EUV driver laser for generating an EUV light-emitting plasma and method for generating an excitation light beam
Patent Information
- Application Number
- PCT/EP2024/056169
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-03-08
- Publication Date
- 2025-10-02
AI Technical Summary
State-of-the-art EUV driver lasers face challenges in increasing the power of excitation light beams due to their complex design and high cost, particularly those using gas mixtures as active media, limiting the scalability and efficiency of EUV light generation.
The EUV driver laser employs solid-state or semiconductor amplifiers with a wavelength range of 1600 nm to 2300 nm, utilizing stimulated emission to generate excitation light beams, which are scalable and less complex, thereby compensating for lower overall conversion efficiency with higher absorptivity of target materials.
This design allows for arbitrarily scalable power output of EUV light generation, reducing manufacturing complexity and costs while maintaining high conversion efficiency, enabling increased throughput of structured wafers.
Smart Images
Figure EP2024056169_02102025_PF_FP_ABST
Abstract
Description
[0001] EUV driver laser for generating an EUV light-emitting plasma and method for generating a driver 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) an amplifier arrangement having a plurality of optical amplifiers configured to amplify the excitation light beam to an amplified excitation light beam such that the amplified excitation light beam can be directed onto the target material to generate EUV light, d) a focusing unit for focusing the amplified excitation light beam onto the target material.
[0005] The invention further relates to a method for generating an excitation light beam for generating an EUV light-emitting plasma of a target material.
[0006] 2. State of the art
[0007] 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).
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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, is used 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 that include xenon (Xe), gold (Au), and / or lithium (Li) are also possible. To increase the amount of emitted EUV light, 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 could be generated using just a single excitation light beam; a second excitation light beam is therefore not absolutely necessary. However, the use of a second excitation light beam increases the so-called conversion efficiency, i.e.the ratio of applied 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 the excitation with the pre-pulse in this known method causes a preconditioning of the target material droplet. Currently, this preconditioning involves the target material droplet developing an approximately disk-like shape due to the input of energy 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 the generation of an ionized gas of the target material – if the target material is a tin material, this creates a tin plasma. This tin plasma then emits the EUV light required to form the pattern in the photoresist.
[0023] 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 increase in volume of the target material droplet, which has been formed into a disc 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.
[0024] 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 distinct from the main pulse in terms of its temporal distribution and intensity distribution over time. In this case, it is often referred to as a "pedestal."
[0025] 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.
[0026] In each of the above cases, the main pulse hits the target material droplet with a power in the range of 20 to 50 kW.
[0027] EUV driver laser
[0028] State-of-the-art EUV driver lasers, sometimes also 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.
[0029] The beam source can generate the excitation light beam, which is then amplified at a specific point along the optical path by the plurality of optical amplifiers into an amplified excitation light beam.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] State-of-the-art problem
[0035] 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 that are required in optical amplifiers that use a gas mixture as the active medium, particularly in CO2 amplifiers. Such lasers are referred to below as CO2 EUV driver lasers. 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 quantitatively, 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, were simply not conceivable until now, since the overall conversion efficiency was comparatively highest in a wavelength range from 10 pm to 11 pm.
[0036] SUMMARY OF THE INVENTION
[0037] The object of the present invention is therefore to provide the EUV driver laser described above, which addresses the above-mentioned disadvantages of the prior art.
[0038] This object is achieved according to the invention by the EUV driver laser mentioned at the outset, in which e) the optical amplifiers comprise at least a first and a second solid-state or semiconductor amplifier, f) the excitation light beam has a wavelength in a range from 1600 nm to 2300 nm.
[0039] 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.
[0040] 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 using at least a first and a second solid-state or semiconductor amplifier, the comparatively lower overall conversion efficiency compared to known CO2 EUV driver lasers can be compensated for without the target material having to be preconditioned.
[0041] It was also discovered 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 shorter 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 easily compensated. 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.
[0042] 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.
[0043] The amplifier arrangement preferably comprises a plurality of solid-state or semiconductor amplifiers. The EUV driver laser can thus theoretically be scaled arbitrarily with respect to the maximum achievable power for the excitation light beam, since the effort required to provide additional optical amplifiers in the amplifier arrangement is comparatively low. Consequently, the power output of generated EUV light can also theoretically be scaled arbitrarily.
