Abstract:
The disclosure relates to an illumination system for EUV lithography, as well as related elements, systems and methods. In some embodiments, an illumination system includes a first optical element and a second optical element. The first optical element can include a plurality of first facet elements configured so that, when impinged by respective partial beams of radiation, the plurality of first facet elements produce secondary light sources. The second optical element can include a second optical element including a plurality of second facet elements. Each of the plurality of second facet elements can be assigned to at least one of the plurality of first facet elements. The plurality of second facet elements can be configured to be impinged by the radiation via the first optical element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of international application PCT/EP2007/003609, filed Apr. 25, 2007, which is a continuation of German Application No. 10 2006 020 734.3, filed May 4, 2006. International application PCT/EP2007/003609 is incorporated by reference herein in its entirety. 
     
    
     FIELD 
       [0002]    The disclosure relates to an illumination system for EUV lithography, as well as related elements, systems and methods. 
       BACKGROUND 
       [0003]    Illumination systems for EUV lithography are known. In some illumination systems, different illumination settings can be produced using displaceable field facet elements. In certain instances, with each of the different illumination settings, the object surface is illuminated with a different distribution of illumination angles. 
       SUMMARY 
       [0004]    In some embodiments, the disclosure provides an illumination system, such as an illumination system for EUV lithography, with which it is possible to change between various illumination settings even when using an optical element with relatively few facet elements. 
         [0005]    In certain embodiments, the disclosure provides an illumination system in which at least two of the first facet elements are configured and oriented such that they are imaged by the optical arrangement in one and the same of at least two partial fields of the illumination field which make up the entire illumination field. An intersection between the partial fields, if such an intersection exists, is always being smaller than each of the partial fields contributing to the intersection. 
         [0006]    It has been found that it is not absolutely necessary to reproduce all the first facet elements in one and the same image area, but that it is possible to configure the illumination field as an assembly of individual partial fields which together make up the complete illumination field. In so doing, the entire illumination field is not illuminated via any of the partial fields. This basic approach allows the second facet elements to be used in a more flexible manner. These second facet elements may be irradiated by a plurality of first facet elements which are at a spatial distance from one another. The partial beams of EUV radiation emanating from the various first facet elements which act on one and the same second facet element are then imaged in the various partial fields. This makes it possible to image the partial beams of EUV radiation, associated with a plurality of first facet elements, by one and the same second facet element. This flexibility allows a second optical element with a relatively small number of second facet elements to be used to produce various illumination settings, while still allowing the production of an illumination setting, using a second optical element of this type, in which illumination setting a 1:1 assignment of the first and second facet elements is provided. Different illumination settings may therefore be produced without a loss of light and without exchanging optical components. A conventional setting, such as a homogeneous illumination of the second optical element may be produced while tightly filling the second optical element. Dividing the illumination field into at least two partial fields makes it possible to preset an EUV intensity profile in the displacement direction of the object. It is possible, for example, to set as the default a Gaussian, Lorenzian, Trapezoidal or other profile. By displacing the object presetting the object surface perpendicularly to the partial field division of the illumination field, it is possible to ensure that the object is illuminated over all partial fields, so that each partial field plays a part in illuminating a predetermined object point. This sequential illumination of object points over the partial fields may be utilised to achieve, for example, a desired activation of a sensitive wafer layer which is an example of an object surface to be illuminated. 
         [0007]    Displaceable first facet elements, which are movable between various setting positions by actuators associated in each case with the selected first facet elements, and which, in each setting position, deflect the partial beam associated in each case therewith to produce one of the predetermined illumination settings, lead to the possibility of automatically changing between various illumination settings. 
         [0008]    The number of partial fields, which corresponds to the maximum number of first facet elements which are able to act on the same second facet element, is at the same time the minimally desired number of partial fields. The result is a compact illumination field. Two, three or more first facet elements may be allocated to a second facet element to act thereon. 
         [0009]    A division of the illumination field into at least two adjacent partial fields in the form of partial field strips, which can have the same surface area, allows a comparatively simply constructed optical arrangement. The partial fields can be adjacent to one another, such as in a scan direction of a projection illumination installation, with the illumination system being the component thereof. This can help ensure that a substrate to be illuminated, for example a wafer which is scanned by the illumination field, passes the at least two partial fields in succession. 
         [0010]    Curved first facet elements and/or partial fields of the illumination system can reduce the restrictions made on the optical arrangement. 
         [0011]    Arranging the facet elements such that at least one of the second facet elements may be acted on by precisely two adjacent first facet elements can reduce restrictions on the imaging, which can avoid large angles between the individual partial beams of the EUV radiation. 
