Abstract:
A projection illumination installation for EUV microlithography includes an EUV synchrotron light source for producing EUV used light. An object field is illuminated with the used light using illumination optics. The object field is mapped into an image field using projection optics. A scanning device is used to illuminate the object field by deflecting the used light in sync with a projection illumination period. The result is a projection illumination installation in which the output power from an EUV synchrotron light source can be used as efficiently as possible for EUV projection illumination.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of, and claims benefit under 35 USC §120 to, international application PCT/EP2008/067594, filed on Dec. 16, 2008, which claims benefit of German Application No. 10 2008 000 967.9, filed Apr. 3, 2008. International application PCT/EP2008/067594 is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     The disclosure relates to a projection illumination installation for EUV microlithography. 
     BACKGROUND 
     A projection illumination installation is known from U.S. Pat. No. 6,859,515 B2 and U.S. Pat. No. 5,439,781. A projection illumination installation for EUV microlithography is known from US 2007/0152171 A1. 
     SUMMARY 
     The disclosure provides a projection illumination installation configured so that the output power of an EUV light source can be used for EUV projection illumination in a highly efficient fashion. 
     In some embodiments, a projection illumination installation for EUV microlithography includes an EUV light source to produce EUV light. The installation also includes illumination optics to illuminate an object field with the EUV light, and projection optics to map the object field into an image field. The installation also includes a scanning device to illuminate the object field by deflecting the EUV light in sync with a projection illumination period. 
     It has been recognized that a scanning device for illuminating the field facet mirror in sync with the projection illumination provides an opportunity to use the illumination optics of the projection illumination installation without having to widen an often small divergence angle for the EUV light source, which is usually associated with losses. A numerical aperture for the illumination optics is filled using the scanning device not all at once but rather sequentially, for example via row-by-row scanning. It has been recognized that many EUV light sources usually have a repetition rate which is so high that such sequential scan illumination of the illumination optics does not result in restrictions for the mapping quality. A projection illumination period has elapsed when the entire object field has been illuminated with a prescribed illumination intensity. 
     In some embodiments, facet mirrors are particularly suitable for providing a defined object field illumination. The facet mirror can be illuminated by the scanning device. Alternatively, the facet mirror may also be part of the scanning device itself. 
     A synchrotron radiation based EUV light source, and particularly a free electron laser (FEL), can provide particularly high beam brilliance. As an alternative to an FEL, it is also possible to use a wiggler or an undulator. The synchrotron radiation based EUV light sources usually have a small divergence angle which can advantageously be widened by the scanning device. Synchrotron radiation based EUV light sources usually have a very high repetition rate, which allows advantageous combination with the scanning device according to the disclosure. 
     By way of example, scanning devices are known in connection with the development of laser RGB displays or laser television sets and have proven themselves for beam deflection even when the highest deflection frequencies are involved. Examples of scanning devices include: at least one activation tiltable mirror that includes a single mirror face; at least one polygonal mirror; and at least one mirror configured to move translationally back and forth, where the at least one mirror has at least two reflection faces configured to tilt relative to one another. 
     An intensity modulator can be used to produce a specific intensity variation during the scanning operation. This can be used particularly for correction purposes. 
     An intensity modulator can be used for specifically influencing or correcting an intensity distribution for the EUV light on the object field. If all the field facets of the field facet mirror have the EUV the light applied with the same intensity distribution, this results in a corresponding intensity distribution over the object field. 
     An intensity modulator can be used for specifically influencing or correcting an illumination angle distribution over the object field. In this case, the synchronization may be such that the pupil facets always have the same intensity distribution applied for the synchronized sweep over the pupil facet mirror. This makes it possible to obtain a prescribed illumination angle distribution which is constant over time. Alternatively, the intensity distribution over the pupil facet mirror can be altered for successive scans. This allows the illumination angle distribution to be varied over time. 
