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
Patent Information
- Application Number
- PCT/EP2024/056185
- 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 the complexity and cost of using gas mixtures as active media, limiting the conversion efficiency and throughput of structured wafers.
An EUV driver laser system that generates excitation light beams with wavelengths between 1600 nm to 2300 nm, using solid-state amplifiers and semiconductor lasers, amplifies these beams independently, and combines them to form a combined excitation light beam, which is then focused onto the target material, enhancing power input without pre-conditioning the target material.
This approach significantly increases the power output of EUV light generation, overcoming the limitations of conventional lasers by achieving higher conversion efficiency and enabling more efficient wafer processing.
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Figure EP2024056185_02102025_PF_FP_ABST
Abstract
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 first beam source configured to generate a first excitation light beam having a first wavelength, b) a second beam source configured to generate a second excitation light beam having a second wavelength, c) a first and a second pump source configured to supply the two beam sources with pump energy accordingly, such that the two excitation light beams are each generated scatter-free by converting the corresponding pump energy, d) a first amplifier arrangement and a second amplifier arrangement, each having an amplifier unit with at least one optical amplifier, wherein the optical amplifiers are configured to amplify the two excitation light beams accordingly to form amplified excitation light beams.
[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. 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, 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.
[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 disk shape by the pre-pulse, to form a target material cloud. The term "cloud" in this context is not synonymous with a gaseous or vapor state of the target material. The use of such a rarefaction pulse further increases the 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 temporally distinct in terms of the 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 kW 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. 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).
[0030] In the known methods, the term "pulse" refers to a light beam that has a comparatively short duration and therefore a starting time and an end time, is part of a multitude 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 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 conversion efficiency is comparatively highest in a wavelength range from 10 pm to 11 pm.
[0036] SUMMARY OF THE INVENTION
[0037] 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.
[0038] 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 excitation light 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 first and the second excitation light beam and / or the two amplified excitation light beams have different wavelengths 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 independently generating at least two excitation light beams, their individual amplification in optical amplifiers and the subsequent merging of the amplified excitation light beams into a combined excitation light beam, the comparatively lower 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.
[0041] It was also recognized according to the invention that excitation light beams with mutually different wavelengths in the range from 1600 nm to 2300 nm have a significantly higher 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 more difficult to compensate. Consequently, the invention is based—in other words—on the finding that the wavelength range according to the invention for the excitation light beams has an optimal overall conversion efficiency and that combining them into a combined excitation light beam is advantageous.
[0042] Advantageously, the excitation light beams have different wavelengths from one another in a range from 1800 nm to 2100 nm, in particular 1900 nm to 2050 nm, particularly preferably in a range from 1900 nm to 2000 nm.
[0043] In this case, a wavelength is understood to be an average wavelength of an emission range of the radiation sources.
[0044] The optical amplifiers are preferably connected in parallel.
[0045] Different wavelength combinations of the excitation light beams within this range are conceivable and possible, for example 1900 nm and 1910 nm, 1910 nm and 1920 nm or 1920 nm and 1930 nm, etc. - whereby the ten-steps are chosen here purely as examples for illustration purposes.
[0046] The EUV driver laser is, of course, not limited to a first and a second beam source (including the components associated with them), but can also comprise a plurality of beam sources. Essentially, this can be a modular system using beam source arrangements and amplifier arrangements that can generate spectrally different amplified excitation light beams, which are subsequently combined by the combination unit to form the combined excitation light beam.
[0047] The specific number of beam sources used and the components assigned to them is theoretically unlimited, meaning that the power of a combined excitation light beam arriving at the target material, which is composed of the excitation light beams, is theoretically scalable as desired. It is particularly advantageous if the EUV driver laser has the following further features: a) the optical amplifiers 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 corresponding amplifier arrangement in a corresponding propagation direction of the two excitation light beams, and / or c) the beam sources each comprise a solid-state and / or semiconductor laser.
[0048] If the amplifier arrays each comprise at least one solid-state optical amplifier, the maximum achievable power for the combined excitation light beam can theoretically be scaled arbitrarily, since the effort required to provide additional optical amplifiers in the amplifier arrays is comparatively low. Consequently, the power output of generated EUV light can also theoretically be scaled arbitrarily.
