DE102020213983A1 - Optical system, in particular in a microlithographic projection exposure system - Google Patents

Abstract

The invention relates to an optical system, in particular in a microlithographic projection exposure system, and to a method for heating an optical element in an optical system. An optical system according to the invention has an optical element (450) and at least one heating unit (100, 200) for heating this optical element by applying electromagnetic radiation to the optical element, the heating unit having at least one mirror with a non-planar optical effective surface.

Description

BACKGROUND OF THE INVENTION

field of invention

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

State of the art

Microlithography is used to produce microstructured components such as integrated circuits or LCDs. The microlithographic process is carried out in a so-called projection exposure system, which has an illumination device and a projection lens. The image of a mask (= reticle) illuminated by means of the illumination device is projected by means of the projection objective onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to project the mask structure onto the light-sensitive coating of the to transfer substrate.

In projection lenses designed for the EUV range, i.e. at wavelengths of around 13 nm or around 7 nm, for example, mirrors are used as optical components for the imaging process due to the lack of availability of suitable transparent refractive materials.

A problem that occurs in practice is that the EUV mirrors, among other things, experience heating as a result of absorption of the radiation emitted by the EUV light source and an associated thermal expansion or deformation, which in turn can result in impairment of the imaging properties of the optical system .

Various approaches are known for avoiding surface deformations caused by heat input into an EUV mirror and optical aberrations associated therewith. Among other things, it is known to use a material with ultra-low thermal expansion ("ultra-low-expansion material") as the mirror substrate material, e.g. a titanium silicate glass sold by Corning Inc. under the name ULE™, and in one of set the so-called zero crossing temperature (= "zero crossing temperature") in the area close to the optical active surface. At this zero-crossing temperature, which is around ϑ = 30°C for ULE™, for example, the temperature dependence of the thermal expansion coefficient shows a zero crossing, in the vicinity of which there is no or only negligible thermal expansion of the mirror substrate material.

Possible further approaches to avoid surface deformations caused by heat input into an EUV mirror include the use of a heating arrangement based on infrared radiation. With such a heating arrangement, an active heating of the mirror can take place in phases of comparatively low absorption of useful EUV radiation, with this active heating of the mirror being correspondingly reduced with increasing absorption of the useful EUV radiation. Furthermore, the EUV mirrors can also be preheated to the above-mentioned zero crossing temperature (= "zero crossing temperature") before actual operation or before exposure to EUV radiation.

The coupling of the infrared radiation into the relevant (EUV) mirror in turn requires the use of suitable optics forming the heating arrangement, which, taking into account the specific conditions in the optical system (in particular the geometric arrangement of the mirror to be heated, its limited absorption of the heating radiation and existing Installation space restrictions) allows a mirror heating that is as uniform as possible in local terms.

A problem that occurs in practice is that with larger dimensions of the mirror to be heated or increasing power of the EUV light source, the heating concept of the mirror heating described above requires the coupling of considerable heating power of the order of 100W or more, which in turn reduces the optically effective surfaces of the heating unit visual optics are exposed to correspondingly high irradiation intensities. This in turn can result in degradation up to and including destruction of the optical elements in the optics forming the heating unit or of the volume and coating materials present in these optical elements. The defects leading to such degradation can be, for example, compaction effects (i.e. local density changes in the volume material and the associated changes in the refractive index), transmission changes and non-linear effects such as self-induced focusing. In addition, contamination particles, which are deposited on the optically effective surfaces of the optical elements in the optics forming the heating unit, can result in an undesired local increase in the absorbed radiation intensity and a thermally induced degradation associated therewith.

The problems described above are particularly serious when the optical elements of the optics forming the heating unit are dimensioned significantly smaller than the (EUV) mirror to be heated for reasons of installation space and/or cost.

As a result, it may be necessary to replace the entire heating unit and, as a result, to interrupt the operation of the microlithographic projection exposure system.

The prior art is only given as an example DE 10 2017 207 862 A1 referred.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical system, in particular in a microlithographic projection exposure system, and a method for heating an optical element in an optical system, which avoids or reduces surface deformations caused by heat input in an optical element and associated optical allow aberrations while at least partially avoiding the problems described above.

This object is achieved by the optical system according to the features of independent patent claim 1 and the method according to the features of independent patent claim 16 .

