CN115244799A

CN115244799A - Device for suppressing radiation feedback and laser apparatus having such a device

Info

Publication number: CN115244799A
Application number: CN202080098031.9A
Authority: CN
Inventors: S·埃哈德; G·施陶彭达尔
Original assignee: Trumpf Laser und Systemtechnik GmbH
Current assignee: Trumpf Laser und Systemtechnik GmbH
Priority date: 2020-03-03
Filing date: 2020-03-03
Publication date: 2022-10-25
Also published as: KR20220134656A; EP4115481A1; WO2021175412A1

Abstract

The invention relates to an optical device (20) for filtering laser radiation. The device (20) has a wavelength-selective element (28), a first Fabry-Perot interferometer (10), a polarizer (36) and a lambda/4 phase shifter (38). The optical axis of the Fabry-Perot interferometer (10) is oriented at an angle of more than 0 DEG and less than 6 DEG with respect to a laser beam arriving at the Fabry-Perot interferometer (10). The Fabry-Perot interferometer (10) is designed not only for coupling out undesired radiation components (30) of a primary laser beam (24) reaching the Fabry-Perot interferometer (10), but also for coupling out undesired radiation components (52) of a secondary laser beam (46) reflected by the laser beam target (40). Preferably, the device (20) comprises at least one further fabry-perot interferometer. Further preferably, the device (20) comprises a telescope device (70) for broadening a primary laser beam (24) reaching the fabry-perot interferometer (10). The invention also relates to a laser device having at least one such device (20). Preferably, the laser device comprises an amplifier system arranged between the two devices (20). Particularly preferably, the laser device is designed to generate EUV radiation. For this purpose, the laser beam target (40) may be in the form of droplets.

Description

Device for suppressing radiation feedback and laser apparatus having such a device

Technical Field

The present invention relates to an optical device for suppressing radiation feedback. The invention also relates to a laser device having such an arrangement.

Background

In many applications of high power lasers, from typical use cases of laser material processing to the generation of EUV radiation and so on, such specific arrangements, two main problems are important: firstly the focusability of the radiation, which is related to the beam quality, and secondly the suppression of the radiation feedback between the laser and the laser beam target. In a characteristic case, the focusing is such that the focal point is located on or at least in the vicinity of the surface of the laser beam target. This means that almost all radiation that is back-reflected or back-scattered from the focal volume in the direction of the focusing element propagates in the direction of the laser in a quasi-optimal parallelized manner. This radiation can reach considerable values, which can be much higher than 10% of the radiation power incident on the workpiece, mainly in the case of highly reflective materials such as copper or aluminum and in the case of flat workpiece surfaces. In particular in the case of EUV devices, back reflections of droplets or of many other flash sources (sources of flicker) occur. Since the out-coupling element of the laser is more or less partially transparent, a significant portion of this radiation reaches the resonator and causes an undesirable reduction in the population inversion (besetzengsingversion). This is undesirable because, firstly, the radiation from the laser beam target has statistical intensity fluctuations and, secondly, the coherence properties of the initial laser radiation have been lost to a large extent. If the radiation is amplified in the resonator, this therefore not only results in a laboriously generated transverse mode structure (most likely a TEM) ₀₀ ) Severe deterioration and also a severe P (t) profile in the case of pulsed lasersThe process becomes worse.

To suppress the feedback mentioned, polarization-sensitive reflective or transmissive structural elements have long been used for a wide variety of laser types (for CO) ₂ A laser, this is usually an arrangement of ATFR mirrors or brewster plates) in combination with a λ/4 phase shifter (typically in the form of a phase shifter mirror). These combinations even have a dual function in that, in addition to radiation decoupling, a generally desired circular polarization of the radiation propagating towards the target of the laser beam is produced. However, the decoupling by means of such devices is not optimal, which can be attributed firstly to imperfections (uvullkommenheit) of the optical components, secondly to imperfections in the adjustment, and thirdly to the influence of the polarization of the returning radiation upon interaction with the laser beam target. For these reasons, a few percent of this radiation always enters the resonator, but in many applications, for example in the cw operation of the laser, in which the inversion is continuously reduced and kept at a relatively low level, this generally has no significant effect on the machining process (Foglen).

In the case of defined beam forming which is external to the definition, i.e. outside the laser resonator, the feedback can likewise lead to a dramatic deterioration of the desired result, even if the laser itself is operated in continuous operation.

This situation proves to be more critical in the case of Q-switched lasers for high peak power. The principle of Q-switching is based on the extreme temporal overshoot of the population inversion and thus on the amplification in the active medium, which is a prerequisite for the targeted generation of powerful radiation pulses. At this point in time of maximum amplification, the system is extremely susceptible to the smallest amount of radiation fed back into the active medium in an undesired manner, whose "round trip" in the resonator can then easily be influenced by a factor of 10 ⁴ To 10 ⁵ And (5) amplifying. It is clear that this seriously affects the pulse generation, since the desired configuration of population inversion is disturbed on a large scale. Here, the above are shownThe "classical" decoupling is no longer sufficient and additional, efficient decoupling variants must be used.

Laser amplifier arrangements for the highest power, in which radiation returning from the laser beam target in the direction of the laser must pass through an amplifier step even before reaching the laser and be amplified sharply there, are similarly sensitive. An example of a current situation for this is an apparatus for generating EUV radiation, in which apparatus CO ₂ The seed laser produces extremely accurately defined primary radiation pulses which are then highly amplified and focused onto a laser beam target (typically in the form of a droplet). The backscattered radiation is not allowed to influence the beam generation process, i.e., the highest decoupling requirements also exist here. It is known from patent application DE 10 2015211 426A1 that an efficient optical isolator, i.e. in practice a "shutter" for short pulses, can be realized by means of a suitable arrangement of a passive polarization-selective structural element (polarization beam splitter) and an active polarization-selective structural element (electro-optical modulator) while taking advantage of the propagation time of the radiation pulses in the amplifier system. Since the propagation time is usually very short (e.g.. Ltoreq.60 ns in the case of propagation paths up to approximately 20 m), the method is limited to correspondingly short pulses.

