DE102010047832A1 - Optical element e.g. lens of microlithographic projection exposure system used in manufacture of integrated circuit, has excitation light mirror that is arranged to irradiate excitation light with respect to region of optical material - Google Patents

Abstract

The optical element has an optical surface (314) consisting of to-be-cooled optical material, that is formed on the substrate (312). An excitation light mirror (360) is arranged to irradiate excitation light with respect to the region of to-be-cooled optical material. The intermediate regions (365,365A) are formed between the cooled optical material and excitation light mirror.

Description

BACKGROUND AND PRIOR ART

The invention relates to an actively coolable optical element of a lens or other beam guidance system of a microlithography projection exposure apparatus, in particular a lens or a front surface mirror, and to an optical system of a microlithography projection exposure apparatus with such an optical element.

Optical elements, such. Mirrors and lenses are used in lenses and other beam guidance systems to deflect, refocus, focus or defocus light. Among other things, when used within a microlithography projection exposure system, high demands are placed on the quality and optical stability of the optical elements and the optical systems constructed therewith.

In microlithographic projection exposure systems, as used for example in the production of highly integrated electrical circuits, the heating of the optical elements by the useful light is sometimes a non-negligible cause of optical disturbances of the plants. The term "useful light" here refers to that of a primary light source ( Useful light source), which is selectively influenced by the optical elements of an optical system according to the structure of the optical system with regard to an intended purpose (eg imaging or illumination) and guided in a Nutzlichtstrahlengang. In microlithographic applications, the wavelengths of useful light are typically in the range of ultraviolet (UV), deep ultraviolet (DUV), or extreme ultraviolet (EUV), to the limit of the soft X-ray range.

In the case of the usually used optical element materials, heating can lead to a change in volume of the optical elements and thus to a change in shape which can directly change the optical properties of the optical elements. For example, an optical surface of an optical element may be deformed by thermal influences. Thermally induced surface deformations can occur in mirrors and lenses and lead to difficult-to-control disturbances of the useful light beam path.

In addition, usually go with the change in shape associated with mechanical stresses in the material that can affect its refractive index. At the microscopic level, a stronger thermal motion leads directly to a change in the refractive index. These influences change the effect of a lens and can, for. B. make noticeable in imaging systems as aberrations.

If the aberrations with respect to the optical axis of an imaging optical system are rotationally symmetric, compensation is often possible, for. B. by means of a readjustment of individual optical elements of the system.

The situation is more difficult with non-rotationally symmetrical aberrations. These can be caused in projection exposure systems, for example, by the use of slit-shaped image fields. This is for example in the US 6,781,668 B2 have been proposed to symmetrize the local temperature distribution in an optical element and then compensate for remaining rotationally symmetric aberrations in a known per se. For this purpose, a cooling gas flow is directed to the relevant optical element. However, this is not always possible due to space limitations.

In optical systems with one or more mirrors, for example in catadioptric or catoptric projection objectives, it is possible to counteract the heating of the mirrors by active cooling of the mirror back or of the mirror substrate. This is possible, for example, by cooling with cooling liquids which are passed through cooling channels in the mirror substrate (cf. WO 2007 051638 A1 ).

The not previously published German patent application DE 10 2009 028 776.6 The applicant of June 18, 2009 describes the use of the so-called "optical refrigeration"("OpticalRefrigeration") for cooling lenses and / or front surface mirrors, such as. B. be used in microlithography projection exposure equipment. The phenomenon of optical cooling is for example from the article "Optical Refrigeration" by Mansoor Sheik-Bahae and Richard I. Epstein, published in Nature Photonics, VOL 1, December 2007 or the article "Laser cooling of solids" by Mansoor Sheik-Bahae and Richard I. Epstein in: Laser & Photon. Rev. (2008) pages 1-18 known. The DE 10 2009 028 776.6 describes optical elements in the form of lenses or front surface mirrors having a substrate on which at least one optical surface (eg lens surface or mirror surface) is formed. The substrate consists at least in part of the substrate volume of an optically coolable material, which is transparent to excitation light from the infrared range and can be cooled by irradiation with the excitation light. It is exploited that there are materials that are almost monochromatic Using an anti-Stokes fluorescence, convert the excitation light into shorter-wave fluorescent light. The necessary energy is removed from the material, which then cools down. One advantage of the optical cooling is that the cooling takes place without contact, so that no electrical, mechanical or fluidic connections to the cooling optical element have to be made for the cooling. As optically coolable substrate materials, rare earth doped glasses or rare earth doped crystals are addressed, in particular ZBLANP: Yb ³⁺ , ZBLAN: Yb ³⁺ , CNBZn: Yb ³⁺ , BIG: Yb ³⁺ , KGd (WO ₄ ): Yb ³⁺ , KY (WO ₄ ) ₂ : Yb ³⁺ , YAG: Yb ³⁺ , Y ₂ SiO ₅ : Yb ³⁺ , KPb ₂ Cl ₅ : Yb ³⁺ , BaY ₂ F ₈ : Yb ³⁺ , ZBLANP: Tm ^{3 +,} BaY ₂ F _8: Tm ^3+, CNBZn: Er ^3+, or KPb ₂ Cl _5: Er ^3+. The spatial concentration distribution of the doping atoms can be homogeneous or inhomogeneous.

Another approach to limiting thermally-induced perturbations in optical systems is to use materials with extremely low thermal expansion coefficients in the fabrication of the optical elements so that thermal expansion-related perturbations are reduced from the beginning to a tolerable level. It is known, for example, to use glass ceramics as substrate material for mirrors. A suitable for the production of mirror substrates for microlithography systems glass ceramic is sold under the trade name ZERODUR ^® (Schott AG). For these glass-ceramics, thermal expansion coefficients of 0 ± 0.10 × 10 ^-6 K ^-1 are specified for the temperature range between 0 ° C and 50 ° C. Even lower coefficients of thermal expansion are achieved with certain titanium silicate glasses, also known as "Ultra Low Expansion Glass". For such a titanium silicate glass, marketed by Corning, Inc. under the trade designation ULE ^®, an average coefficient of thermal expansion of 0 ± 30 × 10 ^-9 K ^-1 is specified for the temperature range between 5 ° C and 35 ° C.

Out US 6,378,321 and WO 2009/018558 are known components which are cooled due to anti-Stokes fluorescence by irradiation with suitable excitation light.

TASK AND SOLUTION

It is an object of the invention to provide an actively coolable optical element of a lens or other beam guiding system of a microlithography projection exposure apparatus which can be efficiently cooled by means of optical cooling. It is a further object to provide an optical system having at least one such optical element.

This object is achieved by an actively coolable optical element having the features of claim 1 and an optical system having such an optical element having the features of claim 10.

Advantageous developments are specified in the dependent claims. The wording of all claims is incorporated herein by reference.

An optical element according to the claimed invention has a substrate on which at least one optical surface is formed. The substrate consists at least in part of the substrate volume of an optically coolable material which is transparent to excitation light and can be cooled by irradiation with the excitation light. An effective for the excitation light mirroring is arranged with respect to the optically coolable material such that excitation light, which has left the optically coolable material after irradiation, is reflected by the mirroring at least once in the region of the optically coolable material back. Between the optically coolable material and the mirror coating is an intermediate region free from optically coolable material or a distance. As a result, the efficiency of the cooling can be increased in comparison to optical elements with no spacing between mirroring and optically coolable material.

