WO2023088544A1

WO2023088544A1 - Back reflection protection by means of a method and a device for interference of a laser pulse

Info

Publication number: WO2023088544A1
Application number: PCT/EP2021/081909
Authority: WO
Inventors: Guenther Krauss
Original assignee: Trumpf Lasersystems For Semiconductor Manufacturing Gmbh
Priority date: 2021-11-17
Filing date: 2021-11-17
Publication date: 2023-05-25
Also published as: CN118435471A; EP4434125A1; KR20240101950A; TW202325452A; TWI842178B

Abstract

The invention relates to a back reflection protection by means of destructive interference of a returning laser pulse. A forward laser pulse (30) is subdivided, a first forward laser pulse portion (30a) passes, unlike a second forward laser pulse portion (30b), through a first beam path (19). The two laser pulse portions (30a, b) are superimposed at least in part. Subsequently, the forward laser pulse (30) is reflected, and the reflected, returning laser pulse passes mainly through the same path as the forward laser pulse (30), but a returning first laser pulse portion is phase-shifted, when passing through the first beam path (19), in such a way that, on its way to the laser beam source (14), it is destructively superimposed at least in part, in particular in full, with a returning second laser pulse portion, so that the laser beam source (14) is protected against the returning laser pulse.

Description

Back reflection protection by means of a method and device for interfering with a laser pulse

Background of the Invention

The invention relates to a method and a device for irradiating an object with a laser beam.

It is known to irradiate an object with a laser beam, in particular to generate extreme ultraviolet light (EUV radiation). The problem, however, is that reflections, particularly from the object, can damage the laser beam source.

Certain variants of a back-reflection protection are already known, but as a rule these can only be used for laser pulses of limited intensity or only for polarized back-reflections. object of the invention

In contrast, the object of the invention is to provide a method and a device that enable back reflection protection for laser pulses of particularly high intensity and/or to protect against non-polarized back reflections.

Description of the invention

This object is achieved according to the invention by a method according to claim 1 and a device according to claim 11. The dependent claims specify preferred developments.

The object according to the invention is thus achieved by a method for irradiating an object with a laser beam, the method having the following method steps:

A) emitting a leading laser pulse from a laser beam source;

B) splitting the leading laser pulse by a first beam splitter into a first leading and a second leading laser pulse part;

C) traversing a first beam path of the first laser pulse portion;

D) constructively interfering the first leading laser pulse portion and the second leading laser pulse portion at a second beam splitter en route towards the object;

E) Reflecting the preceding laser pulse, in particular on the object, and thereby generating a returning laser pulse;

F) dividing the returning laser pulse at the second beam splitter into a first returning laser pulse part and a second returning laser pulse part;

G) the first returning laser pulse part traversing the first beam path, with the first returning laser pulse part experiencing a different phase shift than the first advancing laser pulse part when traversing the first beam path, in that the advancing laser pulse and the returning laser pulse are at least partially i) passing through an actuating element which is traversal by the returning laser pulse is offset from traversal by the leading laser pulse; and or ii) passing through a frequency shifter (23, 25);

H) destructive interference of the returning first laser pulse portion with the returning second laser pulse portion at the first beam splitter on the way to the laser beam source.

At the first beam splitter, the leading laser pulse is divided into a first and a second leading laser pulse part. The first leading laser pulse part is guided along a first beam path, the second leading laser pulse along a second beam path to a second beam splitter. The first leading part of the laser pulse is deflected along the first beam path by preferably two reflecting optical elements and thus covers a longer distance than the second leading part of the laser pulse. The first and second leading laser pulse parts are superimposed again at the second beam splitter. The distance covered by the first leading laser pulse part along the first beam path differs from the distance covered by the second leading laser pulse along the second beam path, preferably by approximately one or a multiple of approximately one wavelength. The consequence of this is that the first and second leading laser pulse parts interfere constructively at the second beam splitter in the direction of the object and destructively in a second direction, for example in the direction of a beam trap.