Disclosure of Invention

The object of the present invention is therefore to provide a highly efficient device for decoupling radiation between a laser as radiation source and a laser beam target loaded with the radiation. In this case, the device should not rely on dynamic components, such as a fast modulator with expensive actuators and optionally a synchronization device synchronized with the pulsed radiation, but should have a fixed, static decoupling effect for short pulses in the ns range as well as for cw radiation. It is also an object of the invention to provide a laser device having such a device.

According to the invention, this object is achieved by a device having the features of claim 1 and a laser device according to claim 9. The dependent claims reflect preferred embodiments.

The object is therefore achieved according to the invention by a device for guiding a primary laser beam in a beam path. The device is also configured to at least partially direct the secondary laser beam reflected by the laser beam target. The device has the following components through which the beam path is directed:

a) A wavelength selective element;

b) A first Fabry-Perot interferometer which is arranged in the beam path of the primary laser beam at an angle of more than 0 ° and less than 6 ° relative to the optical axis of the Fabry-Perot interferometer;

c) A polarizer;

d) A/4 phase shifter.

The novel solution according to the invention is based on the combination of the "classical" decoupling, which is achieved by means of a lambda/4 phase shifter and polarization-sensitive reflective or transmissive structural elements, with the effect of a fabry-perot interferometer (FPI). In addition to the extremely efficient decoupling effect, the device has almost incidentally: the quality of the radiation propagating towards the laser beam target is effectively improved by using specific characteristics of the integrated FPI.

The basic idea of all decoupling methods is based on the following: the returning wave is extinguished by means of a generated (or already existing) difference in the parameters of the outgoing and returning (hin-und der ricklaufenden) waves. In the case of conventional decoupling, the parameter is polarization. However, there are also further differences between these two waves, which may be more or less pronounced. These additional differences are:

1. ) Divergence: in the case of waves propagating toward the laser beam target, this divergence typically corresponds to the divergence of a pure laser beam and is in the order of 1 mrad. Since firstly there is usually (intentionally or unintentionally) no ideal reflection on an ideally flat laser beam target and secondly there is no completely ideal focal position on the surface of the laser beam target (intentionally or unintentionally), the returning wave will have a distinctly different ("poor") divergence characteristic after the focusing lens, in a manner that is averaged over the course of the overall intensity variation.

2. ) Beam direction: since firstly there is no ideal orthogonality (intentionally or unintentionally) between the beam axis and the surface of the laser beam target and secondly there is no ideal focal position (intentionally or unintentionally) on the surface of the laser beam target, a (albeit small) difference in the beam direction of the outgoing and returning bunches occurs.

3. ) Coherence: the original laser beam is usually distinguished by good coherence, which is significantly influenced by reflections or scattering at the surface of the laser beam target, i.e. the returned bunch has significantly poorer coherence.

4. ) Polarization: laser radiation is almost always strictly linearly polarized. Behind the λ/4 phase shifter and thus ideally becomes circularly polarized radiation, which propagates towards the laser beam target. However, due to the fundamental physics responsible for reflection or scattering, the radiation component returning after interaction with the laser beam target is only incompletely circularly polarized, so that after passing through the λ/4 phase shifter, the desired wave, ideally linearly but perpendicularly polarized to the emerging radiation, is not generated and can therefore be destroyed only incompletely by polarization-sensitive structural elements.

5. ) Power: in typical devices for material processing with high-power lasers, the power is at a maximum for the outgoing beam bunching and is more or less greatly reduced for the returning beam bunching due to interaction with the laser beam target and due to optical structural elements. However, in laser amplifier arrangements (for example in EUV systems), the opposite situation may also occur if the point of view of the arrangement which should correspond to the decoupling system is located between the (relatively low-power) seed laser and the high-power amplifier which of course also amplifies the returned radiation.

6. ) Plasma effect: in practice all of the processing tasks discussed herein are associated with the generation of plasma sparks generated directly on the surface of the laser beam target or even with the conversion of the complete laser beam target/droplet to plasma. Since the plasma is close to the laser beam target, the effect of the plasma, for example, on the direction of the focused beam bunch is generally negligible, but is important for the direction, divergence, frequency and power of the returning beam bunch.

7. ) Wavelength (frequency): if the emerging beam is scattered or reflected on a very fast moving laser beam target (e.g. a fast expanding plasma), the returning beam has a certain frequency variation or wavelength variation according to the doppler effect.

All properties listed are important to the transmission and reflection characteristics of the FPI and can be used collectively to suppress the returning wave.

To better understand the most important relationships, the general performance of the FPI is briefly discussed below.

