The invention relates to an imaging optical system comprising a plurality of mirrors, which via a beam path for imaging light image an object field in an object plane in to an image field in an image plane, the imaging optical system comprising a pupil obscuration. Furthermore, the invention relates to a projection exposure installation for microlithography comprising an imaging optical system of this type, a method for producing a microstruc- tured component comprising a projection exposure installation of this type and a microstructured component produced with this method.

Imaging optical systems of the type mentioned at the outset are known from US 2008/0170310 Al, US 2006/0232867 Al and US 6,750,948 B2. Another imaging optical system is known from US 6,975,385 B2.

In particular for use within a projection exposure installation for micro- lithography, in particular for the production of microstructured or nano- structured semiconductor components, there is a need for improved imag- ing properties, for example a greater numerical aperture to achieve a higher resolution or a better correction of imaging errors, in the imaging optical systems mentioned at the outset. Improvement of imaging properties should be achieved without compromise regarding throughput of the imaging optical system. This in particular is demanding regarding imaging opti- cal systems working with EUV imaging light in the range between 5 ran and 30 nm.

Thus, it is the object of the invention to improve an imaging optical system mentioned in the outset regarding imaging properties without compromise in throughput. This object is achieved according to the invention by an imaging optical system having the features as specified in claim 1.

It has been found in accordance with the invention that a concave fourth last mirror in the beam path leads to the possibility of designing the imaging optical system with low maximum angles of incidence on the respective mirrors. High reflection coatings therefore are possible with only low tolerance in band width of allowable angles of incidence. An imaging opti- cal system having low losses and therefore a high throughput is the result. The image plane of the imaging optical system can be an intermediate image plane which is imaged into another image plane via a relay optics comprising e. g. two additional mirrors. A pupil obscuration means that at least one pupil plane of the imaging optical system has an area which is, regard- ing an unfolded beam path, not penetrated by the imaging light. The pupil obscuration may be a central pupil obscuration or a decentred pupil obscuration. The pupil obscuration may be a rotational symmetric pupil obscuration. In particular, the pupil obscuration may be an annular pupil obscuration. The field size on a high apertured side of the imaging optical system which as a rule is the image side, may have an area which is spanned up by two dimensions, the lower of which is at least 1 mm. This lower dimension may be at least 2 mm or may be even larger. In general, a pupil of an imaging optics is defined as an image of an aperture limiting the optical path of the imaging light. Those planes, wherein these aperture images are located, are designated as pupil planes. As the images of the aperture stop are not necessarily plane images, more generally all planes which approximately coincide with these aperture images are designated as pupil planes. The plane of the aperture stop itself also is designated as a pupil plane. If the aperture stop does not define a plane, by definition, as is done in the case of the images of the aperture stop, those planes which approximately coincide with the aperture stop are designated as pupil planes. The mirrors of the imaging optical system may be designed with free-form reflective surfaces which cannot be described by a rotationally symmetrical function. At least one of the mirrors of the imaging optical system may have a free- form reflection surface of this type.

An entrance pupil of the imaging optics is the image of the aperture stop which is formed by imaging the aperture stop via that part of the imaging optics which is located between the object plane and the aperture stop. Consequently, the exit pupil is defined as being the image of the aperture stop which is formed by imaging the aperture stop via that part of the imaging optics which is located between the image plane and the aperture stop.

When the entrance pupil is a virtual image of the aperture stop, i. e., if the entrance pupil plane is located in the beam path of the imaging light before the object field, this is known as a negative input back focal length or a negative back focal length of the entrance pupil. In that case, the chief rays of all object field points propagate as if having an origin from a point being located before the object field, i. e. outside the beam path between the object field and the image field. The chief ray of each object point is defined as a ray connecting a given object point and the center of the entrance pupil. Given a negative input back focal length of the entrance pupil the chief rays of all object field points run divergently at the object field.

A shaded or obscured exit pupil at an image point means that this image point cannot be reached by all rays which originate from a respective object point within the aperture. This means that there is an area within the exit pupil which cannot be reached by rays originating from this field point. This area defines the pupil obscuration.

An alternative definition of a pupil is that area in the optical path of the imaging optics, in which individual rays originating from the object field points intersect, these individual rays being chosen such that they have, in relation to the chief rays originating from these object field points, the same illumination angle, respectively. Regarding this alternative definition, a pupil plane is that plane, in which the intersection points of the individual rays according to this alternative pupil definition are located or is defined as that plane which approximates a spatial distribution of the intersection points which not necessarily must lie exactly in a plane.

