EUV driver laser for generating an EUV light-emitting plasma, system for exposing semiconductor substrates coated with a photosensitive coating to EUV light, and method for generating a combined excitation light beam

Abstract

The invention relates to an EUV driver laser (10) for generating a plasma of a target material (14) emitting EUV light (12). The EUV driver laser (10) has a beam source (16), which is designed to generate an excitation light beam (18), a pump source (20), which is designed to supply the beam source (16) with pump energy, in such a way that the excitation light beam (18) is generated without scattering by converting the pump energy, a beam splitter arrangement (22), which is designed to split the excitation light beam (18) into a plurality of partial beams (24.1 -24.5), and an amplifier arrangement (26) having an amplifier unit (28) which has a plurality of optical amplifiers (30.1-30.5) which are each configured to amplify at least one of the partial beams (24.1-24.5) to form an amplified partial beam (32.1-32.5). According to the invention, the EUV driver laser (10) comprises a combination unit (36), which is designed to combine the amplified partial beams (32.1-32.5) to form a combined excitation light beam (34), and a focusing unit (38), which is designed to focus the combined excitation light beam (18) onto the target material (14). Furthermore, according to the invention, the excitation light beam (18) has a wavelength in a range of 1600 nm to 2300 nm.

Description

[0001] EUV driver laser for generating an EUV light-emitting plasma, system for exposing semiconductor substrates coated with a photosensitive coating to EUV light, and method for generating a combined excitation light beam

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to an EUV driver laser for generating an EUV light-emitting plasma of a target material, comprising a) a beam source configured to generate an excitation light beam, b) a pump source configured to supply the beam source with pump energy such that the excitation light beam is generated scatter-free by converting the pump energy, c) a beam splitter arrangement configured to split the excitation light beam into a plurality of partial beams, d) an amplifier arrangement having an amplifier unit comprising a plurality of optical amplifiers, each configured to amplify at least one of the partial beams to form an amplified partial beam,

[0005] Furthermore, the invention relates to a system for exposing semiconductor substrates coated with a photosensitive coating to EUV light.

[0006] The invention further relates to a method for generating a combined excitation light beam for generating an EUV light-emitting plasma of a target material.

[0007] 2. State of the art

[0008] In the manufacture of integrated circuits, so-called microchips, semiconductor substrates (hereinafter "wafers"), which are often monocrystalline, are coated with a coating in a comparatively early manufacturing step, which is then structured in subsequent manufacturing steps. Such structuring acts as a mask in further downstream manufacturing steps, which makes it possible, for example, to etch a functional layer located beneath the coating, to dope it with foreign atoms (e.g., by ion implantation), or to introduce foreign materials into the structuring (e.g., by LIGA and lift-off).

[0009] The goal of these process steps is, among other things, to produce transistors on the wafers, as well as conductive areas that connect the transistors. The patterning can be incorporated into the coating using various methods.

[0010] In photolithography, for example, the coating is implemented as a photosensitive coating, particularly a photoresist. Here, the photoresist is exposed to light of a specific wavelength, to which the photoresist is sensitive, in a specific pattern. This can influence the chemical properties of the exposed areas of the photoresist, such as its solubility in a developer solution. There are photoresists in which the exposed areas polymerize, meaning the solubility of the exposed areas increases compared to the unexposed areas (so-called negative resists). However, there are also photoresists in which the exposed areas become more soluble than the unexposed areas (so-called positive resists).In any case, in order to form the structuring in the photoresist and thus the masking for the functional layer located beneath the photoresist, the more soluble areas of the photoresist are removed using the developer solution.

[0011] In order to introduce the specific pattern into the photoresist, a masking element (hereinafter "photomask") is usually arranged between the light source and the wafer, which, depending on the photoresist used (negative resist or positive resist), is designed as a negative or positive of the structuring to be formed on the photoresist.

[0012] In conventional photolithography, the photomask either lies directly on the photoresist or is positioned just above the photoresist, i.e., at a distance from it, whereby the photomask and the resulting pattern on the photoresist are in a 1:1 ratio. Since photomasks can only be scaled to a finite size, the size of the pattern is essentially limited to a few hundred nanometers. Because smaller patterns enable microchips with significantly higher performance per unit area or volume (since this allows more transistors to be realized in a consistently small area), there is a general effort to reduce the pattern size to physical absolute limits.For this reason, projection exposure systems are generally used today. These systems usually feature a focusing lens system between the photomask and the photoresist, allowing the photomask to be imaged on the photoresist in a drastically reduced size. This allows photomasks to be produced more cost-effectively, as the photomask structures do not need to be as small. It also allows significantly smaller structures to be imaged on the wafers compared to conventional photolithography. The photomask and the resulting patterning on the photoresist can then be present in a ratio of 5:1 or more, for example. The photomask itself can be designed as an absorptive or reflective photomask.

[0013] Since the reduced image of the photomask on the photoresist cannot cover the entire wafer, the wafer is often exposed in prior art processes using a so-called "step-and-repeat" process at a first exposure position, moved a certain distance, and then exposed again at a second exposure position. This is repeated in these known processes until the wafer is largely covered with instances of the same pattern.

[0014] After the areas of the photoresist that are more soluble than the developer solution have been removed and the underlying functional layer has been treated, for example, by etching, the photoresist is often removed from the functional layer by a process called "stripping."

[0015] For multilayer microchips, the projection exposure process described above can be performed up to a hundred times for one and the same microchip.

[0016] EUV light generation system and EUV generation

[0017] In addition to reducing the size of the photomask and using focusing optics, another way to reduce the size of the pattern on the wafers is to use light with a shorter wavelength for exposure.

