Focusing device having an image plane extending parallel to or coinciding with the target plane

Abstract

The invention relates in particular to a focusing device (10) for producing EUV radiation. The focusing device (10) is configured to irradiate a target material (12) on a target plane (14). The focusing device (10) comprises at least one beam forming element (26, 28) whose optical axis (30, 32) is positioned perpendicular to the target plane (14). This ensures that the image plane of a laser beam (18, 20) guided through at least one of the focusing elements (26, 28) is oriented parallel to the target plane (14). In this way, efficient irradiation of the target material (12) is possible. Perfect imaging without distortion of at least one laser beam (18, 20) can be achieved by offsetting the main axis of said laser beam (18, 20) with respect to the optical axis (30, 32) of the focusing elements (26, 28) through which said laser beam (18, 20) is guided. A third laser beam (50) can be guided by one of the focusing elements (26, 28) for intermediate irradiation of the target material (12).

Description

[Technical field]

[0001] 2. Background of the Invention The present invention relates to a focusing device for focusing at least two laser beams moving within a target area onto a target material for generating EUV (Extreme Ultraviolet) radiation, preferably for lithography, comprising: a) a first focusing element for focusing a first laser beam onto a target material at a first location within a target area; b) a second focusing element for focusing the second laser beam onto the target material at a second location within the target area; and c) a reflective optical element that reflects the EUV radiation generated by the target material; and The present invention relates to a focusing device comprising:

[0002] The invention further relates to an EUV beam generating device comprising such a focusing device.

[0003] Such focusing devices and EUV beam generating devices are known to the applicant, although they may be internal, unpublished prior art.

[0004] Further focusing devices or EUV beam generating devices are known from WO 2015 / 036024 and WO 2015 / 036025.

[0005] A drawback with these focusing or beam generating systems is that each laser beam is focused onto the target material from different directions at different locations within the target area, reducing the efficiency of target illumination.

[0006] Problem to be solved by the invention It is therefore an object of the present invention to provide a focusing device and an EUV beam generating device which allow a particularly efficient irradiation of a target material.

[0007] Description of the invention This object is achieved according to the invention by a focusing device according to claim 1 and by an apparatus for generating an EUV beam according to claim 8. Preferred developments are given in the respective dependent claims.

[0008] Therefore, the above problem is d) the optical axis of the first focusing element and / or the optical axis of the second focusing element is oriented parallel to or coincident with the optical axis of the reflecting optical element, in particular up to ±4°, preferably up to ±2°. The problem is solved by the focusing device mentioned at the beginning, which is characterized in that it has the following features:

[0009] Thereby, the image plane of the first focusing element and / or the image plane of the second focusing element are oriented perpendicular to the optical axis of the reflecting optical element or approximately perpendicular to the optical axis of the reflecting optical element. In particular, the target material can move in a plane that is approximately perpendicular to the optical axis of the reflecting optical element or perpendicular to the optical axis of the reflecting optical element and thus approximately parallel to one of the image planes of the two focusing elements or parallel to this image plane or coincident with this image plane (target plane). By orienting the optical axis of at least one of the focusing optical elements approximately parallel to the optical axis of the reflecting optical element or parallel to the optical axis of the reflecting optical element or coincident with the optical axis of the reflecting optical element, the target material can be illuminated particularly effectively.

[0010] In this case, the reflective optical element is particularly suitable for reflecting EUV radiation emitted when the target material is irradiated with laser radiation, which can be configured, for example, as a near-normal incidence collector mirror having a reflective surface in the form of a spheroid, with the reflective optical element having a first focal point in the vicinity of or within the area where the target material is irradiated with the laser beam(s).

