Laser light source, in particular for use in a microlithography projection exposure system

Abstract

The invention relates to a laser light source, in particular for use in a microlithography projection exposure system. A laser light source has a laser module (105, 305) for generating a plurality of light pulses and an optical pulse stretcher (110, 310) for stretching the pulse length of the light pulses generated by the laser module (105, 305), wherein the optical pulse stretcher (110, 310) has a plurality of mirrors (111, 112, 113, 114, 200, 250, 311, 312, 313, 314), each of which comprises a mirror substrate (210, 250) and a reflective layer system (220, 270), and at least one of the mirrors (250) has an intermediate layer (265) between the mirror substrate (260) and the reflective layer system (270), said intermediate layer forming a diffusion barrier in order to prevent components of the reflective layer system (270) from being diffused to the mirror substrate (260).

Description

[0001] Laser light source, particularly for use in a microlithographic projection exposure system

[0002] The present application claims priority from German patent application DE 10 2023 135 987.8, filed on December 20, 2023. The content of this DE application is incorporated by reference into the present application text.

[0003] BACKGROUND OF THE INVENTION

[0004] Field of the invention

[0005] The invention relates to a laser light source, in particular for use in a microlithographic projection exposure system.

[0006] State of the art

[0007] Microlithography is used to manufacture microstructured electronic components. The microlithography 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 the illumination device is projected by the projection lens onto a substrate (e.g., a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens in order to transfer the mask structure onto the light-sensitive coating of the substrate. In a system designed for operation in the DUV range (e.g.,In projection exposure systems designed for working wavelengths of less than 250 nm, in particular less than 200 nm), laser light sources in the form of excimer lasers are typically used, in particular krypton fluoride excimer lasers at a working wavelength of 248 nm or argon fluoride excimer lasers at a working wavelength of 193 nm.

[0008] The challenges faced in the development of projection exposure systems include, on the one hand, increasing accuracy requirements for the microlithographic production of ever smaller structures and their positioning on the wafer and, on the other hand, a cost-effective increase in the throughput achieved with the respective projection exposure system.

[0009] Regarding the aforementioned increase in throughput achieved with the respective projection exposure system, one problem is that there are limits to the increase in pulse energy and / or repetition rate that is desirable for increasing the output power of the laser light source. Increasing the repetition rate and thus the frequency of electrical discharges within the resonator of the laser light source, which is considered for increasing the output power of the laser light source, can lead to acoustic resonances and ultimately to an increase in the spectral bandwidth beyond an acceptable level. In contrast, increasing the pulse energy leads to higher radiation exposure of optical components within the laser light source and also to greater stress on electronic assemblies.

[0010] A well-known approach is to stretch pulses generated by the laser light source (e.g. 20 ns pulse length) to a longer duration of e.g. (100-450) ns by using optical pulse stretchers, thus reducing the degradation of subsequent optical components accordingly.

[0011] A problem that arises during operation of such an optical pulse stretcher or a corresponding laser light source, particularly when the output power of the laser light source is increased, is the occurrence of undesired wavefront aberrations, which are caused on the one hand by undesired changes in the refractive index of the gas medium near the respective mirror surface of the mirrors heated by thermal absorption when passing through the optical pulse stretcher, and on the other hand by thermally induced deformation or "bulging" of the mirrors themselves. In particular, the heating of purge gas (e.g. nitrogen) located in the area of ​​the respective mirror surface leads to the generation of thermal striae, which disrupts the wavefront of the light passing through the optical pulse stretcher and reflected by the respective mirror.Furthermore, the above-mentioned undesirable change in refractive index upon heating leads to a thermal lens effect and a concomitant defocusing of the light in question.

[0012] For the state of the art, reference is made solely to DE 10 2022 202 241 A1 and DE 10 2022 107 633 B3 as examples.

[0013] SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to provide a laser light source, in particular for use in a microlithographic projection exposure apparatus, which enables an increase in the output power of the laser light source while at least partially avoiding the problems described above.

[0015] This problem is solved according to the features of the independent patent claims.

[0016] According to one aspect of the invention, a laser light source, in particular for use in a microlithographic projection exposure apparatus, comprises

[0017] - a laser module for generating a plurality of light pulses; and

[0018] - an optical pulse stretcher for stretching a pulse length of the light pulses generated by the laser module; - wherein the optical pulse stretcher has a plurality of mirrors, each having a mirror substrate and a reflective layer system; and

[0019] - wherein at least one of these mirrors has an intermediate layer between the mirror substrate and the reflection layer system, which intermediate layer forms a diffusion barrier against diffusion of components of the reflection layer system to the mirror substrate.