[0044] 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, molecules, and / or ions embedded in solids, and / or by spontaneous emission that induces a chain reaction in excited atoms and / or molecules.
[0045] 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.
[0046] It is advantageous if the beam source is a solid-state or semiconductor laser. Compared to gas lasers, especially CC lasers, solid-state or semiconductor lasers are significantly simpler and cheaper to manufacture, since (in the case of gas lasers), for example, there is no need for gas cooling, gas exchange, a gas-tight system, or possibly vacuum pumps. In the field of EUV light generation, solid-state or semiconductor lasers have so far been used exclusively for generating a pre-pulse and / or a dilution pulse, particularly with a wavelength in the range of 900 nm to 1100 nm, since the plasmas generated by them exhibit a comparatively high self-absorption with respect to the emitted EUV light, and thus a satisfactory power output of emitted EUV light from the target material was not possible.
[0047] 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.
[0048] Further preferably, the pump energy is an optical pump energy or an electrical pump energy.
[0049] Advantageously, the first solid-state or semiconductor amplifier is a crystal amplifier having a crystal substrate doped with a first active medium. It is further advantageous if the second solid-state or semiconductor amplifier is a fiber amplifier having an optical waveguide doped with a second active medium.
[0050] 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.
[0051] Preferably, the first and / or second active medium comprises holmium (Ho) and / or thulium (Tm). More preferably, the first and / or second active medium is thulium. The active medium may also comprise other lanthanide materials.
[0052] Depending on which lanthanide materials are included in the active medium and in what ratio they are present to each other, an emission spectrum of the active medium can be adjusted.
[0053] 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 can thus be designed as a Tm:YAG, Tm:YAP, Tm:YLF, Tm:Lu2O3, Tm:LuAG, Tm:LuLF, Ho:YAG, Ho:YAP, Ho:YLF, HO:LU2O3, Ho:LuAG, or Ho:LuLF amplifier, with Tm:YLF being preferred.
[0054] 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 amplifier comprising the optical waveguide can thus be designed as a fiber amplifier. The fiber amplifier can therefore be designed as a Tm:SiO2, TnrrAlO3, Tm:MgO, TnrrB2O3, Tm:CaO, TnrrNa2CO3, Ho:SiO2, Ho:Al2O3, Ho:MgO, HO:B2O3, Ho:CaO, or Ho:Na2CO3 fiber amplifier.
[0055] Preferably, the beam source is a semiconductor laser having a laser emitter, wherein the laser emitter comprises a semiconductor material, wherein the semiconductor material is selected from the following group of semiconductor materials: GaAs, GaP, InAs, InP, InGaAsP.
[0056] Advantageously, the beam source is a main pulse beam source, the excitation light beam is a main pulse, and the pump source is a main pulse pump source, wherein the EUV driver laser 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 and the main pulse for generating the EUV light-emitting plasma of the target material can be directed onto 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. The excitation light beam can thus be shaped temporally, spectrally, and / or with respect to an intensity profile of the excitation light beam before entering the amplifier arrangement. The pulse and / or beam-shaping unit can comprise a phase shift, a Q-switch, an intensity profile modulation, and / or a polarization modulation module. Additionally or alternatively, the pulse and / or beam-shaping unit can comprise a spectral expansion or restriction module.
[0059] Preferably, at least one pulse and / or beam-shaping unit is assigned to at least one optical amplifier of the amplifier arrangement downstream of the optical amplifier in a propagation direction of the excitation light beam. The amplified excitation light beam can thus be shaped temporally, spectrally, and / or with respect to an intensity profile of the amplified excitation light beam after leaving the optical amplifier. The pulse and / or beam-shaping unit can comprise a phase shift, a Q-switch, an intensity profile modulation, and / or a polarization modulation module. Additionally or alternatively, the pulse and / or beam-shaping unit can comprise a spectral expansion or restriction module.
[0060] Advantageously, each optical amplifier of the amplifier arrangement is assigned a pulse and / or beam-forming unit. Furthermore, each optical amplifier of the amplifier arrangement is preferably a solid-state or semiconductor amplifier.
[0061] According to a further aspect of the invention, the problem mentioned at the outset is solved by a system for exposing semiconductor substrates coated with a photosensitive coating to EUV light, wherein the system comprises 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 on a first system level and the focusing unit is arranged on a second system level, and wherein a light beam transport system is provided which can transport the excitation light beam from the amplifier arrangement to the focusing unit.
