Method and device for irradiating object by means of laser beam

Abstract

The invention relates to a method and a device for irradiating an object (15) by means of a laser beam (11), wherein a multi-beam interferometer (12) is used to prevent radiation (16) reflected back on the object (15) from damaging a laser beam source (10).

Description

Background Technology

[0001] The present invention relates to a method for irradiating an object with a laser beam and an apparatus for irradiating an object with a laser beam.

[0002] Irradiating an object with a laser beam allows for processing, such as cutting, welding, or brazing. Another application is the generation of extreme ultraviolet (EUV) light. This is done by irradiating a droplet of molten tin, exciting an expanding plasma within it, which emits EUV radiation. In many cases, the irradiated object reflects a portion of the laser radiation. This reflected component propagates in the opposite direction along the beam path, potentially damaging the laser source.

[0003] Various methods have been proposed to avoid this problem. Therefore, conventional dual acousto-optic modulators can be used in a crossfire configuration, or a single acousto-optic modulator (AOM) can be used in a so-called isolation module to isolate the forward laser beam from the reflected laser beam. However, these solutions have relatively slow switching transitions, which can lead to backward pulse leakage.

[0004] For other wavelengths and power ranges, there are other solutions for isolating the forward beam. These include Faraday isolators or electro-optic modulators. However, these solutions are not available for CO2 laser wavelengths and high power.

[0005] US5126876A describes a device for suppressing reflected beams, which incorporates a Mach-Zehnder interferometer that splits the backward beam into two segments, inducing a phase shift between the segments and superimposing them in a constructive and destructive manner, wherein the constructive superposition component is 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 varying wavelength shifts.

[0006] Invention Task The objective of this invention is to provide a method and an apparatus that reliably protect a pulsed CO2 laser beam source from reflected radiation, requires less installation space than known solutions, and is robust to wavelength variations of reflected radiation. Summary of the Invention

[0007] The solution of the present invention to achieve the above-mentioned task lies in a method for irradiating an object with a laser beam, the method comprising the following steps: a. Emitting a forward laser beam from a laser beam source; b. Guide the forward laser beam through a multibeam interferometer, which is arranged in the optical path between the laser beam source and the object, wherein the frequency of the forward laser beam is equivalent to the transmission frequency of the multibeam interferometer; c. Suppressing the transmission of a backward laser beam, generated by reflection of the forward laser beam onto an object and entering the multibeam interferometer in the opposite direction towards the laser beam source, by disallowing the resonator condition of the multibeam interferometer, is achieved by, - Before the backward laser beam enters the multibeam interferometer, the frequency of the backward laser beam is changed to a frequency different from the transmission frequency of the multibeam interferometer. - and / or, in a time-controlled manner, synchronized with the pulse sequence frequency of the laser beam emitted by the laser beam source, the transmission frequency of the multibeam interferometer is changed such that the forward laser beam is transmitted through satisfying the resonator condition of the multibeam interferometer, and the backward laser beam is suppressed in the direction of the laser beam source by not satisfying the resonator condition of the multibeam interferometer.

[0008] The dependent claims relate to preferred embodiments.

[0009] The method according to the invention utilizes the advantages of a multibeam interferometer over a two-beam interferometer (e.g., a Mach-Zehnder interferometer) to transmit only the forward laser beam and suppress the backward laser beam. Compared to a two-beam interferometer, the multibeam interferometer offers several orders of magnitude higher spectral selectivity while requiring a significantly shorter optical path length. Furthermore, the multibeam interferometer requires fewer optical components.

[0010] Furthermore, compared to other optical isolation devices (such as Faraday isolators), the method according to the invention can optically isolate high-power beams with the aid of simple optical components, and therefore can also be used to protect CO2 lasers.

[0011] According to the method of the invention, only the optical frequency of the forward laser beam is transmitted. Conversely, undesired reflections that re-enter the multibeam interferometer as a backward beam are not transmitted. For this purpose, two techniques can be employed: 1) shifting the frequency of the backward laser beam relative to the transmission frequency of the multibeam interferometer, and / or 2) time-controlling the transmission frequency supported by the multibeam interferometer.

