Abstract:
A method for determining intensity distribution in the focal plane of a projection exposure arrangement, in which a large aperture imaging system is emulated and a light from a sample is represented on a local resolution detector by an emulation imaging system. A device for carrying out the method and emulated devices are also described. The invention makes it possible to improve a reproduction quality since the system apodisation is taken into consideration. The inventive method includes determining the integrated amplitude distribution in an output pupil, combining the integrated amplitude distribution with a predetermined apodization correction and calculating a corrected apodization image according to the modified amplitude distribution.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method wherein an imaging system having a large aperture is emulated and light coming from a sample is imaged onto a spatially resolving detector by means of an emulation imaging system. 
     BACKGROUND OF THE INVENTION 
     In particular, the emulated imaging system may be a photolithographic scanner used to produce microelectronic circuits. For this purpose, photolithographic masks are used whose structures are imaged onto wafers coated with photoresist, such that the resist is exposed and the structure on the wafer is modified at the exposed locations. 
     Emulation imaging systems are used to emulate such a photolithographic scanner. However, whereas the scanner images a reduced-size image of the mask&#39;s structure onto the carrier to be exposed, an emulation imaging system is used in mask inspection systems, in which case an enlarged image of the structure is imaged onto a detector. While both systems have the same numerical aperture on the mask side, they differ on the image side. The image-side numerical aperture of a mask inspection system, on the one hand, is approximately 0. On the other hand, increasingly smaller object structure sizes require increasingly greater image-side numerical apertures of 0.8 and more in the photolithographic scanner. This leads to deviations in the images of the mask inspection system and of the photolithographic scanner, which are no longer negligible, particularly in cases where the ratio of the numerical aperture of the scanner to the refractive index in the photoresist is greater than 0.8/1.7. These deviations, or defects appearing in the scanner system, respectively, also include the so-called apodization. The transmission of light in the photoresist as a function of the angle of incidence is not constant, but increases towards great angles of incidence. 
     In the prior art, the image-side differences between the photolithographic scanner and the emulation imaging system for mask inspection only play a secondary role. However, in the semiconductor industry, the use of immersion systems for manufacturing wafer structures of less than 65 nm is favored for the future. By applying an immersion liquid onto the wafer, numerical apertures NA&gt;1 are achieved on the image-side, allowing smaller structures to be generated at the same wavelength. Thus, for example, using water as the immersion liquid and illumination with light at a wavelength of λ=193 nm, a maximum numerical aperture of 1.4 can be achieved. Even greater numerical apertures can be achieved using other immersion liquids. A reduction factor of 1:4 requires mask structures of 260 nm or 180 nm, respectively, for wafer structures of 65 nm or 45 nm, respectively. Thus, the mask structures are within the range of the imaging wavelength of currently 193 nm. In the case of smaller structures, the analysis of defects in the masks to be used is becoming increasingly important. One example of a mask inspection system suitable for analysis is the AIMS™ (Areal Imaging Measurement System) of Carl Zeiss SMS GmbH. A small area of the mask (site of the defect) is illuminated and imaged under the same conditions of illumination and imaging (wavelength, numerical aperture) as in the photolithographic scanner. In contrast to the photolithographic scanner, however, the image generated by the mask is enlarged and imaged onto a CCD camera. The camera sees the same image as the photoresist on the wafer. Thus, the aerial image can be analyzed without complicated test prints. In this system—just like in the other systems known in the prior art—the deviation in apodization with respect to the photolithographic scanner is not taken into consideration, because the numerical apertures are still so small that the deviations have not played a role so far. However, since the use of larger apertures on the mask side in the photolithographic scanner may be expected in the future, these deviations are becoming more important. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to find ways in which the imaging conditions in terms of the apodization behavior between the mask inspection system, on the one hand, and the photolithographic scanner, on the other hand, are adjusted to each other. 
     In a method of the above-described type, this object is achieved in that a mask is illuminated, the light coming from the mask is directed onto a spatially resolving detector, a signal is recorded by the spatially resolving detector, and the intensity distribution is computed on the basis of the signal. Said computation takes the numerical aperture in the image plane of the projection illumination system into account. 