A beam path according to claim 2 leads to the possibility to guide the im- age light between the third last and the second last mirror near a central optical axis of the imaging optical system leading to smallest possible through-openings in the image side obscuration mirrors of the imaging optical system. Further, this helps to achieve low maximum angles of incidence on the respective mirrors. Said normal to the image field can be a common axis of rotational symmetry of the reflection surfaces of the mirrors of the imaging optical system or a common axis of rotational symmetry of best fit surfaces to approximate these reflection surfaces. The image field may be a ring segment field, i. e. may have the form of a part of an annulus. In that case, the center of such an image field which is the model point for the normal is defined as the middle point of the field range along a mirror axis of symmetry of such a ring segment field.

An intermediate image plane according to claim 3 helps to avoid obstruction problems in certain sections of the beam path as in the vicinity of the intermediate image plane the beam path has a small cross section. Furthermore, an intermediate image generates an additional pupil plane of the imaging optical system that can alternatively be used as a position for an aperture stop or an obscuration defining element.

Locations of the intermediate image plane according to claims 4 to 7 help to avoid obstructions in the beam path prior to the passage through the last mirror which as a rule has a through-opening for pupil plane obscuration. To avoid such obstructions of the imaging light prior to the passage through the mirror, the intermediate image plane may be located at a spatial distance in front of the last mirror which is more than 20 % of the distance between the object plane and the image plane, more than 30 %, more than 33 %, more than 40 %, more than 50 %, more than 60 % and even more than 65 %. The spatial distance according to claim 4 is not measured along the beam path but is the real distance between the intermediate image plane and the last mirror within the imaging optical system.

An intermediate pupil plane according to claim 8 leads to the possibility to arrange a pupil obscuration stop on this mirror.

An imaging optical system having exactly six mirrors leads to a well- balanced compromise between high quality imaging properties on the one hand and high throughput on the other. The imaging optical systems may be a catoptric imaging optical system.

An image side numerical aperture of at least 0,4 leads to a high resolution of the imaging optical system. The image side numerical aperture may be as high as 0,45 or maybe even higher. A large image field according to claims 11 or 12 serves to achieve a high throughput of the imaging optical system.

Maximal angles of incidence according to claims 13 and 14 give the possi- bility of the use of multilayer high reflection coatings on the mirrors. The maximum angle of incidence for the imaging light chief ray of the central object point on the fourth last mirror may be 5 deg at most, 4 deg at most, 3,8 deg at most, 3 deg at most or even 2,3 deg at most. The maximum angle of incidence for the imaging light on the fourth last mirror in the merid- ional plane of the imaging optical system may be 5 deg at most, 4,6 deg at most, 4 deg at most or even 3,5 deg at most.

Optical properties according to claims 15 and 16 give high quality imaging properties. The maximum wave front error (rms) can be as low as 25 mλ. The maximum distortion can be as low as 10 nm, as low as 5 nm, as low as 2 nm or even as low as 1,2 nm.

The advantages of an optical system according to claims 17 and 18 and of a projection exposure installation according to claims 19 and 20 correspond to those previously discussed with regard to the imaging optical system according to the invention. The light source of the projection exposure installation may be in the form of a broadband light source and may have, for example, a bandwidth greater than 1 nm, greater than 10 nm or greater than 100 nm. In addition, the projection exposure installation may be con- structed in such a way that it can be operated with light sources of different wavelengths. Light sources for other wavelengths, in particular wavelengths used for microlithography, can be used in conjunction with the imaging optical system according to the invention, for example light sources with wavelengths of 365 nm, 248 nm, 193 nm, 157 nm, 126 nm and 109 nm, and in particular also with wavelengths which are less than 100 nm.

Corresponding advantages also apply to the production method according to claim 21 and the microstructured or nanostructured component according to claim 22 produced thereby.

Embodiments of the invention will be described in the following in greater detail with reference to the drawings, in which:

Fig. 1 is a schematic view of a projection exposure installation for EUV microlithography having an imaging optical system to image an object field in an object plane into an image field in an image plane;

Fig. 2 to 4 are embodiments of the imaging optical system, each in meridional section.

