Abstract:
A projection objective of a microlithographic projection exposure apparatus has a last optical element on the image side which is plane on the image side and which, together with an image plane of the projection objective, delimits an immersion space in the direction of an optical axis of the projection objective. This immersion space can be filled with an immersion liquid. At least one liquid or solid volume having plane-parallel interfaces can be introduced into the beam path of the projection objective, the optical thickness of the at least one volume being at least substantially equal to the optical thickness of the immersion space. By introducing and removing the volume, it is possible to convert the projection objective from dry operation to immersed operation in a straightforward way, without extensive adjustments to the projection objective or alignment work.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a continuation of international application PCT/EP2005/000641 filed Jan. 24, 2005 and claiming benefit of U.S. provisional application 60/542,924, which was filed Feb. 9, 2004. The full disclosure of these earlier applications is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to a projection objective for microlithographic projection exposure apparatuses, such as those used for the production of large-scale integrated electrical circuits and other microstructured components.  
         [0004]     2. Description of Related Art  
         [0005]     Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers to a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) spectral range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of diffracting structures, which is arranged on a mask, is projected onto the photoresist with the aid of a projection objective. Since the imaging scale is generally less than 1, such projection objectives are also often referred to as reduction objectives.  
         [0006]     After the photoresist has been developed, the wafer is subjected to an etching process so that the layer becomes structured according to the pattern on the mask. The remaining photoresist is then removed from the other parts of the layer. This process is repeated until all the layers have been applied to the wafer.  
         [0007]     One of the essential aims in the development of projection exposure apparatuses used for production is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, and this generally has a favourable effect on the performance of the microstructured components produced with the aid of such systems.  
         [0008]     The size of the structures which can be defined depends primarily on the resolution of the projection objective being used. Since the resolution of the projection objectives is proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths used at present are in the deep ultraviolet (DUV) spectral range, namely 193 nm and 157 nm.  
         [0009]     Another way of increasing the resolution is based on the idea of introducing an immersion liquid with a high refractive index into an intermediate space which remains between a last lens on the image side of the projection objective and the photoresist or other photosensitive layer to be exposed. Projection objectives which are designed for immersed operation, and which are therefore also referred to as immersion objectives, can achieve numerical apertures of more than 1, for example 1.3 or 1.4. The immersion moreover not only allows high numerical apertures and therefore an improved resolution, but also has a favourable effect on the depth of focus. The greater the depth of focus is, the less stringent are the requirements for exact positioning of the wafer in the image plane of the projection objective.  
         [0010]     Carrying out immersed operation, however, requires considerable extra outlay on construction and process technology. For example, it is necessary to ensure that the optical properties of the immersion liquid are spatially homogeneous and constant as a function of time, at least in the volume exposed to the projection light, even if the substrate with the photosensitive layer applied to it moves relative to the projection objective. The technological difficulties associated with this have not yet been resolved satisfactorily.  
         [0011]     It has therefore been considered expedient that projection objectives designed for dry operation, which will be referred to below as “dry objectives” for short, should be operated in immersion only during particularly critical process steps. Of course, with a objective designed for dry operation it is not possible to increase the numerical aperture since this requires a different configuration of the projection objective. Nevertheless, a higher depth of focus is achieved even in the immersed operation of dry objectives, and this can be advantageous in particularly critical process steps. The dry objective may be used without an immersion liquid in the less critical process steps, so that the exposure of the wafer is simplified considerably and, as a general rule, can be carried out more rapidly.  
         [0012]     However, the introduction of an immersion liquid into the immersion space will affect the imaging by the projection objective in such a way that major adjustments to the dry objective have to be carried out before the immersed operation commences. Such adjustments are described in US 2004/109237 A1. The original state has to be restored for a subsequent change to dry operation, which again entails significant costs. Of course, it is also possible to configure the dry objective a priori so that it can be operated in immersion. The numerical aperture must then remain less than 1, since otherwise total reflection would occur at particular optical surfaces during the dry operation. But in this case, too, adjustments are necessary for a change to dry operation since the removal of immersion liquid naturally also affects the imaging.  
       SUMMARY OF THE INVENTION  
       [0013]     It is therefore an object of the invention to provide a projection objective in which a change from dry operation to immersed operation, and vice versa, can be carried out in a straightforward way.  