[0012] In a first configuration of the method, the forward laser beam, after exiting the multibeam interferometer, can be guided through a frequency shifting device that is time-controlled and synchronized with the pulse sequence 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 incident on the object. Therefore, the frequency of the forward laser beam after passing through the multibeam interferometer, or the frequency of the reflected backward beam, can be changed to a frequency suppressed by the multibeam interferometer to prevent the reflected laser beam from transmitting back to the laser beam source. Through time control and synchronization with the pulse sequence frequency of the laser beam source, only the pulses of the forward laser beam or only the pulses of the backward laser beam can be selectively subjected to frequency shifting through the frequency shifting device.

[0013] In another embodiment of the method according to the invention, the resonator length of the multibeam interferometer can be changed in a time-controlled manner in sync with the pulse sequence frequency of the laser beam emitted by the laser source, such that the resonator length is adjusted before each pulse of the forward laser beam enters the interferometer resonator to transmit the frequency of the forward laser beam through the interferometer, and the resonator length is changed after the pulse of the forward laser beam leaves the resonator to suppress the frequency of the backward laser beam through the interferometer in the direction of the laser beam source.

[0014] A multibeam interferometer (such as a Fabry-Perot interferometer) is a relatively simple device that basically consists of two partially reflective mirrors that form a resonator between them.

[0015] The propagation of a beam with wavelength λ in a multibeam interferometer can be explained using the so-called Airy function: .

[0016] Phase by Given, where n is the refractive index of the medium inside the multibeam interferometer, and L is the length of the resonator. By matching the phase... In particular, by changing the resonator length L, the transmission wavelength or frequency can be precisely selected. The frequency that can pass through the multibeam interferometer is also called the "supported transmission frequency". In this case, it is advantageous to change the optical resonator length after the pulse of the forward laser beam has left, so that the change in transmission frequency is higher than the 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 length of the resonator, and F is the precision of the multibeam interferometer.

[0017] As an alternative to changing the length of the optical resonator to affect the transmission frequency, a phase modulator in a multibeam interferometer can be used to adjust the phase in a time-controlled manner, synchronized with the pulse sequence frequency of the forward laser beam, when the pulse enters the resonator. This allows the interferometer to transmit the frequency of the forward laser beam. Furthermore, this phase adjustment after the pulse leaves the resonator allows the interferometer to suppress the frequency of the backward laser beam in the direction of the laser beam source. The advantage of this configuration is that it eliminates the need for mechanically movable parts used to adjust the transmission frequency of the multibeam interferometer.

[0018] Furthermore, the forward-facing laser beam can be guided through an amplifier before it reaches the object. This allows for the achievement of the power required to process the object and compensates for power losses that occur in the interferometer.

[0019] The method according to the invention can be used in all laser processing techniques. It is particularly advantageous to use the forward laser beam to generate extreme ultraviolet (EUV) radiation. This radiation has high power, and therefore its reflection back to the laser beam source is particularly harmful.

[0020] The present invention also relates to an apparatus for illuminating an object with a laser beam, particularly for implementing the method according to the invention, the apparatus comprising a laser beam source, an object on which the laser beam is reflected back, and a multibeam 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 multibeam interferometer and the object, and / or the multibeam interferometer is designed such that its transmission spectrum is variable. With this apparatus, only the forward laser beam generated by the laser beam source can be transmitted through the multibeam interferometer, without transmitting the light reflected back from the object. This can be achieved by frequency shifting either the forward or backward laser beam after the interferometer by the frequency shifting device. The laser beam reflected from the object has the same frequency as the forward laser beam incident on the object. Therefore, one of the two beams must be frequency shifted relative to the transmission frequency of the multibeam interferometer to prevent the reflected light from propagating back to the laser beam source. In applications where the light reflected by the object has a different frequency than the light incident on the object, the frequency shifting device can also be completely omitted.

[0021] However, the back propagation of reflected light can also be prevented by changing the transmission frequency of the interferometer in a time-controlled manner, in sync with the pulse sequence frequency of the laser beam generated by the laser beam source.