     In an advantageous embodiment of the invention, the phase distribution of the incident light is determined in the image plane. On the basis of said phase distribution, the complex amplitude distribution in the exit pupil is reconstructed, the reconstructed amplitude distribution is combined with a predetermined apodization correction, and the modified amplitude distribution is used to compute an image having a corrected apodization. 
     In a further advantageous embodiment of the invention, the complex amplitude distribution is determined by computing the phase distribution of the incident light in the image plane and reconstructing the complex amplitude distribution on the basis of the phase distribution. 
     Determining the phase distribution in the image plane allows computing by known methods, such as two-dimensional Fourier transformation or by phase retrieval, of which of the beams in the exit pupil, where the beams emanating from an object point are all parallel, leads to which point in the image plane. The phase itself can be measured by methods known from the prior art. For example, several images can be recorded in front of within and behind the focal plane, and then the phase values can be mathematically reconstructed by a recursive method of evaluation. Direct measurement of the phase values, for example by the Shack Hartmann method, is also possible. A phase-shifting lateral shearing interferometer can also be used to determine the phase distribution. Knowledge of the phase distribution allows the complex amplitude distribution in the exit pupil to be mathematically reconstructed. This reconstructed amplitude distribution is then combined with a predetermined apodization correction, which results in a modified amplitude distribution. The latter, in turn, finally allows computing of an image in which the correction of the apodization is taken into consideration. 
     In this case, the complex amplitude distribution is advantageously not determined for each point in the pupil plane, but only for a number of points having the coordinates (p,q). The amplitude distribution is combined with the apodization correction only for these points. Common interpolation methods can be used to compute the intermediate values. 
     Depending on the problem to be solved, the apodization correction can be predetermined in different ways. Thus, for example, the predetermined apodization correction may be an apodization which substantially corresponds to the apodization present in the emulation imaging system, and the amplitude distribution can be divided by this apodization. In this manner, the apodization present in the system can be compensated for at least in part. Conveniently, the predetermined apodization correction will be the natural apodization or, if known, the technical apodization, or both combined. 
     However, the predetermined apodization correction will preferably be an apodization which substantially corresponds to the apodization present in the emulated imaging system, and the amplitude distribution will be multiplied therewith. This allows the apodization present in the emulated imaging system to be generated at least approximately, so that the conditions prevailing there are emulated in the best possible way. The natural apodization or the technical apodization, or both apodizations present in the emulated imaging system are conveniently given here as the apodization correction as well. This is important, in particular, if the emulated imaging system is a photolithographic scanner, because this is precisely the manner in which the conditions prevailing in the photolithographic scanner can be emulated best. In order to emulate these conditions as faithfully as possible, it is advantageous to predetermine the apodization correction as a function of the scanner&#39;s numerical aperture and of the emulated photoresist. The apodization of the photolithographic scanner can then be emulated by multiplying the amplitude distribution in the exit pupil by the factor 1/cos θ′, with θ′ being the angle of incidence of the light in the emulated photoresist. The natural apodization T(p,q) of the photolithographic scanner can also be determined according to the term 
               T   ⁡     (     p   ,   q     )       =       (     1     1   -       (     p     n   ′       )     2     -       (     q     n   ′       )     2         )       1   4             
wherein T is the amplitude transmission, n′ is the refractive index of the photoresist used, and (p,q) are the dimension-less pupil coordinates.
 
     Apart from the mathematical consideration of the apodization present in the scanner, the object is also achieved by a method of the above-described type in that, in order to modify the apodization, an intensity-modifying optical element is placed in or connected to the beam path. Using this element, a predetermined intensity distribution is then generated in the exit pupil. Said intensity-modifying optical element can be advantageously controlled to generate a predetermined intensity distribution. For example, this allows different apodizations caused by different photoresists to be taken into account in the photolithographic scanner. Another possibility consists in changing this intensity-modifying optical element, which change can be effected automatically. Conveniently, an intensity distribution substantially corresponding to the natural and/or technical apodization of the emulated imaging system will be given, so that said apodization will be generated at least approximately. In this case, the natural apodization is determined according to the term 
                 n   ⁢           ⁢   cos   ⁢           ⁢   θ         n   ′     ⁢   cos   ⁢           ⁢     θ   ′               
wherein n and θ are the refractive index and the angle of incidence, respectively, of the light in the object space, i.e. in the mask; n′ and θ′ are the corresponding quantities in the image space, i.e. on the wafer, with n′ and θ′ referring to the photoresist.