[0062] According to yet another aspect of the invention, the problem mentioned at the outset is also solved by a method for generating an excitation light beam for generating an EUV light-emitting plasma of a target material, wherein the excitation light beam has a wavelength in the range of 1600 nm to 2300 nm and an EUV driver laser having some or all of the features mentioned above for the EUV driver laser is used.
[0063] Character list
[0064] An embodiment of the invention is explained in more detail below with reference to the drawings. These show:
[0065] 1a to 1d show schematic representations of embodiments of an EUV driver laser according to the invention, each comprising at least one beam source, a pulse and / or beam shaping unit and an amplifier arrangement comprising a plurality of optical amplifiers;
[0066] Fig. 2a to d show a schematic representation of exemplary embodiments of amplifier stages, as shown in Figs. 1b to 1d. The invention is, of course, not limited to the illustrated exemplary embodiment. The exemplary embodiment shown in the figures merely represents a concrete example of the invention. Within the scope of their specialist knowledge, it is possible for a person skilled in the art to recognize exemplary embodiments of the invention as such, even if not explicitly shown.
[0067] List of reference symbols
[0068] 10 EUV driver lasers
[0069] 12 EUV light
[0070] 14 Target material
[0071] 16 Beam source
[0072] 16.1 first beam source
[0073] 16.2 second beam source
[0074] 16.3 third beam source
[0075] 18 Excitation light beam
[0076] 18.1 first excitation light beam
[0077] 18.2 second excitation light beam
[0078] 18.3 third excitation light beam
[0079] 20 Pump source
[0080] 20.1 first pump source
[0081] 20.2 second pump source
[0082] 20.3 third pump source
[0083] 22 Amplifier arrangement
[0084] 22.1 first amplifier arrangement
[0085] 22.2 second amplifier arrangement
[0086] 22.3 third amplifier arrangement
[0087] 24 optical amplifiers
[0088] 24.1 first solid-state or semiconductor amplifier
[0089] 24.2 second solid-state or semiconductor amplifier
[0090] 26 amplified excitation light beam
[0091] 26.1 amplified first excitation light beam
[0092] 26.2 amplified second excitation light beam
[0093] 26.3 amplified third excitation light beam
[0094] 28 Focusing unit
[0095] 30 amplifier module
[0096] 31 Propagation direction
[0097] 32 Pulse shaping unit
[0098] 34 Beam forming unit DESCRIPTION OF PREFERRED EMBODIMENTS
[0099] Figs. 1a to 1d each schematically show 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).
[0100] All embodiments of the EUV driver laser 10 schematically illustrated in Figs. 1a to 1d have in common that 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 pump energy is supplied to the EUV driver laser 10 in such a way that the excitation light beam 18 is generated scatter-free by converting the pump energy.
[0101] Furthermore, the EUV driver laser 10 has an amplifier arrangement 22 with a plurality of optical amplifiers 24 configured to amplify the excitation light beam 18 into an amplified excitation light beam 26. After leaving the last optical amplifier 24 of the amplifier arrangement 22, the amplified excitation light beam 26 has such a high power that the amplified excitation light beam 26 can be directed onto the target material 14 to generate EUV light 12. Typically, the amplified excitation light beam 26 is directed onto the target material 14 with an average power of approximately 30 kW.
[0102] In addition, the EUV driver laser 10 has a focusing unit 28, by means of which the amplified excitation light beam 26 can be focused onto the target material 14. Focusing the amplified excitation light beam 26 is particularly important because the target material 14 must be hit particularly precisely by the amplified excitation light beam 26 to ensure a comparatively high overall conversion efficiency. A comparatively high overall conversion efficiency is understood to mean a comparatively high power output of EUV light 12 in relation to a specific power input by the amplified excitation light beam 26.
[0103] The target material 14 can in principle comprise a tin (Sn), a xenon (Xe), or a lithium (Li) material. In the present embodiments of the EUV driver laser 10, the amplified excitation light beam 26 is directed onto a tin material. The use of a target material 14 comprising tin (Sn) material is advantageous because tin (Sn) has a particularly high absorptivity for the typically used wavelength of the amplified excitation light beam 26, and therefore a particularly high overall conversion efficiency can be achieved through the use of the tin material (Sn). Typically, the tin material (Sn) is shot at a discharge frequency by means of a droplet generator as a target material droplet into a high vacuum comprising a process gas, where it is struck by the amplified excitation light beam 26, and as a result, the plasma of the tin material (Sn) emitting EUV light 12 is generated.