It has been recognized that the exit of the fluorescent light from the optically coolable material plays an important role in the cooling efficiency. The fluorescent light should finally leave the optically coolable material as short as possible, so that the probability of reabsorption in the substrate and thus the probability of heat-generating competition effects, such as the generation of lattice vibrations (phonons), is minimized. Since there is only a slight difference between the wavelengths of the excitation light and the fluorescent light, the mirror coating also has a reflective effect on the fluorescent light and can block the desired exit of the fluorescent light. If an intermediate region without optically coolable material is provided between the optically coolable material and the mirror coating, a reduction in the probability of reabsorption can be achieved.

In some embodiments, the optically coolable material consists of a base material that is transparent to the excitation light, in which dopant atoms from the class of rare earths (SE) are distributed. The intermediate region is formed by the fact that between the doping atoms closest to a mirroring surface and the associated silvering remains a minimum distance without Dotieratome remains.

The size of the minimum distance may vary from application to application. In some embodiments, the reflective coating has an extended reflection surface that has a minimum extension in a direction lying in the reflection surface, wherein the minimum distance of the doping atoms from the mirror coating is at least as large as the minimum dimension. The minimum distance may also be at least twice or at least three times or at least four times as large as the minimum extent. The larger the minimum distance, the smaller is the space angle range blocked by the mirroring, in which no immediate and final fluorescence light decoupling can take place. A maximum distance should not be exceeded so that the desired back reflection effect for the excitation light remains ensured.

In some embodiments, the substrate has a substrate interface facing the reflective coating, and between the substrate interface and the mirror coating is an intermediate region free of substrate material. This can for example be filled with a gas or evacuated, for example in applications in the EUV range. In such embodiments, the doping atoms may extend as far as the edge of the substrate facing the mirroring surface, i. H. reach to the substrate interface, making the entire substrate cross-section accessible for optical cooling.

The mirror coating may, for example, be arranged on an inner side of a mirror ring surrounding the substrate, which has an inner diameter that is larger than an outer diameter of the substrate. The mirror ring may be substantially cylindrically shaped, other shapes adapted to the outer contour of the substrate are possible, for example elliptical shapes.

An intermediate region which is free of optically coolable material can also be created if the mirror coating is applied directly to the substrate interface facing the silvering, for example in the form of a coating of a side surface of the substrate which is reflective for the excitation light. In such cases, it may be provided that the doping atoms distributed in a volume region of the substrate do not reach the mirroring in the radial direction, but that an annular region adjacent to the mirroring, for example, remains free of optically coolable material or free of doping atoms. In the intermediate region there is then no optically coolable material, but a region of the base material which is not provided with doping atoms.

In embodiments for macroscopic optics, z. Example, in optical elements for optical systems of a microlithography projection exposure system, the distance of the doping atoms to the nearest reflective coating (measured perpendicular to the reflection surface) is preferably at least 100 microns, often 500 microns or more and / or 1 mm or more. Corresponding minimum values also apply to the width or the extent of the intermediate region which is free of optically coolable material. By adhering to these distance values, an efficient back reflection of the excitation light is generally possible without excessive blocking of the exit of the fluorescent light.

A combination of several measures which are favorable with regard to the efficiency of the fluorescent light extraction is possible. For example, a mirroring spaced from an outer substrate interface may be provided, and at the same time doping atoms within the base material may still have a radial distance from the substrate interface, so that the solid angle region in which the silvering constitutes a disturbing blockage against the outcoupling of fluorescent light, be kept particularly small can.

The measures presented here for increasing the fluorescence light decoupling can be provided with optical elements of different optical function. An optical element may, for example, be a lens or a front surface mirror.

The measures are also independent of the type of optically coolable material. As substrate materials, all of the substrate materials described in more detail in the present application, in particular those with extremely low thermal expansion coefficients, can be used. For example, the base material for the substrate may be a titanium silicate glass or a glass ceramic. These materials are particularly interesting for mirror substrates.

However, optically coolable substrate materials for lenses or mirrors can also be used with rare-earth-doped glasses or rare-earth-doped crystals, for example the materials ZBLANP: Yb ³⁺ , ZBLAN: Yb ³⁺ , CNBZn: Yb ³⁺ , BIG: Yb ^{3 +} , KGd (WO ₄ ): Yb ³⁺ , KY (WO ₄ ) ₂ : Yb ³⁺ , YAG: Yb ³⁺ , Y ₂ SiO ₅ : Yb ³⁺ , KPb ₂ Cl ₅ : Yb ³⁺ , BaY ₂ F ₈ : Yb ³⁺ , ZBLANP: Tm ³⁺ , BaY ₂ F ₈ : Tm ³⁺ , CNBZn: He ³⁺ , or KPb ₂ Cl ₅ : He ³⁺ , used in the DE 10 2009 028 777.6 are described. The content of this patent application is incorporated herein by reference.

The measures described to increase the efficiency of Fluorescent light decoupling can furthermore be provided independently of how the spatial concentration distribution of the doping atoms is within the base material. The spatial concentration distribution can be homogeneous or inhomogeneous.

The invention also relates to an optical system having at least one optical element according to the claimed invention and at least one device for introducing excitation light into a part of the substrate volume consisting of an optically coolable material.

The foregoing and other features will become apparent from the claims and from the description and drawings. In this case, the individual features may be implemented individually or in the form of subcombinations in one embodiment of the invention and in other areas, and may represent advantageous embodiments. Preferred embodiments will be explained with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

1 shows a schematic for explaining principles of optical cooling;

2 shows in 2A a schematic side view of an optical system having a front surface mirror designed as optical element, and in 2 B the optical element 2A in supervision;

3 shows in 3A a schematic side view of another optical system with a surface mirror designed as an optical element, and in 3B the optical element 3A in supervision;

4 shows in 4A and 4B in the upper circular sub-figures each schematically differently oriented inhomogeneous local distributions of doping atoms in two superimposed separate volume areas and in the sub-figures below schematic concentration diagrams representing the location dependence of the local concentration of the doping atoms in mutually perpendicular directions within the volume ranges;

5 schematically shows the emission of fluorescence photons from doping atoms, which are arranged in the vicinity of a Verspiegelung;

6 schematically shows the emission of fluorescence photons from the in 5 the doping atoms shown, wherein the reflective coating is arranged at a greater distance from the doping atoms and at a distance from the side surface of the substrate; and

7 shows the essential optical components of a microlithography projection exposure apparatus containing optical systems according to this application.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Based on 1 First, some basics of optical cooling are explained. Simplified, the process of optical cooling can be understood as follows: The optically coolable material has energy bands or energy levels E1 and E2> E1, which are separated by a band gap. The material to be cooled is with largely monochromatic excitation light, z. As laser radiation, the frequency ν _L irradiated. The frequency of the excitation light is ideally tuned to the smallest (= reddish) frequency of the energy allowed for the transition between the two energy bands E1 and E2.

If phonons of this frequency are absorbed (corresponding to a photon energy hν _L ), then the upper levels in the energetically lower energy band E1 are emptied and the lowest levels are filled in the energy higher energy band E2. This disturbs the balance of distribution in the bands. This balance is restored by phonon absorption in both bands.

