Abstract:
Illumination devices and masks for microlithography projection exposure systems, as well as related systems and methods, are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to German patent application serial number 10 2006 049 612.4, filed Oct. 20, 2006, which is hereby incorporated by reference.  
       FIELD  
       [0002]     The disclosure relates to illumination devices and masks for microlithography projection exposure systems, as well as related systems and methods.  
       BACKGROUND  
       [0003]     Microlithography exposure systems are known in which structures on a mask are imaged onto a semiconductor wafer disposed in a wafer plane. Typically this involves using an illumination device to illuminate the portion of the mask which is to be imaged.  
       SUMMARY  
       [0004]     The disclosure relates to illumination devices and masks for microlithography projection exposure systems, as well as related systems and methods.  
         [0005]     In one aspect, the disclosure features an illumination device having a mask plane. The illumination device includes a radiation source configured to generate electromagnetic radiation, and illumination optics configured to direct the electromagnetic radiation onto the mask plane. The illumination system has a mask plane where the mask, when present, is located. The illumination device is configured to generate an interference pattern in the mask plane. The illumination device is configured to be used in a microlithography projection exposure system.  
         [0006]     In a further aspect, the disclosure features a microlithography projection exposure system that has a mask plane and an object plane. The system includes an illumination device (e.g., as described in the preceding paragraph) configured to generate an interference pattern in the mask plane. The system also includes projection optics configured to image at least a portion of the interference pattern from the mask plane into the wafer plane.  
         [0007]     In another aspect, the disclosure features a mask that includes at least one mask structure configured to generate a target structure in a radiation sensitive medium. The mask is configured to be used in a microlithography projection illumination system, and the shape of the mask structure differs from the shape of a target structure so that the target structure can be generated in the radiation sensitive medium via illumination of the mask through variable illumination in the mask plane.  
         [0008]     In an additional aspect, the disclosure features a method that includes providing a microlithography projection exposure system that has a mask plane and a wafer plane. The system includes an illumination device configured to illuminate the mask plane with electromagnetic radiation, an object in the wafer plane, and projection optics configured to image an object structure from the mask plane into the wafer plane. The method also includes generating an interference pattern in the mask plane via the illumination device, and imaging at least a portion of the interference pattern via the projection optics onto the object  
         [0009]     In some embodiments, the disclosure provides a microlithography projection exposure system, a mask for such exposure system, and a method for lithographic exposure of an object, whereby structures with great geometric variety can be generated on the object in an efficient manner, and simultaneously the requirements with respect to the dimensional precision of the mask can be kept moderate.  
         [0010]     In certain embodiments, the disclosure provides an illumination device for a microlithography projection exposure system with a radiation source for generating electromagnetic radiation, and illumination optics for directing the electromagnetic radiation onto a mask plane of the microlithography projection exposure system. The illumination device is configured to generate an interference pattern in the mask plane in the state in which the illumination device is mounted in the microlithography projection exposure system. In some embodiments, the disclosure provides a mask for such microlithography projection exposure system. The mask has at least one mask structure for generating a target structure in a radiation sensitive medium, and the shape of the mask structure deviates from the shape of the target structure so that the target structure can be generated in the radiation sensitive medium through illumination of the mask through variable illumination in the mask plane, in particular through an interference pattern, generated through the above mentioned illumination device. In some embodiments, the disclosure provides a microlithography projection exposure system with a mask plane for disposing a mask, a wafer plane for disposing an object to be exposed, in particular a wafer, an illumination device of the above mentioned type for generating an interference pattern in the mask plane, and projection optics for imaging at least a portion of the interference pattern from the mask plane into the wafer plane.  
         [0011]     In certain embodiments, the disclosure provides a method for lithographic exposure of an object through a microlithography projection exposure system. The microlithography projection exposure system includes a mask plane, a wafer plane, an illumination device for illuminating the mask plane with electromagnetic radiation, and projection optics for imaging an object structure from the mask plane into the wafer plane. The method includes disposing the object in the wafer plane, generating an interference pattern in the mask plane through the illumination device and imaging at least a portion of the interference pattern onto the object through the projection optics.  
