WO2004102230A1

WO2004102230A1 - Polarization-optimized axicon system, and an illuminating system for microlithographic projection system having such an axicon system

Info

Publication number: WO2004102230A1
Application number: PCT/EP2003/009613
Authority: WO
Inventors: Gerhard Fürter; Wolfgang Singer; Manfred Maul; Alexander Kohl; Damian Fiolka
Original assignee: Carl Zeiss Smt Ag
Priority date: 2003-05-13
Filing date: 2003-08-29
Publication date: 2004-11-25
Also published as: AU2003264131A1; DE10322375A1

Abstract

An axicon system (250) which can be used, for example, in an illuminating system for a microlithographic projection exposure apparatus serves the purpose of transforming an entrance light distribution striking its entrance surface (255) into an exit light distribution emerging from its exit surface (258) by radial redistribution of light intensity. It has an optical axis (3), a first axicon element (251) with a first axicon surface (255), and at least one second axicon element (252) with a second axicon surface (258). Furthermore, at least one intermediate surface (256, 257) arranged between the first and the second axicon surface is provided which can likewise be designed as an axicon surface, for example. The distribution of the axicon functionality over a plurality of surfaces permits the design of polarization-maintaining axicon systems.

Description

POLARIZATION-OPTIMIZED AXICON SYSTEM, AND AN ILLUMINATING SYSTEM FOR MICROLITHOGRAPHIC PROJECTION SYSTEM HAVING SUCH AN AXICON SYSTEN

The invention relates to an axicon system for transforming an entrance light distribution striking an entrance surface of the axicon system into an exit light distribution emerging from an exit surface of the axicon system by radial redistribution of light intensity, as well as to an illuminating system for an optical device which includes at least one such axicon system.

The efficiency of projection exposure apparatuses for the microlithographic production of semiconductor components and other finely structured subassemblies is determined substantially by the imaging properties of the optical projection system. Moreover, the image quality and the wafer throughput achievable with an apparatus are substantially codetermined by properties of the illuminating system connected upstream of the projection objective. The said system must be capable of preparing the light of a light source with the highest possible efficiency and in the process setting a light distribution which can be precisely defined with reference to the position and shape of illuminated regions, and in the case of which as uniform as possible a distribution of intensity is present within illuminated regions. These requirements are to be fulfilled in equal measure for all settable illuminating modes, for example for conventional settings with various degrees of coherence or in the case of an annular field, dipole or quadrupole illumination.

A requirement becoming ever more important which is placed on illuminating systems consists in that the latter are to be capable of providing output light with a state of polarization which can be defined as precisely as possible. For example, it may be desired for the light falling onto the photomask or into the downstream projection objective to be largely or completely linearly polarized and to have a defined alignment of the preferred direction of polarization. Modern catadioptric projection objectives with a polarization beam splitter (beam splitter cube, BSC) having a theoretical efficiency of 100% at the beam splitter, for example, can operate with linearly polarized input light.

If the illuminating system is used in conjunction with an excimer laser light source, which already provides largely linearly polarized light, linearly polarized output light can be provided by virtue of the fact that the overall illuminating system operates substantially in a fashion maintaining polarization. An optical system operates "in a fashion maintaining polarization" in the meaning of this application when the state of polarization of the light emerging from the optical system corresponds substantially to the state of polarization of the light entering the optical system.

Illuminating systems, in particular those used for microlithography projection exposure apparatuses, normally have a complex design with a multiplicity of different subsystems and components for various functionalities. If it is desired in the case of one illuminating system to be able to switch over between conventional (axial, on-axis) illumination and nonconventional (abaxial, off-axis) illumination, use is preferably made for this purpose of axicon systems which are capable of transforming by radial redistribution of light intensity an entrance light distribution striking an entrance surface of the axicon system into an exit light distribution in the case of which the light intensity outside the optical axis is substantially greater than in the region of the optical axis. These nonconventional illumination settings for producing an abaxial, oblique illumination can serve the purpose, inter alia, of increasing the depth of field by means of two-beam interference, and of increasing the resolving power of projection exposure apparatuses.

EP 747 772 describes an illuminating system having a combined zoom- axicon objective in the object plane of which a first diffractive raster element with a two-dimensional raster structure is arranged. This raster element serves the purpose of increasing the geometrical flux of the striking laser radiation by introducing aperture, and of varying the form of the light distribution such that, for example, an approximated circular distribution (for conventional illumination) or a polar distribution results. In order to alternate between these illumination modes, first raster elements are exchanged as appropriate. The zoom-axicon objective combines a zoom function for infinite adjustment of the diameter of a light distribution with an axicon function for radial redistribution of light intensities. The axicon system has two axicon elements which can be displaced axially relative to one another and have mutually facing conical axicon surfaces which can be moved towards one another until their spacing is zero. Consequently, the annularity of the illumination and the degree of coherence can be adjusted by adjusting the zoom axicon. A second raster element, which is located in the exit pupil of the objective, is illuminated with the corresponding (axial or abaxial) light distribution, and forms a rectangular light distribution whose shape corresponds to the entrance surface of a downstream rod integrator.

Other illuminating systems with axicon systems for radial redistribution of optical energy are shown, for example, in US Patent US 5,675,401 belonging to the applicant, in Patent US 6,377,336 B1 , and in parallel property rights or in Patent US 6,452,663 B1.