     A scanning device may have a similar action to an intensity modulator. Such a scanning device makes it possible to obtain loss-free intensity modulation of the facets of the field facet mirror and/or of the pupil facet mirror, in particular. Regions which are swept over at a relatively fast deflection speed are subject to a lower application of intensity in this case than regions which are swept over at a relatively low deflection speed. 
     In some embodiments, efficient utilization of the entire average power of an EUV synchrotron light source can be achieved. 
     A field facet mirror can be actuated with little complexity via the scanning device. The row and column arrangement does not necessarily require the individual field facets to be rectangular. Another edging shape for the field facets is also possible, for example arcuate edging, particularly part-ring shaped edging. 
     A field shaping mirror, which may be in the form of a single facet, can dispense with a multifaceted design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure are explained in more detail below with reference to the drawing, in which: 
         FIG. 1  shows, schematically in relation to illumination optics, a meridional section through a projection illumination installation for microlithography; 
         FIG. 2  shows a highly schematic illustration of components of a scanning device arranged upstream of the illumination optics in the projection illumination installation; 
         FIG. 3  schematically shows a design for a field facet array of a field facet mirror in the illumination optics; and 
         FIG. 4  schematically shows illumination of a plurality of pupil facet arrays of pupil facet mirrors in the illumination optics with one and the same field facet mirror. 
     
    
    
     DETAILED DESCRIPTION 
     A projection illumination installation  1  for microlithography is used to produce a microstructured or nanostructured electronic semiconductor component. A light source  2  emits EUV radiation in the wavelength range between 5 nm and 30 nm, for example. The light source  2  is in the form of a free electron laser (FEL). This is a synchrotron radiation source, which produces coherent radiation at very high brilliance. Such FELs are known to a person skilled in the art from Pagani et al., Nucl. Instr. &amp; Methods A463 (2001), pp 9 and from Ackermann et al., Nature photonics Vol. 1 (2007), 336 pp. Customization of large FEL installations of this kind to the desired properties of EUV lithography has been presented to a person skilled in the art at the EUVL Source Workshop for the EUVL Symposium of the international organization Sematech in October 2006 (cf. “extreme ultraviolet lithography” (EUVL) Symposium. International. 5 CH 2006. (4 VOLS), published by Curran Associates, INC. in April 2007), particularly in the professional articles by Saldin et al., Hajima et al. and Goldstein. The proceedings of this EUVL source workshop are published at the Internet address http://www.sematech.org/meetings/archives/litho/euv/7855/. A compact FEL, which is suitable as light source  2 , is described in US 2007/0152171 A1. Further considerations regarding the design of an FEL as a light source  2  can be found by a person skilled in the art in the professional article “Design considerations for table-top, laser-based VUV and X-ray free electron lasers” by F. Grüner et al., which is published at the Internet address http://arxiv.org/PS_cache/physics/pdf/0612/0612125v1.pdf. The full scope of the content of the aforementioned prior art will be part of this application. 
     The light source  2  has an average power of 2.5 kW. The pulse frequency of the light source  2  is 30 MHz. Every single radiation pulse then carries energy of 83 μJ. With a radiation pulse length of 100 fs, this corresponds to a radiation pulse power of 833 MW. 
     For the illumination and mapping within the projection illumination installation  1 , a used radiation bundle  3  is used. The used radiation bundle  3  is illuminated within an aperture angle  4 , which matches illumination optics  5  of the projection illumination installation  1 , using a scanning device  6  which is yet to be described. Starting from the light source  2 , the used radiation bundle  3  has a divergence which is less than 5 mrad. The scanning device  6  is arranged in an intermediate focal plane  6   a  of the illumination optics  5 . After the scanning device  6 , the used radiation bundle  3  first of all impinges upon a field facet mirror  7 . Details regarding the scanning device  6  will be explained below with reference to  FIG. 2 . 