[0020] By providing an intermediate layer (according to the invention, e.g., made of Al2O3) between the mirror substrate and the reflective layer system, which intermediate layer forms a diffusion barrier against the diffusion of components of the reflective layer system to the mirror substrate, the selection of the mirror substrate material in the respective mirror can be made without taking into account the materials or components present in the reflective layer system in the specific mirror design. Since the formation of potentially chemically unstable compounds between components of the reflective layer system and components of the mirror substrate is prevented from the outset by the intermediate layer, the risk of potential layer detachments (e.g., due to the undesired formation of gaseous compounds such asSilane) and the associated shortening of the lifetime are avoided, which ultimately opens up the possibility of using mirror substrate materials that were previously unusual in terms of lifetime aspects.

[0021] In addition to the above-described advantageous effect of the intermediate layer from a lifetime perspective, a further advantage is that, from an optical perspective, an undesirable effect of a possibly existing refractive index shift between the respective materials of the mirror substrate (e.g., silicon according to the invention) and the layer of the reflective layer system closest to this mirror substrate (e.g., lanthanum fluoride) can be prevented or at least mitigated. This is based on the consideration that, with a comparatively high refractive index shift, the wavelength-dependent curve of the reflectivity R(λ) may otherwise undesirably limit the usable wavelength range (i.e., the bandwidth).The intermediate layer provided according to the invention and the resulting optical decoupling of the mirror substrate and the reflective layer system in turn create the possibility of making the specific selection of the mirror substrate material without taking into account any refractive index variation between the mirror substrate and the nearest layer of the reflective layer system and thus rather from other aspects - in particular, for example, the optimization of the adhesion properties in the layer structure - which also opens up or expands possibilities for the use of previously unusual mirror substrate materials.

[0022] The invention also involves a deliberate departure from the mirror substrate materials commonly used in optical pulse stretchers of laser light sources. One commonly used mirror substrate material is calcium fluoride (CaF2), which, due to its comparatively large energy band gap and typically high degree of purity, exhibits a fundamentally advantageous low absorption from a lifetime perspective.

[0023] The invention also includes the further concept of designing at least one mirror of the optical pulse stretcher in an optical pulse stretcher of a laser light source with a mirror substrate made of a mirror substrate material which combines a comparatively low thermal expansion coefficient of the mirror substrate material with a comparatively high thermal conductivity.

[0024] The mirror substrate material according to the invention can in particular be silicon (Si) in the form of single-crystalline silicon (Si), polycrystalline silicon (Si) or amorphous silicon (Si).

[0025] The use according to the invention of, for example, silicon (Si) as a mirror substrate material, which is not intuitive in the light of the above background and which, for example, absorbs light reaching the mirror substrate through the reflective layer system to a significant extent at the operating wavelengths in the DUV range (e.g. 193 nm) particularly envisaged according to the invention and which, moreover, differs in its thermal expansion coefficient from the fluoridic materials possibly used in the reflective layer system of the respective mirrors, is now based on the consideration that, due to a significantly improved heat dissipation in the mirror substrate material according to the invention, the disadvantages fundamentally accepted by dispensing with a transparent mirror substrate material can be avoided.

[0026] In other words, the invention accepts that (residual) light which in the respective mirror inevitably reaches the mirror substrate through the reflective layer system is absorbed to a significant extent in the mirror substrate, but the heat generated in the mirror substrate as a result of the conversion of the radiant energy can be dissipated comparatively well, whereby the wavefront aberrations mentioned in the introduction due to both thermal streaking and thermally induced mirror deformations can be at least largely avoided.Furthermore, the invention accepts that the thermal expansion coefficient for the silicon (Si) used according to the invention as the mirror substrate material differs comparatively strongly from the thermal expansion coefficient of the fluoridic materials possibly used in the reflection layer system of the respective mirrors, which fundamentally results in the formation of thermally induced stresses in the coating process.