A projection exposure installation 1 for microlithography has a light source 2 for illumination light. The light source 2 is an EUV light source which produces illumination and imaging light 3 in a wavelength range in particular of between 5 nm and 30 nm. Other EUV wavelengths are also possible. In general, any desired wavelengths, for example visible wavelengths or any other wavelengths which are used, for example, in microlithography and are available for the appropriate laser light sources and/or LED light sources (for example 365 nm, 248 nm, 193 nm, 157 nm, 129 nm or 109 nm), are possible for the illumination light guided in the projection exposure installation 1. A beam path of the illumination light 3 is shown extremely schematically in Fig. 1. An illumination optical system 6 guides the illumination light 3 from the light source 2 to an object field 4 (cf. Fig. 2) in an object plane 5. The object field 4 is imaged into an image field 8 (cf. Fig. T) in an image plane 9, at a pre-specified reduction scale, with a projection optical system or an imaging optical system 7. One of the embodiments shown in Fig. 2 to 7 may be used for the projection optical system 7. The projection optical system 7 of Fig. 2 has a reduction factor of 4. Other reduction scales are also possible, for example 5x, 8x, or even reduction scales that are greater than 8x. In the projection optical system 7 in the embodiments of Fig. 2 to 7, the image plane 9 is arranged parallel to the object plane 5. A portion of the reflective mask 10, also known as a reticle, coinciding with the object field 4 is hereby imaged.

In order to aid the description of the projection exposure installation 1 and the various embodiments of the projection optical system 7, a xyz Cartesian coordinate system is provided in the drawings and indicates the respective locations of the components represented in the figures. In Fig. I ₃ the x direction extends perpendicular to and into the drawing plane. The y direction extends to the right and the z direction extends downwards.

The image field 8 is bent in an arc shape, the y distance between the two arcs which delimit the image field 8 being 2 mm. 2 mm is also the side length of the straight side edges which delimit the image field 8 between the two arcs and which extend parallel to one another in y direction. These two straight side edges of the image field 8 are at a distance of 26 mm from one another. The surface of this curved image field corresponds to a rectangular image field with side lengths of 2 mm x 26 mm. A rectangular image field 8 or any other shape having e. g. these dimensions is also pos- sible, in particular when using a non-rotationally symmetric optical surface of at least one of the mirrors of the imaging optical system 7, a so-called free-form surface, is used.

Imaging takes place on the surface of a substrate 11 in the form of a wafer which is supported by a substrate holder 12. In Fig. 1, a light beam 13 of the illumination light 3 entering the projection optical system 7 is shown schematically between the reticle 10 and said projection optical system, and a light beam 14 of the illumination light 3 exiting from the projection optical system 7 is shown schematically between the projection optical system 7 and the substrate 11.

The projection exposure installation 1 is a scanner-type device. Both the reticle 10 and the substrate 11 are scanned in the y direction during opera- tion of the projection exposure installation 1.

Fig. 2 shows the optical construction of a first embodiment of the projection optical system 7. The beam path of each of three individual rays 15 of the imaging light 3, which proceed in each case from five object field points in Fig. 2 and are distanced from one another in the y direction, is shown. The three individual rays 15, which belong to one of these five object field points, are each associated with three different illumination directions for the five image field points. The individual rays 15, associated with the same illumination direction, of different field points extend divergently proceeding from the object plane 5. This is also referred to in the following as a negative input back focal length or a negative back focal length of the entrance pupil. Thus, an entrance pupil of the projection optical system 7 of Fig. 2 lies not inside the projection optical system 7, but before the object plane 5 in the beam path of the imaging light 3. This makes it possible, for example, to arrange a pupil component of the illumination optical system 6 in the entrance pupil of the projection optical system 7, before the projection optical system 7 in the beam path, without further imaging optical components having to be present between these pupil components and the object plane 5. As alternatives to a negative back focal length of the entrance pupil, variations of the imaging optical system 7 having a positive back focal length of the entrance pupil or an infinite distance between entrance pupil and object plane, i. e. a telecentric object side design, are possible.

The projection optical system 7 of Fig. 2 has a total of six mirrors, which are numbered in the sequence of the beam path, proceeding from the object field 4, as Ml to M6. Fig. 2 shows only calculated reflection surfaces of the mirrors Ml to M6.

The optical data for the projection optical system 7 of Fig. 2 are shown in the following by means of two tables. In the column "radius", the first table shows in each case the radius of curvature of the mirrors Ml to M6. The third column (thickness) describes the distance, proceeding from the object plane 5, to the following surface in each case.

