WO2007031544A1

WO2007031544A1 - Optical system of a microlithographic exposure system

Info

Publication number: WO2007031544A1
Application number: PCT/EP2006/066332
Authority: WO
Inventors: Michael Totzeck; Susanne Beder; Wilfried Clauss; Heiko Feldmann; Daniel KRÄHMER; Aurelian Dodoc
Original assignee: Carl Zeiss Smt Ag
Priority date: 2005-09-14
Filing date: 2006-09-13
Publication date: 2007-03-22
Also published as: CN102207691B; CN102207691A; US20080204877A1; CN101263432A; KR20080043835A; EP2085824A1; EP1924890A1; US8031326B2; CN101263432B; JP2009508170A; TW200717046A

Abstract

An optical system, in particular an illumination system or a projection lens of a microlithographic exposure system, according to one aspect of the present invention has an optical system axis (OA) and at least one element group (200) consisting of three birefringent elements (211 ,212,213) each of which being made of optically uniaxial material and having an aspheric surface, wherein a first birefringent element (211 ) of said group has a first orientation of its optical crystal axis, a second birefringent element (212) of said group has a second orientation of its optical crystal axis, wherein said second orientation can be described as emerging from a rotation of said first orientation, said rotation not corresponding to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof, and a third birefringent element (213) of said group has a third orientation of its optical crystal axis, wherein said third orientation can be described as emerging from a rotation of said second orientation said rotation not corresponding to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof.

Description

OPTICAL SYSTEM OF A MICROLITHOGRAPHIC EXPOSURE SYSTEM

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to an optical system, in particular an illumination system or a projection lens of a microlithographic exposure system.

2. Description of the related art

Microlithography is used in the fabrication of microstruc- tured components like integrated circuits, LCD's and other microstructured devices. The microlithographic process is performed in a so-called microlithographic exposure system comprising an illumination system and a projection lens. The image of a mask (or reticle) being illuminated by said illumination system is projected, through said projection lens, onto a resist-covered substrate, typically a silicon wafer bearing one or more light-sensitive layers and being provided in the image plane of said projection lens, in order to transfer the circuit pattern onto the light- sensitive layers on the wafer.

In order to obtain a good interference contrast of the image created in the image plane of the projection lens, it is advantageous to have a two-beam-interference of light rays each being polarized perpendicular to the plane of incidence. Hereto, it is preferred to have a so-called tangential polarization distribution, where the planes of oscillation of the electrical field vectors of individual linearly polarized light rays in a pupil plane of the system have an orientation that is perpendicular to the radius originating from the optical axis.

Furthermore, the different attempts to enhance the resolution and the optical performance of microlithographic exposure systems are leading to an increasing need for use of optical components made of materials with relatively high refractive index. Herein, a refractive index is re- garded as "high" if its value exceeds, at the used wavelength, the refractive index of Siθ2 which is n«1.56 at 193nm. Such materials are e.g. spinell (n«1.87 at 193nm), sapphire (n«1.93 at 193nm) or magnesium oxide (n«2.02 at 193nm) . However, problems arise from the fact that these materials exhibit the effect of either uniaxial birefringence (e.g. sapphire, being optically uniaxial with Δn«- 0.01 at 193nm) or intrinsic birefringence ("IBR", e.g. spinell with an IBR of «52nm/cm at 193nm or magnesium oxide with an IBR of «70nm/cm at 193nm, or garnets (Ml) ₃ (M2) 7O₁₂ with Ml for instance Y, Sc or Lu, with M2 for instance Al, Ga, In or Tl, and an IBR in a range between 20nm/cm and 80nm/cm) , causing a retardation that disturbs the polarization distribution of the transmitted rays. Further disturbances arise e.g. from stress birefringence in the used optical components, phase shifts occurring at reflecting boundaries etc. Accordingly, countermeasures are needed to at least partially compensate for such disturbances. Many approaches are known in the art, and the following citations are given without being exhaustive and without declaration on their relevance for the present application.

US 6,252,712 Bl discloses to use, in order to provide an arrangement with which local disturbances of the state of polarization can be compensated, at least one birefringent free-form optical element (preferably at least two of such elements with principal axes being rotated with respect to each other) having a thickness that varies irregularly over the cross-section of a light beam such that a disturbance of the distribution of polarization is at least par- tially compensated.

WO 2005/001527 Al discloses a corrective device for compensating disturbances of polarization distribution, wherein said corrective device comprises a corrective mem- ber encompassing two birefringent corrective elements having local irregularities in thickness. The arrangement, thickness and birefringent properties of the corrective elements are selected such that the birefringent effects thereof mutually cancel each other if the local irregu- larities in thickness are not taken into consideration, in order to influence polarization only at points where disturbances are to be compensated.

Furthermore, WO 2005/059645 A2 discloses a microlithogra- phy projection lens wherein at least two different uniaxial crystal materials such as quartz, sapphire, MgF₂, LaF₃ are present in the objective. Further attempts to compensate for the effect of birefringence and/or to avoid disturbances of the state of polarization e.g. include a clocking of lenses (e.g. US 2004/0105170 Al or WO 02/093209 A2 ) , using a varying in- fluence on polarization by form-birefringent layers, or inserting a mosaic tile structure comprising a plurality of birefringent tiles, each tile polarizing a corresponding section of an exposure beam in a particular direction (see e.g. US 6, 191, 880) .

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an op- tical system, in particular an illumination system or a projection lens of a microlithographic exposure system, wherein an arbitrary desired polarization distribution can be effectively created with a simple structure that can be fabricated with a high precision in compliance with the requirements for microlithographic exposure systems. More particularly, the present invention provides an optical system wherein local disturbances of the state of polarization, in particular due the presence of one or more optical elements having a relatively high refractive index and relatively strong birefringence (e.g. due to the presence of uniaxial materials or of materials showing strong intrinsic birefringence), can be effectively compensated. As a further aspect, the present invention provides an optical system wherein a first (e.g. circular or linear) po- larization distribution is transformed into a second (e.g. tangential) polarization distribution. An optical system, in particular an illumination system or a projection lens of a microlithographic exposure system, according to one aspect of the present invention has an optical system axis and at least one element group con- sisting of three birefringent elements each of which being made of optically uniaxial material and having an aspheric surface, wherein: a first birefringent element of said group has a first orientation of its optical crystal axis; - a second birefringent element of said group has a second orientation of its optical crystal axis, wherein said second orientation can be described as emerging from a rotation of said first orientation, said rotation not corresponding to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof; and

- a third birefringent element of said group has a third orientation of its optical crystal axis, wherein said third orientation can be described as emerging from a rotation of said second orientation, said rotation not corresponding to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof.

In the meaning of the present invention, the term "birefringent" or "birefringent element" shall comprise both linear birefringence and circular birefringence (i.e. optical activity, as observed e.g. in crystalline quartz) .

According to a preferred embodiment, the three birefringent elements of said element group are consecutive in such a sense that the second birefringent element is, along the optical system axis or in the light propagation direction, the next birefringent optical element following to the first element, and that the third birefringent element is, along the optical system axis or in the light propagation direction, the next birefringent optical element following to the second element. With other words, the elements of the group are arranged in the optical system in succession or in mutually adjacent relationship along the optical system axis. Furthermore, and still more preferably, the three elements are directly adjacent to each other without any (birefringent or non-birefringent) optical element in between.

According to the present invention, a combination of three birefringent elements is used for achieving a desired compensation of local disturbances of the state of polarization, wherein each of said elements has an aspheric surface and thus a varying strength in its birefringent effect resulting from its thickness profile. The invention is based on the realization that with such a combination of three elements with suitable variations of the thickness profiles and orientations of the respective crystal axes, it is principally possible to achieve any desired distribution of the retardation, which again may be used to at least partially compensate an existing distribution of the retardation due the presence of one or more optical elements in the optical system showing strong retardation caused for instance by using uniaxial media, biaxial media, media with intrinsic birefringence or media with stress induced birefringence. As to the theoretical considerations underlying the present invention, a non-absorbing (= unitary) Jones matrix having the general form

ιa_λ

with can be described by a rotation of the Poin-

care-sphere, wherein points lying on the surface of the Poincare-sphere are describing specific states of polari- zation. The concept of the present invention is based on the fact that said rotation of the Poincare-sphere can be divided into elementary rotations, which again are corresponding to specific Jones-matrices. The suitable combination of three of such Jones-matrices is used to describe a desired rotation of the Poincare-sphere, i.e. a desired non-absorbing (= unitary) Jones matrix.