     The used radiation bundle  3  has, in particular, a divergence which is less than 2 mrad, such as less than 1 mrad. The spot size of the used radiation bundle on the field facet mirror  7  is approximately 4 mm. 
       FIG. 3  shows an example of a facet arrangement, a field facet array, for field facets  8  of the field facet mirror  7 . Only some of the field facets  8  that are actually present are shown. The field facet array of the field facet mirror  7  has 6 columns and 75 rows. The field facets  8  have a rectangular shape. Other shapes for the field facets  8  are also possible, for example an arcuate shape or an annular or partially annular geometry. Overall, the field facet mirror  7  has 450 field facets  8 . Each field facet  8  has an extent of 50 mm in the direction which is horizontal in  FIG. 3  and 4 mm in the direction which is vertical in  FIG. 3 . The entire field facet array accordingly has an extent of 300 mm×300 mm. The field facets  8  are not shown to scale in  FIG. 3 . 
     Following reflection at the field facet mirror  7 , the used radiation bundle  3 , which is split into pencil beams which are associated with the individual field facets  8 , impinges on a pupil facet mirror  9 . Pupil facets of the pupil facet mirror  9 —which are not shown in FIG.  1 —are round. Each pencil beam of the used radiation bundle  3  that is reflected by one of the field facets  8  has an associated one of these pupil facets, so that a respective struck facet pair prescribes, with one of the field facets  8  and one of the pupil facets, a beam guidance channel for the associated pencil beam of the used radiation bundle  3 . The channel-by-channel association of the pupil facets with the field facets  8  is made on the basis of desired illumination by the projection illumination installation  1 . For the purpose of actuating respective prescribed pupil facets, the field facet mirrors  8  are each individually tilted. 
     The pupil facet mirror  9  and downstream transmission optics  13 , including three EUV mirrors  10 ,  11 ,  12 , are used to map the field facets  8  into an object field  14  in an object plane  15  of projection optics  16  in the projection illumination installation  1 . The EUV mirror  12  is in the form of a grazing incidence mirror. 
     In an embodiment of the illumination optics  5  which is not shown, particularly when an entry pupil of the projection optics  16  is in a suitable position, it is also possible to dispense with the mirrors  10 ,  11  and  12 , which results in a corresponding transmission increase in the projection illumination installation for the used radiation bundle  3 . 
     The long side of the field facets  8  is at right angles to the scanning direction y. The aspect ratio of the field facets  8  corresponds to that of the slot-shaped object field  14 , which may likewise be in rectangular or arcuate form. 
     The whole object field  14  experiences a total dose of 24.6 J per complete scan of the field facet mirror  7 . This total dose is multiplied by the total transmission of the illumination optics  5 , on the one hand, and of the projection optics  16 , on the other hand. 
     The object plane  15  in the region of the object field  14  contains a reticle—not shown in FIG.  1 —which reflects the used radiation bundle  3 . 
     The projection optics  16  map the object field  14  into an image field  17  in an image plane  18 . For the projection illumination, this image plane  18  contains a wafer—not shown—which carries a light-sensitive layer which is illuminated during the projection illumination with the projection illumination installation  1 . 
     To facilitate the illustration of positional relationships, an xyz coordinate system is subsequently used. The x-axis is perpendicular to the plane of the drawing in  FIG. 1  and points into it. The y-axis runs to the right in  FIG. 1 . The z-axis runs downward in  FIG. 1 . 
     During the projection illumination, both the reticle and the wafer are scanned in sync in the y-direction in  FIG. 1 . The wafer is scanned at a scanning speed of typically 200 mm/s in the y-direction during the projection illumination. 
       FIG. 2  shows the scanning device  6  for the used radiation bundle  3  in greater detail. An x′-y′ coordinate system is used in  FIG. 2  to facilitate the illustration of positional relationships. The x′-axis, which is parallel to the x-axis, runs to the right in  FIG. 2 . The y′-axis, which lies in the yz-plane, runs upward in  FIG. 2 . 