Conventional axicon systems generally do not operate to maintain polarization. Because of the rotationally symmetrical or radially symmetrical geometry of the axicon surfaces with refracting surfaces inclined obliquely to the optical axis, in the case of linearly polarized input radiation, for example, beams with an identical direction of oscillation of the electric field vector are not incident everywhere in identically oriented incident planes with reference to the refracting axicon surfaces. Consequently, because of Fresnel losses, the entering light experiences an attenuation, dependent on the location of incidence and therefore differing locally, of the p- or s-components of the electric field strength. Here, the s-component is that electric field strength component which runs perpendicular to the plane of incidence which is defined by the surface normal to the axicon surface at the striking location and by the beam entrance direction. The p-component is the electric field strength component which oscillates parallel to the plane of incidence, that is to say in the plane of incidence itself. Consequently, incidences of attenuation can occur which vary on axicon surfaces in the azimuthal direction (circumferential direction). This leads in the case of homogeneous polarized entrance light to a polarization of the light distribution downstream of the axicon system which is no longer homogeneous. In the case of conventional axicon systems and linearly polarized input light, the loss in the linear degree of polarization can certainly be of the order of magnitude of up to approximately 10%.

DE 35 23 641 (corresponding to US 4,755,027) describes a polarizer which uses the polarization-selective action of a plurality of axicon surfaces situated one behind the other to produce tangential or radial polarization. The object of the invention is to provide an axicon system which, by comparison with conventional axicon systems, exhibits a striking reduction in polarization-varying effects. In particular, the aim is to provide an axicon system which operates in a substantially polarization- maintaining fashion.

This object is achieved according to the invention by means of an axicon system having the features of Claim 1. Advantageous developments are specified in the dependent claims. The wording of all the claims is included in the description by reference.

An axicon system according to the invention serves the purpose of transforming an entrance light distribution striking an entrance surface of the axicon system into an exit light distribution emerging from an exit surface of the axicon system by radial redistribution of the light intensity. It comprises: an optical axis; a first axicon element with a first axicon surface; at least one second axicon element with a second axicon surface; and at least one intermediate surface arranged between the first axicon surface and the second axicon surface.

This arrangement permits a distribution of the overall functionality of the axicon system over more than two optical surfaces, such that the polarization-varying effect of the first and second axicon surfaces can be reduced or at least partially compensated. For example, all the surfaces can have a deflecting action, and thus make an effective contribution to the radial redistribution.

In accordance with a development, the intermediate surface is likewise an axicon surface which can be provided on the first and/or on the second axicon element or on an additional, further axicon element. By suitable design of the geometry of the axicon surfaces, it is possible via the refracting action of the intermediate surface to reduce the deflecting action, required for a desired redistribution, of the first and second axicon surfaces to such an extent that their polarization-varying or polarization-selective action is reduced. When the intermediate surface is largely or completely planar, an arrangement downstream of an axicon surface is advantageous, since the intermediate surface is then in the oblique beam path and acts in a fashion that is refracting or deflecting or varies the beam angle.

The axicon surfaces can be fashioned such that incident angles which are in the vicinity of the respectively associated Brewster angle do not occur on any of the axicon surfaces. For all axicon surfaces, the incident angles, occurring on the axicon surfaces, of the penetrating radiation are preferably at an angular spacing of at least 10°, in particular at least 15° or at least 20° or more, from the associated Brewster angle. It is thereby possible to achieve a situation where only a slight and, if appropriate, negligible splitting occurs on such surfaces between s-polarized and p- polarized components of the radiation.

For example, it is possible at all locations of the axicon surfaces for the local inclination angles to be minimized with reference to a radial plane perpendicular to the optical axis such that they are smaller than approximately 30° and, in particular, lie between approximately 20° to 25°. In this case, inclination angles much below 30°, 25° or 20° are preferred. A minimization of incident angles on the axicon surfaces can be 'facilitated, if appropriate, by providing more than one intermediate surface, for example two, three, or four intermediate surfaces, which can all be fashioned as axicon surfaces, if appropriate.

The axicon surfaces and the at least one intermediate surface are expediently each covered by an antireflection coating in order to minimize transmission losses. The anti reflection layer can be designed in this case such that polarization effects are minimized.

In one development, at least one of the axicon surfaces is conical or cone-shaped, all of the axicon surfaces preferably being conical. It is possible as a result, for example, for a circular entrance light distribution to be transformed into a circular, annular exit light distribution, in order to provide annular illumination.

It is also possible for at least one of the axicon surfaces to have the form of a multifaceted pyramid having at least two normally planar pyramid facets inclined to the optical axis. The pyramid facets can be arranged with radial symmetry about the optical axis such that an n-fold radial symmetry is produced, n being the number of the pyramid facets. It is, for example, possible for there to be present two, three, four, five, six, seven, eight, nine, ten, eleven, twelve or more pyramid facets which can have the same shape and size. The larger the number of the pyramid facets, the better it is possible to approximate an annular shape for the exit light distribution. In general, polygonal axicon surfaces with a pyramid shape can produce exit light distributions which are composed of a plurality of largely straight illumination strips at an angle to one another which either merge directly into one another in the circumferential direction, or are at a spacing from one another.

It is also possible for at least one axicon surface to be a quasi-conical optical surface curved about a plurality of axes and having a first curvature, running in the circumferential direction, and a second curvature which lies in a plane of curvature containing the optical axis. Optical elements having such surfaces are also designated here as "aeon lenses" or "lensacon". In preferred embodiments, the axicon system is designed such that light beams of the penetrating light, at each axial position between the entrance surface and the exit surface of the axicon system, run at an angle to the optical axis which is substantially outside the paraxial angular range. There is thus no region with a parallel beam path between the entrance surface and exit surface. In this way, the radial redistribution can be achieved with the aid of a design which is compact in the axial direction.