[0104] To ensure a high throughput of EUV light 12 generated per unit of time, the EUV driver laser 10 is designed as a pulsed EUV driver laser 10, and the pulses of the excitation light beam 18 generated by the pulsed EUV driver laser 10 are time-coordinated with extreme precision in terms of generation frequency to the discharge frequency of the tin material droplets. A temporal offset of a few microseconds or a spatial offset of a few micrometers can significantly impair the throughput of EUV light 12 generated per unit of time and / or the overall conversion efficiency.
[0105] The optical amplifiers 24 comprise at least a first solid-state or semiconductor amplifier 24.1 and at least a second solid-state or semiconductor amplifier 24.2.
[0106] By using solid-state or semiconductor amplifiers 24.1, 24.2, it is theoretically possible to scale the average power of the amplified excitation light beam 26 to any desired level. This is due to the fact that the series connection of solid-state or semiconductor amplifiers 24.1, 24.2 is technically relatively simple to implement. The only limitations in such a setup are the performance limits of the materials used.
[0107] The excitation light beam 18, ie the light beam that can be generated by means of the at least one beam source 16, has a wavelength in a range from 1600 nm to 2300 nm. In the present embodiments, the excitation light beam 18 can be generated directly by the at least one beam source 16.
[0108] State-of-the-art EUV driver lasers generate an excitation light beam with a wavelength of typically 10.6 pm. An amplified excitation light beam with this wavelength results in a significantly higher overall conversion efficiency due to the significantly lower self-absorption of the tin (Sn) plasma. However, due to the gaseous active medium used, such EUV driver lasers are significantly more complex in design, more expensive to manufacture, and more intensive to maintain. Therefore, the average power of the amplified excitation light beam of such EUV driver lasers can only be increased with disproportionate effort. Against this background, the achievable average power of the amplified excitation light beam is currently a limiting factor in the generation of EUV light.
[0109] If the excitation light beam 18 has a wavelength in the range of 1600 nm to 2300 nm according to the invention, the overall conversion efficiency is lower than in prior art methods due to the higher self-absorption of the tin material plasma at the wavelength used. However, as already mentioned, the inventive design allows for theoretically unlimited scalability.
[0110] The EUV driver laser 10 according to the invention makes it possible to scale the average power of the amplified excitation light beam 26 as desired beyond 30 kW.
[0111] The term "scattering-free" used at the beginning means that a major portion of the excitation light beam 18, when the EUV driver laser 10 is in operation, is generated by stimulated emission. In other words, when the EUV driver laser 10 is in operation, a major portion of the excitation light beam 18 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 16 by stimulating excited atoms and / or molecules and / or by spontaneous emission that causes a chain reaction in excited atoms and / or molecules.
[0112] However, the term "scattering-free" does not mean that a major portion of the excitation light beam 18, when the EUV driver laser 10 is in operation, is generated directly or indirectly by inelastic scattering of light by atoms and / or molecules. In other words, the excitation light beam 18 is not said to be generated "scattering-free" if a major portion of the excitation light beam 18 is generated only or predominantly by Raman scattering when the EUV driver laser 10 is in operation.
[0113] In Fig. 1a, a basic construction plan of the EUV driver laser 10 according to the invention is illustrated, whereas Figs. 1b to 1d illustrate further embodiments of the EUV driver laser 10.
[0114] The EUV driver laser 10 of Fig. 1b differs from the EUV driver laser 10 of Fig. 1a only in that, instead of individual optical amplifiers 24, a plurality of amplifier modules 30 are arranged within the amplifier arrangement 22, each comprising an optical amplifier 24 and—as illustrated in Figs. 2a to 2d—optionally a pulse shaping unit 32 (Figs. 2b and 2d) and / or a beam shaping unit 34 (Figs. 2c and 2d) located downstream of the respective optical amplifier 24 in a propagation direction 31. The amplifier modules 30 can thus be arranged essentially freely, as required for an optimal beam shape and / or an optimal temporal sequence of pulses of the amplified excitation light beam 26.
[0115] Furthermore, the present embodiments have in common that a pulse shaping unit 32 and a beam shaping unit 34 are assigned downstream of the beam source 16 in the propagation direction 31 of the excitation light beam.
