Apparatus for suppressing radiation feedback and laser device having the same
By combining wavelength selection elements, a Fabry-Perot interferometer, a polarizer, and a λ/4 phase shifter, the problem of radiation feedback in high-power lasers was solved, achieving efficient radiation decoupling and improving the stability and processing quality of the laser.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- TRUMPF LASER SYSTEMS FOR SEMICONDUCTOR MANUFACTURING GMBH
- Filing Date
- 2020-03-03
- Publication Date
- 2026-06-16
Smart Images
Figure CN115244799B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to an optical device for suppressing radiation feedback. It also relates to a laser device having such a device. Background Technology
[0002] In various applications of high-power lasers, from typical uses in laser materials processing to specific arrangements such as EUV radiation generation, two main issues are important: first, the focusability of the radiation, which is related to beam quality, and second, the suppression of radiative feedback between the laser and the laser beam target. In characteristic cases, focusing is achieved such that the focal point lies on or at least near the surface of the laser beam target. This means that almost all radiation back-reflected or back-scattered from the focal volume along the direction of the focusing element propagates in a quasi-optimally parallel manner along the laser's direction. This radiation can reach considerable values, especially in the case of highly reflective materials such as copper or aluminum, and on flat workpiece surfaces, values that can be well above 10% of the radiation power incident on the workpiece. Particularly in the case of EUV equipment, back-reflection from droplets or numerous other flash sources (scintillation sources) occurs. Since the laser's coupling output element is more or less partially transparent, a large portion of this radiation reaches the resonator and causes an undesirable reduction in population inversion. This is not desirable because, firstly, the radiation from the laser beam target exhibits statistically significant intensity fluctuations, and secondly, it has largely lost the coherence properties of the initial laser radiation. Amplifying this radiation in the resonator would therefore not only disrupt the painstakingly generated transverse mode structure (most likely TEM) 00 The degradation is severe, and even in the case of pulsed lasers, the P(t) change process is severely degraded.
[0003] To suppress the aforementioned feedback, a combination of polarization-sensitive reflective or transmission structural elements (for CO2 lasers, this is typically an ATFR mirror or Brewster plate arrangement) and a λ / 4 phase shifter (typically in the form of a phase shifter mirror) has long been used for various laser types. These combinations even serve a dual purpose, as they generate the generally desired circular polarization of the radiation propagating toward the laser beam target, in addition to radiative decoupling. However, decoupling with such a device is not optimal, primarily due to the imperfections of the optical structural elements, secondly due to deficiencies in tuning, and thirdly due to the influence of the polarization of the returned radiation when interacting with the laser beam target. For these reasons, a few percent of this radiation always enters the resonator; however, in many applications, such as in the cw operation of the laser, the reversal continuously decreases and remains at a relatively low level, which generally has little effect on the processing.
[0004] In the case of defined beamforming that occurs externally, i.e. outside the laser resonator, feedback can also lead to a dramatic deterioration of the desired results, even if the laser itself is operating continuously.
[0005] This situation proves even more critical in the case of Q-switched lasers used for high peak power. The principle of Q-switching is based on an extremely temporary excess of population inversion and therefore on amplification in the active medium, which serves as a prerequisite for the targeted generation of high-power radiation pulses. At this point of maximum amplification, the system is extremely susceptible to even the smallest amount of radiation fed back into the active medium in an undesirable manner, and the "round trip" of this radiation in the resonator can then easily be affected by a factor of 10. 4 Up to 10 5 Magnification. Clearly, this severely impacts pulse generation because it massively disrupts the desired construction of population inversion. Here, the “classical” decoupling shown above is no longer sufficient, and additional, more efficient decoupling variants must be used.
[0006] Laser amplifier devices used for the highest power are similarly sensitive, in which radiation returning from the laser beam target along the direction of the laser must pass through an amplifier stage even before reaching the laser, and is amplified dramatically there. An example currently used for this situation is a device for generating EUV radiation, in which a CO2 seed laser generates a highly precisely defined primary radiation pulse, which is then highly amplified and focused onto the laser beam target (typically in the form of a droplet). Here, backscattered radiation is not allowed to affect the beam generation process; i.e., there is also a high requirement for decoupling. Known from patent application DE 10 2015211 426A1, efficient optical insulators, i.e., "optisches" used in practice for short pulses, can be implemented using the propagation time of the radiation pulse in the amplifier system by means of a suitable arrangement of passive polarization-selective structural elements (polarization beam splitters) and active polarization-selective structural elements (electro-optic modulators). Since the propagation time is typically very short (e.g., ≤60ns in the case of a propagation path up to approximately 20m), the method is limited to correspondingly short pulses. Summary of the Invention
[0007] Therefore, the object of the present invention is to provide a highly efficient device for radiation decoupling between a laser as a radiation source and a target laser beam loaded with that radiation. Here, the device should not rely on dynamic structural elements, such as fast modulators with expensive control mechanisms and, if necessary, synchronization devices for pulsed radiation, but should instead have a fixed, static decoupling effect, applicable not only to short pulses in the nanosecond range but also to cw radiation. A further object of the present invention is to provide a laser device having such a device.