[0027] The acceptance of the above-mentioned disadvantages according to the invention is based on the consideration that the thermal conductivity of, for example, silicon (Si) with a value of about 140 W*nr 1 *K' 1 is significantly greater than the thermal conductivity of calcium fluoride (CaF2) with a value of about 9.71 W*nr 1 *K' 1 or the thermal conductivity of quartz glass (SiO2) with a value of about 1 .38 W*nr 1 *K' 1. Furthermore, the thermal expansion coefficient of silicon (Si) is approximately 2.7*10 -6 K -1 significantly smaller than the thermal expansion coefficient of calcium fluoride (CaF2) with a value of about 18.85*10 -6 K -1 The use of silicon as a mirror substrate material is also advantageous in terms of its comparatively high mechanical stability and robustness (the elastic modulus of silicon is 97.9 GPa compared to 75.8 GPa for calcium fluoride (CaF2)). These properties (i.e., a comparatively low coefficient of thermal expansion, high mechanical stability, and comparatively high thermal conductivity) are each advantageous with regard to the desired minimization of local thermal warping and mirror heating (as well as the associated streaking and thermal lensing).

[0028] For the above-mentioned quartz glass (SiO2), the thermal expansion coefficient is approximately 0.5*10 -6 K -1 even lower than for the silicon (Si) used as the mirror substrate material according to the invention, but for quartz glass (SiO2) as the mirror substrate material, the mirror temperature is increased due to the above-mentioned comparatively low thermal conductivity relative to the silicon (Si) used in the invention, which in turn has a detrimental effect on the contributions to wavefront aberrations discussed at the beginning, namely thermally induced local mirror deformations and thermal streaking.

[0029] The inventive concept of using, for example, silicon (Si) as a mirror substrate material has the further advantage that, due to the particularly efficient heat dissipation, comparatively complex active cooling of the respective mirror (and the associated cooling fluid connections, cooling fluid channels, etc.) can be dispensed with. Furthermore, the invention also allows for relaxing requirements regarding conventionally used directional cooling via a directed gas flow. As described below, such directional cooling can optionally be completely avoided in embodiments of the invention without any detrimental effects.

[0030] A further advantage is that the inventive concept can be implemented in laser light sources of existing (e.g. lithography) systems without significant adaptations.

[0031] Further advantages of using, for example, silicon (Si) as a mirror substrate material result from its wide availability and the relatively low material costs (compared, for example, to the use of calcium fluoride as a mirror substrate material) as well as from a manufacturing perspective from its comparatively good processability.

[0032] According to one embodiment, the intermediate layer is made of Al2O3 or SiO2.

[0033] According to one embodiment, the material of the intermediate layer is selected from the group comprising Al, Ag, Ba, Ca, Cd, Ce, Co, Cs, Dy, Er, Eu, Fe, Ga, Gd, Hf, Ho, In, Ir, K, La, Li, Lu, Mg, Mn, Na, Nb, Nd, Ni, Os, Pb, Pd, Pr, Rb, Re, Rh, Ru, Sc, Sm, Sn, Sr, Ta, Tb, Ti, Ti, Tm, W, Y, Yb, Zn, Zr and the above materials in oxidized or fluoridated form.

[0034] According to one embodiment, the intermediate layer has a thickness of at least 5 nm, in particular in the range from 5 nm to 50 nm.

[0035] According to one embodiment, the mirror substrate consists essentially of a material from the group containing Si, Ge, C, SiC, BN, AIN, GaN, InN, GaP, InP, AlAs, GaAs, InAs, GaSb, AlF3, BaF2, GaFs, GdFs, InFs, KF, MgF2, ScFs, TaFs, YF3, MgO, Al2O3, Y3Al5O12, GdScOs, YAIO3 and KTaOs.

[0036] According to a further aspect of the disclosure, a laser light source, in particular for use in a microlithographic projection exposure apparatus, comprises

[0037] - a laser module for generating a plurality of light pulses; and

[0038] - an optical pulse stretcher for stretching a pulse length of the light pulses generated by the laser module;

[0039] - wherein the optical pulse stretcher comprises a plurality of mirrors, each having a mirror substrate and a reflection layer system; and

[0040] - wherein the mirror substrate of at least one of these mirrors is made of silicon (Si). According to a further aspect of the disclosure, the mirror substrate of at least two, in particular of at least three, and further in particular of all mirrors of the optical pulse stretcher is each made of silicon (Si).

[0041] According to a further aspect of the invention, a laser light source, in particular for use in a microlithographic projection exposure apparatus, comprises

[0042] - a laser module for generating a plurality of light pulses; and

[0043] - an optical pulse stretcher for stretching a pulse length of the light pulses generated by the laser module;

[0044] - wherein the optical pulse stretcher comprises a plurality of mirrors, each having a mirror substrate and a reflection layer system; and

[0045] - wherein the mirror substrate of at least one of said mirrors is made of a material selected from the group consisting of BaF2, MgF2, MgO, Al2O3 and KTaOa.