[0049] The term "scatter-free," used initially in reference to the conversion of the pump energy of the pump sources to the two excitation light beams, means that a major portion of the excitation light beams is generated by stimulated emission when the EUV driver laser is in operation. In other words, when the EUV driver laser is in operation, a major portion of the excitation light beams 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 sources by stimulating excited atoms and / or molecules and / or by spontaneous emission that causes a chain reaction in excited atoms and / or molecules.
[0050] However, the term "scattering-free" does not mean that a major portion of the excitation light beams, 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, excitation light beams are not said to be generated "scattering-free" if a major portion of the excitation light beams is generated only or predominantly by Raman scattering when the EUV driver laser is in operation.
[0051] Advantageously, the optical preamplifiers each comprise at least one solid-state and / or one semiconductor preamplifier. Preferably, the two excitation light beams can each be generated directly by converting the corresponding pump energy. By omitting possible intermediate steps in the generation of the excitation light beams, power losses, for example, due to elastic scattering, can be avoided.
[0052] Furthermore, the pump energy is preferably either an optical pump energy or an electrical pump energy. The pump energy is preferably an electrical pump energy in the form of an electric field. In the case of solid-state lasers, however, optical pump energies are also possible, specifically in the form of pump light beams provided by a laser diode with a wavelength that differs from the wavelength of the excitation light beams emitted by the beam sources.
[0053] It is advantageous if the solid-state amplifiers and / or the optical preamplifiers a) each comprise at least one crystal amplifier or crystal preamplifier having a crystal substrate doped with a first active medium, and / or b) each comprise at least one fiber amplifier or fiber preamplifier having an optical waveguide doped with a second active medium.
[0054] Furthermore, an advantage arises when the first and second active media are identical. If the active media are identical in the crystal (pre)amplifiers and the fiber (pre)amplifiers, it can be ensured, in particular, that the amplified excitation light beams contain highly monochromatic light and, accordingly, have no or only insignificant beam components with wavelengths deviating from the excitation light beams.
[0055] It is advantageous if the beam sources, the pump sources, 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.
[0056] The active medium preferably comprises holmium (Ho) 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 and holmium can emit light in a range from 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.
[0057] Advantageously, the crystal substrates comprise 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 amplifiers or crystal preamplifiers can thus be designed as Tm:YAG, Tm:YAP, Tm:YLF, Tm:LuAG, Tm:LuLF, Ho:YAG, Ho:YAP, Ho:YLF, HOIU2O3, Ho:LuAG (pre-)amplifiers, with Tm:YLF being preferred.
[0058] Advantageously, the optical waveguides comprise 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 amplifiers or fiber preamplifiers can therefore be designed as Tm:SiO2, Al2O3, MgO, B2O3, CaO, or Na2CO3 fiber amplifiers or fiber preamplifiers.
[0059] Alternatively, the beam sources are semiconductor lasers, each having a laser emitter, wherein the laser emitters each comprise 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.
[0060] The beam sources can be configured as a main pulse beam source arrangement, the combined excitation light beam can be a main pulse, and the pump sources can be configured as a main pulse pump source arrangement, wherein the EUV driver laser further comprises: a) a pre-pulse beam source arrangement configured to generate a pre-pulse, b) a pre-pulse pump source arrangement configured to supply the pre-pulse beam source arrangement with pump energy such that the pre-pulse is generated scatter-free by converting the pump energy, wherein c) the pre-pulse beam source arrangement comprises at least one pre-pulse beam source and the pre-pulse pump source arrangement comprises at least one pre-pulse pump source, wherein the pre-pulse pump source is configured to pump the pre-pulse beam source with pump energy,d) 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.