[0008] According to the invention, this task is accomplished by a device having the features of claim 1 and a laser apparatus according to claim 9. The dependent claims reflect preferred extensions.
[0009] Therefore, the objective according to the invention is achieved by means of a device for guiding a primary laser beam along its path. This device is also configured to guide a secondary laser beam, at least partially, reflected from a laser beam target. The device has components through which the beam path is guided:
[0010] a) Wavelength selection element;
[0011] b) A first Fabry-Perot interferometer, which is arranged relative to the optical axis of the Fabry-Perot interferometer at an angle greater than 0° and less than 6° in the beam path of the primary laser beam.
[0012] c) Polarizer;
[0013] d)λ / 4 phase shifter.
[0014] The novel solution according to the invention is based on a combination of "classical" decoupling achieved by means of a λ / 4 phase shifter and polarization-sensitive reflective or transmissive structural elements, and the effect of a Fabry-Perot interferometer (FPI). In addition to its extremely efficient decoupling effect, the device also almost incidentally improves the quality of radiation propagating toward the laser beam target by utilizing the specific characteristics of the integrated FPI.
[0015] The basic idea behind all decoupling methods is based on the following: eliminating the returning wave by utilizing the generated (or existing) differences in the parameters of the emitted and returned waves. In the case of conventional decoupling, this parameter is polarization. However, there are other differences between the two waves, which can be more or less prominent. These other differences are:
[0016] 1.) Divergence: In a wave propagating toward a laser beam target, this divergence typically corresponds to the divergence of a pure laser beam and is on the order of 1 mrad. Since ideal reflection is typically (intentionally or unintentionally) absent on an ideally flat laser beam target, and a perfectly ideal focal point (intentionally or unintentionally) absent on the surface of the laser beam target, the returning wave will have significantly different ("poorer") divergence characteristics after focusing the lens, averaging over the total intensity variation.
[0017] 2.) Beam direction: Since there is no ideal orthogonality between the beam axis and the surface of the laser beam target (intentionally or unintentionally), and there is no ideal focal position on the surface of the laser beam target (intentionally or unintentionally), there is a (though small) difference in the beam direction of the emitted and returned focused beams.
[0018] 3.) Coherence: The outstanding feature of the original laser beam is usually its good coherence. However, good coherence is significantly affected by reflection or scattering on the surface of the laser beam target, meaning that the returned focused beam has significantly poor coherence.
[0019] 4.) Polarization: Laser radiation is almost always strictly linearly polarized. Behind the λ / 4 phase shifter, this ideally becomes circularly polarized radiation, which propagates towards the laser beam target. However, due to the fundamental physical processes responsible for reflection or scattering, the radiation component returning after interacting with the laser beam target is only partially circularly polarized. This means that after passing through the λ / 4 phase shifter, the desired ideally linearly polarized but perpendicularly polarized wave is not produced, and therefore, the wave can only be partially eliminated by polarization-sensitive structural elements.
[0020] 5.) Power: In typical materials processing equipment with high-power lasers, the power is greatest for the emitted beam and more or less significantly reduced for the returning beam due to interaction with the laser beam target and due to optical structural elements. However, in laser amplifier devices (e.g., in EUV systems), if the observation point of the device corresponding to the decoupling system is located between the (relatively weaker) seed laser and the high-power amplifier, which of course also amplifies the returning radiation, the opposite situation may occur.
[0021] 6.) Effects of Plasma: In practice, all the processing tasks discussed here are associated with the generation of plasma sparks, which are generated directly on the surface of the laser beam target, or even with the complete conversion of the laser beam target / droplet into plasma. Because the plasma is close to the laser beam target, its effect, for example, on the direction of the focused beam, is generally negligible, but it is important for the direction, divergence, frequency, and power of the returning beam.
[0022] 7.) Wavelength (frequency): If the emitted beam is scattered or reflected on a very fast-moving laser beam target (e.g., a rapidly expanding plasma), the returned beam will have a certain frequency or wavelength variation according to the Doppler effect.
[0023] All the listed properties are important for the transmission and reflection characteristics of FPI, and can be used together to suppress the returned wave.
[0024] To better understand the most important relationships, the general performance of FPI is briefly discussed below.
