Device and method for amplifying laser radiation
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- TRUMPF LASERSYSTEMS FOR SEMICONDUCTOR MANUFACTURING SE
- Filing Date
- 2026-01-21
- Publication Date
- 2026-07-30
Smart Images
Figure EP2026051484_30072026_PF_FP_ABST
Abstract
Description
[0001] Device and method for amplifying laser radiation
[0002] The present invention relates to a device for amplifying laser radiation, comprising a hollow cylindrical cavity filled with a gaseous laser medium and having two annular end faces on which mirror elements for reflecting a laser beam are arranged such that the laser beam is reflected and thereby deflected in a circumferential direction of the hollow cylindrical cavity, so that the laser beam passes through the hollow cylindrical cavity several times in the area between the two end faces and is thereby amplified.Furthermore, the invention relates to a method for operating a device for amplifying laser radiation, comprising a hollow cylindrical cavity filled with a gaseous laser medium and having two annular end faces on which mirror elements for reflecting a laser beam are arranged such that the laser beam is reflected and thereby deflected in a circumferential direction of the hollow cylindrical cavity, so that the laser beam passes through the hollow cylindrical cavity several times in the area between the two end faces and is thereby amplified.
[0003] Such devices for amplifying laser radiation are also called optical amplifiers and are used in the provision of laser radiation for extreme ultraviolet lithography (EUV lithography).
[0004] In such devices, the laser beam to be amplified is guided through a hollow cylindrical cavity containing a gaseous laser medium. The laser beam can be introduced into the cavity at one of the two annular end faces. The laser beam is reflected by a mirror element on the opposite end face and deflected in the circumferential direction, also known as the azimuthal direction. The reflected laser beam is then reflected by another mirror element on the first end face and again deflected in the circumferential direction. This design offers the advantage that the laser beam is guided multiple times along the entire length of the hollow cylindrical cavity, so that the effective length over which the laser beam travels through the laser medium is many times greater than the length of the cavity.Since the laser beam is deflected circumferentially with each reflection, it travels further along the circumference with each reflection. This allows the hollow cylindrical space of the cavity to be used efficiently and high laser beam amplifications to be achieved.
[0005] A disadvantage of such devices for amplifying laser radiation is that unwanted reflections of the forward-incident radiation can occur, with the radiation being reflected back essentially parallel to the incident radiation. This back radiation can lead to stable oscillations of the laser radiation between the end faces of the hollow cylindrical cavity. This effect, also known as self-lasing, can result in an undesirable amount of energy from the cavity being converted into parasitic optical radiation. This can reduce the maximum amplification of the forward-propagating laser beam.
[0006] Against this background, the task arises to increase the efficiency of a device for amplifying laser radiation of the type mentioned above.
[0007] To solve the problem, a device for amplifying laser radiation is proposed, comprising a hollow cylindrical cavity filled with a gaseous laser medium and having two annular end faces on which mirror elements for reflecting a laser beam are arranged such that the laser beam is reflected and thereby deflected in a circumferential direction of the hollow cylindrical cavity, so that the laser beam passes through the hollow cylindrical cavity multiple times in the region between the two end faces and is thereby amplified, wherein a first aperture device with several recesses is arranged within the hollow cylindrical cavity, in particular arranged such that the laser beam enters in a forward direction through one of the recesses into a region between the aperture device and the mirror elements.The light can be reflected off one of the mirror elements, deflected in the circumferential direction, and then exit through the same opening.
[0008] In the device according to the invention, an aperture device is provided within the hollow cylindrical cavity. This aperture device has several recesses through which the laser beam can enter in the forward direction, be reflected by the mirror element, and then, deflected circumferentially, be reflected back through the recess. The aperture device can reduce unwanted reflections in the backward direction and thus prevent stable oscillations from occurring between mirror elements on opposite end faces of the cavity. Backscattered laser radiation that is not parallel to the optical axis of the forward-propagating laser beam can also be effectively suppressed. The device according to the invention avoids the amplification of unwanted laser radiation in the cavity and increases the efficiency of the device. The amplifier power, or...The excited laser medium can be used to amplify the forward beam, thus enabling higher power output.