[0046] The above-mentioned mirror substrate materials (i.e. BaF2, MgF2, MgO, AI2O3 and KTaOs) are characterized by the fact that an intermediate layer can be omitted to avoid a reaction between components of the reflective layer system on the one hand and the mirror substrate on the other hand, without any risk of layer detachment.

[0047] According to one embodiment, at least some of the mirrors of the optical pulse stretcher are arranged in positions offset from one another with respect to the direction of gravity, wherein the at least one mirror is arranged in the last of these positions with respect to the direction of gravity.

[0048] For a mirror of the optical pulse stretcher located in this position, the design according to the invention with a mirror substrate made of, for example, silicon (Si) is particularly advantageous in that the problem of (purge) gas heating, which is solved according to the invention, is particularly serious for this mirror in view of the gas which may rise and thus reach the remaining optically used area of ​​the optical pulse stretcher.

[0049] In further embodiments, the at least one mirror, manufactured, for example, with silicon (Si) as the substrate material, can also be a concave mirror with a concavely curved optical effective surface facing in the direction of gravity. For a mirror arranged in this way, heated gas in the region of the optical effective surface cannot escape, or can only escape with difficulty, without additional purging, so that the advantageous effect of avoiding such heating achieved according to the invention is particularly effective here as well.

[0050] According to one embodiment, the reflective layer system of the at least one mirror has a multilayer structure composed of a plurality of individual layers, wherein the number of these individual layers is at least twenty, in particular at least thirty, more particularly at least forty, and more particularly at least fifty. These individual layers can be layers of individual layer stacks or functional layers (e.g., in the form of barrier layers or adhesive layers).

[0051] According to one embodiment, the reflection layer system of the at least one mirror has a transmittance of less than 3%, in particular less than 2%, further in particular less than 1%.

[0052] In the above-mentioned embodiments with a comparatively large number of individual layers in the multilayer structure or a comparatively low transmittance of the reflective layer system, the absorbing property of the mirror substrate material used according to the invention is additionally taken into account by reducing the proportion of light passing through the reflective layer system to the mirror substrate. In a further embodiment, in addition to or alternatively to a comparatively large number of individual layers in the multilayer structure, an absorbing layer in the form of a substrate protection layer or a corresponding absorbing layer system can also be provided in the layer structure. According to one embodiment, the reflective layer system of the at least one mirror has a reflectivity of at least 98% over a spectral bandwidth of at least 10 nm.

[0053] According to one embodiment, the at least one mirror has an optical effective surface, wherein at least one surface normal is arranged on this optical effective surface at an angle in the range of 80° to 100°, in particular at an angle in the range of 85° to 95°, further in particular at an angle of 90° to the direction of gravity.

[0054] For such a mirror arranged with its optical effective surface essentially "vertically" (i.e. not "lying"), it applies that a problem solved with conventional directional cooling via a directed gas flow in the case of mirrors arranged essentially "horizontally" or "lying", namely the deposition of particles on the optical effective surface of the mirror, is greatly reduced, so that the inventive dispensation with such directional cooling with directed gas flow can be carried out without any overall losses.

[0055] According to one embodiment, the optical pulse stretcher is arranged such that an angle between the light entry direction into the optical pulse stretcher and the gravitational direction is in the range of 85° to 95°.

[0056] According to one embodiment, the optical pulse stretcher has an output power of more than 1 watt (W), in particular more than 10 watts (W), further in particular more than 40 watts (W).

[0057] At such high output powers of the optical pulse stretcher, the thermal problems described above are particularly relevant, so that the advantages of the inventive design of the optical pulse stretcher, due to the design of one or more mirrors with, for example, silicon (Si) as the mirror substrate material, become particularly apparent. According to one embodiment, the optical pulse stretcher is designed to stretch light pulses generated in the laser light source by the laser module with a pulse length of at least 20 nanoseconds (ns) to a pulse length of at least 100 nanoseconds (ns).

[0058] According to one embodiment, the optical pulse stretcher is designed to stretch light pulses generated in the laser light source by the laser module with a pulse length of at least 20 ns to a pulse length of less than 10,000 ns, in particular less than 1,000 ns, further in particular less than 750 ns.

[0059] According to one embodiment, the laser light source is designed for an operating wavelength of less than 360 nm, in particular for an operating wavelength of less than 250 nm, further in particular for an operating wavelength of less than 200 nm.