[0025] The transmission T of FPI as a function of wavelength λ, plate reflectivity R, and plate spacing a is calculated using the following formula. FPI :
[0026]
[0027] In addition, the following simple relationship applies to lossless FPI:
[0028] P0 = P0(T) FPI +R FPI ) = P T +P R
[0029] and
[0030] R FPI (a)=1-T FPI (a)
[0031] For contrast in transmission, i.e., the ratio of maximum to minimum transmission, it applies to:
[0032]
[0033] Typical FPIs that can be considered for decoupling are those with R = 0.5 and R = 0.8, for which a contrast ratio of 9 or 81 is obtained. Generally, the range of R that can be used meaningfully can be limited to 0.3 ≤ R ≤ 0.9. Later, we will discuss what choices need to be made if decoupling is required at the highest possible power on the order of 1 kW. That is, then other parameters, i.e., at maximum transmission T... FPI The power P inside FPI when = 1 FPI Excessive load plays a significant role in the load capacity of the FPI board.
[0034] P FPI =P o / (1–R) 2
[0035] Finally, a crucial parameter for the decoupling effect sought to be achieved by FPI is its fineness (Finesse) F, which indicates the ratio of the period width to the half-width of the transmission peak:
[0036]
[0037] For R = 0.8, for example, a precision of 14 is obtained, which determines how sensitively the transmission of the FPI is related to the incident direction in conjunction with the plate spacing a.
[0038] The strong angular dependence of FPI transmission readily brings to mind another advantageous property of the device according to the invention: improving the beam quality of the generated laser beam through the spatial filtering effect of the FPI with optimized plate spacing.
[0039] The device can be arranged in a modular form, especially within a common housing.
[0040] The angle between the optical axis of the first Fabry-Perot interferometer and the beam path is preferably less than 5°, particularly less than 4°, and especially preferably less than 3°. This allows the device to operate with particularly high efficiency.
[0041] In a further preferred embodiment of the invention, the device has a first absorber for absorbing the component of the primary laser beam reflected by the first Fabry-Perot interferometer. Alternatively or additionally, the device may have a second absorber for absorbing the component of the secondary laser beam reflected by the first Fabry-Perot interferometer. With one or more absorbers, coupling output against unwanted backscattering can be ensured.
[0042] In order to determine the component of unwanted back radiation, the first absorber and / or the second absorber can be configured as radiation detectors.
[0043] For the primary laser beam, the optical structural elements mentioned are preferably arranged in the following order:
[0044] 1.) Wavelength selection element;
[0045] 2.) First Fabry-Perot interferometer;
[0046] 3.) Polarizer;
[0047] 4.) λ / 4 phase shifter.
[0048] The focusing element can be arranged behind the λ / 4 phase shifter. Particularly preferably, no additional structural elements are arranged between the aforementioned components.
[0049] The first Fabry-Perot interferometer may have two plates made of a transparent material, with the opposing sides of the plates having a reflectivity R between 0.3 and 0.9. Preferably, the sides have a reflectivity R between 0.5 and 0.8. Alternatively or additionally, the plates preferably have a spacing between 1 mm and 1000 mm, particularly between 10 mm and 30 mm. The plates are preferably made of ZnSe or diamond.
[0050] More preferably, the wavelength selection element is constructed in the form of a diffraction grating. The blaze wavelength of the diffraction grating preferably corresponds to the wavelength of the primary laser beam.
[0051] The polarizer is preferably configured as an Absorbing Thin Film Reflector (AFTR). Alternatively or additionally, the λ / 4 phase shifter is preferably configured as a λ / 4 phase-shifting mirror.
[0052] The aforementioned features of the optical structural elements enable the device to be constructed with a simple structure while maintaining high operational capability.
[0053] The device may include a telescope assembly used to broaden the primary laser beam in front of the first Fabry-Perot interferometer. This allows the first Fabry-Perot interferometer to be illuminated with exceptionally high power.
[0054] The broadening factor is preferably between 1.5 and 5, and particularly preferably between 1.5 and 3.
[0055] The device can have a second Fabry-Perot interferometer arranged behind the first Fabry-Perot interferometer. This can significantly increase decoupling efficiency. More preferably, a third Fabry-Perot interferometer is arranged behind the second Fabry-Perot interferometer. The reflectivity R of the plate of the additional Fabry-Perot interferometer is preferably between 0.2 and 0.7, particularly preferably between 0.3 and 0.5. More preferably, interferometers with different reflectivities R are used.
[0056] The objective according to the invention is also achieved by a laser device having a laser beam source for emitting a primary laser beam, a laser beam target, and means described herein for guiding the primary laser beam, wherein the beam path of the means extends between the laser beam source and the laser beam target.