[0061] Furthermore, it is advantageous if the EUV driver laser further comprises: a) a dilution pulse beam source arrangement configured to generate a dilution pulse, b) a dilution pulse pump source arrangement configured to supply the pre-pulse beam source arrangement with pump energy such that the dilution pulse is generated scatter-free by converting the pump energy, wherein c) the dilution pulse beam source arrangement comprises at least one dilution pulse beam source and the dilution pulse pump source arrangement comprises at least one dilution pulse pump source, wherein the dilution pulse pump source is configured to pump the dilution pulse beam source with pump energy, d) the dilution pulse can be directed onto the target material between the pre-pulse and the main pulse to dilute the target material.
[0062] Advantageously, the target material is further preconditioned upon exposure to the dilution pulse, thereby advantageously increasing the absorption coefficient of the target material with respect to the main pulse and, consequently, the conversion efficiency of the applied laser power to the generated EUV power. The dilution pulse can precondition the target material, for example, by changing its shape or by tempering it.
[0063] Preferably, at least one pulse and / or beam-shaping unit is assigned to at least one optical amplifier of the amplifier units in the corresponding propagation direction of the excitation light beams downstream of the corresponding optical amplifier. In the propagation directions 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 excitation light beams can thus be shaped temporally, spectrally, and / or with regard to an intensity profile either before entering or after leaving the amplifier arrangements.
[0064] The pulse and / or beam-forming unit may comprise 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 broadening or narrowing module, by means of which the spectral range comprising the excitation light beams and / or the amplified excitation light beams and / or the combined excitation light beam can be broadened or narrowed.
[0065] Preferably, the combination unit is configured to spectrally combine the amplified excitation light beams.
[0066] Preferably, each amplifier unit can be assigned a combination unit, each of which is configured to spectrally combine the amplified excitation light beams assigned to it.
[0067] According to an advantageous embodiment, at least one beam splitter arrangement can be provided, by means of which at least one of the excitation light beams can be split into two or more partial beams. The beam splitter arrangement can also split two or, if additional beam sources are present, several excitation light beams into two or more partial beams. In such a case, the partial beams can be individually amplified into amplified partial beams in amplifier arrangements assigned to the respective beam sources.
[0068] The amplified excitation light beams and / or 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 overlap, or b) they superimpose one another spatially and temporally in such a way that the beams are superimposed.
[0069] The amplified excitation light beams can, for example, be spectrally combined in such a way that the partial beams do not overlap, or only slightly, but rather run parallel to each other and alongside each other along a common propagation direction. Furthermore, it is also possible to spectrally combine the beams in such a way that the amplified excitation light beams overlap spatially and temporally.
[0070] When spectrally combining the amplified excitation light beams, it may be advantageous to additionally arrange one or more polarization modulation units per beam source within corresponding beam paths of the amplified excitation light beams in order to reduce power losses due to possible adverse, i.e. attenuating, interactions between the amplified excitation light beams due to different polarization.
[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 sources, the beam sources and the amplifier arrangements are arranged in a first system area and the focusing unit is arranged in a second system area 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 two beam sources that can generate a first and a second excitation light beam with different wavelengths, wherein the beam sources can each be supplied with pump energy from a pump source, two amplifier arrangements that can amplify the two excitation light beams accordingly to amplified excitation light beams, a combination unit and a focusing unit;
[0077] Fig. 2 shows a second embodiment of the EUV driver laser, in which, in comparison to the first embodiment, a further beam source, a further pump source and a further amplifier arrangement are present and by means of beam splitter arrangements the excitation light beams generated by the beam sources can each be split into several partial beams;
[0078] Fig. 3 shows a third embodiment of the EUV driver laser, in which, compared to the second embodiment, three further beam sources, three further pump sources and further combination units are present;
[0079] Fig. 4a to 4c show a possible embodiment of a combination unit for the spectral “side-by-side” combination of partial beams;
[0080] Fig. 5 to 9 show several possible designs of combination units by means of which a spectral "filled aperture" combination of partial beams is possible.