In other words, any unitary Jones matrix can be expressed as a matrix product of three matrix functions,

with a suitable choice of the ,,Euler angles" α, β, and γ. Each of the matrix functions R_λ(a) , R₂(a) , R₃(a) is taken from the set

JY cos α -sin αΛ fexp(-ιa) 0 λ f cosa -i sin a H sin α cosα J'l 0 exp(/α) )\- ι sin a cosa

which describes a rotator, a retarder with 0° orientation and a retarder with 45° orientation, the strength of which are specified by α. This decomposition of any unitary Jones matrix is always possible under the condition that

R_λ(a)≠R₂(a) and R₂(a) ≠ R₃(a) (3)

The above feature that, in the element group of three bi- refringent elements according to the present invention, the orientation of the optical crystal axis in the second (or third, respectively) birefringent element can be de- scribed as emerging from a rotation of the orientation of the optical crystal axis in the first (or second, respectively) birefringent element by an angle not corresponding to 90° or an integer multiple thereof guarantees the independency of the three birefringent elements in the above sense. This considers the fact that two elements each having an aspheric surface and such an orientation of their optical crystal axis, that the two orientations of these two elements are rotated by e.g. an angle of 90° to each other, are in so far not independent in their polarizing effect as one of these elements can be substituted by the other if, at the same time, the sign of the respective aspheric surface (or the thickness profile) is inverted.

With other words, the element group according to the pre- sent invention comprises three birefringent elements, wherein two subsequent birefringent elements of the optical group according to the present invention have different orientations of their optical crystal axis. Further, two such orientations are only regarded as being different from each other if one of these orientations cannot achieved by a rotation around the optical system axis by an angle of 90° (or an integer multiple thereof) . With still other words, the orientations of two subsequent birefringent elements of the optical group according to the present invention have to be, in deciding whether they are really different in their polarizing effect, compared to each other "modulo 90°". Accordingly, in a different wording the present aspect of the invention may be defined in that if the optical crystal axes of two subsequent birefringent elements of the optical group are lying in a plane perpendicular to the optical system axis, the "angle modulo 90°" between the two orientations of these optical crystal axes is not zero. As an example, two orientations lying in a plane perpendicular to the optical system axis with an angle of 90° to each other are regarded, according to the present invention, as equal or as not independent, whereas two orientations lying in a plane perpendicular to the optical system axis with an angle of 95° to each other yield an angle of "95° modulo 90°" = 5° and thus are regarded as not equal or as independent from each other.

If a bundle of light rays passes such an element group of three birefringent elements whose optical crystal axes meet the above criterion, it becomes possible to compensate, for suitable selections of the aspheric surfaces or thickness profiles of these birefringent elements, any disturbance of the polarization distribution in the optical system, e.g. projection lens of a microlithography exposure system.

Generally, in order to provide at a predetermined position a predetermined phase retardation of Δφ, a thickness d is needed as given by 2πAn

In the context of the present invention a significant compensation of birefringent effects in a projection lens will typically have to correspond to a retardance of at least λ-Δφ > 5 nanometers (nm) . In order to provide such a compensation, the variation Δd of the thickness due to the aspheric surface corresponding to such a retardance effect will, for a typical value of Δn for e.g. MgF₂ of 0.0024 and a typical wavelength of λ« 193nm, amount to Δd «

5nm/ (2-π-Δn) « 331nm. Accordingly, the lower limit for a typical quantitative level of the thickness profile variation in the aspheric surfaces can be estimated, for a wavelength of λ« 193nm, to Δd_min « 0.3 μm. In terms of the achieved phase retardation Δφ, a lower limit Δφ_min corresponding to a significant compensation of birefringent effects can be given by the criterion Δφ > (5nm/193nm), so that a lower limit Δφ_min of the phase retardation can be estimated as Δφ_min « 0.025 or Δφ_min « 25 mrad. Therefore, according to an embodiment of the invention, each of said birefringent elements has such a variation of its thickness profile that a minimum phase retardation of Δφ_min « 25 mrad is obtained at a given operating wavelength of said optical system.

According to an embodiment of the invention, the optical crystal axes of all of said three birefringent elements are orientated different from each other. Such an arrangement enables to realize the above concept of the three crystal orientations in configurations where the first and third birefringent element have their crystal axes orien- tated perpendicular to each other. This is advantageous in so far, as in case if the desired polarization effect to be compensated (i.e. to be provided by the element group) is an at least almost pure retardance (without or with only a small amount of elliptical components), the respective aspheric surfaces of the first and third element may have aspheric surfaces of substantially identical height profiles with opposite signs, leading to an at least partial compensation of the scalar effects of these surfaces.

According to a further embodiment of the invention, the optical crystal axes of the first birefringent element and the third birefringent element are substantially parallel to each other. Such an arrangement favours to manufacture these two elements with identical aspheric surfaces or height profiles, which is favourable with respect to a significant simplification of the manufacturing process and the use of identical test optics for these elements.

According to a further embodiment of the invention, the optical crystal axes of all three birefringent elements are orientated perpendicular to the optical system axis, wherein the optical crystal axes of the first birefringent element and the third birefringent element are each ro- tated around said optical system axis with respect to the optical crystal axis of the second birefringent element of said group by an angle in the range of 30° to 60°, more preferably 40° to 50°, and still more preferably by an angle of 45°. This is advantageous in so far as the respec- tive elements having their optical crystal axes orientated under an angle of 45° correspond to rotations of the Poin- care-sphere around axes being perpendicular to each other, i.e. linearly independent rotations, which makes it possi- ble to achieve a specific desired compensation effect with a more moderate height profile and smaller surface deformation .

According to a preferred embodiment, an optical crystal axis in each of said optical elements is either substantially perpendicular or substantially parallel to said optical system axis. Here and in the following, the wording that the optical crystal axis is either "substantially perpendicular" or "substantially parallel" to said optical system axis shall express that small deviations of the exact perpendicular or parallel orientation are covered by the present invention, wherein a deviation is regarded as small if the angle between the optical crystal axis and the respective perpendicular or parallel orientation does not exceed ±5 ° .

According to an embodiment of the invention, said bire- fringent elements have on average essentially no refract- ing power. This wording is to be understood, in the meaning of the present invention, such that in case of an approximation of the surfaces of the respective element by a best-fitting spherical surface, the refractive power of the so approximated element is not more than 1 diopter (1 Dpt = Im^"1) . The property of the birefringent elements to have "on average essentially no refracting power" may be alternatively achieved by an additional compensation plate for one or more of said optical elements or may already result from the surface relief of the respective element being only marginal, i.e. being essentially similar to a plane-parallel plate. According to one embodiment, said compensation plate may be made of a non-birefringent material, e.g. fused silica. According to a further aspect of the invention, an optical system, in particular an illumination system or a projection lens of a microlithographic exposure system, has an optical system axis and at least one element group consisting of three element pairs each of which consisting of one birefringent element and one attributed compensation element, said birefringent element being made of optically uniaxial material and having an aspheric surface, wherein each birefringent element and the attributed compensation element supplement each other to a plan-parallel geometry of said element pair, wherein:

- a first birefringent element of said group has a first orientation of its optical crystal axis; - a second birefringent element of said group has a second orientation of its optical crystal axis, wherein said second orientation can be described as emerging from a rotation of said first orientation, said rotation not corresponding to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof; and a third birefringent element of said group has a third orientation of its optical crystal axis, wherein said third orientation can be described as emerging from a rotation of said second orientation, said rotation not corresponding to a rotation around the optical system axis (OA) by an angle of 90° or an integer multiple thereof.

Accordingly, the optical system or the optical element group in this aspect are analogous to optical system or the optical element group described before and differ only in so far as said element group comprises for each of said birefringent elements an attributed compensation element such that said birefringent element and said attributed compensation element add up to a plan-parallel geometry. The advantageous effect additionally achieved in this aspect is that a detrimental influence of the element group on the so-called scalar phase can be kept low and, in the ideal case, made equal to the effect caused by a plan- parallel plate on the scalar phase. The compensation ele- ment is preferably also made of an optically uniaxial material having an optical crystal axis which is orientated in the plane perpendicular to the optical system axis and orientated perpendicular to the optical crystal axis of the attributed birefringent element. As to preferred em- bodiments and advantages of the optical system or the optical element group in this aspect, reference can be made to the preferred embodiments and advantages mentioned and discussed with respect to the optical system or the optical element group according to the first aspect.

According to a further preferred embodiment, said combined element or said element group is arranged in a pupil plane of said optical system.

This arrangement is advantageous in so far as light beams entering the image-sided last lens element of the projection lens under the same angle (and therefore are subjected to a birefringence of similar strength) are passing the element group or the combined element, respectively, at substantially the same position and will be identically compensated with regard to their polarization state. According to a further preferred embodiment, said combined element or said element group is arranged at a position

where the relation 0.8< — A^L<1.0 is met, with Di being a di-

2 ameter of a light bundle at said position and D₂ being a total optically used diameter at said position.