         [0014]     This object is achieved by a projection objective having a last optical element on the image side which is plane on the image side and which, together with an image plane of the projection objective, delimits an immersion space in the direction of an optical axis of the projection objective, which can be filled with an immersion liquid. At least one liquid or solid volume having plane-parallel interfaces can be introduced into the beam path of the projection objective, the optical thickness of the at least one volume being at least substantially equal to the optical thickness of the immersion space.  
         [0015]     The invention is based on the idea that the immersion liquid introduced into the immersion space is comparable to a plane-parallel plate in terms of its optical effect. The relocation of a plane-parallel plate does not affect the focal length when such a relocation takes place only over optical elements without any refractive power. The correction of a wide variety of imaging errors is likewise not compromised by such a relocation of a plane-parallel plate. If, instead of the immersion liquid, a volume which has the same optical thickness as the volume previously filled with immersion liquid in the immersion space, that is to say the same product of refractive index and geometrical thickness, is introduced into the beam path of the projection objective, this is therefore equivalent to merely relocating a plane-parallel plate along the optical axis. If the immersion liquid is removed and the said volume is introduced into the beam path to replace it when changing from immersed operation to dry operation, this therefore does not affect the focal length of the projection objective. Similar considerations apply to the converse case, that is to say when changing from dry operation to immersed operation. Here, the volume is removed from the beam path and immersion liquid is introduced into the immersion state space instead.  
         [0016]     Since a plane-parallel plate can be divided into a plurality of thinner individual plates without changing the optical effect, it is also possible to introduce a plurality of volumes into the beam path instead of just one volume with the aforementioned properties when the immersion liquid is removed. The total optical thickness of all the volumes which are introduced in the direction of the optical axis should then be substantially equal to the dimension of the immersion space in this direction.  
         [0017]     It is preferable for the refractive index of the at least one volume to be substantially equal to the refractive index of the immersion liquid, and for the sum of the dimensions of all the volumes which can be introduced in the direction of the optical axis to be at least equal to the dimension of the immersion space in this direction. Preferably, the sum of the dimensions of all the volumes which are introduced in the direction of the optical axis differs by at most 10 nm, preferably at most 1 nm, more preferably at most 0.1 nm, from the dimension of the immersion space in this direction.  
         [0018]     As mentioned above, a relocation of a plane-parallel plate has no optical effect if the relocation takes place only over optical elements without any refractive power. Therefore, only refractive surfaces which are plane and extend parallel to the image plane should be arranged in the beam path between the image plane and the volume furthest away from the image plane. The last optical element on the image side is therefore preferably a plane-parallel terminating plate. Some refractive surfaces between the intermediate space and the image plane could nevertheless be reprocessed in order to achieve a corrective effect, so that this condition is then no longer fulfilled, or at least no longer exactly fulfilled.  
         [0019]     It is particularly straightforward to introduce or remove the said volume into and from the beam path when the volume is liquid and the projection objective has a sealable intermediate space, for holding at least one liquid volume, between two optical elements whose mutually facing interfaces are plane-parallel. An intermediate space can be filled very completely with liquids in a straightforward way. Furthermore, fewer refractive surfaces overall need to be processed with high accuracy than in the case of an additional plane-parallel plate, which is intended to replace the immersion liquid in terms of its effect on the focal length.  
         [0020]     In the simplest case, such an intermediate space which can be filled with liquid is bounded on the image side by the terminating plate. The intermediate space which can be filled with liquid therefore lies on the rear side of the terminating plate; in order to bound the intermediate space on the object side, it is then necessary to have another refractively acting optical element whose surface on the image side is plane.  
         [0021]     Since the optical properties of the liquids with which the immersion space and the intermediate space are alternately filled must be exactly equal, an identical liquid should be used for filling the immersion space and the intermediate space. It is even feasible to use the same liquid. This would mean that the liquid introduced into the immersion space was previously in the intermediate space, and vice versa, so that a relocation of the liquid is in fact involved. Since the liquid comes in contact with the photosensitive layer during immersed operation, the liquid may become contaminated during immersed operation. The liquid should therefore be purified before it is returned to the intermediate space.  
         [0022]     In principle, the intermediate space as well as the immersion space may be filled and emptied by hand, for example with the aid of a pipette. Preferably, however, an immersion device by which the intermediate space and/or the immersion space can be filled with liquid and emptied is provided for this purpose. Since the liquid introduced in each case should have a maximally uniform temperature, such an immersion device may also comprise a liquid circuit in which the liquid is continuously circulated. In this way, the liquid can be progressively purified and brought to the intended temperature.  
