This application claims priority of German Patent Application DE 10 2020 213 983.0 filed on November 6, 2020. The content of this application is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to an optical system, in particular in a microlithographic projection exposure apparatus, and to a method for heating an optical element in an optical system.

Prior art

Microlithography is used for production of microstructured components, such as integrated circuits or LCDs, for example. The microlithography process is conducted in what is called a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (= reticle) illuminated by means of the illumination device is in this case projected by means of the projection lens onto a substrate (e.g., a silicon wafer) coated with a lightsensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate. In projection lenses designed for the EUV range, i.e. , at wavelengths of, e.g., approximately 13 nm or approximately 7 nm, mirrors are used as optical components for the imaging process owing to the lack of availability of suitable light-transmissive refractive materials.

One problem which arises in practice is that, as a result of absorption of the radiation emitted by the EUV light source among other reasons, the EUV mirrors heat up and undergo an associated thermal expansion or deformation, which in turn can negatively affect the imaging properties of the optical system.

Various approaches are known for avoiding surface deformations caused by heat inputs into an EUV mirror and optical aberrations associated therewith. It is known, inter alia, to use a material with ultra-low thermal expansion (“Ultra Low Expansion Material”), for example a titanium silicate glass sold by Coming Inc. with the name ULE ^TM , as the mirror substrate material and to set what is known as the zero-crossing temperature in a region near the optical effective surface. At this zero-crossing temperature, which lies at around &= 30°C for example for ULE ^TM , the coefficient of thermal expansion, in its temperature dependence, has a zero crossing in the vicinity of which no thermal expansion or only negligible thermal expansion of the mirror substrate material takes place.

Possible further approaches for avoiding surface deformations caused by heat inputs into an EUV mirror include the use of a heating arrangement on the basis of infrared radiation. With such a heating arrangement, active mirror heating can take place in phases of comparatively low absorption of EUV used radiation, wherein said active mirror heating is correspondingly decreased as the absorption of the EUV used radiation increases. Furthermore, the EUV mirrors can also be preheated to the abovementioned zero-crossing temperature prior to actual operation or prior to application of EUV radiation.

The coupling of the infrared radiation into the relevant (EUV)-mirror in turn requires the use of a suitable optical unit forming the heating arrangement, which optical unit, taking account of the concrete conditions in the optical system (in particular the geometric arrangement of the mirror to be heated, the limited absorption of the heating radiation by said mirror, and structural space limitations present), enables as uniform a mirror heating as possible from a spatial standpoint.

One problem that occurs in practice is that when the mirror to be heated has larger dimensions or as the power of the EUV light source increases, the abovedescribed heating concept of mirror heating requires the input coupling of considerable heating powers of an order of magnitude of 100W or more, as a result of which in turn the optically effective surfaces of the optical unit forming the heating unit are subjected to correspondingly high irradiation intensities. This can in turn result in degradation through to destruction of the optical elements in the optical unit forming the heating unit or the volume and coating materials present in said optical elements. The defects that result in such degradation may be, for example, compaction effects (i.e. local changes in density in the volume material and refractive index changes associated therewith), transmission changes and also nonlinear effects such as self-induced focussing. In addition, contamination particles that deposit on the optically effective surfaces of the optical elements in the optical unit forming the heating unit can result in an undesired local increase in the absorbed radiation intensity and thermally induced degradation associated therewith.

The problems described above are particularly serious in particular if the dimensions of the optical elements of the optical unit forming the heating unit are significantly smaller in comparison with the (EUV) mirror to be heated, for reasons of structural space and/or for cost reasons.

As a result, exchange of the entire heating unit and an associated interruption of the operation of the microlithographic projection exposure apparatus may become necessary.

Regarding the prior art, reference is made merely by way of example to DE 10 2017 207 862 A1. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical system, in particular in a microlithographic projection exposure apparatus, and a method for heating an optical element in an optical system which make it possible to avoid or to reduce surface deformations caused by heat inputs in an optical element and optical aberrations associated therewith, while at least partly avoiding the problems described above.

This object is achieved by means of the optical element and respectively the method in accordance with the features of the independent claims.