An excited state can, with a certain probability, return to the ground state by a radiating transition with the emission of a fluorescence photon, corresponding to an anti-Stokes emission. The energy hν _{L of} the fluorescence photon (hν _f = hν + kT) is greater than the photon energy hν of the excitation light required for the excitation. Since more energy is emitted in this elementary process than is absorbed, optical cooling results.

However, there is also the possibility of a non-radiative (non-radiative) transition through multi-phonon (MP) relaxation. This MP relaxation is all the more likely the greater the maximum phonon energy of the material compared to the energy gap of the transition, that is, the fewer phonons needed for the transition. A bright relaxation transition can cool the material by the energy of one or more phonons, whereas an MP relaxation transition would heat the material by the full energy of the band gap.

In a real system are the conditions under which effective cooling takes place can, much more complex. Among other things, there is a certain likelihood that fluorescence photons do not escape, but are reabsorbed in the material and that other absorption processes reduce the potential cooling effect. In order to achieve cooling, the proportion (η _abs · η _ext ) of absorbed photons of the excitation light, which escape as a fluorescent photon from the body to be cooled, must be greater than (1-k _B · T / hν _f ), ie η _abs · η _ext > 1 - k _B · T / hν _f

• (1) W_rad ist der Anteil an Fluoreszenzphotonen.
• (2) W_nr ist der Anteil an nicht-radiativen Zerfällen.
• (3) Die Extraktionseffizienz (extraktion efficiency) η_e beschreibt den Anteil der Fluoreszenzphotonen, die dem Körper komplett entweichen können. Dieser Parameter hängt u. a. von der Geometrie und der Reabsorptionswahrscheinlichkeit ab.
• (4) Die Quanteneffizienz (quantum efficiency) W_rad/(W_rad + W_nr) beschreibt den Anteil der angeregten Zustände, die als Fluoreszenzphotonen zerfallen. Dieser Parameter hängt u. a. von der maximalen Phononenenergie des Trägermaterial und der Energie des Fluoreszenzübergangs ab
• (5) Die externe Quanteneffizienz (external quantum efficiency) η_ext = η_eW_rad (η_eW_rad + W_nr) beschreibt den Anteil der Fluoreszenzphotonen, die dem Körper komplett entweichen können, an den insgesamt zerfallenden angeregten Zuständen
• (6) Die Absorptionseffizienz (absoption efficiency) η_abs = α_r/(α_r + α_p) ist das Verhältnis zwischen der resonanten/gewünschten Absorption und der gesamten Absorption. Dieser Parameter hängt vom resonanten Absorptionskoeffizienten α_r und vom parasitären Absorptionskoeffizienten α_p ab.
• (7) Die Kühleffizienz (cooling efficiency) η_cool = P_kühl/P_abs beschreibt das Verhältnis zwischen der Kühlleistung und der absorbierten Leistung des Anregungslichts.

For the understanding of this "cooling condition", the following definitions apply in the context of this application:

• (1) _rad is the proportion of fluorescence photons.
• (2) W _nr is the proportion of non-radiative decays.
• (3) The extraction efficiency η _e describes the proportion of fluorescence photons that can completely escape the body. This parameter depends, among other things, on the geometry and the probability of reabsorption.
• (4) The quantum efficiency _Wrad / (W _rad + W _nr ) describes the proportion of excited states that decay as fluorescence photons. This parameter depends inter alia on the maximum phonon energy of the carrier material and the energy of the fluorescence transition
• (5) External quantum efficiency η _ext = η _e W _rad (η _e W _rad + W _nr ) describes the proportion of fluorescence photons that can completely escape the body from the total decaying excited states
• (6) The absoption efficiency η _abs = α _r / (α _r + α _p ) is the ratio between the resonant / desired absorption and the total absorption. This parameter depends on the resonant absorption coefficient α _r and on the parasitic absorption coefficient α _p .
• (7) The cooling efficiency η _cool = P _cool / P _abs describes the relationship between the cooling power and the absorbed power of the excitation light.

The cooling condition for an ideal case without undesirable effects can then be stated as follows: η _cool = (hv _f - hv _L) / hv _f = λ _L / λ _f - 1, where λ _L and λ _{f are} the wavelengths of excitation photon and fluorescence photon, respectively. The cooling efficiency then depends only on the frequency of the excitation light, ν _L , and on the frequency of the fluorescence, ν _f , and it is cooled for η _cool > 0.

Since, however, (i) not all the excitation light leads to the excitation of the doping atoms, but a part is undesirably absorbed, (ii) not all excited doping atoms decay by fluorescence but also by MP processes and (iii) not all fluorescence photons also to be cooled Leaving the body, the cooling efficiency is reduced η _cool = η _abs · η _ext λ _L / λ _f - 1

Furthermore, since the frequency of the excitation light for effective absorption should not be detuned too far below the mean fluorescence frequency, hν _L hν _f -k _B * T holds. With η _cool > 0 you get the above mentioned cooling condition η _abs · η _ext > 1 - k _B · T / hν _f

With this in mind, advantageous possibilities for the use of optical cooling in the field of optics are explained using the example of optical elements for optical systems of a microlithography projection exposure apparatus.

The optically coolable material is in each case a base material which is transparent to excitation light from the infrared range (IR) and in which doping atoms which can be excited by the excitation light to give an anti-Stokes emission are present in a suitable spatial concentration distribution. Although the doping atoms in the base material are in ionized form and therefore can also be referred to as doping ions, the term "doping atoms" is used here for reasons of clarity. The term "transparent" in this context means that the base material for the excitation light is so permeable that the excitation light can reach the doping atoms with as little attenuation as possible and can excite the doping atoms. By the excitation of the doping atoms a certain absorption of the excitation light is given, so that the optically coolable material is not 100% transparent, but shows a partial absorption at least due to the doping atoms.

2A shows a schematic representation as a side view of an optical system 200 with an optical element acting as a front surface mirror 210 is executed. The below also simply as a mirror 210 designated front surface mirror has a substrate 212 , whose concave curved front surface 214 is formed as an optical surface processed with optical quality. The mirror 210 is intended for an EUV microlithography projection exposure machine and therefore has its front surface 214 a suitable multilayer coating 216 which is optimized for a useful wavelength in the range of 5 to 15 nm. The coating can be constructed, for example, with molybdenum / silicon alternating layers. It can have uniform or locally varying layer thicknesses.

The EUV useful light 230 During operation, it falls on the reflective coating from the side facing away from the substrate and is thereby reflected on the coated front surface. The mirror 210 is exemplified as a concave mirror with positive refractive power. However, it can also be embodied as a convex mirror with negative refractive power or as a plan mirror without refractive power. The optical surface may be designed as a spherical or aspherical rotationally symmetrical surface, but possibly also as a non-rotationally symmetrical (rotationally asymmetric) free-form surface. The diameter of the mirror is adapted to the respective application. In a projection objective for a microlithography projection exposure apparatus, the diameter is typically between 100 mm and 300 mm.

The substrate 212 consists of a substrate material with an extremely low coefficient of thermal expansion, which is less than 0.1 · 10 ^-6 K ^-1 in the region around room temperature (at 20 ° C), the thermal expansion coefficient preferably being at least half an order of magnitude lower, e.g. B. at a maximum of 5 · 10 ^-8 K ^-1 . Such materials are sometimes referred to as ultra-low-expansion materials.