     The scanning device  6  is a scanning mirror which reflects the used radiation bundle  3  in a sweeping manner and which can be tilted about a row advance axis  19 , which is parallel to the x′-axis, and about a column scan axis  20 , which is at right angles to the row advance axis. Both axes  19 ,  20  are situated in a reflective mirror face  21  of the scanning device  6 . The row advance axis  19  is parallel to the x′-axis in  FIG. 2 . The column scan axis  20  is parallel to the y′-axis in  FIG. 2 . 
       FIG. 2  shows the field facet mirror  7  schematically as a 4×4 array with four horizontal rows of four field facets  8  each. The subsequent frequency and time data relate to the illumination of the field facet mirror  7  with the 6×75 array, already described in connection with  FIG. 3 . The tilting about the column scan axis  20  is effected at the row frequency of 7.5 kHz. In this case, the mirror face  21  is tilted about +/−4.5°, which results in a deflection angle for the used radiation bundle  3  of +/−9°. Accordingly, the dwell time for the used radiation bundle  3  on a respective row of the field facet mirror  7  is 133.3 μs. The row advance is effected via synchronized tilting about the row advance axis  19 , so that the 75 rows are scanned using the correct row interval, with the tilting about the row advance axis  19  also ensuring that the used radiation bundle  3  returns from the last scanned field facet  8   z  to the first field facet  8   a  which is to be scanned. The mirror face  21  is therefore additionally tilted about the row advance axis  19  at a frequency of 100 Hz. The dwell time per individual field facet  8  is 22.2 μs. During the dwell time on a field facet  8 , 660 EUV radiation pulses thus impinge on the field facet  8 . 
     The interval between the mirror face  21  and the field facet mirror  7  is approximately 1 m. 
     Instead of tilting about the column scan axis  20 , the row advance can also be produced using a polygon scanner—not shown—which rotates about the column scan axis  20 . This polygon scanner has a total of 40 polygon facets for the mirror tilt variation about +/−4.5°, that is to say is designed as a regular 40-gon about its axis of rotation in the circumferential direction. A row frequency of 7.5 kHz is achieved with a rotation frequency for the polygon scanner of 187.5 Hz. When the scanning device  6  is designed with the polygonal mirror—not shown—the latter has a tilt mirror arranged upstream or downstream of it which, as described above, can be tilted about the row advance axis  19 . 
     The object field  14  has a slot width parallel to the scanning direction y of 2 mm and a slot width at right angles to the scanning direction, that is to say in the x-direction, of 26 mm. At a dose of 24.6 J on the reticle and transmission of the projection optics  16  of 0.3%, a dose of 74 mJ on the wafer is obtained per complete scan of the field facet mirror  7 . The face-related dose on the object field  14  is 150 mJ/cm 2 . For an assumed sensitivity of the light-sensitive layer of the wafer of 10-20 mJ/cm 2 , the light source  2  typically provides a factor of 7.5 to a factor of 15 more light than is used for illuminating a light-sensitive layer of this kind. In principle, it is thus possible to use one and the same light source  2  to illuminate a plurality of object fields  14  simultaneously. 
     In a first variant of such simultaneous illumination of a plurality of object fields  14 , the used radiation bundle  3  is fanned out by a polygonal mirror  22 —shown in dashes in FIG.  1 —in the yz-plane through a fan angle  23  of 45° immediately after leaving the light source  2 . The used radiation bundle  3  fanned out in this manner is then distributed over a total of 10 illumination optics  5 , which each accept 1/10 of the whole fan angle. Each of the ten beam paths for the used radiation bundle  3  then contains a scanning device  6  in the manner of that which has already been explained above. 
     If the light source  2  has a repetition rate of 30 MHz, fanning out the used radiation bundle  3  in order to use a total of ten illumination optics  5  through a fan angle  23  of 45° involves a polygonal mirror  22  with a total of 16 evenly distributed polygon facets which rotates at a rotation frequency of 87.5 kHz. 