[0011] In other words, the present invention proposes that at least one optical axis, preferably both optical axes, of the two focusing elements are oriented perpendicular or approximately perpendicular to the flight direction or flight trajectory of the target material, where the target material is moved from a first position to a second position in the target area. In this case, the first laser beam can be guided to the target area in a first beam path of the focusing device, and the second laser beam can be guided to the target area in a second beam path of the focusing device. In this case, the main axis of the first beam path and / or the main axis of the second beam path can be offset or tilted parallel to the optical axis of the focusing element. In contrast, in the case of "classical" imaging, the optical axis is coaxial with the main axis of the beam path of the laser beam. For this reason, in order to separate the laser beams positionally in the near field and overlap them in the far field, the two optical axes must be tilted relative to each other.

[0012] In a preferred embodiment, the focusing elements can have parallel or identical optical axes, in particular up to ±4°, preferably up to ±2°, which results in approximately parallel or parallel or coincident image planes for both focusing elements, which allows a particularly effective illumination of the target material.

[0013] The first focusing element can be in the form of an element group. Alternatively or additionally, the second focusing element can be in the form of an element group.

[0014] The main axis of the first beam path preferably extends non-coaxially with respect to the optical axis of the first focusing element, and the main axis of the second beam path preferably extends non-coaxially with respect to the optical axis of the second focusing element.

[0015] The main axes of the beam paths may be collimated and collinear upstream of the focusing element. In a possible alternative configuration, the individual collimated laser beams may not be positioned parallel to one another.

[0016] If one or more of the laser beams are imaged obliquely in the beam path upstream of the respective focusing element (e.g. passing through a collimating lens with an optical axis oriented obliquely with respect to the propagation direction of the laser beam), the focusing element is preferably tilted relative to a plane perpendicular to the main axis of the beam path of the laser beam according to the Scheimpflug principle, so that the image plane is oriented in the plane in which the target material moves. Even if the individual collimated laser beams are not positioned parallel to one another, the focusing element, in particular its main plane, is preferably tilted relative to a plane perpendicular to the main axis of the beam path of the laser beam according to the Scheimpflug principle, so that the focus plane is oriented in the plane in which the target material moves.

[0017] The target material is preferably in the form of tin droplets, which can be irradiated with a laser beam to produce a plasma that emits EUV radiation.

[0018] More preferably, the main axis of the first beam path and / or the main axis of the second beam path are offset, in particular parallel, with respect to the optical axis of the respective focusing element, whereby one or both focusing elements are configured rotationally asymmetric with respect to the respective optical axis, whereby a distortion-free projection in the image plane can be obtained. For example, one or more laser beams with a circular cross section can have a circular projection.

[0019] Particularly preferably, the at least one focusing element is designed in the form of a rotationally symmetrical focusing element section, which allows the focusing device to be constructed in a particularly simple manner and allows the beam path to be calculated relatively easily.

[0020] If the main axes of the beam paths are offset, the moving target material can be illuminated by the two laser beams offset in time. In other words, the moving target material can be offset in time (and therefore in position) by shifting the optical axes of the two focusing optical elements parallel to each other so that each optical axis passes through each target material. The optical axes of the focusing elements are preferably offset from each other by less than 3 mm, in particular by less than 1 mm.

[0021] More preferably, at least one focusing element, in particular the two beamforming elements, can be in the form of a lens or a mirror. In the case of a mirror, in particular an off-axis parabolic mirror or a combination of an off-axis parabolic mirror and an off-axis elliptical mirror can be used.

[0022] The first beam forming element and / or the second beam forming element may also be disposed in a vacuum chamber, which may have a first aperture for allowing the first laser beam to enter the vacuum chamber and / or a second aperture for allowing the second laser beam to enter the vacuum chamber.