[0018] In methods known from the prior art, extreme ultraviolet light (EUV light) with a wavelength in a range of approximately 8 to approximately 15 nm is generated for this purpose, with a major portion of the EUV light being at 13.5 nm, which is directed onto the photoresist by means of an EUV projection optics and an EUV focusing optics of the projection exposure system.

[0019] An EUV light generation system used for this purpose essentially comprises an EUV driver laser, by means of which at least one excitation light beam can be generated, a target material generator, and an EUV light generation chamber. Typically, a pulsed high-power laser is used as the EUV driver laser, and a droplet generator, by means of which droplets of a target material can be shot into the EUV light generation chamber, serves as the target material generator. To prevent damage to the system due to particle contamination, a high vacuum under a process gas atmosphere can prevail within the EUV light generation chamber in implementations known from the prior art. Hydrogen (H2) or helium (He) are particularly suitable as the process gas.

[0020] Tin (Sb) is often used as the target material in processes known from the prior art. In addition to tin (Sb), target materials such as xenon (Xe), gold (Au), and / or lithium (Li) are also possible. In order to increase the amount of EUV light emitted, the droplets generated by the droplet generator can, for example, first be excited with a first excitation light beam, the so-called pre-pulse, and then with a second excitation light beam, the so-called main pulse. The two excitation light beams can be generated as two independent light beams, but can also be generated by splitting a single light beam. In principle, the EUV light can be generated using just a single excitation light beam, so a second excitation light beam is not absolutely necessary. However, the use of a second excitation light beam in processes known from the prior art increases the so-calledConversion efficiency, i.e. the ratio of input laser power to generated EUV power, increases significantly.

[0021] Since the conversion efficiency depends on a multitude of factors, for the sake of clarity, we will refer to the overall conversion efficiency below. This clarifies that the net ratio of the power of the excitation light beam to the power of the generated EUV light is addressed here.

[0022] This is because, in these known methods, excitation with the pre-pulse preconditions the target material droplet. Currently, this preconditioning involves the target material droplet developing an approximately disk-like shape due to the energy input from the pre-pulse, thereby comparatively enlarging the surface area of ​​the target material droplet that can be excited by the second excitation light beam. The main pulse then impinges on the preconditioned target material droplet, exciting it. In the known methods, this excitation involves generating an ionized gas of the target material – if the target material is tin, this generates a tin plasma. This tin plasma then emits the EUV light required to form the pattern in the photoresist.In another known method, for example, a third excitation light beam in the form of a so-called rarefaction pulse can be used for further preconditioning. This pulse strikes the target material droplet between the pre-pulse and the main pulse. In this known method, the rarefaction pulse causes a dilution or volume increase of the target material droplet, which has been formed into a disk shape by the pre-pulse, into a target material cloud. The term "cloud" in this case is not synonymous with a gaseous or vapor state of the target material. The use of such a rarefaction pulse further increases the overall conversion efficiency.

[0023] The dilution pulse can be generated by splitting the first excitation light beam into the pre-pulse and the dilution pulse. It is also possible to use a separate beam source for the dilution pulse or even to split a single excitation light beam into the pre-pulse, the dilution pulse, and the main pulse. Furthermore, the dilution pulse can be configured as a section of the main pulse that precedes the main pulse in terms of time and is temporally distinct in terms of the intensity distribution over time. In this case, it is often referred to as a "pedestal."

[0024] EUV driver lasers are also conceivable, in which more than three excitation light beams are used to excite the target material and generate EUV light.

[0025] In each of the above cases, the main pulse hits the target material droplet with a power in the range of 20 kW to 50 kW.

[0026] EUV driver laser

[0027] State-of-the-art EUV driver lasers, sometimes referred to as EUV drive lasers, may have a beam source, a pump source, an amplifier arrangement with a plurality of optical amplifiers, and a focusing unit.

[0028] The beam source can generate the excitation light beam, which is then amplified at a certain point along the optical path by the plurality of optical amplifiers into an amplified excitation light beam.

[0029] If, in the prior art methods, more than one light beam is used to generate the EUV light, individual beam sources, i.e., a pre-pulse beam source, a dilution pulse beam source, and a main pulse beam source, can also be present for each excitation light beam (pre-pulse, dilution pulse, and main pulse). In the known methods, the term "pulse" refers to a light beam that has a comparatively short duration and, accordingly, a start time and an end time, is part of a plurality of temporally successive light beams of the same type, and is generated by a pulsed laser. The duration of the pulses can be in the micro-, nano-, pico-, or femtosecond range. Lasers operated in continuous wave (CW) mode must be distinguished from this.

[0030] If one, two, three or more individual beam sources are used to generate the excitation light beam or the pre-pulse, dilution pulse and / or main pulse, these can generate the excitation light beams with the same or different wavelength and the same or different intensity, mode, beam caustics and / or polarization, or the excitation light beams can impinge on the target material droplet with the same or different wavelength and the same or different intensity, mode, beam caustics and / or polarization.

[0031] For conventional EUV driver lasers, the use of a solid-state laser is known, for example, as a pre-pulse beam source. Such a well-known solid-state laser generates the pre-pulse with a wavelength of approximately 1 pm. A solid-state laser can also be used as a dilution pulse beam source, which, for example, for metrological purposes, preferably has a different wavelength than the pre-pulse, also approximately 1 pm.

[0032] A CO2 laser is often used as the main pulse beam source, which generates the main pulse with a wavelength of about 10.6 pm.