In the exemplary embodiment, the substrate consists essentially of a titanium-silicate glass base material with a high proportion of at least 90% by weight of SiO ₂ and a predominant proportion of titanium oxide (TiO ₂ ) (for example about 7% by weight). ). As base material suitable glass materials are z. B. branded ULE ^® glass (Corning, Inc) available. In this application, the base material of titanium-silicate glass is also referred to as (SiO ₂ -TiO ₂ ) glass. For the temperature range between 5 ° C and 35 ° C, a mean thermal expansion coefficient of 0 ± 30 · 10 ^-9 K ^{-1 is} given.

In a non-illustrated embodiment, the base material of the substrate is a glass-ceramic containing crystalline phase portions distributed in a glass matrix. By combining the thermal characteristics of the different phases extremely low coefficients of thermal expansion can be achieved, which in some temperature ranges can even be zero or slightly negative.

The substrate 212 has a (in the figure by dashed lines limited) volume range 220 on, which consists predominantly of a material which excites with suitable excitation light 245 Anti-Stokes fluorescence shows. This material is formed by doping atoms within the volume range in the base material 225 are distributed, which are excitable by means of the excitation light to an anti-Stokes fluorescence. The glassy base material ((SiO ₂ -TiO ₂ ) glass) is here doped with Ytterbiumionen Yb ³⁺ . The optically coolable material formed thereby is referred to in this application as (SiO ₂ -TiO ₂ ): Yb ³⁺ . The local concentration of dopant atoms (Yb ³⁺ ) is inhomogeneous and decreases from the center of the substrate to the edges in all radial directions.

The optical system 200 includes a device 240 which the excitation light 245 provides. The device 240 comprises a tunable diode-pumped Yb: YAG laser which generates laser light having a wavelength in a narrow sub-range between 950 nm and 1050 nm, in particular at the longer wavelengths. The narrow-band wavelength range is adapted to the wavelength or frequency range in which the optically coolable material (SiO ₂ -TiO ₂ ): Yb ³⁺ shows a fluorescence transition.

The side surface 218 of the substrate is cylindrical, wherein the cylinder axis with the element axis 213 (Symmetry axis) of the mirror coincides. The volume range 220 has the shape of a flat cylinder whose lateral surface lies on the side surface or lateral surface of the mirror substrate. The cylindrical volume area 220 lies completely within the mirror substrate, with areas of the mirror substrate between the volume area 220 and the front surface 214 or between the volume area and the flat rear area 215 , ie areas outside of the with doping atoms 225 doped volume range, not from the doped base material, but consist of the base material without doping atoms. These areas therefore show when irradiated with the excitation light 245 no anti-Stokes fluorescence.

At the outer edge of the volume area 220 is a mirror coating on the side surface of the mirror substrate 260 intended. The mirroring is optimized for the wavelength of the excitation light. The mirror coating may consist of gold or a suitable dielectric multilayer coating, whereby z. B. a reflectivity for the excitation light of more than 99.5% is achieved. The mirroring 260 has an inlet opening 262 on, through which the excitation light 245 can enter the volume region doped with doping atoms.

2 B shows the optical system of 2A in supervision. It becomes clear that the device 240 a focusing device 242 that's the one from the laser 244 emitted excitation light 245 on the entrance opening 262 focused, whereby at the entrance to the cylindrical volume area a predetermined angular expansion of the excitation light 220 is produced. In this case, a continuous angle spectrum between the angle of incidence 0 ° at incidence along the surface normal of the side surface at the inlet opening 262 and generates a maximum angle of incidence. The rays of the excitation light 245 be at the mirroring 260 Reflected back and forth several times until it is from one of the doping atoms 225 or absorbed elsewhere.

The optical system 200 has only a single device 240 for coupling the excitation light 245 on. In order to improve the homogenous illumination of the optically coolable material in the volume region, one or more further devices can be provided along the circumference of the substrate which directs the excitation light into the same volume region over a corresponding number of inlet openings 220 radiate. Some possible arrangements are in the DE 10 2009 028 776.6 the content of which is incorporated herein by reference.

The incident useful light 230 can cause heating of the optical element 210 lead, especially by absorption of the incident useful light in the reflective coating 216 , For cooling, the optical element or provided with optically coolable material volume range 220 with excitation light 245 irradiated. The excitation light excites the doping atoms 225 for anti-Stokes fluorescence, whereby short-wave fluorescent light is generated. The energy gain between the absorbed excitation photon and the re-emitted fluorescence photon can contribute to the cooling of the optical element.

The fluorescence radiation is emitted in all directions. So that the fluorescence radiation does not lead again to the heating of the optical element or other in the useful beam path preceding or subsequent optical elements, it is advantageous if the fluorescent radiation with suitable measures such. B. absorption traps outside the Nutzstrahlengangs is eliminated as much as possible.

Depending on its arrangement in the optical path of an optical system and of the beam characteristics of the useful light used (eg intensity profile), an optical element, such as, for example, an optical element can be used. B. the mirror 210 , be exposed to very uneven thermal loads, which may vary depending on the application time. As a result, z. B. in a mirror, starting from the coated with the reflective coating front surface thermal gradient both in the depth, as well as in the lateral direction (perpendicular to the element axis or axis of symmetry) form. Due to thermal expansion, these can lead to uneven surface deformations, among other things.

For substrate materials with extremely low coefficients of thermal expansion, the absolute values of dimensional changes are usually very small. Since precision requirements can be placed on the surfaces of the optical elements in the sub-nanometer range, in particular in the EUV range, disturbing aberrations due to inhomogeneous heating can still result.

An analysis based on partly available, partly estimated parameters shows that a thermal redistribution is possible for the effective elimination or reduction of thermal gradients with the help of optical cooling. The estimation was made for a cylindrical body of (SiO ₂ -TiO ₂ ) glass with 20 cm radius and 5 cm thickness. These values correspond to typical magnitudes of mirror substrates in EUV systems.

In the case of anti-Stokes emission, the energy of the emitted fluorescence photon is greater than the photon energy of the excitation light necessary for the excitation. This results in a cooling effect. In particular, multi-phonon relaxation, which is more likely the greater the maximum phonon energy of the material compared to the energy gap of the transition, is considered as a detrimental competitive process for cooling.

For (SiO ₂ TiO ₂₎ No information on the maximum phonon energy were available. Therefore, as an approximation, SiO _{2 was} considered as a majority component of the base material. SiO ₂ has a max. Phonon energy of about 1100 cm ^-1 . Furthermore, it was assumed that if the condition was fulfilled: band gap <8 · max. Phonon energy a multi-phonon relaxation has such a high probability that practical no effective cooling can be achieved anymore.

When evaluating the rare earths holmium (Ho), thulium (Tm), erbium (Er) and ytterbium (Yb) or their ions, which are particularly suitable as doping atoms, ytterbium has a suitable doping atom because of its large band gap relative to the maximum phonon energy of the base material exposed. Maybe Thulium or Erbium are still suitable.

In an estimation of influences that can lead to an undesired reabsorption, it was simulated, among other things, which part of the fluorescence photons can not leave the substrate directly or not after a finite number of internal reflections due to total reflection at substrate interfaces. This can be used to estimate the probability of reabsorption. For Reabsorption also plays a role in the purity of the material. This should be as high as possible.

Furthermore, in the case of a mirror, the reflective coating should be designed so thick that virtually no useful light passes through this coating into the substrate material.

