Method and apparatus for irradiating an object with a laser beam

Abstract

The invention relates to a method and a device for irradiating an object (15) with a laser beam (11). A multiple-beam interferometer (12) is used to prevent that radiation (16) reflected back by the object (15) can damage the laser source (10).

Description

[0001] Method and device for irradiating an object with a laser beam

[0002] Background of the invention

[0003] The invention relates to a method and a device for irradiating an object with a laser beam.

[0004] Irradiating an object with a laser beam can be used for processing such as cutting, welding, or soldering. Another application is the generation of extreme ultraviolet (EUV) light. For this purpose, a tin droplet is irradiated as the object, exciting an expanding plasma within it that emits the EUV radiation. In many cases, the irradiated object reflects a portion of the laser radiation. This reflected portion propagates in the opposite direction along the beam path and can therefore damage the laser beam source.

[0005] Various approaches have been proposed to avoid this problem. For example, conventional dual acousto-optical modulators in a crossfire configuration or a single acousto-optical modulator (AOM) in a so-called isolation module can be used to isolate the forward laser beam from the reflected laser beam.

[0006] However, these solutions have relatively slow switching transitions, which can lead to reverse pulse leakage.

[0007] For other wavelength and power ranges, there are other solutions for isolating a forward-directed optical beam. These include Faraday isolators or electro-optical modulators. However, these are not available for CO2 laser wavelengths and high powers.

[0008] US Pat. No. 5,126,876 A describes a device for suppressing a reflected beam, which comprises a Mach-Zehnder interferometer that splits the backward-directed beam into two partial beams, induces a phase shift between the partial beams, and superimposes the partial beams both constructively and destructively, with the constructively superimposed portion being deflected away from the laser source. However, this solution requires a relatively large installation volume and is not suitable for applications where the reflected beam has a varying wavelength offset.

[0009] Object of the invention

[0010] The invention is based on the object of providing a method and a device which reliably protect pulsed CO2 laser beam sources from reflected radiation and which require a smaller installation space than known solutions, as well as being robust against a change in the wavelength of the reflected radiation.

[0011] Description of the invention

[0012] This object is achieved according to the invention by a method for irradiating an object with a laser beam, comprising the steps of: a. emitting a forward-directed laser beam from a laser beam source; b. guiding the forward-directed laser beam through a multi-beam interferometer arranged in a light path between the laser beam source and the object, wherein the frequency of the forward-directed laser beam corresponds to a transmission frequency of the multi-beam interferometer; c. suppressing the transmission of a laser beam directed backwards in the direction of the laser beam source, which laser beam is generated by reflection of the forward-directed laser beam at the object and enters the multi-beam interferometer in the opposite direction, by not adhering to the resonator conditions of the multi-beam interferometer, by

[0013] - the frequency of the backward-directed laser beam is changed before it enters the multi-beam interferometer to a frequency different from a transmission frequency of the multi-beam interferometer,

[0014] - and / or by changing the transmission frequency of the multi-beam interferometer in a time-controlled manner and synchronized with the pulse repetition frequency of the laser beam emitted by the laser beam source so that the forward-directed laser beam is transmitted by maintaining the resonator conditions of the multi-beam interferometer and the backward-directed laser beam is suppressed in the direction of the laser beam source by not maintaining the resonator conditions of the multi-beam interferometer.

[0015] The subclaims relate to preferred embodiments.

[0016] The method according to the invention exploits the advantages of a multi-beam interferometer compared to a dual-beam interferometer, such as a Mach-Zehnder interferometer, for unidirectional transmission of only the forward-directed laser beam and suppression of the backward-directed laser beam. In contrast to dual-beam interferometers, the spectral selectivity of multi-beam interferometers is orders of magnitude higher, while the required optical path lengths are significantly shorter than with dual-beam interferometers. Furthermore, a multi-beam interferometer requires fewer optical components.

[0017] Compared to other optical isolation devices, such as Faraday isolators, the method according to the invention also enables the optical isolation of high-power beams with simple optical components and can therefore also be used to protect CO2 lasers.

[0018] According to the inventive method, only the optical frequency of the forward-directed laser beam is transmitted. Unwanted reflections, which re-enter the multibeam interferometer as a backward-directed beam, are not transmitted. Two techniques can be used for this purpose: 1) a shift in the frequency of the backward-directed laser beam relative to the transmission frequency of the multibeam interferometer and / or 2) a temporal control of the transmission frequencies supported by the multibeam interferometer.