Although it is possible for the axicon system to be designed only for a specific redistribution geometry, it is preferred when an axial spacing between the first axicon element and the second axicon element is adjustable. The extent of the achievable radial displacement of light intensity can thereby be adjusted, preferably infinitely, in order, for example, to facilitate abaxial (off-axis) illumination of a surface to be illuminated with the aid of incident angles of the illuminating light which can be specifically set.

In some embodiments, all the axicon elements and, if appropriate, also an intermediate element forming the intermediate surface are formed in each case by unipartite optical components which can consist of a transparent material. There are also embodiments in the case of which at least one axicon element is assembled from a plurality of separate individual elements arranged about the optical axis of the axicon system and in each case having at least one boundary surface serving as pyramid facet. The normally wedge-shaped individual elements can be fixed on one another or be immovable relative to one another. In some embodiments, the individual elements are, however, mounted movably in such a way that they can be tilted about an axis perpendicular to a radial direction of the optical axis. It is thereby possible for the inclination angles of the optically active partial surfaces of the axicon surface

(pyramid facets) to be adjusted by tilting the elements. If the tilting axis is situated close beside or on the optical axis, it is possible in this way to form an optical system with an "umbrella mechanism" by way of example, which can operate in the manner of a Brewster telescope. Arrangements are also possible in which the tilting axes are arranged at a greater spacing from the optical axis.

In another variant of the invention, a polarization rotator for rotating the polarization of the penetrating light by approximately 90° is arranged between the first and the second axicon element. In this case, the at least one intermediate surface can be formed on the polarization rotator. The polarization rotator can comprise an optical delay system (optical retarder) which effects a delay of half a wavelength or an odd multiple of half a wavelength between two mutually perpendicular states of polarization (s- and p-component).

As a result of a polarization rotator of this type, it is possible for the field strength component which is stronger upstream of the polarization rotator to be weaker downstream of the polarization rotator, and vice versa. The polarization-splitting action of the first axicon surface in the light path can be compensated largely or completely, since a stronger polarization component arriving at the second axicon surface following the polarization rotator is attenuated more strongly in the correct position than the associated weaker polarization component. Consequently, the p-component and the s-component of the entrance light distribution are influenced overall with approximately the same intensity by the axicon system, and so essentially no change occurs in the state of polarization upon passage through the axicon system. The degree of polarization present upstream of the entrance surface can thus be largely or completely transmitted by the axicon system.

The optical delay system can be formed, for example, by a single, unipartite or assembled optical element which has the action of a λ/2 plate. It is favourable when the entrance and exit surface of the delay system has the same rotationally symmetrical or pyramidal or polygonal symmetry as the neighbouring axicon surfaces. In the case of a conical first and second axicon surface, the delay element can be designed, for example, as a doubly conical element, ideally with the same conical angle. The delay element can also be assembled from a plurality of segments, for example from triangular plates resembling pieces of cake and made from a suitable birefringent material. Coming into consideration as birefringent materials are, for example, magnesium fluoride, crystalline quartz or a fluoride crystalline material such as calcium fluoride, which can have intrinsic birefringence of sufficient intensity given suitable orientation relative to the transirradiation direction (<1 10> axis substantially parallel to the transirradiation direction). It is also possible on the basis of the low angular loading of the birefringent material between the axicon surfaces to use multiorder delay elements which can be produced with relatively large thicknesses, for example more than 1 mm, and are thus mechanically relatively stable and can be mounted with an acceptable structural outlay.

It can be provided in all embodiments that at least one axicon element, if appropriate the entire axicon system, is mounted rotatably about the optical axis. It is possible in this way, if appropriate, to achieve an optimal orientation of the axicon system with reference to a preferred direction of polarization of the incoming radiation.

The invention also relates to an illuminating system for an optical device, in particular for a projection exposure apparatus for microlithography, which includes at least one axicon system in accordance with the invention. The illuminating system can furthermore include at least one light mixer, which can be arranged upstream or downstream of the axicon system. It is favourable when the light mixer is one which largely maintains polarization. The light mixer can include, for example, one or more honeycomb condensers (fly eyes lenses), diffusers or one or more integrator rods or a combination of such light mixing elements. Embodiments of polarization-maintaining and angle-maintaining light mixers with rod integrator are to be gathered from the still unpublished Patent Application DE 102 06 061.4 belonging to the applicant, such that the disclosure content thereof is to this extent incorporated in the content of the description.

The use of axicon systems according to the invention is not, however, limited to illuminating systems for microlithography. Axicon systems according to the invention can also be used in the illuminating system of microscopes or generally as beam widening devices, which can be set variably if appropriate, for other applications. Depending on the direction of transirradiation, a radial redistribution in the direction of the optical axis is also possible.

An alternative form of a polarization-maintaining axicon system has two axicon elements which can be displaced axially relative to one another and have mutually facing conical or polygonal axicon surfaces which can be moved towards one another until their spacing is zero, and which are the sole axicon surfaces of the axicon pair. In order to reduce or avoid the problems of polarization explained at the beginning, the inclination angles of the axicon surfaces are substantially smaller than customary to date. For example, they can be less than 30° or 25° or under, such that the incident angles occurring have a large spacing of, for example, more than 10° or 15° or 20° from the Brewster angle. In order to permit a sufficiently large radial redistribution of light, the settable maximum spacing of the axicon elements must be enlarged, if appropriate.

The above features and further ones emerge not just from the claims but also from the description and the drawings, it being possible for the individual features to be respectively implemented on their own or collectively in the form of subcombinations in the case of an embodiment of the invention and to be implemented in other fields, and for them to constitute advantageous designs and designs which can be protected in themselves.