• - ein optisches Element; und
• - wenigstens eine Heizeinheit zum Heizen des optischen Elements durch Beaufschlagen des optischen Elements mit elektromagnetischer Strahlung;
• - wobei die Heizeinheit wenigstens einen Spiegel mit nicht-planer optischer Wirkfläche aufweist.

An optical system, in particular in a microlithographic projection exposure system, has:

• - 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 has at least one mirror with a non-planar optical effective surface.

The invention is based in particular on the concept of using one or more optics to implement the heating concept described above (i.e. the coupling of heating radiation into an optical element with the aim of avoiding thermally induced deformations during operation of the optical system comprising this element). to realize mirroring. The invention is based on the consideration that if the optics forming the heating unit are designed at least partially as a mirror system, heat can be dissipated comparatively quickly and effectively (relative to a purely refractive lens system) by suitable thermal connection and/or active cooling of the mirror or mirrors in question (s) can be done. In particular, in contrast to lenses, the optically unused rear side of the mirror and/or the mirror substrate can be used for heat dissipation. Due to the effective heat dissipation within the optics forming the heating unit, which is thus made possible according to the invention, the degradation effects described at the outset can be significantly reduced or even completely avoided.

Furthermore, the comparatively good heat conduction, for example within the mirror substrate of the at least one mirror used according to the invention within the heating unit, also has an advantageous effect in that locally varying heating that may unavoidably accompany the deposition of contamination particles can be tolerated more easily, since such locally varying radiation absorption due to the improved heat conduction does not result in a temperature rise above the respective damage limit.

As a result, significantly more stable and robust optics of the heating unit can be provided according to the invention compared to an embodiment with a purely refractive lens system.

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

According to the invention, disadvantages are deliberately accepted with the (at least partial) configuration of the optics forming the heating unit as a mirror system in order to achieve the advantages described above with regard to efficient heat dissipation and the resulting stability and robustness of the system. This relates in particular to the comparatively significantly larger dimensions of the optically effective surface(s), which typically and above all are given when the relevant mirror is in an inclined position in relation to the optical beam path. Another disadvantage that is accepted according to the invention relates to the significantly greater sensitivity of mirror surfaces to manufacturing and/or adjustment errors compared to lens surfaces. Overall, greater difficulties are accepted according to the invention in order to realize the heating unit with one or more mirrors under the typically given strict installation space restrictions and at the same time to limit undesired aberrations to an acceptable level.

According to one embodiment, the mirror is part of an optical collimator.

According to one embodiment, the heating unit has a telescope made up exclusively of mirrors.

According to one embodiment, this telescope does not have a continuous optical axis.

According to one embodiment, the at least one mirror is an aspheric mirror.

According to one embodiment, the at least one mirror has an optically effective free-form surface.

According to one embodiment, the at least one mirror has a mirror substrate which has a mirror substrate material with a thermal conductivity coefficient of at least 10 Wm ^-1 K ^-1 , in particular at least 50 Wm ^-1 K ^-1 , further in particular at least 100 Wm ^-1 K ^-1 .

According to one embodiment, the at least one mirror is thermally coupled to a heat dissipation component made of a material with 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 .

According to one embodiment, the optical system has at least one cooler for dissipating heat from the at least one mirror.

According to one embodiment, the at least one mirror has at least one cooling channel that can be charged with a cooling fluid.

According to one embodiment, the optical element to be heated is a mirror.

According to one embodiment, the optical element to be heated is designed for a working wavelength of less than 30 nm, in particular less than 15 nm.

According to one embodiment, the optical system is an optical system of a microlithographic projection exposure system, in particular an illumination device or a projection lens.

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

The invention further relates to a method for heating an optical element, the optical element having an optical active surface, the optical element being exposed to a beam of electromagnetic radiation via at least one heating unit, and a heating unit having the features described above being used.

With regard to advantages and further preferred configurations of the method, reference is made to the above statements in connection with the optical system according to the invention.

Further configurations of the invention can be found in the description and in the dependent claims.