[0033] State-of-the-art problem

[0034] A disadvantage of the EUV driver lasers known from the state of the art is that the power of the excitation light beam(s) can only be increased at particularly high development and cost expenditure due to the complexity of the laser and the special structural measures required for optical amplifiers that use a gas mixture as the active medium, particularly CO2 amplifiers. Increasing the power of the excitation light beam has been a long-held desire, particularly because a comparatively increased power input into the target material droplet results in a comparatively increased power output of generated EUV light. If more EUV light is available, the throughput of structured wafers per unit of time can be increased.Accordingly, there is a desire to move away from conventional methods in which optical amplifiers are used with a gas mixture as the active medium. However, alternatives to the state-of-the-art EUV driver lasers, which generate excitation light beams with a wavelength of 10.6 pm, have simply not been the focus of development to date, since the overall conversion efficiency is comparatively highest in a wavelength range from 10 pm to 11 pm.

[0035] SUMMARY OF THE INVENTION

[0036] The object of the invention is therefore to provide the EUV driver laser described above, which counteracts the above-mentioned disadvantages of the prior art.

[0037] This object is achieved according to the invention by the EUV driver laser mentioned at the outset, in which e) the EUV driver laser comprises: a combination unit which is configured to combine the amplified partial beams into a combined excitation light beam, a focusing unit which is configured to focus the combined excitation light beam onto the target material, f) the excitation light beam has a wavelength in a range from 1600 nm to 2300 nm.

[0038] In an EUV driver laser that generates an excitation light beam with a wavelength in the range from 1600 nm to 2300 nm, the overall conversion efficiency is significantly lower than that of the CCh-EUV driver lasers known from the prior art and explained above due to a wavelength-specific increase in the self-absorption of the generated plasma compared to EUV light. In other words, with the same power input, comparatively less net EUV light can be provided when exciting the target material with light of lower wavelengths for the exposure of wafers, thus also for the range according to the invention. However, with the known CCh-EUV driver lasers, it was therefore necessary to precondition the target material accordingly in order to achieve this comparatively high overall conversion efficiency.

[0039] However, it was recognized according to the invention that the target materials commonly used for these lower wavelengths have a higher absorptivity than the longer wavelengths known from the prior art and that by splitting the excitation light beam into several partial beams, their individual amplification in optical amplifiers and the subsequent combination of the amplified excitation light beams into a combined excitation light beam, the comparatively lower overall conversion efficiency compared to known CCh-EUV driver lasers can be compensated and even far more power can be introduced into the target material by the combined excitation light beam than with previously known EUV driver lasers and corresponding methods, without the target material having to be preconditioned.

[0040] Likewise, it was recognized according to the invention that an excitation light beam with a wavelength in the range of 1600 nm to 2300 nm has a significantly higher overall conversion efficiency than an excitation light beam with a wavelength in a range of, for example, 900 nm to 1100 nm, since in this range the self-absorption of the generated plasma for the generated EUV light is even higher and thus less compensable. Consequently, the invention is based—in other words—on the finding that the wavelength range according to the invention has an optimal overall conversion efficiency and that combining them into a combined excitation light beam is advantageous.

[0041] Advantageously, the excitation light beam has a wavelength in a range of 1800 nm to 2100 nm, in particular 1900 nm to 2050 nm, particularly preferably in a range of 1900 nm to 2000 nm.

[0042] The optical amplifiers are preferably connected in parallel.

[0043] It is particularly advantageous if the EUV driver laser has the immediately following further developing features: a) the optical amplifiers comprise at least one solid-state amplifier, in particular a fiber amplifier, and / or b) at least one optical amplifier is arranged in front of the amplifier arrangement in a propagation direction of the excitation light beam, and / or c) the beam source comprises a solid-state or semiconductor laser.

[0044] If the EUV driver laser includes at least one solid-state optical amplifier, the maximum achievable power for the excitation light beam can theoretically be scaled arbitrarily, since the effort required to add additional optical amplifiers to the amplifier array is comparatively low. Consequently, the power output of the generated EUV light can also theoretically be scaled arbitrarily.

[0045] The term "scatter-free," used initially in relation to the conversion of the pump energy of the pump source to the excitation light beam, means that a major portion of the excitation light beam, when the EUV driver laser is in operation, is generated by stimulated emission. In other words, when the EUV driver laser is in operation, a major portion of the excitation light beam is generated by photons with wavelengths in the range of 1600 nm to 2300 nm being generated in a state of population inversion within the beam source by stimulating excited atoms and / or molecules and / or by spontaneous emission that causes a chain reaction in excited atoms and / or molecules.

[0046] However, the term "scattering-free" does not mean that a major portion of the excitation light beam, when the EUV driver laser is in operation, is generated directly or indirectly by inelastic scattering of light by atoms and / or molecules. In other words, an excitation light beam is not said to be generated "scattering-free" if a major portion of the excitation light beam is generated only or predominantly by Raman scattering when the EUV driver laser is in operation.

[0047] Advantageously, the optical preamplifier comprises a solid-state and / or a semiconductor preamplifier.

[0048] Preferably, the excitation light beam can be generated directly by converting the pump energy. By omitting possible intermediate steps in the generation of the excitation light beam, power losses, for example, due to elastic scattering, can be reduced.

[0049] Furthermore, the pump energy is preferably an optical pump energy or an electrical pump energy. Preferably, the pump energy is an electrical pump energy in the form of an electric field.

[0050] It is advantageous if the at least one solid-state amplifier and / or the at least one optical preamplifier a) is a crystal amplifier or crystal preamplifier having a crystal substrate doped with a first active medium and / or b) is a fiber amplifier or fiber preamplifier having an optical waveguide doped with a second active medium.

[0051] Furthermore, it is advantageous if the first and second active media are identical. If the active media are identical in the Krista II amplifier and the fiber amplifier, it can be ensured that the amplified excitation light beam contains highly monochromatic light and, accordingly, has no or only insignificant beam components with wavelengths that deviate from the excitation light beam.