After the preceding laser pulse has been reflected by the object, the returning laser pulse first passes through the second beam splitter. At the second beam splitter, the returning laser pulse is divided into a first and a second returning laser pulse part. After passing through the first and second beam paths, the first and second returning laser pulse parts are now superimposed again at the first beam splitter, specifically—without considering the invention—constructively in the direction of the laser beam source and destructively in another direction. In order to isolate the returning laser pulse and thus protect the laser beam source, the first and second returning laser pulse parts must be superimposed destructively on the first beam splitter in the direction of the laser beam source and constructively in another direction, e.g. in the direction of another beam trap. This is achieved in that the first returning laser pulse part experiences a different phase shift when traversing the first beam path than the first leading laser pulse part when traversing the first beam path. If the phase of the first returning laser pulse part is shifted by 180° along the first beam path compared to the phase of the first leading laser pulse part, destructive interference occurs at the first beam splitter in the direction of the laser beam source and constructive interference in the other direction, e.g. in the direction of the other Beam source instead.

So that the first and second returning laser pulse parts are destructively superimposed in the direction of the laser beam source, the advancing and/or returning laser pulse or the first advancing and/or returning laser pulse part can pass through an actuating element.

The adjusting element can be provided along the first beam pad, through which the first advancing or returning laser pulse part passes between the two beam splitters. The control element can preferably be designed as an active control element. The actuating element has in particular a first and a second position. The provision of a preferably active control element, which is able to be switched actively in the time between the passing through of the leading first laser pulse part and the passing through of the returning first laser pulse part, is technically demanding, but offers the possibility of back-reflection protection for higher laser pulses Performance independent of the polarization of the back reflection.

The adjusting element can be designed to change the path length covered by the first forward and/or backward running laser pulse part when passing through the first beam path. As a result, the leading first laser pulse can travel a shorter or longer path in the first position of the actuating element than the returning laser pulse in the second position of the actuating element. The returning first laser pulse portion is phase-shifted in the second position of the actuator compared to the leading first laser pulse portion in the first position of the actuator preferably by 180 ° + n * 360 °, where n is an integer (including zero). In other words, the path length that the preliminary extending first laser pulse part along the first beam path between the two beam splitters, from the path length that the returning first laser pulse part traverses along the first beam path between the two beam splitters by an odd multiple of half a wavelength of the returning or leading laser pulse part. The path length difference is typically a few microns.

The actuating element preferably has a piezoelectric element for the path length variation. As a result, very fast switching times can be achieved in a structurally simple manner.

In this way, complete destructive interference of the returning laser pulse on the way to the laser beam source can be achieved in order to optimally protect the laser beam source.

As an alternative or in addition to this, the actuating element can have an optical element whose refractive index can be varied in an actively controllable manner. For example, the actuating element can have an electro-optical crystal for phase shifting. In the first position of the actuating element, the leading first laser pulse part then experiences a different phase shift than the returning first laser pulse part in the second position of the actuating element. The phase shift in the first position or second position can also be zero (no phase shift). Preferably, the returning first laser pulse portion in the second position of the actuator is phase-shifted (or vice versa) by 180°+n*360° relative to the leading first laser pulse portion in the first position of the actuator, where n is an integer (including zero).

Alternatively or additionally, a frequency shifting device can shift the frequency of the returning laser pulse and/or the frequency of the leading laser pulse.

The first or second beam path is traversed by the first or second leading laser pulse part with a different frequency than that of the first th or second returning laser pulse part. In other words, the first and second leading laser pulse parts traverse the first and second beam paths at a first frequency, respectively, the first and second returning laser pulse parts traverse the first and second beam paths at a second frequency.

In order to achieve destructive interference in the direction of the laser beam source, the path lengths along the first and second beam path must be chosen such that the following two conditions are met: the difference between the path length that the first returning laser pulse travels along the first beam path, and the path length that the second returning laser pulse travels along the second beam path must be an integer multiple plus or minus half a wavelength of the laser pulses returning at the second frequency. At the same time, the difference between the path length that the first laser pulse travels along the first beam path and the path length that the second laser pulse travels along the second beam path must be an integer multiple of the wavelength of the laser pulses that travel at the first frequency.