Figure 1 shows a diagrammatic overview of an embodiment of an illuminating system for a microlithography projection exposure apparatus in which an embodiment of an axicon system according to the invention is installed;

Figure 2 shows a diagrammatic illustration of a first embodiment of an axicon system according to the invention;

Figure 3 shows a diagrammatic illustration of a second embodiment of an axicon system according to the invention;

Figure 4 shows a diagrammatic illustration of a third embodiment of an axicon system according to the invention;

Figure 5 shows a diagrammatic illustration of a fourth embodiment of an axicon system according to the invention;

Figure 6 shows an axial top view of a facetted axicon surface of a fifth embodiment of an axicon system according to the invention;

Figure 7 shows a diagrammatic illustration of an embodiment of an axicon system according to the invention having an axicon element which includes a plurality of individual elements which can be moved relative to one another and are arranged in a first configuration; Figure 8 shows the embodiment in accordance with Figure 7 in a second configuration with tilted individual elements;

Figure 9 shows a diagrammatic illustration of an embodiment of an axicon system according to the invention having a polarization rotator between axicon surfaces; and

Figure 10 shows another embodiment of an axicon system according to the invention having a polarization rotator between consecutive axicon surfaces.

Figure 1 shows an example of an illuminating system 1 of a microlithographic projection exposure apparatus which can be used in the production of semiconductor components and other finely structured subassemblies, and operates for the purpose of achieving resolutions of up to fractions of micrometres by using light from the deep ultraviolet region. Serving as light source 2 is an F₂ excimer laser with an operating wavelength of approximately 157 nm, whose light beam is aligned coaxially with the optical axis 3 of the illuminating system. Other UV light sources, for example ArF excimer lasers with a 193 nm operating wavelength, KrF excimer lasers with a 248 nm operating wavelength or light sources with wavelengths below 157 nm, are likewise possible.

The linearly polarized light from the light source 2 firstly enters a beam expander 4, which can be designed, for example, as a mirror arrangement in accordance with DE 41 24 311 , and serves the purpose of reducing coherence and enlarging the beam cross section. An optionally provided closure is replaced in the case of the embodiment shown by an appropriate pulse controller of the laser 2.

A first diffractive, optical raster element 5 serving as beam shaping element is arranged in the object plane 6 of an objective 7 which is arranged downstream thereof in the beam path and in whose image plane 8 or exit pupil a refractive second optical raster element 9 is arranged which likewise serves as beam shaping element.

A coupling optical system 10 arranged downstream thereof transmits the light onto the entrance plane 11 of a light mixer 12 which mixes and homogenizes the penetrating light. Situated directly at the exit plane 13 of the light mixer 12 is an intermediate field plane in which a reticle/masking system (REMA) 14 is arranged which serves as an adjustable field stop. The subsequent objective 15 images the intermediate field plane with the masking system 14 onto the reticle 16 (mask, lithographical original) and includes a first lens group 17, a pupil intermediate plane 18, into which filters or stops can be introduced, a second and a third lens group 19 and 20 and, therebetween, a deflecting mirror 21 which permits the large illuminating device (approximately 3 m long) to be installed horizontally and the reticle 16 to be mounted horizontally.

Together with a catadioptric projection objective (not shown) with a polarization-selective beam splitter (beam splitter cube, BSC) and an adjustable wafer holder which holds the reticle 16 in the object plane of the projection objective, this illuminating system forms a projection exposure apparatus for the microlithographic production of electronic subassemblies, but also of optically diffractive elements and other microstructured parts.

The design of the parts upstream of the light mixer 12, in particular of the optical raster elements 5 and 9, is selected such that a rectangular entrance surface of the light mixer is illuminated largely homogeneously and with the highest possible efficiency, that is to say without substantial light losses apart from the entrance surface. For this purpose, the parallel light beam coming from the beam expander 4 and having a rectangular cross section and a divergence lacking rotational symmetry is firstly varied by the first diffractive raster element 5 with the introduction of photoconductance with reference to divergence and shape. The linear polarization of the laser light remains largely constant in this case.

The first optical raster element 5, arranged in the front focal plane (object plane) of the objective 7, prepares together with the objective 7 an illuminating spot of variable size and light distribution in the exit pupil or image plane 8 of the optical system 7. Arranged here is the second optical raster element 9, which is designed in the example as a refractive optical element with a rectangular emission characteristic. This beam shaping element produces the main component of the photoconductance and adapts the photoconductance via the optical coupling system 10 to the field coverage, that is to say to the rectangular shape of the entrance surface of the rod integrator 12.

The design of the previously described illuminating system with the exception of the objective 7 can correspond, for example, to the design described in EP 0 747 772, whose disclosure content is to this extent incorporated in the content of this description by reference.

The objective 7, which is also designated below as a zoom/axicon system, includes a polarization-maintaining axicon system 50 which can be set variably and serves the purpose of transforming an entrance light distribution striking its entrance surface into an exit light distribution emerging from an exit surface by radial redistribution of light intensity, and also includes a zoom system 40, likewise settable, for variably setting the diameter of a light distribution output by the zoom system. Consequently, an essentially round illuminating spot of largely uniform intensity and with a settable diameter or a desired light distribution having an intensity which is increased outside the optical axis relative to the axial region can optionally be produced at the entrance surface of the raster element 9, for example in the form of a ring with a variable inside and outside diameter.

Figure 2 shows a first embodiment of a substantially polarization- maintaining axicon system 250. It comprises a first axicon element 251 and a second axicon element 252, which is arranged downstream thereof in the direction in which the light runs and whose axial spacing can be infinitely adjusted by means of an adjusting device (not shown) and, if appropriate, can be reduced to a spacing of zero. The first axicon element 251 has a concave conical entrance surface 255 which forms the entrance surface and first axicon surface of the axicon system, as well as a substantially planar exit surface 256. The inclination angle α of the axicon surface, which is measured between a plane perpendicular to the optical axis and a tangent plane at the cone, is approximately 20° to 25°. The second axicon element 252 has a substantially planar entrance surface 257 and a convex conical exit surface 258 which forms the exit surface of the axicon system and at the same time its second axicon surface. The inclination angle is likewise in the range from 20° to 25°. The planar surfaces 256, 257 are intermediate surfaces of the axicon system.