         [0012]    Second facet elements, which may be displaced by an actuator associated in each case with the selected second facet elements for displacing the latter between various setting positions, increase the flexibility of the illumination system. 
         [0013]    An illumination system in which at least one of the group of first and second facet elements are configured as reflective elements can have low losses. 
         [0014]    An arrangement in which the number of the first facet elements is identical to the number of the second facet elements can allow a 1:1 assignment of the first facet elements to the second facet elements. 
         [0015]    Intersections between the partial fields of the illumination field which are less than 95% (e.g., less than 90%, less than 80%, less than 60%, less than 40%, less than 20%) of the smallest partial field contributing to the intersection, have proved to be advantageous for the practical realisation of the illumination system, such as for presetting a desired sequential illumination for the object surface. 
         [0016]    Various embodiments of the illumination system disclosed herein can provide similar advantages. 
         [0017]    In some embodiments, the disclosure provides a first as well as a second optical element for the illumination system. 
         [0018]    The advantages of these optical elements can be the same as those already mentioned above with reference to the illumination system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Embodiments of the disclosure are described in more detail hereinafter with reference to the drawings, in which: 
           [0020]      FIG. 1  schematically the beam path in an illumination system for EUV microlithography; 
           [0021]      FIG. 2  schematically components of the illumination system according to  FIG. 1  with the second facet elements illuminated to prepare a homogeneous, conventional illumination setting; 
           [0022]      FIG. 3  a view similar to that of  FIG. 2  with the second face elements illuminated to prepare an annular setting of a large diameter; 
           [0023]      FIG. 4  a view similar to that of  FIG. 2  with the second facet elements illuminated to prepare a homogeneous illumination setting with small illumination angles compared to the illumination setting of  FIG. 2 ; 
           [0024]      FIG. 5  a view similar to that of  FIG. 2  with second facet elements illuminated to prepare an illumination setting in the manner of an x-dipole; 
           [0025]      FIG. 6  a view similar to that of  FIG. 2  with the second facet elements illuminated to prepare an illumination setting in the manner of a y-dipole; 
           [0026]      FIG. 7  a view similar to that of  FIG. 3  having curved first facet elements; 
           [0027]      FIG. 8  a view similar to that of  FIG. 2  having a different magnification; and 
           [0028]      FIG. 9  a view similar to that of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION  
       [0029]      FIG. 1  shows an illumination system  1  for illuminating a predetermined illumination field  2  of an object surface  3  with extreme ultraviolet (EUV) radiation  4 . 
         [0030]    A plasma source can be used as the source  5  for the EUV radiation  4 . The wavelength of the EUV radiation is, for example, between 10 and 20 nm. 
         [0031]    A Cartesian coordinate system (x, y, z) is used in  FIGS. 1 and 2 , reference being made hereinafter to the coordinates (x, y, z). In  FIG. 1 , the x direction extends perpendicularly to the plane of projection, the y direction extends to the right-hand side and the z direction extends downwards. 
         [0032]    The EUV radiation emitted from the source  5  is initially collected by a collector  6  which reflects the EUV radiation like all the following steel guide components. The EUV radiation  4  emitted from the source  5  impinges on a first optical element  7 , also termed a field scanning element. The first optical element  7  is used to produce secondary light sources in the illumination system  1 . A reflecting surface of the first optical element  7 , on which the EUV radiation  4  impinges, is divided into a plurality of first facet elements, of which four first facet elements  8  to  11  are shown by way of example in  FIG. 1 . The latter are associated with partial beams  12  to  15  of the EUV radiation  4 . The first facet elements are rectangular, the extent thereof being substantially greater in the x direction than in the y direction. A typical aspect ratio of the first face elements  8  to  11  (x:y) is 20:1. 
         [0033]    Each of the first facet elements  8  to  11  may be tilted between various setting positions about axes parallel to the x direction and y direction. For this purpose, each of the first facet elements  8  to  11  is connected to an associated actuator. In  FIG. 1 , an exemplary actuator  16  is shown which, as indicated in dashed lines at  17 , is mechanically connected to the first facet element  11  for tilting the facet element  11  selectively about one of the two axes (x/y). The actuator  16  is connected to a central control device  19  via a control line  18 . The control device  19  is connected with all the other actuators associated with the first facet elements  8  to  11  and with the first facet elements which are not shown via corresponding control lines (not shown). 
         [0034]    Examples of arrangements of first facet elements are provided in  FIG. 7 to 14  of US 2003/0086524 A1, which is hereby incorporated by reference in its entirety. 