The invention is explained in more detail below with reference to exemplary embodiments illustrated in the attached figures.

character list

• 1 eine schematische Darstellung des möglichen Aufbaus einer Heizanordnung zum Heizen eines optischen Elements in einem optischen System;
• 2 eine schematische Darstellung des Aufbaus einer Heizanordnung gemäß einer weiteren Ausführungsform;
• 3 eine schematische Darstellung eines gemäß der Erfindung ausgestalteten Kollimators für die Heizanordnung von 1 bzw. 2 in einer möglichen Ausführungsform;
• 4 eine schematische Darstellung eines gemäß der Erfindung ausgestalteten Teleskops für die Heizanordnung von 1 bzw. 2 in einer möglichen Ausführungsform;
• 5 eine schematische Darstellung der Bestrahlung eines EUV-Spiegels unter Verwendung eines in einer Heizanordnung gemäß der Erfindung vorhandenen Teleskops;
• 6 eine schematische Darstellung eines möglichen Einsatzszenarios einer Heizanordnung gemäß der vorliegenden Erfindung; und
• 7 eine schematische Darstellung des möglichen Aufbaus einer für den Betrieb im EUV ausgelegten mikrolithographischen Projektionsbelichtungsanlage.

Show it:

• 1 a schematic representation of the possible structure of a heating arrangement for heating an optical element in an optical system;
• 2 a schematic representation of the structure of a heating arrangement according to a further embodiment;
• 3 a schematic representation of a configured according to the invention collimator for the heating arrangement of FIG 1 respectively. 2 in one possible embodiment;
• 4 a schematic representation of a designed according to the invention telescope for the heater assembly of 1 respectively. 2 in one possible embodiment;
• 5 a schematic representation of the irradiation of an EUV mirror using a present in a heating arrangement according to the invention telescope;
• 6 a schematic representation of a possible application scenario of a heating arrangement according to the present invention; and
• 7 a schematic representation of the possible structure of a designed for operation in the EUV microlithographic projection exposure system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

7 shows a schematic meridional section of the possible structure of a microlithographic projection exposure system designed for operation in the EUV.

According to 7 the projection exposure system 1 has 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 via an illumination optics 4 . In this case, 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 can be displaced in particular in a scanning direction via a reticle displacement drive 9 . In 7 a Cartesian xyz coordinate system is drawn in for explanation. The x-direction runs perpendicular to the plane of the drawing. The y-direction is horizontal and the z-direction is vertical. The scan direction is in 7 along the y-direction. The z-direction runs perpendicular to the object plane 6.

The projection lens 10 is used to image the object field 5 in an image field 11 in an image plane 12. A structure on the reticle 7 is imaged on a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is Wafer holder 14 held. The wafer holder 14 can be displaced in particular along the y-direction via a wafer displacement drive 15 . The displacement of the reticle 7 via the reticle displacement drive 9 on the one hand and the wafer 13 on the other hand via the wafer displacement drive 15 can be synchronized with one another.

The radiation source 3 is an EUV radiation source. The radiation source 3 emits in particular EUV radiation, which is also referred to below as useful radiation or illumination radiation. In particular, the useful radiation has a wavelength in the range 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 (“free-electron laser”, FEL). act. The illumination radiation 16 emanating from the radiation source 3 is bundled by a collector 17 and propagates through an intermediate focus in an intermediate focal plane 18 into the illumination optics 4. The illumination optics 4 has a deflection mirror 19 and a first one downstream of this in the beam path Facet mirror 20 (with facets 21 indicated schematically) and a second facet mirror 22 (with facets 23 indicated schematically).

The projection lens 10 has a plurality of mirrors Mi (i=1, 2, . . . ) which are numbered consecutively according to their arrangement in the beam path of the projection exposure system 1. At the in the 7 In the example shown, the projection lens 10 has six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or another number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16 . The projection objective 10 is a doubly obscured optics. The projection objective 10 has a numerical aperture on the image side which is greater than 0.5 and which can also be greater than 0.6 and which can be 0.7 or 0.75, for example.

During operation of the microlithographic projection exposure system 1, the electromagnetic radiation impinging on the optical effective surface of the mirror is partially absorbed and, as explained above, leads to heating and an associated thermal expansion or deformation, which in turn results in impairment of the imaging properties of the optical system can.

The inventive concept for heating an optical element can thus be particularly advantageous for any mirror of the microlithographic projection exposure system 1 of 7 be applied.

1 shows a schematic representation to explain the possible structure of a heating arrangement according to the invention for heating an optical element in a first embodiment.

