EUV light source with a separation device

Description

Background of the invention

[0001] The invention relates to an EUV light source, comprising: a supply device for supplying a target material, a pre-pulse laser source for emitting at least one pre-pulse laser beam at at least one pre-pulse wavelength, a main pulse laser source for emitting a main pulse laser beam at a main pulse wavelength different from the at least one pre-pulse wavelength, a pre-pulse beam guiding device for supplying the at least one pre-pulse laser beam from the pre-pulse laser source into a radiation generation chamber and for focused irradiation of the target material within the radiation generation chamber with at least one pre-pulse of the at least one pre-pulse laser beam,and a main pulse beam guiding device for supplying the main pulse laser beam from the main pulse laser source into the radiation generation chamber and for focused irradiation of the target material within the radiation generation chamber with a main pulse of the main pulse laser beam, wherein the target material is designed to emit EUV radiation as a result of the irradiation.

[0002] An EUV light source is a radiation source used to emit EUV radiation. EUV radiation refers to electromagnetic radiation with a wavelength between 5 nm and 30 nm. EUV radiation is used primarily in the semiconductor industry. Compared to currently common lithography systems, which operate at wavelengths in the UV wavelength range, the use of EUV radiation for microlithographic manufacturing allows for the reliable production of components with significantly smaller feature sizes, thus leading to a corresponding increase in performance.

[0003] The EUV light source is suitable for generating EUV radiation using an LPP ("Laser Produced Plasma") process. The target material, which is typically a metal, in particular tin, is preferably provided in droplet form by the provision device. Each droplet is first irradiated with one, two, or possibly more than two pre-pulses. The pre-pulse(s) serve to prepare the droplet for irradiation with the main pulse, in particular to heat the droplet, expand it, vaporize it, and / or generate a plasma. The subsequent irradiation of each droplet with a main pulse serves to convert the target material into the plasma state, resulting in the emission of EUV radiation.

[0004] The pre-pulses are laser pulses of the at least one pre-pulse laser beam, while the main pulses are laser pulses of the main pulse laser beam. The wavelength of the respective pre-pulse laser beam and thus of the pre-pulses is referred to as the pre-pulse wavelength, while the wavelength of the main pulse laser beam and thus of the main pulse is referred to as the main pulse wavelength. The main pulse laser source is, for example, a CO2 laser, and the pre-pulse laser source is, for example, a solid-state laser. The use of a pre-pulse laser source in the form of a solid-state laser has the advantage of higher conversion efficiency compared to the use of a pre-pulse laser source in the form of another CO2 laser. The pre-pulse beam guiding device and the main pulse beam guiding device usually each have a plurality of optical elements, for example lenses and / or mirrors.

[0005] The pre-pulse laser source can have exactly one laser source for generating a pre-pulse laser beam at a pre-pulse laser wavelength. However, the pre-pulse laser source can also have two or more laser sources for generating two or more pre-pulse laser beams, which can have the same or different pre-pulse laser wavelengths, as described, for example, in US 10,932,350 B2. If two different pre-pulse laser wavelengths are used, these typically do not differ greatly from one another in order to enable joint guidance of the pre-pulse laser beams via the optical elements of the pre-pulse beam guidance device. If the pre-pulse laser source has two or more laser sources for generating two or more pre-pulse laser beams, the pre-pulse laser beams are generally combined in the pre-pulse laser source and jointly guided via the optical elements of the pre-pulse beam guidance device.The pre-pulse laser beams can pass through the optical elements of the pre-pulse beam guidance device collinearly, but this is not absolutely necessary, see US 10,932,250 B2.

[0006] During irradiation of the target material in the form of droplets, a portion of the main pulses is scattered by the target material. Furthermore, the target material emits broadband electromagnetic radiation as a result of the irradiation. A portion of this scattered or emitted radiation enters the pre-pulse beam guidance device as electromagnetic interference, where it causes undesirable effects, particularly heating of optical elements of the pre-pulse beam guidance device and associated thermal lensing effects. A further particular challenge arises from the fact that, due to the use of two separate beam guidance devices for the pre-pulses and the main pulses, there is typically little space available for structural modifications to the EUV light source.