The arrangement shown permits the beam deflection, required for the axicon function, away from the optical axis to be distributed radially outwards to more than the previously customary surfaces. The angles of incidence or incident angles at the individual surfaces can thereby be reduced. The problem of polarization with conventional axicon systems which was explained at the beginning is thereby reduced.

In detail, the function of the axicon system 250 is explained with the aid of two beams 260, 261 radiated onto the axicon system in an axially parallel fashion. The beams running parallel to the optical axis are deflected radially outwards by refraction at the first axicon surface 255, the deflection angle being yielded by the law of refraction. Inside the first axicon element, the radiation therefore already runs no longer in an axially parallel fashion at an angle differing substantially from 0° to the optical axis, and strikes the planar exit surface 256, which is perpendicular to the optical axis, at this angle. Since the radiation striking the planar intermediate surface 256 from the optically dense medium already strikes at an angle to this planar surface, the first planar intermediate surface 256 also has a refracting action and deflects the light at a larger deflection angle radially outwards. This light running obliquely to the optical axis now strikes the planar intermediate surface 257 running perpendicular to the optical axis, the radiation being deflected towards the normal during transition from the optically thin to the optically denser medium. A further deflection is performed when the beam exits at the axicon surface 258, which is the exit surface of the axicon system, its inclination angle being dimensioned such that the emerging radiation runs substantially parallel again to the optical axis.

This axicon system therefore has four optical surfaces which in each case effect a substantial deflection of the penetrating radiation. Thus, four surfaces contribute to the required overall deflection, and this relaxes the requirements placed on each individual surface. As a result, above all at the conical surfaces 255, 258, the radiation can strike these surfaces with incident angles which have a large spacing of, for example, 15°, 20° or 25° from the associated Brewster angle. As a result, by comparison with conventional systems, the splitting of the polarization at the axicon surfaces is substantially reduced such that the state of polarization of the emerging light does not differ, or differs to an only negligibly slight extent, from the state of polarization of the entrance light. The axicon system thus acts largely to maintain polarization and outputs largely linearly polarized light. In the second embodiment, shown in Figure 3, of an axicon system 350, the corresponding surfaces and elements bear the same reference numerals as in Figure 2, increased by 100. By contrast with the first embodiment, both the first axicon element 351 and the second axicon element 352 are double axicon elements, because both the respective entrance surfaces 355, 357 and the respective exit surfaces 356, 358 are conical axicon surfaces. The maximum inclination angles of the conical surfaces are in turn of the order of magnitude of between 20° and 25°. As in the case of the above embodiment, light arriving in parallel at all the optical surfaces 355, 356, 357 and 358 of the axicon system is deflected substantially, the two first deflections at the concave first axicon surface 355 and the concave first intermediate surface 356 leading to an enlargement of the angle between the light beam and optical axis, while the two last deflections at the convex second intermediate surface 357 and the convex axicon exit surface 358 reduce the beam angle again to zero in stages. Here, again, the distribution of the deflecting action over four surfaces leads to a substantial reduction in the polarization-splitting action of the individual surfaces, and so the axicon system 350 acts largely to maintain polarization.

In the third embodiment of an axicon system 450 in Figure 4, corresponding reference numerals are increased again by 100. Here, the first axicon element 451 has a concave conical entrance surface 455 and a convex spherical (or slightly aspheric, if appropriate) exit surface 456, while the subsequent second axicon element 452 has a concave spherical (or slightly aspheric, if appropriate) entrance surface 357 corresponding to the surface 456, and a conical exit surface 458. Because of the spherically curved intermediate sur aces 456, 457, whose spacing can be adjusted, the deflecting action, which leads to a radial redistribution of light intensity, further has superimposed on it a focusing action of the axicon system such that the radiation emerging from the .exit surface 458 of the axicon system runs convergently. An axicon system with an overall "positive refractive power" is thus created. Here, again, the beam deflection is advantageously distributed over all four optical surfaces of the axicon system, such that large incident angles are avoided.

The fourth embodiment of an axicon system 550 in Figure 5 can be understood as a variant of the third embodiment. Whereas, as in the case of the third embodiment, the entrance surface 555 is concavely conical with an inclination angle between approximately 20° and 25°, the convex exit surface 556 of the first axicon element 551 , which forms the first intermediate surface of the axicon system, has a complex shape. A spherical radius is admittedly likewise impressed on the exit surface 556, but in such a way that the apex of a cone is produced on the optical axis. Such an optical element, in the case of which at least one surface is curved about a plurality of axes and has a first curvature running in a circumferential direction as well as a second curvature which lies in a plane of curvature including the optical axis, is also designated here as aeon lens or lensacon. The concave entrance surface 557 of the second axicon element 552 has a corresponding lensacon shape such that upon minimization of the spacing of the two axicon elements the lensacon surfaces can lie properly on one another. The exit surface 558 of the second axicon element or the axicon system is likewise formed in the manner of a lensacon, but with a slightly different inclination angle. Even in the case of this system, a slightly focusing action is produced in addition to the radial redistribution of the incoming optical radiation.