The second table describes the precise surface form of the reflection surfaces of the mirrors Ml to M8, where the constants K and A to G are to be put into the following equation for the sagittal height z (h):

z(Λ) = + Ah ^A + Bh ⁶ + CA ⁸ + Dh ¹⁰ + Eti ² + Fh ¹⁴ + Gh ¹⁶ In this case, h represents the distance from an optical axis 16. Therefore: h ² = x ² + y ² . The reciprocal of "radius" is used for c.

Surface Radius Thickness Mode

Object INFINITY 467,886

Mirror 1 -510,946 -324,122 REFL

Mirror 2 3365,394 753,754 REFL

Mirror 3 -647,263 -480,301 REFL

Mirror 4 -222,580 1255,523 REFL

Mirror 5 4218,871 -227,647 REFL

STOP INFINITY -542,276

Mirror 6 1084,616 814,923 REFL

Image INFINITY 0,000

Surface K A B C

Mirror 1 0,000000E+00 1.436904E-10 -1 ,602611 E-16 -9.147843E-20

Mirror 2 0,OOOOOOE+00 1.416920E-08 -1.117413E-12 2.275422E-16

Mirror 3 0,000000E+00 -1 , 236913E-11 -1.820658E-16 2.300485E-21

Mirror 4 0,000000E+00 4,44471 OE-09 -3.135012E-13 -7.757698E-17

Mirror 5 0,OOOOOOE+00 3.973934E-10 6.412337E-16 1 , 482318E-21

Mirror 6 O.OOOOΌOΈ+OO 2.123517E-11 2.309893E-17 2.197829E-23

Surface D E F G

Mirror 1 3.887094E-24 -8.181187E-29 6.797763E-34 0,000000E+00

Mirror 2 -1.998702E-20 -3.279633E-34 -1.670474E-27 0,OOOOOOE+00

Mirror 3 -2.577499E-26 1.435734E-31 -3,354166E-37 0,OOOOOOE+00

Mirror 4 2.411333E-20 -3.562294E-24 2.033322E-28 0,OOOOOOE+00

Mirror 5 4,731862E-27 -1.087252E-32 4,275112E-37 -2.496780E-42

Mirror 6 1.996269E-29 2,24891 OE-35 -2,392841 E-43 5.656688E-47

The projection optical system 7 has exactly six mirrors Ml to M6. The projection optical system 7 is a catoptrical optical system.

The mirrors M5 and M6 are obscured, i. e. these mirrors M5, M6 have through-openings 17, 18, respectively. Due to this obscuration, an exit pupil of the projection optical system 7 in an exit pupil plane 19 is obscured, i. e., regarding the unfolded beam path, in a portion, in this case in a central area, is not penetrated by the imaging light 3. Mirrors Ml, M3 and M6 are concave. In particular, mirror M3 which is the fourth last mirror in the beam path of the projection optical system 7, is concave.

Mirrors M4 and M5 are convex. Mirror M2 also is concave but has a very large radius of curvature as compared to its used reflective surface thus appearing as an approximately plane mirror in Fig. 2.

The image field-side numerical aperture of the projection optical system 7 in accordance with Fig. 2 is 0,45. This is not reproduced to scale in Fig. 1.

The beam path between the third last mirror M4 and the second last mirror M5 passes the fourth last mirror M3 at a distance y _B to the optical axis 16, i. e. to a normal to the image plane 9 which is smaller than a distance y _M3 of the fourth last mirror M3 to said normal, i. e. to the optical axis 16, to the image plane 9. Thus, the beam path passes the last mirror M6 approximately symmetrical to the optical axis 16 minimizing the width of the through-opening 18 and therefore minimizing the pupil obscuration of the projection optical system 7. Comparing the distances y _B , y _M3 ^as mentioned above, it can be seen clearly from Fig. 2 that also the distance of the beam path between the third last mirror M4 and the second last mirror M5, where this beam path passes the fourth last mirror M3, to a normal to the center of the image field 8 is smaller than the distance of the fourth last mirror M3 to said normal to the image field 8.