         [0012]     Put differently, in certain embodiments, an illumination device for a microlithography projection exposure system is provided, through which a structured exposure or an exposure with varied illumination in the mask plane can be created in the mask plane through interference. The generated interference pattern can be formed in particular as a standing wave. Contrary to many instances of interference lithography, no interference pattern is created in the wafer plane, but created in the mask plane of the microlithography projection exposure system.  
         [0013]     When a mask is disposed in the mask plane, portions of the interference pattern can be blanked out through respective mask structures. Compared to the structural dimension of the interference pattern, the desired properties for sizing of the mask structures of such mask are relatively less pronounced. A relatively “coarse” mask is thus irradiated with a very finely structured illumination. The tolerances of the mask structures can thus be increased. Also, the mask structures can be possibly sized larger. Through projection optics, the portions of the interference pattern, which are passed through by the mask, can be imaged into the wafer plane. The desired properties for the mask precision can therefore be reduced. Furthermore, it is also possible to operate the microlithography projection exposure system without a mask, and to image the structures of the interference patterns from the mask plane directly into the wafer plane. In any case, small size interference structures are used to generate an image with small structures in the wafer plane.  
         [0014]     This can reduce the efforts associated with creating a mask. Possibly, it can even be omitted to provide an optical reduction through projection optics when imaging the structures from the mask plane into the wafer plane, whereby the manufacturing cost of the projection optics can be reduced. Through the possibility to additionally dispose a mask in the mask plane, and to possibly modify the interference pattern, generated in the mask plane accordingly, a multitude of structures can be generated on the object or the wafer.  
         [0015]     In some embodiments, the illumination device is configured to generate at least two single waves, which are coherent to each other, and to generate the interference pattern through superposition of the single waves in the mask plane. The two planar single waves can interfere in the mask plane. This can be achieved, for example, by irradiating into the mask the single waves at an acute angle relative to each other. The two single waves can form identical angles in the mask plane with the optical axis of the projection optics. The single waves can thus symmetrically impact the mask plane.  
         [0016]     In certain embodiments, the illumination optics include a single wave generation element (e.g., a beam splitter) and at least one reflection element. During the operation of the illumination device the single wave generation element splits the electromagnetic radiation into the two single waves and the reflection element redirects one of the two single waves, so that the two single waves generate the interference pattern through superposition in the mask plane. It can be advantageous, when the single wave generation element is provided as a beam splitter, through which the arriving electromagnetic radiation is split into two single waves with different propagation directions. Through the at least one reflection element, the single waves are then joined again in the mask plane. Alternatively, the single wave generation element can also be provided in the form of a beam expansion element and can serve the purpose of expanding the electromagnetic radiation generated by the radiation source into a beam with expanded diameter. This beam with expanded diameter then includes single waves. The reflection element in this case is used for redirecting a portion of the expanded beam, this means, one of its single waves, so that it interferes with another portion of the expanded beam, this means, with another single wave of the beam in the mask plane. For this purpose, for example, also a so-called corner-cube arrangement can be selected, in which the reflection element is disposed at a side surface of the corner-cube, and the mask plane is disposed at another side surface of the corner-cube.  
         [0017]     Furthermore, it can be advantageous, when the illumination device is configured to displace the interference pattern through changing the relative phase of the single waves of the interference pattern from a first position into a second position in the mask plane. Thus, the interference pattern is displaced transversal to the optical axis of projection optics of an associated microlithography projection exposure system. In some embodiments, the illumination system for multiple exposure of at least one portion of an object through the microlithography projection exposure system is configured to generate the interference pattern in the first position during a first exposure and to generate the interference pattern in the second position during a second exposure. Furthermore, it can be advantageous for the interference pattern to include periodically occurring intensity maxima, and for the first position of the interference pattern to be displaced relative to the second position of the interference pattern by at least one quarter period (e.g., by a half period of the interference pattern in the mask plane). Through displacing the interference pattern through changing the relative phase of the single waves, it is possible to overcome the limited resolution of the projection optics, defined by the wave length of the electromagnetic radiation, through multiple exposures of an object or a wafer. After a first exposure of the object with the interference pattern in the first position, thereafter a second exposure of the interference pattern in the second position can be performed. Thus, structures are written between structures generated already in the first exposure. It is also possible to perform a multiple exposure of the object through at least one mask. Thus, illuminated structures of the mask are imaged onto the object in a first exposure through the interference pattern disposed in the first position and in the second exposure structures of the mask illuminated through the interference pattern disposed in the second position are projected onto the object.  