 
     In a further embodiment of the method, an intensity distribution substantially corresponding to the inverse natural and/or inverse technical apodization of the emulation imaging system is predetermined as an alternative or in addition, so that said apodization is compensated for at least in part. This allows elimination of the apodization effects of the emulation imaging system almost completely and thus emulates the apodization of the emulated imaging system in a substantially realistic manner. 
     As a rule, intensity-attenuating elements, such as filters, will be used to modify intensity. However, an embodiment of the method is also conceivable wherein intensity is enhanced by the intensity-modifying optical element. 
     If a photolithographic scanner is emulated, the intensity distribution will be advantageously predetermined as a function of the numerical aperture of the scanner and of the emulated photoresist. 
     Further, in microscopic imaging systems for carrying out the method according to some embodiments of the invention, by which optical imaging systems having a great aperture are emulated, the object is achieved in that one or more intensity-modifying optical elements are provided to vary apodization. In the simplest case, a filter is provided as the intensity-modifying optical element. Even several filters can be provided. The filters may also be exchangeable, so that a different filter can be used or connected depending on the emulated system. For example, the filters can be arranged on a disk and then respectively introduced into the beam path via a user interface. 
     If the filter is exchangeable, the technical apodization can be emulated for different scanners. The filters can be adapted to different refractive indices of the photoresist to be emulated. Moreover, such a filter can also be designed such that it covers only a numerical aperture which is smaller than the maximum possible numerical aperture. In return, it will then have a higher total transmission. A disadvantage of exchangeable filters is the high mechanical precision required. Thus, the mount must be very precisely engineered. In addition to said attenuation, the absorption layer will also cause a certain phase deviation. In a non-exchangeable filter, such phase deviation has to be compensated for in the filter itself. On the other hand, in a non-exchangeable filter, the phase deviation cannot be automatically removed while the objective is being adjusted. 
     Therefore, in a preferred embodiment of the invention, the filter is applied directly onto one of the objective lenses. This can be done, for example, by vapor deposition. 
     In a further preferred embodiment, at least two intensity-modifying optical elements, preferably filters, are provided. One of said elements has a strong intensity-modifying effect and corrects most of the apodization, for example. In this case, the phase deviation is also removed by adjustment. The exchangeable element(s) then also allow(s) adaptation to the particular photoresist being emulated. In this case, the intensity-modifying effect is substantially weaker. Therefore, the phase deviation plays only a secondary role. 
     Instead of a filter, an intensity-enhancing medium is provided as the intensity-modifying optical element in a preferred embodiment of the invention. Particularly preferably, a laser, in particular a solid state laser which is preferably connectable, is used. Such a laser can be realized, for example, by placing a thin, doped crystal in the pupil plane and pumping it, perpendicular to the optical axis, from at least one side. In order for imaging to take place, the photons have to be superimposed in-phase in the diffraction orders. When the photons pass through the crystal, further photons are stimulated by emission with the correct orientation and in-phase. Which order of diffraction is enhanced to what extent depends on the crystal doping profile and the optical pumping process. The use of an active, intensity-enhancing medium prevents energy losses such as those generated when using filters. However, particularly in microscopes the energy density is already relatively low so that a further reduction by filters might have negative effects in certain circumstances. 