     If a plurality of illumination optics  5  are intended to be used, the demands on the polygon scanner  22  can be reduced by reducing the total number of field facets 8 to 100 field facets, for example. 
     A further variant of the design of the projection illumination installation  1  for illuminating a plurality of object fields  14  involves the use of one and the same field facet mirror  24 , an embodiment of which is shown in  FIG. 4 . The field facet mirror  24  has facet mirror sections  25 ,  26 ,  27 ,  28 ,  29 ,  30 , the facet arrangement of which respectively corresponds to that of the field facet mirror  7 . The field facet mirror  24  thus has a total of 24 columns of 75 rows of individual field facets  8  each. For each facet mirror section  25  to  30 , there may also be fewer than 450 field facets  8  provided. 
     Each of the facet mirror sections  25  to  30  illuminates an associated pupil facet mirror  31  to  36 . This is achieved by appropriately tilting the field facets of the respective facet mirror sections  25  to  30 . 
     The pupil facet mirror  31 , which is associated with the facet mirror section  25 , is illuminated completely in a round region. This is what is known as a conventional illumination setting, in which a pupil of the illumination optics  5  is filled uniformly. 
     The pupil facet mirror  32 , which is associated with the facet mirror section  26 , is illuminated annularly, that is to say in a ring-shaped manner. 
     The pupil facet mirror  33 , which is associated with the facet mirror section  27 , is illuminated like the pupil facet mirror  31 , with the difference that there is a central cutout of a star  37  with four points. In line with this star shape, the object field  14  which is illuminated by the pupil facet mirror  33  lacks illumination directions. 
     The pupil facet mirror  34 , which is associated with the facet mirror section  28 , is illuminated annularly, that is to say in a ring-shaped manner, in comparable fashion to the pupil facet mirror  32 , the ring width for the illumination of the pupil facet mirror  34  being approximately half the magnitude of the ring width for the illumination of the pupil facet mirror  32 , given the same ring diameter. 
     The pupil facet mirror  35 , which is associated with the facet mirror section  29 , is illuminated in a manner comparable to the pupil facet mirror  33 , the pupil facet mirror  35  having a central cutout of a star-shaped region  38  with a total of five points, that is to say that the region is not illuminated. 
     The pupil facet mirror  36 , which is associated with the facet mirror section  30 , is illuminated in a manner comparable to the pupil facet mirror  31 , but with a reduced illumination diameter, so that the maximum illumination angles which are achieved with the illumination optics  5  with the pupil facet mirror  36  are reduced in comparison with the maximum illumination angle of the other illumination optics  5  with the pupil facet mirrors  31  to  35 . 
     Besides the embodiments which have been explained above in connection with the pupil facet mirrors  31  to  36 , other embodiments are also possible, for example dipole, quadrupole or other shapes of multipole embodiments. 
     A row advance, that is to say a change between the different rows of the field facet mirror  7 , can alternatively also be achieved via reflection at a facet mirror  39  displaced parallel to the columns of the field facet mirror  7 —shown at the bottom right in  FIG. 2 . 
     The displacement direction of the facet mirror  39  is parallel to the y-direction. Depending on the point of appearance of the used radiation bundle  3  on one of the facets  40  of the facet mirror  39 , the used radiation bundle  3  is deflected into another row of the field facet mirror  7 .  FIG. 2  shows a facet mirror  39  with five facets  40 . This illustration is simplistic. Illuminating the facet mirror  7  shown in  FIG. 3  involves a facet mirror  39  with 75 facets. This faceting using 75 facets can then also be combined with a faceting in the circumferential direction around the column scan axis  20 , so that the facet mirror  39  is provided with the shape of a faceted convex body, particularly a faceted sphere. Instead of the faceting, sufficiently small used radiation bundles  3  allow the facet mirror  39  also to be produced with a continuous contour, the facet faces having continuous transitions and no sharp edges between them. The planar sections between these soft transitions have the same normal vectors as in the embodiment with the sharp edges. 