         [0018]     Furthermore, it can be advantageous, when the reflection element is movable, so that the relative phase of the single waves can be changed through moving the reflection element, wherein in particular, the moving direction in assembled state of the illumination device is aligned in parallel with the mask plane. In particular, the movement direction of the reflection element is aligned transversal to the optical axis of the projection optics of the associated microlithography projection exposure system. Such a movable reflection element allows a change of the relative phase of the single waves in a particularly precise and simple manner.  
         [0019]     Furthermore, it can be advantageous, when the single wave generation element is provided as a diffractive beam splitter, and movable to change the relative phase of the single waves, in particular movable transversal to the mask plane. Such a diffractive beam splitter can e.g. include a linear grid. The partial beams created by the grid in opposite diffraction order are then superimposed for generating the interference pattern. Such diffractive beam splitter can have the advantage that the irradiation strength pattern generated by the beam splitter is approximately independent of the wave length of the incoming light. Furthermore, the required relative phase movement can be accomplished in a simple manner through moving the beam splitter perpendicular to the optical axis.  
         [0020]     In certain embodiments, the illumination optics include at least one aperture to define an illuminated area in the mask plane, wherein the aperture is disposed in the beam path of the illumination optics in front of the single wave generation element. Furthermore, it can be advantageous when a lens is disposed between the aperture and the single wave generation element, through which the aperture is imaged into the plane of the single wave generation element. The at least one aperture can be provided as a variable aperture and it is used for adjusting the size of the image field generated on the object during the exposure step. Through imaging the aperture into the plane of the single wave generation element or the beam splitter through the lens the requirements with respect to the imaging precision of the lens are kept moderate. In case the aperture is imaged into the mask plane, it is complicated to image the interference pattern optically with little distortion.  
         [0021]     Furthermore, it can be advantageous, when the illumination device is configured to operate in a microlithography projection exposure system configured as a scanner, in which the exposed object, in particular, a wafer, is continually moved in a wafer plane of the microlithography projection exposure system during exposure, wherein the illumination device is furthermore configured to move the interference pattern during the illumination of the object in the mask plane, so that the image of the interference pattern in the wafer plane follows the motion of the object. In such a microlithography projection exposure system configured as a scanner, an illumination slot or a scanner slot is moved over the object for illuminating a field on the object. The field on the object is thus continually “written” through moving the illumination slot relative to the object. In conventional microlithography projection exposure systems, the mask is moved in the mask plane together with the movement of the object or the wafer, so that the image of the mask in the wafer plane follows the movement of the object. However, the interference pattern constitutes at least a portion of the structures to be imaged onto the object. Therefore, the interference pattern in the mask plane can be displaced so that its image in the wafer plane follows the movement of the object. Thus, the interference pattern is moved transversal to the optical axis of the projection optics. In some embodiments, the movement is performed through changing the relative phase of the single waves generating the interference pattern. Alternatively, also the illumination device can be moved relative to the projection optics of the microlithography projection exposure system.  