This arrangement is advantageous in view of the improved compensation which may be obtained in case of a field- dependency of the polarization effect caused by the image- sided last lens element (due to different geometrical path length within the said last lens element belonging to different field positions of the light beams), since said field dependency can be better considered with a displacement of the element group or combined element respec- tively, with respect to the pupil plane.

According to a further preferred embodiment, the optical system comprises at least two combined elements or element groups, which are both arranged at a position where the

relation 0.5 < —-≤l.O is met, with Di being a diameter of a

2 light bundle at the respective position being a total optically used diameter at the respective position. Such an arrangement considers that the achieved compensation is particularly effective at positions being at least closed to the pupil plane. In particular, these two element groups, or combined element group, can be symmetrically arranged with regard to the pupil plane, i.e. at positions along the optical system optics having the same relation O₁ZO₂, but on opposite sides on the pupil plane. According to a preferred embodiment, the element group or combined element, respectively, is arranged in the first pupil plane along the light propagation of the optical system. This position is advantageous particularly with respect to the enhanced possibilities to vary this pupil plane in the design in the whole optical system with regard to the corrective effect and the geometrical size of the compensation element (or element group) placed therein. This is because the first pupil plane is arranged at a position where the numerical aperture (NA) is relatively low compared to the last (i.e. image-sided) pupil plane and where the numerous optical elements being arranged downstream of this first pupil plane provide sufficient possibilities to correct and optimize the optical imaging.

According to a further preferred embodiment, said combined element or said element group have a maximum axial length along the optical system axis being not more than 50%, preferably not more than 20%, and still more preferably not more than 10 % of the average optically effective diameter of the element group. Such a small axial length may be obtained by arranging the birefringent elements of the group close to each other, by making each optical element with a relatively small thickness and/or by arranging the birefringent elements (or element pairs, respectively) directly adjacent to each other without any other optical elements in between. Such a compact design of the optical element group is advantageous in so far as a divergence of light beams which are passing the same inclined to the optical system axis is reduced or minimized, so that light beams passing the element with the same distance to the optical system axis experience at least approximately the same polarization effect.

In a further aspect, the present invention also relates to an optical element comprising a first lens component embedded in a second lens component, wherein said first lens component is made from spinell and wherein said second lens component is made from an optically isotropic material. An advantageous effect of such a structure of the optical element is that the first lens component may be made relatively thin, and any deterioration of the optical performance of the optical system due to effects of said element (in particular uniaxial or intrinsic birefringence as well as absorption) may be kept small. Such an optical element can be realized in combination with or also independent of an optical system as outlined above.

The present invention also relates to a microlithographic projection exposure apparatus, an illumination system or a projection lens for a microlithographic projection exposure apparatus, the use of an optical system for microlithographic projection exposure, a method of microlithographic structuring a substrate and a microstructured device .

Further aspects and advantageous embodiments of the present invention result from the following description as well as the further appended claims whose content is made part of the description in its entirety by reference. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail with reference to the following detailed description and based upon preferred embodiments shown in the drawings, in which:

Figure 1 shows a meridional section of a microlitho- graphy projection lens according to an exem- plarily embodiment of the present invention;

Figures 2 schematically shows a principal structure of an optical element group in a side view (Fig. 2a and 2b) and in a top view (Fig. 2c and 2d) on each of the three elements according to embodiments of the present invention;

Figure 3a-c shows height profiles (in micrometres, μm) for specific birefringent elements in an element group according to the embodiments of Fig. 2a-2d;

Figure 4a-b shows the retardation of the projection lens of Fig. 1 without (Fig. 4a) and with an optical element group according to the present invention;

Figure 5a-f schematically show principal structures of further embodiments of an optical element group according to Fig. 2a in a top view on each of the three elements;

Figure 6 shows a meridional section of a microlitho- graphy projection lens according to a further exemplarily embodiment of the present invention;

Figures 7a-d show principal structures of an optical ele- and 8a-b ment group according to further embodiments of the present invention;

Figures 9a-c show height profiles for birefringence elements in the optical group according to fig- ure 7 and 8 according to an embodiment of the invention;

Figure 10a-b show the respective retardance pupil map for the projection lens with (Fig. 10a) and with- out (Fig. 10b) an element group according to

Fig. 7-9;

Figure 11 shows a meridional section of a microlitho- graphy projection lens according to a further exemplarily embodiment of the present invention;

Figure 12 shows a detail of the microlithography projection lens of Fig. 11;

Figure 13a-c show height profiles (in micrometres, μm) of three optical elements in an element group according to the present invention that is used in order to partially compensate for the Jones-Pupil of Figure 14a-b;

Figure 14a-b show by way of an example a Jones-Pupil in a microlithography projection lens comprising a spinell-100-lens , wherein Fig. 14a shows the distribution of the absolute value of retardation (in nm) and wherein Fig. 14b shows the direction of the fast axis; and

Figure 15a-b show the retardation profile in radiant of each of the three optical elements in an element group that is used according to the invention to transform a circular polarization distribution (Fig. 15a) or linear polarization distribution (Fig. 15b) into a tangential polarization distribution as a function of the azimuth angle.

DETAILED DESRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 shows a meridional overall section through a complete catadioptric projection lens 100 in accordance with an embodiment of the invention. The design data of the projection lens 100 are set out in Table 1. In this Table, column 1 includes the number of the respective, reflective or otherwise distinguished optical surface, column 2 includes the radius of this surface (in mm) , column 3 the distance (also named as thickness, in mm) of this surface from the next following surface, column 4 the material following to the respective surface, column 5 the refractive index of this material at λ= 193nm and column 6 the optically usable, free half diameter of the optical compo- nent (in mm) .

The surfaces which are identified in Fig. 1 by short horizontal lines and which are specified in Table 2 are aspherically curved, the curvature of those surfaces being given by the following aspheric formula:

In that formula (5), P denotes the sagitta of the surface in question in parallel relationship with the optical axis, h denotes the radial spacing from the optical axis, r denotes the radius of curvature of the surface in ques- tion, K denotes the conical constant and Cl, C2, ... denote the aspheric constants set out in Table 2.

The projection lens 100 comprises, along an optical system axis OA and between an object (or reticle) plane OP and an image (or wafer) plane IP, a first subsystem 110 comprising refractive lenses 111-114 and 116-119, a second subsystem 120 comprising a first concave mirror 121 and a second concave mirror 122 which are each cut at the appropriate positions to enable the passing of light rays there through, and a third subsystem 130 comprising refractive lenses 131-143. The image-sided last lens 143 of the third subsystem is a plano-convex lens made from LU₃AI5O₁₂ (= "LuAG") and having a [100] -orientation, i.e. the optical system axis OA of the projection lens 100 is parallel to the [100] -crystal axis of the lens 143. The image-sided last lens 143 is adjacent to an immersion liquid being present between said last lens 143 and the light-sensitive layer on the wafer being arranged, during the operation of the projection lens 100, in the image plane IP. Said im- mersion liquid 143 has, in the illustrated embodiment, a refraction index of n_imm«l .65. A suitable immersion liquid is e.g. "Decalin". A further suitable immersion liquid is e.g. Cyclohexane (n_imm«1.57 at λ« 193nm) .

In the sense of the present application, the term ' subsys- tern' always denotes such an arrangement of optical elements, by which a real object is imaged in a real image or intermediate image. In other words, each subsystem starting from a given object or intermediate image plane always includes all optical elements to the next real image or intermediate image.

The first subsystem 110 images the object plane OP onto a first intermediate image IMIl, the approximate position of which being marked in Fig. 1 with an arrow. This first in- termediate image IMIl is imaged, by the second subsystem 120, into a second intermediate image IMI2, the approximate position of which is also marked in Fig. 1 with an arrow. The second intermediate image IMI2 is imaged, by the third subsystem 130, into the image plane IP.

At a position marked by arrow 115 in Fig. 1 and close to the pupil plane PPl within the first subsystem 110, an element group is provided whose structure is explained in the following with reference to Fig. 2a-d and Fig. 3.

The element group 200 consists, according to Fig. 2a, of three birefringent elements 211-213 each being made of optically uniaxial sapphire (Al₂O₃) . The optical crystal axes of the optically uniaxial material in the three elements 211-213 are, according to Fig. 2c, oriented different from each other. Furthermore, each of the three elements 211- 213 comprises an aspheric surface only schematically illustrated in Fig. 2a and as explained in more detail with respect to Fig. 3. It is emphasized that the schematic illustration of Fig. 2a only serves to symbolize that each of the elements 211-213 has a varying thickness profile, while a more quantitative description of the shape of said thickness profile can be gathered from the corresponding height profiles of Fig. 3.