The frequency shifting device can in particular be an accumulator-optical modulator (AOM) or a Stimulated Brillouin Scattering (SBS) mirror. The frequency can also be shifted on a moving element, such as an expanding plasma. In particular, the object itself can be designed as a moving, expanding plasma, on which the laser pulse is reflected, so that the frequency of the laser pulse is shifted when it is reflected on the object itself. In this case, one or more frequency shifting devices can be arranged in the beam path of the forward and/or backward running laser pulse.

The frequency shifting device can in particular be designed to be switchable. If a switchable frequency shifting device, for example an AOM, is used, the leading laser pulse can pass through it in a first, for example switched off position, in which the frequency of the laser pulse is not influenced. The returning laser pulse passes through the frequency shift device then in a second position in which the frequency of the laser pulse is shifted. The disadvantage of this is that the frequency-shifted returning laser pulse no longer runs on the same optical axis as the leading laser pulse. However, the frequency-shifted returning laser pulse can be deflected back to the first and/or second beam splitter, in particular by beam-deflecting optical elements.

The frequency shifting device is preferably designed to shift the frequency of the forward and/or backward running laser pulse by at most 1000 MHz and at least 20 MHz, preferably between 600 MHz and 30 MHz, in particular 40 MHz.

In particular, the leading laser pulse can pass through a first frequency shifting device before passing through the first beam splitter and a second frequency shifting device after passing through the second beam splitter. The returning laser pulse then passes through the second frequency shifter before passing through the second beam splitter and through the first frequency shifter after passing through the first beam splitter. In this way it is possible to fully or partially compensate for the frequency shift which the second frequency shift device effects after passing through the second beam splitter. In this case, the first frequency shifting device shifts the frequency of the preceding laser pulse by the same or approximately the same amount as the second frequency shifting device, only with an opposite sign. This is particularly advantageous when the leading laser pulse is amplified in one or more amplifiers after passing through the second beam splitter. These have an optimal gain range at one or more frequencies. Laser beams with frequencies that deviate from these are not optimally amplified. With two frequency shifting devices, which are arranged in front of the first and behind the second beam splitter and which shift the frequency of the forward and backward running laser pulse by compensating amounts, a comparatively large frequency shift can now be selected without the preceding current laser pulse is amplified less efficiently in a subsequent amplifier.

The greater the amount of frequency shift of the advancing versus the returning laser pulse between the two beam splitters, the smaller the difference between the distance that the first advancing or returning laser pulse travels along the first beam path compared to the second advancing or returning laser pulse along the second beam path can be chosen to ensure destructive interference in the direction of the laser beam source.

The splitting of the laser pulses at the beam splitters preferably takes place 50% in each case, i.e. into equal parts. The two beam splitters are preferably part of a Mach-Zehnder interferometer. In one of the embodiments, the actively actuable control element is then arranged in a beam path of the Mach-Zehnder interferometer.

The leading laser beam is preferably emitted from a laser beam source in the form of a seed source.

The first beam splitter and/or the second beam splitter can each be in the form of a partially transparent mirror. The beam splitters can be designed in the form of glass plates that are metal-coated on one side.

In particular, the first advancing or returning laser pulses can pass through telescopes or other beam-shaping elements between the first and second beam splitter, so that any deviations that occur in the beam size and/or divergence of the first and second advancing or returning laser pulses can be compensated for . In this way, it should be ensured that the interference of the first and second advancing or returning laser pulses is not spatially influenced and the first and second advancing or returning laser pulses interfere as completely as possible in a destructive or constructive manner. A beam trap can be arranged on the first beam splitter and/or on the second beam splitter to receive constructively interfering laser pulse parts.

The laser pulses preferably pass through an amplifier between the object and the second beam splitter.

The amplifier can be in the form of an amplifier chain.

In order to achieve more time for switching the actuating element, a delay line can be provided between the second beam splitter and the object.

In a particularly preferred embodiment of the invention, the leading laser pulse generates EUV radiation. For this purpose, the object can be in the form of a tin droplet, in which an expanding plasma is excited by the preceding laser pulse, which leads to the emission of EUV radiation.