[0007] EUV beam generation devices with two beam sources for emitting two laser beams (pre-pulse laser beam and main pulse laser beam) with different wavelengths, each entering a vacuum chamber through an opening, are described in WO 2015 / 036024 A1, WO 2015 / 036025 A1, and US 2008 / 0149862 A1. In one example of WO 2015 / 036024 A1, the openings in the vacuum chamber are each sealed gas-tight with a window that transmits the respective laser beam (pre-pulse or main pulse laser beam). The window transmitting the main pulse laser beam has a coating on its vacuum-side surface that reflects the pre-pulse laser beam. This serves to superimpose the pre-pulse and main pulse laser beams and to guide the beam together within the vacuum chamber onto the target material. A superimposition in the reverse manner is also described.In US 2008 / 0149862 A1, in one example, an optical isolator is arranged along a beam path to reduce the amount of photons reflected from droplets of the target material towards the pre-pulse laser source and / or the main pulse laser source, since these can cause self-lasing in the laser-active medium. Object of the invention

[0008] In contrast, it was the object of the invention to provide an EUV light source of the type mentioned above, in which adverse effects of interference radiation entering the pre-pulse beam path are avoided, in particular in a space-saving manner. Subject of the invention

[0009] This object is achieved by an EUV light source of the type mentioned at the outset, in which the pre-pulse beam guiding device has at least one separation device which is designed to reflect interference radiation entering the pre-pulse beam guiding device from the radiation generation chamber in at least one wavelength range which does not include the at least one pre-pulse wavelength, in a focused manner back into the radiation generation chamber or in a focused manner into at least one beam trap.

[0010] The separation device comprises one or more optical elements and forms part of the pre-pulse beam guidance device. It is arranged in the beam path of the pre-pulse laser beam before it enters the radiation generation chamber. The separation device, or more precisely at least a part of the separation device, exhibits wavelength-dependent behavior, whereby the pre-pulses and the interference radiation are influenced differently. This wavelength dependence can be the result of interference effects, for example. The separation device can, for example, comprise a diffraction grating or a dichroic mirror. A dichroic mirror is an optical element that has a layer stack of at least two dielectric materials with different refractive indices and therefore reflects or transmits radiation depending on the wavelength.Due to the wavelength-dependent behavior of the separation device, the pre-pulses can be fed into the radiation generation chamber and focused onto the target material, while the electromagnetic interference radiation in at least one wavelength range that does not include the at least one pre-pulse wavelength is focused back into the radiation generation chamber or reflected into at least one beam trap. As a result, the further pre-pulse beam guidance device, i.e., the part of the pre-pulse beam guidance device that lies in the beam path of the pre-pulses upstream of the separation device, is no longer affected by the electromagnetic interference radiation.

[0011] Reflecting the interfering radiation back into the radiation generation chamber has the advantage of being particularly space-saving, as no additional space is required for a beam trap. This is particularly advantageous in the present case of two separate beam guidance devices, as space is very limited. Focusing the interfering radiation reflected back into the radiation generation chamber results in it passing unhindered through an opening in the radiation generation chamber and re-entering the radiation generation chamber. The radiation diverging beyond the focus is then absorbed within the radiation generation chamber.

[0012] If reflected into at least one beam trap, the electromagnetic interference is absorbed in the beam trap. Due to the focusing into the beam trap, installation space can be saved in this case as well. To absorb high power, the beam trap typically has a suitable cooling device.

[0013] Instead of a single separation device, multiple separation devices can be used, for example, to focus the interfering radiation in different wavelength ranges back into the radiation generation chamber or to focus it into at least one beam trap. The individual separation devices can also be constructed differently.

[0014] In one embodiment, the separation device is designed to reflect interfering radiation in the form of a portion of the main pulses backscattered by the target material in a focused manner back into the radiation generation chamber or in a focused manner into the at least one beam trap. Since the main pulse laser beam for converting the target material into the plasma state has a high power of typically more than approximately 30-40 kW, it is particularly important to reduce the influence of the backscattered portion of the main pulses on the pre-pulse beam guidance device. For this purpose, the wavelength dependence of the separation device is specifically adapted to the main pulse wavelength.

[0015] In a further embodiment, the separation device is designed to reflect interfering radiation in the form of radiation emitted as a result of the irradiation of the target material in at least one wavelength range that does not include the at least one pre-pulse wavelength, focused back into the radiation generation chamber or focused into the at least one beam trap. The irradiation of the target material typically results in a broadband emission of radiation, which also includes components in the UV / VIS range, corresponding to wavelengths from 100 nm to 780 nm, and has a detrimental effect upon entry into the pre-pulse beam guidance device.

[0016] In one embodiment, the separation device is designed to reflect the pre-pulses. In this case, the separation device can serve, for example, as a (planar) deflection mirror for the pre-pulses.