[0052] It is advantageous if the beam source, the pump source, and / or one, several, or all optical amplifiers or optical preamplifiers are doped with an active medium. It is advantageous if the active medium is the same for the beam source, the pump source, and / or one, several, or all optical amplifiers.

[0053] The active medium preferably comprises holmium (Ho) and / or thulium (Tm). In the case of a first and a second active medium, these active media can also comprise holmium (Ho) and / or thulium (Tm). Pure thulium (Tm) doping is preferred, but mixed doping with thulium (Tm) and holmium (Ho) in different relative concentrations of the dopants in the material to be doped is also conceivable and possible. When excited, thulium (Tm) can emit light with a wavelength in a range of approximately 1600 nm to 2300 nm. The active medium can also comprise other lanthanide materials besides the two mentioned. Depending on which lanthanide materials are comprised in the active medium and their ratio to one another, the emission spectrum of the active medium, the beam quality, and / or the beam profile of the emitted excitation light beam can be adapted.

[0054] Advantageously, the crystal substrate comprises a crystalline material from the following group of crystalline materials: YAG, YAP, YLF, LU2O3, LuAG, LuLF. Preferably, the crystal substrate comprises YLF as the crystalline material. The crystal amplifier or crystal preamplifier can thus be designed as a Tm:YAG, Tm:YAP, Tm:YLF, Tm:LuZOs, Tm:LuAG, Tm:LuLF, Ho:YAG, Ho:YAP, Ho:YLF, Ho:LuZOs, Ho:LuAG, or Ho:LuLF (pre-)amplifier, with Tm:YLF being preferred.

[0055] Advantageously, the optical waveguide comprises at least one amorphous material transparent to the excitation light beam from the following group of amorphous materials: SiO2, Al2O3, MgO, B2O3, CaO, Na2CO3. The fiber amplifier or fiber preamplifier can therefore be designed as a Tm:SiO2, Tn:AlO3, Tm:MgO, Tm:B2O3, Tm:CaO, Tm:Na2CO3, Ho:SiO2, Ho:Al2O3, Ho:MgO, Ho2Os, Ho:CaO, or Ho:Na2CO3 fiber amplifier or fiber preamplifier.

[0056] Alternatively, the beam source is a semiconductor laser having a laser emitter, wherein the laser emitter comprises a semiconductor material or an alloy of this semiconductor material, wherein the semiconductor material is selected from the following group of semiconductor materials: GaAs, GaP, InAs, InP, InGaAsP. The beam source can be a main pulse beam source, the excitation light beam a main pulse, and the pump source a main pulse pump source, wherein the EUV driver laser then further comprises: a) a pre-pulse beam source configured to generate a pre-pulse, b) a pre-pulse pump source configured to supply the pre-pulse beam source with pump energy such that the pre-pulse is generated scatter-free by converting the pump energy, wherein c) the pre-pulse can be directed onto the target material for preconditioning the target material and the main pulse can be directed onto the target material for generating the EUV light-emitting plasma of the target material.

[0057] Furthermore, it is advantageous if the EUV driver laser further comprises: a) a dilution pulse beam source configured to generate a dilution pulse, b) a dilution pulse pump source configured to supply the dilution pulse beam source with pump energy such that the dilution pulse is generated scatter-free by converting the pump energy, wherein c) the dilution pulse can be directed onto the target material between the pre-pulse and the main pulse to dilute the target material.

[0058] Preferably, at least one pulse and / or beam-shaping unit is arranged downstream of the beam source in a propagation direction of the excitation light beam. In particular, a first pulse and / or beam-shaping unit can be arranged upstream of the beam splitter arrangement in the propagation direction. In the propagation direction downstream of the beam splitter arrangement, a second pulse and / or beam-shaping unit can additionally be assigned to at least one, preferably several or all, optical amplifiers. The partial beams can thus be shaped temporally, spectrally, and / or with regard to an intensity profile either before entering or after leaving the amplifier arrangement. The pulse and / or beam-shaping unit can have a phase shift, a Q-switch, an intensity modulation, and / or a polarization module.Additionally or alternatively, the pulse and / or beam-forming unit may comprise a spectral expansion or restriction module by means of which the spectral range from which the excitation light beam and / or the plurality of (amplified) partial beams are composed can be expanded or restricted.

[0059] The partial beams can preferably be combined coherently with each other.

[0060] If multiple amplifier units are present, each amplifier unit can preferably be assigned a first combination unit, which is configured to coherently combine the amplified partial beams then correspondingly assigned to them into a combined excitation light beam or into a combined amplified partial beam. The combined excitation light beam can then also be referred to as the final combination beam. In a propagation direction of the combined amplified partial beams downstream of the combination units, in the case of combined amplified partial beams, at least one second combination unit can be present, which in turn can combine the combined amplified partial beams emerging from the corresponding first combination units into the final combination beam.

[0061] Such splitting of the excitation light beam, parallel amplification, and stepwise combination into a combined excitation light beam or final combination beam is particularly advantageous when using fiber amplifiers or fiber preamplifiers that have certain absolute power limits for certain material combinations. Such a power limit means that only light up to a certain maximum power can pass through the fiber amplifier without irreversibly damaging it or significantly impairing the beam quality.

[0062] The amplification unit can, for example, comprise individual fiber amplifiers, i.e., individual fiber amplifiers doped with an active medium such as thulium (Tm). However, it is also possible for the amplification unit to be designed as a multi-core fiber. Such multi-core fibers can comprise at least two, preferably several, optical fiber channels formed within a common fiber body and each functioning as an optical amplifier, wherein a partial beam can be guided and amplified individually through the optical fiber channels, essentially unaffected by the other optical fiber channels.