[0009] According to an advantageous embodiment of the invention, the first aperture device is arranged in the region of a first of the two end faces in front of the mirror elements. Such an arrangement in the region of the first end face allows the suppression of unwanted back radiation to occur immediately after its reflection at the mirror elements of the first end face. Furthermore, this also allows misaligned laser radiation, i.e., radiation that does not propagate along at least a portion of a conceived propagation path in the hollow cylindrical cavity, to be reflected away from the mirror elements of the first end face, i.e., before any reflection at these mirror elements would occur.
[0010] According to an advantageous embodiment of the invention, a second aperture device with several recesses is arranged in front of the mirror elements within the hollow cylindrical cavity in the region of a second end face opposite the first end face. The second aperture device suppresses unwanted reflections at the second end face in the same way as at the first end face. This further reduces the amplification of unwanted laser radiation within the cavity and increases the efficiency of the device even more.
[0011] Alternatively, the first aperture device can be arranged in a region between 0.4 and 0.6 times the distance between the two end faces, for example, midway between the two end faces. Furthermore, in addition to a first aperture device in the region of the first end face and, if applicable, a second aperture device in the region of the second end face, a third aperture device can be arranged in a region between 0.4 and 0.6 times the distance between the two end faces.
[0012] Preferably, the first and optionally the second aperture device comprises a base body, wherein the recesses are designed as through holes in the base body. The recesses can, for example, be designed as through-holes. The recesses in the aperture device can be identical. The base body is preferably annular in shape so that it can close the annular end face of the cavity.
[0013] Preferably, the recesses in the aperture device are provided in a surface of the base body that is inclined relative to a cylinder axis of the hollow cylindrical cavity at an angle between 25° and 55°, in particular between 30° and 50°, preferably between 35° and 45°, for example at 40°. The inclined surface preferably faces the respective opposite end face of the cavity, i.e., that the inclined surface of the first aperture device faces the second end face and the inclined surface of any second aperture device faces the first end face. The inclined surface of the aperture device can be inclined such that the laser radiation is deflected in a direction away from an outer surface of the hollow cylindrical cavity that bounds it outwards.
[0014] According to an advantageous embodiment of the invention, the first and, optionally, the second aperture device comprises a base body in which the recesses are arranged distributed along a circular path. The circular distribution of the recesses in the base body allows the aperture device to be adapted to the path of the laser beam in the forward direction, where each reflection causes a deflection in the circumferential direction, so that the laser beam strikes the opposite end face with a circumferential offset. This results in the laser beam passing through the recesses of the aperture device multiple times. Unwanted back radiation may also pass through the recesses of the aperture device multiple times and is thereby at least partially suppressed.The arrangement of the cutouts acts like a distributed filter on the unwanted backward radiation, filtering out unwanted radiation.
[0015] According to an advantageous embodiment of the invention, the recesses are oval in shape, with the recesses having a greater extent in the circumferential direction of the hollow cylindrical cavity than in a radial direction of the hollow cylindrical cavity.
[0016] According to an alternative, advantageous embodiment of the invention, the recesses are circular. This circular shape allows unwanted laser radiation to be suppressed essentially uniformly in all directions perpendicular to the optical axis of the laser beam. The circular shape also enables improved adaptation of the recess to the shape of a circular laser beam to be amplified.
[0017] According to an advantageous embodiment of the invention, the laser beam has a beam diameter, and the recesses each have an extent, in particular a diameter, that lies in the range between 1.3 and 2.5 times the beam diameter, for example, between 1.7 and 2.1 times the beam diameter, or further, for example, between 1.9 and 2.0 times the beam diameter. The beam diameter of the laser beam can be determined according to DIN EN ISO 11146-1:2021-11. By selecting the extent of the recesses, in particular the diameter of the recesses, to be larger than the beam diameter by the aforementioned factor, unwanted laser radiation in the rear direction can be effectively reduced without significantly affecting or reducing the amplification of the laser radiation in the forward direction.
[0018] According to an advantageous embodiment of the invention, the recesses each have an extent, in particular a diameter, in the range of 5 mm to 8 mm, preferably between 6 mm and 7 mm. Recesses of this dimension have proven particularly effective in suppressing back radiation from laser beams with a beam diameter in the range of 3 mm to 4 mm. The beam diameter of the laser beam can be determined according to DIN EN ISO 11146-1:2021-11.