[0017] In a further development of this embodiment, the separation device comprises a dichroic mirror and a curved deflection mirror, wherein the dichroic mirror serves as a deflection mirror for reflecting the pre-pulses and is designed to transmit the interfering radiation in at least one wavelength range that does not include the at least one pre-pulse wavelength, and wherein the curved deflection mirror is designed to reflect the interfering radiation transmitted by the dichroic mirror, preferably through the dichroic mirror, in a focused manner back into the radiation generation chamber or in a focused manner into the at least one beam trap. The curvature of the deflection mirror serves to focus the interfering radiation. The orientation of the curved deflection mirror is selected such that the interfering radiation is reflected in the desired direction.

[0018] To avoid adverse effects of the interfering radiation on the pre-pulse beam guidance device, it does not necessarily have to be reflected in a focused manner back into the radiation generation chamber or in a focused manner into the at least one beam trap. Alternatively, a dichroic mirror can serve as a deflecting mirror for reflecting the pre-pulses and be designed to transmit the interfering radiation in at least one wavelength range that does not include the at least one pre-pulse wavelength. In this case, a beam trap is arranged such that the interfering radiation transmitted by the dichroic mirror is absorbed in the beam trap. A disadvantage of this solution, which is not part of the claimed invention, is that there is typically insufficient space available for it.

[0019] In a further development of this embodiment, the separation device has a base body that transmits the interference radiation, with the dichroic mirror being formed as a coating on a front side of the base body and the deflection mirror being formed as a coating on a back side of the base body. In this case, the dichroic mirror and the deflection mirror are not two separate optical elements, but rather form a common separation element. An advantage of this embodiment is its particularly small space requirement. The optical separation element is based on a dielectric base body that transmits the interference radiation and is made of, for example, zinc selenide (ZnSe), diamond, or SiC.The coating on the front side of the base body should have the highest possible reflectance at the at least one pre-pulse wavelength and the highest possible transmittance in the at least one wavelength range that does not include the at least one pre-pulse wavelength, in particular at the main pulse wavelength. The coating on the back side of the base body should have the highest possible reflectance for the interference radiation transmitted by the dichroic mirror, in particular at the main pulse wavelength. For this purpose, the coatings on the front and back sides comprise, for example, a plurality of dielectric thin layers of at least two materials with different refractive indices, which are applied to the base body of the optical separation element by means of a suitable deposition process.The coating on the back of the base body can also have a highly reflective metallic layer or coating instead of a dielectric coating.

[0020] In a further embodiment, the separation device is designed to transmit the pre-pulses.

[0021] In a further development of this embodiment, the separation device comprises a base body that transmits the pre-pulses and has an entry surface through which the pre-pulses enter the base body, and a curved exit surface through which the pre-pulses exit the base body. The curved exit surface has a coating that is designed to transmit the pre-pulses and to reflect the interfering radiation in at least one wavelength range that does not include the at least one pre-pulse wavelength, focused back into the radiation generation chamber or focused into the at least one beam trap. In this case, too, the separation device forms a single optical separation element. In this case, the optical separation element is based on a dielectric base body, which consists, for example, of quartz glass or sapphire.The curved exit surface typically has a concave shape to focus the interference radiation. The reflectance of the coating of the exit surface should be as high as possible in the at least one wavelength range that does not include the at least one pre-pulse wavelength, particularly at the main pulse wavelength, while the transmittance should be as high as possible at the at least one pre-pulse wavelength. The coating of the curved exit surface can preferably also be designed as a bandpass filter, which transmits the pre-pulses as well as possible and reflects all other wavelengths as well as possible. The coating of the curved exit surface can comprise a plurality of thin dielectric layers applied to the base body by means of a suitable vapor deposition process, for example by means of a process based on the principle of physical vapor deposition.

[0022] In a further development of this embodiment, the entrance surface and the exit surface are curved such that the optical separation element serves as a focusing lens for the pre-pulses. A concave shape of the exit surface of the optical separation device, considered in itself, leads to a dispersion of the exiting pre-pulses. However, if the entrance surface is convex and more strongly curved than the exit surface, i.e., if the radius of curvature of the entrance surface is smaller than the radius of curvature of the exit surface, the optical separation element as a whole acts as a focusing lens for the pre-pulses. Other configurations of the curvature of the entrance surface or the exit surface are also possible, which result in the optical separation element serving as a focusing lens for the pre-pulses.

[0023] In a further embodiment, the entrance surface has an anti-reflective coating to increase the transmission of the pre-pulses. This prevents back reflections of the pre-pulses upon entering the optical separation element and the associated problems, and increases the efficiency of the EUV light source. The transmittance at at least one pre-pulse wavelength should therefore generally be as high as possible. In this case, the anti-reflective coating can also be implemented, for example, in the form of a layer stack of dielectric thin layers.