The transmission T of the FPI as a function of wavelength λ, plate reflectivity R and plate spacing a is calculated according to the following equation _FPI ：

Furthermore, the following simple relationship applies to lossless FPI:

P ₀ ＝P ₀ (T _FPI +R _FPI )＝P _T +P _R

and is

R _FPI (a)＝1-T _FPI (a)

For the contrast in transmission, i.e. the ratio of maximum transmission to minimum transmission, the following applies:

typical FPIs that can be considered for decoupling are those with R =0.5 and R =0.8, forThe contrast was found to be 9 or 81 on the FPI. In general, the R range that can be meaningfully used can be limited to 0.3 ≦ R ≦ 0.9. It will be discussed later which choices need to be made if the highest possible power, on the order of 1kW, needs to be decoupled. I.e. then another parameter, i.e. at the maximum transmission T _FPI Power P inside FPI in case of =1 _FPI Plays an important role in the loading of the FPI panel:

P _FPI ＝P _o /(1–R) ²

finally, a very important parameter for the intended decoupling effect of an FPI is the Finesse (Finesse) F of the FPI, which indicates the ratio of the period width to the half-value width of the transmission peak:

for R =0.8, for example, a fineness of 14 results, which decisively determines at the same time how sensitively the transmission of the FPI is dependent on the direction of incidence in conjunction with the plate spacing a.

The strong angular dependence of the FPI transmission makes it easy for one to think of another advantageous property of the device according to the invention: the beam quality of the produced laser beam bunching is improved by the spatial filtering effect of the FPI with optimized plate spacing.

The device may be arranged in the form of a unit, in particular in a common housing.

The angle of the optical axis of the first fabry-perot interferometer with respect to the beam path is preferably less than 5 °, in particular less than 4 °, particularly preferably less than 3 °. This makes it possible to operate the device particularly efficiently.

In a further preferred embodiment of the invention, the device has a first absorber for absorbing a component of the primary laser beam reflected by the first fabry-perot interferometer. Alternatively or additionally thereto, the device may have a second absorber for absorbing a component of the secondary laser beam reflected by the first fabry-perot interferometer. Coupling out of undesired back radiation can be ensured by one or more absorbers.

In order to be able to determine the undesired component of the back radiation, the first absorber and/or the second absorber can each be designed in the form of a radiation detector.

For the primary laser beam, the optical components mentioned are preferably arranged in the following order:

1. ) A wavelength selective element;

2. ) A first Fabry-Perot interferometer;

3. ) A polarizer;

4. ) A/4 phase shifter.

The focusing element may be arranged behind the lambda/4 phase shifter. Particularly preferably, no further structural elements are arranged between the components mentioned.

The first fabry-perot interferometer may have two plates of transparent material, the sides of the plates that lie opposite one another having a reflectivity R of between 0.3 and 0.9. The side preferably has a reflectivity R between 0.5 and 0.8. Alternatively or in addition thereto, the plates preferably have a spacing of between 1mm and 1000mm, in particular between 10mm and 30 mm. The plate is preferably made of ZnSe or diamond.

Further preferably, the wavelength selective element is configured in the form of a diffraction grating. The Blaze (Blaze) wavelength of the diffraction grating preferably corresponds to the wavelength of the primary laser beam.

The polarizer is preferably constructed in the form of an Absorbing Thin Film Reflector (AFTR). Alternatively or additionally thereto, the λ/4 phase shifter is preferably designed in the form of a λ/4 phase shifting mirror.

The above-described features of the optical construction element enable a structurally simple construction of the device with high operating capacity.

The device may have a telescopic device for widening (Aufweitung) the primary laser beam in front of the first fabry-perot interferometer. This makes it possible to illuminate the first fabry-perot interferometer with particularly high power.

The coefficient of broadening is preferably between 1.5 and 5, particularly preferably between 1.5 and 3.

The device may have a (nachgeordnet) second fabry-perot interferometer arranged behind the first fabry-perot interferometer. Thereby significantly increasing the decoupling efficiency. It is further preferred that a third fabry-perot interferometer is arranged behind the second fabry-perot interferometer. The reflectivity R of the plates of the additional fabry-perot interferometer is preferably between 0.2 and 0.7, particularly preferably between 0.3 and 0.5. Further preferably, interferometers with different reflectivities R are used.

The object is also achieved according to the invention by a laser device having a laser beam source for emitting a primary laser beam, a laser beam target and the device described here for guiding the primary laser beam, wherein a beam path of the device extends between the laser beam source and the laser beam target.

The laser beam source is preferably designed in the form of a high-power laser beam source for emitting a primary laser beam, the average power of which is at least 1kW. The laser beam source is especially configured as CO ₂ In the form of a laser beam source. The laser beam source may be arranged for emitting a primary laser beam having a beam width 10 ^-9 s and 10 ^-6 A form of radiation pulse of pulse duration between s. The laser device is thus particularly well suited for generating EUV radiation.

The laser device may have an amplifier system which is arranged in the beam path between the laser beam target and the laser beam source, wherein the device is arranged between the amplifier system and the laser beam source. In addition to this, the laser device may have a further apparatus described herein, which is arranged between the amplifier system and the laser beam target. Alternatively or additionally thereto, the laser device may have a further λ/4 phase shifter, which is arranged between the amplifier system and the further arrangement. By means of one or more of these features, a laser device with particularly excellent performance and a compact design can be achieved.

In a particularly preferred embodiment of the invention, the laser beam target is designed to emit EUV radiation when irradiated with the primary laser beam. The laser beam target may be configured in the form of a droplet. The laser beam target is preferably designed in the form of a tin droplet.

The invention further relates to a device as described herein and/or to the use of a laser apparatus as described herein in an apparatus for generating EUV radiation, in particular an EUV lithography apparatus.

Further advantages of the invention emerge from the description and the drawings. Likewise, the features mentioned above and those yet further explained can be applied in accordance with the invention either individually per se or in any combination of a plurality of forms. The embodiments shown and described are not to be understood as an exhaustive enumeration but rather have exemplary character for the description of the invention.