Figure 6 shows an axial top view of an axicon surface 655 of an axicon system 650 of a fifth embodiment. The axicon element comprises a plurality of separate individual elements which are arranged circularly about the optical axis and in each case have the form of wedges. Such a wedge element of triangular shape is marked by bold lines. The individual elements arranged in the shape of a rosette can be fixed on one another and thus form with an axicon surface a pyramid element which has a polygonal pyramid shape which is assembled from a plurality of planar pyramid facets 670 which border one another or are arranged at a slight spacing from one another. It is thereby possible to create a multiple prism with a number of prism elements which are arranged in a circular arrangement such that light which comes from a point light source arranged on the optical axis runs through the prism elements and after the passage a number of component beams which corresponds to the number of prism elements is present. If, for example, four such wedge elements in the shape of a cross are used, a quadrupole distribution can be produced downstream of such a pyramid arrangement. Dipole illumination can be produced if only two prism wedges are provided. If a sufficiently high number of individual elements are provided, for example substantially more than 4, for example between 10 and 20 such individual elements, it is possible to produce a quasi-annular illumination in the circumferential direction, interrupted by small gaps if appropriate.

Illustrated in Figures 7 and 8 in a diagrammatic side view is an axicon system 650 which has two axicon elements 651 , 652 which are arranged one behind another and comprise pyramidal individual elements. The refracting wedges are shaped and arranged with reference to the optical axis such that all the planar refracting surfaces are at an inclination angle to the optical axis. The action of such an axicon system on an abaxially incident beam of diameter D is explained in Figure 7. Upon passage through the axicon element 651 on the entrance side, this beam is deflected from the axially parallel course into a course inclined outwards relative to the axis, while the second axicon element 652 cancels this deflection by means of two further refractive operations such that a light beam of unchanged beam diameter D is present on the exit side, its spacing from the optical axis 3 being enlarged, however

(compare Figure 3). If, for example, at least the rear wedges are now fastened in a holder device which is designed in such a way that each of the wedge elements can be rotated about a tilting axis 670 which runs perpendicular to the radial direction in a plane running perpendicular to the optical axis, it is possible, for example, to switch over between the configuration shown in Figure 7 and the configuration shown in Figure 8 with the aid of swivelled individual elements of the second axicon element 652. In the case of the example, the swivelling reduces the inclination angle of the entrance surfaces 655 of the second axicon element 652, while the inclination angle of the corresponding exit surfaces 658 is enlarged. An effect of this switchover is that during the adjustment the diameter d of the exit light distribution varies continuously in accordance with the tilting angles set such that a suitable aspect ratio D/d can be set via the tilting angle of the wedge elements. In this variant as well, the deflecting action of the axicon is distributed over four surfaces, four (subdivided) pyramid faces with individual, triangular planar pyramid facets being formed in the case of the example. The arrangement shown with swivelable wedge-shaped or prismatic individual elements is also designated here as a "Brewster telescope axicon with an umbrella mechanism". It is particularly advantageous with this arrangement when both the front segments of the pyramid axicon element and the rear individual elements can be swivelled in order to construct per angular segment of the pyramid multifaceted element a complete Brewster telescope for beam widening or beam focusing.

It can, if appropriate, be regarded as a disadvantage of the system that it is no longer possible for any closed annular field to be produced, since gaps occur for geometrical reasons between the wedges of a rosette. However, this disadvantage can be taken into account by providing a suitable large number of individual elements (corresponding to a high number of "poles" produced), or it can even be advantageous for the imaging of semiconductor structures which are arranged on the structure-carrying mask, predominantly in a horizontal or vertical orientation.

When designing individual angular segments of the pyramid axicon element in the manner of a Brewster telescope, it can therefore be advantageous that individual segments or symmetrically opposite segments of neighbouring pairs of segments, with particular preference arranged rotated by 90°, can be displaced or rotated differently in the axial direction in order to illuminate different structural widths or line densities in the different directions of the structure of the mask to be imaged, with matched directions of illumination in each case.

In all the embodiments shown, but also in the case of the axicon surfaces of conventional axicon systems, it is possible to implement the functionality of individual surfaces, be they conical, spherical or lensacon surfaces, in each case by a Fresnel zone plate or a refractive optical raster element. Compact axicon systems having particularly short length dimensions which can satisfy most stringent design space requirements are thereby possible. The axial shortening of axicon systems in conjunction with maintaining the basic axicon function by replacing individual surfaces or all of them by Fresnel elements is countered as a disadvantage only by an increased layout on production for these elements. If appropriate, it is also possible further to produce a higher level of scattered light which is to be taken into account in the design.

In the case of axicon systems, in particular of an embodiment of axicon systems according to the invention, the functionality of an axicon can also be replaced by diffractive optical elements (DOEs), that is to say such optical elements whose light-influencing action is effected substantially by diffraction (by contrast with refraction of light). It is thereby possible to produce particularly compact axicon systems, in particular having small axial dimensions. Moreover, the path through the optical material is reduced, and it is possible, furthermore, to make use for this purpose if appropriate of material which is more cost-effective. Given a not too complicated shape of the diffracting structures, it is possible, if appropriate, for axicon elements to be produced more simply together with a corresponding saving in costs by comparison with conventional systems. Typical grating periods of diffractive structures could be, for example, in the range between approximately 0.3 and 0.4 μm. Diffractive optical elements can be designed, for example, as computer-generated holographic elements (CGHs). Consequently, the invention also covers an axicon system in which at least one axicon element is designed as a diffractive or refractive optical element, in particular with a two-dimensional raster structure.

The person skilled in the art can further transfer the designs according to the invention to reflective systems without any additional inventive input, such that at least one of the surfaces is of reflective design with, for example, a multiple coating. The angles of incidence of the optical elements redistributing the light intensity in the radial direction can therefore be minimized, in order also to minimize the polarization effects occurring during the reflection at multiple reflection layers such as occur, for example, in EUV lithography with wavelengths of 13.5 nm.