[0024] In a further embodiment, the separation device is designed to reflect the interfering radiation back into the radiation generation chamber in a focused manner in at least one wavelength range that does not include the at least one pre-pulse wavelength, wherein a focus of the back-reflected interfering radiation is offset from the target material and, in particular, a focal plane of the back-reflected interfering radiation lies in front of a target plane. This has the advantage that the plasma generation is subsequently not or only slightly influenced by the back-reflected interfering radiation.

[0025] The focal plane of the back-reflected interference radiation, in which the focus of the back-reflected interference radiation lies, and the target plane, in which the target material is irradiated in a focused manner, are planes perpendicular to the beam axis of the pre-pulse laser beam. The further the focal plane of the interference radiation reflected back into the radiation generation chamber is in front of the target plane, the more the back-reflected interference radiation is expanded in the target plane and the smaller the impact on plasma generation. Alternatively or additionally, the focus of the back-reflected interference radiation can not lie on the beam axis of the pre-pulse laser beam, but have a slight angular offset. As a result, the back-reflected electromagnetic interference radiation does not directly hit the target material after its focus.

[0026] The focus of the interfering radiation reflected back into the radiation generation chamber is not necessarily ideal. Rather, it may be advantageous to spatially smear the corresponding power density distribution. In this case, the beams of the interfering radiation reflected back into the vacuum chamber do not intersect at a single point. This can be achieved, for example, by appropriately designing the curved exit surface or the curved deflection mirror.

[0027] In a further embodiment, the at least one pre-pulse wavelength is less than 1.5 µm, and the main pulse wavelength is more than 10 µm. Compared to conventional pre-pulse wavelengths of around 10 µm, the use of correspondingly short pre-pulse wavelengths leads to higher conversion efficiency. This is due, among other things, to the fact that this allows for short pulse durations and sharp focusing of the pre-pulses on the target material, as well as the fact that the pre-pulses exhibit different absorption behavior than the main pulses. For example, the pre-pulse wavelength can be 1030 nm if a Yb:YAG laser serves as the pre-pulse laser source.

[0028] In a further embodiment, the EUV light source comprises a vacuum chamber in which the radiation generation chamber is arranged, wherein the vacuum chamber has a first opening with a first window for the passage of the at least one pre-pulse laser beam and a second opening with a second window for the passage of the main pulse laser beam. The sealing of the vacuum prevailing in the radiation generation chamber from the environment typically occurs not at one or more openings in the radiation generation chamber itself, but at a vacuum chamber in which the radiation generation chamber is arranged. This makes it possible to create a sufficient distance between the windows and the target material, so that contamination of the windows by the target material can be avoided or at least greatly reduced.Due to the different pre-pulse wavelength(s) and the main pulse wavelength, two windows are typically required for vacuum sealing. These windows are made of a different material that is transmissive for the pre-pulse wavelength(s) and the main pulse wavelength, respectively. With the aid of the separation device described above, optical elements located in the section of the beam path of the pre-pulse beam guiding device that is located within the vacuum chamber (i.e., in the vacuum) can also be protected from interference radiation. Further features and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. drawing

[0029] Examples of embodiments are shown in the schematic drawing and are explained in the following description. Fig. 1 a schematic representation of an EUV light source with a pre-pulse beam guiding device, which has a separation device in order to reflect interference radiation entering the pre-pulse beam guiding device from a radiation generation chamber back into the radiation generation chamber in a focused manner, Fig. 2 a simplified schematic detailed view of the in Fig. 1 shown EUV light source, in which the separation device is designed to reflect a pre-pulse laser beam and Fig. 3 a schematic detailed view of an EUV light source with a separation device which is designed to transmit a pre-pulse laser beam.

[0030] In Fig. 1 is an EUV light source 1 which shows a radiation generating chamber 2, a provisioning facility 3 to provide a target material 4, a pre-pulse laser source 5, a main pulse laser source6, a pre-pulse beam guidance device 7 and a main pulse beam guidance device 8 The target material 4 is tin, which is provided in the form of droplets 4 by the provision device 3 within the radiation generation chamber 2. The droplets 4 emerging from the provision device 3 move within the radiation generation chamber 2 along a predetermined, approximately rectilinear trajectory. In principle, however, the target material 4 can also be a different material, for example, a different metal. Furthermore, the provision by the provision device 3 does not have to be in droplet form.