         [0022]     It can be advantageous for the interference pattern generated by the illumination device to have a stripe pattern with straight stripes, which are disposed periodically and wherein the rims of the stripe pattern extend along a straight line respectively, wherein the maximum deviation of the rims from the respective straight line amounts to less than one twentieth of the stripe period. Thereby, it can be assured that the distortion of the stripes of the interference pattern is so small that the single printed structures do not run into each other during a double exposure with interference patterns displaced by a half stripe period. At an exemplary illumination wave length of 193 nm and a stripe period of approximately 100 nm, the stripe distortion can thus only be in the nm range, in order for the stripe distortion of the stripes printed onto the object through the double exposure to amount to less than one tenth of the stripe distance from the object. For this purpose, the incoming planar wave in this section has to be flat and the single wave generation element and a possible additional optical component also have to comply with this specification.  
         [0023]     In order to increase the geometric variety of the structures, which can be imaged onto the object, it can be advantageous for the illumination device to be configured to generate the interference pattern in different orientations, in particular in orientations perpendicular to each other, in the mask plane. Thus, structures with different orientations can be imaged onto the object with several exposures or exposure steps. Furthermore, it is possible during the use of an additional mask to selectively illuminate differently oriented structures on the mask. Thus for example, only horizontally disposed lines can be illuminated on the mask in a first illumination step, and in a second illumination step, vertically oriented lines can be illuminated on the mask.  
         [0024]     In certain embodiments, the microlithography projection exposure system is configured to expose at least one portion of the object, in particular of the wafer, multiple times, wherein in a first exposure, the interference pattern is disposed in a first position in the mask plane, and in a second exposure, the interference pattern is disposed in a second position, offset relative to the first position in the mask plane. Thus, a double exposure of the object can be performed, through which the printable line frequency can be doubled, as described above. In order to move the interference pattern from the first position into the second position, as already described before also, for example, the relative phase of the single waves can be changed, or also the illumination device and the projection optical system can be moved relative to each other.  
         [0025]     Furthermore it can be advantageous for the interference pattern to provide periodically occurring intensity maxima and the first position of the interference pattern, is offset relative to the second position of the interference pattern by at least one quarter period (e.g., by half a period of the interference pattern in the mask plane). In this case, the structures generated through the interference pattern, disposed in the second position, can be printed on the object between the structures generated during a first exposure through the interference pattern disposed in the first position. Thus, the stripe frequency relative to a single exposure can be doubled.  
         [0026]     In certain embodiments, the projection optics are configured for operation with electromagnetic radiation in a certain wave length range, thereby including a resolution limit for imaging an even stripe pattern from the mask plane into the wafer plane, wherein a minimum distance between neighboring stripes of a stripe pattern, which can still be imaged by the projection optics, is defined through the resolution limit, and wherein the first position of the interference pattern is offset relative to the second position of the interference pattern by less than the minimum distance, in particular, by half the minimum distance in the mask plane. The resolution limit of projection optics is generally proportional to a ratio of the illumination wave length and the numerical aperture of the projection optics. The stripe frequency of a stripe pattern, which can be printed at the resolution limit onto the object with projection optics, can be doubled, when the first position of the interference pattern is offset relative to the second position by less than the minimum distance (e.g., by half the minimum distance in the mask plane). In this case, it is possible, to project a stripe pattern with a still printable stripe distance onto the object, through the first exposure, and thereafter project a stripe pattern with a similar stripe period through the second exposure, between the stripe pattern printed through the first exposure.  
         [0027]     Furthermore, it can be advantageous for the projection optical system to have an optical axis, and for the illumination device and the projection optics to be movable relative to each other in a direction transverse to the optical axis. Thus in particular the interference pattern in the mask plane can be moved from the first position into the second position for performing a double exposure. Furthermore, the movable support can be used to move the interference pattern in the mask plane during the scanner motion, when the microlithography projection exposure system is configured as a scanner, as will be subsequently described in more detail.  
         [0028]     Furthermore, it can be advantageous for the microlithography projection exposure system to be configured as a scanner and to have a moving platform for the object, which will subsequently be designated as wafer stage, for continuous movement of the object in the wafer plane during exposure, wherein the microlithography projection exposure system is furthermore configured to move the interference pattern in the mask plane during the exposure of the object, so that the image of the interference pattern in the wafer plane follows the movement of the object. As already described previously, the movement of the interference pattern can be performed through changing the relative phase of the single waves generating the interference pattern, or also through a movement of the illumination device and the projection optics relative to each other. This function of a scanner has already been described previously. The interference pattern is continuously moved according to this embodiment during the exposure in the mask plane, so that its projection in the wafer plane during the exposure of a field on the object or the wafer is moved with the movement of the wafer stage.  