A first intermediate pupil plane in the beam path of the projection optical system 7 is located in the vicinity of mirror M2. An intermediate image 20 in the beam path between the object plane and the image plane 9 is located between the fifth last mirror M2 and the fourth last mirror M3 in an inter- mediate image plane 21. Thus, the intermediate image 20 is located in the beam path in front of the third last mirror M4 and also in front of the fourth last mirror M3. The intermediate image 20 is located at a spatial distance z _;i in front of the last mirror M6, which is more than 10 % of the distance Z ₀₁ between the object plane 5 and the image plane 9 which is also known as track length. In the embodiment of Fig. 2, the ratio Z _ϋ /z _oi is about 0,33.

A maximum angle of incidence for an imaging light chief ray of a central object point on the fourth last mirror M3 is 2,3 deg. A chief ray is defined as a ray in the beam path of the projection optical system 7 which centrally passes the pupil planes of the projection optical system 7. Of course, the beam paths of these chief rays are not used as they are obscured.

A maximum angle of incidence for the imaging light 3 on the fourth last mirror M3 in the meridional plane of the projection optical system 7 is 3, 5 deg.

The projection optical system 7 has a maximum wavefront error of 25 mλ with respect to a wavelength of 13,5 nm.

The projection optical system 7 has a maximum distortion of 1,2 nm.

Fig. 3 shows a further embodiment of a projection optical system 7. Components and features which correspond to those which have previously been described with reference to Fig. 1 and 2 have the same reference numerals and will not be discussed in detail again. The optical data for the projection optical system 7 of Fig. 3 are shown in the following by means of two tables, which correspond in layout to the tables for Fig. 2.

Surface K A B C

Mirror 1 0,000000E+00 -6.649307E-12 -4.302304E-18 -8,848113E-24

Mirror 2 O.OOOOOOE+00 6.282448E-09 6.011360E-13 -3,791310E-17

Mirror 3 O.OOOOOOE+00 -6.195561 E-12 -2.166405E-17 4,405461 E-22

Mirror 4 0,OOOOOOE+00 -1.260058E-10 3.471757E-15 -1.438560E-19

Mirror 5 O.OOOOOOE+00 3,711197E-09 3.364407E-14 3.306640E-19

Mirror 6 O.OOOOOOE+00 6.440745E-11 1.979430E-16 5.542475E-22

Surface D E F G

Mirror 1 5,943741 E-29 -2.958093E-34 5.813740E-40 O.OOOOOOE+00

Mirror 2 1.010546E-21 -1 ,002068E-33 -6.370370E-27 1.558060E-30

Mirror 3 -6.624002E-27 5,921233E-32 -2.942683E-37 6.268230E-43

Mirror 4 3.141242E-24 -1 ,800849E-29 -5,645134E-34 8.168772E-39

Mirror 5 7.350550E-24 -1.994692E-28 -2,272188E-33 3.986656E-37

Mirror 6 8.640666E-28 1 , 466217E-32 -1.024646E-37 5,046001 E-43

In the projection optical system of Fig. 3, mirrors Ml, M3 and M6 are concave. Mirrors M2 and M5 are convex. Mirror M4 has a very large radius and therefore appears to be approximately plane in Fig. 3.

Mirrors M2 and M4 lie approximately at the same z-position of projection optical system 7 of Fig. 3.

The projection optical system 7 of Fig. 3 has an image side numerical aper- ture of O,45. The beam path between mirrors M4 and M5 passes mirror M3 between the mirror M3 and the optical axis 16. Consequently, also in the projection optical system 7 of Fig. 3 the beam path between the third last mirror M4 and the second last mirror M5 passes the fourth last mirror M3 at a distance ys to the optical axis 16 which is smaller than a distance y _M3 of the fourth last mirror M3 to the optical axis 16. Also, the beam path between the third last mirror M4 and the second last mirror M5 passes the fourth last mirror M3 at a distance to a normal to the image field 8 which is smaller than a dis- tance of the fourth last mirror M3 to this normal to the center of the image field 8 of the projection optical system 7 of Fig. 3.

The intermediate image 20 of the projection optical system 7 of Fig. 3 lies in the beam path between mirrors M4 and M5. This intermediate image 20 is located at a spatial distance Z;; in front of the last mirror M6 which is more than 10 % of the distance z _O i between the object plane 5 and the image plane 9. In the embodiment of Fig. 3, the ratio Zii/z _oi is about 0,65.

A first intermediate pupil plane in the beam path between the object plane 5 and the image plane 9 lies in the vicinity of mirror M2. On this mirror M2 a pupil obscuration stop can be placed to define the pupil obscuration of the projection optical system 7 of Fig. 3.