According to one aspect, an optical system, in particular in a microlithographic projection exposure apparatus, comprises:

- an optical element; and

- at least one heating unit for heating the optical element by applying electromagnetic radiation to the optical element;

- wherein the heating unit comprises, as part of an optical collimator and/or as part of a telescope, at least one mirror having a non-plane optical effective surface.

The invention is based on the concept, in particular, of realizing an optical unit serving to implement the above-described heating concept (i.e. the input coupling of heating radiation into an optical element with the aim of avoiding thermally induced deformations during operation of the optical system comprising said element), using one or more mirrors. In this case, the invention is based on the consideration that in the case where the optical unit forming the heating unit is at least partly configured as a mirror system, comparatively rapid and effective heat dissipation (relative to a purely refractive lens element system) can be effected by suitable thermal linking and/or active cooling of the relevant mirror(s). In particular, in contrast to lens elements, the optically unused mirror rear side and/or the mirror substrate can be used for heat dissipation. On account of the effective heat dissipation thus made possible according to the invention within the optical unit forming the heating unit, the degradation effects described in the introduction can be significantly reduced or even completely avoided.

Furthermore, the comparatively good heat conduction for instance within the mirror substrate of the at least one mirror used within the heating unit according to the invention also has an advantageous effect in so far as locally varying heating that is possibly unavoidably associated with the depositing of contamination particles can rather be afforded tolerance, since such locally varying radiation absorption, owing to the improved heat conduction, does not result in a temperature increase beyond the respective damage limit.

As a result, a significantly stabler and more robust optical unit of the heating unit can be provided according to the invention in comparison with a configuration comprising a purely refractive lens element system.

In particular, according to the invention, within the optical unit forming the heating unit, those optically effective surfaces can be configured in reflective fashion at which the maximum irradiation intensities to be expected during operation of the optical system or the microlithographic projection exposure apparatus exceed a specific threshold value (e.g. 1W/mm ² , in particular 5W/mm ² , more particularly 10W/mm ² ).

In this case, according to the invention, with the (at least partial) configuration of the optical unit forming the heating unit as a mirror system, disadvantages are deliberately accepted in order, in return, to achieve the above-described advantages with regard to more efficient heat dissipation and resultant stability and robustness of the system. This concerns in particular the comparatively significantly larger dimensions of the optically effective surface(s) afforded typically and primarily when the relevant mirror is inclined in relation to the optical beam path. A further disadvantage that is accepted according to the invention concerns the significantly greater sensitivity of a mirror surface to manufacturing faults and/or alignment errors in comparison with lens element surfaces. Overall, greater difficulties are accepted according to the invention in order to realize the heating unit with one or more mirrors under the strict structural space limitations typically given and at the same time to limit undesired aberrations to an acceptable level.

In accordance with one embodiment, the heating unit comprises an optical collimator that is constructed exclusively from mirrors. In embodiments, the heating unit may, in addition thereto, comprise a telescope which can either be constructed from lenses or which can be constructed from mirrors.

In accordance with one embodiment, the heating unit comprises a telescope that is constructed exclusively from mirrors. In embodiments, the heating unit may, in addition thereto, comprise an optical collimator which can either be constructed from lenses or which can be constructed from mirrors.

In accordance with one embodiment, the heating unit comprises an optical collimator that is constructed exclusively from mirrors as well as a telescope that is constructed exclusively from mirrors.

In accordance with one embodiment, said telescope does not have a continuous optical axis.

In accordance with one embodiment, the at least one mirror is an aspherical mirror.

In accordance with one embodiment, the at least one mirror comprises an optically effective freeform surface.

In accordance with one embodiment, the at least one mirror has a mirror substrate comprising a mirror substrate material having a thermal conductivity coefficient of at least 10 Wm’ ¹ K’ ¹ , in particular at least 50 Wm’ ¹ K’ ¹ , more particularly at least 100 Wm’ ¹ K’ ¹ . In accordance with one embodiment, the at least one mirror is thermally coupled to a heat dissipating component composed of the material having a thermal conductivity coefficient of at least 10 Wm’ ¹ K’ ¹ , in particular at least 50 Wm’ ¹ K’ ¹ , more preferably at least 100 Wm’ ¹ K’ ¹ .