[0057] The laser beam source is preferably configured as a high-power laser beam source for emitting a primary laser beam, the primary laser beam having an average power of at least 1 kW. The laser beam source is particularly configured as a CO2 laser beam source. The laser beam source can be configured to emit a primary laser beam, the primary laser beam having a power of at least 10 kW. -9 s and 10 -6 The radiation pulse is in the form of a pulse duration between s. Laser devices are therefore particularly well-suited for generating EUV radiation.
[0058] The laser device may include an amplifier system 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, the laser device may have other devices described herein, arranged between the amplifier system and the laser beam target. Alternatively or additionally, the laser device may have an additional λ / 4 phase shifter arranged between the amplifier system and the additional device. By utilizing one or more of these features, a laser device with particularly high performance and a compact structure can be achieved.
[0059] In a particularly preferred configuration of the invention, the laser beam target is configured to emit EUV radiation when irradiated by a primary laser beam. The laser beam target may be configured in the form of a droplet. Preferably, the laser beam target is configured in the form of a tin droplet.
[0060] The present invention also relates to the application of the apparatus and / or laser device described herein in a device for generating EUV radiation, particularly an EUV lithography device.
[0061] Other advantages of the invention become apparent from the specification and drawings. Similarly, according to the invention, the features mentioned above and further explained can be applied individually or in any combination of multiple forms. The embodiments shown and described should not be construed as an exhaustive enumeration, but rather as exemplary features used in the narrative of the invention. Attached Figure Description
[0062] Figure 1 shows a measurement of the effect of suppressing radiative feedback on the construction of a defined radiative pulse;
[0063] Figure 2 The basic structure of a Fabry-Perot interferometer (FPI) is shown.
[0064] Figure 3 The transmission curves of FPI for the reflectivity R of the three selected plates are shown.
[0065] Figure 4 The separation of incident and reflected laser beams on the FPI is schematically shown.
[0066] Figure 5 The calculation of the relative position of the returning wave offset at a frequency of 100MHz is shown for an FPI with a plate spacing a = 10cm and a fineness of 14.
[0067] Figure 6 An illustrative example is shown of the measurement of radiation filtering using FPI;
[0068] Figure 7 The basic structure of the device according to the invention is shown in a highly schematic manner.
[0069] Figure 8 The invention illustrates the construction of a laser device in the form of a laser amplifier apparatus;
[0070] Figure 9 This demonstrates the broadening of high-power radiation techniques using telescopes;
[0071] Figure 10 This illustrates the basic structure of an FPI series connection. Detailed Implementation
[0072] Figure 1 illustrates a characteristic example of the necessity of suppressing radiative feedback, where a defined pulse should be formed from the cw radiation of a CO2 laser using an interferometric laser radiation modulator. Without suppressing feedback, the resulting pulse... Figure 1a The almost statistically significant P(t) change process shown in the figure, when decoupled using an AFTR mirror and a λ / 4 phase-shifting mirror, produces a result based on... Figure 1b The clean, desired pulse function. The example shown in Figure 1 involves a specific application of the defined radiation pulse in finishing using a CO2 laser. Here, the desired pulse train is generated from the inherently continuous laser radiation using a specific interferometric laser radiation modulator (ILM) based on a rapidly and continuously tunable FPI. This mechanism is susceptible to interference without radiation decoupling. More or less statistically undesirable pulses occur, where, in this example, an important factor becomes apparent when using an interferometer: the strong dependence of FPI transmission on the radiation wavelength. In the example, pulse generation is based on the rapid switching of the ILM between maximum transmission and maximum reflection, which, of course, only works for the defined wavelength. However, without radiation decoupling and without a specific wavelength selection element (e.g., a grating) to ensure the defined wavelength, parasitic oscillations can occur even if the FPI is blocked for the truly desired wavelength, such as with a high-power CO2 laser, in the case of which, potentially, a large rotational vibrational transition between 9 μm and 11 μm is available. This is in Figure 1a This is illustrated heuristically. Even in the solution according to the invention, the fact must be considered that the system must be fixed to the selected wavelength by integrating a wavelength selection element. In these considerations... It is generally important to note that, despite all losses along the beam path, the end mirror of the laser resonator on one side and the laser beam target on the other side form a resonator. This introduces the risk of parasitic oscillations, in which, under certain conditions, the radiation will precisely "pick up" the wavelength for which the gain-to-loss ratio is maximized. A frequency-selective element, which may be integrated into the laser resonator itself if necessary, precisely prevents this possibility.