Drawings

Fig. 1 shows a measurement of the effect of suppressing radiation feedback on the formation of defined radiation pulses;

fig. 2 shows a basic configuration of a fabry-perot interferometer (FPI);

FIG. 3 shows transmission curves for FPI for three selected panel reflectivities R;

FIG. 4 schematically illustrates the separation of incident and reflected laser beam bunching on an FPI;

fig. 5 shows the calculation of the relative position for an FPI with plate spacing a =10cm and fineness of 14 for a returned wave shifted at a frequency of 100 MHz;

FIG. 6 schematically shows the measurement of radiation filtering by means of FPI;

FIG. 7 shows a highly schematic basic configuration of the device according to the invention;

fig. 8 shows a configuration for a laser device in the form of a laser amplifier arrangement according to the invention;

fig. 9 shows the widening of the radiation technique with high power by means of a telescope;

FIG. 10 shows the principle construction of an FPI tandem.

Detailed Description

FIG. 1 shows a characteristic example of the necessity of suppressing radiation feedback, wherein the CO should be removed by means of an interferometric laser radiation modulator ₂ A defined pulse is formed in the cw radiation of the laser. Without suppressing the feedback, the almost statistical P (t) profile shown in fig. 1a is produced, and with the decoupling achieved by means of the AFTR mirror and the λ/4 phase-shifting mirror, a clean desired pulse function according to fig. 1b is produced. The example shown in fig. 1 relates to the defined radiation pulses being carried out by means of CO ₂ Particular application in the finishing by a laser. In this case, the desired pulse train is generated from the inherently continuous laser radiation by means of a special interferometric laser radiation modulator (ILM) which is based on a rapidly continuously adjustable FPI. This mechanism is susceptible to interference without radiation decoupling. More or less statistically undesirable pulses occur, wherein, in this example, an important factor in the case of the use of interferometers has become apparent: the FPI transmission is strongly dependent on the wavelength of the radiation. In the example, the pulse generation is based on a fast switching of the ILM between maximum transmission and maximum reflection, which of course only works for defined wavelengths. However, without radiation decoupling and without specific wavelength selective elements (e.g. grids) to ensure this defined wavelength, then even if the FPI is blocked for the really desired wavelength, e.g. high power CO ₂ Parasitic oscillations of the laser may also occur at this high power CO ₂ In the case of lasers, potentially a large number of rotational vibration transitions between 9 μm and 11 μm are available. This is shown heuristically in fig. 1 a. Even in the solution according to the invention, the fact must be taken into account that the system must be fixed to the selected wavelength by means of an integrated wavelength selective element. In these respects

It is generally noted that, despite all losses in the beam path, the laser resonates on one sideThe end mirror of the device and the laser beam target on the other side form a resonator. This creates a risk of parasitic oscillations, wherein, with corresponding possibilities, the radiation will "pick up" exactly the following wavelengths: the ratio of gain to loss is greatest for this wavelength. This possibility is just prohibited by the frequency-selective element, which may possibly already be integrated into the laser resonator itself.

Fig. 2 shows the basic configuration of an

FPI10 having plates

12a and 12b with a spacing a to illustrate the performance enumerated above. What is often important in the solution according to the invention is that high powers, for example average powers in the range of 1kW or more, are handled, in which solution an FPI10 with the shown configuration will advantageously be used, in which there is air between the two decisive faces with reflectivity R. This reduces the adverse effect of excessive power in the FPI on the R coating itself, especially when using a larger R (R)>0.5 ) are used. A more or less large basic absorption of the carrier material of the

FPI panels

12a,12b plays a subordinate role. CO generally discussed in detail herein ₂ In laser systems, the carrier material is typically ZnSe or diamond. In fig. 2, the antireflective coating is labeled with "AR".

Fig. 3 shows the theoretical transmission of RFPI10 as a function of plate spacing a for three selected reflectivities. The figure illustrates the strong R-dependence of contrast and finesse, both of which are crucial for the FPI 10's effect for suppressing radiation feedback between the laser beam target and the laser beam source. This not only relates to the sensitivity with respect to the minimum angular change of the radiation incident on the FPI10, but also to the frequency change or wavelength change of the radiation returning from the laser beam target, which is generated, for example, in the case of reflection or scattering on a rapidly expanding plasma, which is characteristic for the generation of EUV radiation.

However, in addition to the reflectivity R, two individual parameters are decisive for the FPI effect. One is the plate spacing a and the other is the angle of incidence δ of the radiation with respect to the FPI axis. The latter has a large practical significance, since it must of course be ensured that the radiation reflected by the FPI (which relates not only to the radiation propagating from the laser beam source to the laser beam Target (Target) but also to the returned radiation) is removed from the main beam path. Especially in laser amplifier systems, it is necessary to prevent: the FPI10 forms a mirror which oscillates the amplifier chain (anskwingen).

In order to meet these requirements, a minimum angle δ is required which is sufficient for separating the incident and reflected beams after a predetermined distance s (see fig. 4). Preferably, however, two factors in the FPI function need to be noted at the same time. For an efficient decoupling effect of the FPI10, a sufficiently large plate spacing a is preferred. The optimum size of the plate spacing is decisively dependent on the respective parameter of the returning radiation, which first of all becomes effective for decoupling. Due to the high selectivity of the FPI10 with respect to the incident angle δ, if the finesse of the FPI is assumed to be 14, then to suppress the incoming beam having an axis with respect to the FPI>A radiation component with a divergence or tilt of 1mrad, a plate spacing a in cm has been preferred. A significantly larger plate separation a is preferred if the parameter affecting the decoupling is a frequency shift due to the doppler effect on a fast moving laser beam target. For an efficient decoupling effect a should preferably be at least 10cm (see fig. 5), for example if a doppler shift on a very rapidly expanding plasma on the irradiated droplets is assumed to be about 100MHz at EUV generation. However, if a frequency offset of 100MHz is taken into account, for example CO ₂ Of lasers or CO ₂ This requirement is relative, since the optical gain of the active medium of the amplifier is significantly reduced, so that a second effect occurs here, which contributes to the suppression of the returning wave. This fact is very advantageous for the solution of the "optimal" FPI10, since it has to be taken into account that the ideal FPI characteristic shown in fig. 3, in which case normal incidence and monochromatic radiation are assumed, deteriorates as a increases and even more strongly as δ increases simultaneously. In particular, the maximum transmission is further and further away from the ideal value 1, i.e. the losses for the working beam increase. As use ofThe general optimization rule for the plate spacing a of the FPI follows that a should be chosen as large as needed, but as small as possible.