[0019] In a first embodiment of the method, the forward laser beam, after leaving the multi-beam interferometer, can be passed through a frequency shifter that is time-controlled and synchronized with the pulse repetition frequency of the forward laser beam emitted by the laser beam source. The reflected, backward beam typically has the same frequency as the forward beam striking the object. Therefore, either the frequency of the forward laser beam after passing through the multi-beam interferometer or the frequency of the reflected, backward beam can be changed to a frequency suppressed by the multi-beam interferometer to prevent transmission of the reflected laser beam back to the laser beam source.Through timing and synchronization with the pulse repetition frequency of the laser beam source, either only the pulses of the forward-directed laser beam or only the pulses of the backward-directed laser beam can be selectively frequency shifted by the frequency shifting device.

[0020] In a further embodiment of the method according to the invention, the

[0021] The length of the resonator of the multi-beam interferometer can be changed in a time-controlled manner and synchronized with the pulse repetition frequency of the laser beam emitted by the laser source in such a way that before each pulse of the forward-directed laser beam entering the resonator of the interferometer, the resonator length is adjusted so that the frequency of the forward-directed laser beam is transmitted through the interferometer, and after the pulse of the forward-directed laser beam leaves the resonator, the resonator length is changed so that the frequency of the backward-directed laser beam is suppressed by the interferometer in the direction of the laser beam source.

[0022] Multi-beam interferometers such as Fabry-Perot interferometers are relatively simple devices that essentially consist of two partially reflecting mirrors that form a resonator between them.

[0023] The transmission of a beam of wavelength X in a multi-beam interferometer can be described by the so-called Airy function:

[0024] A(L,n) = 1 / (1 + (2F / K) 2 *sin 2 (4>(L, n))).

[0025] The phase is given by <|)(L, n) = n*27t*L / X, where n is the refractive index of the medium inside the multi-beam interferometer and L is the length of the resonator. By adjusting the phase < , in particular by changing the resonator length L, the transmitted wavelengths or frequencies can be precisely selected. The frequencies that can pass through the multi-beam interferometer are also called "supported transmission frequencies." It is advantageous if the optical resonator length is changed after a pulse of the forward-directed laser beam leaves such that the transmission frequency changes by more than one optical bandwidth of the interferometer d = c / (2nLF), where c is the speed of light, n is the refractive index of the resonator, L is the resonator length, and F is the finesse of the multi-beam interferometer.

[0026] As an alternative to varying the optical resonator length to influence the transmission frequencies, a phase modulator in the multi-beam interferometer can be used to adjust the phase in a time-controlled manner and synchronized with the pulse repetition frequency of the forward-directed laser beam. Upon entry of a pulse of the forward-directed laser beam into the resonator, the phase can be adjusted so that the interferometer transmits the frequency of the forward-directed laser beam. After the pulse leaves the resonator, the phase can be changed so that the interferometer suppresses the frequency of the backward-directed laser beam in the direction of the laser beam source. This design has the advantage that mechanically moving parts for adjusting the transmission frequencies of the multi-beam interferometer are dispensed with.

[0027] Furthermore, the forward-directed laser beam can be passed through an amplifier before hitting the object. This allows the required power to be achieved for processing the object and compensates for power losses occurring in the interferometer.

[0028] The method according to the invention can be used for all laser processing processes. Particular advantages arise when the forward-directed laser beam is used to generate extreme ultraviolet (EUV) radiation. This radiation has such high power that reflections of this radiation back into the laser beam source are particularly damaging.

[0029] The invention also relates to a device for irradiating an object with a laser beam, in particular for carrying out a method according to the invention, which device comprises a laser beam source and an object from which laser light is reflected back, as well as a multi-beam interferometer arranged in the beam path between the laser beam source and the object, wherein a frequency shifting device is arranged in the beam path between the multi-beam interferometer and the object and / or the multi-beam interferometer is designed such that its transmission frequency spectrum is variable. With this device, it is possible for the multi-beam interferometer to transmit only the forward-directed laser beam generated by the laser beam source, but not light reflected back from the object.This can be achieved by using a frequency shifter downstream of the interferometer to perform a frequency shift of either the forward-directed laser beam or the backward-directed laser beam. The laser beam reflected from the object has the same frequency as the forward-directed laser beam incident on the object. Therefore, one of the two beams must be frequency-shifted relative to the transmission frequency of the multi-beam interferometer to prevent the reflected light from being transmitted back to the laser beam source. In applications where the light reflected from the object has a different frequency than the light directed at the object, a frequency shifter can be omitted entirely.