In accordance with one embodiment, the optical system comprises at least one cooler for dissipating heat from the at least one mirror.

In accordance with one embodiment, the at least one mirror comprises at least one cooling channel to which a cooling fluid can be applied.

In accordance with one embodiment, the optical element to be heated is a mirror.

In accordance with one embodiment, the optical element to be heated is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

In accordance with one embodiment, the optical system is an optical system of a microlithographic projection exposure apparatus, in particular an illumination device or a projection lens.

The invention furthermore also relates to a microlithographic projection exposure apparatus comprising an optical system having the features described above, and to a heating unit for use in such an optical system.

The invention furthermore relates to a method for heating an optical element, wherein the optical element comprises an optical effective surface, wherein a beam of electromagnetic radiation is applied to the optical element by way of at least one heating unit, and wherein a heating unit having the features described above is used. With regard to advantages and further preferred configurations of the method, reference is made to the above explanations in association with the optical system according to the invention.

Further configurations of the invention can be gathered from the description and the dependent claims.

The invention is explained in greater detail below on the basis of exemplary embodiments illustrated in the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures:

Figure 1 shows a schematic illustration of the possible construction of a heating arrangement for heating an optical element in an optical system;

Figure 2 shows a schematic illustration of the construction of a heating arrangement in accordance with a further embodiment;

Figure 3 shows a schematic illustration of a collimator configured according to the invention for the heating arrangement from Figure 1 or respectively 2 in one possible embodiment;

Figure 4 shows a schematic illustration of a telescope configured according to the invention for the heating arrangement from Figure 1 or respectively 2 in one possible embodiment;

Figure 5 shows a schematic illustration of the irradiation of an EUV mirror using a telescope present in a heating arrangement according to the invention; Figure 6 shows a schematic illustration of one possible scenario of use of a heating arrangement according to the present invention; and

Figure 7 shows a schematic illustration of the possible construction of a microlithographic projection exposure apparatus designed for operation in the EUV.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 7 schematically shows in meridional section the possible construction of a microlithographic projection exposure apparatus designed for operation in the EUV.

In accordance with Fig. 7, the projection exposure apparatus 1 comprises an illumination device 2 and a projection lens 10. The illumination device 2 serves to illuminate an object field 5 in an object plane 6 with radiation from a radiation source 3 by way of an illumination optical unit 4. Here, a reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable by way of a reticle displacement drive 9, in particular in a scanning direction. For purposes of explanation, a Cartesian xyz-coordinate system is depicted in Fig. 7. The x-direction runs perpendicularly to the plane of the drawing into the latter. The y-direction runs horizontally, and the z-direction runs vertically. The scanning direction runs along the y-direction in Fig. 7. The z- direction runs perpendicularly to the object plane 6.

The projection lens 10 serves for imaging the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a lightsensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable by way of a wafer displacement drive 15, in particular along the y-direction. The displacement on the one hand of the reticle 7 by way of the reticle displacement drive 9 and on the other hand of the wafer 13 by way of the wafer displacement drive 15 can take place in such a way as to be synchronized with one another.

The radiation source 3 is an EUV radiation source. The radiation source 3 emits, in particular, EUV radiation 16, which is also referred to below as used radiation or illumination radiation. In particular, the used radiation has a wavelength in the range of between 5 nm and 30 nm. The radiation source 3 can be for example a plasma source, a synchrotron-based radiation source or a free electron laser (FEL). The illumination radiation 16 emanating from the radiation source 3 is focussed by a collector 17 and propagates through an intermediate focus in an intermediate focal plane 18 into the illumination optical unit 4. The illumination optical unit 4 comprises a deflection mirror 19 and, disposed downstream thereof in the beam path, a first facet mirror 20 (having schematically indicated facets 21 ) and a second facet mirror 22 (having schematically indicated facets 23).