Fig. 4 again illustrates the described geometry scale. As marginal conditions, it should be satisfied here that: firstly, it must be ensured that the incident laser beam bunch 14 having the diameter d is separated cleanly from the component 16 reflected on the FPI10 having, for example, the same diameter, and secondly, the required path distance s should not be too large here, i.e. the angle δ should not be too small. Third, the effect of δ (roller) is relatively complex: in the sense of a high filtering effect on the undesired radiation and a high efficiency in the decoupling of the radiation, δ should likewise not be too small, on the other hand, as mentioned above, the maximum transmission, which of course should be close to 1 for the working beam, deteriorates with increasing δ. In fig. 4, the laser beam bunch 14 exits from the wavelength selective element 18.

Therefore, δ should preferably be optimized according to the specific situation, in particular according to the characteristics of the laser beam itself. This optimization is closely related to the spacing a of the

FPI plates

12a,12b, since ultimately two geometry parameters a and δ, in addition to the plate reflectivity R, determine the effectiveness of the described device. If it is assumed that the divergence of the laser radiation is about 1mrad, the depicted requirements can be met by means of "reasonable" values, which for a, as described above, in the range up to about 10cm, for D about 10mm, for δ =2.5 °, and for s from about 20 cm. Then, if R =0.8 is assumed, i.e. with a fineness of 14, three important parameters, namely beam divergence, inclination and frequency offset with respect to the working beam, can be calculated in the desired sense with the aid of the high effectiveness of the FPI 10. In particular, in the setting state corresponding to T ≈ 1 for the working beam, the beam components to be "extinguished" (i.e. those in the outgoing (hinlaufe) beam which have a low beam mass and therefore a higher divergence than the typical 1 mrad) and those in the returning beam which have a divergence with respect to the FPI axis>Radiation components with a divergence or tilt of 1mrad, or due to the doppler effect on a rapidly moving laser beam targetThe resulting frequency shift on the order of 100 MHz) can be shifted to a range (R) with high reflection _FPI >0.8 ) and can therefore be easily eliminated. In terms of the effect of the frequency shift in question on the FPI effect in the decoupling of radiation, it should be noted that the radiation components backscattered or reflected on the rapidly expanding plasma also have divergence and direction fluctuations, whose effect is superimposed on the effect of the frequency shift in the sense of a significantly larger overall effect.

Fig. 5 shows the "filtering effect" of the FPI10 on the returned wave shifted at a frequency of 100MHz, where the plate spacing a =10cm for the FPI10 and the fineness is 14. In fig. 5, the transmission T is plotted as a function of the frequency f.

Fig. 6 additionally shows the results of a measurement which clearly illustrates the effect of an optimized FPI10 as a radiation filter in the sense of the present invention. Research low pressure CO ₂ The influence of the gas discharge current I of the laser on the beam quality. The latter being characterised by a quotient P _T /P _R The quotient reflecting the useful power P _T (i.e. radiation transmitted by the FPI) and the power component P reflected by the FPI10 to be filtered out _R The ratio of (a) to (b). Since the laser is operated in a Regime (Regime) intended to cause overheating of the laser gas relatively quickly, the beam quality (i.e. quotient P) is seen very heuristically in the graph _T /P _R ) How to assume a significant maximum at a relatively low current in order to drop sharply in the event of a current exceeding this maximum and thus a significant overheating. In the example, the filtered-out radiation component increases from approximately 17% to approximately 25% of the incident power, which ultimately means that, although the "original beam" of the laser has lost its quality significantly, the quality of the useful beam remains almost constant (but in the case of a power drop). By continuously monitoring the filtered-out radiation component which is continuously available for measurement during the machining process, important variables for diagnosing the machining process are obtained.

Fig. 7 shows a highly schematic basic configuration of the device 20 according to the invention. In the apparatus 20, first, a laser beam source is used22, the laser beam source may be a Q-switched CO, for example ₂ A high power laser. The primary laser beam 24 of the laser beam source is characterized by a defined wavelength λ, a linear polarization in a defined direction 26 and a more or less good beam quality K. Since the components according to the invention for beam filtering on the one hand and for suppressing radiation feedback on the other hand are to be designed for the precisely defined wavelengths λ mentioned, the system in the beam path "laser beam source — beam guidance and beam formation — laser beam Target" must be prevented from selecting further wavelengths in order to ensure their function. This risk is present, for example, in high-power CO ₂ In the case of a laser, the high-power CO ₂ The laser can oscillate at a large number of wavelengths between 9 μm and 11 μm. The wavelength-selective element 28 (for example a diffraction grating or a prism) ensures that the system is reliably fixed to a predetermined lambda. It should be noted at this point that the wavelength selective element 28 may also be already integrated into the laser, i.e. the laser beam source 22, and is then naturally no longer necessary outside the laser. However, the situation is slightly different in high power laser amplifier devices as used for example in EUV systems (see fig. 8). In the second device for suppressing radiation feedback, which may be necessary there, the wavelength-selective element is preferably not omitted.