[0060] According to one embodiment, the laser light source is designed for an operating wavelength of more than 120 nm, in particular more than 150 nm.

[0061] The invention further relates to a microlithographic projection exposure system, comprising a laser light source, an illumination device and a projection lens, wherein the illumination device illuminates an object plane of the projection lens with light from the laser light source during operation of the projection exposure system and the projection lens images this object plane onto an image plane, wherein the laser light source is designed according to the features described above.

[0062] Further embodiments of the invention can be found in the description and the dependent claims.

[0063] The invention is explained in more detail below with reference to exemplary embodiments illustrated in the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] They show:

[0065] Figure 1 is a schematic representation to explain the possible basic structure of a laser light source according to the invention;

[0066] Figure 2a is a schematic diagram to explain the possible structure of a mirror present in an optical pulse stretcher of a laser light source according to the invention;

[0067] Figure 2b is a schematic representation to explain the possible structure of another mirror present in an optical pulse stretcher of a laser light source according to the invention;

[0068] Figure 3 is a schematic diagram to explain the possible basic structure of a laser light source according to the invention in a further embodiment; and

[0069] Figure 4 is a schematic diagram to explain the possible structure of a microlithographic projection exposure system designed for operation in DUV.

[0070] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0071] Embodiments of the present invention are explained below with reference to the schematic representations of Figs. 1 - 3.

[0072] Fig. 1 initially shows a schematic representation of the possible basic structure of a laser light source 100 according to the invention. The laser light source 100 has a laser module 105, which is only indicated schematically and is designed in a conventional manner, for generating a plurality of light pulses. These light pulses can, merely by way of example (and without the invention being limited thereto), each have a pulse length of the order of 30 nanoseconds (ns) and enter an optical pulse stretcher designated "110". The optical pulse stretcher 110 has a beam splitter 115 and a plurality of mirrors 111 -114, of which only four mirrors are shown in Fig. 1 for the sake of simplicity. The invention is not limited to a specific number of mirrors within the optical pulse stretcher, although this number of mirrors can typically be eight to twelve in embodiments, merely by way of example.The arrow “101” indicates the direction of gravity.

[0073] As a result of the repeated reflection of the light component coupled out by the beam splitter 115 (dashed arrows) at the respective optical effective surfaces of the mirrors 111-114, as indicated in Fig. 1, this coupled out light component experiences a time delay compared to the light component transmitted through the beam splitter 115, before said coupled out light component follows the transmitted light component after reflection at the beam splitter 115. The overall temporal stretching achieved can be carried out in a conventional manner by connecting several stages in series according to this principle, each with a different propagation delay. For the sake of simplicity, only two beams emerging from the optical pulse stretcher 110 are shown in the schematic representation in Fig. 1, although in practice the number of revolutions or partial beams can be considerably higher.

[0074] By way of example only (and without the invention being limited thereto), the stretching of the pulse length achieved in the optical pulse stretcher 110 can be carried out to a pulse length in the range of 100 nanoseconds (ns) to 750 nanoseconds (ns).

[0075] Fig. 2a shows a schematic representation to explain a possible embodiment of a mirror 200 in the optical pulse stretcher 110 of a laser light source 100 according to the invention. The mirror 200 according to the invention according to Fig. 2a has a mirror substrate 210 and a reflection layer system 220, wherein additionally further functional layers (not shown), e.g. in the form of barrier layers or adhesive layers, can be provided in the layer structure of the mirror 200.

[0076] The reflective layer system 220 comprises a plurality of alternating individual layers 221, 222 in a multilayer structure. These individual layers may, for example, comprise a fluoride material in a manner known per se. This may, for example, be aluminum fluoride (AlF3), gadolinium fluoride (GdFs), lanthanum fluoride (LaFs), magnesium fluoride (MgFa), sodium aluminum fluoride (NaAlFa), or ytterbium fluoride (YbFa).

[0077] The number of individual layers is typically (without the invention being limited thereto) at least twenty and, in embodiments of the invention, can preferably be at least thirty, more particularly at least forty, and more particularly at least fifty. These individual layers can be layers of individual layer stacks or functional layers (e.g., in the form of barrier layers or adhesive layers).