[0063] Preferably, the amplified partial beams can be combined with one another by the combination unit in such a way that a) they propagate parallel to one another and next to one another along a propagation direction without any interference, or b) they superimpose one another spatially and temporally in such a way that the amplified partial beams interfere with one another constructively.

[0064] The amplified partial beams can be coherently combined, for example, "side-by-side" (side-by-side combination) in the sense of the immediately preceding feature a). This means that the amplified partial beams then do not overlap spatially or only insignificantly, but propagate parallel to each other and next to each other along a propagation direction.

[0065] Furthermore, in accordance with the immediately preceding feature b), it is also possible for the amplified partial beams to be coherently combined using a so-called filled aperture. A filled aperture is understood here to be an optical arrangement in which the partial beams are superimposed, particularly spatially and temporally.

[0066] Such a filled aperture can be created, for example, by a mirror arrangement comprising multiple mirror zones, wherein the mirror zones have different reflectivities from one another and wherein the reflectivities can range from almost entirely transmissive to almost entirely reflective. Ideally, a first mirror zone has a reflectivity of almost 0% and is, in particular, coated with an anti-reflective coating, and a final mirror zone ideally has a reflectivity that is matched to the power of the amplified partial beam that has already been coherently combined up to this mirror zone. The number of mirror zones can advantageously correspond to the number of amplified partial beams to be combined.Furthermore, the reflectivities of the one or more mirror zones arranged between the first mirror zone and the last mirror zone can then also be adapted to the power of the amplified partial beams already combined up to the corresponding mirror zones and in particular can be smaller than the reflectivity of the last mirror zone.

[0067] Such a filled aperture can also be realized by at least one, preferably several, diffractive optical elements (DOEs), for example, diffraction gratings or diffraction lenses. First diffractive optical elements can diffract the amplified partial beams such that they propagate to at least one second diffractive optical element, through which the amplified partial beams can be diffracted such that they are coherently combined into a single combined excitation light beam or combined amplified partial beam. A combined amplified partial beam can then be another combined amplified partial beam for a further combination.

[0068] When coherently combining the partial beams, it is advantageous to arrange one or more phase modulation units within the beam path of one, several, or all partial beams to compensate for possible propagation time differences between the partial beams by means of phase modulation. This advantageously ensures that the partial beams interfere constructively with each other; destructive interference, which would lead to the mutual cancellation of at least two partial beams, would be disadvantageous in this regard.

[0069] The combined amplified partial beams do not necessarily have to be coherent with each other, but can, for example, have different polarizations with the same or different phases. For this purpose, polarization modulation units can be arranged upstream of the second combination unit in the propagation direction of the combined amplified partial beams.

[0070] In this case too, one or more phase modulation units can be arranged within a beam path of one, several or all combined amplified partial beams in order to adapt the temporal course (phase) of the combined excitation light beams to one another, so that the combined excitation light beam or the final combination beam is the result of constructive interference between the combined amplified partial beams or amplified combination beams.

[0071] The problem mentioned at the outset is solved according to a further aspect of the invention by a system for exposing semiconductor substrates coated with a photosensitive coating to EUV light, characterized in that the system has an EUV driver laser with some or all of the features mentioned above for the EUV driver laser, wherein the pump source, the beam source and the amplifier arrangement are arranged in a first system region and the focusing unit is arranged in a second system region, and wherein a light beam transport system is present which can transport the combined excitation light beam from the amplifier arrangement to the focusing unit.

[0072] System areas can be system levels that are located at different heights of the system and are separated from each other by, for example, a system ceiling or a system floor.

[0073] According to yet another aspect of the invention, the problem mentioned at the outset is also solved by a method for generating a combined excitation light beam for generating an EUV light-emitting plasma of a target material, in which an EUV driver laser with some or all of the features mentioned above for the EUV driver laser is used.

[0074] Figures

[0075] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings:

[0076] Fig. 1 shows a first embodiment of an EUV driver laser with a beam source, a pump source for the beam source, a beam splitter arrangement, an amplifier arrangement, a combination unit and a focusing unit;

[0077] Fig. 2 shows a second embodiment of the EUV driver laser, in which more partial beams can be generated compared to the first embodiment and the amplifier arrangement accordingly comprises several amplifier units, wherein the partial beams can be combined by means of several combination units to form combined amplified partial beams;

[0078] Fig. 3 shows a third embodiment of the EUV driver laser, in which even more partial beams can be generated compared to the second embodiment and the amplifier arrangement accordingly comprises further amplifier units, wherein the partial beams can be combined by means of first combination units to form combined amplified partial beams and the combined amplified partial beams can be combined by means of second combination units to form amplified combination beams;

[0079] Fig. 4a to 4c show a possible embodiment of a combination unit for the “side-by-side” combination of partial beams;

[0080] Fig. 5 to 7 show several possible embodiments of combination units by means of which a "filled aperture" combination of partial beams is possible, wherein Fig. 5 shows a first embodiment and Fig. 6 shows a second embodiment of such a combination.