The term "light" in this application is therefore intended also to cover radiation in the soft x-ray region at suitable points.

Figures 9 and 10 are used to explain further variants of axicon systems according to the invention, in the case of which at least one intermediate surface is arranged between a first and a second axicon surface. In these exemplary embodiments, at least one polarization rotator which is bounded by intermediate surfaces is provided between axicon elements (that is to say optical elements with axicon surfaces). The axicon system 750 in Figure 9 has a first axicon element 751 with a conical entrance surface 755 and planar exit surface 756, as well as a second axicon element 752 with a planar entrance surface 757 and conical exit surface 758, it being possible for the inclination angles or conical angles of the axicon surfaces 755, 758 to be equal or different. A polarization rotator 770 in the form of a λ/2 plate consisting of a birefringent material is arranged between the axicon elements.

In order to explain the function, there is illustrated in Figure 9 a light beam 780 which traverses the axicon arrangement with an integrated polarization rotator. The electric field strength E of the light beam is composed of a component E_s, which oscillates perpendicularly to the plane of incidence lying in the plane of the paper, and a component E_p, which oscillates parallel to the said plane of incidence. The light beam incident parallel to the optical axis is deflected radially outwards by a deflection angle through refraction at the entrance surface 755 of the axicon system, thereafter traverses the planar exit surface of the first axicon element while being further deflected outwards, thereafter traverses the delay plate 770 while producing a slight parallel offset, thereafter strikes the planar entrance surface of the second axicon element 752, is deflected at said entrance surface with reduction of the beam angle relative to the optical axis, and emerges from the second axicon surface 758, being in the process deflected again at the latter in the direction of the optical axis such that it runs in a substantially axially parallel fashion downstream of the axicon system, but at a greater spacing from the optical axis. The components E_s and E_p are attenuated differently in accordance with the Fresnel losses (reflection losses) at the entrance surface 755, which can, if appropriate, have an inclination angle in the range of the associated Brewster angle. A similar thing happens in the attenuated form upon exit from the first axicon element, since here the light strikes the boundary surface at a lesser angle of incidence. In the absence of the polarization rotator 770, the rectified components would be attenuated anew upon entry into the planar entrance surface of the second axicon element and to a greater extent upon exit from this element.

As a result of the polarization rotator 770, the stronger p-component upstream of the λ/2 plate 770 and the weaker s-component upstream of the plate are respectively rotated by 90°. Before entrance into the second axicon element, the weaker component then runs parallel to the plane of incidence, while the stronger component runs perpendicular to the latter. Upon traversing the two boundary surfaces of the second axicon element, the stronger field component is now respectively more strongly attenuated than the weaker one, such that after passage through the axicon system in the ratio of the field strengths E_p and E_s no variation, or only a negligible one, has resulted overall. Both components are thus attenuated overall with an approximately equal strength by the axicon system, such that no change is produced in the spatial distribution of the state of polarization, and the axicon system consequently acts to maintain polarization.

The axicon system 850 in accordance with Figure 10 likewise acts in a substantial way to maintain polarization by means of a corresponding measure (arrangement of a polarization rotator 870 between the first axicon surface 856 and the second axicon surface 857). By contrast with the above embodiment, however, the axicon surfaces 856, 857 face one another. Consequently, they can be moved towards one another up to a slight axial spacing, as a result of which very small annular diameters of the exit radiation are also rendered possible. The polarization rotator 870 is designed in the case of the example as an axially thin, doubly conical element. Sufficiently thick and thus mechanically stable and reliably producible doubly conical elements are permitted by fabrication from only slightly birefringent material, for example from <1 10>-orientated calcium fluoride and/or by a design as a multiorder plate with a delay of an odd multiple of λ/2.

Alternatively, the λ/2 plate can be applied directly by means of coating methods to one of the intermediate surfaces, or a polarization- dependent delaying action which corresponds to the delaying action of a λ/2 plate can be set on at least one of the layers applied to a plurality of intermediate layers by means of coating methods.

If the production of rotationally symmetrical conical lenses is not desired, it is possible to go over to a polygonal design of "conical lenses" which can be fashioned in accordance with the multifaceted shape of the axicon surfaces in Figure 6, for example hexagonally or with more or fewer than six wedge faces. The λ/2 plates can also be snapped on in this case.

A delay element having only four pyramid faces can suffice given complete linear polarization at the entrance of the axicon system with the direction of oscillation of the electric field strength perpendicular or parallel to one of the axicon surfaces. It is possible to arrange for the light to have only a pure s-component or a pure p-component at a surface and the respective opposite axicon surface given suitable alignment of these surfaces relative to the preferred direction of polarization. Consequently, no rotation of the polarization is required at these surfaces. Moreover, given a pure p-polarization, the Fresnel losses are minimized in the case of the reflections at the associated axicon surfaces, above all when the incident radiation strikes this surface substantially at the Brewster angle.

It can be favourable to arrange in the optical delay system a further optical delay element which introduces a delay of half a wavelength between two mutually orthogonal states of polarization. The optical delay element can be, for example, a half-wave plate, a birefringent optical element or a coating on an optical element whose action corresponds to that of a half-wave plate. The fast axis of the first optical delay element should in this case enclose an angle of 45° ± 5° with the fast axis of the second optical delay element. An angle of 45° is ideal. The term "fast axis" is known from polarization optics. The advantage of two mutually rotated delay elements consists in that two mutually orthogonal states of polarization of a beam are transposed by the optical delay system, this being done specifically irrespective of the state of polarization of the incident beam. Thus, in the case of a beam pencil with beams having different states of polarization, it is possible in the case of all the beams to transpose the two mutually orthogonal states of polarization. A single appropriately orientated delay element would suffice if all the beams of the beam pencil have the same state of polarization. If use is made of two delay elements, the latter can, for example, be joined seamlessly or wrung onto one another.