The projection lens 10 comprises a plurality of mirrors Mi (i= 1 , 2, ... ), which are consecutively numbered in accordance with their arrangement in the beam path of the projection exposure apparatus 1 . In the example illustrated in Fig. 7, the projection lens 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or any other number of mirrors Mi are similarly possible. The penultimate mirror M5 and the last mirror M6 each have a through opening for the illumination radiation 16. The projection lens 10 is a doubly obscured optical unit. The projection lens 10 has an image-side numerical aperture that is greater than 0.5 and may also be greater than 0.6, and may be for example 0.7 or 0.75.

During operation of the microlithographic projection exposure apparatus 1 , the electromagnetic radiation incident on the optical effective surface of the mirrors is partly absorbed and, as explained in the introduction, results in heating and an associated thermal expansion or deformation, which can in turn result in an impairment of the imaging properties of the optical system. The concept according to the invention for heating an optical element can thus be applied in particular advantageously to any desired mirror of the microlithographic projection exposure apparatus 1 from Fig. 7.

Fig. 1 shows a schematic illustration for elucidating the possible construction of a heating arrangement according to the invention for heating an optical element in a first embodiment.

In accordance with Fig. 1 , a beam generated by a radiation source (not illustrated), which can be e.g. a fibre laser for generating IR radiation having an exemplary wavelength of approximately 1000 nm, emerges at a fibre end designated by “101" and firstly passes through an optical collimator 120, for which a concrete exemplary embodiment is described merely by way of example below with reference to Fig. 3. In accordance with Fig. 1 (but without the invention being restricted thereto) the collimated beam emerging from the collimator 110 firstly enters an optical component 120 comprising a polarization beam splitter 121 and a deflection mirror 122. A function of said optical component 120 is to provide two partial beams, each of which is linearly polarized, from the laser beam (originally still unpolarized upon entering the component 120), wherein said linearly polarized partial beams can be used for input coupling - optimized with regard to absorption - of heating radiation into the optical element to be heated in each case (e.g. an EUV mirror of the microlithographic projection exposure apparatus from Fig. 7).

In accordance with Fig. 1 , the two partial beams each having linear polarization emerge from the optical component 120 along the original light propagation direction (i.e. along the z-direction in the coordinate system depicted) along two separate parallel beam paths and each successively pass through an optical retarder 123 and 125, respectively, a diffractive optical element (DOE) 124 and 126, respectively, and an optical telescope 130 and 140, respectively. Refractive optical elements or reflective optical elements or mirrors can also be used instead of the diffractive optical elements 124, 126. The DOEs 124, 126 serve as beam shaping units for impressing an individual heating profile into the optical element to be heated by way of beam shaping of the IR radiation to be directed onto the optical element.

By means of the optical retarders 123, 125 (which can be configured e.g. as lambda/2 plates), it is possible to achieve a suitable setting of the respective polarization direction in order to set suitably (i.e. with regard to maximum absorption) the respective polarization direction in both separate beam paths for the input coupling of the IR radiation into the optical element to be heated or the EUV mirror. In further embodiments, instead of the retarders, an optical rotator for suitably rotating the polarization direction can also be used in each case (wherein for example a 90° rotator can be produced, in a manner known per se, from two lambda/2 plates rotated relative to one another by 45° with respect to their fast axis of birefringence). Furthermore, in embodiments of the invention, it may also be sufficient to use an optical retarder or rotator in only one of the two separate beam paths. This is the case in particular if a rotation of the polarization direction in only one of the two separate beam paths and by an angle of 90° is advisable since the splitting - achieved by means of the polarization beam splitter 121 - into two radiation portions polarized orthogonally to one another can be assumed to be substantially perfect and the aperture of the optical beam path in the region of the polarization beam splitter 121 can be assumed to be small.

The optical telescopes 130 and respectively 140, for which a concrete exemplary embodiment is described likewise merely by way of example below with reference to Fig. 4, serve for providing a suitable additional beam deflection before the IR radiation is coupled into the optical element to be heated or the EUV mirror.