[0081] The invention is, of course, not limited to the embodiments shown. The embodiments shown in the figures merely represent concrete examples of the invention. Within the scope of their specialist knowledge, it is possible for a person skilled in the art to recognize embodiments of the invention not explicitly shown as such. List of reference symbols

[0082] 10 EUV driver lasers

[0083] 12 EUV light

[0084] 14 Target material

[0085] 16 Beam source

[0086] 18 Excitation light beam

[0087] 20 Pump source

[0088] 22 beam splitter arrangement

[0089] 24 partial beams; 24.1 to 24.5

[0090] 26 amplifier arrangement

[0091] 28 Amplifier unit

[0092] 30 optical amplifiers; 30.1 to 30.5

[0093] 32 amplified partial beams; 32.1 to 32.5

[0094] 34 combined excitation light beam

[0095] 36 combination unit

[0096] 38 Focusing unit

[0097] 40 optical fibers

[0098] 42 Propagation direction

[0099] 44 multi-core fiber

[0100] 46 fiber bodies

[0101] 48 light guide channel

[0102] 50 Pulse and / or beam forming unit

[0103] 52 second combination unit

[0104] 54 combined amplified partial beam; 54.1 to 54.3

[0105] 56 final combination beam

[0106] 58 reflective optical element

[0107] 60 first beam splitter unit

[0108] 62 second beam splitter unit

[0109] 64 third combination unit

[0110] 66 reinforced combination beam

[0111] 67 Side-by-side combination

[0112] 68 filled aperture

[0113] 70 mirror arrangement

[0114] 71a first mirror unit

[0115] 71 b second mirror unit

[0116] 72 mirror zones; 72.1 to 72.5

[0117] 74 diffractive optical element DESCRIPTION OF PREFERRED EMBODIMENTS

[0118] Figs. 1 to 3 each schematically show an embodiment of an EUV driver laser, designated overall by 10. The EUV driver laser 10 is configured to generate a plasma of a target material 14 that emits EUV light 12. The technical literature describes methods in which a target material plasma is generated by applying light power to a target material 14, which in turn generates EUV light 12. The target material plasma is occasionally referred to as LPP (laser-produced plasma).

[0119] The EUV driver laser 10 has at least one beam source 16 configured to generate an excitation light beam 18. In addition, the EUV driver laser 10 has at least one pump source 20 configured to supply the beam source 16 with pump energy. The beam source 16 is supplied with pump energy in the EUV driver laser 10 by generating the excitation light beam 18 in a scatter-free manner by converting the pump energy. In particular, the excitation light beam 18 can be generated directly by converting the pump energy. This means that, for example, no optical parametric oscillator (OPO) is used or so-called high harmonics are generated to generate the excitation light beam 18.

[0120] In addition, the EUV driver laser 10 has a beam splitter arrangement 22 configured to split the excitation light beam 18 into several partial beams 24.1 to 24.5. For this purpose, the beam splitter arrangement 22 may comprise one or more beam splitters (not specifically shown).

[0121] Furthermore, the EUV driver laser 10 has an amplifier arrangement 26 with an amplifier unit 28, which is only shown in Figs. 2 and 3. The amplifier unit 28 has a plurality of optical amplifiers 30.1 to 30.5, connected in parallel in this case, each of which is configured to amplify at least one of the partial beams 24.1 to 24.5 into an amplified partial beam 32.1 to 32.5. In other words, each optical amplifier 30.1 to 30.5 can amplify at least one of the partial beams 24.1 to 24.5 assigned to it into an amplified partial beam 32.1 to 32.5.

[0122] In order to generate a combined excitation light beam 34 from the amplified partial beams 32.1 to 32.5, the EUV driver laser 10 further comprises a combination unit 36 ​​according to the invention. To accomplish this, the combination unit 36 ​​can comprise one or more beam combiners (not specifically shown). Furthermore, according to the invention, the EUV driver laser 10 comprises a focusing unit 38, by means of which the combined excitation light beam 34 can be focused onto the target material 14.

[0123] According to the invention, the excitation light beam 18 has a wavelength in a range from 1600 nm to 2300 nm. Compared to known EUV driver lasers 10, which largely use a CC gas mixture as the active medium and can generate an excitation light beam with a wavelength of approximately 10.6 pm, the so-called overall conversion efficiency, i.e. the efficiency of converting the optical power introduced into the target material by the excitation light beam into EUV power, is significantly lower. However, the EUV driver laser 10 according to the invention can be operated significantly more energy-efficiently. This is due, among other things, to the fact that no radio frequency generators are required to excite the CC gas mixture and no complex gas circulation and renewal circuit is required.

[0124] It was recognized according to the invention that the lower overall conversion efficiency of the EUV driver laser 10 can be compensated for by splitting the excitation light beam 18, amplifying the resulting partial beams 24.1 to 24.5 individually and independently by optical amplifiers 30.1 to 30.5, and combining the resulting amplified partial beams 32.1 to 32.5 into a combined excitation light beam 34 by a combination unit 36. This allows a much higher power of the combined excitation light beam 34 to be achieved than with the EUV driver lasers known from the prior art, exceeding the absolute physical load limits of the individual optical amplifiers 30.1 to 30.5.

[0125] State-of-the-art laser powers ranging from 20 kW to 50 kW are known, which ultimately reach and are introduced into the target material 14. By splitting the excitation light beam 18 and individually and independently amplifying it, theoretically any desired laser power can be achieved, limited only by, for example, the absorption properties of the optics used within the EUV driver laser 10.

[0126] In the present embodiments of Fig. 1 to 3, the optical amplifiers 30.1 to 30.5 are designed as fiber amplifiers and the beam source 16 as a solid-state laser. Accordingly, the pump source 18 in the present embodiments is an optical pump source in the form of a semiconductor laser diode, which can supply the solid-state laser 18 with optical pump energy. In the present case, the solid-state laser 18 is designed, for example, as a Tm:YLF laser. In embodiments not specifically shown, the solid-state laser can also comprise a different dopant:crystal combination. In further embodiments not specifically shown, the beam source 16 is designed as a semiconductor laser in the form of a laser diode, which is then pumped not optically, but electrically by means of an electric field.