The object according to the invention is also achieved by a device for irradiating an object, in particular for carrying out a method described here. The device has the following features: a) a laser beam source; b) a first beam splitter for dividing the leading laser pulse into a first leading laser pulse part and a second leading laser pulse part; c) a first beam path traversable by the first leading portion of the laser pulse; d) a second beam splitter for at least partially constructive interference of the first leading laser pulse part with the second leading laser pulse part on the way to the object; e) the object for reflecting the leading laser pulse and generating a returning laser pulse; wherein the second beam splitter is designed to divide the returning laser pulse into a first returning laser pulse part and a second returning laser pulse part; wherein the device has an actuating element and/or a frequency shifting device which are set up to bring about a different phase shift in the first returning laser pulse part than in the first leading laser pulse part and the returning first laser pulse part with the returning second laser pulse part at the first beam splitter on the way to the laser beam source can be at least partially destructively interfered with.

In particular, an actuating element can be arranged along the first beam path between the first and the second beam splitter. The actuating element can in particular be switchable between a first and a second position. In this case, the actuating element can be designed to change the path length that the first forward or backward running laser pulse part travels along the first beam path. For this purpose, the actuating element preferably has a piezoelectric element. As a result, very fast switching times can be achieved in a structurally simple manner. In particular, the adjusting element can be designed to change the path length of the first beam path by half a wavelength or an odd multiple of half a wavelength of the first forward or backward running laser pulse.

Alternatively or additionally, the actuating element can have an optical element whose refractive index can be changed in an actively controllable manner, for example an electro-optical phase modulator. This adjusting element is also preferably arranged along the first beam path. This causes the first returning laser pulse part to experience a different phase shift than the first leading laser pulse part. In particular, the optical element can be designed to shift the phase of the first returning laser pulse part by 180°+n*360° (or vice versa), where n is an integer (including zero).

The frequency shifting device can be designed in the form of an accumulator-optical modulator (AOM), a Stimulated Brillouin Scattering (SBS) mirror and/or a moving, reflecting element, for example an expanding plasma. The device can also have several frequency shifters. In particular, viewed in the beam direction of the preceding laser pulse, a first frequency shifting device can be arranged in front of the first beam splitter and a second frequency shifting device can be arranged after the second beam splitter.

The first and second frequency shifting device is designed in particular to increase the frequency of the preceding laser pulse by an amount before the first beam splitter and to decrease it by an amount, in particular the same or almost the same, after the second beam splitter (or vice versa). This has the advantage that the frequency difference between the forward and backward running laser pulse can be selected to be comparatively large without the preceding laser pulse being amplified less efficiently in a subsequent amplifier. Furthermore, the greater the frequency shift of the leading laser pulse compared to the returning laser pulse, the difference in path length between the distance covered by the first leading or returning laser pulse along the first beam path compared to the second leading or returning laser pulse along the second beam path , be chosen smaller.

In this case, one or more telescopes can be arranged in the first beam path, so that any deviations that may occur in the beam size and/or divergence of the first and second laser pulses running forwards or backwards can be compensated for. In this way, it should be ensured that the interference of the first and second advancing or returning laser pulses is not spatially influenced and the first and second advancing or returning laser pulses interfere as completely as possible in a destructive or constructive manner.

The device can have an amplifier between the second beam splitter and the object. In particular, this can involve one or more CO2 amplifiers connected in series. In case the object itself is in the form of a plasma as a frequency shifter, the device can also be placed between the amplifier and the object.

The device can have at least one mirror, in particular a plurality of mirrors, for deflecting the laser beam. In particular, the first forward or backward running laser pulse part can be guided through the first beam path via a pair of mirrors.

The object is preferably designed to emit EUV radiation. The object is particularly preferably in the form of a tin droplet.

Further advantages of the invention result from the description and the drawing. Likewise, the features mentioned above and those detailed below can be used according to the invention individually or collectively in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather have an exemplary character for the description of the invention.