     Since the beam path in the microscope is usually rotationally symmetric, the intensity-modifying optical element is conveniently embodied such that it modifies intensity substantially with rotation symmetry about the optical axis. Depending on the application, a filter can be embodied such that its transmission either increases or decreases from the center to the periphery. For example, if the emulation in a scanner system is to be emulated, transmission has to increase towards the periphery. In the form which is easiest to realize, the intensity-modifying optical element is conveniently arranged in or near a pupil plane in which the beams coming from an object point are parallel or beams coming from different object points at the same angle converge at the same location. In this manner the natural apodization of a photolithographic scanner can be emulated, for example, if the intensity-modifying optical element is designed to modify intensity substantially according to the following term: 
             I   =     C       1   -       (     rNA       r   0     ⁢     n   r         )     2                 
wherein I is the transmission of intensity, r 0  is the radius of the imaging pupil, r is the distance from the optical axis, n r  is the refractive index in the emulated photoresist, NA is the numerical aperture of the photolithographic scanner, and C is a selectable constant. If a value of 1 is selected as the constant, the intensity-modifying optical element has to enhance intensity. However, if the constant is selected to be less than or equal to the maximum of the term below the fraction bar, the optical element can be designed as purely attenuating—in this case, transmission will not exceed 100%.
 
     In another preferred embodiment of the invention, the intensity-modifying optical element is arranged in an area under high angular load. For example, a filter can be vapor-deposited on a curved lens surface. Transmission through this element will then become angle- and polarization-dependent. A suitable coating allows emulation of Fresnel reflections which appear at the boundary surface at the transition from the air or the immersion liquid to the photoresist. However, there is no restriction to apply the filter to a lens surface. Alternatively, the filter can also be introduced into the beam path at a location where the rays have a high numerical aperture. 
     When arranging the optical element in an area under high angular load, two major effects can be utilized and the material can be designed accordingly: On the one hand, there is a volume absorption which depends on the angle of incidence and is generated because inclined beams travel longer distances in plane-parallel plates; and, on the other hand, there is a dependence of the reflection at the surface on the angle of incidence of the light. The first effect can also be utilized, of course, in the case of lenses with positive or negative diffraction, where rays entering the lens near the optical axis travel a longer or a shorter distance therein, respectively, than peripheral beams. 
     Instead of modifications of apodization in the emulation imaging system, one or more intensity-modifying optical elements can also be provided in the beam path of a photolithographic scanner itself, i.e. of the emulated system, so as to vary apodization. In this manner, natural and/or technical apodizations of the scanner system can be corrected such that a correction in the emulation system will become superfluous in certain circumstances. Preferably, the optical element is exchangeably arranged in or near a pupil plane here as well. The optical element conveniently corrects the natural and/or technical apodization according to the photoresist used. In a preferred embodiment of the invention, the optical element has an amplitude transmission T(p, q) with the pupil coordinates (p, q) according to the term 
               T   ⁡     (     p   ,   q     )       =       (       1   -       (     β   ⁢           ⁢   p     )     2     -       (     β   ⁢           ⁢   q     )     2         1   -       (     p     n   ′       )     2     -       (     q     n   ′       )     2         )       1   4             
wherein β is the magnification and n′ is the refractive index of the photoresist used.
 
     The invention will be explained in more detail below with reference to exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the respective drawings, 
         FIG. 1   a  depicts a photolithographic scanner of the type known from the prior art; 
         FIG. 1   b  depicts a prior art mask inspection system; 
         FIG. 2  depicts the emulation imaging system, i.e. that part of the mask inspection system—as it currently exists in the prior art—which is located on the image side with respect to the mask; 
         FIG. 3   a  depicts a first embodiment of the invention comprising a pupil filter; 
         FIG. 3   b  depicts the transmission of the latter for emulation of natural apodization; 
         FIG. 4  depicts a further embodiment of the invention, wherein the optical element is arranged in an area under high angular load; 
         FIG. 5  depicts a further embodiment of the invention, wherein the optical element is also arranged in an area under high angular load; 
         FIG. 6  depicts an alternative to the intensity modification of optical elements in areas under high angular load, 
         FIG. 7  depicts an embodiment utilizing the different path lengths covered in one of the lenses; 
         FIG. 8  depicts an emulation imaging system according to another embodiment of the invention; and 
         FIG. 9  depicts another emulation imaging system according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1   a  shows a photolithographic scanner as known from the prior art. Light beams  1  from a source of illumination (not shown) are incident on an object or a mask  2 , which is arranged in a field plane. The beams then enter the scanner imaging system  3 , which images the mask  2 , reduced by a factor of 4 in the example shown here, into a photoresist layer  4  applied onto a wafer. The photoresist is exposed at the light-transmitting locations. By subsequent processing, the image of the mask  2  is reproduced on the wafer. 