According to 1 a beam generated by a radiation source (not shown), which can be, for example, a fiber laser for generating IR radiation with an exemplary wavelength of about 1000 nm, exits at a fiber end labeled “101” and first passes through a optical collimator 120, for which further reference is made to FIG 3 a concrete exemplary embodiment is described merely by way of example. The collimated beam emerging from the collimator 110 occurs according to FIG 1 (However, without the invention being restricted thereto) first into an optical component 120 which has a polarization beam splitter 121 and a deflection mirror 122 . A function of this optical component 120 is the provision of two linearly polarized partial beams from the laser beam (originally still unpolarized upon entry into component 120), said linearly polarized partial beams for coupling of heating radiation into the respective optical element to be heated, which is optimized in terms of absorption (e.g. an EUV mirror of the microlithographic projection exposure system from 7 ) can be used.

The two partial beams, each with linear polarization, occur according to 1 from the optical component 120 along the original direction of light propagation (ie along the z-direction in the coordinate system shown) along two separate parallel beam paths and successively pass through an optical retarder 123 or 125, a diffractive optical element (DOE) 124 or 126 and an optical telescope 130 or 140. Instead of the diffractive optical elements 124, 126, refractive optical elements or reflective optical elements or mirrors can also be used.

The DOE's 124, 126 serve as beam shaping units for impressing an individual heating profile on the optical element to be heated by beam shaping of the IR radiation to be directed onto the optical element.

About the optical retarder 123, 125 (which can be configured as lambda / 2 plates, for example) a suitable setting of the respective polarization direction can be achieved to the respective polarization direction in two separate beam paths for coupling the IR radiation into the optical to be heated Element or the EUV mirror (ie, with regard to maximum absorption) to adjust appropriately. In further embodiments, instead of the retarder, an optical rotator can also be used in each case for suitable rotation of the polarization direction (whereby, for example, a 90° rotator consists, in a manner known per se, of two lambda/2 plates rotated by 45° with respect to their fast axis of birefringence can be made). Furthermore, in embodiments of the invention, the use of an optical retarder or rotator in only one of the two separate beam paths can also be sufficient. This is particularly the case when a rotation of the polarization direction is required in only one of the two separate beam paths and by an angle of 90°, since the polarization beam part The splitting achieved by ler 121 into two mutually orthogonally polarized radiation components can be assumed to be essentially 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 140, for which further with reference to 4 a specific exemplary embodiment is also described merely by way of example, serve to provide 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 limited to the application in the specific embodiment of 1 limited. Rather, embodiments should also be considered to be covered by the invention in which the generation of two separate partial jets described above is dispensed with (and thus only a "single heating head" in contrast to the "double heating head" of 1 provided). A corresponding, in 2 Schematically shown heating arrangement has only a single telescope 230, which moreover compared to 1 analogous or essentially functionally identical components are denoted by reference numbers increased by “100”. In comparison to 1 the use of both the optical component 120 and the optical retarder 123, 125 is dispensed with. With "223" is in 2 a polarizer suitable for generating the appropriate polarization state, and "224" denotes a DOE.

The coupling of linearly polarized (partial) rays via the heating unit into the optical element to be heated has the advantage that even if the heating radiation generated is coupled in at comparatively large angles of incidence in relation to the respective surface normal ("grazing incidence") “) sufficient absorption of the heating radiation can be achieved. Such a coupling of the heating radiation with "grazing incidence" can turn out to be advantageous or even necessary in the specific application situation under installation space aspects if - as is often the case - there is insufficient installation space within the projection exposure system in the direction perpendicular to the surface of the optical element to be heated is available. Furthermore, by means of said coupling of the heating radiation with grazing incidence, it can be ensured, depending on the specific application situation, that the heating arrangement is arranged outside of the actual useful beam path.

Each of the in the optical system (such as the projection exposure system from 7 ) heating arrangements used, an individual, via the respective DOE ("124" or "126" in 1 ) generate a predetermined heating profile on the respective optical element or EUV mirror. By switching the heating arrangement having the respective DOE on or off, it is determined whether the respective heating profile assigned to this DOE is set on the optical element or the EUV mirror is correspondingly actively heated or not. In embodiments, several, independently controllable heating arrangements with the basis of 1 be provided in the structure described and assigned to one and the same optical element in order to be able to set a suitable heating profile in the optical element or EUV mirror depending on the currently selected illumination setting.

The optical telescope 130 or 140 according to 1 and the optical telescope 230 according to FIG 2 can be constructed according to the invention as a mirror system. An exemplary embodiment of such a mirror system, which according to in 1 respectively. 2 shown beam path causes the conversion of an incoming collimated beam with a comparatively large beam cross section into an outgoing diverging beam with a comparatively small beam cross section, is in 4 shown.