[0031] The pre-pulse laser source 5 emits a subsequently as pre-pulse laser beam 9 referred to as pulsed laser beam, while the main pulse laser source 6 produces a hereinafter referred to as main pulse laser beam 10The laser pulses of the pre-pulse laser beam 9 are called pre-pulses 11, the laser pulses of the main pulse laser beam 10 as main pulses 12 The main pulse laser source 6 shown is a CO2 laser. The wavelength of the main pulse laser beam 10, referred to as the skin pulse wavelength λH, is approximately 10.6 µm. However, the main pulse laser source 6 can also be a different laser. The pre-pulse laser source 5 shown is a Yb:YAG laser. The wavelength of the pre-pulse laser beam 9, referred to as the pre-pulse wavelength λV, is 1030 nm. However, the pre-pulse laser source 5 can also be a different laser, for example a different solid-state laser, a fiber laser, etc.

[0032] The pre-pulse laser source 5 can also have two or possibly more than two laser sources, each designed to generate a pre-pulse laser beam 9. In this case, the pre-pulse laser beams are combined in the pre-pulse laser source 5. The pre-pulse laser beams 9 generated by the pre-pulse laser source 5 can have the same pre-pulse wavelength λ V , but it is also possible for the pre-pulse laser beams to have different pre-pulse wavelengths λ V , which generally differ only slightly so that they can be guided using the common pre-pulse beam guiding device 7.

[0033] By means of the pre-pulse beam guiding device 7, the pre-pulse laser beam 9 is fed from the pre-pulse laser source 5 to the radiation generation chamber 2. The pre-pulse laser beam 9 passes through a first opening 13 in a vacuum chamber 14,in which the radiation generation chamber 2 is arranged. Furthermore, the pre-pulse laser beam 9 is focused onto the tin droplets 4 by means of the pre-pulse beam guiding device 7. For these purposes, the pre-pulse beam guiding device 7 shown comprises several optical elements in the form of lenses and mirrors. However, the pre-pulse beam guiding device 7 can also comprise other transmissive and / or reflective optical elements for these purposes.

[0034] By means of the main pulse beam guiding device 8, the main pulse laser beam 10 is fed from the main pulse laser source 6 to the beam guiding chamber 2. The main pulse laser beam 10 passes through a second opening 15in the vacuum chamber 14. Furthermore, the main pulse laser beam 10 is focused onto the target material 4, more precisely onto one of the tin droplets, by means of the main pulse beam guiding device 8. For this purpose, the main pulse beam guiding device 8 shown comprises several optical elements in the form of mirrors. Alternatively, the main pulse beam guiding device 8 may also comprise other reflective and / or transmissive optical elements.

[0035] Particularly within the vacuum chamber 14, the pre-pulse beam guiding device 7 and the main pulse beam guiding device 8 can also coincide, at least in sections. The pre-pulse laser beam 9 and the main pulse laser beam 10 are then guided in a respective section via the same optical elements and form a common beam guiding device. In the example shown, the pre-pulse laser beam 9 and the main pulse laser beam 10 are guided via separate openings. 16, 16'into the radiation generation chamber 2. However, it is understood that the pre-pulse laser beam 9 and the main pulse laser beam 10 can alternatively be guided into the radiation generation chamber 2 via a common opening.

[0036] Since the pressure inside the vacuum chamber 14 is typically significantly lower than outside the vacuum chamber 14, the openings 13, 15 are sealed gas-tight, in the example shown by means of a window each 17, 18. The first window 17 is made of quartz glass and exhibits high transmission at the pre-pulse wavelength λ V . The second window 18 is a diamond window, which exhibits high transmission at the main pulse wavelength λ H . However, the two windows 17, 18 can also be made of other materials. The openings 16, 16' through which the pre-pulse laser beam 9 and the main pulse laser beam 10 enter the radiation generation chamber 2, respectively, do not have windows.

[0037] The tin droplets are first irradiated with two pre-pulses 11 and then with a focused main pulse 12. The respective focus position of the pre-pulses 11 or the main pulse 12 during focused irradiation does not necessarily correspond to the position of the tin droplet. Rather, the respective focus position of the pre-pulses 11 or the main pulse 12 can be located in front of or behind the tin droplet in the beam path. Compared to irradiation with only one main pulse 12, this achieves a higher conversion efficiency. As a result of the irradiation, the tin droplets are converted into the plasma state, and EUV radiation is emitted. 19, which is used by a collector mirror 20which is arranged in the radiation generation chamber 2. The radiation generation chamber 2 serves as a holder or support frame for the collector mirror 20. The collector mirror 20 has an opening through which the pre-pulses 11 and the main pulse 12 pass. The provision device 3 is attached to the radiation generation chamber 2 and serves to provide the target material 4 within the radiation generation chamber 2.