         [0023]     Instead of an intermediate space being filled with a liquid, the said volume may also be formed by a plane-parallel plate which has the same refractive index as the immersion liquid, with which the immersion space can be filled. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     Various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing in which:  
         [0025]      FIG. 1  shows a meridian section through a microlithographic projection exposure apparatus according to a first exemplary embodiment of the invention, in a schematic representation which is not true to scale;  
         [0026]      FIG. 2   a  shows an enlarged detail of the end on the image side of the projection objective as shown in  FIG. 1 , during dry operation;  
         [0027]      FIG. 2   b  shows the end on the image side in  FIG. 2   a , but during immersed operation;  
         [0028]      FIG. 3   a  shows a detail corresponding to  FIG. 2   a , of a projection objective according to a second exemplary embodiment of the invention during dry operation;  
         [0029]      FIG. 3   b  shows the end on the image side in  FIG. 3   a , but during immersed operation;  
         [0030]      FIG. 4   a  shows a detail corresponding to  FIG. 2   a , of a projection objective according to a third exemplary embodiment of the invention during dry operation;  
         [0031]      FIG. 4   b  shows the end on the image side in  FIG. 4   a , but during immersed operation;  
         [0032]      FIG. 5   a  shows a detail corresponding to  FIG. 2   a , of a projection objective according to a fourth exemplary embodiment of the invention during dry operation;  
         [0033]      FIG. 5   b  shows the end on the image side in  FIG. 5   a , but during immersed operation;  
         [0034]      FIG. 6   a  shows a detail corresponding to  FIG. 2   a , of a projection objective according to a fifth exemplary embodiment of the invention during dry operation;  
         [0035]      FIG. 6   b  shows the end on the image side in  FIG. 6   a , but during immersed operation. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0036]      FIG. 1  shows a meridian section through a microlithographic projection exposure apparatus, denoted overall by  10 , during immersed operation in a highly simplified representation which is not true to scale. The projection exposure apparatus  10  has an illumination device  12  for the generation of projection light  13 , which comprises a light source  14 , illumination optics indicated by  16  and a diaphragm  18 . In the exemplary embodiment which is represented, the projection light  13  has a wavelength of 193 nm.  
         [0037]     The projection exposure apparatus  10  furthermore includes a projection objective  20  which contains a multiplicity of lenses, only some of which denoted by L 1  to L 4  are represented by way of example in  FIG. 1  for the sake of clarity. The projection objective  20  furthermore contains a plane-parallel plate  21  as the last optical element, as well as a likewise plane-parallel terminating plate  23  which terminates the projection objective  20  on the image side. The lenses L 1  to L 4 , the plate  21  and the terminating plate  23  consist of quartz glass. Nevertheless, other materials which have sufficient optical transparency at the wavelength of the projection light  13  may also be selected as the material.  
         [0038]     The projection objective  20  is used to project a reduced image of a mask  24 , which is arranged in an object plane  22  of the projection objective  20 , onto a photosensitive layer  26 . The layer  26 , which for may example consist of a photoresist, is arranged in an image plane  28  of the projection objective  20  and is applied to a support  30 .  
         [0039]     The support  30  is fastened on the bottom of an open-topped container  32  in the form of a trough, which can be displaced (in a way which is not represented in detail) parallel to the image plane  28  with the aid of a displacement device. The container  32  is filled sufficiently with an immersion liquid  34  so that the projection objective  20  is immersed with its terminating plate  23  in the immersion liquid  34  during operation of the projection exposure apparatus  10 . The immersion liquid  34  may, for example, be highly pure deionised water or a halogen-free oil. The immersion liquid  34  chosen in the represented exemplary embodiment is nitrobenzene, whose refractive index at the wavelength of the projection light  13  being used is approximately equal to the refractive index of quartz glass, of which the plate  21  and the terminating plate  23  are made.  
         [0040]     Via a feed line  36  and a discharge line  38 , the container  32  is connected to a treatment unit  40  which contains, inter alia, a circulating pump and a filter for purifying the immersion liquid  34 . The treatment unit  40 , the feed line  36 , the discharge line  38  and the container  32  together form an immersion device denoted by  42 , in which the immersion liquid  34  circulates while being purified and kept at an at least approximately constant temperature.  
         [0041]     The immersion device  42  furthermore includes another line  44 , which leads directly to the projection objective  20 . Via the line  44 , when changing from immersed operation to dry operation and vice versa, immersion liquid  34  can be pumped into or removed from an intermediate space which is located between the terminating element  23  and the plane-parallel plate  21 .  