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

Although the use of at least one mirror within the heating arrangement according to the invention in 1 or 2 If the application example shown preferably takes place on the part of the optical telescope 130, 140 or 230 with regard to the comparatively large values of the maximum irradiation intensity to be expected there, the invention is not restricted to this. In particular, in other applications, one or more mirrors can also be used at a different position within the optics forming the heating unit, for example in the area of the optical collimator 110 or 210, in addition or as an alternative.

This in 3 The exemplary embodiment shown of an optical collimator has two mirrors 311, 312. The mirror 311 has a toric optical active surface with 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 flip angle in source coordinates is 11.97°. The mirror 312 also has a toric optical active surface with 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 flip angle in source coordinates is 41.261°.

This in 4 The exemplary embodiment of a telescope 430 shown has four mirrors 431-434, each with an aspherical optical effective surface, and forms the angular spectrum of the DOE (which, for example, corresponds to the DOE 224 2 corresponds and in 4 not shown, but is arranged on the entry side of the mirror 431) on the mirror to be heated. Such a mirror is in 5 indicated, arranged tilted in the optical beam path and labeled "450". In the exemplary embodiment, the spot size on the mirror to be heated is approximately 30 μm. Only half of the area to be heated (without vignetting) is exposed to radiation. The magnification of the telescope 430 is β= 5, with angles up to 3° being supported.

Tabelle 1: n m C _nm 2 1 -2.32*10^-5 mm^-3 0 3 5. 10*10^-5 mm^-3 4 0 1.31*10^-5 mm^-4 2 2 2.83*10^-6 mm^-4 0 4 1.35*10^-5 mm^-4 The mirror 431 has a toric optical active surface with 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 gives the asphere coefficients up to the 4th order for the mirror 431. The free-form surface formed by the optical effective surface is described here by a fourth-order polynomial according to $$f\left(x,y\right)={\displaystyle \sum _{n=0}^4{\displaystyle \sum _{m=0}^4{c}_{nm}{x}^n{y}^m}}$$

Table 1: n m C _nm 2 1 -2.32*10 ^{-5mm -3} 0 3 5. 10*10 ^{-5mm -3} 4 0 1.31*10 ^{-5mm -4} 2 2 2.83*10 ^{-6mm -4} 0 4 1.35*10 ^{-5mm -4}

The mirror 432 has a toric optical active surface with 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 shows the asphere coefficients up to the 4th order for the mirror 432 analogously to Table 1: Table 2: n m C _nm 2 1 3.51*10 ^{-4mm -3} 0 3 -9.55*10 ^{-5mm -3} 4 0 -3.48*10 ^{-4mm -4} 2 2 -5.03*10 ^{-5mm -4} 0 4 -1.84*10 ^{-5mm -4}

The mirror 433 has a toric optical active surface with 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°. In analogy to Table 1, Table 3 gives the asphere coefficients up to the 4th order for the mirror 433: Table 3: n m C _nm 2 1 1. 36*10 ^{-4mm -3} 0 3 2.60*10 ^{-4mm -3} 4 0 -2 .24*10 ^{-7mm -4} 2 2 1.75*10 ^-5mm -4 0 4 1.23*10 ^{-5mm -4}

The mirror 434 has a toric optical active surface with 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 shows the asphere coefficients up to the 4th order for the mirror 434 analogously to Table 1: Table 4: n m C _nm 2 1 -1.35*10 ^{-3mm -3} 0 3 -3.11*10 ^{-3mm -3} 4 0 3.53*10 ^{-5mm -4} 2 2 -1.33*10 ^{-4mm -4} 0 4 3.67*10 ^{-5mm -4}

the in 3-4 Possible configurations of a mirror system used according to the invention within the heating unit, which are shown as examples, have in common that they do not have a continuous optical axis, which is particularly advantageous with regard to the required minimization of the installation space required in each case—in comparison to a Schwarzschild design with a continuous optical axis that can also be used in principle according to the invention is. Furthermore, the mirror system in question preferably (but also without the invention being limited to this) has at least one aspherically shaped optically active surface, whereby image errors associated with the advantageous renunciation of a rotationally symmetrical structure for reasons of installation space can be effectively corrected.