[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 first beam source
[0086] 18 second beam source
[0087] 20 first excitation light beam
[0088] 22 second excitation light beam
[0089] 24 first pump source
[0090] 26 second pump source
[0091] 28 first amplifier arrangement
[0092] 30 second amplifier arrangement
[0093] 32 amplifier unit
[0094] 34.1 optical amplifiers
[0095] 36 first amplified excitation light beam
[0096] 38 second amplified excitation light beam
[0097] 40 combined excitation light beam
[0098] 42 combination unit
[0099] 43 Focusing unit
[0100] 44.1 other radiation sources
[0101] 46.1 other pump sources
[0102] 48.1 Further amplifier arrangements
[0103] 50.1 further excitation light beams
[0104] 52.1 Beam splitter arrangements
[0105] 54.1 Partial beams
[0106] 56.1 amplified partial beams
[0107] 58 optical fibers
[0108] 60 Propagation direction
[0109] 62 multi-core fiber
[0110] 64 fiber bodies
[0111] 66 Fiber optic channel
[0112] 68.1 Pulse and / or beam forming units
[0113] 70 second combination unit
[0114] 72.1 combined amplified partial beams
[0115] 74 final combination beam
[0116] 76 reflective optical element
[0117] 78 third combination unit 8O.i reinforced combination beams
[0118] 82 Side-by-side combination
[0119] 84 filled aperture
[0120] 86 Diffraction arrangement 88 Diffractive optical element
[0121] 90 second diffractive optical element
[0122] 92a first focusing optical element
[0123] 92b second focusing optical element
[0124] 94 stepped reflective optical element 96 third focusing optical element
[0125] 98 Mirror arrangement
[0126] 100 dichroic mirrors
[0127] DESCRIPTION OF PREFERRED EMBODIMENTS
[0128] 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).
[0129] The EUV driver laser 10 has a first beam source 16 and a second beam source 18, wherein the first beam source 16 is configured to generate a first excitation light beam 20 and the second beam source 18 is configured to generate a second excitation light beam 22. In addition, the EUV driver laser 10 has a first pump source 24 and a second pump source 26, wherein the first pump source 24 is configured to supply the first beam source 16 with pump energy and the second pump source 26 is configured to supply the second beam source 18 with pump energy. The beam sources 16, 18 are supplied in the EUV driver laser 10 in such a way that the excitation light beams 20, 22 are each generated scatter-free by converting the corresponding pump energy.This means that, for example, no optical parametric oscillator (OPO) is used here or so-called high harmonics are generated to generate a major portion of the excitation light beams 16, 18.
[0130] For reasons of clarity, identical structures in the figures are not consistently provided with reference symbols.
[0131] Furthermore, the EUV driver laser 10 has a first amplifier arrangement 28 and a second amplifier arrangement 30, each of which has an amplifier unit 32 (shown only in Figs. 2, 3, and 4a) and at least one optical amplifier 34 (likewise shown only in these figures). The optical amplifiers 34 are configured to amplify the excitation light beams 20, 22 to amplified excitation light beams 36, 38, respectively.
[0132] In order to generate a combined excitation light beam 40 from the amplified excitation light beams 36, 38, the EUV driver laser 10 further comprises a combination unit 42 according to the invention. To accomplish this, the combination unit 42 can comprise one or more beam combiners (not specifically shown).
[0133] Furthermore, according to the invention, the EUV driver laser 10 has a focusing unit 43, by means of which the combined excitation light beam 40 can be focused onto the target material 34. The first and second excitation light beams 16, 18 and in this case also the two amplified excitation light beams 36, 38 have different wavelengths 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 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, but 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.
[0134] It was recognized according to the invention that the lower conversion efficiency of the EUV driver laser 10 can be compensated for by amplifying at least two excitation light beams 16, 18, which have different wavelengths in the range from 1600 nm to 2300 nm, individually and independently of one another in optical amplifiers 34 of amplifier arrangements 28, 30, and combining the resulting amplified excitation light beams 36, 38 into the combined excitation light beam 40 by the combination unit 42. This allows a much higher power of the combined excitation light beam 40 than with the EUV driver lasers known from the prior art, exceeding the absolute physical load limits of the individual optical amplifiers 34.
[0135] Laser powers in the range of 20 kW to 50 kW are known from the prior art, which ultimately reach and are introduced into the target material 14. By generating at least two excitation light beams 16, 18, amplifying them individually, and then combining them using the combination unit 42, theoretically any desired laser power can be achieved, which is only limited, for example, by the absorption properties of the optics used within the EUV driver laser 10.