The invention is not restricted to the application in the concrete configuration from Fig. 1. Rather, the invention is also intended to encompass embodiments in which the above-described generation of two separate partial beams is dispensed with (and thus only a “single heating head” is provided in contrast to the “double heating head” from Fig. 1 ). A corresponding heating arrangement illustrated schematically in Fig. 2 comprises only a single telescope 230, wherein components that are analogous or substantially functionally identical in comparison with Fig. 1 are moreover designated by reference numerals increased by "100". In this case, in comparison with Fig. 1 , the use both of the optical component 120 and of the optical retarders 123, 125 is dispensed with. In Fig. 2, “223” denotes a polarizer suitable for generating the suitable polarization state, and “224” denotes a DOE.

The input coupling of linearly polarized (partial) beams via the heating unit into the optical element to be heated has the advantage that even in the case of input coupling of the generated heating radiation at comparatively large angles of incidence relative to the respective surface normal (“grazing incidence”) a sufficient absorption of the heating radiation can be achieved. Such input coupling of the heating radiation with “grazing incidence” in turn may prove to be advantageous or even necessary in the concrete application situation in respect of structural space aspects if - as is often the case - sufficient structural space is not available within the projection exposure apparatus in the direction perpendicular to the surface of the optical element to be heated. Furthermore, said input coupling of the heating radiation with grazing incidence, depending on the concrete application situation, makes it possible optionally to ensure that the heating arrangement is arranged outside the actual used beam path.

During operation, each of the heating arrangements used in the optical system (such as e.g. the projection exposure apparatus from Fig. 7) can generate an individual heating profile predefined by way of the respective DOE (“124” and “126” in Fig. 1 ) on the respective optical element or EUV mirror. Switching the heating arrangement comprising the respective DOE on or off stipulates whether or not the respective heating profile assigned to said DOE on the optical element is set or the EUV mirror is correspondingly actively heated. In embodiments, a plurality of mutually independently controllable heating arrangements having the construction described with reference to Fig. 1 can also be provided and assigned to one and the same optical element in order, depending on the illumination setting currently chosen, to be able to set a suitable heating profile in the optical element or EUV mirror.

The optical telescope 130 and respectively 140 in accordance with Fig. 1 and the optical telescope 230 in accordance with Fig. 2 can be constructed as a mirror system according to the invention. One exemplary embodiment of such a mirror system is illustrated in Fig. 4, which mirror system, according to the beam path illustrated in Fig. 1 or respectively Fig. 2, causes an incoming collimated beam having a comparatively large beam cross section to be converted into an outgoing divergent beam having a comparatively small beam cross section.

A suitable material for the configuration of the reflection layer of the at least one mirror used in the heating unit according to the invention is in particular gold (Au), which, for the wavelength of e.g. approximately 1000 nm used in the heating unit, provides a sufficiently high reflectance of more than 95% depending on the angle of incidence.

Even if the use - effected according to the invention - of at least one mirror within the heating arrangement in the application example shown in Fig. 1 or Fig. 2 is effected preferably on the part of the optical telescope 130, 140 or respectively 230 in view of the comparatively high values there of the maximum irradiation intensity to be expected, the invention is not restricted thereto. In particular, in other applications, additionally or alternatively, one or more mirrors can also be used at some other position within the optical unit forming the heating unit, e.g. in the region of the optical collimator 110 or respectively 210.

The exemplary embodiment of an optical collimator shown in Fig. 3 comprises two mirrors 311 , 312. The mirror 311 comprises a toric optical effective surface having two radii Rx = -35.202216 mm and Ry = -36.850281 mm. The coordinates of the origin in source coordinates are (0, 0, 7.6320000), and the tilt angle in source coordinates is 11.97°. The mirror 312 likewise comprises a toric optical effective surface having two radii Rx= -36.005011 mm and Ry = -39.555388 mm. The coordinates of the origin in source coordinates are (0, 2.2439621 , 2.5777410), and the tilt angle in source coordinates is 41 .261 °.

The exemplary embodiment of a telescope 430 as shown in Fig. 4 comprises four mirrors 431-434 each having an aspherical optical effective surface and images the angular spectrum of the DOE (which e.g. corresponds to the DOE 224 from Fig. 2 and is not illustrated in Fig. 4, but is arranged on the entrance side of the mirror 431 ) onto the mirror to be heated. Such a mirror is indicated in Fig. 5, is arranged in tilted fashion in the optical beam path and is designed by “450". The spot size on the mirror to be heated is approximately 30 pm in the exemplary embodiment. Radiation is applied to only half of the area to be heated (without vignetting). The magnification of the telescope 430 is [3= 5, angles of up to 3° being supported.