It is also possible within the scope of the invention for the axicon system to be supplemented by one or more radially polarization-rotating arrangements which can comprise delay elements which are arranged in a raster and have a suitable orientation of their crystallographic axes, as are disclosed, for example, in EP 0 764 858. In the case of such a raster arrangement of delay elements, the individual delay elements can also be shaped like pieces of cake in a number which corresponds to the number of individual elements of a polygonal axicon. If such a raster arrangement is fitted on the exit side of an axicon system, an approximated radial or tangential distribution of polarization can be produced in the output radiation. The possibility of installing and removing such λ/2 triangular plates in a flexible way, for example by means of an automatic changing device, then permits a changeover between radial and tangential output polarization of the axicon system.

Claims

Patent Claims

1. Axicon system for transforming an entrance light distribution striking an entrance surface of the axicon system into an exit light distribution emerging from an exit surface of the axicon system by radial redistribution of light intensity, having: an optical axis; a first axicon element with at least one first axicon surface; at least one second axicon element with at least one second axicon surface; and at least one intermediate surface arranged between the first axicon surface and the second axicon surface.

2. Axicon system according to Claim 1 , in which the intermediate surface is a further axicon surface.

3. Axicon system according to Claim 1 or 2, in which the intermediate surface is a substantially planar or substantially spherical optical surface arranged in the light path downstream of an axicon surface.

4. Axicon system according to one of the preceding claims, in which incident angles of the penetrating radiation which are in the vicinity of the Brewster angle do not occur on any of the axicon surfaces.

5. Axicon system according to one of the preceding claims, in which, for all axicon surfaces, an incident angle, occurring on the axicon surfaces, of the penetrating radiation is at an angular spacing of at least 10°, in particular at least 15°, from the associated Brewster angle.

6. Axicon system according to one of the preceding claims, in which, at all locations of the axicon surfaces, local inclination angles are smaller than 30°, in particular smaller than 25°, with reference to a radial plane perpendicular to the optical axis.

7. Axicon system according to one of the preceding claims, in which at least one of the axicon surfaces is conical, all of the axicon surfaces preferably being conical.

8. Axicon system according to one of the preceding claims, in which at least one of the axicon surfaces has the form of a multifaceted pyramid face having at least two pyramid facets inclined to the optical axis, preferably planar pyramid facets with n-fold radial symmetry being arranged about the optical axis, n being the number of the pyramid facets.

9. Axicon system according to Claim 8, in which more than four pyramid facets are provided.

10. Axicon system according to one of the preceding claims, in which at least one axicon surface is a quasi-conical optical surface curved about a plurality of axes and having a first curvature, running in the circumferential direction, and a second curvature which lies in a plane of curvature containing the optical axis.

11. Axicon system according to one of the preceding claims which is designed such that light beams of the penetrating light, at each axial position between the entrance surface and the exit surface of the axicon system, run at an angle to the optical axis such that no region with a substantially parallel beam path is present between the entrance surface and exit surface.

12. Axicon system according to one of the preceding claims, in which an axial spacing between the first axicon element and the second axicon element is, preferably infinitely, adjustable.

13. Axicon system according to one of the preceding claims, in which at least one axicon element is assembled from a plurality of separate individual elements arranged about the optical axis of the axicon system.

14. Axicon system according to Claim 13, in which at least one of the individual elements is mounted movably in such a way that it can be tilted about an axis perpendicular to a radial direction of the optical axis.

15. Axicon system according to one of the preceding claims, in which a polarization rotator for rotating the polarization of the penetrating light by approximately 90° is arranged between the first and the second axicon element.

16. Axicon system according to Claim 15, in which the polarization rotator comprises an optical delay system which effects a delay of half a wavelength or an odd multiple of half a wavelength between two mutually perpendicular states of polarization.

17. Axicon system according to one of Claims 15 or 16, in which the optical delay system comprises two optical delay elements which in each case effect a delay of half a wavelength or an odd multiple thereof between two mutually perpendicular states of polarization, a first optical delay element having a first fast axis, and the second optical delay element having a second fast axis, and the first and the second fast axes enclosing an angle of 45° + 5°.

18. Axicon system according to one of Claims 15 to 17, in which the polarization rotator is arranged directly between an axicon surface of an upstream axicon element and an axicon surface of a downstream axicon element, the polarization rotator preferably being of double- axicon shape

19. Axicon system according to one of Claims 15 to 17, in which the polarization rotator is arranged directly between a substantially planar exit surface of an upstream axicon element and a substantially planar entrance surface of a downstream axicon element, the polarization rotator preferably having a plane-parallel shape.

20. Axicon system according to one of the preceding claims, in which at least one axicon element, in particular the entire axicon system, is mounted rotatably about the optical axis.

21. Axicon system according to one of the preceding claims, in which the at least one axicon element is designed as a diffractive or refractive or reflective element.

22. Illuminating system for an optical device, in particular for a projection exposure apparatus for microlithography, characterized by at least one axicon system according to one of the preceding claims.

23. Illuminating system according to Claim 22, which has at least one light mixer, which is preferably arranged downstream of the axicon system.

24. Microlithographic projection exposure apparatus having: a light source; an illuminating system in accordance with one of Claims 22 or 23; and a projection objective.

25. Microlithographic projection exposure apparatus according to

Claim 24, in which the light source is designed for outputting linearly polarized light, and the projection objective is a catadioptric projection objective with a polarization-selective physical beam splitter.