[0023] The first and / or second beam path can be provided with deflection mirrors, in particular double mirror arrangements, which provide a significant, preferably complete, reduction of astigmatism (see Seunghyuk Chang: Linear astigmatism of confocal off-axis reflective imaging systems with N-conic mirrors and its elimination; Journal of the Optical Society of America A, Vol.32, No.5 / May 2015, p.852-859). However, in such an arrangement, the imaged laser beam is not guided on the theoretical optical axis that provides astigmatism-free imaging, but is guided offset parallel to the theoretical optical axis at the input side (collimated input beam) in order to obtain the desired effect of an inclined main axis at the output side with an image plane parallel to the target plane. In the substitution model of lens optics, this corresponds to a shift of the main axis of the beam path of the laser beam with respect to the optical axis of the lens. Here, the angle of the main axis of the beam path of the laser beam at the output side corresponds to the tangent of the distance of the main axis of the beam path of the laser beam at the input side divided by the effective focal length EFL.

[0024] The focusing device can have a third beam path for guiding the third laser beam to the target area. On this path, the target material can be irradiated first by the first laser beam, then by the third laser beam and finally by the second laser beam. The third laser beam can be incident on the first focusing element at an angle of more than 2°, in particular more than 4°, relative to the first laser beam. Thereby, the third beam path and thus also the third laser beam have an image plane that extends approximately parallel to the plane in which the target material moves or parallel to this plane or coincident with this plane. The main axis of the third beam path and thus of the third laser beam, in particular after passing through the first focusing element, preferably extends non-coaxially with respect to the optical axis of the first focusing element. Alternatively, the third beam path or another beam path can have a third focusing element, thereby having a positionally separated optical axis with the same boundary conditions as the two first beam paths.

[0025] The object of the present invention is further solved by an EUV beam generating apparatus comprising a vacuum chamber, in which a target material can be introduced to a target area in the vacuum chamber for generating EUV radiation, the EUV beam generating apparatus comprising a focusing device as described in the present specification, a first beam source generating a first laser beam, and a second beam source generating a second laser beam.

[0026] The main axis of each laser beam preferably corresponds to the main axis of each beam path along which each laser beam is guided.

[0027] The laser beams can have different wavelengths. In particular, the first laser beam can have a wavelength of 500 nm to 1200 nm, and the second laser beam can have a wavelength of 8 μm to 12 μm. The first optical element, particularly in the case of the aforementioned wavelengths, is preferably formed in the form of a lens made of quartz glass, borosilicate crown glass, sapphire glass, or in the form of a mirror made of aluminum, silicon carbide, or copper. The second optical element, particularly in the case of the aforementioned wavelengths, is preferably formed in the form of a lens made of zinc selenide or (artificial) diamond, or in the form of a mirror made of copper. The materials mentioned here have particularly good optical properties while having good processability and good cooling properties.

[0028] In a particularly preferred configuration of the invention, the first laser beam and / or the second laser beam are pulsed, which allows, inter alia, for the energy introduced into the target material and thus the power of the emitted EUV radiation to be particularly high.

[0029] The EUV beam generating device may include a third beam source generating a third laser beam, where a main axis of the third laser beam preferably corresponds to a main axis of the third beam path.

[0030] The third laser beam can also be pulsed and can be applied onto the target material to generate intermediate pulses between the pulses from the first laser beam and the pulses from the second laser beam.

[0031] Further advantages of the invention emerge from the description and the drawings. Likewise, the above and following features can be used according to the invention either individually or in any combination. The illustrated and described embodiments should not be understood as a restrictive enumeration, but rather as illustrative examples of the invention. [Brief description of the drawings]

[0032] [Figure 1] FIG. 1 shows a schematic diagram of a focusing device according to the prior art; [Diagram 2] 1 shows a schematic diagram of a first embodiment of a focusing device according to the present invention; [Diagram 3] 1 is a schematic diagram illustrating an embodiment of an EUV beam generating apparatus according to the present invention;

[0033] Detailed description of the invention and drawings 1 shows a prior art focusing apparatus 10. This is prior art known to applicant, but not necessarily publicly available.