A maximum angle of incidence for an imaging light chief ray of a central object point on the fourth last mirror M3 is 3,8 deg.

A maximum angle of incidence for the imaging light 3 on the fourth last mirror M3 in the meridional plane of the projection optical system 7 of Fig. 3 is 4,6 deg. The projection optical system 7 of Fig. 3 has a maximum wave front error of 47 mλ. The projection optical system 7 of Fig. 3 has a maximum distortion of 5 nm.

Fig. 4 shows a further embodiment of a projection optical system 7. Components and features which correspond to those which have previously been described with reference to Fig. 1 to 3 have the same reference numerals and will not be discussed in detail again.

The optical data for the projection optical system 7 of Fig. 4 are shown in the following by means of two tables, which correspond in layout to the tables for Fig. 2.

Surface K A B C

Mirror 1 0,000000E+00 2.115219E-09 1.569479E-14 8.536232E-19

Mirror 2 0,000000E+00 -2.973633E-10 7.102254E-16 -4.274832E-21

Mirror 3 0,000000E+00 1.555252E-10 4.516776E-16 -1.374152E-20

Mirror 4 0,OOOOOOE+00 -4.815282E-09 5.040254E-12 -1.401315E-15

Mirror 5 0,000000E+00 3.122473E-10 4.443090E-16 8.155899E-22

Mirror 6 0,000000E+00 2.819557E-11 3.077126E-17 3.054059E-23

Surface D E F G

Mirror 1 -4.412914E-22 4.162560E-31 1.960635E-29 O.OOOOOOE+00

Mirror 2 2.467142E-25 -6,671143E-30 7.419380E-35 0,000000E+00

Mirror 3 3.711994E-25 -5.054858E-30 2.911858E-35 0,000000E+00

Mirror 4 1.768816E-19 1.916495E-30 -1.428438E-27 0,000000E+00

Mirror 5 1.958046E-27 2.405983E-33 2.873232E-38 -8.029814E-45

Mirror 6 2.752062E-29 4.111797E-35 -3.595583E-41 1.870027E-46 In the projection optical system of Fig. 4, mirrors M2, M3 and M6 are concave. Mirrors M4 and M5 are convex. Mirror Ml has a very large radius and therefore appears to be approximately plane in Fig. 4.

The image field-side numerical aperture of the projection optical system 7 of Fig. 4 is 0,45.

The beam path between mirrors M4 and M5 passes mirror M3 between the mirror M3 and the optical axis 16. Consequently, also in the projection optical system 7 of Fig. 4 the beam path between the third last mirror M4 and the second last mirror M5 passes the fourth last mirror M3 at a distance ye to the optical axis 16 which is smaller than a distance y _M3 of the fourth last mirror M3 to the optical axis 16. Also, the beam path between the third last mirror M4 and the second last mirror M5 passes the fourth last mirror M3 at a distance to a normal to the center of the image field 8 which is smaller than a distance of the fourth last mirror M3 to this normal to the image field 8 of the projection optical system 7 of Fig. 4.

The intermediate image 20 of the projection optical system 7 of Fig. 4 lies in the vicinity of mirror M4. This intermediate image 20 is located at a spatial distance Z _n in front of the last mirror M6 which is more than 10 % of the distance z _oi between the object plane 5 and the image plane 9. In the embodiment of Fig. 4, the ratio is about 0,24.

The individual rays 15, associated with the same illumination direction, of different field points extend convergently proceeding from the object plane 5 of the imaging or projection optical system 7 of Fig. 4. This is also referred to as a positive back focal length of the entrance pupil. The first intermediate pupil plane in the beam path between the object plane 5 and the image plane 9 lies in the vicinity of mirror Ml. On this mirror Ml a pupil obscuration stop can be placed to define the pupil obscura- tion of the projection optical system 7 of Fig. 4.

The chief ray angles for the central field point on each of the mirrors Ml to M6 are 9,7 deg, 6,9 deg, 6,6 deg, 9,6 deg, 1,8 deg and 0,9 deg, respectively. With respect to the central field point all chief ray angles of incidence on the mirrors Ml to M6 are smaller than 10 deg. The wave front error of the system has a composite rms value of 16 mλ. Distortion is corrected to values below 1 nm.

The projection optical system of Fig. 4 has a demagnification ratio of 8x, while the field size at wafer level, i. e. the image field size, is still 2 mm x 26 mm.