[0116] The embodiment of the EUV driver laser 10 illustrated in Fig. 1c differs from the preceding embodiments in that the beam source 16 is embodied as a first beam source 16.1 and a second beam source 16.2 is present. In this case, the first beam source 16.1 is the one that generates a first excitation light beam 18.1, which is configured to be directed onto the target material 14 formed as a tin material to generate a plasma emitting EUV light 12. In this embodiment, the first beam source 16.1 is thus embodied as the main pulse beam source, the excitation light beam 18.1 as the main pulse, and a first pump source 20.1, which supplies the first beam source 16.1 with pump energy, is embodied as the main pulse pump source.
[0117] In the present embodiment, the second beam source 16.1 is designed as a pre-pulse beam source that generates a second excitation light beam 18.2, designed as a pre-pulse, which is configured to be directed onto the target material 14 for preconditioning the target material 14, which is formed as a tin material. In the present case, preconditioning is generally understood as "preparing" the tin material 14 for the main pulse, which temporally follows shortly after the pre-pulse, in order to increase the overall conversion efficiency.
[0118] Specifically, this preconditioning is realized in the present embodiment in such a way that the tin material 14, in response to the exposure to the prepulse 18.2, forms a disk-like or plate-like shape, which provides a larger surface for the main pulse 18.1.
[0119] The embodiment of Fig. 1c further comprises, in addition to a first amplifier arrangement 22.1, a second amplifier arrangement 22.2, which amplifies the pre-pulse 18.2 to form an amplified second excitation light beam 26.2 in the form of an amplified pre-pulse. The first amplifier arrangement 22.1 amplifies the main pulse 18.1 to form an amplified first excitation light beam 26.1 in the form of an amplified main pulse. In the embodiment of the EUV driver laser 10 illustrated in Fig. 1d, in addition to the two beam sources 16.1, 16.2, a third beam source 16.3 is also present, which is supplied with pump energy by a third pump source 20.3. The third beam source 16.3 is designed here as a dilution pulse beam source, a third excitation light beam 18.3 generated by it as a dilution pulse and the third pump source 20.3 as a dilution pulse pump source.Dilution is understood here as further "preparing" the tin material 14 for the main pulse 16.1 in order to further increase the overall conversion efficiency.
[0120] Specifically, this dilution means not only increasing the surface area of the tin material 14 for the main pulse 18.1, but also its volume.
[0121] The embodiment shown in Fig. 1d further comprises a third amplifier arrangement 22.3, which amplifies the dilution pulse 18.3 to an amplified third excitation light beam 26.3 in the form of an amplified dilution pulse.
[0122] In addition, the invention also encompasses further embodiments not specifically shown, in which a plurality of beam sources, each having its own pump source or alternatively being able to be supplied with pump energy by one or more pump sources.
[0123] Common to all present embodiments is that the excitation light beam(s) 18.1 to 18.3 can be generated directly by converting the pump energy. This means that no further energy conversion takes place between the respective beam source as such and the respective pump source as such.
[0124] The pump sources 20.1 to 20.3 can each be designed, for example, as a current generator or as a laser diode and / or as a laser diode stack or laser diode array, the pump energies accordingly as electrical pump energy in the form of an electric field or as optical pump energy in the form of a diode laser beam.
[0125] In the illustrated embodiments, the first solid-state or semiconductor amplifier 24.1 can be embodied as a crystal amplifier having a crystal substrate doped with a first active medium. The second solid-state or semiconductor amplifier 24.2, on the other hand, can be embodied as a fiber amplifier having an optical waveguide doped with a second active medium.
[0126] While it is advantageous if the first active medium and the second active medium are identical, this is not technically mandatory. In general, when generating light using excited doped materials, the principle is that the specific doping determines the specific emitted wavelength of the light. However, the active media can be doped slightly differently, so that the generated excitation light beams 18.1 to 18.3 and the amplified excitation light beams 26.1 to 26.3 each exhibit multiple spectral components.
[0127] In the embodiments shown, the first and / or the second active medium comprise holmium (Ho) and / or thulium (Tm).