     Since manufacture of the masks is very complex and costly, and since these costs increase as the size of the structures used decreases, the masks are usually tested for errors before being used in photolithographic scanners. For this purpose, the mask inspection system shown by way of example in  FIG. 1   b , e.g. AIMS™ of Carl Zeiss SMS GmbH, can be used. Up to the dashed line through the imaging system, which line is also indicated in  FIG. 1   a , both systems are identical, i.e. both systems use the same wavelength, a corresponding source of illumination and the same illumination polarization. The mask-side numerical apertures are also the same. However, whereas the image of the mask  2  is imaged into the photoresist layer  4  with a reduction in size in the scanner imaging system  3 , the mask  2  is imaged—in the example shown here—on a CCD camera  6  by the emulation imaging system  5  with a magnification of 450 times chosen as an example here. It goes without saying that other magnifications are also possible. Thus, whereas the numerical aperture is very large on the image side in the case of the scanner, the numerical aperture in the mask inspection system is very small. This difference results in a different apodization. 
     Said apodization can be compensated for in different ways in the mask inspection system. First of all,  FIG. 2  shows the emulation imaging system  5  again in more detail, comprising two lens systems  7  and  8 , between which the beams are approximately parallel. 
     In a first embodiment of the invention, shown in  FIG. 3   a , a filter  9  is introduced in a pupil plane or a plane near the pupil plane—in this case between the lens systems  7  and  8 —which filter modifies transmission of the intensity with rotation symmetry depending on the distance from the optical axis according to the formula 
             I   =     C       1   -       (     rNA       r   0     ⁢     n   r         )     2                 
as shown in  FIG. 3   b . In this case, the constant C&lt;1 has been selected such that transmission is 100% at the periphery of the pupil. In the example shown, the numerical aperture is 1.4 and the refractive index in the photoresist is 1.72. Instead of the filter, an intensity-enhancing medium can also be employed.
 
     A second embodiment of the invention is shown in  FIG. 4 . In this case, the filter  10  was vapor-deposited on a strongly arcuate surface of the lens system  7 , which is shown here in a mask-side position. However, it is not limited to this position and may also be situated at other locations. The surface is arcuate and also exhibits a high angular load, i.e. the beams coming from an object point are incident on the filter  10  at large angles. The only decisive factor in selecting a surface is the angular load. As an alternative, a filter  11  having a planar surface can also be inserted between the mask  2  and the lens system  7  or any other position at which there is a large numerical aperture. In both cases, the coating must be selected such that it influences the transmission of the light according to the angle of incidence. For example, a layer can be used whose reflection behavior depends on the angle of incidence of the light. 
       FIG. 6  shows a further embodiment of the invention. Instead of a coating whose filter properties depend on the angle of incidence of the light, a plane-parallel plate  12  is inserted here at the same location—and also under a high angular load—in the beam path between the mask  2  and the lens system  7 . This filter element utilizes the effect that beams incident at different angles cover different path lengths in the plate. The greater the angle of incidence is with respect to the surface normal, the greater will be the distance to be covered and the higher will be the absorption in the volume, too. 
     Alternatively, as shown in  FIG. 7 , a lens  13 —of the lens system  8  in this example—can also be used for volume absorption. As in the case of the above-mentioned plate  12 , advantage is taken here also of the effect of the different path lengths and of the related differences in absorption. However, the lens can also be inserted at a position in the system where beam divergence is low or even approaches zero. The different path lengths are then due to the shape of the lens. For example, in a convex lens having positive refraction the beams impinging on the lens near the optical axis cover a greater distance in said lens than beams impinging on the lens at the periphery of the lens. For concave lenses having negative refraction the exact opposite is the case. 