Fig. 5 shows a further embodiment of the projection optical system 7. Components which correspond to those which have already been described above with reference to the projection optical system 7 from Fig. 2 to 4 have the same reference numerals and are not discussed again in detail.

All six mirrors Ml to M6 of the projection optical system 7 are designed as free-form faces which cannot be described by a rotationally symmetrical function. Other embodiments of the projection optical system 7 are also possible, in which at least one of the mirrors Ml to M6 has a free-form reflection face of this type. A free-form face of this type may also be produced from a rotationally symmetrical reference face. Free-form faces of this type for reflection faces of the mirrors of projection optical systems of projection exposure installation for microlithography are known from US 2007-0058269 Al .

The free-form face can be described mathematically by the following equation:

wherein there applies:

_ (m + n) ² + m + 3n _{+ 1}

Z is the rising height (sagitta) of the free-form face at the point x, y

(χ ² + y ² = r ² ). c is a constant, which corresponds to the vertex curvature of a corresponding asphere. k corresponds to a conical constant of a corresponding asphere. C _j are the coefficients of the monomials X ^m Y ⁿ . Typically, the val- ues of c, k und C _j are determined on the basis of the desired optical properties of the mirror within the projection optical system 7. The order of the monomial, m + n, may be varied as desired. A monomial of a higher order may lead to a design of the projection optical system with improved image error correction, but is more complex to calculate, m + n may adopt values of between 3 and more than 20.

Free-form faces can also be described mathematically by Zernike polynomials, which are described, for example, in the manual of the optical design programme CODE V ^® . Alternatively, free-form faces may be described with the aid of two-dimensional spline surfaces. Examples of this are Bezier curves or non-uniform rational basis splines (NURBS). Two- dimensional spline surfaces may, for example, be described by a network of points in an xy-plane and associated z- values or by these points and gradients associated with them. Depending on the respective type of spline surface, the complete surface is obtained by interpolation between the network points using, for example, polynomials or functions, which have specific properties with regard to their continuity and differentiability. Exam- pies of this are analytical functions.

The mirrors Ml to M6 have multiple reflection layers to optimise their reflection for the impinging EUV illumination light 3. The reflection can be optimised all the better, the closer the impingement angle of the individual beams 15 on the mirror surface lies to the perpendicular incidence. The projection optical system 7 has small reflection angles overall for all the individual beams 15.

The optical design data of the reflection faces of the mirrors Ml to M6 of the projection optical system 7 can be inferred from the following tables. The first of these tables gives, for the optical surfaces of the optical components and for the aperture diaphragm, the respective reciprocal value of the vertex curvature (radius) and a spacing value (thickness), which corresponds to the z-spacing of adjacent elements in the beam path, proceeding from the object plane. The second table gives the coefficients C _j of the monomials X ^m Y ⁿ in the free-form face equation given above for the mirrors Ml to M6. Nradius is in this case a standardisation factor. According to the second table, the amount is still given in mm, along which the respective mirror, proceeding from a mirror reference design, has been decentred (Y- decentre) and rotated (X-rotation). This corresponds to a parallel displacement and a tilting in the free-form face design method. The displacement takes place here in the y-direction and the tilting is about the x-axis. The rotation angle is given in degrees here.

Coefficient Mt M2 M3 M4 M5 M6

K -5 356880E-02 8 027989E-02 2 215137E+00 -5 038784E-01 4 907814E-02 1 120410E-01

Y 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X2 -4 675062E-04 -2 669898E-05 1 839305E-03 -2 126518E-04 -9 700568E-04 7 232599E-06

Y2 -1 320900E-03 -5 452804E-04 7 618071E-04 -4431611E-04 -8 012468E-04 1 044012E-05

X2Y 1 229247E-06 4 121203E-07 -2 381957E-06 9 694733E-08 -1 199882E-06 -5 510251E-09

Y3 -7 423640E-08 -8 613636E-08 4979224E-08 -2 077214E-08 1 739231 E-07 -1 476529E-09

X4 -2 550932E-10 -5 351417E-11 1 2B1481E-08 -4 388472E-11 -2 188996E-09 2 529523E-13

X2Y2 -2 178964E-10 -2 067816E-10 3488864E-09 -1 254954E 10 2 292180E-09 2 688243E-13

Y4 -8 677437E-10 -8 009019E-10 1 878436E-09 -1 372181E-10 -1 320127E-09 1 277954E-12

X4Y -3 215827E-13 1 670802E-13 -1 372762E-10 4 754789E-14 -6 365774E-12 -2679463E-16

X2Y3 -1 228117E-12 3 874349E-13 -2 437182E-11 -1 718101E-13 -6 070058E 12 -1 679816E-15

Y5 -1 148542E-13 2 500543E-13 1 577671E-12 5 427575E-13 -6 442309E-12 -1 838497E-15

X6 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X4Y2 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X2Y4 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