[0127] In the present case, the optical amplifiers 30.1 to 30.5 designed as fiber amplifiers each have an optical waveguide 40 in the form of an optical fiber, which can each independently and individually receive one of the partial beams 24.1 to 24.5 and guide it over a corresponding distance along a propagation direction 42.

[0128] Alternatively, the amplifier unit 28, instead of several individual fiber amplifiers

[0129] 30.1 to 30.5, as shown in Figs. 4a and 4b, can also be designed as a multi-core fiber 44, which comprises a fiber body 46, within which light guide channels 48 are formed, each functioning as an optical fiber. In such a case, the light guide channels 48 function not only as light-guiding elements, but also as optical amplifiers 30.1 to 30.5. Such use of multi-core fibers 44 is advantageous, since the structural effort required to combine the partial beams 24.1 to 24.5 into the optical amplifiers

[0130] 30.1 to 30.5 is significantly reduced. Furthermore, this allows for considerable space savings. With appropriate design of the multi-core fibers 44, the partial beams 24.1 to 24.5 can be guided through the multi-core fibers 44 in such a way that they are essentially unaffected by one another to form amplified partial beams.

[0131] 32.1 to 32.5 can be amplified.

[0132] In the present case, each optical waveguide 40 or the multi-core fiber 44 is doped with an active medium by means of which the partial beams 24.1 to 24.5 or possibly also the combined excitation light beam 34 can be optically amplified.

[0133] The active medium can comprise, in particular, thulium (Tm), holmium (Ho), or a combination of these lanthanides. Thulium (Tm) has proven advantageous with regard to beam generation and amplification, which is why the active medium is doped with thulium (Tm) in this case.

[0134] In embodiments not specifically shown, the EUV driver laser 10 comprises a second beam source in addition to the beam source 16, wherein the first beam source is embodied as a main pulse beam source and the second beam source as a pre-pulse beam source. In further embodiments not specifically shown, the EUV driver laser 10 comprises a third beam source embodied as a dilution pulse beam source. Accordingly, these EUV driver lasers 10 then comprise a main pulse pump source, a pre-pulse pump source, and optionally a dilution pulse pump source, wherein the respective pump sources supply the beam sources with pump energy accordingly. The pre-pulse beam source can generate a light beam called a pre-pulse, while the dilution pulse beam source can generate a light beam called a dilution pulse.Both beams can be directed onto the target material 14 in time before the main pulse in order to increase the power output of generated EUV light by preconditioning the target material 14.

[0135] In the embodiments of Figs. 2 and 3, a pulse and / or beam-shaping unit 50 is assigned to each optical amplifier 30.1 to 30.5 in the propagation direction 42 downstream of the beam splitter arrangement 22, specifically in the present case downstream of the combination unit 36 ​​in the propagation direction 42. In addition, the pulse and / or beam-shaping units 50 can also be assigned to the amplifier units 28 or the amplifier arrangement 26 in embodiments not specifically illustrated. It is also possible to provide the pulse and / or beam-shaping units 50 downstream of the beam splitter arrangement 22 and upstream of the amplifier arrangement 26 in the propagation direction 42. The partial beams 24.1 to 24.5 can thus be shaped temporally, spectrally and / or with regard to an intensity profile of the partial beams 24.1 to 24.5 before entering or - as in the present case - after leaving the amplifier arrangement 26.

[0136] The following will now specifically address the embodiment of Fig. 2. In this embodiment, the EUV driver laser 10 has three amplifier units 28, each having five individual optical amplifiers 30.1 to 30.5 designed as fiber amplifiers. In the propagation direction 42 downstream of the amplifier units 28, a second combination unit 52 is arranged for each amplifier unit 28 between the amplifier arrangement 26 and the combination unit 36 ​​for the combined excitation light beam 34, by means of which second combination unit 52 the amplified partial beams 32.1 to 32.5 emerging from the amplifier units 28 can each be combined to form a combined amplified partial beam 54.1 to 54.3. The combined amplified partial beams 54.1 to 54.3 are subsequently combined by means of the combination unit 36 ​​to form the combined excitation light beam 34, which in the case of such step-by-step combination can also be referred to as the final combination beam 56.

[0137] The combination unit 36 ​​has a plurality of reflective optical elements 58 for combining the combined amplified partial beams 54.1 to 54.3.

[0138] The following will now look at the exemplary embodiment of the EUV driver laser 10 according to Fig. 3. In this exemplary embodiment, the beam splitter arrangement 22 has a first beam splitter unit 60 and two second beam splitter units 62, which split the excitation light beam 18, in the present case by way of example, into thirty partial beams 24. Amplifier units 28 are each assigned to the partial beams, wherein the amplifier units can each comprise individual optical amplifiers 30.1 to 30.5 or can each be designed as a multi-core fiber 44 with optical amplifiers 30.1 to 30.5 designed as light guide channels 48, wherein the light guide channels 48 are formed within a common fiber body 46. In the propagation direction 42, after the amplifier arrangement 26 and before the combination unit 36 ​​for the combined excitation light beam 34 orIn the present embodiment, several second combination units 52 are arranged in the final combination beam 56, each of which can combine five partial beams 24.1 to 24.5 into a combined amplified partial beam 54.1 to 54.3. Following this in the propagation direction 42 are two third combination units 64, each of which can combine three combined amplified partial beams 54.1 to 54.3 into amplified combination beams 66.1 and 66.2.

[0139] In the embodiment of Fig. 3, pulse and / or beam forming units 50 are arranged between the second combination units 52 and the third combination units 64.

[0140] As can be seen from Figs. 4a to 7, the amplified partial beams 24.1 to 24.5 can be combined with one another in various ways. As can be seen from Figs. 4a to 4c, the beams can propagate essentially without interference, parallel to one another and side by side along the propagation direction 42 (Fig. 4c). In this case, one can speak of a "side-by-side" combination 67.