     In a further variant of the projection illumination installation  1 , the light source  2  is equipped with an intensity modulator  41  for modulating the intensity of the used radiation bundle  3 . The intensity modulator  41  has a signaling connection to the scanning device  6  and possibly to the actuation of the polygonal mirror  22  for the purpose of synchronization. The intensity modulator  41  can be operated such that the intensity of the used radiation bundle  3  is influenced while a single field facet  8  of a field facet mirror, for example the field facet mirror  7 , is being swept over. If this influencing is effected in the same way in the case of all the field facets  8  of the field facet mirror  7  which are swept over, the result is corresponding influencing for the intensity distribution of the illumination in the object field  14 . 
     As an alternative or in addition, the intensity modulation via the intensity modulator  41  can be effected in sync with the scanning device  6  and possibly with the polygonal mirror such that the intensity of the used radiation bundle  3  is influenced in sync with the sweeping-over of a pupil facet mirror. This makes it possible to achieve correction of the illumination angle distribution over the object field  14 . 
     An effect which corresponds to the intensity modulation with the intensity modulator  41  can also be achieved by varying the deflection speed of the deflection of the used radiation bundle  3  when sweeping over the field facet mirror  7 . If, by way of example, the field facet mirror  7  is scanned at varying deflection speeds such that the center of each field facet  8  is swept over more quickly than the two edges at the right-hand and left-hand edges of the field facet  8 , the result is that the used radiation bundle  3  is applied to the object field  14  and is applied less intensely to the center in comparison with the edge. 
     By way of example, intensity modulation of the light source  2  is possible by shutting off radiation pulses, by disturbing the laser function, for example by specifically detuning the resonator, and/or by modulating the laser frequency. 
     Radiation pulses can be shut off by actuating a Q-switch or by electro-optical or acousto-optical modulators or deflectors (EOM, AOM) arranged inside the resonator or outside the resonator. The laser function can be disturbed by additionally connected electromagnetic fields, for example. A wavelength for the EUV used radiation can be detuned by subsequent scatter on a relativistic electron beam via the inverse Compton effect, for example. For this detuning, it is possible to use a portion of the electrons to be recycled from an FEL. The detuning of the wavelength of the EUV used radiation is greater than a bandwidth for EUV reflection coatings on the mirror elements of the illumination optics  5 . Such a reflection coating may be in the form of a multilayer coating. 
     Pulsed actuation of the light source  2  allows specific prescribing firstly of an intensity distribution and secondly of an illumination angle distribution over the object field  14 . 
     Each field point in the object field  14  has appropriately conjugated points on the individual field facets  8  associated with it. Insofar as the radiation pulses of the used radiation bundle  3  are suppressed whenever they are otherwise applied to these facet points, which are associated with a particular field point, it is possible to influence the illumination intensity at the relevant field point. If N field facets  8  are being used for the overlaying illumination of the object field  14 , a correction with a relative accuracy of 1/N can be achieved by suppressing the illumination at a facet point of one of the field facets  8 . 
     For a given point on the object field  14 , each illumination channel, which is associated with one of the field facets  8  via the pupil facet mirror  9 , corresponds to a particular illumination angle. Accordingly, intensity modulation of the radiation pulses of the used radiation bundle  3  such that entire illumination channels of this kind are suppressed allows the illumination angle distribution over the object field  14  to be influenced. 
     In principle, instead of a field facet mirror which has a plurality of field facets, it is also possible for the field facet mirror to be provided as part of the scanning device itself. In that case, the field facet mirror has precisely one facet which is activation tiltable through two degrees of freedom and, as a result, also directly illuminates the pupil facets of the pupil facet mirror or the object field, for example.