     The intensity-modifying optical elements, which have been considered here as examples of the emulation imaging system in the mask inspection system, can be used with comparable properties in a similar manner at basically the same locations in the scanner imaging system  3 . 
     As a rule, intensity-attenuating elements, such as filters, will be used to modify intensity. However, an embodiment of the method is also conceivable wherein intensity is enhanced by the intensity-modifying optical element depicted in  FIG. 8 . 
     If a photolithographic scanner is emulated, the intensity distribution will be advantageously predetermined as a function of the numerical aperture of the scanner and of the emulated photoresist. 
     Further, in microscopic imaging systems for carrying out the method by which optical imaging systems having a great aperture are emulated, the object is achieved in that one or more intensity-modifying optical elements are provided to vary apodization. In the simplest case, a filter  9 ,  10 ,  11 ,  16  is provided as the intensity-modifying optical element. Even several filters  9 ,  10 ,  11 ,  16  can be provided. The filters  16  may also be exchangeable as depicted in  FIG. 9 , so that a different filter  16  can be used or connected depending on the emulated system. For example, the filters  16  can be arranged on a disk and then respectively introduced into the beam path via a user interface. 
     If the filter  16  is exchangeable, the technical apodization can be emulated for different scanners. The filters  9 ,  10 ,  11 ,  16  can be adapted to different refractive indices of the photoresist to be emulated. Moreover, such a filter  9 ,  10 ,  11 ,  16  can also be designed such that it covers only a numerical aperture which is smaller than the maximum possible numerical aperture. In return, it will then have a higher total transmission. A disadvantage of exchangeable filters  16  is the high mechanical precision required. Thus, the mount must be very precisely engineered. In addition to said attenuation, the absorption layer will also cause a certain phase deviation. In a non-exchangeable filter  9 ,  10 ,  11 , such phase deviation has to be compensated for in the filter itself. On the other hand, in a non-exchangeable filter  16 , the phase deviation cannot be automatically removed while the objective is being adjusted. 
     Therefore, in a preferred embodiment of the invention, the filter  10  is applied directly onto one of the objective lenses. This can be done, for example, by vapor deposition. 
     In a further preferred embodiment, at least two intensity-modifying optical elements, preferably filters  9 ,  10 ,  11 ,  16 , are provided. One of said elements has a strong intensity-modifying effect and corrects most of the apodization, for example. In this case, the phase deviation is also removed by adjustment. The exchangeable element(s) then also allow(s) adaptation to the particular photoresist being emulated. In this case, the intensity-modifying effect is substantially weaker. Therefore, the phase deviation plays only a secondary role. 
     Instead of a filter  9 ,  10 ,  11 ,  16 , an intensity-enhancing medium is provided as the intensity-modifying optical element in a preferred embodiment of the invention. Particularly preferably, a laser  15 , in particular a solid state laser which is preferably connectable, is used. Such a laser  15  can be realized, for example, by placing a thin, doped crystal  14  in the pupil plane and pumping it, perpendicular to the optical axis, from at least one side. In order for imaging to take place, the photons have to be superimposed in-phase in the diffraction orders. When the photons pass through the crystal, further photons are stimulated by emission with the correct orientation and in-phase. Which order of diffraction is enhanced to what extent depends on the crystal doping profile and the optical pumping process. The use of an active, intensity-enhancing medium prevents energy losses such as those generated when using filters. However, particularly in microscopes the energy density is already relatively low so that a further reduction by filters might have negative effects in certain circumstances. 
     LIST OF REFERENCE SYMBOLS 
     
         
           1  Light beam 
           2  Mask 
           3  Scanner imaging system 
           4  Photoresist layer 
           5  Emulation imaging system 
           6  CCD camera 
           7 ,  8  Lens system 
           9 ,  10 ,  11  Filter 
           12  Plane-parallel plate 
           13  Lens