[0038] During irradiation, a portion 21 The main pulses 12 are scattered back by the tin droplets and reach the pre-pulse beam guiding device 7 as interference radiation 21. The tin droplets can essentially be considered as a point light source. The pre-pulse beam guiding device 7 has a separation device 22to reflect the interference radiation 21 in the form of the backscattered portion of the main pulses 12 back into the radiation generation chamber 2 in a focused manner. The interference radiation 21' reflected back into the radiation generation chamber 2 is absorbed within the radiation generation chamber 2. This serves to avoid adverse effects, in particular thermal effects, for example thermal lens effects, which the interference radiation 21 can cause in the pre-pulse beam guiding device 7. For the pre-pulses 11, the Fig. 1 The separation device 22 shown here serves as a deflecting mirror. Between the separation device 22 and the radiation generation chamber 2, there is also an additional deflecting mirror, which is not specifically provided with a reference numeral. Alternatively, one or more other optical elements, or no additional optical element, may be located between the separation device 22 and the radiation generation chamber 2.

[0039] Alternatively, the separation device 22, as also shown in Fig. 1 shown, be designed to focus the interference radiation 21 into a beam trap 24 to reflect or, unlike in Fig. 1 shown to reflect focused into more than one beam trap 24.

[0040] The interference radiation 21 is not limited to the backscattered portion 21 of the main pulses 12. For example, the irradiation of the target material 4 results in the emission of radiation 25which is typically very broadband and also enters the pre-pulse beam guiding device 7 as interfering radiation 21. In this case, too, the target material 4 can essentially be described as a point light source. By means of the separation device 22, the interfering radiation 21 can also be focused back into the vacuum chamber 2 or reflected in a focused manner into at least one beam trap 24 in more than one wavelength range, which does not include the at least one pre-pulse wavelength λ V .

[0041] In the example shown, the additional deflection mirror serves to absorb the radiation 25 emitted by the target material 4 as a result of the irradiation, which enters the pre-pulse beam guidance device 7 as interfering radiation 21, primarily in the UV / VIS range. The additional deflection mirror is suitably cooled for this purpose. Alternatively, the additional deflection mirror can transmit the interfering radiation 21 so that it can be absorbed by a suitably arranged beam trap.

[0042] In addition to the separation device 22, further separation devices 22 can also be arranged in the pre-pulse beam guiding device 7, for example in order to reflect the interference radiation 21 in different wavelength ranges back into the radiation generation chamber 2 or in a focused manner into at least one beam trap 24.

[0043] Alternatively, the additional deflecting mirror can also absorb the interfering radiation 21 as completely as possible, while reflecting the pre-pulses 11 as completely as possible. For this purpose, the additional deflecting mirror can have a coating that acts as a bandpass filter and is highly reflective only for the pre-pulse wavelength λ V, while the coating transmits or absorbs all other wavelengths. If the substrate material of the additional deflecting mirror is absorbent for wavelengths deviating from the pre-pulse wavelength λ V, the interfering radiation 21 will be absorbed—with appropriate cooling of the additional deflecting mirror. The back of the additional deflecting mirror can also have an absorption layer, which, with appropriate cooling of the additional deflecting mirror, can also be used as a "beam sump."A disadvantage of this solution is a thermal deformation of the additional deflection mirror caused by the absorption and a consequent impairment of the quality of the pre-pulse beam guidance device 7.

[0044] In Fig. 2 is the Fig. 1 The EUV light source 1 shown is shown in a detailed view. For simplification, the additional deflection mirror between the separation device 22 and the first opening 13 in the radiation generation chamber 2 is not shown.

[0045] In the example shown, the separation device is a single optical separation element 22. The optical separation element 22 has a dielectric base body 25 which in the example shown is made of quartz glass. A coating 26 in the form of a layer stack of dielectric layers on a front side 27of the optical separation element 22 serves as a dichroic mirror 26. A coating 28 in the form of a metal layer on a back 29 of the optical separation element 28 serves as a curved deflection mirror 28. The coatings 26, 28 do not have to be realized as a layer stack of dielectric layers or as a thin metal layer.

[0046] The dichroic mirror 26 serves as a deflection mirror for reflecting the pre-pulses 11. Furthermore, the interfering radiation 21 entering the pre-pulse beam guiding device 7 from the radiation generation chamber 2 is transmitted by the dichroic mirror 26 in at least one wavelength range that does not include the pre-pulse wavelength λ V . The transmitted interfering radiation 21 strikes the curved deflection mirror 28 and is reflected back into the radiation generation chamber 2 by the dichroic mirror 26, focused.

