part and leaks into the exit side, and the reference wave front deviation as a result of interference between the leaking light and the diffracted light from the aperture. On the other hand, the second mask  40  uses the multilayer membranes, such as the first membrane  47 , the second membrane  48  and the third membrane  49 , to reduce the unshielded, transmitting light, even when the aperture diameter reduces so as to reduce the influences of the optical axis offset and the wave front aberration of the incident light. Therefore, the influences of the optical axis offset and the wave front aberration of the incident light can be reduced. Even when the aperture is made of a slit, the reference wave front deviation reduces when the slit width of the first membrane  47  is smaller than that of the second membrane  48 .  
         [0092]     As discussed, the highly precise measurement is possible when the first mask  20  and the second mask  40  that can generate a reference wave front with a small error from the ideal wave front, for the reference wave front in an interferometer, such as a PDI and LDI, is used for measuring the wave front precision of the optical system and the surface precision of the optical element. Therefore, the measuring apparatus  1  can measure the optical performances of the optical system and the optical element that are required to have high precision.  
         [0093]     In addition, the highly precise measurement of the wave front aberration caused by mounting the inventive measuring apparatus  1  onto an exposure apparatus maintains the stable exposure performance, and improves the yield of the semiconductor device and the maintainability.  
         [0094]     Referring now to  FIG. 18 , a description will be given of an exemplary exposure apparatus  500  that includes the inventive measuring apparatus  1 . Here,  FIG. 18  is a schematic block diagram showing a structure of the exposure apparatus  500  as one aspect of the present invention.  
         [0095]     The inventive exposure apparatus  500  is a projection exposure apparatus that uses the EUV light, e.g., with a wavelength of 13.4 nm as illumination light to expose a circuit pattern of the reticle  520  onto the plate  540 , e.g., in a step-and-repeat or a step-and-scan manner. Such an exposure apparatus is suitable for a submicron or quarter-micron lithography process, and this embodiment discusses a step-and-scan exposure apparatus (which is also called “a scanner”) as an example. The “step-and-scan manner”, as used herein, is an exposure method that exposes a mask pattern onto a wafer by continuously scanning the wafer relative to the mask, and by moving, after a shot of exposure, the wafer stepwise to the next exposure area to be shot. The “step-and-repeat manner” is another mode of exposure method that moves a wafer stepwise to an exposure area for the next shot every shot of cell projection onto the wafer.  
         [0096]     Referring to  FIG. 18 , the exposure apparatus  500  includes the measuring apparatus  1 , an illumination apparatus  510 , a reticle  520 , a reticle stage  525  mounted with the reticle  520 , a projection optical system  530 , an object to be exposed  540 , a wafer stage  545  mounted with the object  540 , an alignment detection mechanism  550 , and a focus position detection mechanism  560 .  
         [0097]     As shown in  FIG. 18 , at least the optical path through which the EUV light travels (or the entire optical system) should preferably be maintained in a vacuum atmosphere, since the EUV light has low transmittance in air and causes contaminations when reacting with components of residual gas, such as oxygen, carbon dioxide, and water vapor.  
         [0098]     The measuring apparatus  1  measures the optical performance of the projection optical system  530  in this embodiment.  110  in  FIG. 18  denotes an illumination unit, mounted on a stage  115 .  120  denotes a light receiving unit, mounted on a stage  125 . In measuring the optical performance of the projection optical system  530 , the illumination unit  110  and the light receiving unit  120  are driven for each stage, and replaced with the reticle  520  and the object  540 . The light split from the illumination optical system  514  is introduced to the illumination unit  110 , and used to measure the optical performance of the projection optical system  530 . The measuring apparatus  1  can apply any of the above or other configurations, and a detailed description thereof will be omitted. Alternatively, instead of providing the exposure apparatus  500  with the measuring apparatus  1 , the exposure apparatus  500  may utilize a projection optical system whose optical performance is greater than a predetermined value through a measurement by the measuring apparatus  1 .  
         [0099]     The illumination apparatus  510  uses the arc-shaped EUV light, for example, with a wavelength of 13.4 nm corresponding to an arc-shaped field of the projection optical system  530  to illuminate the reticle  520 , and includes an EUV light source  512  and an illumination optical system  514 .  
         [0100]     The EUV light source  512  employs, for example, a laser plasma light source. It generates high temperature plasma by irradiating a pulsed laser beam with high intensity onto a target material in a vacuum chamber, and uses the EUV light, for example, with a wavelength of about 13 nm, which has been emitted from the plasma. The target material may use a metallic film, gas jets, liquid drops, etc. Preferably, the pulse laser is driven with a higher repetitive frequency of usually several kHz for increased average intensity of the radiated EUV light.  
         [0101]     The illumination optical system  514  includes a condenser mirror  514   a , and an optical integrator  514   b . The condenser mirror  514   a  serves to collect the EUV light that is isotropically irradiated from the laser plasma. The optical integrator  514   b  serves to uniformly illuminate the reticle  520  with a predetermined NA. The illumination optical system  514  further includes an aperture  514   c  at a position conjugate with the reticle  520  to limit an illuminated area to an arc shape.  
         [0102]     The reticle  520  is a reflection mask that has a circuit pattern or image to be transferred, and supported and driven by the reticle stage  525 . The diffracted light from the reticle  520  is reflected by the projection optical system  530  and projected onto the object  540 . The reticle  520  and the object  540  are arranged optically conjugate with each other. The exposure apparatus  500  is a scanner, and projects a reduced size of the pattern of the reticle  520  onto the object  540  by scanning the reticle  520  and the object  540 .  