Detailed description of the invention and drawings

1a shows a schematic view of a first embodiment of a device with a path length adjusting element with a leading laser pulse.

FIG. 1b shows the device from FIG. 1a with the laser pulse returning.

2a shows a schematic view of a second embodiment of a device with a refractive index setting element with a leading laser pulse.

FIG. 2b shows the device from FIG. 2a with a returning laser pulse.

FIG. 3 shows a schematic view of a third embodiment of a device with a frequency shifter.

FIG. 4 shows a schematic view of a fourth embodiment of an apparatus, in which an irradiated object represents a frequency shifter. Fig. 5 shows a schematic view of a fifth embodiment with a frequency shifter arranged upstream of an interferometer and a frequency shifter arranged downstream of the interferometer.

Fig. la shows a device 10 for irradiating an object 12. The device 10 has a laser beam source 14, a first beam splitter 16, a second beam splitter 18, a first beam path 19, an actuating element 20, a first beam trap 22, a second beam trap 24 and an amplifier 26 on.

The beam splitters 16, 18 are preferably designed in the form of glass plates that are metal-coated on one side, so that a reflected light beam experiences a phase jump or no phase jump depending on the side on which it is incident on the beam splitter 16, 18. Such beam splitters are known from the design of a Mach-Zehnder interferometer.

The laser beam source 14 emits a laser beam 28. The laser beam 28 has a leading laser pulse 30. FIG. The leading laser pulse 30 is split at the first beam splitter 16 into a first leading laser pulse part 30a and a second leading laser pulse part 30b. The first leading laser pulse part 30a is guided to the second beam splitter 18 via the first beam path 19 and the adjusting element 20 , whereas the second leading laser pulse part 30b is guided to the second beam splitter 18 without passing the adjusting element 20 .

Due to its reflection at the first beam splitter 16, the first leading laser pulse part 30a experiences a phase shift of 180° or pi, whereas the second leading laser pulse part 30b at the first beam splitter 16 does not experience a phase shift.

The first leading laser pulse part 30a partially passes through the second beam splitter 18 to the second beam trap 24 without a phase jump. Partly becomes the first leading laser pulse part 30a is reflected at the second beam splitter 18 to the amplifier 26 and experiences a phase jump of 180°.

The second leading laser pulse part 30b passes through the first beam splitter 16 without a phase jump. Partly the second leading laser pulse part 30b then passes through the second beam splitter 18 to the amplifier 26 without a phase jump and partly towards the second beam trap 24 likewise without a phase jump.

From the second beam splitter 18 to the amplifier 26, the first leading laser pulse portion 30a and the second leading laser pulse portion 30b therefore interfere constructively, and from the second beam splitter 18 to the second beam trap 24, the first leading laser pulse portion 30a and the second leading laser pulse portion 30b interfere destructively. Thus, the at least partially recombined leading laser pulse 30 is directed towards the amplifier 26 .

Laser pulse 30 is amplified in amplifier 26, which is preferably in the form of an amplifier chain, and then strikes object 12.

In the present case, the object 12 is in the form of a tin droplet. A plasma is generated in the tin droplet by the preceding laser pulse 30, which leads to the emission of extreme ultraviolet light (EUV) radiation 32.

1b shows the device 10 with a returning laser pulse 34 reflected by the object 12. The returning laser pulse 34 is split at the second beam splitter 18 into a first returning laser pulse part 34a and a second returning laser pulse part 34b. The first returning laser pulse part 34a experiences a phase jump of 180° at the second beam splitter 18, whereas the second returning laser pulse part 34b at the second beam splitter 18 experiences no phase jump.

The first returning laser pulse part 34a passes through the actuating element 20 on its way to the first beam splitter 16 along the first beam path 19, whereas Gen the second returning laser pulse part 34b does not pass through the actuating element 20 on its way to the first beam splitter 16.

The first returning laser pulse part 34a partially passes through the first beam splitter 16 to the first beam trap 22 without a phase jump. The first returning laser pulse part 34a is partly reflected at the first beam splitter 16 towards the laser beam source 14 and experiences a phase jump of 180° in the process.