An estimate assuming realistic estimates for the above-mentioned parameters W _rad , W _nr , η _e , α _r and α _p gave a cooling power of about 58 mW for 1 W power of the excitation light assuming complete absorption of the excitation light ,

For cooling the entire substrate, extremely high powers of the excitation light would then be required for the optically coolable material (SiO ₂ -TiO ₂ ): Yb ³⁺ . At lower excitation light powers, however, cooling capacities can be achieved which, with the aid of locally different degrees of cooling, can effect an effective redistribution of thermal energy in the optical element. In particular, for the minimization of aberrations that can occur due to temperature and therefore volume or refractive index gradients on lenses or mirrors, an optical cooling can be effective, even if this total, integrated over the entire body, causing almost no cooling effect.

Optionally, cooling efficiency may be enhanced by measures to lower the phonon energy of the base material. For example, the base material can still be mixed with elements which change the local environment of the doping atoms in the sense of a reduction in the phonon energy of the base material (cf. US 7,003,981 B2 ).

A first example of a spatially inhomogeneous optical cooling is in 2 B shown. 2 B shows the optical system 200 in supervision. In the volume area 220 takes the concentration of the doping atoms 225 from the middle in the area of the element axis 213 towards the edge in all radial directions. This largely rotationally symmetrical, radially outwardly sloping doping is due to the radially decreasing density of the doping atoms 225 indicated. This distribution of doping may, for. B. be advantageous if the mirror 210 due to the EUV useful light, especially in the middle, ie around the element axis 213 around, heated more than in the border areas. Since a significant proportion, z. B. about 30% of the EUV useful light in the multilayer coating 216 is absorbed and at the same time the mirror is in the ready-mounted state in high vacuum, a cooling of EUV mirrors is advantageous. With homogeneous irradiation of the excitation light, the cooling effect in the central region is greater because of the higher spatial Dotieratomdichte than in the edge regions, so that any resulting, all sides extending in the radial direction temperature gradients can be at least partially compensated again. Thermal induced aberrations are less pronounced, as they are at least partially compensated by spatially inhomogeneous optical cooling.

Depending on the application and the construction of an optical system, it is possible that certain characteristic defects, for example defects on the surface shape of an optical surface (lens surface or mirror surface), result on an optical element due to heat generation. Such characteristic errors can sometimes, like the aberrations of a wavefront, be described by means of Zernike polynomials or another ideally complete polynomial set describing the viewing optics. During operation of a projection exposure apparatus, temporal changes of thermally induced errors may also result, for example due to changes in the so-called illumination setting.

For example, depending on the application, an optical system may have a more or less disturbing astigmatism, possibly with temporally variable symmetry alignment. With the help of a variable location-dependent effective optical cooling can be counteracted these disorders.

It can be attempted to couple a spatially varying power of the excitation light by suitable control of the angular distribution and / or intensity distribution of the excitation light irradiated by one or more lasers. However, this will usually be relatively expensive and it may not be possible to generate all the useful location-dependent cooling energy distributions. Based on the following figures, the example of the compensation of an astigmatic deformation explains how a compensating location-dependent cooling with the aid of inhomogeneously distributed doping atoms is possible.

3A shows this in a schematic side view designed as a concave mirror front surface mirror 310 , The substrate 312 , whose concave curved front surface 314 prepared in optical quality and with a reflective coating for EUV Nutzlicht 316 is occupied, has a front surface opposite the rear surface 315 and a cylindrical side surface 318 , The substrate may have the same base material as the mirror substrate according to the embodiment 2 , In that regard, reference is made to the description there. It could also be another transparent to infrared light material, for. B. a glass ceramic. Also the optically coolable materials used in the DE 10 2009 028 776.6 may be used.

A special feature is that the doping atoms present in the base material, which are used for the optical cooling, in two (relative to the element axis 313 ) are arranged axially spaced-apart volume regions. A first flat cylindrical volume area 320-1 lies at a relatively small distance from the coated front surface 314 , In the first volume region, first doping atoms are in a very thin, planar layer 325-1 distributed spatially inhomogeneous. A second volume range 320-2 lies a little further away from the front surface and also at a distance from the first volume region 320-1 closer to the back surface 315 , In the second volume range 320-2 are second dopant atoms 325-2 inhomogeneously distributed in a thin, even layer. In the example, the first dopant atoms 325-1 and the spatially separated second doping atoms 325-2 of the same type, e.g. Ytterbium. In other embodiments, dopant atoms of different types may be present in the different volume regions.

A first device 340-1 with an IR laser light source is provided, excitation light in the first volume range 320-1 to radiate in the direction perpendicular to the side surface. Only the first doping atoms can be excited by this excitation light 325-1 but not the second doping atoms 325-2 to be stimulated. A second device 340-2 , which may be identical in construction with the first device, emits its excitation light exclusively in the second volume range 320-2 to order the second doping atoms 325-2 but not first dopant atoms 325-1 to stimulate. By this arrangement, the first doping atoms 335-1 independent of the second dopant atoms 325-2 Excitable and thus usable for optical cooling.

Also in this embodiment is a mirror coating 360 is provided, which is optimized for the wavelength of the excitation light, to reflect back not yet absorbed excitation light again in the volume region to be excited and in this way to increase the probability of exciting a cool-effective anti-Stokes emission. However, the reflective coating is not applied directly to the side surface (lateral surface) of the substrate, but has a radial distance 365 to. For this purpose, a ring mirror is provided as a reflective coating, whose inner radius is significantly larger than the outer radius of the substrate, so that in concentric arrangement of substrate and annular mirror ( 3B ) a radial distance on all sides 365 between the reflective inside of the annular mirror and the side surface of the substrate remains. The ring mirror has two inlet openings 362 at which the beam of the device 340-1 respectively. 340-2 can be coupled.

For example, for a typical application of an EUV mirror, the following typical dimensions may apply: mirror diameter 20 cm; Center height 5 cm; axial distance of the "doping plane" with the first doping atoms to the reflective coating 5 mm; Distance between the doping planes 5 mm. The term "doping plane" does not refer to a mathematical plane, but to a layer-like, planar, voluminous part region which is very flat in comparison to its lateral extent. The doping planes may, for example, be less than 1 mm thick in the vertical direction, so that light emitted in the vertical direction passes through it a maximum of twice and is thus absorbed with negligible probability. The ring mirror or mirror ring may, for example, have a distance of 4 cm from the side surface of the substrate. A single mirror ring can cover both doping regions. However, it can also be provided with two spaced apart superimposed mirror bands, which may be, for example, each 3 mm wide. Although geometrically optically a width in the order of 1 mm would be sufficient, but due to the diffraction, a larger mirror width is advantageous.

Both the first doping atoms and the second doping atoms are not homogeneous in their volume region or their doping plane, but are spatially distributed in an inhomogeneous manner. In this case, the inhomogeneous first spatial distribution of the first doping atoms differs 325-1 from the inhomogeneous second spatial distribution with which the second doping atoms 325-2 in second volume range 320-2 are housed. The inhomogeneous local distributions are designed by way of example for the correction of an astigmatic deformation with a time-variable alignment of the symmetry axis of the astigmatism. For explanation shows 4A schematically in the upper part of the figure, the spatial distribution of the first dopant atoms 325-1 in the first volume range 320-1 and below schematically the concentration profiles for the doping atoms in the x-direction and the y-direction perpendicular thereto, perpendicular to each other and to the axis of symmetry 313 of the mirror. Analog shows 4B in the upper part of the figure, the schematic spatial distribution of the second doping atoms 325-2 in the second volume range 320-2 and below that the concentration profiles of the second doping atoms in the x and y directions.