As to the different orientations of the optical crystal axes and more specifically, these optical crystal axes, which are named as ca-1, ca-2 and ca-3 in Fig. 2c, are all oriented in a plane perpendicular to the optical axis OA (= z-axis) of the projection lens 100, i.e. in the x-y- plane according to the coordinate system shown in Fig. 2c. Further, according to Fig. 2c, the optical crystal axis ca-1 of element 211 is oriented parallel to the y-axis, the optical crystal axis ca-2 of element 212 is clockwise rotated around the optical axis OA (i.e. the z-axis) with respect to the crystal axis ca-1 by an angle of 45°, and the optical crystal axis ca-3 of element 213 is also clockwise rotated around the optical axis OA (i.e. the z- axis) with respect to the crystal axis ca-2 by an angle of 45° (i.e. by an angle of 90° with respect to the y-axis) .

More generally, the orientation of the optical crystal axis ca-2 in the second optical element 212 can be described as emerging from a rotation of the orientation of the optical crystal axis ca-1 in the first optical element 211 around the optical axis 100 of the projection lens 100, said rotation not corresponding to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof. Furthermore, the orientation of the optical crystal axis ca-3 in the third optical element 213 can be described as emerging from a rotation of the orien- tation of the optical crystal axis ca-2 in the second optical element 212 around the optical axis OA of the projection lens 100, said rotation also not corresponding to a rotation around the optical system axis OA by an angle of 90° or an integer multiple thereof.

As to the aspheric surface provided on each of said elements 211-213, Fig. 3a shows the height profile (in micrometres, μm) of the first element 211, Fig. 3b for the sec- ond element 212 and Fig. 3c for the third element 213. It can be seen that the height profiles of the first element 211 and the third element 213 are of opposite sign and, in the illustrated example, identical in amount.

To illustrate the effect of the element group 200 in the projection lens 100, Fig. 4a shows the retardation (in nanometers, nm) caused by the image-sided last lens element 143 for the case without said optical element group 200 at the position 115, while Fig. 4b shows the retardation of the projection lens 100 with the optical element group 200 at the position 115. It can be seen that the retardation in Fig. 4a has maximum values of approximately 180nm, whereas the maximum retardation in Fig. 4b is significantly reduced to very low values of approximately 0.5nm, which is more than sufficient for typical lithography applications .

Fig. 2d shows a further example of an element group of elements 221-223, wherein the orientations of the optical crystal axes ca-1 and ca-3 in the first element 221 and the third element 223 are identical and differ from the orientation of the optical crystal axis ca-2 in the second element 222. More specifically and as illustrated in Fig. 2d, the optical crystal axes ca-1 and ca-3 of elements 221 and 223 are both oriented parallel to the y-axis, whereas the optical crystal axis ca-2 of element 212 is rotated around the optical axis OA (i.e. the z-axis) with respect to the crystal axis ca-1 by an angle of 45°.

As a common feature with the embodiment of Fig. 2c, the orientation of the optical crystal axis ca-2 in the second optical element 222 can be described as emerging from a rotation of the orientation ca-1 of the optical crystal axis ca-1 in the first optical element 221 around the optical axis OA of the projection lens 100, said rotation not corresponding to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof. Furthermore, the orientation of the optical crystal axis ca-3 in the third optical element 223 can be described as emerging from a rotation of the orientation of the optical crystal axis ca-2 in the second optical element 222 around the optical axis OA of the projection lens 100, said rota- tion also not corresponding to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof .

As to the aspheric surface provided on each of said ele- ments 221-223, Fig. 3a shows the height profile (in micrometres, μm) of the first element 221 and the third element 223, whereas Fig. 3b shows the height profile for the second element 222. Accordingly, in this specific example the height profiles of the first element 221 and the third element 223 are identical, which means that this element is suitable to compensate, in the projection lens 100, a retardation without elliptical components. However, the invention is not limited thereto, so the invention also comprises groups of optical elements 221-222c with the principal structure of Fig. 2c, but with different height profiles of the first and third element 221 and 223.

Although the elements 211-213 and 221-223 of the embodiments described with reference to Fig. 2-3 are all made from sapphire (Al₂O₃) , the invention is not limited to this, and other optically uniaxial materials having sufficient transparency in the used wavelength region, for ex- ample but not limited to magnesium-fluoride (MgF₂) , lanthanum-fluoride (LaF₃) and crystalline quartz (SiO₂) can be alternatively used. Furthermore, the invention is not restricted to a realization of all the three elements 211- 213 or 221-223 from the same material, so that also dif- ferent combinations of materials may be used.

Fig. 5a-f show principal structures of further embodiments of an optical element group according to Figure 2a in a top view on each of the three elements.

To generalize these different embodiments of element groups according to Fig. 5 and like in Fig. 2c and Fig. 2d, for any of these element groups, the orientation of the optical crystal axis ca-2 in the respective second op- tical element 512-562 can be described as emerging from a rotation of the orientation ca-1 of the optical crystal axis ca-1 in the respective first optical element 511-561 around the optical axis 100 of the projection lens 100, said rotation not corresponding to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof. Furthermore, the orientation of the optical crystal axis ca-3 in the respective third optical element 513-563 can be described as emerging from a rotation of the orientation of the optical crystal axis ca-2 in the respective second optical element 512-563 around the optical axis OA of the projection lens 100, said rotation also not corresponding to a rotation around the optical system axis by an angle of 90° or an integer multiple thereof.

As a further common feature of these elements groups and like in Fig. 2c and Fig. 2d, the optical crystal axes "ca- 1" and "ca-3" of two of the respective three elements (e.g. element 511 and element 513 in Fig. 5a) are orientated differently from the optical crystal axis of the third element (e.g. element 512 in Fig. 5a) .

More specifically according to Fig. 5a, the optical crys- tal axis "ca-2" of element 512 is running into the y- direction in the coordinate system illustrated in the figure, while the optical crystal axes ca-1 and ca-3 are both rotated around the optical system axis OA and with respect to said optical crystal axis ca-2 by 45°. All elements 511-513 may e.g. be made from magnesium-fluoride (MgF₂), sapphire (AI2O3) or another suitable optically uniaxial material .

According to Fig. 5b, the optical crystal axis ca-2 of element 522 is running into the y-direction in the coordinate system illustrated in the figure, while the optical crystal axes ca-1 and ca-3 of elements 521 and 523 are running parallel to the optical system axis OA (i.e. into z-direction) . Element 522 is e.g. made from magnesium- fluoride (MgF₂) , while elements 521 and 523 are made from optically active quartz. According to Fig. 5c, the optical crystal axis ca-2 of element 532 is running parallel to the optical system axis OA (i.e. into z-direction) , while the optical crystal axes ca-1 and ca-3 of elements 531 and 533 are running into the y-direction in the coordinate system illustrated in the figure. Elements 531 and 533 are e.g. made from magnesium- fluoride (MgF₂) , while element 532 is made from optically active quartz.

According to Fig. 5d, the optical crystal axis ca-2 of element 542 is running perpendicular to the optical system axis OA and is rotated with respect to the y-direction by 45°, while the optical crystal axes ca-1 and ca-3 of elements 541 and 543 are running parallel to the optical sys- tern axis OA (i.e. the z-direction in the coordinate system illustrated in the figure) . Element 542 is e.g. made from magnesium-fluoride (MgF₂), while elements 541 and 543 are made from optically active quartz.

According to Fig. 5e, the optical crystal axis ca-2 of element 552 is running parallel to the optical system axis OA (i.e. the z-direction in the coordinate system illustrated in the figure) , while the optical crystal axes ca-1 and ca-3 of elements 551 and 553 are running perpendicular to the optical system axis OA and are rotated with respect to the y-direction by 45°. Elements 541 and 543 are made from magnesium-fluoride (MgF₂), while element 542 is made from optically active quartz.

According to Fig. 5f, the optical crystal axis ca-1 of element 561 is running parallel to the optical system axis "OA" (i.e. into z-direction) . The optical crystal axis ca- 2 of element 562 is running into the y-direction. The op- tical crystal axis ca-3 of element 563 is running perpendicular to the optical system axis OA and is rotated with respect to the y-direction by 45°. Elements 562 and 563 are e.g. made from magnesium-fluoride (MgF₂), while ele- ment 561 is made from optically active quartz. Accordingly, in the embodiment shown in Fig. 5f, the optical crystal axes of all of said three optical elements 561-563 are, like in the embodiment of Fig. 2c, orientated different from each other. Of course, the embodiment shown in Fig. 5f is not limited to the illustrated order of elements 561-563 but comprises all possible permutations of these elements (with e.g. element 563 being arranged between elements 561 and 562 etc.) .