[0073] Figure 2The basic structure of FPI10 is shown to illustrate the performance described above. This FPI has plates 12a and 12b with a spacing a. In the solutions according to the invention, it is often important to handle high power, such as average power in the range of 1 kW or greater. Advantageously, in this solution, an FPI10 with the structure shown is used, in which air exists between the two decisive surfaces having reflectivity R. This reduces the adverse effects of excessive power within the FPI on the R coating itself, especially when using a larger R (R > 0.5). The more or less large fundamental absorption of the carrier material of the FPI plates 12a and 12b plays a subordinate role. In the CO2 laser systems generally discussed in detail here, the carrier material is typically ZnSe or diamond. Figure 2 In the middle, the anti-reflective coating is marked with "AR".
[0074] Figure 3 The figure shows the theoretical transmission of the RFPI10 as a function of the plate spacing *a* for three selected reflectivities. This illustrates the strong R-dependency of contrast and fineness, both of which are crucial to the effectiveness of the FPI10 in suppressing radiative feedback between the laser beam target and the laser beam source. This involves not only the sensitivity to the minimum angular variation of radiation incident on the FPI10, but also the frequency or wavelength variation of the radiation returning from the laser beam target, which occurs, for example, in cases of reflection or scattering on a rapidly expanding plasma, as is characteristic of the generation of EUV radiation.
[0075] However, besides reflectivity R, two other parameters are crucial to the effectiveness of the FPI. One is the plate spacing α, and the other is the incident angle δ of the radiation relative to the FPI axis. The latter has significant practical implications because it is essential to ensure the removal of radiation reflected by the FPI from the main beam path (this involves not only the radiation propagating from the laser beam source to the laser beam target, but also the returning radiation). Especially in laser amplifier systems, it is necessary to prevent the FPI from acting as a reflector that causes amplifier chain oscillation.
[0076] To meet these requirements, a minimum angle δ is needed, which, after a pre-given road segment s, is sufficient to separate the incident and reflected beams (see...). Figure 4However, preferably, two factors need to be considered simultaneously regarding the functionality of the FPI. For effective decoupling of the FPI10, a sufficiently large plate spacing 'a' is preferred. The optimal size of this plate spacing is decisively related to the corresponding parameters of the returned radiation, which first become effective for decoupling. Due to the high selectivity of the FPI10 with respect to the incident angle δ, assuming an FPI precision of 14, a plate spacing 'a' in cm is already preferred to suppress radiation components in the returned beam with divergence or tilt relative to the FPI axis >1 mrad. If the parameter affecting decoupling is due to frequency shift caused by the Doppler effect on a rapidly moving laser beam target, a significantly larger plate spacing 'a' is preferred. For example, if the Doppler shift on the rapidly expanding plasma on the irradiated droplet is assumed to be approximately 100 MHz during EUV generation, then for effective decoupling, 'a' should preferably be at least 10 cm (see...). Figure 5 However, this requirement is relative when considering the significant reduction in optical gain of the active medium in a CO2 laser or CO2 amplifier at a frequency shift of 100 MHz, resulting in a secondary effect that helps suppress the returning wave. This fact greatly favors the “optimal” FPI10 scheme because it is necessary to consider the simultaneous increase of a and even more drastically with δ. Figure 3 The ideal FPI characteristics shown deteriorate under the assumption of perpendicular incidence and monochromatic radiation. In particular, the maximum transmission will deviate further and further from the ideal value of 1, i.e., the loss of the working beam increases. As a general optimization rule for the plate spacing 'a' for FPI, the following should be followed: 'a' should be chosen to be as large as needed, but as small as possible.
[0077] Figure 4 To reiterate the described geometric proportions, the following conditions must be met: First, it must be ensured that the incident laser beam 14 with diameter d is cleanly separated from the component 16 reflected on FPI 10 with, for example, the same diameter. Second, the required path length s should not be too large, i.e., the angle δ should not be too small. Third, the role of δ is relatively complex: δ should also not be too small in the sense of high filtering effect in undesirable radiation and high efficiency in radiation decoupling; on the other hand, as mentioned above, the maximum transmission deteriorates with increasing δ, and this maximum transmission for the working beam should naturally be close to 1. Here, in Figure 4 In the process, the laser beam focusing 14 is emitted from the wavelength selection element 18.