The first dopant atoms 325-1 (please refer 4A ) have in the middle of the volume range in the region of the axis of symmetry 313 (x = 0, y = 0) one mean concentration C _M. The concentration or spatial density continuously decreases parallel to the x direction towards both edges, down to a minimum value which may be 0% or more. In the y-direction perpendicular thereto, the concentration of the first doping atoms increases from the center in both directions toward the edge, so that the absolutely largest concentrations of the first doping atoms are present along the y-direction in the radially outer regions. The maximum value can be z. B. in the range of several percent, z. B. at about 3%. In a three-dimensional representation, the concentration distribution would appear as a saddle-shaped surface.

The second dopant atoms 325-2 in the second volume range 320-2 ( 4B ) are distributed in the same spatial pattern relative to each other as the first dopant atoms. Again, there is a saddle-shaped astigmatic spatial concentration profile around the center. However, the distributions are oriented 90 ° relative to each other with respect to the symmetry axis. It is therefore the case that the second doping atoms, starting from an average concentration C _M in the middle region parallel to the x-direction, become increasingly denser radially outward, so that the concentration of the doping atoms increases radially outward. In contrast to the y-direction, on the other hand, starting from the mean value C _M , there is a continuous decrease in the concentration towards both radial sides.

In both volume regions, the location-dependent distribution density or concentration of the doping atoms (eg ytterbium ions) can thus be described by a function which has an angular dependence (in the circumferential direction) and a radial dependence within the thin layers each provided with doping atoms. For the upper first dopant atoms let the function be given by A · r ² cos (2θ). For the lower second dopant atoms by A · r ² sin (2θ). Here, r represents the normalized radius, A is the maximum density at radius 1 and θ is the angle in the azimuthal direction. The first and the second spatial distribution thus have with respect to the axis of symmetry 313 a 2-fold rotational symmetry. The first and the second spatial distribution or concentration distribution of doping atoms correspond to two linearly independent astigmatic polynomials.

If now one of these volume regions is largely homogeneously irradiated with excitation light, the result is a spatially inhomogeneous cooling power which depends on the spatial distribution of the doping atoms and which can be referred to as astigmatic cooling distribution. With homogeneous illumination of the first doping atoms 325-1 The areas of maximum cooling power are in the vicinity of the outer edge of the doped region in the y direction. An average cooling capacity results in the center. In the x-direction, on the other hand, the cooling power decreases from the middle to the two edges. If cooling is effected only with the first doping atoms, the effective axis of the cooling geometry would then form, for example, at the angle θ = 0 °, ie along the y direction, if the axis of greater cooling is referred to as the effective axis.

If, in contrast, only the lower, second doping plane is irradiated, the axis of the cooling geometry would be at θ = 90 °. By setting suitable ratios of the intensities of the excitation light for the first and second doping atoms, the effective cooling angle or the orientation of the axis of the cooling geometry between these extreme values can be steplessly about the symmetry axis 313 to be turned around. If, for example, both layers of doping atoms are uniformly illuminated with excitation light with the same intensity, an alignment of the effective axis of the astigmatic cooling geometry occurs at θ = 45 °.

According to the basic principle of the superposition of the inhomogeneous cooling effect of a plurality of groups of excited doping atoms, other characteristic thermally induced deformations can also be corrected. If, for example, the irradiation with useful light on the optical element yields a global temperature gradient that extends across the illuminated area from one side to the other of the element axis, the first spatial distribution and / or the second spatial distribution may, for example, be in a direction perpendicular to the axis of symmetry Have concentration gradient of the doping atoms. In this way, characteristic tilt-type deformations can be compensated. Correspondingly, other uniaxial symmetry errors could also be corrected, for example coma-type errors.

In the following, some measures implemented in embodiments will be explained, which help to improve the efficiency of the optical cooling by increasing the fluorescence light outcoupling. These measures can, regardless of the type of optically coolable material and / or the distribution of doping atoms within this material in all mentioned in this application optically coolable materials, but also in other materials, eg. B. in the DE 10 2009 028 776.6 used materials.

An important role in optical cooling is played by the exit of the fluorescent light from the solid. The excitation light radiated into the substrate should ideally be absorbed to 100% of the substrate material. For this purpose, an effective for the excitation light reflection is provided in the embodiments, with respect to the optically coolable material is arranged such that the excitation light is reflected back before exiting the substrate by the mirror coating at least once in the region of the optically coolable material. This multiple reflection is in 2 B illustrated schematically by way of example. The fluorescent light, on the other hand, should leave the substrate as quickly as possible, ie by the shortest possible route, so that the probability of reabsorption is as low as possible. As described above, reabsorption could not only be cooling for e.g. B. to reduce a Schwingungsquant, but possibly also heat the body by a multiple thereof. A reduction of the reabsorption probability is therefore advantageous.

It has been recognized that the mirroring advantageous in terms of increasing the absorption rate for the excitation light may be disadvantageous in terms of the reabsorption probability. Since the wavelengths of the excitation light and the fluorescent light are very similar to each other, the reflective coating also has a reflective effect on the fluorescent light and can block the desired exit of the fluorescent light.

A significant improvement in terms of a more efficient extraction of fluorescent light is achieved in embodiments by the fact that between the optically coolable material and the mirroring is an intermediate region free of optically coolable material or a distance. In the embodiments, the optically coolable material is made of the excitation light transparent base material doped with rare earth (SE) atoms. The said intermediate region can then be created by leaving a suitable minimum distance without doping atoms between the doping atoms closest to a silvering surface and the associated silvering.

Namely, if the fluorescent light from the related 1 When the anti-Stokes fluorescence processes are described and the light is emitted from the doping atoms (cooling centers) in all spatial directions, then fluorescent light originating from doping atoms closer to the mirror will be less likely to directly strike the material leave, as fluorescent light, which comes from a doping atom, which is located farther away from the mirror coating.

For example, a reflective coating has a flat, one-dimensionally or two-dimensionally curved reflection surface which has a minimum extent in a direction lying in the surface. For example, the reflection surface may have the shape of a flat cylinder jacket whose height corresponds to the minimum extent. In some embodiments, the minimum distance of the doping atoms from the silver coating is at least as great as the minimum extent of the silver coating. The minimum distance can z. B. at least twice as large or at least three times as large or at least four times as large as the minimum extent. A maximum distance should not be exceeded so that the desired back reflection effect for the excitation light remains ensured. The larger the distance or the wider the intermediate area, the smaller the solid angle area blocked by the mirroring, in which no immediate and final fluorescence light decoupling can take place.

Based on 5 and 6 is qualitatively explained why a dopant-free intermediate region between the optically coolable material and a mirror coating in terms of a more efficient fluorescence light decoupling is favorable. 5 shows a section of a radial edge region of a mirror in the region of a doping layer, the cylindrical side surface 518 with a mirroring 560 is provided. The curved front surface 514 and the flat back surface 515 are also shown. Two dopant atoms A, B are in a doped volume region within the substrate 512 near the mirroring 560 , Of the excited doping atoms, fluorescent light rays emanate in all directions, of which some excellent rays are shown. The towards the side surface 518 outgoing beam A1 can leave the mirror substrate and strike the side surface 518 at an angle corresponding to the critical angle of total reflection. Rays that continue towards the front surface 514 run meet at a shallower angle on the side surface and are therefore totally reflected on the side surface. Beam A2 can just leave the solid because it is at the top of the mirroring 560 passes. Beam A3 runs in the direction of mirroring and is reflected there. Also beam A4 is reflected at the mirroring.