[0128] Furthermore, in the illustrated embodiments, the crystal substrate comprises a crystalline material from the following group of crystalline materials: YAG, YAP, YLF, LU2O3, LuAG, LuLF. The optical waveguide, on the other hand, comprises at least one amorphous material transparent to the excitation light beam from the following group of amorphous materials: SiCh, Al2O3, MgO, B2O3, CaO, Na2CO3.
[0129] In the present embodiments, one or more of the beam sources 16.1 to 16.3 may be designed as a semiconductor laser in the form of a laser diode, wherein the semiconductor laser has a laser emitter, wherein the laser emitter comprises a semiconductor material, wherein the semiconductor material is selected from the following group of semiconductor materials: GaAs, GaP, InAs, InP, InGaAsP.
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) an amplifier arrangement (22) having a plurality of optical amplifiers (24) configured to amplify the excitation light beam (18) to an amplified excitation light beam (26) such that the amplified excitation light beam (26) can be directed onto the target material (14) to generate EUV light (12); d) a focusing unit (28) for focusing the amplified excitation light beam (26) onto the target material (14); marked,that e) the optical amplifiers (24) comprise at least a first and a second solid-state or semiconductor amplifier (24.1, 24.2), 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 the beam source (16) is a solid-state or semiconductor laser.
3. EUV driver laser (10) according to claim 1 or 2, characterized in that the excitation light beam (18) can be generated directly by converting the pump energy.
4. EUV driver laser (10) according to one of the preceding claims, characterized in that the pump energy is an optical pump energy or an electrical pump energy.
5. EUV driver laser (10) according to one of the preceding claims, characterized in that the first solid-state or semiconductor amplifier (24.1) is a crystal amplifier having a crystal substrate doped with a first active medium.
6. EUV driver laser (10) according to one of the preceding claims, characterized in that the second solid-state or semiconductor amplifier (24.2) is a fiber amplifier having an optical waveguide doped with a second active medium.
7. EUV driver laser (10) according to claim 5 or 6, characterized in that the first and the second active medium are the same.
8. EUV driver laser (10) according to claim 5 and 6 or according to claim 7, characterized in that the first and / or the second active medium comprise holmium (Ho) and / or thulium (Tm).
9. EUV driver laser (10) according to claim 5, 7 or 8, characterized in that the crystal substrate comprises a crystalline material from the following group of crystalline materials: YAG, YAP, YLF, LU2O3, LuAG, LuLF.
10. EUV driver laser (10) according to claim 6, characterized in that the optical waveguide comprises at least one amorphous material transparent to the excitation light beam (18) from the following group of amorphous materials: SiCh, Al2O3, MgO, B2O3, CaO, Na2CO3.
11. EUV driver laser (10) according to claim 2, characterized in that the beam source (16) is a semiconductor laser having a laser emitter, wherein the laser emitter comprises a semiconductor material, wherein the semiconductor material is selected from the following group of semiconductor materials: GaAs, GaP, InAs, InP, InGaAsP.
12. EUV driver laser (10) according to one of the preceding claims, characterized in that the beam source (16) is a main pulse beam source (16.1), the excitation light beam (18) is a main pulse (18.1) and the pump source (20) is a main pulse pump source (20.1), wherein the EUV driver laser (10) further comprises: a) a pre-pulse beam source (16.2) which is configured to generate a pre-pulse (18.2), b) a pre-pulse pump source (20.2) which is configured to supply the pre-pulse beam source (16.2) with pump energy such that the pre-pulse (18.2) is generated scatter-free by converting the pump energy, wherein c) the pre-pulse (18.2) for preconditioning the target material (14) and the main pulse (18.1) for generating the EUV light (12) emitting plasma of the target material (14) can be directed onto the target material (14).
13. EUV driver laser (10) according to one of the preceding claims, characterized in that at least one pulse and / or beam shaping unit (32; 34) is arranged downstream of the beam source (16) in a propagation direction (31) of the excitation light beam (18).
14. EUV driver laser (10) according to one of the preceding claims, characterized in that at least one pulse and / or beam shaping unit (32; 34) is assigned to at least one optical amplifier (24) of the amplifier arrangement (22) in a propagation direction (31) of the excitation light beam (18) downstream of the optical amplifier (24).
15. A method for generating an excitation light beam (18) for generating a plasma of a target material (14) emitting EUV light (12), characterized in that the excitation light beam (18) has a wavelength in the range from 1600 nm to 2300 nm and an EUV driver laser (10) according to one of claims 1 to 14 is used.