[0034] The focusing device 10 comprises a target material 12 moving in a flight direction 16 in a target plane 14. The target material 12 is irradiated by a first laser beam 18 and a second laser beam 20. By irradiating the target material 12, EUV radiation is generated, which is reflected by a reflecting optical element 70. The reflecting optical element 70 has an optical axis 72 on which in particular a first focus and a second focus are located. In particular, the target material 12 is irradiated such that EUV radiation is generated at the first focus and is focused by the reflecting optical element 70 to the second focus. The target material moves in the target plane 14, which preferably extends perpendicularly to the optical axis 72 of the reflecting optical element 70. The first laser beam 18 is guided in a first beam path 22 and the second laser beam 20 is guided in a second beam path 24. A first focusing element 26 is arranged in the first beam path 22. A second focusing element 28 is arranged in the second beam path 24. The focusing elements 26, 28 are each embodied in the form of a focusing optical element. The focusing elements 26, 28 can each have a different focal length.

[0035] The focusing elements 26, 28 have optical axes 30, 32 which extend coaxially with respect to the main axes of the beam paths 22, 24 of the laser beams 18, 20. The laser beams 18, 20 are non-superimposable due to their wavelength and / or polarization. The optical axes 30, 32 are therefore situated at an angle of more than 4° relative to one another and to the optical axis 72 of the reflecting optical element 70. This results in the image planes 34, 36 also being situated at an angle of more than 4° relative to the target plane 14. This results in a distortion of the image 38 of the laser beams 18, 20 on the target material 12 (when viewed in the plane of the flight direction). When the target material 12 is observed in the line of sight 40, if the target material 12 is not spherical but deformed, the laser beams 18, 20, which have, for example, a circular cross section, are imaged as an ellipse. The deformations in this case are caused in particular by the irradiation. This results in an ineffective irradiation of the target material 12.

[0036] A first embodiment of a focusing device 10 according to the invention is shown in Figure 2. The structure of the focusing device 10 corresponds to the structure of the known focusing device 10 of Figure 1, except for the differences discussed below, so that in order to avoid repetition, reference is made to Figure 1 for such structure.

[0037] As can be seen from Fig. 2, the optical axes 30, 32 lie parallel to the optical axis 72 of the reflecting optical element 70. Alternatively, only one of the two optical axes 30, 32 may lie parallel to the optical axis 72 of the reflecting optical element 70. This ensures that the image planes 34, 36 of the laser beam(s) 18, 20 extend parallel to or coincident with the plane containing the flight trajectory of the target material. The image 38 is distortion-free and therefore allows for effective illumination.

[0038] Furthermore, in this embodiment, the main axes 42, 44 of the beam paths 22, 24, along which the laser beams 18, 20 are guided after passing through the focusing elements 26, 28, are offset with respect to the optical axes 30, 32. Alternatively, only one of the main axes 42, 44 of the beam paths 22, 24 may extend offset with respect to the respective optical axis 30, 32. The focusing elements 26, 28 are preferably rotationally symmetric with respect to their respective optical axes 30, 32, but rotationally asymmetric with respect to the main axes 42, 44, as shown by dashed lines in FIG. 2. Alternatively, only one of the focusing elements 26, 28 may preferably be rotationally symmetric with respect to the respective optical axis 30, 32, and rotationally asymmetric with respect to the main axes 42, 44.

[0039] The focusing elements 26, 28 are preferably formed in the form of focusing optics, in particular as a focusing lens or a focusing mirror, respectively. Alternatively, only one of the focusing elements 26, 28 can be preferably formed in the form of a focusing optics, in particular as a focusing lens or a focusing mirror.

[0040] In one or both beam paths 22, 24, deflection mirrors 46, 48 can be arranged, which are only diagrammatically shown in Fig. 2. According to the invention, such deflection mirrors 46, 48 do not affect the optical image. The deflection mirrors 46, 48 can also be provided at least in pairs, i.e. in the form of at least one double mirror arrangement each (not shown).

[0041] In the embodiment according to Fig. 2, the target material 12 is irradiated by a third laser beam 50 guided on a third beam path 52 with a main axis 54. The third beam path 52 passes through the first beam-forming element 26. The main axis 54 of the third beam path 52 has an angle of more than 2°, in particular more than 4°, relative to the main axis 42 of the first beam path 22.