[0136] The embodiments shown in Figs. 2 and 3 furthermore have, merely by way of example, at least one further beam source 44.i, at least one further pump source 46.i, and at least one further amplifier arrangement 48.i, where i corresponds to a consecutive number of further beam sources 44.i, pump sources 46.i, or amplifier arrangements 48.i. Accordingly, a first further beam source is designated, for example, by reference numeral 44.1, a second further beam source by reference numeral 44.2, etc. For the sake of clarity, elements subsequently designated by this consecutive number are not provided with reference numerals throughout the figures. If further reference numerals are assigned below using the "Xi" scheme, the same principles apply.
[0137] Furthermore, the amplifier units 32 of the embodiments of the EUV driver laser 10 illustrated in Figs. 2 and 3 comprise a plurality of optical amplifiers 34.i connected in parallel, where i also corresponds to a consecutive number of optical amplifiers. Each additional beam source 44.i generates an additional excitation light beam 50.i, wherein the additional excitation light beams 50.i, as well as the two excitation light beams 20, 22, have different wavelengths in the range from 1600 nm to 2300 nm and are generated in a scatter-free manner by converting the pump energy of the pump sources 46.i.
[0138] The embodiments of Figs. 2 and 3 furthermore comprise a beam splitter arrangement 52.i for each beam source 16, 18, 44.i, each of which is configured to split the excitation light beams 20, 22, 50.i generated by the beam sources 16, 18, 44.i into a plurality of partial beams 54.i. For this purpose, the beam splitter arrangements 52.i can each comprise one or more beam splitters (not specifically shown).
[0139] In these embodiments, each optical amplifier 34.i is configured to amplify at least one of the partial beams 54.i into an amplified partial beam 56.i. In other words, each optical amplifier 34.i can amplify at least one of the partial beams 54.i assigned to it into an amplified partial beam 56.i.
[0140] In all present embodiments, the optical amplifiers 34.i are embodied as fiber amplifiers and the beam sources 16, 18, 44.i as solid-state lasers. Accordingly, the pump sources 24, 26, 46.i are optical pump sources in the form of semiconductor laser diodes that can supply the solid-state lasers 16, 18, 44.i with optical pump energy. In the present case, the solid-state lasers 16, 18, 44.i are embodied, for example, as Tm:YLF lasers. In embodiments not specifically illustrated, the solid-state lasers 16, 18, 44.i can also comprise a different dopant:crystal combination. In further embodiments not specifically illustrated, the beam sources 16, 18, 44.i are each embodied as semiconductor lasers in the form of a laser diode, which is then pumped not optically but electrically by means of an electric field.
[0141] In the present case, the optical amplifiers 34.i designed as fiber amplifiers each have an optical waveguide 58 in the form of an optical fiber, each of which can independently and individually receive one of the partial beams 54.i and guide it over a corresponding distance along a propagation direction 60. Alternatively, instead of comprising several individual fiber amplifiers 34.i, as shown in Figs. 4a and 4b, the amplifier units 32 can also be designed as a multi-core fiber 62 comprising a fiber body 64 within which light guide channels 66 are formed, each functioning as an optical fiber. In such a case, the light guide channels 66 function not only as light-guiding elements, but also as optical amplifiers 34.i. Such use of multi-core fibers 62 is advantageous because it significantly reduces the structural complexity of coupling the partial beams 54.i into and out of the optical amplifiers 34.i.This also allows for considerable space savings. With appropriate design of the multi-core fibers 62, the partial beams 54.i can be guided through the multi-core fibers 62 in such a way that they can be amplified to form amplified partial beams 56.i essentially unaffected by one another.
[0142] In the present case, each optical waveguide 58 or the multi-core fiber 62 is doped with an active medium by means of which the partial beams 54.i or possibly also the combined excitation light beam 40 can be amplified.
[0143] The active medium can, in particular, comprise 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 in this case comprises thulium (Tm).