Partly the second returning laser pulse part 34b passes through the first beam splitter 16 towards the laser beam source 14 without a phase jump and partly towards the first beam trap 22 likewise without a phase jump.

In principle, it would therefore be expected that the first returning laser pulse part 34a and the second returning laser pulse part 34b interfere constructively from the first beam splitter 16 towards the laser beam source 14 and interfere destructively towards the first beam trap 22 . The laser beam source 14 would be damaged as a result.

The actuating element 20 is therefore switched from its first position, shown in FIG. 1a, for the leading laser pulse 30 to its second position, shown in FIG. The actuating element 20 consequently changes the path length for the returning laser pulse 34 compared to the preceding laser pulse 30 . For this purpose, the actuating element 20 can have a piezoelectric element.

The change in path length of the actuating element 20 is dimensioned such that the returning laser pulse 34 is phase-shifted by 180°+n*360° (with an integer n, where n=0 can be) in relation to the leading laser pulse 30 . As a result, the first returning laser pulse part 34a and the second returning laser pulse part 34b interfere destructively from the first beam splitter 16 to the laser beam source 14 and constructively to the first beam trap 22 . The laser beam source 14 is protected from the returning laser pulse 34 .

2a and FIG. 2b correspond to those shown in FIGS. la, b shown devices 10. However, in Figs. 2a, b shown actuating element 20 an electric tro-optical crystal 36 for changing the phase of the first returning laser pulse 34 in relation to the first leading laser pulse 30 .

It should be noted with regard to FIGS. 1a, b and 2a, b that the distances for the first laser pulse parts 30a, 34a and the second laser pulse parts 30b, 34b are the same—apart from any phase shifts. The distances are drawn with different lengths in the figures only for reasons of representability. The distances can be executed as desired, so that they are the same length in the end.

Devices 10 in Figs. 3, 4 and 5 correspond to the devices 10 shown in FIGS. 1a and b or 2a and b. However, the devices 10 in FIGS. 3, 4 and 5 no actuating element 20 along the first beam path 19.

For this purpose, a frequency shifting device 23 is provided between the second beam splitter 18 and the object 12 in FIG. 3 . The frequency of the preceding laser pulse is shifted by the frequency shifting device 23 after it has passed through the second beam splitter 18 . A path length along the first beam path 19 and along a second beam path 21, which is traversed by second forward and backward running laser pulse parts 30b, 34b between the two beam splitters 16, 18, is chosen so that the frequency-shifted first and second returning laser pulse parts 30a, 30b experience destructive interference towards the laser beam source 14 at the first beam splitter 16, while the non-frequency-shifted first and second leading laser pulse parts 34a, 34b experience constructive interference towards the object 12.

In FIG. 4, the object 12 is designed as a frequency shifting device 23, for example as an expanding plasma. The frequency of the leading laser pulse 30 is shifted on the object 12 . The returning laser pulse 34 is already destructively interfered in the direction of the laser beam source 14 before passing through the amplifier 26 . 5 shows a device 10 with two frequency shifting devices 23, 25 which are arranged on the one hand between the laser beam source 14 and the first beam splitter 16 and on the other hand between the second beam splitter 18 and the object 12. The frequency of the leading laser pulse 30 is shifted by a certain amount before passing through the first beam splitter 16 and shifted back again by the same amount after passing through the second beam splitter 18 . After reflection on the object 13, the frequency of the returning laser pulse 34 is also shifted by a specific amount before passing through the second beam splitter 18 and is also shifted back again by the same amount after passing through the first beam splitter 16. The amplifier 26 can thus be operated at the frequency of the laser pulse generated in the laser beam source 14 .

In the embodiment according to FIGS. 3, 4 and 5, the distances covered by the first and second laser pulse parts (30a, b, 34a, b) along the first and second beam paths 19, 21 are of different lengths. The two beam paths 19, 21 are designed such that a time difference between the time that the first laser pulse part (30a, 34a) needs to run through the first beam path 19 and a time that the second laser pulse part (30b, 34b) needs to run through the second Beam path 21 required is short compared to the pulse duration. This ensures that only a small part of the two laser pulse parts (30a, b, 34a, b) does not interfere.