[0078] Fig. 2b shows a schematic representation to explain a further embodiment of a mirror in the optical pulse stretcher 110 of a laser light source 100 according to the invention. Compared to Fig. 2a, analogous or essentially functionally identical components are designated by reference numerals increased by "50". The mirror according to Fig. 2b has an intermediate layer 265 between the mirror substrate 260 and the reflective layer system 270, which forms a diffusion barrier against the diffusion of components of the reflective layer system 270 to the mirror substrate 260. The intermediate layer 265 can in particular have a thickness of at least 5 nm, in particular in the range from 5 nm to 50 nm, and can be made of Al2O3 or SiO2.In further embodiments, the material of the intermediate layer is selected from the group comprising Al, Ag, Ba, Ca, Cd, Ce, Co, Cs, Dy, Er, Eu, Fe, Ga, Gd, Hf, Ho, In, Ir, K, La, Li, Lu, Mg, Mn, Na, Nb, Nd, Ni, Os, Pb, Pd, Pr, Rb, Re, Rh, Ru, Sc, Sm, Sn, Sr, Ta, Tb, Ti, Ti, Tm, W, Y, Yb, Zn, Zr and the above materials in oxidized or fluoridated form. The at least one mirror 200 (according to Fig. 2a) or 250 (according to Fig. 2b) provided according to the invention in the optical pulse stretcher of the laser light source 100 is characterized in particular in that a different material is used as the mirror substrate material of the mirror substrate 210, 260 than is conventionally used in an optical pulse stretcher of a laser light source (wherein a conventionally used mirror substrate material is in particular calcium fluoride (CaF2)).

[0079] In addition to other advantages already discussed above, this achieves a significant reduction of wavefront aberrations by reducing the occurrence of the disturbing factors of thermally induced local mirror deformations and thermal streaking, which were also discussed in the introduction.

[0080] Wavefront experiments conducted by the inventors show that the local thermally induced deformation or bulging, for example, for silicon (Si) as a mirror substrate material is significantly reduced when compared to calcium fluoride (CaF2) as a mirror substrate material when irradiated with laser light having an operating wavelength of approximately 193 nm. In these wavefront experiments, the surface of the respective samples was scanned with a 633 nm laser beam, and the wavefront of the light reflected from the respective sample was measured with a wavefront sensor. Despite the comparatively greater total absorption (approximately 0.2%) of the sample made of silicon (Si) compared to the total absorption of the sample made of calcium fluoride (CaF2) (approximately 0.1%), the silicon (Si) sample shows a significantly lower thermally induced deformation or bulging. In the experiment, the (each in arbitrary units,= arbitrary units) PV value (peak-to-valley) of 3.9 for the calcium fluoride (CaF2) sample was reduced to a PV value of 0.43 for the silicon (Si) sample.

[0081] Due to the presence of the intermediate layer 265 in the layer structure of the mirror 250 according to Fig. 2b, the selection of the mirror substrate material for the substrate 260 can be made without regard to the materials or components present in the reflective layer system 270. Due to the diffusion barrier provided by the intermediate layer 265, the formation of potentially chemically unstable compounds between components of the reflective layer system 270 and components of the mirror substrate 260 is prevented from the outset, thus also avoiding potential layer delamination and the associated shortening of the service life.In addition, from an optical point of view, an undesirable effect of a possibly existing refractive index shift between the respective materials of mirror substrate 260 and the layer layer of the reflection layer system 270 that is closest to this mirror substrate 260 can be prevented or at least mitigated.

[0082] According to Fig. 2a, the mirror substrate 210 can be made of a material from the group consisting of BaF2, MgF2, MgO, Al2O3, and KTaOa. The aforementioned mirror substrate materials (i.e., BaF2, MgF2, MgO, Al2O3, and KTaOa) are characterized by the fact that an intermediate layer can be omitted to prevent a reaction between components of the reflective layer system, on the one hand, and the mirror substrate, on the other hand, without creating a risk of layer detachment.

[0083] Fig. 3 shows a schematic representation to explain the possible basic structure of a laser light source according to the invention in a further embodiment, wherein compared to Fig. 1, analogous or essentially functionally identical components are designated by reference numerals increased by "200". According to Fig. 3, the optical pulse stretcher 310 is rotated by 90° compared to Fig. 1, which according to Fig. 3 is achieved merely by way of example using two additional deflecting mirrors 316, 317 in the optical beam path. As a result of this arrangement, the mirrors 311 - 314 in the optical pulse stretcher 310 are arranged with their respective optical effective surface essentially "vertically" (i.e. not "lying"). The result of this is that with conventional directional cooling via a directed gas flow with essentially "horizontal" orThe problem solved by "horizontally" arranged mirrors, namely the deposition of particles on the optical effective surface of the mirror, is greatly reduced, so that the inventive omission of such directional cooling with a directed gas flow can be achieved without any overall detrimental effects. Fig. 4 shows a fundamentally possible structure of a microlithographic projection exposure system 400 designed for operation in the DUV as an application example of a laser light source according to the invention.