         [0029]     Furthermore, it can be advantageous, when the microlithography projection exposure system has a moving platform for the mask, which is subsequently designated as reticle stage for continuous movement of the mask in the mask plane with a movement coupled to the movement of the wafer stage, wherein the illumination device is configured to move the interference pattern in the mask plane synchronous with the movement of the reticle stage, while the object is exposed. The reticle stage moves depending on the movement of the wafer stage, and on the reduction factor of the projection optics (e.g., opposite to the movement direction of the wafer stage). At a reduction factor of e.g. four, the reticle stage moves four times as fast as the wafer stage. In any case, the movement of the reticle stage is adjusted to the movement of the wafer stage, so that the image of a mask mounted onto the reticle stage, projected into the wafer plane, moves with an object located on the wafer stage. Since at least a portion of the interference pattern characterizes a structure to be imaged onto the object, it can be advantageous for the interference pattern to be moved with the reticle stage during exposure. Alternatively, the microlithography projection exposure system can be configured as a stepper in which the exposure of a field on a wafer is performed in a static manner. In this case, a movement of the interference pattern in the mask plane during an exposure of the object is not required.  
         [0030]     Furthermore, it can be advantageous for the microlithography projection exposure system to include a mask disposed in the mask plane with at least two mask structures offset from each other. Optionally, the two mask structures are provided line shaped and extend in parallel to each other. One of the two mask structures can be illuminated through the interference pattern, while the other mask structure thereby remains not illuminated. Furthermore it can be advantageous, when the distance between the two mask structures is adjusted to the interference pattern, so that in the first position of the interference pattern an intensity maximum of the interference pattern falls onto the first mask structure and an intensity minimum of the interference pattern falls onto a second mask structure and in the second position of the interference pattern, the intensity maximum falls onto the second mask structure, and the intensity minimum falls onto the first mask structure. Thus, as already described above, only the first mask structure can be imaged through a first exposure of an object, and during a second exposure of an object, the second mask structure can be printed at an offset from the first mask structure, which cannot be achieved through a single exposure.  
         [0031]     In some embodiments, an object, in particular a wafer, is disposed in the wafer plane, which is coated with a two photon resist. Such two photon resist includes molecules, whose energy levels are adapted to be excited through the absorption of two irradiated photons of the illumination wave length. When imaging an interference pattern with an intensity distribution in sin 2 (x)-form, depending on a coordinate x in the wafer plane, the radiation intensity absorbed by the two photon resist has a sin 4 (x)-, or a cos 4 (x)-distribution. To the contrary, the radiation intensity absorbed by a conventional one photon resist has a sin 2 (x)- or cos 2 (x)-distribution. When using a one photon resist and a double exposure of the object with an interference pattern in sin 2 (x)-distribution moved by half a period, the absorbed intensity adds up to a value, which is constant along the location coordinate. In this case, a sin 2 (x)-intensity distribution is absorbed in the resist during a first exposure, and during a second exposure a cos 2 (x)-intensity distribution is absorbed in the resist. The resulting total intensity distribution does not include a modulation. To the contrary, when using a two photon resist, e.g. a sin 4 (x)-shaped intensity distribution is absorbed during the first exposure, and during the second exposure a cos 4 (x)-shaped intensity distribution is absorbed. The entire intensity absorbed by two photon resist thus has the distribution sin 4 (x)+cos 4 (x). This total intensity distribution is not constant over the location, but it has modulations with twice the spatial frequency of a single interference pattern in the wafer plane. When the resistive sensitivity is adapted to the irradiation dosage accordingly, thus wafer structures with the described double spatial frequency can be generated without an additional development step between the two exposures. In an alternative use of a one photon resist it can be advantageous to perform a chemical development after the first exposure, and to perform the second exposure thereafter through the offset interference pattern.  