[0019] According to an advantageous embodiment of the invention, the device comprises an inner electrode arranged in the region of an inner surface of the hollow cylindrical cavity, and an outer electrode arranged in the region of an outer surface. A field, particularly a high-frequency field, can be provided by means of the electrodes to excite the gaseous laser medium.
[0020] Another aspect of the invention is an EUV lithography system with an excitation laser for generating EUV radiation, wherein the excitation laser includes a device for amplifying laser radiation as described above. In such an EUV lithography system, a target material, for example, a tin droplet, is excited by the laser radiation provided by the excitation laser. Undesired backscattering or reflection of laser radiation can occur at the target material, which is fed back into the excitation laser, in particular into the device for amplifying laser radiation. The laser radiation amplification device according to the invention can significantly reduce this backscattered laser radiation and thereby increase the efficiency of the excitation laser.
[0021] To solve the aforementioned problem, a method for operating a device for amplifying laser radiation is further proposed, comprising a hollow cylindrical cavity filled with a gaseous laser medium and having two annular end faces on which mirror elements for reflecting a laser beam are arranged such that the laser beam is reflected and thereby deflected in a circumferential direction of the hollow cylindrical cavity, so that the laser beam passes through the hollow cylindrical cavity multiple times in the region between the two end faces and is thereby amplified, and wherein a first aperture device with several recesses is arranged within the hollow cylindrical cavity and the laser beam enters in a forward direction through one of the recesses into a region between the aperture device and the mirror elements.is reflected off one of the mirror elements and deflected in the circumferential direction, then exits through the same opening.
[0022] The same advantages can be achieved with the method according to the invention as have been explained in connection with the device according to the invention.
[0023] According to an advantageous embodiment of the method, the first aperture device is arranged in the area of a first of the two end faces in front of the mirror elements.
[0024] According to an advantageous embodiment of the method, it is provided that within the hollow cylindrical cavity, in the area of a second end face opposite the first end face of the two end faces, a second aperture device with several recesses is arranged in front of the mirror elements, and the laser beam enters in the forward direction through one of the recesses into an area between the second aperture device and the mirror elements, is reflected at one of the mirror elements and thereby deflected in the circumferential direction and then exits through the same recess.
[0025] According to an advantageous embodiment of the method, the laser beam has a beam diameter and the recesses have an extent, in particular a diameter, which is in the range between 1.3 times and 2.5 times the beam diameter, for example between 1.7 times and 2.1 times the beam diameter, and further for example between 1.9 times and 2.0 times the beam diameter.
[0026] Alternatively or in addition to the advantageous embodiments explained above, the advantageous embodiments and features explained in connection with the device according to the invention can be used in the method.
[0027] Further details and advantages of the invention will be explained below with reference to the exemplary embodiments shown in the figures. These show:
[0028] Fig. 1 shows a device for amplifying laser radiation according to an embodiment of the invention in a schematic, perspective view;
[0029] Fig. 2 shows a diaphragm device of the device for amplifying laser radiation according to Fig. 1 in a schematic top view;
[0030] Fig. 3 shows a diaphragm device according to a second embodiment of the invention in a schematic, perspective view;
[0031] Fig. 4 shows the aperture device according to Fig. 3 in a side view;
[0032] Fig. 5 shows the aperture device according to Fig. 3 in a sectional view along the section line AA shown in Fig. 4; and
[0033] Fig. 6 shows an embodiment of an EUV lithography system according to the invention in a schematic block diagram.
[0034] The illustration in Fig. 1 shows an embodiment of a device 2 for amplifying laser radiation 11 according to an embodiment of the invention. The device 1 comprises a hollow cylindrical cavity 20 containing a gaseous laser medium. The laser medium can be, for example, a laser-active gas, e.g., consisting of carbon dioxide (CO2), nitrogen (N2), and helium (He). The gaseous laser medium can be excited by radio frequency (RF) signals applied to the laser medium via electrodes 21. Fig. 1 shows an inner electrode 21 arranged in the region of an inner surface of the hollow cylindrical cavity 20. The inner electrode has a substantially cylindrical shape and can define the inner boundary of the cavity 20 in a radial direction. The outer boundary of the cavity 20 is defined by an outer electrode, which is shown in Fig.