[0141] However, it is also possible, as shown in Figs. 5 to 7, to use the amplified partial beams

[0142] 24.1 to 24.5 spatially and temporally in such a way that the amplified partial beams

[0143] 24.1 to 24.5 essentially interfere constructively with each other. In this case, one can speak of a "filled aperture" 68. In the present case, the filled aperture is realized by means of a mirror arrangement 70 (Fig. 6), which has a first mirror unit 71a with several mirror zones 72.1 to 72.5 and a second mirror unit 71b. The mirror zones

[0144] 72.1 to 72.5 have different reflectivities, which, starting from the first mirror zone 72.1 to the last mirror zone 72.5, increase proportionally with the power of the amplified partial beam combined up to the corresponding mirror zone 72.

[0145] However, the filled aperture 68 can also be realized by means of at least one diffractive optical element 74, onto which the partial beams 24 are directed.

Claims

Claims 1. An EUV driver laser (10) for generating a plasma of a target material (14) emitting EUV light (12), comprising: a) a beam source (16) configured to generate an excitation light beam (18); b) a pump source (20) configured to supply the beam source (16) with pump energy such that the excitation light beam (18) is generated scatter-free by converting the pump energy; c) a beam splitter arrangement (22) configured to split the excitation light beam (18) into a plurality of partial beams (24.1-24.5); d) an amplifier arrangement (26) having an amplifier unit (28) comprising a plurality of optical amplifiers (30.1-30.5), each configured to amplify at least one of the partial beams (24.1-24.5) to form an amplified partial beam (32.1-32.5); characterized in that e) the EUV driver laser (10) comprises: a combination unit (36) which is designed to combine the amplified partial beams (32.1 -32.5) to form a combined excitation light beam (34), a focusing unit (38) which is designed to focus the combined excitation light beam (18) onto the target material (14), f) the excitation light beam (18) has a wavelength in a range from 1600 nm to 2300 nm.

2. EUV driver laser (10) according to claim 1, characterized in that a) the optical amplifiers (30.1 - 30.5) comprise at least one solid-state amplifier, in particular a fiber amplifier, and / or b) at least one optical preamplifier is arranged in front of the amplifier arrangement (26) in a propagation direction (42) of the excitation light beam (18), and / or c) the beam source (16) comprises a solid-state or semiconductor laser.

3. EUV driver laser according to claim 2, characterized in that the optical preamplifier comprises a solid-state and / or a semiconductor preamplifier.

4. EUV driver laser according to one of the preceding claims, characterized in that the excitation light beam (18) can be generated directly by converting the pump energy.

5. EUV driver laser according to one of the preceding claims, characterized in that the pump energy is an optical pump energy or an electrical pump energy.

6. EUV driver laser according to one of claims 1 to 3, characterized in that the at least one solid-state amplifier and / or the at least one optical preamplifier a) is a crystal amplifier or crystal preamplifier having a crystal substrate doped with a first active medium and / or b) is a fiber amplifier or fiber preamplifier having an optical waveguide (40) doped with a second active medium.

7. EUV driver laser according to claim 6, characterized in that the first and second active medium are the same.

8. EUV driver laser according to one of the preceding claims, characterized in that the beam source (16), the pump source (20) and / or one, several or all optical amplifiers (30.1 - 30.5) or optical preamplifiers are doped with an active medium.

9. EUV driver laser according to one of the preceding claims, characterized in that the beam source (16) is a main pulse beam source, the excitation light beam (18) is a main pulse and the pump source (20) is a main pulse pump source, wherein the EUV driver laser (10) further comprises: a) a pre-pulse beam source configured to generate a pre-pulse, b) a pre-pulse pump source configured to supply the pre-pulse beam source with pump energy such that the pre-pulse is generated scatter-free by converting the pump energy, wherein c) the pre-pulse for preconditioning the target material (14) and the main pulse (18) for generating the EUV light (12) emitting plasma of the target material (14) can be directed onto the target material (14).

10. EUV driver laser according to one of the preceding claims, characterized in that at least one pulse and / or beam shaping unit (50) is arranged downstream of the beam source (16) in a propagation direction (42) of the excitation light beam (18).

11. EUV driver laser according to one of the preceding claims, characterized in that at least one pulse and / or beam shaping unit (50) is assigned to at least one optical amplifier (30.1 - 30.5) of the amplifier unit (28) in a propagation direction (42) of the excitation light beam (18) downstream of the optical amplifier (30.1 - 30.5).

12. EUV driver laser according to one of the preceding claims, characterized in that the combination unit (36) is configured to coherently combine the partial beams (24.1 - 24.5) with one another.

13. EUV driver laser according to claim 7, characterized in that the amplified partial beams (32.1 - 32.5) can be combined with one another by the combination unit (36) in such a way that a) they propagate essentially without overlap parallel to one another and next to one another along a propagation direction (42), or b) they superimpose one another spatially and temporally in such a way that the amplified partial beams (32.1 - 32.5) constructively interfere with one another.

14. System for exposing semiconductor substrates coated with a photosensitive coating with EUV light (12), characterized in that the system has an EUV driver laser (10) according to one of claims 1 to 13, wherein the pump source (20), the beam source (16) and the amplifier arrangement (26) are arranged in a first system area and the focusing unit (38) is arranged in a second system area and wherein a light beam transport system is present which can transport the combined excitation light beam (18) from the amplifier arrangement (26) to the focusing unit (38).

15. A method for generating a combined excitation light beam (18) for generating a plasma of a target material (14) emitting EUV light (12), characterized in that an EUV driver laser (10) according to one of claims 1 to 13 is used.

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