[0030] However, the retransmission of the reflected light can also be prevented by changing the transmission frequency of the interferometer in a time-controlled manner and synchronized with the pulse repetition frequency of the laser beam generated by the laser beam source.

[0031] In a preferred embodiment of the device, the multi-beam interferometer may comprise two wedge-shaped, semi-transparent mirrors and one opaque, highly reflective mirror.

[0032] To adjust the transmission frequency of the multi-beam interferometer, the opaque, highly reflective mirror can be adjusted in position using a piezoelectric transducer. This adjustment changes the resonator length of the interferometer and thus also the transmission frequency spectrum of the interferometer. Piezoelectric transducers are capable of performing the rapid oscillating movements required here, synchronized with the pulse repetition frequency of the laser beam generated by the laser beam source. In contrast to Mach-Zehnder interferometers, the required mirror adjustment ranges are orders of magnitude smaller.

[0033] Alternatively, a phase modulator can be arranged in a beam path of the multi-beam interferometer to change the interferometer's transmission frequency in a time-controlled manner and synchronized with the pulse repetition frequency of the laser beam. This design has the advantage that no optical component of the interferometer needs to be moved mechanically. Furthermore, phase modulators operate over a wide electrical bandwidth. The modulators are available for frequencies in the UV range up to the MIR range. Possible switching frequencies include up to 100 MHz.

[0034] Further advantages arise when the multi-beam interferometer has a beam trap for falsely optically isolated beam portions of the forward-directed laser beam and a beam trap for the isolated, backward-directed laser beam. This prevents scattered radiation from leaving the interferometer.

[0035] The multi-beam interferometer can be a classic Fabry-Perot interferometer, which is characterized by its simple design. Preferably, however, the multi-beam interferometer can have a ring-shaped resonator. This prevents reflections into the original beam path. The resonator can also be filled with nitrogen or other media with low radiation attenuation to minimize beam attenuation. Alternatively, the resonator can be operated under vacuum.

[0036] The frequency shifting device can preferably be an acousto-optical modulator. However, other embodiments are also conceivable. To achieve the required power levels when irradiating the object, an amplifier can be conveniently arranged in the beam path in front of the object.

[0037] Further features and advantages of the invention will become apparent from the description, the claims, and the drawings. According to the invention, the above-mentioned and further-described features can be used individually or in combination in any convenient way. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention.

[0038] Detailed description of the invention and drawing

[0039] Fig. 1 shows a schematic view of a first device for irradiating an object;

[0040] Fig. 2 shows a schematic view of a second device for

[0041] Irradiation of an object;

[0042] Fig. 3 shows a schematic view of a third device for

[0043] Irradiation of an object.

[0044] Fig. 1 shows a schematic representation of a laser beam source 10 that generates a forward-directed laser beam 11. This beam is passed through a multi-beam interferometer 12 and then through a frequency shifter 13 and an amplifier 14 before it impinges on an object 15. The object 15 reflects a portion of the laser beam 11, thus generating a backward-directed laser beam 16. This laser beam 16 has the same frequency as the incident laser beam. The multi-beam interferometer 12 is designed to suppress all frequencies except the frequency of the originally generated laser beam 11. Thus, shifting the frequency of either the forward-directed laser beam 11 or the frequency of the backward-directed laser beam 16 in the frequency shifter 13 results in the backward-directed laser beam 16 being suppressed in the multi-beam interferometer 12.In the example shown, the forward-directed laser beam 11 undergoes a frequency change in the frequency shift device 13 and exits the frequency shift device 13 as a forward-directed laser beam 11'. The frequency shift device can preferably be an acousto-optical modulator and perform a frequency shift of, for example, 80 MHz. This frequency shift is time-controlled and synchronized with the pulse repetition frequency of the forward-directed laser beam 11, so that only the incoming forward-directed laser beam 11, but not the backward-directed laser beam 16, is frequency-shifted.