[0078] Therefore, depending on the specific circumstances, and especially on the characteristics of the laser beam itself, δ should preferably be optimized. This optimization is closely related to the spacing a of the FPI plates 12a and 12b, because ultimately, in addition to the plate reflectivity R, the two geometric parameters a and δ determine the effectiveness of the described device. If we assume that the dispersion of the laser radiation is approximately 1 mrad, the described requirements can be met with “reasonable” values, which, as mentioned above, are approximately 10 mm for D in the range up to approximately 10 cm, δ = 2.5° for δ, and from approximately 20 cm for s. Then, assuming R = 0.8, i.e., with a fineness of 14, three important parameters can be calculated with the high effectiveness of the FPI10 in the desired sense: beam divergence, tilt relative to the working beam, and frequency offset. Specifically, in the adjustment state corresponding to T≈1 for the working beam, the beam components to be "eliminated" (i.e., the beam components in the outgoing beam with low beam quality and therefore higher divergence than a typical 1 mrad, and the radiation components in the returning beam with divergence or tilt >1 mrad relative to the FPI axis, or frequency shifts on the order of 100 MHz due to the Doppler effect on a fast-moving laser beam target) can be shifted to a range with high reflectivity (R FPI >0.8) and therefore can be easily eliminated. Regarding the effect of the frequency shift on the FPI effect during radiation decoupling, it should be noted that the backscattered or reflected radiation components on a rapidly expanding plasma also have divergence and directional fluctuations, the effects of which are superimposed on the effect of the frequency shift in a significantly larger sense of the total effect.
[0079] Figure 5 The image shows the "filtering effect" of the FPI10 on a returning wave offset by 100 MHz, where the FPI10's board spacing a = 10 cm and the fineness is 14. Figure 5 In the diagram, the transmission T is plotted as a function of frequency f.
[0080] Figure 6 The results of measurements are also shown, clearly illustrating the effectiveness of the optimized FPI10 as a radiation filter in the context of this invention. The effect of the gas discharge current I of the low-pressure CO2 laser on the beam quality was investigated. The latter is characterized by the quotient P... T / P R The quotient reflects the useful power P. T (i.e., the radiation transmitted by the FPI) and the power component P to be filtered out, reflected by the FPI10. RThe ratio. Since the laser is intentionally operated in a system (Regime) that leads to relatively rapid overheating of the laser gas, it is quite revealing in this graph that the beam quality (i.e., the quotient P) is... T / P R How can we initially assume a significant maximum value at relatively low currents, so that it drops sharply when the current exceeds that maximum and thus causes significant overheating? In the example, the filtered radiation component increases from approximately 17% to approximately 25% of the incident power, which ultimately means that while the laser's "raw beam" has significantly lost quality, the quality of the useful beam remains almost constant (however, under power reduction). By continuously monitoring the filtered radiation component, which is constantly used for measurements during processing, important parameters for diagnosing the processing are obtained.
[0081] Figure 7 A highly schematic basic structure of the device 20 according to the invention is shown. In this device 20, the process begins with a laser beam source 22, which may, for example, be a Q-switched high-power CO2 laser. The primary laser beam 24 of this laser beam source is characterized by a defined wavelength λ, linear polarization in a defined direction 26, and more or less good beam quality K. Since the components according to the invention, used for beam filtering on one hand and for suppressing radiation feedback on the other hand, should be designed for the precisely defined wavelength λ, it is necessary to prevent the system from selecting an alternative wavelength in the beam path "laser beam source – beam guiding and beamforming – laser beam target" in order to ensure its functionality. This risk exists, for example, in the case of a high-power CO2 laser, which can oscillate over a wide range of wavelengths between 9 μm and 11 μm. A wavelength selection element 28 (e.g., a diffraction grating or prism) ensures that the system is reliably fixed to the pre-given λ. It should be noted that the wavelength selection element 28 can also be integrated into the laser, i.e., the laser beam source 22, and thus is naturally no longer necessary outside the laser. However, the situation is slightly different in high-power laser amplifier devices, such as those used in EUV systems (see...). Figure 8 In the second device that may be necessary there for suppressing radiative feedback, the wavelength selection element is preferably not omitted.
[0082] On its alternative 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 embodiment. The first task of the FPI 10, set for maximum transmission at wavelength λ, is to filter out the radiation component 30, which reduces beam quality, thus reducing the focusability of the radiation and ultimately reducing processing quality. Typically, this is the following radiation component: the radiation component that deviates more or less strongly from the fundamental mode TEM. 00 The radiation component is eliminated by absorber 32 in its simplest case, which, as here, can be configured as a radiation detector. The measurement signal from the absorber provides information about the magnitude of the parasitic radiation component 30, but also, when recording the time-varying process, 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.
[0083] Subsequently, the radiation component 34 transmitted and filtered by the FPI10 is transmitted via a “classical” combination of polarizer 36 and λ / 4 phase shifter 38 for radiation decoupling between the laser (laser beam source 22) and the target (laser beam target 40), wherein a generally desired conversion of linearly polarized radiation to circularly polarized radiation 42 is performed simultaneously, which can be transmitted to the laser beam target 40, for example, by means of focusing element 44.