For the lower dopant B, excellent rays B1 to B4 are also shown. The rays B1 and B2 hit the mirroring 560 and are therefore reflected back into the substrate. Beam B3 runs at the bottom of the mirror coating and can just leave the substrate. Beam B4 hits the back surface 515 below the critical angle for total reflection and can just leave the substrate. Rays which strike with a flatter angle of incidence are reflected back into the substrate due to total reflection. It can be seen that due to the proximity of the doping atoms for mirroring, a relatively large solid angle range is blocked for the direct fluorescence photon emission.

6 shows the corresponding situation of the two doping atoms A and B, in which case, however, the reflective surface of the mirroring a distance 665 from the side surface 618 Has. The distance is several times greater than the measured parallel to the side surface height of the mirroring. The beams A1 to A4 and B1 to B4 correspond to those in terms of their angular orientation 5 , It can be seen immediately that due to the greater distance between the doping atoms and the mirroring, the solid angle range over which fluorescence photons can escape directly finally has become substantially larger, or the solid angle range which leads to a return reflection into the mirror substrate has become considerably smaller. For example, jet A3, which in the above case ( 5 ) still encounters the mirroring, finally leaving the substrate because it is the lower edge of the mirroring 660 can happen. Also beam A4 can now exit. Rays, at a shallower angle to the lateral surface 618 hit as beam A4, would be totally reflected. Thus, for the rays emitted by the doping atom A, a spatial solid angle range is obtained, which is represented by α in the illustration. For the fluorescence photons emanating from the doping atom B, a space angle range obtained, which is denoted by β, results on the basis of the corresponding considerations. Overall, therefore, in the configuration of 6 With increased distance between doping atoms and mirroring significantly more fluorescence photons the substrate immediately and finally escape, so that the extraction efficiency increases.

In the embodiments, in particular two measures are implemented, which increase the efficiency of the cooling due to the fluorescence decoupling in comparison to embodiments in which the doping atoms come close to directly to the mirroring.

In the embodiment according to 3 is different from the embodiment according to 2 , the mirroring 360 is not applied directly to the side surface of the substrate, but is in the form of a mirror ring or ring mirror before, whose reflective surface for fluorescent light and excitation light inside a radial distance 365 to the cylindrical side surface of the substrate. It is therefore provided a spaced from the material of the substrate mirroring for the excitation light. In this arrangement, an intermediate region free of optically coolable material would be present even if the dopant atoms in the volume regions 320-1 and 320-2 to the radial side surfaces, ie to the lateral surface of the cylindrical substrate, would suffice (see also explanation to 5 and 6 ).

In the embodiment of 2A Another measure is implemented to provide an optically coolable material free intermediate area between this material and the mirroring. In this embodiment, the mirror coating 260 formed by a mirror coating applied to the cylindrical side surface of the substrate. The in the flat-cylindrical volume range 220 distributed doping atoms 225 However, in the radial direction, they do not reach as far as the mirror coating, but in the production of the substrate or during the introduction of the doping atoms, the procedure has been such that an annular outer region of the volume region 220 remains free of doping atoms. In the annular intermediate region whose radial thickness is the distance 265 corresponds, the substrate is not made of optically coolable material, but from the undoped base material of the substrate.

In the embodiment of 3A Both measures are combined, as between the radially outermost dopant and the cylindrical side surface 318 a considerable distance 365A lies and also the mirroring 360 at a distance 365 lies to the side surface.

In order to qualitatively determine the effectiveness of these measures, it was determined by a simulation of the geometric optical beam path, according to which path length l _F in the substrate material a beam of the fluorescence light emitted in any direction by a doping atom, with a predetermined probability, for. B. 95%, the solid has left. It was assumed that an isotropic emission. Simulations were performed on many dopant atoms with different positions relative to the outer surfaces of the substrate (considering total reflection) and relative to a mirror coating present on a side surface to quantify how much inferior dopant atoms near the mirror image the fluorescent light in the Can decouple compared to more distant Dotieratomen. Using data for material properties such as the absorption coefficient for the wavelength of fluorescence, a threshold for the path l _F was determined in each case, according to the example, 95% of the radiation must have left the solids so that a cooling effect is still present. Thus, those positions were calculated for dopant atoms that do not fulfill this condition. At these locations, the solid should then remain free of doping.

In an example simulation, a flat cylinder with a radius of 10 cm and a thickness of 3 cm was assumed as the solid (substrate), whose cylindrical side surface (lateral surface) and a top surface are mirrored. Positions for dopant atoms are in a thin planar layer in the Middle of the cylinder height. The absorption coefficient of the reabsorption of the fluorescent light was assumed to be so small that 100 internal paths or 99 internal reflections (corresponding to a path length of about 7 m) can be tolerated, so that it is still possible to effectively cool. The simulation showed that the cylinder should only be doped up to a cylinder radius of approx. 6.5 cm to avoid unfavorable high reabsorption rates. The radial width of the intermediate region which is free of optically coolable material is thus 35% of the radius of the cylinder in this example case.

The estimation of the extraction coefficient μ _e was supported by simulations. It was similar to a concave mirror 2A with dopant atoms in a flat planar layer spaced from the mirrored front surface. As the base material of the substrate, the above titanium-silicate glass ultra-low-expansion glass was adopted. The following parameters were used in the simulation: mirror diameter 20 cm; Mirror thickness in the middle 5 cm; Radius of curvature of the concave front surface 100 cm; Thickness of the doping layer 2 mm; IR reflectance of the front surface 100%; IR absorption of the front surface 0%; IR reflectivity of the cylindrical side surface (lateral surface) at the edge of the doping layer (corresponds to mirroring) 100%; IR absorption in other areas of the lateral surface 0%; Absorption coefficient of the substrate material for excitation light 0.0005 / mm and refractive index of the substrate material for excitation light 1.47.

The fluorescence of the rare earth dopant atoms was also simulated. Key results can be summarized as follows.

For doping atoms, which are located radially in the center of the doping layer, it was qualitatively shown that most of the radiation (99.995%) can escape the optical element after a maximum of 100 reflections. For dopant atoms, which are closer to the cylindrical side surface, the proportion of fluroszene photons, which can escape, falls sharply towards the lateral surface. While 92.9% of the fluorescence radiation of doping atoms located 3 cm away from the lateral surface escape after a maximum of 100 reflections, only 66.9% of the fluorescence photons are located at doping atoms which are only 5 mm away from the lateral surface ,

These simulated values are given assuming vanishing absorption in the substrate material. If the absorption in the substrate material is taken into account, slight changes occur. For an absorption coefficient of 0.005 cm ^{-1 of} the fluorescent light in the substrate material, the abovementioned values are reduced by the factor 0.995, for the doping atoms from the middle region of the doping layer, for example to 99.49%.

7 shows the essential optical components of a microlithography projection exposure apparatus 700 , The system includes a lighting system 710 and a projection lens 730 and becomes with the radiation of a light source 714 operated. The light source 714 may be, inter alia, a laser plasma source or a discharge source. Such light sources generate radiation 720 in the EUV range, that is to say with wavelengths between 5 nm and 15 nm. In this wavelength range, a lighting system and a projection objective have mainly reflective components.