         [0032]     In some embodiments of the method, described above, at least one portion of the object is exposed multiple times by generating the interference pattern in a first position in the mask plane through a first exposure, and at least partially imaging it onto the object, and through generating the interference pattern in a second position, offset relative to the first position in the mask plane, in a second exposure, and at least partially imaging it onto the object. Furthermore, it can be advantageous to coat the object with a radiation sensitive medium, in particular a resist, before exposures, and to chemically develop the radiation sensitive medium between the first exposure and the second exposure. In a chemical development of the radiation sensitive medium, portions of the medium are removed, which were either sufficiently irradiated during the preceding exposure, or alternatively, portions of the medium can also be removed, which were not sufficiently irradiated during the preceding exposure. In the present case, it can be advantageous to remove the exposed resist components after the first exposure to perform a second exposure with accordingly moved interference pattern thereafter, and to perform a second development step thereafter.  
         [0033]     Furthermore, it can be advantageous, to coat the object, or the wafer before the exposures with a two photon resist. As already previously described, in this case, a chemical development step between two exposures can be omitted, whereby the throughput of the microlithography projection exposure system can be increased.  
         [0034]     Furthermore, it can be advantageous for the object to be moved continuously in the wafer plane while imaging at least a portion of the interference pattern onto the object, thus during the exposure of the object, to move the image of the interference pattern in the mask plane simultaneously, so that the image of the interference pattern in the wafer plane follows the motion of the object.  
         [0035]     The features described with respect to the previously described embodiments of the illumination device, or the microlithography projection exposure system can be transferred accordingly to the method and vice versa. The resulting embodiments of the method resulting there from shall be explicitly included herein. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0036]     Microlithography projection exposure systems are described with reference to the appended schematic drawings, showing in:  
         [0037]      FIG. 1  is a schematic cut view of a microlithography projection exposure system;  
         [0038]      FIG. 2  is a schematic side view for visualizing the principle for generating an interference pattern in the mask plane of the microlithography projection exposure system;  
         [0039]      FIG. 3  is an illustration of a dissection of a target structure, to be generated on the wafer, into single partial structures, which can be generated through single exposures;  
         [0040]      FIG. 4  is a visualization of the generation of the single partial structures in the mask plane through structured mask illumination;  
         [0041]      FIG. 5  is an illustration of the microlithography projection exposure system in the form of a scanner;  
         [0042]      FIG. 6  is an illustration of a portion of the illumination optics of a microlithography projection exposure system;  
         [0043]      FIG. 7  is an illustration of a single wave generation element configured as a diffractive beam splitter;  
         [0044]      FIG. 8  is an illustration of an intensity distribution generated through the microlithography projection exposure system in a two photon resist through double exposure. 
     
    
     DETAILED DESCRIPTION  
       [0045]      FIG. 1  shows a microlithography projection exposure system  10  that includes an illumination device  12  and projection optics  18  for imaging structures from a mask plane  16  into a wafer plane  22 . The illumination device  12  is used for generating an interference pattern in the mask plane  16 . Furthermore, the microlithography projection exposure system  10  includes a reticle stage, through which a mask  14  can be disposed in the mask plane  16 , when required. Furthermore, a wafer stage for disposing an object shaped as a wafer  20  is provided in the wafer plane  22 . The illumination device  12  includes a radiation source  24  in the form of a laser for generating electromagnetic radiation  25 , e.g. UV radiation, in the 193 nm wave length range.  
         [0046]     The electromagnetic radiation  25  propagates along an optical axis  27  of the microlithography projection exposure system  10 . The optical axis  27  coincides with the optical axis of the projection optics  18 . After leaving the radiation source  24 , the electromagnetic radiation  25  enters into illumination optics  26 . In the illumination optics  26 , an electromagnetic radiation  25  initially passes through a spatial filter  28 , including a focusing lens  30 , and a pinhole or a hole aperture  32 . Thereafter, the radiation  25  is directed from a collimator  34  to an aperture  36 . The aperture  36  can also be provided as so-called rema aperture and can have several aperture elements. A field area is defined through the aperture  36 , which is illuminated through the electromagnetic radiation  25  in the mask plane  16 .  