As a further common feature of the above described element groups, each of them comprises three optical elements being made of an optically uniaxial material and having a varying thickness profile along said optical system axis, wherein an optical crystal axis in each of said optical elements is either substantially perpendicular or substantially parallel to said optical system axis, and wherein the optical crystal axes of at least two of said three optical elements are orientated different from each other.

In the embodiments of Fig. 2d and 5a, all of said three optical elements have an optical crystal axis which is substantially perpendicular to said optical system axis, wherein the optical crystal axes of a first optical element and a second optical element (namely elements 211 and 213 or 511 and 513, respectively) of said group are substantially parallel to each other and rotated around said optical system axis with respect to the optical axis of a third optical element (namely elements 212 or 512, respectively) of said group.

In the embodiments of Fig. 5b-f, only one or two of said optical elements (namely elements 522, 531, 533, 542, 551, 553) of said group have an optical crystal axis which is substantially perpendicular to said optical system axis, wherein the other optical element (s) (namely elements 521, 523, 532, 541, 543, 552, 561) of said group have an opti- cal crystal axis which is substantially parallel to said optical system axis. In these embodiments, the elements having an optical crystal axis which is substantially parallel to the optical system axis OA are made from an optically active material, e.g. quartz.

In the embodiment according to Fig. 5f, the optical crystal axes of all of said three optical elements 561-563 are orientated different from each other. The element having an optical crystal axis which is substantially parallel to the optical system axis OA is made from an optically active material, e.g. crystalline quartz.

Fig. 2b shows a further preferred embodiment for an element group according to the present invention, which has the advantageous effect that a detrimental influence of said element group on the so-called scalar phase can be kept low. According to the concept schematically illustrated in Fig. 2b, intermediate spaces 216, 218 between different birefringent elements 215, 217 and 219 are filled with a liquid in order to reduce the shift in refractive index occurring when the light passing the optical group enters a light entrance surface or leaves a light exit surface of any of said birefringent elements. In the specific embodiment according to Fig. 2b, each of the birefringent elements 215, 217 and 219 is made of MgF2, and the intermediate spaces 216 and 218 are filled with water (H₂O) .

At a typical operating wavelength of λ« 193.38 nm, the ordinary refractive index of MgF₂ is n_o«1.4274, and the extraordinary refractive index is n_e«1.4410, corresponding to an average refractive index n = (n_o+n_e) /2«1.4342. The re- fractive index of water (H₂O) at λ« 193.38 nm is 1.4366. Accordingly, the shift in refractive index occurring between the birefringent elements 215, 217 and 219 and the intermediate spaces 216 and 218 amounts (for the averaged index in MgF₂) to Δn« 0.0024. For comparison, the shift in refractive index, if the intermediate spaces 216 and 218 are filled with a typical filling gas as e.g. nitrogen (N₂) at λ« 193.38 nm, is Δn« 0.439. Accordingly, the shift in refractive index occurring between the birefringent elements 215, 217 and 219 and the intermediate spaces 216 and 218 is reduced, for the embodiment of Fig. 2b, approximately by a factor of 180.

Of course, the above concept of filling the intermediate spaces between the birefringent element with a suitable liquid in order to reduce the shift in refractive index occurring at light entrance surfaces and/or light exit surfaces of said birefringent elements is not limited to the above combination of MgF₂ with H₂O. In general, a liquid may be regarded as suitable to significantly improve the above index-shift-situation between the birefringent elements of the inventive element group, and thus reduce a detrimental influence of said element group on the so- called scalar phase, if a gap between at least two of said birefringent elements is at least partially filled with a liquid having a refraction index that differs not more that 30%, preferably not more than 20%, and still more preferably not more than 10% of the refraction indices of said two birefringent elements. Depending on the refractive indices of the material in the adjacent birefringent elements, such suitable liquids may also be so-called high-index immersion liquids which are also used as immersion liquids in the region between the image-sided last lens and the light-sensitive layer being present on the wafer, such as e.g. "Decalin" (n_irrm« 1.65 at λ« 193nm) or Cyclohexane (n_imm«1.57 at λ« 193nm) .

Fig. 6 shows a meridional overall section through a com- plete catadioptric projection lens 600 in accordance with a further embodiment of the invention. The design data of the projection lens 600 are set out in Table 3, with the surfaces specified in Table 4 are aspherically curved.

The projection lens 600 has a similar, catadioptric design as the projection lens 100 of Fig. 1, and comprises along the optical axis OA a first subsystem 610 with lenses 611- 617, a second subsystem 620 with two mirrors 621 and 622 and a third subsystem 630 with lenses 631-642.

The projection lens 600 also comprises, at a position marked with an arrow and closed to the pupil plane PP2 within the third subsystem 630, an element group 650 according to a further preferred embodiment of the inven- tion, embodiments of which being described in the following with reference to Fig. 7 and 8. The advantageous effect achieved by these embodiments is that a detrimental influence of said element group on the so-called scalar phase can be kept low and, in the ideal case, made equal to the effect caused by a plan-parallel plate on the scalar phase.

To this, the element group 650 as schematically illustrated in Fig. 6a comprises three birefringent elements 651, 652 and 653, each of which being composed of two plates 651a and 651b, 652a and 652b, or 653a and 653b, respectively. Each of the respective plates being attributed to each other has an aspheric surface and a plane surface, wherein the aspheric surfaces of the plates being attributed to each other are complementary and add up to a plan- parallel geometry of the such-formed birefringent element 651, 652 or 653, respectively. With other words, the thickness of each formed birefringent element 651, 652 or 653, respectively, is constant over its cross-section.

Furthermore, as can be seen in Fig. 8a which is showing all six plates 651a-653b in an exploded way of illustra- tion just for a better representation of the optical crystal axes, the optical crystal axes of the respective plates 651a and 651b, 652a and 652b, or 653a and 653b, respectively being attributed to each other are orientated perpendicular to each other. Apart from said orientation of the optical crystal axes, the plates of each pair 651a and 651b, 652a and 652b, or 653a and 653b, respectively, and preferably all six plates 651a-653b are made of the same optically uniaxial material, e.g. Al₂O₃, MgF₂ or LaF₃.

As a consequence of the plan-parallel geometry of the birefringent elements 651-653, each of said birefringent elements 651, 652 and 653 does not disturb or affect the scalar phase of light passing though said element group 650, since the aspheric boundaries which are present within each birefringent element 651, 652 and 653 at the position where the two plates complementary abut on each other with their aspheric surface are only boundaries be- tween regions of identical refractive indices. The embodiment shown in Fig. 8a is just exemplarily, and further embodiments to realize the general concept of Fig. 7 can be constructed by composing an element group as follows: As to the respective first plates 651a, 652a and 653a of each birefringent element 651, 652 and 653, these plates are arranged according the optical axis OA according to the principal structure of Fig. 5a. Similarly, the other embodiments described above and illustrated in Fig. 2c-d and Fig. 5b-f may be modified by replacing, in each of said embodiments, at least one (and preferably all) of those birefringent elements which have their optical crystal axis orientated in a plane perpendicular to the optical system axis OA by a pair of plates as described before with reference to Fig. 7-8, i.e. by plates being pairwise complementary to each and adding up to a plan-parallel geometry of the such-formed birefringent element and having optical crystal axes being orientated pairwise perpendicular to each other.

Although the three birefringent elements 651-653 of Fig. 7a of the optical group 650 are shown separated from each other, they may be, according to a preferred embodiment shown in Fig. 7b, joined together to form a common optical element 650' , which is favourable in view of the mechani- cal stability of the arrangement taking into consideration the relatively low thickness of the plates 651a-653b, which is typically much less than 1 mm and may e.g. be in the range of several micrometers. In further embodiments, one or more support plates of a significantly larger thickness are used as schematically illustrated in Fig. 7c and 7d. More specifically, Fig. 7c shows two such support plates 660 and 670, one of each being arranged between each neighboured birefringent elements 651 and 652 or 652 and 653, respectively, to form a common element 650''. Fig. 7d shows all three birefringent elements 651-653 joined together as already shown in Fig. 7b and supported by a single support plate 680 to form a common element 650' ' ' . A perspective view of this embodiment is shown in Fig. 8b. Such one or more support plates 660, 670 and 680 are preferably made from an optically isotropic material such as fused silica (SiO₂) . Although the thicknesses of such support plates are principally arbitrary, typical thicknesses are in the range of several millimetres .