[0084] The projection exposure system 400 according to Fig. 4 has an illumination device 410 and a projection lens 420. The illumination device 410 serves to illuminate a structure-bearing mask (reticle) 415 with light from a light source unit 405, which comprises a laser light source, for example in the form of an ArF excimer laser for an operating wavelength of approximately 193 nm (or also in the form of a XeF excimer laser for an operating wavelength of approximately 351 nm, in the form of a KrF excimer laser for an operating wavelength of approximately 248 nm, or in the form of an F2 excimer laser for an operating wavelength of approximately 157 nm), as well as beam-shaping optics generating a parallel light beam.

[0085] The illumination device 410 has an optical unit 411, which, in the example shown, includes a deflection mirror 412. The optical unit 411 can, for example, have a diffractive optical element (DOE) and a zoom axicon system to generate different illumination settings (ie, intensity distributions in a pupil plane of the illumination device 410). In the light propagation direction downstream of the optical unit 411, a light mixing device (not shown) is located in the beam path, which, for example,in a manner known per se, an arrangement of micro-optical elements suitable for achieving light mixing, as well as a lens group 413, behind which is a field plane with a reticle masking system (REMA), which is imaged by a REMA objective 414 following in the direction of light propagation onto the structure-bearing mask (reticle) 415 arranged in a further field plane, thereby delimiting the illuminated area on the reticle. The structure-bearing mask 415 is imaged by the projection objective 420 onto a lens substrate or a wafer 430 provided with a light-sensitive layer (photoresist). The projection objective 420 can be designed in particular for immersion operation, in which case an immersion medium is located in front of the wafer or its light-sensitive layer with respect to the direction of light propagation. Furthermore, it can, for example, have a numerical aperture NA greater than 0.85, in particular greater than 1 .1 .

[0086] Although the invention has been described with reference to specific embodiments, numerous variations and alternative embodiments will become apparent to those skilled in the art, e.g., by combining and / or interchanging features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are encompassed by the present invention, and the scope of the invention is limited only by the appended claims and their equivalents.

Claims

Patent claims 1. Laser light source, in particular for use in a microlithographic projection exposure apparatus, with • a laser module (105, 305) for generating a plurality of light pulses; and • an optical pulse stretcher (110, 310) for stretching a pulse length of the light pulses generated by the laser module (105, 305); • wherein the optical pulse stretcher (110, 310) comprises a plurality of mirrors (111, 112, 113, 114, 200, 250, 311, 312, 313, 314), each having a mirror substrate (210, 260) and a reflection layer system (220, 270); and • wherein at least one of these mirrors (250) has an intermediate layer (265) between the mirror substrate (260) and the reflection layer system (270), which intermediate layer forms a diffusion barrier against diffusion of components of the reflection layer system (270) to the mirror substrate (260).

2. Laser light source according to claim 1, characterized in that the intermediate layer (265) is made of Al2O3 or SiO2.

3. Laser light source according to claim 1, characterized in that the material of the intermediate layer (265) is selected from the group which contains Al, Ag, Ba, Ca, Cd, Ce, Co, Cs, Dy, Er, Eu, Fe, Ga, Gd, Hf, Ho, In, Ir, K, La, Li, Lu, Mg, Mn, Na, Nb, Nd, Ni, Os, Pb, Pd, Pr, Rb, Re, Rh, Ru, Sc, Sm, Sn, Sr, Ta, Tb, Ti, TI, Tm, W, Y, Yb, Zn, Zr and the above materials in oxidized or fluoridated form.

4. Laser light source according to one of claims 1 to 3, characterized in that the intermediate layer (265) has a thickness of at least 5 nm, in particular in the range from 5 nm to 50 nm.

5. Laser light source according to one of the preceding claims, characterized in that the mirror substrate (260) consists essentially of a material from the group which contains Si, Ge, C, SiC, BN, AlN, GaN, InN, GaP, InP, AlAs, GaAs, InAs, GaSb, AlFa, BaFa, GaFa, GdFa, InFa, KF, MgFa, ScFa, TaFs, YF3, MgO, Al2O3, Y3Al5O12, GdScOa, YAO3 and KTaOa.