[0042] The target material 12 may be partially vaporized by the laser beams 18, 20, 50. For this reason, movement of the target material 12 can often only be performed substantially within the target plane 14. However, deviations of the flight trajectory of the target material 12 from the target plane 14 are shown in a schematic and exaggerated manner in the figures.

[0043] In Fig. 3 a second embodiment of the focusing device 10 according to the invention is shown as part of an EUV beam generating device 55. The structure of the focusing device 10 corresponds to the known structure of the focusing device 10 of Fig. 2 except for the differences discussed below, so that in order to avoid repetition reference is made to Fig. 2 for such structure.

[0044] The first laser beam 18 and the optional third laser beam 50 shown in Fig. 3 are incident on the beam forming element 26 at an angle of more than 2°, in particular more than 4°, relative to the optical axis 30 of the first focusing element 26. Here, the main axis 42 of the first beam path 22 and the main axis 54 of the optional third beam path 52 intersect with the optical axis 30 of the first focusing element 26 at the first focusing element 26. The main axes 42, 54 are therefore not offset at the first focusing element 26 relative to the optical axis 30 of the first focusing element 26. This results in a slight distortion of the image 38 (and coma in the far field). However, even in this embodiment, the beam paths 22, 52 have a common or parallel image plane according to the invention in a plane containing the flight trajectory of the target material.

[0045] FIG. 3 shows generally a first beam source 56 for generating a first laser beam 18, a second beam source 58 for generating a second laser beam 20, and an optional third beam source 60 for generating a third laser beam 50.

[0046] 3 further shows that the target material 12 is introduced into a target area 62 of a vacuum chamber 64. The vacuum chamber 64 may have a first aperture 66 for the first laser beam 18 or first beam path 22 to enter. An optional third laser beam 50 or third beam path 52 may also enter the vacuum chamber 64 through the first aperture 66. Alternatively or additionally, the vacuum chamber 64 may have a second aperture 68 for the second laser beam 20 or second beam path 24 to enter.

[0047] Summarizing with reference to all figures of the drawing together, the invention relates in particular to a focusing device 10 for producing EUV radiation. The focusing device 10 is configured for irradiating a target material 12 on a target plane 14. The focusing device 10 has at least one beam-forming element 26, 28, whose optical axis 30, 32 is located perpendicular to the target plane 14. This ensures that an image plane is oriented parallel to the target plane 14 by a laser beam 18, 20 guided through one of the focusing elements 26, 28. This allows for an efficient irradiation of the target material 12. A perfect imaging without distortion of the at least one laser beam 18, 20 can be achieved by offsetting the main axis of the beam path of said laser beam 18, 20 with respect to the optical axis 30, 32 of the focusing element 26, 28 through which said laser beam 18, 20 is guided. A third laser beam 50 can be guided by one of the focusing elements 26, 28 for intermediate illumination of the target material 12. [Explanation of symbols]

[0048] 10 Focusing device 12 Target material 14 Target Plane 16 Flight Direction 18 First Laser Beam 20 Second Laser Beam 22 First Beam Path 24 Second Beam Path 26 First focusing element 28 Second focusing element 30 Optical axis of the first focusing element 26 32 Optical axis of second focusing element 28 34 Focusing plane of first focusing element 26 36 focusing plane of second focusing element 28 38 statue 40 View direction 42 main axis of first beam path 22 44 Main axis of second beam path 24 46 Deflection Mirror 48 Deflection Mirror 50 Third Laser Beam 52 Third Beam Path 54 Main axis of third beam path 52 55 EUV Beam Generator 56 First Beam Source 58 Second Beam Source 60 Third Beam Source 62 Target area 64 Vacuum Chamber 66 First aperture of vacuum chamber 62 68 Second aperture of vacuum chamber 62 70 Reflective Optical Elements 72 Optical axis of reflecting optical element 70