The height profiles of the birefringent elements according to Fig. 8 are shown in Fig. 9. A quantitative description of the height profiles of the birefringent elements can be given e.g. based on the commercially available software "CODE V 9.6" (October 2005) of "OPTICAL RESEARCH ASSOCIATES", Pasadena, California (USA) , according to which the respective free-form surfaces, as described in the corresponding Release Notes of this software, are described via a polynomial approximation using the equation

wherein z denotes the sagitta of the surface parallel to the z-axis, c denotes the vertex curvature, k denotes the conical constant, Z₃ denotes the j^th Zernike polynomial (standard Zernike polynomials in radial coordinates, i.e. Zi = 1, Z₂= R-cosθ, Z₃= R-sinθ, Z4= R²-cos2θ, etc.) and C_{] +}i denotes the coefficient for Z₁.

For the embodiment of Fig. 9a-9c, Table 5 gives for each of the free-form surfaces 41, 43 and 45 the corresponding coefficients of the above Zernike polynomials, wherein ZPi=C₂ denotes the coefficient of term 1-zernike- polynomial, ZP₂=C₃ denotes the coefficient of term 2- zernike-polynomial, ..., ZP₆₃=C₆₄ denotes the coefficient of term 63-zernike-polynomial etc.

The effect of the corresponding optical group is shown in Fig. 1Oa-IOb by way of the respective retardance pupil map for the projection lens with (Fig. 10a) and without (Fig. 10b) an element group according to Fig. 7-9. It can be seen that the element group effects a significant reduction of the retardance (note the different scales in Fig. 10a and 10b) .

Fig. 11 shows a meridional overall section through a complete catadioptric projection lens 900 in accordance with a further embodiment of the invention. The projection lens 900 has a similar design as the projection lens 100 of Fig. 1, and comprises along the optical axis OA a first subsystem 910 with lenses 911-917 , a second subsystem 920 with two mirrors 921 and 922 and a third subsystem 930 with lenses 931-942.

In order to compensate for a disturbance of the polarization within the projection lens 900, the projection lens 900 again comprises, in the first pupil plane "PPl" and at a position marked with arrow, a correction element 950 formed of an element group of three birefringent elements as has been described above, with the height profiles of three optical elements being discussed below with refer- ence to Fig. 13a-13c.

As a further aspect of the projection lens 900 of Fig. 11, the last lens 942 of the third partial system 930 (i.e. the lens closest to the image plane IP) comprises a first lens component 942a embedded in a second lens component 942b as described below in more detail with reference to the enlarged schematic diagram of Fig. 12.

It is to be noted that the realization of this "embedded lens"-configuration is of course not limited to a combination with the compensation concept of making use, for compensation of a disturbance of polarization, of an optical group or correction element composed of at least three birefringent elements with aspheric surfaces. Accordingly, the aspect illustrated in Fig. 12 also covers other designs (without such correction element or optical group) where an optical lens, which may particularly be an image- sided last element, i.e. an optical element being most close to the image plane, is realized by embedding a first lens component in a second lens component, as described in the following.

Generally, the arrangement shown in Fig. 11 and 10 is advantageous if the first lens component 942a is made from an optically uniaxial material or a material of cubic crystal structure with strong intrinsic birefringence, and the second lens component 942b is made from an optically isotropic material. Beside a cubic crystal like spinell, the material of the first lens component can e.g. be selected from the group that consists of magnesium-fluoride (MgF₂) , lanthanum-fluoride (LaF₃) , sapphire (AI2O3) and crystalline quartz (SiO₂) . An advantageous effect of the above structure of the optical element is that the first lens component 942a may be made relatively thin, and any deterioration of the optical performance of the optical system due to effects of said element (in particular uniaxial or intrinsic birefringence as well as absorption) may be kept small.

In the exemplarily embodiment of the image-sided last lens 942 of Fig. 11 and 12, the first lens component 942a is made from (100) -spinell, and the second lens component 942b is made from fused silica (SiO₂) . In the specific example of Fig. 11 and 12, the lens 942 is described by the following parameters of Table 6:

Table 6:

Furthermore, the arrangement of Fig. 12 is preferably realized by a close contact between the light entrance surface of the first lens component 942a and the light exit surface of the second lens component 942b. Alternatively, an immersion liquid layer or a small air-gap may be ar- ranged between the light entrance surface of the first lens component 942a and the light exit surface of the second lens component 942b.

Referring again to the correction element 950 mentioned above, the correction element is used in the projection lens 900 for compensating the Jones-Pupil illustrated in Fig. 14a-b, wherein said Jones-Pupil has been determined for a microlithography projection lens comprising a spinell-100-lens . More specifically, Fig. 14a shows the distribution of the absolute value of retardation (in nm) and Fig. 14b shows the direction of the fast axis of retardation .

Fig. 13a-c show the height profiles of the first, second and third optical element, respectively, being arranged according to the general structure of Fig. 2a. In the illustrated embodiment, each of the optical elements 951-953 is made of magnesium-fluoride. These height profiles are determined by first determining, for each of the first, second and third optical element, the retardation distribution needed in order to achieve the desired compensation effect, and then calculating the corresponding height profile. Generally, in order to provide at a predetermined position a predetermined retardation of Δφ, a thickness d is needed as given in the (already above-mentioned) equation (4) .

d=™*- (4^'

2πAn As to the general shape of the Jones-Pupil illustrated in Fig. 14, the distribution of retardation shown in Fig. 14a has a fourfold symmetry as it is characteristic for the spinell- [100 ] -lens to be compensated for in the exempla- rily embodiment. Furthermore, it can be seen that for each of said first, second and third optical element, the height profile has a mirror symmetry with two axes as well as a sign-change with rotation by an angle of 90°.

According to a further aspect of the invention, a group of optical elements as outlined above with reference to Fig. 1-12 may be used to generally transform a first (e.g. circular or linear) polarization distribution into a second (e.g. tangential) polarization distribution. To this, ref- erence can be made e.g. to the general configuration of Fig. 2d, i.e. with the optical crystal axes of all bire- fringent, elements 211-213 being perpendicular to the optical system axis, and with the optical crystal axis of the second element ca-2 being rotated around the optical system axis OA and with respect to the optical crystal axes ca-1 and ca-3 of the first and the second optical element by 45°. All three elements are again made of optically uniaxial material and may e.g. be made of magnesium- fluoride (MgF₂) .

If the three birefringent elements of such a group have the retardation profiles illustrated in Fig. 15a, this element group may be used to transform a circular polarization distribution into a tangential polarization distri- bution. In Fig. 15a and 15b, curve "Tl" illustrates the retardation profile a function of the azimuth angle & for the first element 201, curve "T2" illustrates the retardation profile for the second element 202 and curve "T3" il- lustrates the retardation profile for the third element 203. The respective retardation profiles may be constant in the radial direction. If the three elements of the element group show the retardation profiles illustrated in Fig. 15b, this element group may be used to transform a linear polarization distribution into a tangential polarization distribution.

The above description of preferred embodiments has been given by way of example. A person skilled in the art will, however, not only understand the present invention and its advantages, but will also find suitable modifications thereof. Therefore, the present invention is intended to cover all such changes and modifications as far as falling within the spirit and scope of the invention as defined in the appended claims and the equivalents thereof.

Table 1: DESIGN DATA for Fig. 1 (NA = 1.55 ; wavelength λ= 193 run)

Table 1 (continued) : DESIGN DATA for Fig. 1

Table 2: ASPHERICAL CONSTANTS for Fig. 1

Table 2 (continued) : ASPHERICAL CONSTANTS for Fig. 1

SRF 31 33 36 38 40

K 0 0 0 0 0

C1 3 .570488E-09 -1 108288E-08 1 098120E-08 3 498535E-09 4 009017E-08

C2 -2 899790E-13 -5 556755E-13 -8 319264E-13 1 277784E-12 -5 714125E-12

C3 1 081327E-16 -3 884368E-18 3 311901 E-17 -7 357487E-17 6 202718E-16

C4 -1 172829E-20 1 842426E-21 7 733186E-23 1 115535E-21 -5 344939E-20

C5 2 404194E-25 3 001406E-27 -1 051458E-26 2 894369E-25 3 354852E-24

C6 1 461820E-29 -7 804121 E-30 -4 556477E-30 -1 579978E-29 -1 359158E-28

C7 -5 103661 E-34 2 042295E-34 1 779547E-34 3 499951 E-34 2 690400E-33

SRF 42

K 0

C1 6 190270E-09

C2 1 866031 E-11

C3 -3 186549E-15

C4 5 219881 E-19

C5 -6 008898E-23

C6 4 502251 E-27

C7 -1 632255E-31

Table 3: DESIGN DATA for Fig. 6 (NA = 1.55 ; wavelength λ= 193 run)

Table 3 (continued) : DESIGN DATA for Fig. 6

Table 4: ASPHERICAL CONSTANTS for Fig. 6

SURFACE NR. π

K -1.4693

C] -2.06488339e-008

C2 1.16939811e-014

C3 -1.28854467e-0i8

C4 -2.18667724e-024

C5 -?.]1424143e-029

C6 -2.63669751e-033

C7 O.OOOOOOOOe+000

C8 0. OOOOOOOOe+000

C9 O.OOOOOOOOe+000

SURFACE NR. 18

K -1.4756

Cl 1.81J 34384e-008

C2 4.18803124e-014

C3 1.13727194e-018

C4 1.0b429895e-023

C5 -7.51318112e-029

C6 5.73990187e-033

C7 0. OOOOOOOOe+000

C8 O.OOOOOOOOe+000

C9 0. OOOOOOOOe+000

SURFACE NR. 21

K 0.0000

( 1 -o ^0/75113e-008

C2 -1.42875005e-012

C3 2 44348063C-017

C4 2.6934947Be-02¹.