The of the light source 714 outgoing radiation 720 is done by means of a collector 715 collected and into the lighting system 710 directed. The lighting system here comprises a mixing unit 712 consisting of two faceted mirrors 713 and 714 , a telescope optics 716 and a field-forming mirror 718 , The projection lens 730 serves to create an object field 752 in the object plane 750 on a picture frame 762 in the picture plane 760 Here, it has 6 mirrors M1, M2, M3, M4, M5 and M6.

Individual mirrors of the microlithography projection exposure apparatus, in particular at least one mirror of the illumination system 710 and / or at least one mirror of the projection lens 730 , are embodied as optically cooled optical elements according to embodiments of this disclosure.

Critical components are in particular the mirrors of the projection objective 730 which, due to the heating by the EUV useful light, change their shape only to a small extent, whereby, however, the image of the object in the image plane 760 is disturbed sensitively. In order to minimize heat-induced deformations of the mirrors, all mirrors M1 to M6 have a mirror substrate made of a titanium silicate glass with a thermal expansion coefficient of a few 10 ^-9 K ^-1 (ie few parts per billion per Kelvin). In particular, a material can be used for which a mean thermal expansion coefficient of 0 ± 30 10 ^-9 K ^{-1 is} given for the temperature range between 5 ° C and 35 ° C.

The heating of the mirror of the projection lens 730 depends on the one hand on their order in the beam path. Thus, the integral power of the useful light decreases from mirror to mirror due to absorption in the multi-layer coatings. On the other hand, the heating also depends on the diameter of the mirror. If it is a small mirror, then the integral light output hits a smaller area like a large one Mirror, so that smaller mirrors are heated more. This is the case in particular in the case of the third mirror M3 in the light direction and the fifth mirror M5. Here can counteract a relatively strong large-scale effective cooling.

Furthermore, the heating of the mirrors does not always take place homogeneously over the mirror surface. Thus, in particular, the second mirror M2 of the projection objective in the light direction becomes 730 , which is arranged in the pupil plane, depending on the so-called lighting setting usually have a non-homogeneous illumination. The lighting setting defines the angle spectrum with which an object to be imaged within the object field 752 from the lighting system 710 is illuminated. In other words, the illumination setting describes the local intensity distribution of the illumination of the entrance pupil of the projection objective. This can be, for example, a conventional illumination setting (corresponding to an axis-centered circular illumination area in the entrance pupil), an annular, a dipole, a quadrupole or another multipole illumination setting.

In a dipole illumination setting, the illumination of a pupil plane is characterized by two diametrically opposed intensity maxima lying outside the reference axis of the optical system. Thus, the mirror M2 arranged in the pupil plane is heated by absorption in the multilayer coating mainly in two diametrically opposite regions. This can result in an astigmatic deformation of the mirror surface, in which the orientation of the connecting line of the particularly warm zones depends on the orientation of the dipole. The heat distribution can therefore essentially have a 2-fold rotational symmetry.

The mirror M2 can in principle be similar to 3 be shown constructed and similar as related to 3 and 4 be operated described. The doping atoms are inhomogeneous in each case in two schematically shown, mutually separated, axially successive volume areas 702-1 and 702-2 distributed. Two independently controllable laser sources operate in the devices for emitting excitation light. A control device 745 This is configured to be the first device 740-1 for introducing excitation light into the first volume region 702-1 independent of the second device 740-2 for introducing excitation light into the second volume region 702-2 can be controlled. By setting an appropriate ratio of the power of the excitation light radiated into the volume areas, the orientation of the more cooled zones is adjusted so that it is more strongly cooled in the areas of the illumination poles than in other areas. As a result, temperature gradients in the region of the mirror surface degrade or reduce, so that the inhomogeneous cooling leads to a reduction of thermally induced aberrations.

If a different or differently oriented inhomogeneous heat load on the mirror surface occurs after a change in the illumination setting, the axis of maximum cooling in the substrate can be rotated in the substrate by changing the ratios of the powers of the excitation light in the volume regions and adapted to the new alignment.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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.

Cited patent literature

• US 6781668 B2 [0007]
• WO 2007051638 A1 [0008]
• DE 102009028776 [0009, 0009, 0058, 0076, 0088]
• US 6378321 [0011]
• WO 2009/018558 [0011]
• DE 102009028777 [0026]
• US 7003981 B2 [0071]

Cited non-patent literature

• "Optical Refrigeration" by Mansoor Sheik-Bahae and Richard I. Epstein, published in Nature Photonics, VOL 1, December 2007 [0009]
• "Laser cooling of solids" by Mansoor Sheik-Bahae and Richard I. Epstein in: Laser & Photon. Rev. (2008) pp. 1-18 [0009]

Claims (10)

Optical element of a lens or other beam guidance system of a microlithography projection exposure apparatus with a substrate ( 212 . 312 ), on which at least one optical surface ( 214 . 314 ), wherein the substrate consists of an optically coolable material at least in a part of the substrate volume, which is transparent to excitation light and can be cooled by irradiation with the excitation light, and a mirror coating () which acts on the excitation light ( 260 . 360 ), which is arranged in relation to the optically coolable material in such a way that excitation light is reflected back by the mirroring at least once in the region of the optically coolable material, wherein between the optically coolable material and the mirroring an intermediate area of optically coolable material ( 265 . 365 . 365A ) lies An optical element according to claim 1, wherein the optically coolable material comprises a base material transparent to the excitation light, in which dopant atoms are distributed from the rare earth class, the intermediate region being formed by interposing between the dopant atoms nearest the mirror and the associated dopant Mirroring a minimum distance without Dotieratome remains. An optical element according to claim 2, wherein the reflective coating has an extended reflection surface which has a minimum extension in a direction lying in the reflection surface, wherein the minimum distance of the doping atoms from the reflection is at least as large as the minimum extension. An optical element according to claim 2 or 3, wherein a distance of the doping atoms to the nearest reflective coating, measured perpendicular to the reflection surface of the mirror coating, is at least 1 mm. An optical element according to any one of the preceding claims, wherein the substrate ( 312 ) one of the mirroring ( 360 ) facing substrate interface ( 318 ) and between the substrate interface and the mirroring an intermediate region ( 365 ) lies. An optical element according to claim 5, wherein the mirror coating ( 360 ) on the inside of a substrate ( 312 ) is disposed surrounding the mirror ring having an inner diameter which is larger than an outer diameter of the substrate. An optical element according to claim 5 or 6, wherein the intermediate region ( 365 ) has a width of at least 1 mm. An optical element according to any one of claims 2 to 7, wherein the doping atoms extend as far as the substrate interface of the substrate facing the mirror coating. Optical element according to one of claims 2 or 3, wherein the mirror coating ( 260 ) directly on the mirror surface facing the substrate interface ( 218 ) is applied and distributed in a volume region of the substrate doping atoms ( 225 ) in the radial direction do not reach as close as possible to the silvering, so that an outer region of the base material adjacent to the silvering is free of doping atoms. Optical system of a microlithography projection exposure apparatus with at least one optical element according to one of claims 1 to 9, and with at least one device ( 240 . 340-1 . 340-2 ) for introducing excitation light into a part of the substrate volume consisting of an optically coolable material.

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