[0035] Figure 1 is not shown in order to avoid obstructing the view of the internal structure of the device 2. The outer electrode is arranged in the region of an outer surface of the cavity 20. The outer electrode can have a substantially cylindrical shape and is preferably arranged concentrically to the inner electrode 21. Thus, an electric field, in particular an RF field, can be generated between the inner electrode 21 and the outer electrode to excite the laser medium.
[0036] A laser beam 11, supplied by an external radiation source 1 (shown only in Fig. 6), is introduced into the cavity 20 at a first annular end face 23 of the cavity 20. Within the cavity 20, the laser beam travels substantially parallel to a cylinder axis of the hollow cylindrical cavity 20 and strikes the opposite, second end face 22 of the cavity 20. The end faces 22, 23 each comprise several mirror elements 24, 25. Each of these mirror elements 24, 25 is configured to reflect the incident laser beam and deflect it in a circumferential direction U of the hollow cylindrical cavity 20. Additionally, each of these mirror elements 24, 25 has a concavely curved surface, such that the caustic of the laser beam within the cavity 20 is adapted to the distance between the inner electrode 21 and the outer electrode.In this case, the laser beam is focused approximately midway between end face 22 and end face 23 due to the concavely curved surface. In other words, the waist of the laser beam is located approximately midway between the two end faces 22 and 23 due to the concavely curved surfaces. Within the cavity 20, the laser beam thus travels back and forth multiple times between the first end face 22 and the second end face 23, being deflected by a certain angle in the circumferential direction U each time. The forward-traveling laser beam is labelled 26 in Fig. 1. The positions on one of the two end faces 22, 23, where the reflection of this reciprocating laser beam 26 takes place, lie on a circular path around the cylinder axis of the cavity 20. The path of the laser beam 26 in the longitudinal direction of the cavity thus moves further with each reflection in the circumferential direction U.In the embodiment of device 2, the laser beam can therefore be guided multiple times, in particular between 50 and 60 times, between the two end faces 22, 23. The length of the cavity 20 is preferably in the range of 0.6 m to 1 m, so that an effective length of the laser beam 26 through the cavity 20 in the range of 30 m to 60 m results.
[0037] To increase the efficiency of the device 2, special measures have been taken according to the exemplary embodiment. Within the hollow cylindrical cavity 20, an aperture device 27, 28 with several recesses 29 is arranged in front of the respective mirror elements 24, 25 in the region of both end faces 22, 23. The aperture devices 27, 28 can reduce unwanted reflections in the rear direction and thus prevent stable oscillations from forming between mirror elements 24, 25 on opposite end faces 22, 23 of the cavity 20. Furthermore, the aperture devices 27, 28 also suppress laser radiation backscattered in the rear direction that is not parallel to the optical axis of the forward-propagating laser beam 26.Such backscattered laser radiation can, for example, be laser radiation that has been backscattered from a target material which is excited with the laser radiation to generate EUV light.
[0038] Fig. 2 shows a schematic top view of an aperture device 27, 28, which can be used with the device 2 from Fig. 1. It can be seen that the aperture device 27, 28 has an annular base body 30, which, however, is only provided with a reference numeral in Figs. 3 to 5, in which the recesses 29 are arranged distributed along a circular line. This circular line is selected according to the positions on the corresponding end faces 22, 23 where the laser beam 26 is reflected by the respective mirror elements 24, 25. The recesses 29 are oval and have a greater extent in the circumferential direction U than in the radial direction of the hollow cylindrical cavity 20.
[0039] Figures 3, 4, and 5 show a further, alternative embodiment of an aperture device 27, 28, which can be used with the device 2 from Figure 1. This aperture device 27, 28 also comprises an annular base body 30 in which several recesses 29 are arranged distributed along a circular path. In this embodiment, the recesses 29 are circular and formed as through holes in the base body 30. Furthermore, in this embodiment, all recesses 29 are identical, i.e., they have an identical cross-section. However, it is conceivable that the recesses could have different cross-sections, deviating from this embodiment.