[0144] In embodiments not specifically shown, the EUV driver laser 10 comprises, in addition to the beam sources 16, 18, 44.i, at least one further beam source, wherein the beam sources 16, 18, 44.i are configured jointly as a main pulse beam source arrangement and the at least one further beam source is configured as a pre-pulse beam source arrangement. In further embodiments not specifically shown, the EUV driver laser 10 comprises at least one further independent beam source, which is configured as a dilution pulse beam source arrangement. Accordingly, these EUV driver lasers 10 then comprise a main pulse pump source arrangement, a pre-pulse pump source arrangement, and optionally a dilution pulse pump source arrangement, wherein the respective pump source arrangements supply the beam sources of the corresponding beam source arrangements with pump energy accordingly.The pre-pulse pump source arrangement can generate a light beam called a pre-pulse, while the dilution pulse beam source arrangement can generate a light beam called a dilution pulse. Both beams can be directed onto the target material 14 prior to the main pulse to increase the power output of the generated EUV light 12.
[0145] In the embodiments of Figs. 2 and 3, a pulse and / or beam-shaping unit 68.i is assigned to each optical amplifier 34.i in the propagation direction 60 downstream of the beam splitter arrangements 52.i, specifically in the present case downstream of the combination units 42 in the propagation direction 60. In addition, the pulse and / or beam-shaping units 68.i can also be assigned to the amplifier units 32 or the amplifier arrangements 28, 30, 48.i in embodiments not specifically shown. It is also possible to provide the pulse and / or beam-shaping units 68.i downstream of the beam splitter arrangements 52.i and upstream of the amplifier arrangements 28, 30, 48.i in the propagation direction 60. The partial beams 54.i can thus be shaped as amplified partial beams 56.i temporally, spectrally and / or with regard to an intensity profile of the amplified partial beams 56.i before entering or - as in the present case - after leaving the amplifier arrangements 28, 30, 48.i.
[0146] The following will now specifically address the embodiment shown in Fig. 2. In this embodiment, the EUV driver laser 10 comprises three beam sources 16, 18, 44.1, each capable of generating an individual excitation light beam 20, 22, 50.1 by converting the pump energy provided by the corresponding pump source 24, 26, 46.1 without scattering. Accordingly, the EUV driver laser comprises five individual optical amplifiers 34.1 to 34.5 configured as fiber amplifiers or, optionally, an amplifier unit 32 configured as a multi-core fiber 62 with light guide channels 66 formed within the multi-core fiber 62. In the propagation direction 60 after the amplifier units 32 of three amplifier arrangements 28, 30, 48.1, which are each assigned to the beam sources 16, 18, 44.1, there is a gap between the respective amplifier arrangement 28, 30, 48.1 and the respective combination unit 42 for the combined excitation light beam 40, a second combination unit 70 is arranged for each amplifier unit 32, by means of which the amplified partial beams 56.i emerging from the amplifier units 32 can each be combined to form an amplified excitation light beam 36, 38, in this case designed as a combined amplified partial beam 72.i. The combined amplified partial beams 72.i are subsequently combined by means of the combination unit 42 to form the combined excitation light beam 40, which, with such step-by-step combination, can also be referred to as the final combination beam 74.
[0147] The combination unit 42 has a plurality of reflective optical elements 76 for combining the combined amplified partial beams 72.i.
[0148] The following will now discuss the embodiment of the EUV driver laser 10 according to Fig. 3. In this embodiment, a total of six individual beam sources 16, 18, 44.1 to 44.4 are used, each of which can generate an individual excitation light beam 20, 22, 50.1 to 50.4. The excitation light beams 20, 22, 50.1 to 50.4 each have different wavelengths in a range from 1600 nm to 2300 nm. The beam sources 16, 18, 44.1 to 44.4 are supplied with pump energy by the pump sources 24, 26, 46.1 to 46.4, which is converted directly without scattering.