On its other path, the primary laser beam 24 passes through the central element of the solution according to the invention, namely the fabry-perot interferometer (FPI) 10 optimized according to the above-described embodiment. A first task for FPI10, in which the wavelength λ is set to maximum transmission, consists in filtering out the radiation component 30, which reduces the beam quality, thus the focusability of the radiation and ultimately the processing quality. Typically, this is the following radiation component: the radiation component deviates more or less strongly from the fundamental mode TEM ₀₀ . In the simplest case, the radiation component is eliminated by an absorber 32, which, as here, can be designed in the form of a radiation detector. The measuring signal of the absorber provides information on the one hand about the magnitude of the parasitic radiation component 30, but at the same timeThe recording of the time profile furthermore provides information about the stability of the laser beam source 22 and/or the pulse characteristics of the laser, and can therefore also be used for diagnostic and control purposes.

The radiation component 34 transmitted and filtered by the FPI10 is then transmitted by a "classical" combination of a polarizer 36 and a λ/4 phase shifter 38 for radiation decoupling of the laser (laser beam source 22) and the target (laser beam target 40), wherein a generally desired conversion of linearly polarized radiation into circularly polarized radiation 42, which can be transmitted, for example, by means of a focusing element 44 onto the laser beam target 40, takes place at the same time.

In addition to this first conversion, the effect of the

elements

36 and 38 is that a secondary laser beam 46 returning from the laser beam target 40 in the direction of the laser beam source 22 is first converted in the λ/4 phase shifter 38 in a second conversion into partially linearly polarized radiation 48, which is typically more or less partially circularly polarized, the polarization direction of which is perpendicular to the polarization direction of the primary laser beam 24. This is a prerequisite for the following: this radiation 48 is largely eliminated and removed from the beam path "laser beam source 22-laser beam target 40", depending on the degree of linear polarization and the quality of the polarizer 36.

In CO ₂ In the example of a laser, a brewster plate (ATFR mirror) is usually used for the

elements

36 and 38, or an ATFR mirror, which absorbs radiation to be extinguished in the "wrong" polarization direction, and a λ/4 phase shifter mirror. However, for the reasons mentioned above, the returned radiation is not completely eliminated. A radiation component 50 which must be in the range of the percentage of the primary laser beam 24 would propagate without further precautionary measures in the direction of the laser beam source 22 and would lead to interference with the process of radiation formation in the laser beam source 22. These disturbances are all the more severe the higher the population inversion in the active medium and hence the optical gain of the laser. This is the case in particular in systems of Q-switches, whose principle is based on maximizing the population inversion directly before the pulse generationAnd (4) melting. At this point in time, small signal amplification in the active medium

Can absolutely reach the order of magnitude of 10 ⁴ I.e. even a very small amount of fed back radiation can seriously affect the construction of a powerful pulse.

According to the invention, this dilemma is solved by the effect of a specifically optimized fabry-perot interferometer 10. The FPI10, arranged at the mentioned small angle δ, attenuates the radiation component 50 in a multiple manner to a scale that is not dangerous for the laser function. In this case, the difference in the parameters of the outgoing radiation component 34 and the returning radiation component 50 is decisive. The most important differences have already been mentioned, which result in high reflections on the FPI10 and thus in the desired elimination of the radiation component 50. It has also been said that in practice it is not possible to carry out an accurate quantitative determination of all the effects which work together. It makes sense for qualitative considerations to divide the effects into coherence-independent effects and coherence-constrained effects. We want to understand the following effects as coherence-constrained effects: the effect causes interference and is therefore closely related to the characteristics of the FPI10 discussed above. It is important with this property that the divergence, direction and (possibly) wavelength of the radiation component 50 differs from the initial beam 34 so much that the FPI10, which is set for optimal transmission for the beam 34, contributes significantly to the beam 50 as close as possible to the reflection maximum and to the radiation component 52.

For example, a simple reflection at the FPI surface of the reflectivity R, which already reflects approximately 90% of the radiation component 50 in the direction of the component 52, for example when R =0.8, taking into account two FPI surfaces without interference effects, is independent of the coherence. The stronger the initial good coherence of the beam 34 is destroyed by interaction with the laser beam target 40, the more important the reflection of this radiation component becomes, which is largely incoherent. The total radiation component 52, which ideally corresponds approximately in terms of power to the radiation component 34, is extinguished in the second absorber 54 and finally an efficient blocking (abbocken) of the returning radiation is ensured.

In this case, the total attenuation of the returning radiation reaches a value which may be at 10 ³ And 10 ⁴ Coefficient of (d) between. The efficiency of this total attenuation is at least 1 to 2 orders of magnitude higher than that of a conventional combination consisting of a polarizer and a lambda/4 phase shifter, which is suitable for the following radiation decoupling: the highest requirements are placed on this radiation decoupling.

In this connection, it should again be pointed out that the preferred sequence of the

elements

10, 36 and 38 is important, in particular, in terms of the power sensitivity of the FPI10 and is decisive, in particular, in the case of a laser amplifier arrangement (see also the explanations of fig. 8, 9 and 10). That is, if the radiation power to which the FPI10 is exposed, which ultimately relates to the sum of the emitted and returned radiation, is above a critical value, which is typically in the range of a few kW, thermal effects occur which, in an uncontrolled manner, for example, shift a carefully set working point (T =1 for radiation propagating towards the laser beam target 40) and thus may deteriorate or even jeopardize the function of the overall system. In the arrangement of the

elements

10, 36 and 38 according to the invention, the power load of the FPI10 is in any case minimized for the following reasons: first, whether the FPI is placed in front of or behind the

elements

36 and 38, the FPI10 is always exposed to the full power of the radiation propagating toward the laser beam target 40, since it can be very approximately assumed that losses in the

elements

36 and 38 are minimal. Second, therefore, the difference in radiation exposure of the FPI10 for the two arrangement variants is in practice only related to the power of the returned radiation reaching the FPI 10. In the inventive sequence of the

elements

10, 36 and 38, however, this returned radiation is decisively attenuated by the decoupling effect of the

elements

36 and 38, so that even in the case of very powerful returned radiation components, which may occur in high-power laser amplifier devices, the FPI function 10 is not endangered by these radiation components.