         [0042]     This will be explained in more detail below with reference to  FIGS. 2   a  and  2   b , which show the end on the image side of the projection objective  20  in an enlarged representation during dry operation and immersed operation, respectively. It can be seen in  FIG. 2   a  that an intermediate space  46 , which remains between the plane-parallel plate  21  and the terminating plate  23 , is sealed tightly all around. An annular sealing element  48 , which is clamped between two element frames indicated by  50 ,  52 , is used for this purpose. The element frames  50 ,  52  make it possible to align the plane-parallel plate  21  and the terminating plate  23 , as is known per se in the prior art. The line  44  extends through the sealing element  48  into the intermediate space  46 .  
         [0043]     The plane-parallel plate  21  and the terminating plate  23  are aligned so that their mutually facing plane interfaces  56 ,  58  are aligned exactly parallel with a distance d 1  between them. When the intermediate space  46  is filled with immersion liquid  34  during dry operation, as represented in  FIG. 2   a , the intermediate space acts like a plane-parallel plate made of a material whose refractive index is equal to that of the immersion liquid  34 .  
         [0044]     Peripheral rays of the projection light  13 , which pass through the projection objective  20  onto the photosensitive layer  26 , are indicated by  60  and  62  in  FIG. 2   a . Since the refractive index of the immersion liquid  34  is substantially equal to the refractive indices of the plane-parallel plate  21  and of the terminating plate  23 , the projection light  13  is almost not refracted at all when it passes through the intermediate space  46  filled with immersion liquid  34 . The maximum aperture angle at which the peripheral rays  60 ,  62  meet at points on the photosensitive layer  26  is denoted by a in  FIG. 2 .  
         [0045]     If a change is then to be made to the immersed operation as shown in  FIGS. 1 and 2   b , the immersion liquid  34  will be pumped out of the intermediate space  46  via the line  44 . After having been purified in the immersion device  42 , the immersion liquid  34  is then introduced via the feed line  36  into the container  32  where it enters an immersion space  64 , which is formed between the terminating plate  23  and the photosensitive layer  26 . The projection objective  20  is in this case designed so that the height d 1  of the intermediate space  46  is equal to the height d 2  of the immersion space  64 . The change from dry operation to immersed operation can therefore be readily understood as entailing relocation of a plane-parallel “plate” of immersion liquid  34  from the intermediate space  46  into the immersion space  64 . Since this “plate” does not thereby change its thickness and since the optical element lying in between, namely the terminating plate  23 , has plane-parallel interfaces, even after the change to immersed operation the peripheral rays denoted by  60 ′ and  62 ′ in  FIG. 2   b  still meet at a point in the image plane  28  where the photosensitive layer  26  is arranged.  
         [0046]     As can be seen in  FIG. 2   b , the maximum aperture angle denoted by α′ has become smaller because of the change from dry operation to immersed operation. This is associated with an increase in the depth of focus since, when the photosensitive layer  26  is moved out of the image plane  28 , the broadening of the focal point into a focal spot is less than the case of the dry operation as represented in  FIG. 2   a.    
         [0047]     A second exemplary embodiment of a projection objective will be explained below with reference to  FIGS. 3   a  and  3   b , which are representations corresponding to  FIGS. 2   a  and  2   b . Parts which are the same are denoted by identical reference numerals, and parts which correspond to one another are denoted by reference numerals increased by  100 . Wherever reference numerals increased by  100  are not explicitly mentioned in the text, then the comments made above about  FIGS. 2   a  and  2   b  apply accordingly.  
         [0048]     The projection objective  120  differs from the projection objective  20  as shown in  FIGS. 2   a  and  2   b  essentially in that the projection objective  120  does not have a plane-parallel plate  21 . Instead, the last optical element with non-zero refractive power in the projection objective  120  as shown in  FIGS. 3   a  and  3   b  is a planoconvex lens L 104 . Together with an opposing plane surface  158  of a terminating plate  123 , its plane surface  156  on the image side forms an intermediate space  146  of height d 1 , which can be filled with immersion liquid  34 . In terms of function, the projection objective  120  does not substantially differ from the projection objective  20  as shown in  FIGS. 2   a  and  2   b . In particular, the intended reduction of the aperture angle α and therefore the increase of the depth of focus also take place here when changing to immersed operation.  