[0045] The multi-beam interferometer 12 has two wedge-shaped, partially transparent mirrors 17, 18 and an opaque, highly reflective mirror 19. The partially transparent mirrors 17, 18 and the mirror 19 form a resonator. The incoming laser beam 11 is transmitted through the partially transparent mirror 17 if the frequency of the laser beam 11 satisfies the resonator condition of the multi-beam interferometer 12. If, however, the resonator's resonance condition for the frequency of the laser beam 11 is not met, the laser beam 11 is guided into a beam trap 22 by reflection from the partially transparent mirror 18 and the mirror 19. This prevents damage caused by an incorrectly adjusted resonator.

[0046] The backward-directed laser beam 16, however, does not fulfill the resonance condition of the multi-beam interferometer due to the frequency shift in the frequency shift device 13. It is therefore guided into a beam trap 21 by reflection from the mirrors 17 and 19.

[0047] Fig. 2 shows an alternative embodiment of a device according to the invention, wherein identical elements are provided with the same reference numerals compared to the embodiment in Fig. 1. Here too, the laser beam source 10 generates a forward-directed laser beam 11, which is passed through a multi-beam interferometer 12'. However, the device in Fig. 2 does not have a frequency shifter 13. After leaving the multi-beam interferometer 12', the laser beam 11 is fed directly to the amplifier 14 and then strikes the object 15. Reflections on the object 15 generate a backward-directed laser beam 16 with the same frequency as the forward-directed laser beam 11.To prevent this backward-directed laser beam 16 from passing through the multi-beam interferometer 12' and striking the laser beam source 10, the multi-beam interferometer 12' has a means for time-controlled variation of its transmission frequency. This is achieved by arranging the opaque mirror 19 so that it can be adjusted in the direction of the double arrow 23. The mirror 19 and the wedge-shaped, partially transparent mirrors 17, 18 form a resonator between them, the length L of which is varied by adjusting the position of the mirror 19. However, this also changes the transmission frequency of the resonator and thus of the multi-beam interferometer 12'.The adjustment of the mirror 19 is time-controlled: whenever a forward-directed pulse arrives, the resonator length L is adjusted so that it is transmitted through the interferometer 12', and whenever a backward-directed pulse arrives, the resonator length L is adjusted so that the beam 16 is guided into the beam trap 21. The adjustment of the mirror 19 can preferably be carried out by a piezoelectric converter.

[0048] The variant shown in Fig. 3 corresponds in external design to the variant in Fig. 2. However, the internal design of the multi-beam interferometer 12" differs. The mirror 19 of the multi-beam interferometer 12" is not adjustable. Here, the adjustment of the transmission frequency is effected by a phase modulator 24, which is arranged in the beam path between the semi-transparent mirror 17 and the mirror 19. When a forward-directed pulse of the laser beam 11 arrives, the frequency is adjusted by the phase modulator 24 such that the laser beam 11 is transmitted. If, on the other hand, a backward-directed pulse of the reflected laser beam 16 arrives, the frequency and thus the resonance condition of the multi-beam interferometer 12" is changed such that no transmission of the beam 16 takes place, but rather it is guided into the beam trap 21.

Claims

Patent claims 1. A method for irradiating an object (15) with a laser beam, comprising the steps of: a. emitting a forward-directed laser beam (11) from a laser beam source (10); b. guiding the forward-directed laser beam (11) through a multi-beam interferometer (12, 12', 12") arranged in a light path between the laser beam source (10) and the object (15), wherein the frequency of the forward-directed laser beam (11) corresponds to a transmission frequency of the multi-beam interferometer (12, 12', 12"); c. Suppressing the transmission of a laser beam (16) directed backwards in the direction of the laser beam source (10), which laser beam is generated by reflection of the forward-directed laser beam (11, 11') at the object (15) and enters the multi-beam interferometer (12, 12', 12") in the opposite direction, by not complying with the resonator condition of the multi-beam interferometer (12, 12', 12"), by - the frequency of the backward-directed laser beam (16) is changed before it enters the multi-beam interferometer (12, 12', 12") to a frequency different from a transmission frequency of the multi-beam interferometer (12, 12', 12"), - and / or by changing the transmission frequency of the multi-beam interferometer (12, 12', 12") in a time-controlled manner and synchronized with the pulse repetition frequency of the laser beam emitted by the laser beam source, such that the forward-directed laser beam (11) is transmitted by maintaining the resonator condition of the multi-beam interferometer (12, 12', 12") and the backward-directed laser beam (16) is transmitted by not maintaining the Resonator condition of the multi-beam interferometer (12, 12', 12") in the direction of the laser beam source (10) is suppressed.