In particular, to simplify the following embodiment, the structural elements essential for the beam filtering and radiation decoupling according to the invention are integrated into the device 20.

Fig. 8 shows a laser device 56 according to the invention in the form of a laser amplifier arrangement. The amplifier system 58 can also be composed of a plurality of stages, for example an additional preamplifier or a plurality of high-power amplifiers. In all multi-stage systems, the following problems arise: how often and at which locations filtering and decoupling should be performed. Regardless of the specific requirements, the filtering should be carried out as directly as possible behind the laser output, and decoupling should be considered significantly more complex. This is clearly shown in figure 8. The radiation 22 generated in the laser 22 and having precisely defined characteristics, for example radiation pulses having a defined duration and repetition frequency (Folgefrequenz), is brought to a power level at which the highly demanding application can be realized in the amplifier system 58. Thus, problems arise which far exceed those of the simple laser 32-laser beam target 40-arrangement according to fig. 7, due to the very high optical gain in the active medium of the amplifier system 58. Thus, for example, it is to be taken into account that, on the one hand, the radiation reflected back from the laser beam target 40, in particular in the form of droplets, should not pass through the amplifier system 58 anyway, since otherwise it would be extremely amplified and would therefore significantly impede the decoupling of the laser 22. Besides, in the case of high power amplifiers, the following parts of the superradiation should be considered: this portion propagates in the direction of laser 22 and occurs in any event and independently of the radiation feedback through laser beam target 40.

Fig. 8 schematically shows how this problem can be solved according to the invention. Accordingly, the laser radiation 24 first passes through the first device 20a, which assumes all the tasks discussed with respect to fig. 7, namely firstly beam filtering and secondly efficient suppression of the radiation feedback into the laser 22. The radiation component 60 consists of two components: firstly, the component reflected or scattered by the laser beam target 40, which is only incompletely extinguished by the second device 20b and highly amplified in the amplifier system 58, and secondlyA component which occurs even in the case of ideal functioning of the second device 20b, namely unidirectional superradiation (ASE) of the amplifier system 58 itself. The latter is substantially unpolarized and is therefore reduced to only about half by the cell constituted by the polarizer 36 and the λ/4 phase shifter 38 (see fig. 7) of the first arrangement 20 a. However, this is not sufficient in many cases, so that the effect of the FPI10 is particularly important. After passing through the first means 20a, the radiation in the amplifier system 58 is typically amplified to a very high value of the pulse peak power, but also to a very high value of the average power. The latter is for example used in EUV generated CO ₂ In laser amplifier installations, in the region of several kW, and therefore special precautions are required for the second device 20b, which should ensure that the amplifier system 58 is safely shielded with respect to the radiation component 62 returning from the laser beam target 40.

The effect of the second device 20b, in particular with respect to radiation decoupling, should here be completely similar to the effect of the first device 20 a. For this purpose, the amplified, circularly polarized radiation 64 is first converted into linearly polarized radiation again. This is done by means of a lambda/4 phase shifter 66. That is, for the second arrangement 20b, it should be noted in comparison with the first arrangement 20a that firstly the incident radiation has a significantly higher power value and secondly the linear polarization of this incident radiation is rotated by 90 ° with respect to the laser radiation 24. The latter is not a problem, and in principle it is possible to rotate only 90 ° for this purpose by the polarizer 36 and the λ/4 phase shifter 38 (see fig. 7).

It is significantly more critical that the high average power is safely controlled due to the sensitivity of the FPI10 (see fig. 7) due to the excessive power inside the FPI. This high average power is already the incident power P, for example, in the case of R =0.8 and maximum FPI transmission T =1 ₀ 25 times of and thus P in the kW range ₀ Even when diamond optics are used, is in the limit of the maximum allowable load.

For effective and flexible remedial measures, there are two possibilities. Fig. 9 shows the simplest possibility of widening the diameter of the beam bunch 68 by means of a telescope arrangement 70 immediately in front of the FPI10, which has a larger free aperture matched to the widened beam 72. The intensity in the R plane can thus be set within wide limits in such a way that the FPI function and thus the beam 74 proceeding from the FPI10 are not impaired by thermal effects.

The remedial measures according to the invention can also suggest an FPI cascade if it is desired to avoid changes to the beam diameter. FIG. 10 shows the base version, i.e., FPI Tandem (Tandem). The solution is based here on the assignment of the FPI effect for suppressing the radiation feedback to a plurality of FPIs, here a first FPI10 and a second FPI76, which in particular has a reduced R. The numerical example should illustrate this. For example, if the effect of an FPI with R =0.8 should be achieved equivalently, but a power overshoot by a factor of 25 as discussed above is unacceptable, the beam 78 may be sent to a first FPI10 with R =0.5 (which is equivalent to a power overshoot by a factor of only 4), and the beam 80 leaving the first FPI10 is sent onto a (in particular identical) second FPI 72. The desired overall effect is then produced, but the loading of the R layers of the two

FPIs

10, 76 will be reduced by a factor of 6.25. The basic principle can be extended and optimized, for example, by using more than 2 interferometers in order to further reduce the radiation load, or by using a plurality of interferometers with optimized, different R in order to maximize the decoupling efficiency while minimizing the radiation load.