         [0035]    A second optical element  20 , also termed a pupil scanning element, is positioned at the location of the secondary light sources generated by the first optical element  7 , generally in an image plane of the source  5 . The EUV radiation  4  impinges the second optical element  20  via the first optical element  7 . The surface of the second optical element  20  impacted by the EUV radiation  4  is divided into a plurality of second facet elements, of which four facet elements  21  to  24  are shown by way of example in  FIG. 1 . The second facet elements  21  to  24  are each assigned to one of the first facet elements  8  to  11 , so that a respective secondary light source is generated at the location of the respectively charged second facet elements  21  to  24 . In the illustration according to  FIG. 1 , the second facet element  21  is assigned to the first facet element  8 , the second facet element  23  is assigned to the first facet element  9 , the second facet element  22  is assigned to the first facet element  10  and the second facet element  24  is assigned to the first facet element  11 . 
         [0036]    Like the first facet elements  8  to  11 , the second facet elements  21  to  24  and the other facet elements of the second optical element  20  which are not shown may be tilted via actuators about axes parallel to the x and y directions.  FIG. 1  schematically shows an exemplary actuator  25  which is associated with the second facet element  21  and which, as shown in dashed lines at  26 , is mechanically connected to the second facet element  21  to tilt the second facet element  21 . The actuator  25  is also in a signal connection with the control device  19  via a control line  18 . 
         [0037]    The first facet elements as well as the second facet elements are typically reflective elements. 
         [0038]    The second optical element  20  is part of an optical arrangement  27  which includes a plurality of optical components and forms an image of the first optical element  7  in a plane  30  predetermined by the object surface  3 . Two other reflecting elements  28 ,  29  for EUV radiation belong to the optical arrangement  27 . The reflecting element  28  is downstream of the second optical element  20  and reflects the EUV radiation at a small angle of incidence, for example an incidence angle of 30°. The reflecting element  29  is positioned in the subsequent beam path of EUV radiation  4  and reflects the EUV radiation by glancing incidence. 
         [0039]      FIG. 2  shows in a highly schematic manner allocation relations between the first and the second facet elements during image formation in the illumination field  2 . In this respect, the plane of projection of  FIG. 2  is parallel to the x-y plane, and this also applies to the planes of projection of the following  FIG. 3 to 9 . For illustration purposes, the components shown in  FIGS. 2 to 9  have been rotated into the x-y plane. These components may in practice also be oriented in a different way. Components in  FIGS. 2 to 9  which are the same as those which have already been described above with reference to  FIG. 1 , have the same reference numerals and will not be described again in detail. 
         [0040]      FIG. 2  shows the four facet elements  8  to  11  as representative of all the first facet elements of the first optical element  7 . Representative of all the second facet elements of the second optical element  20 , the second facet elements  21  to  24  are arranged as follows in  FIG. 2  for illustration purposes where the second facet elements  21  and  24  are components of an inner ring made of second facet elements. The second facet elements  22  and  23  are components of an outer ring of second facet elements. The overall round second optical element  20  has a plurality of such concentrically arranged rings of second facet elements. The second facet elements may be arranged in the manner of an evenly distributed x/y raster. FIG. 15 of US 2003/0086524 A1 provides an example of this. 
         [0041]    The allocation of the first facet elements  8  to  11  to the second facet elements  21  to  24  is such that it produces a homogeneous, conventional illumination setting. Together with other second facet elements which are not shown, all the second facet elements of the second optical element  20  (not only the illustrated second facet elements  21  to  24 , but also all other second facet elements of the associated raster) are illuminated by a respective first facet element. The second optical element  20  is thus illuminated homogeneously. 
         [0042]    The first facet elements  8  and  10  are configured and oriented in such a manner that they are imaged in a lower partial field  31  of the illumination field  2  by the optical arrangement  27  in the arrangement of  FIG. 2 . The first facet elements  9  and  11  are configured and oriented in such a manner that they are imaged in the upper partial field  32  of the illumination field  2  by the optical arrangement  27 . The two partial fields  31 ,  32  have the same surface area. The partial fields  31 ,  32  have the same aspect ratio as the first facet elements  8  to  11 . The two partial fields  31 ,  32  are slightly curved arcuately due to the imaging properties of the optical arrangement  27 . The two partial fields  31 ,  32  make up the complete illumination field  2 . In the idealised example shown in  FIG. 2 , the two partial fields  31 ,  32  are immediately adjacent to one another, without overlapping. In practice, the two partial fields  31 ,  32  can overlap in such a manner that the intersection of the two partial fields  31 ,  32  (the overlap region) is always smaller than the area of each partial field  31 , 32 . 