2. Method according to claim 1, characterized in that the forward-directed laser beam (11), after leaving the multi-beam interferometer (12, 12', 12"), is guided through a frequency shifting device (13) which is time-controlled and synchronized with the pulse repetition frequency of the forward-directed laser beam (11) emitted by the laser beam source (10).

3. Method according to claim 2, characterized in that the frequency of the forward-directed laser beam (11) or the frequency of the backward-directed laser beam (16) is changed by the frequency shifting device (13) to a frequency (11') deviating from a transmission frequency of the multi-beam interferometer (12, 12', 12").

4. Method according to one of the preceding claims, characterized in that the length (L) of the resonator of the multi-beam interferometer (12, 12', 12") is changed in a time-controlled manner and synchronized with the pulse repetition frequency of the laser beam (11) emitted by the laser source (10) in such a way that before each pulse of the forward-directed laser beam (11) entering the resonator of the interferometer (12, 12', 12"), the resonator length (L) is adjusted such that the frequency of the forward-directed laser beam (11) is transmitted through the interferometer, and after the pulse of the forward-directed laser beam (11) leaves the resonator, the resonator length (L) is changed such that the frequency of the backward-directed laser beam (16) is suppressed by the interferometer (12, 12', 12") in the direction of the laser beam source (10).

5. Method according to claim 4, characterized in that the resonator length (L) is changed after leaving a pulse of the forward-directed laser beam (11) such that the transmission frequency changes by more than an optical bandwidth d of the interferometer (12, 12', 12") d = c / (2nLF), where c is the speed of light, n is the refractive index of the medium within the resonator, L is the resonator length and F is the Airy finesse of the multi-beam interferometer (12, 12', 12").

6. Method according to one of the preceding claims, characterized in that with the aid of a phase modulator (24) in the multi-beam interferometer (12, 12', 12"), in a time-controlled manner and synchronized with the pulse repetition frequency of the forward-directed laser beam (11), when a pulse of the forward-directed laser beam (11) enters the resonator, the phase is adjusted such that the interferometer (12, 12', 12") transmits the frequency of the forward-directed laser beam (11), and after the pulse leaves the resonator, the phase is changed such that the interferometer (12, 12', 12") suppresses the frequency of the backward-directed laser beam (16) in the direction of the laser beam source (10).

7. Method according to one of the preceding claims, characterized in that the forward-directed laser beam (11, 11') is passed through an amplifier (14) before impinging on the object (15).

8. Method according to one of the preceding claims, characterized in that the forward-directed laser beam (11, 11') is used to generate extreme ultraviolet radiation (EUV).

9. Method according to one of the preceding claims, characterized in that the laser beam source (10) is operated in a pulsed manner.

10. A device for irradiating an object (15) with a laser beam, in particular for carrying out a method according to one of the preceding claims, which device has a laser beam source (10) and an object (15) from which laser light is reflected back, as well as a multi-beam interferometer (12, 12', 12") arranged in the beam path between the laser beam source (10) and the object (15), wherein a frequency shifting device (13) is arranged in the beam path between the multi-beam interferometer (12, 12', 12") and the object (15) and / or the multi-beam interferometer (12, 12', 12") is designed such that its transmission frequency spectrum is variable.

11. Device according to claim 10, characterized in that the multi-beam interferometer (12, 12', 12") has two wedge-shaped, semi-transparent mirrors (17, 18) and one opaque, highly reflective mirror (19).

12. Device according to claim 10 or 11, characterized in that the opaque, highly reflective mirror (19) is adjustable in its position by means of a piezo converter.

13. Device according to one of claims 10 to 12, characterized in that a phase modulator (24) is arranged in a beam path of the multi-beam interferometer (12, 12', 12").

14. Device according to one of claims 10 to 13, characterized in that the multi-beam interferometer (12, 12', 12") has a beam trap (20) for erroneously optically isolated beam components of the forward-directed laser beam (11) and a beam trap (21) for the isolated backward-directed laser beam (16).

15. Device according to one of claims 10 to 14, characterized in that the multi-beam interferometer (12, 12', 12") has an annular resonator.

16. Device according to one of claims 10 to 15, characterized in that the multi-beam interferometer (12, 12', 12") is a Fabry-Perot interferometer.

17. Device according to one of claims 10 to 16, characterized in that the frequency shifting device (13) is an acousto-optical modulator.

18. Device according to one of claims 10 to 17, characterized in that an amplifier (14) is arranged in the beam path in front of the object (15).

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