         [0047]     An imaging objective  38  follows after the aperture  36  in the beam path, wherein the imaging lens can be provided as so-called rema objective. The imaging objective  38  images the aperture  36  onto a single wave generation element  40 . The single wave generation element  40  is provided in the form of a beam splitter and provided as a diffractive beam splitter in the shown embodiment. The beam splitter can alternatively also be provided as a conventional beam splitter.  
         [0048]     In  FIG. 7 , the principle of such diffractive beam splitter is illustrated. The beam splitter includes a linear grating. The incoming electromagnetic radiation  25  is split through the grating into different refractive orders. Thus, the resulting refractive orders +1 and −1 are subsequently designated as single waves  42   a  and  42   b , and are processed further in the illumination optics  26 . Through the translatoric movement of the beam splitter  40  along a translatoric direction  43  transversal to the optical axis  27 , the relative phase difference between the single waves  42   a  and  42   b  can be moved. The single waves  42   a  and  42   b  impact, as furthermore shown in  FIG. 1 , the reflection elements  44   a  or  44   b  formed as mirrors. The reflection elements  44   a  and  44   b  are adjusted so that the single waves  42   a  and  42   b  superimpose in the mask plane  16 , forming an interference pattern  46  in the shape of a standing wave. Optionally, as described above, furthermore a mask  14  can be disposed in the mask plane  16 . The interference pattern  46 , or portions of the interference pattern  46 , which are passed through by the mask  14 , are imaged onto the wafer  20  by the projection optics.  
         [0049]     In  FIG. 2 , the method of wafer exposure using an additional mask  14  is illustrated in more detail. The interference pattern  46  formed as a standing wave is generated on the mask  14 . The mask  14  includes radiation permeable mask structures  49 , which are shown in  FIG. 2  with cross hatch and radiation permeable mask structures  50 . In the disposition of the interference pattern  46 , relative to the mask  14 , shown in  FIG. 2 , a mean intensity maximum  48   a  is passed through the mask  14 , while both adjacent intensity maxima  48   a  of the interference pattern  46  are blocked by the mask  14 .  
         [0050]     In  FIG. 3 , a target structural pattern  60  is depicted, which is to be generated on the wafer  20 . The single partial structures  61  included therein, either include horizontal, or vertical orientation. The target structure pattern  60  is to be understood as a fictitious mask and includes the structures to be imaged onto the wafer  20  with dimensions adapted to the mask plane. The target structural pattern  60  is thus enlarged relative to the pattern generated in the wafer plane  22  by the inverse imaging scale β −1  of the projection optics  18 . The minimum distance between the structures of the target structural pattern  60  is calculated from half of the multiple of β −1  and the stripe distance printable in the wafer plane  22  through the projection optics  18 . This means, the distance between the single partial structures  61  in the target structural pattern  60  is half the size, which can be imaged by the projection optics  18 . The target structural pattern  60  is initially broken down into two first partial structural patterns  62   a  and  62   b , one of which has all the horizontal partial structures  61  of the structural target pattern  60 , and the other one has all the vertical partial structures  61  of the structural target pattern  60 . Thereafter, the two first target structural patterns  62   a  and  62   b  are divided into two additional second partial structural patterns  64   a  and  64   b , or  64   c  and  64   d  respectively. The second partial structural patterns  64   a ,  64   b ,  64   c , and  64   d , each have a doubled minimum distance, compared to the first partial structural patterns  62   a  and  62   b . Thus, each of the second partial structural patterns  64   a  and  64   b  includes a set of partial structures  61 , having at least the distance from each other, which can be imaged via the projection optics  18 . The second partial structural patterns  64   a ,  64   b ,  64   c , and  64   d  thus form structural patterns, which can be imaged onto the wafer  20  through single exposures. Thus, the target structural pattern  60  can be generated on the wafer  20  in the respective dimension through multiple exposures.  