C5 -6.45183994e-026

C6 -1.06542172e-030

C7 O.OOOOOOOOe+000

C8 O.OOOOOOOOe+000

C9 O.OOOOOOOOe+000

SURFACE NR. 22

K 0.0000

C_^ 3.2565657CΘ-008

C2 -9.8015] 931e-012

C3 4.72663722e-016

C4 -3.37084211e-020

C5 5.44443713e-024

C6 -2.69886851e-028

C7 O.OOCOOOOOe+000

C8 O.OOOOOOOOe+000

C9 O.OOOOOOOOe+000

SURFACE NR. 26

K 0.0000

Cl -1.25873172e-007

C2 5.07 /29011e-013

C3 -4. ^] 5968C4Θ-0J 6

C4 3. O710175e-020

C5 -1.093714240-024

C6 7._^9441882e-029

C7 0. OOOOOOOOe+000

C8 0. OOOOOOOCe+000

C9 O.OOOOOOOOe+000 Table 4 (continued) : ASPHERICAL CONSTANTS for Fig. 6

Table 5: Coefficients for Zernike polynomial-terms for free- form surfaces of Fig. 9

Surface Specifications and Birefringence data

41 ZP2: 1.3464E-04 ZP6: 7.0720E-03 ZP7 : -3.0436E-04

ZP8: -8.6148E-05 ZP14 -2.7788E-03 ZP15 9.9238E-05

ZP16: 1.6627E-04 ZP17 9.6187E-05 ZP18 1.6835E-04

ZP26: 7.2238E-04 ZP27 -5.4027E-05 ZP31 -8.2896E-05

ZP32: 9.2226E-05 ZP42 -1.3009E-04 ZP50 2.2443E-05

NRADIUS: 1.4442E+02

BIREFRINGENCE: -0.01130

CRYSTAL AXIS: 0.707107 -0.707107 0.000000

42 BIREFRINGENCE: -0.01130

CRYSTAL AXIS: 0.000000 1. ( 300000 0.000000

43 ZPl: -2.2103E-05 ZP3 3.1465E-05 ZP4 -2.6569E-03

ZP5: 1.2076E-05 ZP9 -2.0832E-04 ZPlO 2.4878E-04

ZPIl: -1.1947E-04 ZP12 2.2720E-03 ZP13 -4.8980E-05

ZP19: -1.6463E-05 ZP20 2.6678E-04 ZP21 1.2347E-04

ZP23: -1.0043E-04 ZP24 -7.8608E-04 ZP25 -4.9355E-05

ZP33: -8.3815E-05 ZP34 2.9550E-04 ZP40 6.6448E-04

ZP41: -3.2893E-05 ZP51 -8.6576E-05 ZP61 -1.4676E-06

NRADIUS: 1.4437E+02

BIREFRINGENCE: -0.01130

CRYSTAL AXIS: 1.000000 0.( 300000 0.000000

44 BIREFRINGENCE: -0.01130

CRYSTAL AXIS: 0.707107 0.7 C 37107 0.000000

45 ZP2: 9.9565E-05 ZP6: 7.1135E-03 ZP7 : -5.2388E-04

ZP8: -1.9099E-04 ZP14 -2.7880E-03 ZP15 5.6141E-05

ZP16: -1.2722E-04 ZP17 1.0277E-04 ZP18 -2.1371E-04

ZP26: 6.8543E-04 ZP27 -1.0003E-04 ZP31 -5.4322E-06

ZP32: -2.5020E-04 ZP42 -1.4399E-04 ZP50 -1.2186E-04

NRADIUS: 1.4433E+02

BIREFRINGENCE: -0.01130

CRYSTAL AXIS: 0.707107 -0.7 C 37107 0.000000

51 INTRINSIC BIREFRINGENCE 0.3010E-05

CUBIC AXIS ORIENTATION: Y : 0.707107 0.7C 7107 0.000000

2 -0.707107 0.70 7107 0.000000

Claims

1. An optical system, in particular an illumination system or a projection lens of a microlithographic exposure system, said optical system having an optical system axis (OA) and at least one element group (200) consisting of three birefringent elements each of which being made of optically uniaxial material and having an aspheric surface, wherein:

• a first birefringent element (211, 221, 511, 521, 531, 541, 551, 561) of said group has a first orientation of its optical crystal axis (ca-1);

• a second birefringent element (212, 222, 512, 522, 532, 542, 552, 562) of said group has a second orientation of its optical crystal axis (ca-2) , wherein said second orientation can be described as emerging from a rotation of said first orientation (ca-1), said rotation not corresponding to a rotation around the optical system axis (OA) by an angle of 90° or an integer multiple thereof; and

• a third birefringent element (213, 223, 513, 523, 533, 543, 553, 563) of said group has a third orientation of its optical crystal axis (ca-3), wherein said third orientation can be described as emerging from a rotation of said second orientation, said rotation not corresponding to a rotation around the optical system axis (OA) by an angle of 90° or an integer multiple thereof.

2. An optical system, in particular an illumination system or a projection lens of a microlithographic exposure system, said optical system having an optical system axis (OA) and at least one element group (650, 650', 650'', 650''') consisting of three element pairs (651, 652, 653) each of which consisting of one birefringent element (651a, 652a, 653a) and one attributed compensa- tion element (651b, 652b, 653b), said birefringent element being made of optically uniaxial material and having an aspheric surface, wherein each birefringent element and the attributed compensation element supplement each other to a plan-parallel geometry of said element pair (651, 652, 653), wherein:

• a first birefringent element (651a) of said group

(650) has a first orientation of its optical crystal axis (ca-1) ;

• a second birefringent element (652a) of said group (650) has a second orientation of its optical crystal axis (ca-2) , wherein said second orientation can be described as emerging from a rotation of said first orientation (ca-1), said rotation not corresponding to a rotation around the optical system axis (OA) by an angle of 90° or an integer multiple thereof; and

• a third birefringent element (653a) of said group

(650) has a third orientation of its optical crystal axis (ca-3), wherein said third orientation can be described as emerging from a rotation of said second orientation, said rotation not corresponding to a rotation around the optical system axis (OA) by an angle of 90° or an integer multiple thereof.

3. The optical system according to claim 2, characterized in that each compensation element (651b, 652b, 653b) is also made of optically uniaxial material, wherein the optical crystal axis of each compensation element (651b, 652b, 653b) is orientated perpendicular to the optical system axis (OA) and perpendicular to the optical crystal axis (ca-1, ca-2, ca-3) of the attributed birefrin- gent element (651a, 652a, 653a) .

4. The optical system according to anyone of the preceding claims, characterized in that each of said birefringent elements has such a variation of its thickness profile that a minimum phase retardation of Δφ_min « 25 mrad is obtained at a given operating wavelength of said optical system.

5. The optical system according to anyone of the preceding claims, characterized in that the optical crystal axes of all of said three birefringent elements are orien- tated different from each other.

6. The optical system according to anyone of the claims 1 to 4, characterized in that the optical crystal axes of the first birefringent element (211, 511) and the third birefringent element (213, 513) are substantially parallel to each other.

7. The optical system according to anyone of the preceding claims, characterized in that said optical crystal axis of each of said first, second and third birefringent elements is either perpendicular or parallel to said optical system axis.

8. The optical system according to anyone of the claims 1 to 5, characterized in that the optical crystal axis of one of said birefringent elements is substantially parallel to said optical system axis (OA) , and that the optical crystal axes of the remaining two birefringent elements are substantially perpendicular to said optical system axis (OA) and rotated to each other around said optical system axis (OA) by an angle in the range of 30° to 60°, more preferably 40° to 50°, and still more pref- erably by an angle of 45°.

9. The optical system according to anyone of the claims 1 to 7, characterized in that the optical crystal axes (ca-1, ca-2, ca-3) of said birefringent elements are perpendicular to said optical system axis (OA) , wherein optical crystal axis (ca-1) of the first birefringent element (211, 511, 651a) and the optical crystal axis (ca-3) of the third birefringent element (213, 513, 653a) are each rotated around said optical system axis (OA) with respect to the optical crystal axis (ca-2) of the second birefringent element (212, 512, 652a) by an angle in the range of 30° to 60°, more preferably 40° to 50°, and still more preferably by an angle of 45°.