[0047] Unlike in Fig. 2 As shown, the interfering radiation 21' reflected by the curved deflection mirror 28 does not have to pass through the dichroic mirror 26 again. The transmitted interfering radiation 21 can also be reflected by the curved deflection mirror 28 onto at least one beam trap 24. Furthermore, the separation device 22 does not have to be a single optical separation element 22. Rather, the separation device 22 can also consist of several optical elements, among which its function is distributed. For example, the dichroic mirror 26 and the curved deflection mirror 28 can be separate optical elements.

[0048] In Fig. 3 an EUV light source 1 is shown in a detailed view, which essentially corresponds to the one already shown in Fig. 1 EUV light source 1 shown. Deviating from the Fig. 1The EUV light source 1 shown here has a pre-pulse beam guiding device 7 with a separation device 22 which is designed to transmit the pre-pulses 11.

[0049] In the example shown, the separation device is a single optical separation element 22. The optical separation element 22 has a dielectric base body 25, which in the example shown is made of quartz glass. The pre-pulses 11 pass through an entrance surface 30 into the optical separation element 22 and through a curved exit surface 31 The curved exit surface 31 has a coating 32which, in the example shown, is, but not necessarily, a layer stack of dielectric layers of two materials with different refractive indices. The coating 32 of the curved exit surface 31 is designed to transmit the pre-pulses 11 and to reflect the interference radiation 21 in a focused manner back into the radiation generation chamber 2 in at least one wavelength range that does not include the pre-pulse wavelength λ V. In particular, the coating 32 of the curved exit surface 31 can be designed such that, in addition to the main pulse wavelength λ H , other wavelength ranges, for example in the UV / VIS range, which do not include the pre-pulse wavelength λ V , are also reflected.

[0050] The curvature of the entrance surface 30 of the Fig. 3The optical separation element 22 shown is adapted to the curvature of the exit surface 32 of the optical separation element 22 such that the optical separation element 22 serves as a focusing lens for the pre-pulses 11. For this purpose, in the example shown, the entrance surface 30 is convex and more curved than the exit surface 31. The entrance surface 30 of the optical separation element 22 also has an anti-reflective coating 33 to increase the transmittance at the pre-pulse wavelength λ V.

[0051] Deviating from the representation in Fig. 3 The optical separation element 22 does not need to serve as a focusing lens for the pre-pulses 11, and the entrance surface does not need to have an anti-reflection coating 33. The separation device is not necessarily a single optical separation element 22, but can also comprise multiple optical elements.

[0052] In Fig. 1-3 is a focus34 of the interference radiation 21' reflected back into the radiation generation chamber 2 is offset from the target material 4. More precisely, a focal plane 35 the interference radiation 21' reflected back into the radiation generation chamber 2 clearly in front of a target plane 36,in which the target material 4 is provided. Both the pre-pulses 11 or the pre-pulse laser beam 9 and the main pulses 12 or the main pulse laser beam 10 are focused on the target plane 36. Due to the offset between the focal plane 34 of the back-reflected interference radiation 21' and the target plane 36, the back-reflected interference radiation 21' is expanded so strongly in the target plane 36 that the plasma generation is not affected. Deviating from this, the focus 34 of the back-reflected interference radiation 21' may not lie on the beam axis of the pre-pulse laser beam 9 or the pre-pulses 11, but may be arranged laterally offset to the beam axis, ie the beam axis of the back-reflected interference radiation 21' may be aligned at a (small) angle to the beam axis of the pre-pulse laser beam 9 in order to prevent the back-reflected interference radiation 21' from directly hitting the location of the irradiation of the target material 4 in the radiation generation chamber 2.

Claims

1. An EUV light source (1), comprising: - a provisioning device (3) for providing a target material (4), - a pre-pulse laser source (5) for emitting at least one pre-pulse laser beam (9) at at least one pre-pulse wavelength (λν), - a main pulse laser source (6) for emitting a main pulse laser beam (10) at a main pulse wavelength (λH) that differs from the at least one pre-pulse wavelength (λν), - a pre-pulse beam guidance device (7) for supplying the at least one pre-pulse laser beam (9) from the pre-pulse laser source (5) into a radiation generation chamber (2) and for the focused irradiation of the target material (4) within the radiation generation chamber (2) with in each case at least one pre-pulse (11) of the at least one pre-pulse laser beam (9) and - a main pulse beam guidance device (8) for supplying the main pulse laser beam (10) from the main pulse laser source (6) to the radiation generation chamber (2) and for irradiating the target material (4) within the radiation generation chamber (2) in a focused manner with a main pulse (12) of the main pulse laser beam (10) in each case, wherein the target material (4) is formed to emit EUV radiation (19) as a result of the irradiation, wherein the pre-pulse beam guidance device (7) has at least one separation device (22), characterized in that the separation device (22) is formed to reflect interference radiation (21) entering from the radiation generation chamber (2) into the pre-pulse beam guidance device (7) in at least one wavelength range which does not comprise the at least one pre-pulse wavelength (λν) back into the radiation generation chamber (2) in a focused manner or into at least one beam trap (24) in a focused manner.