When summarizing all the figures of the drawing, the invention generally relates to an

optical device

20, 20a,20b for filtering laser radiation. The

arrangement

20, 20a,20b has a wavelength-selective element 28, a first fabry-perot interferometer 10, a polarizer 36 and a λ/4 phase shifter 38. The optical axis of the fabry-perot interferometer 10 is oriented at an angle of more than 0 ° and less than 6 ° with respect to the laser beam arriving at the fabry-perot interferometer 10. The fabry-perot interferometer 10 is designed not only for coupling out undesired radiation components 30 of the primary laser beam 24 reaching the fabry-perot interferometer 10, but also for coupling out undesired radiation components 52 of the secondary laser beam 46 reflected by the laser beam target 40. Preferably, the

arrangement

20, 20a,20b comprises at least one further fabry-perot interferometer 76. It is further preferred that the

arrangement

20, 20a,20b comprises a telescope arrangement 70 for broadening the primary laser beam 24 reaching the fabry-perot interferometer 10. The invention also relates to a laser device 56 having at least one

such apparatus

20, 20a,20 b. Preferably, the laser device 56 comprises an amplifier system 58 arranged between the two

apparatuses

20a,20 b. Particularly preferably, the laser device 56 is configured for generating EUV radiation. For this purpose, the laser beam target 40 may be in the form of a droplet.

List of reference numerals

10. First Fabry-Perot interferometer

12a,12b Fabry-Perot interferometer plate

14. Laser beam bunching

16. Reflected component of laser beam bunching

18. Wavelength selective element

20 20a,20b apparatus for beam filtering and radiation decoupling

22. Laser beam source

24. Primary laser beam

26. Direction of polarization

28. Wavelength selective element

30. Radiation component filtered from the primary laser beam

32. First absorber

34. Filtered radiation component of the primary laser beam

36. Polarizer

38. Lambda/4 phase shifter

40. Laser beam target

42. Circularly polarized radiation

44. Focusing element

46. Secondary laser beam

48. Partially linearly polarized radiation

50. Returned attenuated radiation component

52. Returned (secondary) radiation component cancelled by FPI

54. Second absorber

56. Laser device

58. Amplifier system

60. Radiation component propagating from the amplifier system in the direction of the laser beam source

62. Component of radiation returned from laser beam target

64. Amplified, circularly polarized radiation

66. Lambda/4 phase shifter

68. Beam bunching

70. Telescope device

72. Widened beam

74. Beam from FPI

76. Second FPI

78. Beams arriving at the first FPI10

80. Beams leaving the first FPI10

Claims

1. A device (20, 20a, 20b) for guiding a primary laser beam (24) and a reflected secondary laser beam (46) on a beam path, wherein the device (20, 20a, 20b) has components through which the beam path is guided:

a. a wavelength selective element (28);

b. a first Fabry-Perot interferometer (10) arranged in the beam path at an angle of more than 0 ° and less than 6 ° with respect to an optical axis of the Fabry-Perot interferometer (10);

c. a polarizer (36);

d. a lambda/4 phase shifter (38).

2. The device according to claim 1, wherein the device (20, 20a, 20b) has the following:

a first absorber (32) for absorbing a component (30) of the primary laser beam (24) reflected by the first Fabry-Perot interferometer (10); and/or

A second absorber (54) for absorbing a component (52) of the secondary laser beam (46) reflected by the first Fabry-Perot interferometer (10).

3. The device according to claim 2, wherein the first absorber (32) and/or the second absorber (54) are each configured in the form of a radiation detector.

4. The device according to any of the preceding claims, wherein for the primary laser beam (24) the wavelength selective element (28) is arranged in front of the first fabry-perot interferometer (10), the fabry-perot interferometer (10) is arranged in front of the polarizer (36), and the polarizer (36) is arranged in front of the λ/4 phase shifter (38).

5. The device according to any of the preceding claims, wherein the first fabry-perot interferometer (10) has two plates (12a, 12b) made of transparent material, the sides of the two plates opposite each other having a reflectivity R between 0.3 and 0.9.

6. The device according to any one of the preceding claims, wherein the wavelength selective element (28) is configured in the form of a diffraction grating.

7. The device according to one of the preceding claims, having a telescope device (70) for widening the primary laser beam (24) in front of the first fabry-perot interferometer (10).

8. The device according to any of the preceding claims, having a second fabry-perot interferometer (72) arranged behind the first fabry-perot interferometer (10).

9. A laser device (56) having a laser beam source (22) for emitting a primary laser beam (24), a laser beam target (40) and an apparatus (20, 20a, 20b) for guiding the primary laser beam (24) according to any of the preceding claims, wherein a beam path of the apparatus (20, 20a, 20b) extends between the laser beam source (22) and the laser beam target (40).

10. Laser device according to claim 9, having an amplifier system (58) which is arranged between the laser beam target (40) and the laser beam source (22) on the beam path, wherein the device (20 a) is arranged between the amplifier system (58) and the laser beam source (22), wherein the laser device (56) has a further device (20 b) according to one of claims 1 to 8 which is arranged between the amplifier system (58) and the laser beam target (40), wherein the laser device (56) has a further λ/4 phase shifter (66) which is arranged between the amplifier system (58) and the further device (20 b).

11. The laser device according to claim 9 or 10, wherein the laser beam target (40) is configured to emit EUV radiation upon irradiation by means of the primary laser beam (24).

12. The laser device according to claim 11, wherein the laser beam target (40) is configured in the form of a droplet.