[0040] In the second embodiment, the recesses 29 are arranged in a funnel-shaped region 31 of the aperture device 30, i.e., they are located in a surface of the base body that is inclined relative to the cylinder axis of the hollow cylindrical cavity 20. This inclination has an angle relative to the cylinder axis in the range between 35° and 45°, for example, 40°. Within the cavity 20, the respective aperture devices 27, 28 are preferably arranged such that their inclined surface faces the respective opposite end face 22, 23 of the cavity 20, i.e., that it points towards the center of the cavity 20.
[0041] In this embodiment, the size of the recesses 29 is adapted to the laser beam 11 fed into the device. The laser beam 11 to be amplified has a beam diameter that can be determined according to the standard DIN EN ISO 11146-1:2021-11. The recesses 29 are dimensioned such that they have a diameter in the range between 1.3 and 2.5 times the beam diameter, for example, between 1.7 and 2.1 times the beam diameter, or further for example, between 1.9 and 2.0 times the beam diameter. For example, the laser beam 11 can have a beam diameter in the range between 3.5 mm and 3.6 mm, and the recesses can have a diameter in the range between 5 mm and 8 mm, preferably between 6 mm and 7 mm.
[0042] Figure 6 shows an embodiment of an EU lithography system 101 with an excitation laser 100, comprising a radiation source 1 for providing, in particular pulsed, laser radiation 11 and a device 2 for amplifying the laser radiation 11, as explained in connection with Figures 1 to 5. Although only one radiation source 1 is shown in Figure 6, two radiation sources 1 can be provided, for example, to generate prepulses and main pulses of the laser radiation 11, which follow each other in quick succession. The laser radiation generated by the radiation source 1 is amplified by the device 2 for amplifying the laser radiation 11.
[0043] The laser radiation emitted by the excitation laser 100 is focused onto a target area 40 in a focusing device 3. A target material is arranged in the target area, which emits EUV radiation 42 when irradiated with the laser radiation. According to the exemplary embodiment, the target area 40 is arranged in a target chamber 4 in which vacuum conditions prevail. The target material can, for example, be tin, which is provided in droplet form. The tin droplet can be heated by a prepulse, for example pulsed laser radiation 11 with a wavelength of 1 micrometer, so that the tin droplet expands, vaporizes, ionizes, and / or generates a weak or possibly strong plasma.A main pulse following the pre-pulse, for example pulsed laser radiation 11 with a wavelength of 10.6 micrometers, can convert the substantial part of the material affected by the pre-pulse into the plasma state, thereby generating the EUV radiation 42. This EUV radiation 42 is then supplied to an exposure unit 5 of the lithography system 10, where the EUV radiation 42 can be used to expose semiconductor substrates.
[0044] By means of the exemplary embodiments of the device 2 according to the invention for amplifying laser radiation 11 described above, the amplification of unwanted laser radiation in the cavity 20 can be avoided and the efficiency of the device 2 can be increased. The amplifier power or the excited laser medium can be used to amplify the main beam in the forward direction 26, so that a higher power can be achieved.
[0045] 1 radiation source
[0046] 2 Device for amplifying laser radiation 3 Focusing device
[0047] 4 Target chamber
[0048] 5 Exposure equipment
[0049] 11 Laser radiation
[0050] 20 cavities
[0051] 21 inner electrode
[0052] 22 Front
[0053] 23 Front
[0054] 24 mirror elements
[0055] 25 mirror elements
[0056] 26 Main beam in forward direction
[0057] 27 Aperture device
[0058] 28 Aperture device
[0059] 29 Exclusion
[0060] 30 basic shapes
[0061] 31 funnel-shaped area
[0062] 40 Target area
[0063] 42 EUV radiation
[0064] 100 excitation lasers
[0065] 101 EUV lithography system
[0066] U circumferential direction
[0067] x x-direction
[0068] y y-direction
Claims
Patent claims:
1. Device (2) for amplifying laser radiation (11) comprising a hollow cylindrical cavity (20) filled with a gaseous laser medium and having two annular end faces (22, 23) on which mirror elements (24, 25) for reflecting a laser beam are arranged such that the laser beam is reflected and thereby deflected in a circumferential direction (U) of the hollow cylindrical cavity (20), so that the laser beam passes through the hollow cylindrical cavity (20) multiple times in the area between the two end faces (22, 23) and is thereby amplified, characterized in that a first aperture device (27) with several recesses (29) is arranged within the hollow cylindrical cavity (20), in particular such that the laser beam can enter in a forward direction through one of the recesses (29) into an area between the aperture device (27) and the mirror elements (24), be reflected at one of the mirror elements (24) and thereby be deflected in the circumferential direction (U) and then exit through the same recess (29).