6. Laser light source, in particular for use in a microlithographic projection exposure apparatus, with • a laser module (105, 305) for generating a plurality of light pulses; and • an optical pulse stretcher (110, 310) for stretching a pulse length of the light pulses generated by the laser module (105, 305); • wherein the optical pulse stretcher (110, 310) comprises a plurality of mirrors (111, 112, 113, 114, 200, 250, 311, 312, 313, 314), each having a mirror substrate (210, 260) and a reflection layer system (220, 270); and • wherein the mirror substrate (210, 260) of at least one of these mirrors (111, 112, 113, 114, 200, 250, 311, 312, 313, 314) is made of a material from the group comprising AlF3, BaFa, GaFa, GdFa, InFa, KF, MgFa, ScFa, TaFs, YF3, MgO, Al2O3 and KTaOa.

7. Laser light source according to one of the preceding claims, characterized in that at least some of the mirrors of the optical pulse stretcher (110, 310) are arranged in positions offset from one another with respect to the direction of gravity, wherein the at least one mirror (111, 112, 113, 114, 200, 250, 311, 312, 313, 314) is arranged in the last of these positions with respect to the direction of gravity.

8. Laser light source according to one of the preceding claims, characterized in that the reflection layer system (220) of the at least one mirror (1 1 1, 1 12, 1 13, 1 14, 200, 250, 31 1, 312, 313, 314) has a multilayer structure comprising a plurality of individual layers (221, 222, 271, 272), wherein the The number of these individual layers (221, 222, 271, 272) is at least twenty, in particular at least thirty, further in particular at least forty, further in particular at least fifty.

9. Laser light source according to one of the preceding claims, characterized in that the reflection layer system (220, 270) of the at least one mirror (111, 112, 113, 114, 200, 250, 311, 312, 313, 314) has a transmittance of less than 3%, in particular less than 2%, further in particular less than 1%.

10. Laser light source according to one of the preceding claims, characterized in that the reflection layer system (220, 270) of the at least one mirror (1 1 1, 1 12, 113, 114, 200, 250, 31 1, 312, 313, 314) has a reflectivity of at least 98% over a spectral bandwidth of at least 10 nm. 1 1. Laser light source according to one of the preceding claims, characterized in that the at least one mirror (31 1 , 312, 313, 314) has an optical active surface, wherein at least one surface normal on this optical active surface is arranged at an angle in the range of 80° to 100°, in particular at an angle in the range of 85° to 95°, further in particular at an angle of 90° to the direction of gravity.

12. Laser light source according to one of the preceding claims, characterized in that the optical pulse stretcher (110, 310) is arranged such that an angle between the light entry direction into the optical pulse stretcher (110) and the gravitational direction is in the range of 85° to 95°.

13. Laser light source according to one of the preceding claims, characterized in that the optical pulse stretcher (110, 310) has an output power of more than 1 watt (W), in particular of more than 10 watts (W), further in particular of more than 40 watts (W).

14. Laser light source according to one of the preceding claims, characterized in that the optical pulse stretcher (110, 310) is designed to stretch light pulses generated in the laser light source (100, 300) by the laser module (105, 305) with a pulse length of at least 20 nanoseconds (ns) to a pulse length of at least 100 nanoseconds (ns).

15. Laser light source according to one of the preceding claims, characterized in that the optical pulse stretcher (110, 310) is designed to stretch light pulses generated in the laser light source (100, 300) by the laser module (105, 305) with a pulse length of at least 20 ns to a pulse length of less than 10,000 ns, in particular less than 1,000 ns, further in particular less than 750 ns.

16. Laser light source according to one of the preceding claims, characterized in that it is designed for an operating wavelength of less than 360 nm, in particular for an operating wavelength of less than 250 nm, further in particular for an operating wavelength of less than 200 nm.

17. Laser light source according to one of the preceding claims, characterized in that it is designed for an operating wavelength of more than 120 nm, in particular more than 150 nm.

18. A microlithographic projection exposure system, comprising a laser light source (100, 300, 405), an illumination device (410) and a projection lens (420), wherein the illumination device (410) illuminates an object plane of the projection lens (420) with light from the laser light source (100, 300, 405) during operation of the projection exposure system (400) and the projection lens (420) images this object plane onto an image plane, characterized in that the laser light source (100, 300, 405) is designed according to one of the preceding claims.

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