10. The optical system according to anyone of the preceding claims, characterized in that one or two of said birefringent elements (521-523, 531-533, 541-543, 551-553, 561-563) of said group have a optical crystal axis which is substantially perpendicular to said optical system axis, and wherein the other birefringent element (s) of said group have a optical crystal axis which is substantially parallel to said optical system axis (OA) .

11. The optical system according to claim 10, character- ized in that said birefringent element (s) of said group having a optical crystal axis which is substantially parallel to said optical system axis are made of an op- tically active material, preferably crystalline quartz (SiO₂) .

12. The optical system anyone of the preceding claims, characterized in that a birefringent material of at least one of said birefringent elements (211-213, 511- 513, 521-523, 531-533, 541-543, 551-553, 561-563, 651a, 652a, 653a) is selected from the group consisting of magnesium-fluoride (MgF₂) , lanthanum-fluoride (LaF₃) , sapphire (Al₂Oa) and crystalline quartz (SiO₂) .

13. The optical system according to anyone of the preceding claims, characterized in that at least two, more preferably all of said three birefringent elements (211- 213, 511-513, 521-523, 531-533, 541-543, 551-553, 561- 563, 651a, 652a, 653a) of said element group are united to a combined element.

14. The optical system according to anyone of the preced- ing claims, characterized in that said element group or said combined element has a maximum axial length along the optical system axis (OA) being not more than 50%, preferably not more than 20%, and still more preferably not more than 10 % of an average optically effective di- ameter of the element group or combined element, respectively.

15. The optical system according to anyone of the preceding claims, characterized in that said birefringent ele- ments have on average essentially no refracting power.

16. The optical system according to anyone of the preceding claims, characterized in that it comprises at least one fourth optical element (143, 642, 942) that causes a disturbance of the distribution of polarization over the cross section of a light beam, wherein said first, second and third aspheric surfaces are selected such that said disturbance of the distribution of polarization is at least partially compensated.

17. The optical system according to claim 16, characterized in that said distribution of polarization caused by said fourth optical element (143, 642, 942) has a k-fold symmetry around said optical system axis (OA) , and wherein said aspheric surfaces of said birefringent elements (211-213, 511-513, 521-523, 531-533, 541-543, 551- 553, 561-563, 651a, 652a, 653a) of said element group have at least one mirror symmetry axis.

18. The optical system according to claim 16 or 17, characterized in that said distribution of polarization caused by said fourth optical element has a 4-fold sym- metry around said optical system axis, and wherein said aspheric surfaces of said at least one birefringent element of said group has two mirror symmetry axes.

19. The optical system according to anyone of the claims 16 to 18, characterized in that said fourth optical element (143, 642, 942) causing a disturbance of the distribution of polarization over the cross section of a light beam comprises a cubic crystal material providing a birefringence magnitude of at least lOnm/cm at an op- erating wavelength of 193nm.

20. The optical system according to anyone of the claims 16 to 19, characterized in that said fourth optical ele- ment (143, 642, 942) is a (100) -cubic crystal-lens or a (111 ) -cubic-crystal-lens .

21. The optical system according to anyone of the claims 16 to 20, characterized in that said fourth optical element (143, 642, 942) is made of a material selected from the group that consists of spinell, in particular MgAl₂O₄ and garnets, in particular Lu₃Al₅Oi₂ and Y₃Al₅Oi₂.

22. The optical system according to anyone of the claims 16 to 21, characterized in that said fourth optical element (942) comprises a first lens component embedded in a second lens component, wherein said second lens component is made from an optically isotropic material, pref- erably fused silica (SiO₂) .

23. The optical system according to claim 22, wherein said first lens component is made from a crystal material providing a retardation of at least lOnm/cm at an operating wavelength of 193nm.

24. The optical system according to claim 22 or 23, characterized in that said first lens component is made from a crystal material with a refractive index larger than 1.7 at an operating wavelength of 193nm.

25. The optical system according to anyone of the claims 22 to 24, characterized in that said crystal material of said first lens component is an optically uniaxial mate- rial.

26. The optical system according to claim 25, characterized in that said optically uniaxial material is se- lected from the group that consists of magnesium- fluoride (MgF₂) , lanthanum-fluoride (LaF₃) , sapphire (AI2O3) and crystalline quartz (SiO₂) .

27. The optical system according to anyone of the claims 22 to 24, characterized in that said crystal material of said first lens component is a cubic crystal material.

28. The optical system according to claim 27, character- ized in that said crystal material of said first lens component is selected from the group that consists of spinell, in particular MgAl₂C>4 and garnets, in particular Lu₃Al₅Oi₂ and Y₃Al₅Oi₂.

29. The optical system according to anyone of the claims 22 to 28, characterized in that said second lens component is a meniscus lens and said first lens component is arranged at a concave light exit side of said meniscus lens to form a substantially plan-convex lens together with said second lens component.

30. The optical system according to anyone of the preceding claims, characterized in that at least one of said aspheric surfaces is arranged at least near a pupil plane of said optical system.

31. The optical system according to anyone of the preceding claims, characterized in that said pupil plane is the first pupil plane along a light propagation direc- tion of said optical system.

32. The optical system according to anyone of the preceding claims, characterized in that a gap between at least two of said birefringent elements is at least partially filled with a liquid having a refraction index that differs not more that 30%, preferably not more than 20%, and still more preferably not more than 10% of the re- fraction indices of said two birefringent elements.

33. The optical system according to claim 32, characterized in that said birefringent elements are made from magnesium-fluoride (MgF₂) , wherein a gap between at least two of said birefringent elements is at least partially filled with water (H₂O) .

34. The optical system according to anyone of the preceding claims, characterized in that it comprises at least two, more preferably at least three of said element groups or combined elements, respectively.

35. The optical system according to anyone of the preceding claims, characterized in that said combined element or said element group is arranged in a pupil plane of said optical system.

36. The optical system according to anyone of the claims 1 to 34, characterized in that said combined element or said element group is arranged at a position where the

relation 0.8< — A <1.0 is met, with Di being a diameter of

a light bundle at said position and D₂ being a total optically used diameter at said position.

37. The optical system according to anyone of the claims 1 to 34, characterized in that it comprises at least two combined elements or element groups, which are both ar- ranged at a position where the relation 0.5< —-≤l.O is

2 met, with Di being a diameter of a light bundle at the respective position being a total optically used diameter at the respective position.

38. An optical system, in particular an illumination system or a projection lens of a microlithographic exposure system, said optical system having at least one element group, wherein said element group consists of: • a first birefringent element having a first aspheric surface and a first orientation of its optical crystal axis;

• a second birefringent element having a second aspheric surface and a second orientation of its op- tical crystal axis; and

• a third birefringent element having a third aspheric surface and a third orientation of its optical crystal axis, ;

• wherein said first, second and third orientations are different from each other.

39. Microlithography projection lens comprising at least one optical system according to anyone of the claims 16 to 38, characterized in that said fourth optical element is among the three elements that lie closest to an image plane of said microlithography projection lens, and more preferably the last element of said microlithography projection lens.

40. Illumination system, in particular for a microlithographic projection exposure apparatus, comprising for providing a predetermined polarization distribution at least one optical system according to anyone of the claims 1 to 38.

41. Illumination system according to claim 40, wherein said predetermined polarization distribution is a tangential polarization distribution.

42. Illumination system according to claim 40 or 41, wherein said birefringent elements of said optical ele- ment group are exchangeable for changing said predetermined polarization distribution.

43. An optical element comprising a first lens component embedded in a second lens component, wherein said first lens component is made from spinell and wherein said second lens component is made from an optically isotropic material.

44. An optical element comprising a first lens component embedded in a second lens component, wherein said first lens component is made from a cubic crystal material with a refractive index larger than 1.7 at λ« 193nm and wherein said second lens component is made from an optically isotropic material.

45. A microlithographic projection exposure apparatus comprising an illumination system and a projection lens, wherein said illumination system and/or said projection lens is an optical system according to anyone of the claims 1 to 38 or comprises an optical element according to claim 43 or 44.

46. Use of an optical system according to anyone of the claims 1 to 38 for microlithographic projection exposure .

47. Method of microlithographic structuring a substrate comprising the steps of illuminating a mask with light and projecting an image of said mask onto said substrate, wherein said step of illuminating a mask with light and/or said step of projecting an image of said mask onto said substrate uses an optical system according to anyone of claims 1 to 38.

48. A microstructured device, characterized in that it is fabricated according to the method of claim 47.