2. Device (2) according to claim 1, characterized in that the first aperture device (27) is arranged in the area of a first of the two end faces (22) in front of the mirror elements (24).
3. Device (2) according to claim 2, characterized in that within the hollow cylindrical cavity (20) in the area of a second end face (23) opposite the first end face (22) of the two end faces (22, 23) a second aperture device (28) with several recesses (29) is arranged in front of the mirror elements (25).
4. Device (2) according to one of the preceding claims, characterized in that the first and optionally second aperture device (27, 28) has a base body (30), in particular annular, in which the recesses (29) are arranged distributed along a circular line.
5. Device (2) according to one of the preceding claims, characterized in that the recesses (29) are circular.
6. Device (2) according to one of the preceding claims, characterized in that the laser beam (11) has a beam diameter and the recesses each have an extent, in particular a diameter, which lies in the range between 1.3 times and 2.5 times the beam diameter, for example between 1.7 times and 2.1 times the beam diameter, further for example between 1.9 times and 2.0 times the beam diameter.
7. Device (2) according to one of the preceding claims, characterized in that the recesses (29) each have an extent, in particular a diameter, which is in the range between 5 mm and 8 mm, preferably between 6 mm and 7 mm.
8. Device (2) according to one of the preceding claims, characterized in that it has an inner electrode (21) which is arranged in the region of an inner surface of the hollow cylindrical cavity (20), and an outer electrode which is arranged in the region of an outer surface.
9. EU V lithography system (101 ) with an excitation laser (100) for exciting EU V radiation (42), characterized in that the excitation laser (100) has a device (2) for amplifying laser radiation according to one of the preceding claims.
10. Method for operating a device (2) for amplifying laser radiation (11), comprising a hollow cylindrical cavity (20) filled with a gaseous laser medium and having two annular end faces (22, 23) on which mirror elements (24, 25) are arranged for reflecting a laser beam, wherein the laser beam is reflected and thereby deflected in a circumferential direction (U) of the hollow cylindrical cavity (20), so that the laser beam passes through the hollow cylindrical cavity (20) multiple times in the area between the two end faces (22, 23) and is thereby amplified, and wherein a first aperture device (27) with several recesses (29) is arranged within the hollow cylindrical cavity (20) and the laser beam enters in a forward direction through one of the recesses (29) into an area between the aperture device (27) and the mirror elements (24), is reflected at one of the mirror elements (24) and is thereby deflected in the circumferential direction (U) and then exits through the same recess (29).
11. Device (2) according to claim 10, characterized in that the first aperture device (27) is arranged in the area of a first of the two end faces (22) in front of the mirror elements (24).
12. Device (2) according to claim 11, characterized in that within the hollow cylindrical cavity (20) in the region of a second end face (23) opposite the first end face (22) of the two end faces (22, 23) a second aperture device (28) with several recesses (29) is arranged in front of the mirror elements (25) and the laser beam enters in the forward direction through one of the recesses (29) into an area between the second aperture device (28) and the mirror elements (25), is reflected at one of the mirror elements (25) and is thereby deflected in the circumferential direction (U) and then exits through the same recess (29).
13. Method according to one of claims 10 to 12, characterized in that the laser beam (11) has a beam diameter and the recesses have an extent, in particular a diameter, which is in the range between 1.3 times and 2.5 times the beam diameter, for example between 1.7 times and 2.1 times the beam diameter, further for example between 1.9 times and 2.0 times the beam diameter.