[0149] In the present embodiment, several second combination units 70 are arranged in the propagation direction 60 downstream of the amplifier arrangements 28, 30, 48.1 to 48.4 and upstream of the combination unit 42 for the combined excitation light beam 40 or the final combination beam 74, each of which can combine five partial beams 54.i into a combined amplified partial beam 72.i. Following this in the propagation direction 60 are two third combination units 78, each of which can combine three combined amplified partial beams 72.1 to 72.3 into amplified excitation light beams 36, 38 formed as amplified combination beams 80.1 and 80.2.
[0150] In the embodiment of Fig. 3, pulse and / or beam forming units 68.i are arranged between the second combination units 70 and the third combination units 78.
[0151] As can be seen from Figs. 4a to 9, the amplified partial beams 56.i can be combined in different ways using the combination units 42, 70, 78. As can be seen from Figs. 4a to 4c, after combination, the beams can propagate essentially parallel to each other and side by side along the propagation direction 60 (Fig. 4c). In this case, one can speak of a "side-by-side" combination 82.
[0152] However, it is also possible, as shown in Figures 5 to 9, to spatially and temporally superimpose the amplified partial beams 56.i. Because the excitation light beams 20, 22, 50.i and the amplified partial beams 56.i have different wavelengths, essentially no interference occurs during this spatial and temporal superimposition—neither constructive nor destructive. This does, however, result in the disadvantage that the beams cannot mutually reinforce each other through constructive interference. However, this also has the advantage that the beams cannot destructively interfere with each other, provided they exhibit a corresponding phase difference. In these embodiments, one speaks of a "filled aperture" 84.In the case of Figures 5, 6 and 7, the combination units 42, 70, 78 have a diffraction arrangement 86 by means of which the filled aperture 84 is realized, wherein the diffraction arrangement 86 has at least one diffractive optical element 88, for example a diffraction grating.
[0153] In the embodiment of Fig. 5, the filled aperture 84 is realized by directing the amplified partial beams 56.i at different angles onto the diffractive optical element 88. The amplified partial beams 56.i are shown in this figure and in the following figures merely as examples and for the sake of clarity. These embodiments can, of course, also relate to amplified excitation light beams 36, 38, combined amplified partial beams 72.i and / or amplified combination beams 80.i. Statements made below only with regard to amplified partial beams 56.i accordingly also apply to the other beams mentioned. By diffraction, for example at a grating, the different wavelengths are deflected to different degrees and thus transferred into the filled aperture 84.
[0154] In the embodiment of Fig. 6, amplified partial beams 56.i running parallel to one another are focused via the diffractive optical element 88 onto a second diffractive optical element 90, which, in the specific example, converts the amplified partial beams 56.i into the combined excitation light beam 40, a combined amplified partial beam 72.i or an amplified combination beam 80.i.
[0155] In the embodiment of Fig. 7, focusing optical elements 92a, 92b in the form of lenses are located upstream and downstream of the diffractive optical element 88 in the propagation direction. By means of these lenses, parallel amplified partial beams 56.i are directed onto a stepped reflective optical element 94, in this case in the form of a stepped mirror. By means of a third focusing optical element 96 following the stepped reflective optical element 94 in the propagation direction 60, the amplified partial beams 56.i are directed onto the second diffractive optical element 90, whereby the amplified partial beams 56.i are converted into the combined excitation light beam 40, a combined amplified partial beam 72.i, or an amplified combination beam 80.i. By means of this arrangement, a so-called "chirping" that may be imposed on the amplified excitation light beams 56.i can be reduced.
[0156] In the embodiment of Fig. 8, however, a mirror arrangement 98 having a plurality of dichroic mirrors 100 is used to combine the amplified partial beams 56.i into the combined excitation light beam 40, a combined amplified partial beam 56.i or an amplified combination beam 80.i.