The mirror 431 comprises a toric optical effective surface having two radii Rx= - 21.44069 mm and Ry= -627.03403 mm. The coordinates of the origin in DOE coordinates are (0, 0, 10), and the tilt angle in DOE coordinates is 22.529543°. Table 1 indicates the asphere coefficients up to the 4th order for the mirror 431 . In this case, the freeform surface embodied by the optical effective surface is described by a fourth-order polynomial in accordance with

(1 )

Table 1 : The mirror 432 comprises a tone optical effective surface having two radii Rx= 7.445997 mm and Ry= -460.285833 mm. The coordinates of the origin in DOE coordinates are (0, 5.365294085389801 , 4.645760386650385), and the tilt angle in DOE coordinates is 21.492417°. Table 2, analogously to table 1 , indicates the asphere coefficients up to the 4th order for the mirror 432:

Table 2:

The mirror 433 comprises a toric optical effective surface having two radii Rx= - 49.353522 mm and Ry= -32.780797 mm. The coordinates of the origin in DOE coordinates are (0, 6.120438429146021 , 25.49553182682347), and the tilt angle in DOE coordinates is -18.58314900000001 °. Table 3, analogously to table 1 , indicates the asphere coefficients up to the 4th order for the mirror 433:

Table 3:

The mirror 434 comprises a toric optical effective surface having two radii Rx= 28.872586 mm and Ry= 7.455634 mm. The coordinates of the origin in DOE coordinates are (0, 0.3285899881067937, 17.25213274393808), and the tilt angle in DOE coordinates is -17.50913°. Table 4, analogously to table 1 , indicates the asphere coefficients up to the 4th order for the mirror 434: Table 4:

The possible configurations - illustrated by way of example in Fig. 3-4 of a mirror system used within the heating unit according to the invention have in common the fact that they do not have a continuous optical axis, which is advantageous in particular with regard to the advisable minimization of the structural space required in each case - in comparison with a Schwarzschild design having a continuous optical axis as is likewise usable, in principle, according to the invention. Furthermore, the relevant mirror system comprises preferably (but likewise without the invention being restricted thereto) at least one aspherically shaped optically effective surface, whereby image aberrations associated with dispensing with a rotationally symmetrical construction in a way that is advantageous for reasons of structural space can be effectively corrected.

In accordance with Fig. 6, the input coupling of electromagnetic (heating) radiation into an optical element 600 to be heated (here in the form of a concave mirror having an optical effective surface 601 ) for the purpose of heating said optical element 600 can also be effected by way of a first heating unit 610 and a second heating unit 620. In this case, the heating units 610, 620 can each have the construction described above with reference to Figs 1 -5. In accordance with Fig. 6, a beam generated by the first heating unit 610 impinges on that region - designated by “A" - of the optical effective surface 601 which lies nearer to the second heating unit 620 (i.e. faces the second heating unit 620). By contrast, the beam generated by the second heating unit 620 impinges on that region - designated by “B" - of the optical effective surface 601 which lies nearer to the heating unit 610. In other words, the heating units 610, 620 are arranged relative to the optical element 600 or the optical effective surface 601 thereof in such a way that the beams respectively generated overlap one another or the centroid rays assigned to the respective beams cross one another on their way to the optical effective surface 610, whereby the “compliance” with the angle-of- incidence range suitable for minimizing or limiting reflections can be achieved across the entire optical effective surface.

Even if, in the embodiments described above, the optical element to be heated is in each case a mirror (in particular designed for operation in the EUV range), the invention is not restricted thereto. In further embodiments, the optical element to be heated can also be a mirror designed for other operating wavelengths (e.g. for the DUV range, i.e. for wavelengths of less than 250 nm, in particular less than 200 nm) or else a lens element.

Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments will be apparent to a person skilled in the art, for example through combination and/or exchange of features of individual embodiments. Accordingly, it goes without saying for a person skilled in the art that such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is restricted only within the meaning of the appended claims and the equivalents thereof.