Y6 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X6Y 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X4Y3 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X2Y5 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

Y7 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X8 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X6Y2 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X4Y4 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X2Y6 0 OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

Y8 0 OO0000E+O0 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X8Y O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X6Y3 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X4Y5 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X2Y7 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

Y9 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X1D O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 o ooooooε+oo

X8Y2 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X6Y4 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O QOOOOOE+00 O OOOOOOE+00

X4Y6 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

X2Y8 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

Y10 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00 O OOOOOOE+00

Nradius 1 OOOOOOE+00 O OOOOOOE+00 1 OOOOOOE+00 1 OOOOOOE+00 1 OOOOOOE+00 1 OOOOOOE+00

Coefficient M1 M2 M3 M4 M5 MS Image plane

Y-decentre 172 287 276 185 336 270 167 562 379 550 267 210 0 000 X-rotation -3 535 -5 840 10 868 13 825 9306 4 657 0,000 The mirror M5 of the embodiment of the projection optical system 7 of Fig. 5 has no opening for passage of the imaging light 3.

The object field 4 and the mirror M5 are arranged on different sides of a main plane 22. The main plane 22 is defined as a plane being perpendicular to the plane of projection of Fig. 5 and intersecting this plane of projection at the optical axis 16.

The mirrors Ml and M4 on the one hand, and the mirrors M3 and M6, on the other hand, in the projection optical system 7 according to Fig. 5, are arranged back to back.

The mirrors Ml, M3 and M6 are concave. The mirror M5 is convex. The mirrors M2 and M4 have a radius of curvature which is so great that they appear virtually to be a planar mirror in Fig. 5.

In the projection optical system 7 according to Fig. 5, an aperture diaphragm or stop may be arranged in the region of a pupil plane 23 between the mirrors M2 and M3.

The central pupil obscuration, in the projection optical system 7 according to Fig. 5, is 4.0 %.

The numerical value for the pupil obscuration is produced by the ratio of the area within the exit pupil masked because of the pupil obscuration relative to a total area of an exit pupil of the imaging optical system. A pupil obscuration, which is less than 5%, makes possible a pupil obscured imaging optical system with a particularly high light throughput. Furthermore, the small obscuration according to the invention may lead to a small or negligible influence on an imaging quality of the imaging optical system, in particular on the imaging contrast. The pupil obscuration may be less than 10%. The pupil obscuration may, for example, be 4.4% or 4.0%. The pupil obscuration may be less than 4%, may be less than 3%, may be less than 2% and may even be less than 1%. The pupil obscuration of the imaging optical system may be predetermined by one of the mirrors, for example by a through-opening thereof or by an outer edging thereof, or by an obscuration stop or diaphragm, which is arranged in the beam path of the imaging light between the object field and the image field.

The working spacing d _w between the image plane 9 and the portion of the used reflection face of the mirror M5 closest to the image plane is 85 mm in the projection optical system according to Fig. 5. A ratio of this working spacing d _w to the overall length of the projection optical system 7 accord- ing to Fig. 5 is 4.25 %. The angle of incidence of the individual beams 15 on the mirror M5 in the meridional plane, which is shown in Fig. 5, is a maximum of 15.9°.

The intermediate image plane 20 of the projection optical system 7 of Fig. 5 is arranged near the through-opening 18 in mirror M6 for the passage of the imaging light 3 through the mirror M6. The exit pupil plane 19 in the light path between intermediate image 21 and the image field 8 is located near the reflection of the illumination light 3 at the mirror M5.

To produce a microstructured or nanostructured component, the projection exposure installation 1 is used as follows: Initially, the reflection mask 10, or the reticle and the substrate, or the wafer 11 is prepared. Subsequently, a structure on the reticle 10 is projected onto a light-sensitive layer of the wafer 11 by means of the projection exposure installation 1. By develop- ing the light-sensitive layer, a microstructure on the wafer 11, and thus the microstructured component, are then produced.