         [0051]      FIG. 4  again shows the second partial structural patterns  64   a  and  64   b  again, which are achieved through dividing the only horizontally oriented partial structures  61  in the form of the first partial structural pattern  62   a , including lines. The left section of  FIG. 4  shows a method of generating the partial structural pattern  64   a  through irradiation of a mask  14  with the interference pattern  46   a  disposed in a first position is shown. The mask  14  includes light impermeable mask structures  49  for blocking components of the irradiated light. The requirements with respect to the precision and tolerances of the mask  14  are reduced, compared to a conventional imaging of a mask with an even irradiation strength distribution. The mask  14  is thus designated as a simplified mask in  FIG. 4 .  
         [0052]     The partial structural pattern  64   a  is thus imaged onto the wafer in a first exposure with the interference pattern  46  disposed in a first position  46   a . Thereafter, the interference pattern  46  is moved into a second position  46   b . In the second position  46   b  the stripes  47  of the interference pattern  46  are moved relative to a first position  46   a  by a half stripe period  47   a  of the interference pattern  46 . For generating the partial structural pattern  64   b  in the illustrated case, a second mask  14  is exposed with the interference pattern  46   b . But it is also possible to perform the second exposure with the interference pattern  46  moved into the second position  46   b  in connection with the same mask, as in the first exposure. In further exposure steps, then also the partial structural patterns  64   c  and  64   d  can be imaged onto the wafer  20 . For this purpose the interference pattern  46  is rotated by 90°, and also projected into two different positions in the mask plane  16 .  
         [0053]     In  FIG. 5 , the operation of a microlithography projection exposure system  10  in the form of a scanner is shown. In a scanner, the wafer  20  is moved on a wafer stage during the exposure in a scan movement direction  54  transversal to the optical axis  27  of the projection optics  18 . Simultaneously, the mask  14  is moved through a reticle stage in a scan movement direction  52  opposite to the scan movement direction  54  of the wafer. Below the mask plane  14  there is a so-called scanner slot, disposed in a fixed location. During the exposure only a section of the mask  14  is imaged through the scanner slot  58  onto the wafer  20 . The interference pattern  46  is moved during the exposure of the wafer  20  synchronously with the reticle stage, so that the interference pattern  46  does not move with respect to the mask  14 , when a mask  14  is used.  
         [0054]      FIG. 6  shows an embodiment of the illumination optical system  26  of the illumination device  12 , which is configured to move the interference pattern  46  continuously during exposure. For this purpose, the reflection element  44   a  is moved in a movement direction  56 , parallel to the scan movement direction  52  of the mask. This causes a movement of the interference pattern  14  in a movement direction  57 , aligned in parallel to the scan movement direction  52 . Other alternatives for moving the interference pattern  56  during the exposure include a relative translatoric movement between the illumination optics  26  and the projection optics  18 .  
         [0055]     Optionally, the wafer  20  is coated with a two photon resist. Such a two photon resist includes molecules, whose energy level is adapted to be excited through the absorption of two irradiated photons of the illumination wave length.  FIG. 8  shows a distribution of a radiation intensity absorbed through the double exposure in a two photon resist. Only the absorbed photons are chemically effective in the resist. The intensity distribution absorbed through the interference pattern  46  in the first position  46   a  in the two photon resist follows the function cos 4 (x) along a location coordinate in the mask plane. The intensity distribution absorbed through the interference pattern  46  in the second position  46   b  in the two photon resist follows the function sin 4 (x). The superposition of the two intensity distributions follows the function sin 4 (x)+cos 4 (x) and includes a modulation in the direction of the local coordinate, which is sufficient to structure the resist. When using such two photon resist, a chemical development step between a first exposure and a second exposure of the wafer is not necessary. In an alternative embodiment, the wafer is coated with a two photon resist. In this case, the resist is chemically developed between a first exposure and a second exposure.  
         [0056]     Other embodiments are in the claims.