[0022] In a preferred configuration of the device, the multibeam interferometer may have two wedge-shaped semi-transparent mirrors and one opaque, highly reflective mirror.

[0023] In this case, to adjust the transmission frequency of the multibeam interferometer, the position of an opaque, highly reflective mirror can be adjusted using a piezoelectric transducer. This adjustment changes the resonator length of the interferometer, thereby altering its transmission spectrum. The piezoelectric transducer can implement the required rapid oscillation motion synchronized with the pulse sequence frequency of the laser beam generated by the laser beam source. In this case, the required adjustment stroke of the mirror is several orders of magnitude smaller compared to a Mach-Zehnder interferometer.

[0024] As an alternative, a phase modulator can be placed in the beam path of the multibeam interferometer to allow the interferometer's transmission frequency to be changed in a time-controlled manner, synchronized with the pulse sequence frequency of the laser beam. The advantage of this configuration is that it eliminates the need for mechanically moving the interferometer's optical components. Furthermore, the phase modulator operates over a wide electrical bandwidth spectrum. These modulators can be used in frequencies ranging from the ultraviolet (UV) to the mid-infrared (MIR) range. Achievable switching frequencies are particularly high, up to 100 MHz.

[0025] A further advantage is that the multibeam interferometer has a beam trap for optically isolating the forward laser beam component and a beam trap for isolating the backward laser beam. This avoids scattered radiation leaving the interferometer.

[0026] A multibeam interferometer can be a classic Fabry-Perot interferometer, characterized by its simple structure. However, a multibeam interferometer preferably incorporates a ring resonator. This prevents reflections into the original beam path. Furthermore, the resonator can be filled with nitrogen or other media that have low damping for radiation, thus keeping beam damping to a minimum. Alternatively, the resonator can operate in a vacuum.

[0027] The frequency shifting device is preferably an acousto-optic modulator. However, other implementation methods may also be used.

[0028] In order to achieve the required power when irradiating an object, an amplifier can be advantageously arranged in the beam path in front of the object.

[0029] Other features and advantages of the invention will become apparent from the specification, claims, and drawings. According to the invention, the features mentioned above and further elaborated herein may be used individually or in any advantageous combination of several. The illustrated and described embodiments should not be construed as an exhaustive enumeration, but rather as having exemplary features for describing the invention.

[0030] Detailed description of the implementation method and accompanying drawings Figure 1 A schematic diagram of a first device for irradiating an object; Figure 2 This is a schematic diagram of a second device for irradiating an object; Figure 3 This is a schematic diagram of a third device used to irradiate an object.

[0031] Figure 1A laser beam source 10 generating a forward laser beam 11 is illustrated schematically. Before this laser beam is incident on an object 15, it is guided through a multibeam interferometer 12, and then through a frequency shifter 13 and an amplifier 14. The object 15 reflects a portion of the laser beam 11, thereby generating a backward laser beam 16. This laser beam 16 has the same frequency as the incident laser beam. The multibeam interferometer 12 is designed to suppress all frequencies except the frequency of the originally generated laser beam 11. In this respect, shifting the frequency of either the forward laser beam 11 or the backward laser beam 16 in the frequency shifter 13 causes the backward laser beam 16 to be suppressed in the multibeam interferometer 12. In the example shown, the forward laser beam 11 undergoes a frequency change in the frequency shifter 13 and exits the frequency shifter 13 as the forward laser beam 11'. The frequency shifter is preferably an acousto-optic modulator and performs a frequency shift of, for example, 80 MHz. In this case, the frequency shift is time-controlled and synchronized with the pulse sequence frequency of the forward laser beam 11, so that only the arriving forward laser beam 11 is shifted, while the backward laser beam 16 is not shifted.

[0032] The multibeam interferometer 12 has two wedge-shaped semi-transparent mirrors 17 and 18 and an opaque, highly reflective mirror 19. The semi-transparent mirrors 17 and 18 and the mirror 19 form a resonator. When the frequency of the laser beam 11 satisfies the resonator condition of the multibeam interferometer 12, the arriving laser beam 11 is transmitted through the semi-transparent mirror 17. Conversely, if the frequency of the laser beam 11 does not satisfy the resonator condition, the laser beam 11 is guided into the beam trap 22 by reflection from the semi-transparent mirrors 18 and 19. This avoids damage caused by an incorrectly adjusted resonator.