2. The EUV light source (1) according to claim 1, characterized in that the separation device (22) is formed to reflect interference radiation (21) in the form of a proportion (21) of the main pulse (12) re-emitted at the target material (4) back into the radiation generation chamber (2) in a focused manner or into the at least one beam trap (24) in a focused manner.

3. The EUV light source (1) according to one of the preceding claims, characterized in that the separation device (22) is formed to reflect interference radiation (21) in the form of radiation (25) emitted as a result of the irradiation of the target material (4) in at least one wavelength range which does not comprise the at least one pre-pulse wavelength (λν), back into the radiation generation chamber (2) in a focused manner or into the at least one beam trap (24) in a focused manner.

4. The EUV light source (1) according to one of the preceding claims, characterized in that the separation device (22) is formed to reflect the pre-pulses (11).

5. The EUV light source (1) according to claim 4, characterized in that the separation device (22) comprises a dichroic mirror (26) and a curved deflection mirror (28), wherein the dichroic mirror (26) serves as a deflection mirror for reflecting the pre-pulses (11) and is formed to transmit the interference radiation (21) in at least one wavelength range which does not comprise the at least one pre-pulse wavelength (λν), and wherein the curved deflection mirror (28) is formed to reflect the interference radiation (21) transmitted by the dichroic mirror (26), preferably through the dichroic mirror (26), back into the radiation generation chamber (2) in a focused manner or into the at least one beam trap (24) in a focused manner.

6. The EUV light source (1) according to claim 5, characterized in that the separation device (22) has a base body (25) which transmits the interference radiation (21), wherein the dichroic mirror (26) is provided as a coating (26) on a front side (27) of the base body (25) and the deflection mirror (28) is formed as a coating (28) on a rear side (29) of the base body (25).

7. The EUV light source (1) according to one of claims 1 to 3, characterized in that the separation device (22) is formed to transmit the pre-pulses (11).

8. The EUV light source (1) according to claim 7, characterized in that the separation device (22) comprises a base body (25) which transmits the pre-pulses (11), which has an entrance surface (30), through which the pre-pulses (11) enter the base body (25) and a curved exit surface (31), through which the pre-pulses (11) exit the base body (25), wherein the curved exit surface (31) has a coating (32) which is formed to transmit the pre-pulses (11) and to reflect the interference radiation (21) in at least one wavelength range which does not comprise the at least one pre-pulse wavelength (λν), back into the radiation generation chamber (2) in a focused manner or into the at least one beam trap (24) in a focused manner.

9. The EUV light source (1) according to claim 8, characterized in that the entrance surface (30) and the exit surface (31) are curved in such a way that the optical separation device (22) serves as a focusing lens for the pre-pulses (11).

10. The EUV light source (1) according to claim 8 or 9, characterized in that the entrance surface (30) has an antireflection coating (33) for increasing the transmission of the pre-pulses (11).

11. The EUV light source (1) according to one of the preceding claims, characterized in that the separation device (22) is formed to reflect the interference radiation (21) in at least one wavelength range which does not comprise the at least one pre-pulse wavelength (λν), back into the radiation generation chamber (2) in a focused manner, wherein a focus (34) of the back-reflected interference radiation (21') is offset with respect to the target material (4) and, in particular, with a focal plane (35) of the back-reflected interference radiation (21') being arranged in front of a target plane (36).

12. The EUV light source (1) according to one of the preceding claims, characterized in that the at least one pre-pulse wavelength (λν) is less than 1.5 µm and the main-pulse wavelength (λH) is more than 10 µm.

13. The EUV light source (1) according to one of the preceding claims, characterized by a vacuum chamber (14) in which the radiation generation chamber (2) is arranged, wherein the vacuum chamber (14) has a first opening (13) having a first window (17) for the passage of the at least one pre-pulse laser beam (9) and a second opening (15) having a second window (18) for the passage of the main pulse laser beam (10).