[0084] In addition to this first conversion, the effect of elements 36 and 38 is that the secondary laser beam 46 returning from the laser beam target 40 along the direction of the laser beam source 22 is first converted in the λ / 4 phase shifter 38 into partially linearly polarized radiation 48 in the second conversion. This secondary laser beam is typically more or less circularly polarized, and the polarization direction of this partially linearly polarized radiation is perpendicular to the polarization direction of the primary laser beam 24. This is based on the premise that 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.
[0085] In examples of CO2 lasers, Brewster plates or ATFR mirrors and λ / 4 phase shifter mirrors are typically used for elements 36 and 38, where the ATFR mirror absorbs radiation to be eliminated in the "wrong" polarization direction. However, for the reasons mentioned above, the returned radiation is not completely eliminated. Radiation component 50, which is certainly within a percentage range of the primary laser beam 24, will propagate along the direction of the laser beam source 22 without further precautions, and will cause interference with the radiation formation process in the laser beam source 22. The higher the population inversion in the active medium and therefore the higher the optical gain of the laser, the more severe these interferences become. This is particularly true in Q-switched systems, which are based on maximizing population inversion directly before pulse generation. At this point, small signals in the active medium are amplified. It can definitely reach the order of 10. 4 That is, even a very small amount of feedback radiation can significantly affect the construction of high-power pulses.
[0086] According to the invention, this dilemma is resolved by the effect of a specially optimized Fabry-Perot interferometer 10. The FPI10, arranged at the aforementioned small angle δ, attenuates the radiation component 50 to a scale that is not dangerous for the laser's function in multiple ways. Here, the difference in parameters between the emitted radiation component 34 and the returned radiation component 50 is decisive. The most important difference has already been mentioned, which leads to high reflectivity on the FPI10 and thus to the elimination of the desired radiation component 50. It has also been stated that in practice, it is impossible to accurately quantify all the effects that work together. For qualitative considerations, it is meaningful to divide the effects into those unrelated to coherence and those constrained by coherence. We wish to understand the following effects as coherence-constrained: those that cause interference and are therefore closely related to the characteristics of the FPI10 discussed above. In this case, it is important that the divergence, direction and (possibly) wavelength of the radiation component 50 differ so greatly from the initial beam 34 that the FPI10, which is set to be the best for transmission to the beam 34, is as close as possible to the maximum reflection value for the beam 50 and makes a significant contribution to the radiation component 52.
[0087] For example, simple reflection on an FPI surface with reflectivity R is independent of coherence; such simple reflection, for example, at R = 0.8, already reflects approximately 90% of radiation component 50 along the direction of component 52, considering two FPI surfaces without interference effects. The more strongly the initial good coherence of the beam 34 is disrupted by interaction with the laser beam target 40, the more important the reflection of this largely incoherent radiation component becomes. Ideally, the total radiation component 52, which corresponds almost in power to radiation component 34, is eliminated in the second absorber 54, ultimately ensuring efficient blocking of the returned radiation.
[0088] Here, the total attenuation of the returned radiation may reach 10. 3 With 10 4 The coefficient between them. The efficiency of this total attenuation is at least one to two orders of magnitude higher than that of a conventional combination of polarizer and λ / 4 phase shifter, and this total attenuation is suitable for the following radiation decoupling: the one that places the highest demands on radiation decoupling.
[0089] At this point, the preferred order of components 10, 36, and 38 should be reiterated. This preferred order is important, especially in terms of the power sensitivity of FPI10, and is particularly decisive in the case of laser amplifier devices (see also the section on...). Figure 8 , Figure 9 and Figure 10 (The explanation is as follows). That is, if the radiation power to which FPI10 is exposed—which ultimately involves the sum of the emitted and returned radiation—exceeds a critical value, typically in the range of several kW, a thermal effect occurs that shifts the carefully set operating point (T=1 for radiation propagating toward the laser beam target 40) in an uncontrolled manner and may therefore degrade or even jeopardize the function of the overall system. In the arrangement of elements 10, 36, and 38 according to the invention, the power load of FPI10 is minimized in any case for the following reasons: First, regardless of whether the FPI is placed in front of or behind elements 36 and 38, 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 the losses in elements 36 and 38 are minimal. Therefore, secondly, the difference in radiation exposure of FPI10 for the two arrangement variations is in practice only related to the power of the returned radiation reaching FPI10. However, in the order of elements 10, 36, and 38 according to the invention, the returned radiation is decisively attenuated by the decoupling effect of elements 36 and 38, such that even in the case of very strong returned radiation components, such as those that may occur in high-power laser amplifier devices, the FPI function 10 is not harmed by these radiation components.