         [0049]     A third exemplary embodiment of a projection objective will be explained below with reference to  FIGS. 4   a  and  4   b , which are representations corresponding to  FIGS. 2   a  and  2   b . Parts which are the same are denoted by identical reference numerals, and parts which correspond to one another are denoted by reference numerals increased by  200 . Wherever reference numerals increased by  200  are not explicitly mentioned in the text, then the comments made above about  FIGS. 2   a  and  2   b  apply accordingly.  
         [0050]     The projection objective  220  as shown in  FIGS. 4   a  and  4   b  differs from the projection objective  120  as shown in  FIGS. 3   a  and  3   b  essentially in that the terminating plate  123  is divided into two individual plane-parallel plates  223   a  and  223   b . The respective thicknesses a a  and a b  of the plates  223   a  and  223   b  now add up to the thickness a of the terminating plate  123  as shown in  FIGS. 3   a  and  3   b.    
         [0051]     The division of the terminating plate  123  into two individual plane-parallel plates  223   a  and  223   b  also partitions the immersion space  146  above the terminating plate  123  into two intermediate spaces  246   a  and  246   b , the respective heights d 1a  and d 1b  of which add up to equal the height d 1  of the intermediate space  146  and therefore to equal the distance d 2  between the terminating plate  223   b  on the image side and the image plane  28 . On account of the separate intermediate spaces  246   a  and  246   b , there are also two separate lines  244   a ,  244   b  by which the intermediate spaces  246   a ,  246   b  can be filled with immersion liquid  34 , or this can be pumped out of them. When changing from dry operation to immersed operation, as indicated between  FIGS. 4   a  and  4   b , the maximum aperture angle α is here again reduced so that the depth of focus of the projection is improved.  
         [0052]     A fourth exemplary embodiment of a projection objective will be explained below with reference to  FIGS. 5   a  and  5   b , which are representations corresponding to  FIGS. 2   a  and  2   b . Parts which are the same are denoted by identical reference numerals, and parts which correspond to one another are denoted by reference numerals increased by  300 . Wherever reference numerals increased by  300  are not explicitly mentioned in the text, then the comments made above about  FIGS. 2   a  and  2   b  apply accordingly.  
         [0053]     The projection objective  320  as shown in  FIGS. 5   a  and  5   b  differs from the projection objective  120  as shown in  FIGS. 3   a  and  3   b  essentially in that the intermediate space  146  between the terminating plate  123  and the planoconvex lens L 104  arranged above it cannot be filled with a liquid. Instead, the projection objective  320  is designed so that a plane-parallel plate  334  which also consists of quartz glass, and which therefore has approximately the same refractive index as the immersion liquid  34 , can be introduced into the intermediate space  346  (see arrow P). The thickness d 1  of the plate  334  is in this case the same as the height d 2  of the immersion space  64 . The plate  334  is removed from the beam path when changing to immersed operation.  
         [0054]     A fifth exemplary embodiment of a projection objective will be explained below with reference to  FIGS. 6   a  and  6   b , which are representations corresponding to  FIGS. 2   a  and  2   b . Parts which are the same are denoted by identical reference numerals, and parts which correspond to one another are denoted by reference numerals increased by  400 . Wherever reference numerals increased by  400  are not explicitly mentioned in the text, then the comments made above about  FIGS. 2   a  and  2   b  apply accordingly.  
         [0055]     The projection objective  420  as shown in  FIGS. 6   a  and  6   b  differs from the projection objective  320  as shown in  FIGS. 5   a  and  5   b  essentially in that the lens L 404  is a meniscus lens instead of a planoconvex lens. During dry operation, therefore, the plane-parallel plate  334  is not inserted into an intermediate space between two plane and parallel interfaces as in the case of the projection objective  320  in  FIGS. 5   a  and  5   b , but merely rests on the plane surface  458  on the object side of the terminating plate  423  (which is thinner in this case).  
         [0056]     The advantage of the projection objectives  320  and  420 , in which respective plane-parallel plates  334  and  434  are introduced into the beam path for dry operation, is primarily that it is not necessary to provide specially dimensioned intermediate spaces as was the case for the projection objectives  20 ,  120  and  220 . Complete reconfiguration of the projection objectives is therefore unnecessary. Instead, it is sufficient to start off with “normal” dry operation and then accommodate the additional plates  334  or  434  by modifications such as those described, for example, in US 2004/109237 A1. Such a modification can be carried out with the aid of liquid lenses whose refractive power is variable.