[0033] Conversely, due to the frequency shifting in frequency shifting device 13, the backward laser beam 16 does not satisfy the resonance condition of the multibeam interferometer. Therefore, the backward laser beam is guided into beam trap 21 by reflection on mirrors 17 and 19.

[0034] Figure 2 An alternative configuration of the device according to the invention is shown, wherein... Figure 1 Compared to the embodiments described herein, the same components are labeled with the same reference numerals. Here, the laser beam source 10 also generates a forward laser beam 11, which is guided by a multibeam interferometer 12'. However, Figure 2The device in this example does not have a frequency shifting device 13. The laser beam 11 is fed directly to the amplifier 14 after leaving the multibeam interferometer 12', and then incident on the object 15. Reflection on the object 15 generates a backward laser beam 16 with the same frequency as the forward laser beam 11. To prevent the backward laser beam 16 from passing through the multibeam interferometer 12' and incident on the laser beam source 10, the multibeam interferometer 12' has a scheme to change its transmission frequency in a time-controlled manner. This is achieved by arranging an opaque mirror 19 in an adjustable manner in the direction of the double arrow 23. The mirror 19 and wedge-shaped translucent mirrors 17 and 18 form a resonator therebetween, the length L of which is changed by adjusting the position of the mirror 19. However, this also changes the transmission frequency of the resonator, thereby changing the transmission frequency of the multibeam interferometer 12'. In this case, the reflector 19 is adjusted in a time-controlled manner: each time a forward pulse arrives, the resonator length L is adjusted so that the pulse is transmitted through the interferometer 12'; and each time a backward pulse arrives, the resonator length L is adjusted so that the beam 16 is guided into the beam trap 21. The adjustment of the reflector 19 can preferably be performed by a piezoelectric transducer.

[0035] Figure 3 The variant shown is equivalent in external structure to Figure 2 The variant shown is different. In contrast, the internal structure of the multibeam interferometer 12'' is different. The mirror 19 of the multibeam interferometer 12'' is not adjustable. Here, the transmission frequency is adjusted by a phase modulator 24, which is arranged in the beam path between the translucent mirror 17 and the mirror 19. When the forward pulse of the laser beam 11 arrives, the frequency is adjusted by the phase modulator 24 so that the laser beam 11 is transmitted. Conversely, when the backward pulse of the reflected laser beam 16 arrives, the frequency and resonance condition of the multibeam interferometer 12'' are changed so that the beam 16 is not transmitted, but is guided into the beam trap 21.

Claims

1. A method for irradiating an object (15) with a laser beam, comprising the following steps: a. A forward laser beam (11) is emitted from the laser beam source (10); b. Guide the forward laser beam (11) through a multibeam interferometer (12, 12', 12''), the multibeam interferometer being arranged in the optical path between the laser beam source (10) and the object (15), wherein the frequency of the forward laser beam (11) is equivalent to the transmission frequency of the multibeam interferometer (12, 12', 12''); c. By not satisfying the resonator conditions of the multibeam interferometers (12, 12', 12''), the transmission of the backward laser beam (16) generated by the reflection of the forward laser beams (11, 11') onto the object (15) and entering the multibeam interferometers (12, 12', 12'') in the opposite direction toward the laser beam source (10) is suppressed, in such a way as follows: - Before the backward laser beam (16) enters the multibeam interferometer (12, 12', 12''), the frequency of the backward laser beam is changed to a frequency different from the transmission frequency of the multibeam interferometer (12, 12', 12''). - and / or in a manner thereof, the transmission frequency of the multibeam interferometer (12, 12', 12'') is changed in a time-controlled manner in sync with the pulse sequence frequency of the laser beam emitted by the laser beam source, such that the forward laser beam (11) is transmitted by satisfying the resonator conditions of the multibeam interferometer (12, 12', 12''), and the backward laser beam (16) is suppressed toward the laser beam source (10) by not satisfying the resonator conditions of the multibeam interferometer (12, 12', 12'').