Claims
Claims 1. An EUV driver laser (10) for generating a plasma (14) of a target material that emits EUV light (12), comprising a) a first beam source (16) configured to generate a first excitation light beam (20) having a first wavelength, b) a second beam source (18) configured to generate a second excitation light beam (22) having a second wavelength, c) a first and a second pump source (24, 26) configured to supply the two beam sources (16, 18) with pump energy, such that the two excitation light beams (20, 22) are each generated scatter-free by converting the corresponding pump energy, d) a first amplifier arrangement (28) and a second amplifier arrangement (30), each having an amplifier unit (32) with at least one optical amplifier (34), wherein the optical amplifiers (34.i) are configured to amplify the two excitation light beams (20, 22) accordingly to form amplified excitation light beams (36, 38), characterized in that e) the EUV driver laser (10) comprises: a combining unit (42) configured to combine the amplified excitation light beams (36, 38) to form a combined excitation light beam (40), a focusing unit (43) configured to focus the combined excitation light beam (40) onto the target material (14), f) the first and second excitation light beams (20, 22) and / or the two amplified excitation light beams (36, 38) have different wavelengths in a range from 1600 nm to 2300 nm.
2. EUV driver laser according to claim 1, characterized in that a) the optical amplifiers (34.i) comprise at least one solid-state amplifier, in particular a fiber amplifier, and / or b) in the corresponding propagation direction (60) of the two excitation light beams (20, 22), at least one optical preamplifier is arranged in front of the corresponding amplifier arrangements (28, 30), and / or c) the beam sources (16, 18) each comprise a solid-state and / or semiconductor laser.
3. EUV driver laser according to claim 2, characterized in that the optical preamplifiers each comprise at least one solid-state and / or one semiconductor preamplifier.
4. EUV driver laser according to one of the preceding claims, characterized in that the two excitation light beams (20, 22) are each generated directly by converting the corresponding 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 2 to 5, characterized in that the solid-state amplifiers and / or the optical preamplifiers a) each comprise at least one crystal amplifier or crystal preamplifier having a crystal substrate doped with a first active medium, and / or b) each comprise at least one fiber amplifier or fiber preamplifier having an optical waveguide (58) 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 sources (16, 18, 44.i), the pump sources (24, 26, 46.i) and / or one, several or all optical amplifiers (34.i) or optical preamplifiers are provided with a active medium.
9. EUV driver laser according to one of the preceding claims, characterized in that the beam sources (16, 18, 44.i) are designed as a whole as a main pulse beam source arrangement, the combined excitation light beam (40) is a main pulse and the pump sources (24, 26, 46.i) are designed overall as a main pulse pump source arrangement, wherein the EUV driver laser (10) further comprises: a) a pre-pulse beam source arrangement which is configured to generate a pre-pulse, b) a pre-pulse pump source arrangement which is configured to supply the pre-pulse beam source arrangement with pump energy such that the pre-pulse is generated in a scatter-free manner by converting the pump energy, wherein c) the pre-pulse beam source arrangement has at least one pre-pulse beam source and the pre-pulse pump source arrangement has at least one pre-pulse pump source, wherein the pre-pulse pump source is configured to supply the pre-pulse beam source with pump energy, d) the pre-pulse for preconditioning the target material (14) and the main pulse for generating the plasma of the target material (14) emitting EUV light (12) can be directed onto the target material (14).
10. EUV driver laser according to one of the preceding claims, characterized in that in the corresponding propagation direction (60) of the excitation light beams (20, 22, 50.i) at least one pulse and / or beam shaping unit (68.i) is arranged after the corresponding beam source (16, 18, 44.i).
11. EUV driver laser according to one of the preceding claims, characterized in that at least one pulse and / or beam shaping unit (68.i) is assigned to at least one optical amplifier (34.i) of the amplifier units (32) in the corresponding propagation direction (60) of the excitation light beams (20, 22, 50.i) downstream of the corresponding optical amplifier (34.i).
12. EUV driver laser according to one of the preceding claims, characterized in that the combination unit (42) is configured to spectrally combine the amplified excitation light beams (36, 38) with one another.
13. 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 sources (24, 26, 46.i), the beam sources (16, 18, 44.i), the amplifier arrangements (28, 30, 48.i) and the combination unit (42) in a first system area and the A focusing unit (43) is arranged in a second system area, and a light beam transport system is present that can transport the combined excitation light beam (40) from the combining unit (42) to the focusing unit (43).
14. A method for generating a combined excitation light beam (40) 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 12 is used.