         [0043]    In the embodiment according to  FIG. 2 , the first facet elements  8  to  11  are imaged in the image plane  30  with a negative magnification. 
         [0044]    The division of the illumination field  2  into the two partial fields  31 ,  32  results in a portion of every second facet element being illuminated in each of the partial fields  31 ,  32 , so that in each partial field, illumination takes place at selected illumination angles of all illumination angles which are present in the illumination setting according to  FIG. 2 . Only a superpositioning of both partial fields  31 ,  32  results in the object surface  3  being illuminated at all illumination angles of the illumination setting. This superpositioning is carried out by displacing the object surface  3  in the y direction. This displacement may be carried out continuously or in steps, in which case a step length should not be greater than the y extent of a partial field. In this way, each point on the object surface  3 , which point passes through both partial fields  31 ,  32 , integrates the exposure effect of the EUV radiation radiated therein, so that following the passage, an illumination has been carried out from all illumination angles possible in the illumination setting. 
         [0045]      FIG. 3  shows another illumination setting which is possible with the arrangement of  FIG. 1 . This illumination setting is sometimes called a large annular setting. Starting from the situation according to  FIG. 2 , this large annular setting is set by tilting the first facet element  8  about its longitudinal axis parallel to the x direction and by tilting the second facet element  11  about its longitudinal axis parallel to the y direction. As a result of these tilting actions, the first facet element  8  now acts on the second facet element  23  and the first facet element  11  acts on the second facet element  22 . The second facet element  22  is thus associated with two first facet elements, more specifically the first facet elements  10  and  11 . Accordingly, the second facet element  23  is associated with the first facet elements  8  and  9 . The second facet elements  22 ,  23  are arranged in such a manner that in the illumination setting according to  FIG. 3 , they are thus acted on respectively by two different adjacent first facet elements, more specifically on the one hand the first facet elements  10  and  11  and on the other hand by the first facet element  8 ,  9 . The second facet elements  21 ,  24  are not acted on by the first optical element  7 . 
         [0046]    In  FIG. 3 , as in the setting according to  FIG. 2 , the lower partial field  31  is impacted by radiation from the first facet element  8  and  10 . The upper partial field  32  is impacted by radiation from the first facet elements  9  and  11 . 
         [0047]    It is not necessary to displace the second facet elements  22 ,  23  when changing the illumination settings between those shown in  FIGS. 2 and 3 . 
         [0048]    In the annular setting according to  FIG. 3 , the illumination angle varies between a minimum illumination angle different from zero and a maximum illumination angle. 
         [0049]      FIG. 4  shows another illumination setting which may be produced with the arrangement according to  FIGS. 1 and 2 . This is sometimes call a small conventional setting with a maximum illumination angle which is smaller than the minimum illumination angle of the annular setting according to  FIG. 3 . 
         [0050]    Compared to the setting of  FIG. 2 , in  FIG. 4  the two first facet elements  9  and  10  are tilted in the x and y direction respectively. The first facet element  9 , together with the first facet element  8  now acts on the second facet element  21 . The first facet element  10 , together with the first facet element  11 , now impacts on the second facet element  24 . The second facet elements  22 ,  23  are not acted on by the first optical element  7 . 
         [0051]    As in the setting according to  FIG. 2 , in  FIG. 4  the lower partial field  31  is impacted by radiation from the first facet elements  8  and  10 . The upper partial field  32  is impacted by radiation from the first facet elements  9  and  11 . 
         [0052]    It is unnecessary to displace the second facet elements  21  and  24  when changing between the illumination settings according to  FIGS. 2 and 4 . 
         [0053]      FIG. 5  shows another illumination setting which may be produced with the arrangement according to  FIGS. 1 and 2 . This is an illumination setting in the manner of what is sometimes called an x-dipole. In this case, illumination takes place in the x-z plane over a range of illumination angles which corresponds to the conventional setting according to  FIG. 2 . Perpendicularly (in the y-z plane), illumination takes place in a region of smaller illumination angles which may increase outwardly. 
         [0054]    In the setting according to  FIG. 5 , the beam path, starting from the first facet elements  10 ,  11 , corresponds to the beam path of the setting according to  FIG. 3 . The radiation emanating from the first facet elements  8  and  9  jointly impacts the second facet element  24 . The radiation emanating from the first facet element  8  is imaged in the lower partial field  31 , and the radiation emanating from the first facet element  9  is imaged in the upper partial field  32 . 