Taking a synopsis of all figures of the drawing, the invention relates in summary to back reflection protection by destructive interference of a returning laser pulse 34. A leading laser pulse 30 is divided, a first leading laser pulse part 30a, in contrast to a second leading laser pulse part 30b, runs through a first beam path 19. The two Laser pulse parts 30a, b are at least partially constructively superimposed. Thereafter, the leading laser pulse 30 is reflected and the reflected, returning laser pulse 34 basically traverses the same path as the leading laser pulse 30, but a returning first laser pulse part 34a is phase-shifted when passing through the first beam path 19 so that it coincides with a returning second laser pulse part 34b is at least partially, in particular completely, destructively superimposed on its way to the laser beam source 14, so that the laser beam source 14 is protected from the returning laser pulse 34.

Reference character list

10 device

12 object

14 laser beam source

16 first beam splitter

18 second beam splitter

19 first ray path

20 actuator

21 second beam path

22 first beam trap

23 frequency shifter

24 second beam trap

25 frequency shifter

26 amplifiers

28 laser beam

30 leading laser pulse

30a first laser pulse part

30b second laser pulse part

32 Extreme Ultraviolet Light (EUV) Radiation

34 returning laser pulse

34a first laser pulse part

34b second laser pulse part

36 electro-optic crystal

Claims

Claims Method for irradiating an object (12) with a laser beam (28), with the method steps:

A) emitting a leading laser pulse (30) of the laser beam (28) from a laser beam source (14);

B) dividing the leading laser pulse (30) into a first leading laser pulse part (30a) and a second leading laser pulse part (30b) by a first beam splitter (16);

C) the first leading laser pulse portion (30a) traversing a first beam path (19);

D) constructively interfering the first laser pulse portion (30a) and the second laser pulse portion (30b) by a second beam splitter (18);

E) reflecting the at least partially recombined leading laser pulse (30) off the object (12) and thereby generating a returning laser pulse (34);

F) splitting the returning laser pulse (34) into a first returning laser pulse portion (34a) and a second returning laser pulse portion (34b) by the second beam splitter (18);

G) Traversing the first beam path (19) of the first returning laser pulse part (34a), the first returning laser pulse part (34a) undergoing a different phase shift than the first leading laser pulse part (30a) by the leading laser pulse (30) and the returning laser pulse (34) at least partially i) pass through an adjusting element (20) which is adjusted when passing through the returning laser pulse (34) compared to passing through the leading laser pulse (30); and/or ii) pass through a frequency shifter (23, 25);