         [0103]     The reticle stage  525  supports the reticle  520  and is connected to a moving mechanism (not shown). The reticle stage  525  may use any structure known in the art. A moving mechanism (not shown) may include a linear motor etc., and drives the reticle stage  525  at least in a direction X and moves the reticle  520 . The exposure apparatus  500  assigns the direction X to scan the reticle  520  or the object  540 , a direction Y perpendicular to the direction X, and a direction Z perpendicular to the reticle  520  or the object  540 .  
         [0104]     The projection optical system  530  uses plural multilayer membrane mirrors  530   a  to project a reduced size of the pattern of the reticle  520  onto the object  540 . The number of mirrors  530   a  is about four to six. For a wide exposure area with the small number of mirrors, the reticle  520  and object  540  are simultaneously scanned to transfer a wide area that is an arc-shaped area or ring field apart from the optical axis by a predetermined distance. The projection optical system  530  has a NA of about 0.1 to 0.2. The inventive measuring apparatus  1  is applicable to a measurement of the optical performance, such as a wave front aberration, of the projection optical system  530 . As a result of the measurement of the optical performance by the measuring apparatus  1 , the projection optical system  530  having a measurement value in a permissible range is used for superior imaging performance. The optical performance of the projection optical system  530  is adjusted based on the measurement result of the measuring apparatus  1 .  
         [0105]     The instant embodiment uses a wafer as the object to be exposed  540 , but it may include a liquid crystal plate and a wide range of other objects to be exposed. Photoresist is applied onto the object  540 .  
         [0106]     An object to be exposed  540  is held onto the wafer stage  545  by a wafer chuck  545   a . The wafer stage  545  moves the object  540 , for example, using a linear motor in XYZ directions. The reticle  520  and the object  540  are synchronously scanned. The positions of the reticle stage  525  and wafer stage  545  are monitored, for example, by a laser interferometer, and driven at a constant speed ratio.  
         [0107]     The alignment detection mechanism  550  measures a positional relationship between the position of the reticle  520  and the optical axis of the projection optical system  530 , and a positional relationship between the position of the object  540  and the optical axis of the projection optical system  530 , and sets positions and angles of the reticle stage  525  and the wafer stage  545  so that a projected image of the reticle  520  may be positioned in place on the object  540 .  
         [0108]     The focus detection optical system  560  measures a focus position in the direction Z on the object  540  surface, and control over a position and angle of the wafer stage  545  may always maintain the object  540  surface at an imaging position of the projection optical system  530  during exposure.  
         [0109]     The EUV light source  512  and the illumination optical system  514  in the illumination apparatus  510  in this embodiment may serve as the illumination optical system  10  in the measuring apparatus  1 .  
         [0110]     In exposure, the EUV light emitted from the illumination apparatus  510  illuminates the reticle  520 , and images a pattern of the reticle  520  onto the object  540  surface. This embodiment uses an arc or ring shaped image plane, scans the reticle  520  and object  540  at a speed ratio corresponding to a reduction ratio to expose the entire surface of the reticle  520 . The exposure apparatus  500  uses the projection optical system  530  whose optical performance is greater than a predetermined value through a measurement of the measuring apparatus  1 , realizes superior exposure performance, and provides devices (e.g., a semiconductor device, a LCD device, an image pickup device (such as a CCD), and a thin film magnetic head) with good throughput and economical efficiency.  
         [0111]     Referring now to  FIGS. 19 and 20 , a description will now be given of an embodiment of a device manufacturing method using the above exposure apparatus  500 .  FIG. 19  is a flowchart for explaining a fabrication of devices (i.e., semiconductor chips such as IC and LSI, LCDs, CCDs, etc.). Here, a description will be given of a fabrication of a semiconductor chip as an example. Step  1  (circuit design) designs a semiconductor device circuit. Step  2  (mask fabrication) forms a mask having a designed circuit pattern. Step  3  (wafer preparation) manufactures a wafer using materials such as silicon. Step  4  (wafer process), which is referred to as a pretreatment, forms actual circuitry on the wafer through photolithography using the mask and wafer. Step  5  (assembly), which is also referred to as a post-treatment, forms into a semiconductor chip the wafer formed in Step  4  and includes an assembly step (e.g., dicing, bonding), a packaging step (chip sealing), and the like. Step  6  (inspection) performs various tests for the semiconductor device made in Step  5 , such as a validity test and a durability test. Through these steps, a semiconductor device is finished and shipped (Step  7 ).  
         [0112]      FIG. 20  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  500  to expose a reticle pattern onto the wafer. Step  17  (development) develops the exposed wafer. Step  18  (etching) etches parts other than a developed resist image. Step  19  (resist stripping) removes disused resist after etching. These steps are repeated, and multilayer circuit patterns are formed on the wafer. This embodiment can provide higher-quality semiconductor devices than the prior art. Thus, the device manufacturing method that uses the exposure apparatus  500 , and its resultant (intermediate and final) products also constitute one aspect of the present invention.  
         [0113]     Furthermore, the present invention is not limited to these preferred embodiments and various variations and modifications may be made without departing from the scope of the present invention.  
         [0114]     As discussed, the present invention can provide not only a measuring apparatus that reduces an error between the ideal wave front and the reference wave front generated by the fine aperture, and precisely measures the optical performance, such as a wave front aberration, of an optical system, but also an exposure apparatus having the measuring apparatus.  
         [0115]     This application claims a foreign priority benefit based on Japanese Patent Applications No. 2004-126049, filed on Apr. 21, 2004, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.