Claims

1. A focusing device (10) for focusing at least two laser beams onto a target material (12) for generating EUV radiation, which moves within a target area (62), comprising: a) a first focusing element (26) for focusing a first laser beam (18) onto the target material (12) at a first position within the target area (62); b) a second focusing element (28) for focusing a second laser beam (20) onto the target material (12) at a second position within the target area (62); c) a reflective optical element (70) for reflecting the EUV radiation generated by the target material (12); wherein, in the focusing device (10), d) the optical axis (30) of the first focusing element (26) and / or the optical axis (32) of the second focusing element (28) is / are oriented substantially parallel to or coincident with the optical axis (72) of the reflective optical element (70). The focusing device (10) is characterized by this.

2. The focusing device is configured such that e) the first laser beam (18) is guided between the first focusing element (26) and the target area (62) along a first main axis (42) offset and / or inclined with respect to the optical axis (30) of the first focusing element (26), and / or f) the second laser beam (20) is guided between the second focusing element (28) and the target area (62) along a second main axis (44) offset and / or inclined with respect to the optical axis (32) of the second focusing element (28). The focusing device according to claim 1, which is configured as such.

3. The first beam-forming element (26) is formed in the form of a section of a focusing element configured rotationally symmetrically with respect to its own optical axis (30), and / or The second beam-forming element (28) is formed in the form of a section of a focusing element formed rotationally symmetrically with respect to its own optical axis (32). The focusing device according to claim 2.

4. The optical axis (30) of the first focusing element (26) and the optical axis (32) of the second focusing element (28) are oriented substantially parallel to the optical axis (72) of the reflective optical element (70) or parallel to the optical axis (72) of the reflective optical element (70) or coincident with the optical axis (72) of the reflective optical element (70), the optical axis (30) of the first focusing element (26) and the optical axis (32) of the second focusing element (28) are offset from each other by less than 3 mm, The focusing device according to any one of claims 1 to 3.

5. The first beam forming element (26) is formed in the form of a lens or a mirror, and / or The second beam forming element (28) is formed in the form of a lens or a mirror, The focusing device according to any one of claims 1 to 3.

6. A double mirror device is arranged downstream of the first focusing element (26) when viewed in the beam direction, and / or A double mirror device is arranged downstream of the second focusing element (28) when viewed in the beam direction, The focusing device according to any one of claims 1 to 3.

7. The focusing device is configured to guide a third laser beam (50) from the first beam forming element (26) to the target area (62) along a third principal axis (54) that forms an acute angle greater than 2°, particularly greater than 4°, with respect to the first principal axis (42) of the first laser beam (18). The focusing device according to any one of claims 1 to 3.

8. An EUV beam generating device (55) comprising a vacuum chamber (64), in which a target material (12) can be introduced to generate EUV radiation into a target area (62) within the vacuum chamber (64), The EUV beam generating device (55) includes the focusing device according to any one of claims 1 to 3 for focusing at least two laser beams onto the target material (12) moving within the target area (62), a first beam source (56) for generating a first laser beam (18), and a second beam source (58) for generating a second laser beam (20). EUV beam generating device (55).

9. The first laser beam (18) has a first wavelength, and the second laser beam (20) has a second wavelength different from the first wavelength. The EUV beam generating apparatus according to claim 8.

10. The first wavelength is 500 nm to 1200 nm, and the second wavelength is 8 μm to 12 μm. The EUV beam generating apparatus according to claim 9.

11. The first laser beam (18) is pulsed and / or the second laser beam (20) is pulsed. The EUV beam generating apparatus according to claim 8.

12. The EUV beam generating apparatus according to claim 8, comprising a third beam source (60) for generating a third laser beam (50), and the focusing apparatus according to claim 8.

13. The third laser beam (50) is pulsed and / or has a wavelength different from that of the second laser beam (20) and / or has a polarization different from that of the second laser beam (20). The EUV beam generating apparatus according to claim 12.

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