         [0055]    During the transition between the illumination settings according to  FIGS. 2 and 5 , the second facet element  24  can be tilted to ensure that the radiation emanating from the first facet elements  8  and  9  is in fact correctly imaged in the illumination field  2 . 
         [0056]      FIG. 6  shows another illumination setting which may be produced in the arrangement according to  FIGS. 1 and 2 . This illumination setting is sometimes called y-dipole mode. According to that carried out in connection with the setting of  FIG. 5 , the illumination angles in the illumination field are distributed in the setting of  FIG. 6  in such a manner that there is an angle distribution in the y-z plane as in the conventional setting according to  FIG. 2 , and there is an illumination angle distribution in the x-z plane which corresponds to that in the y-z plane of the setting according to  FIG. 5 . 
         [0057]    Compared to the setting of  FIG. 2 , to adjust the setting according to  FIG. 6 , the first facet elements  8 ,  10  and  11  should be tilted and also the second facet element  21  should be tilted so that the radiation emanating from the first facet elements  10 ,  11  is correctly imaged in the illumination field  2 . 
         [0058]    In the setting according to  FIG. 6 , the first facet elements  8 ,  9  jointly eradiate the second facet element  23  and the first facet elements  10 ,  11  jointly eradiate the second facet element  21 . 
         [0059]      FIG. 7  shows an illumination system like the illumination system of  FIGS. 1 and 2 , adjusted to an illumination setting like the setting of  FIG. 3 . Components corresponding to those which have already been described above with reference to  FIG. 1 to 6 , have the same reference numerals and will not be described again in detail. In contrast to the rectangular first facet elements of the embodiments according to  FIGS. 1 and 2 , the first optical element  7  of the arrangement according to  FIG. 7  has slightly arcuately curved first facet elements  8  to  11 . 
         [0060]    The curved first facet elements  8  to  11  are imaged in the curved partial fields  31 ,  32  by the optical arrangement  27 . 
         [0061]      FIGS. 8 and 9  show an illumination system. Components which correspond to those already described above with reference to  FIG. 1 to 7  have the same reference numerals and are not described again in more detail. 
         [0062]    In contrast to the embodiments according to  FIG. 1 to 7 , the optical arrangement  27  according to  FIGS. 8 and 9  has a positive magnification. The second optical element  20  as well other reflective elements like the elements  28  and  29  (not shown) belong to the optical arrangement  27 . In  FIGS. 8 and 9 , the first facet elements  9  and  11  are imaged in the lower partial field  31  of the illumination field  2 . The first facet elements  8  and  10  are imaged in the upper partial field  32 . 
         [0063]      FIG. 8  shows a conventional setting corresponding to that of  FIG. 2 . With respect to the guidance of the partial beams  12  to  15 , the first facet element  8  is associated with the second facet element  23 , the first facet element  9  is associated with the second facet element  21 , the first facet element  10  is associated with the second facet element  22  and the first facet element  11  is associated with the second facet element  24 . 
         [0064]      FIG. 9  shows another illumination setting produced by tilting the first facet elements  9  and  11 , so that a large annular setting like the setting of  FIG. 3  is produced. The second facet element  23  is jointly acted on by the two first facet elements  8  and  9 . The second facet element  22  is jointly acted on by the two first facet elements  10  and  11 . 
         [0065]    The illumination system  1  is part of a projection illumination installation for microlithography, with which an object having the object surface (e.g., a mask or a reticle) is imaged on a wafer to produce integrated components, for example microprocessors or memory chips. 
         [0066]    The first optical element  7  may be configured such that only specific first facet elements may be tilted by actuators, while other first facet elements are stationary. The second optical element  20  may also be equipped accordingly with tiltable and stationary second facet elements. It also possible to equip the second optical element  20  in general with stationary second facet elements, in other words, not to provide a tilting option there. 
         [0067]    In some embodiments, the number of first facet elements of the first optical element  7  is identical to the number of the second facet elements of the second optical element  20 . Alternatively, in certain embodiments, the number of facet elements of the second optical element  20  is greater or smaller than the number of the first facet elements of the first optical element. 
         [0068]    Although in embodiments described above a maximum of two first facet elements  8  to  11  are associated with the second facet elements  21  to  24 , it is possible for more than two first facet elements to be associated in each case with a second facet element. The minimum number of the partial fields constructing the illumination field can increase accordingly. 
         [0069]    In principle, it is possible also to configure at least individual components of the illumination system as transmissive components. 
         [0070]    Other embodiments are in the claims.