H) at least partially combining the first laser pulse part (34a) of the returning laser pulse (34) with the second laser pulse part (34b) of the returning laser pulse (34) by the first th beam splitter (16) towards the laser beam source (14), wherein the first laser pulse portion (34a) and the second laser pulse portion (34b) interfere destructively. Method according to Claim 1, in which, in method steps C) and G), the advancing or returning first laser pulse part (30a, 34a) passes through an actuating element (20) which is switched into a first and into a second position, the Actuating element (20) is traversed in method step C) by the first leading laser pulse part (30a) in a first position and in method step G) by the first returning laser pulse part (34a) in a second position. Method according to Claim 1 or 2, in which the adjusting element (20) is designed to adjust a path length along the first beam path (19), so that the leading first laser pulse part (30a) running through the first beam path (19) is in the first position of the The adjusting element (20) travels a shorter or longer path than the returning first laser pulse part (34a) in the second position of the adjusting element (20), the path length that the advancing first laser pulse part (30a) travels differing from the path length that the returning first laser pulse part (34a) passes through, in particular by an odd multiple of half a wavelength of the returning laser pulse part (34a). Method according to one of the preceding claims, in which the actuating element (20) has a piezoelectric element for varying the path length between the first position and the second position. Method according to one of the preceding claims, in which the actuating element (20) has an electro-optical crystal (36) for phase shifting, so that the leading first laser pulse part (30a) passing through the actuating element (20) has a different phase shift in the first position of the actuating element (20). experiences as the returning first laser pulse part (34a) in the second position of the actuating element (20), the phase senshift corresponds in particular to an odd multiple of half a wavelength of the returning laser pulse part (34a). Method according to one of the preceding claims, wherein the leading laser pulse (30) before method step B and/or after method step D and/or the returning laser pulse (34) before method step E and/or after method step H has a frequency shifting device (23, 25) , in particular an accumulator-optical modulator (AOM), a Stimulated Brillouin Scattering (SBS) mirror and/or an expanding plasma. Method according to one of the preceding claims, in which the frequency shifting device (23, 25) shifts the frequency of the leading laser pulse (30) and/or the returning laser pulse (34) by at most 1000 MHz and at least 20 MHz, preferably by less than 100 MHz . Method according to one of the preceding claims, in which the leading laser pulse (30) in method step D) is introduced into an amplifier (26) and the returning laser pulse (34) in method step F) coming from the amplifier (26) onto the second beam splitter ( 18) is conducted. Method according to one of the preceding claims, in which in method step D) the leading laser pulse (30) after the constructive interference of the first laser pulse part (30a) and the second laser pulse part (30b) runs through a delay path which is also separated by the returning laser pulse (34) in the method step E) is run through. Method according to one of the preceding claims, in which the preceding laser pulse (30) is used to generate extreme ultraviolet (EUV) radiation (32). Device (10) for irradiating an object (12), which is designed in particular for emitting EUV radiation, with a laser beam (28), in particular for carrying out a method according to one of the preceding claims, having the following features: a) a laser beam source ( 14); b) a first beam splitter (16) for dividing a leading laser pulse (30) from the laser beam source (14) into a first leading laser pulse part (30a) and a second leading laser pulse part (30b); c) a first beam path (19) which can be traversed by the first laser pulse part (30a); d) a second beam splitter (18) on which the first laser pulse part (30a) can be at least partially constructively interfered with by the second laser pulse part (30b); e) an object (12) for reflecting the leading laser pulse (30) and generating a returning laser pulse (34); wherein the returning laser pulse (34) can be divided at the second beam splitter (18) into a first returning laser pulse part (34a) and a second returning laser pulse part (34b); wherein the device has an actuating element (20) and/or a frequency shifting device (23, 25), which are set up to bring about a different phase shift in the first laser pulse part (34a) of the returning laser pulse (34) than in the first laser pulse part (30a) of the leading laser pulse (30) and wherein the first returning laser pulse part (34a) can be at least partially destructively interfered with with the second returning laser pulse part (34b) at the first beam splitter (16) towards the laser beam source (14). Device according to Claim 11, in which the actuating element (20) can be switched into a first and into a second position. Apparatus according to claim 11 or 12, wherein the adjusting element (20) along the first beam path (19) is arranged so that the first The leading first laser pulse part (30a) running through the beam path (19) in the first position of the adjusting element (20) travels a shorter or longer path than the returning first laser pulse part (34a) in the second position of the adjusting element (20); and/or the adjusting element (20) is arranged along the first beam path (19) and has an electro-optical crystal (36) for phase shifting, so that the first laser pulse part (30a) running through the first beam path (19) is in the first position of the adjusting element ( 20) undergoes a different phase shift than the returning first laser pulse part (34a) in the second position of the actuating element (20). Device according to one of claims 11 to 13, in which the frequency shifting device (23, 25) as an accumulator-optical modulator (AOM), Stimulated Brillouin Scattering (SBS) mirror and/or as a moving object, in particular a moving, expanding plasma, wherein the frequency shifting device (23, 25) is arranged in particular after the second beam splitter (18) and/or before the first beam splitter (16), viewed in the beam direction of the preceding laser pulse (30). Device according to one of Claims 11 to 14, in which the device (10) has an amplifier (26) to which the leading laser pulse (30) can be routed after the second beam splitter (18) and from which the returning laser pulse (34) to the second beam splitter (18) can be guided.