According to 6 electromagnetic (heating) radiation can also be coupled into an optical element 600 to be heated (here in the form of a concave mirror with an effective optical surface 601) for the purpose of heating this optical element 600 via a first heating unit 610 and a second heating unit 620. In this case, the heating units 610, 620 can each have the previously referred to 1-5 have the structure described. According to 6 a beam of rays generated by the first heating unit 610 strikes that region of the effective optical surface 601 denoted by “A” which is closer to the second heating unit 620 (ie faces the second heating unit 620). On the other hand, the bundle of rays generated by the second heating unit 620 impinges on the area of the effective optical surface 601 labeled “B”, which is closer to the heating unit 610 . In other words, the heating units 610, 620 are arranged relative to the optical element 600 or its optical effective surface 601 in such a way that the beam bundles generated in each case overlap one another or the centroid rays assigned to the respective beam bundles cross one another on their way to the optical effective surface 610, as a result of which the "Compliance" with the angle of incidence range suitable for minimizing or limiting reflection can be achieved over the entire optical effective area.

Although in the previously described embodiments the optical element to be heated is in each case a mirror (designed in particular for operation in the EUV range), the invention is not restricted to this. In further embodiments, the optical element to be heated can also be a mirror designed for other working wavelengths (e.g. for the DUV range, i.e. for wavelengths smaller than 250 nm, in particular smaller than 200 nm) or also a lens.

Although the invention has also been described on the basis of specific embodiments, numerous variations and alternative embodiments will become apparent to the person skilled in the art, for example by combining and/or exchanging features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are intended to be encompassed by the present invention, and the scope of the invention is limited only in terms of the appended claims and their equivalents.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102017207862 A1 [0011]

Claims (17)

Optical system, in particular in a microlithographic projection exposure system • an optical element (450); and • at least one heating unit (100, 200) for heating the optical element (450) by subjecting the optical element (450) to electromagnetic radiation; • wherein the heating unit (100) has at least one mirror (311, 312, 431, 432, 433, 434) with a non-planar optical effective surface. Optical system after claim 1 , characterized in that this mirror (311, 312) is part of an optical collimator (110, 310). Optical system after claim 1 or 2 , characterized in that the heating unit has a telescope (130, 140, 230, 430) constructed exclusively of mirrors. Optical system after claim 3 , characterized in that this telescope (130, 140, 230, 430) has no continuous optical axis. Optical system according to one of the preceding claims, characterized in that the at least one mirror (431, 432, 433, 434) is an aspherical mirror. Optical system according to one of the preceding claims, characterized in that the at least one mirror has an optically effective free-form surface. Optical system according to one of the preceding claims, characterized in that the at least one mirror (311, 312, 431, 432, 433, 434) has a mirror substrate which is a mirror substrate material with a thermal conductivity coefficient of at least 10 Wm ^-1 K ^-1 , in particular at least 50Wm ^-1 K ^-1 , more particularly at least 100Wm ^-1 K ^-1 . Optical system according to one of the preceding claims, characterized in that the at least one mirror (311, 312, 431, 432, 433, 434) is attached to a heat dissipation component made of a material with 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 100Wm ^-1 K ^-1 , is thermally coupled. Optical system according to one of the preceding claims, characterized in that it has at least one cooler for dissipating heat from the at least one mirror (311, 312, 431, 432, 433, 434). Optical system according to one of the preceding claims, characterized in that the at least one mirror (311, 312, 431, 432, 433, 434) has at least one cooling channel to which a cooling fluid can be applied. Optical system according to one of the preceding claims, characterized in that the optical element (450) to be heated is a mirror. Optical system according to one of the preceding claims, characterized in that the optical element (450) to be heated is designed for a working wavelength of less than 30 nm, in particular less than 15 nm. Optical system according to one of the preceding claims, characterized in that this is an optical system of a microlithographic projection exposure system, in particular an illumination device or a projection objective. Microlithographic projection exposure system with an optical system according to one of Claims 1 until 13 . Heating unit for use in an optical system according to any one of Claims 1 until 13 . Method for heating an optical element in an optical system, wherein the optical element (450) via at least one heating unit (100, 200) with a beam of electromagnetic Radiation is applied, characterized in that as a heating unit (100, 200) according to a heating unit claim 15 is used. procedure after Claim 16 , characterized in that the heating of the optical element (450) takes place in such a way that a spatial and/or temporal variation in a temperature distribution in the optical element (450) is reduced.

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