2. The method according to claim 1, characterized in that, After leaving the multibeam interferometer (12, 12', 12''), the forward laser beam (11) is guided through a frequency shifting device (13), which is time-controlled and synchronized with the pulse sequence frequency of the forward laser beam (11) emitted by the laser beam source (10).

3. The method according to claim 2, characterized in that, The frequency of the forward laser beam (11) or the frequency of the backward laser beam (16) is changed by the frequency shifting device (13) to a frequency (11') that is different from the transmission frequency of the multibeam interferometer (12, 12', 12'').

4. The method according to any one of the preceding claims, characterized in that, The resonator length (L) of the multibeam interferometer (12, 12', 12'') is changed in a time-controlled manner in sync with the pulse sequence frequency of the laser beam (11) emitted by the laser source (10). This is done so that the resonator length (L) is adjusted before each pulse of the forward laser beam (11) enters the resonator of the interferometer (12, 12', 12''), thereby transmitting the frequency of the forward laser beam (11) through the interferometer. After the pulse of the forward laser beam (11) leaves the resonator, the resonator length (L) is changed so that the frequency of the backward laser beam (16) is suppressed by the interferometer (12, 12', 12'') in the direction of the laser beam source (10).

5. The method according to claim 4, characterized in that, After the pulse of the forward laser beam (11) departs, the resonator length (L) is changed such that the change in transmission frequency is greater than the 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 in the resonator, L is the length of the resonator, and F is the Airy fineness of the multibeam interferometer (12, 12', 12'').

6. The method according to any one of the preceding claims, characterized in that, With the aid of the phase modulator (24) in the multibeam interferometer (12, 12', 12''), the phase is adjusted in a time-controlled manner in sync with the frequency of the pulse sequence of the forward laser beam (11) as the pulse of the forward laser beam (11) enters the resonator, such that the interferometer (12, 12', 12'') transmits the frequency of the forward 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 laser beam (16) in the direction of the laser beam source (10).

7. The method according to any one of the preceding claims, characterized in that, The forward laser beam (11, 11') is guided through an amplifier (14) before being incident on the object (15).

8. The method according to any one of the preceding claims, characterized in that, The forward laser beams (11, 11') are used to generate extreme ultraviolet (EUV) radiation.

9. The method according to any one of the preceding claims, characterized in that, The laser beam source (10) operates in a pulsed manner.

10. An apparatus for irradiating an object (15) with a laser beam, particularly for carrying out the method according to any one of the preceding claims, the apparatus having a laser beam source (10), an object (15) on which the laser beam is reflected back, and a multibeam 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 multibeam interferometer (12, 12', 12'') and the object (15) and / or the multibeam interferometer (12, 12', 12'') is designed such that its transmission spectrum is variable.

11. The apparatus according to claim 10, characterized in that, The multibeam interferometer (12, 12', 12'') has two wedge-shaped semi-transparent mirrors (17, 18) and an opaque, highly reflective mirror (19).

12. The apparatus according to claim 10 or 11, characterized in that, The opaque, highly reflective mirror (19) can be positioned by means of a piezoelectric transducer.

13. The apparatus according to any one of claims 10 to 12, characterized in that, A phase modulator (24) is arranged in the beam path of the multibeam interferometer (12, 12', 12'').

14. The apparatus according to any one of claims 10 to 13, characterized in that, The multibeam interferometer (12, 12', 12'') has a beam trap (20) for the erroneously optically isolated beam component of the forward laser beam (11) and a beam trap (21) for the isolated backward laser beam (16).

15. The apparatus according to any one of claims 10 to 14, characterized in that, The multibeam interferometer (12, 12', 12'') has a ring resonator.

16. The apparatus according to any one of claims 10 to 15, characterized in that, The multibeam interferometers (12, 12', 12'') are Fabry-Perot interferometers.

17. The apparatus according to any one of claims 10 to 16, characterized in that, The frequency shifting device (13) is an acousto-optic modulator.

18. The apparatus according to any 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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