[0090] In particular, to simplify the following embodiments, structural elements important for beam filtering and radiation decoupling according to the invention are incorporated in device 20.
[0091] Figure 8 A laser device 56 according to the invention is shown, which takes the form of a laser amplifier device. Here, the amplifier system 58 can also consist of multiple stages, such as additional preamplifiers or multiple high-power amplifiers. In all multi-stage systems, the following issues arise: how frequently and at what locations should filtering and decoupling be performed. Regardless of specific requirements, filtering should ideally be performed as directly after the laser output as possible, while decoupling requires significantly more complex considerations. Figure 8 This is clearly illustrated. The radiation 22 generated in laser 22, with precisely defined characteristics, is here brought to a power level in amplifier system 58 capable of achieving demanding applications. These precisely defined characteristics are, for example, radiation pulses with a defined duration and repetition frequency. Therefore, due to the very high optical gain in the active medium of amplifier system 58, power levels far exceeding those required for demanding applications are achieved. Figure 7 The problems arise from the simple arrangement of the laser 32 and the laser beam target 40. Therefore, it is necessary to consider, for example, that radiation reflected from the laser beam target 40, especially from a droplet-shaped laser beam target, should in any way pass through the amplifier system 58, because otherwise, the radiation would be extremely amplified and thus significantly hinder the decoupling of the laser 22. Furthermore, in the case of high-power amplifiers, the following portion of super-radiation should be considered: this portion propagates along the direction of the laser 22 and, in any case, appears independently of the radiative feedback through the laser beam target 40.
[0092] Figure 8 This schematically illustrates how the problem can be solved according to the present invention. Accordingly, laser radiation 24 first passes through a first device 20a, which is responsible for targeting... Figure 7 The entire task under discussion is, firstly, beam filtering, and secondly, efficiently suppressing the feedback of radiation into laser 22. This radiation component 60 consists of two components: first, a component reflected or scattered by the laser beam target 40, which is only partially eliminated by the second device 20b and highly amplified in the amplifier system 58; and second, a component that appears even under ideal conditions of the second device 20b, namely the unidirectional superradiation (ASE) of the amplifier system 58 itself. The latter is essentially unpolarized and thus passes through the polarizer 36 and λ / 4 phase shifter 38 of the first device 20a (see...). Figure 7The unit is reduced to approximately half its original size. However, this is insufficient in many cases, making the effect of FPI10 particularly important. After passing through the first device 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, in a CO2 laser amplifier device used for EUV generation, in the range of several kW, and therefore requires special precautions for the second device 20b, which should ensure the safe shielding of the amplifier system 58 relative to the radiation component 62 returning from the laser beam target 40.
[0093] The effect of the second device 20b, especially regarding radiation decoupling, should be exactly similar to that of the first device 20a. To this end, the amplified, circularly polarized radiation 64 is first converted back into linearly polarized radiation. This is done using a λ / 4 phase shifter 66. That is, for the second device 20b, compared to the first device 20a, it should be noted that the incident radiation firstly has a significantly higher power value, and secondly, the linear polarization of this incident radiation is rotated by 90° relative to the laser radiation 24. The latter is not a problem; in principle, this can be achieved simply by using polarizer 36 and λ / 4 phase shifter 38 (see...). Figure 7 Rotate 90°.
[0094] More critically, due to FPI10 (see...) Figure 7 The high average power should be attributed to the excessively high sensitivity of the FPI internally, requiring safe control of the high average power. This high average power is already 25 times the incident power P0, for example, with R = 0.8 and maximum FPI transmission T = 1, and therefore, even with diamond optics, it is within the limits of the maximum permissible load for P0 in the kW range.
[0095] There are two possibilities for effective and flexible remedies. Figure 9 The simplest possibility is shown by widening the diameter of the beam focusing 68 by means of a telescope device 70 immediately in front of the FPI10, which has a larger free aperture to match the widened beam 72. This allows the intensity on the R-plane to be set within a wide limit such that the function of the FPI and therefore the beam 74 emanating from the FPI10 are not impaired by thermal effects.
[0096] If it is desired to avoid changes to the beam diameter, an FPI cascade can also be proposed according to the remedy of the present invention. Figure 10The basic version is shown, namely the FPI tandem. This solution is based on distributing the FPI effect used to suppress radiative feedback across multiple FPIs, here a first FPI 10 and a second FPI 76, the second FPI having a particularly reduced R. Numerical examples should illustrate this. For instance, if the equivalent effect of an FPI with R = 0.8 is to be achieved, but the excessive power of the coefficient 25 discussed above is unacceptable, beam 78 can be sent to the first FPI 10 with R = 0.5 (which is equivalent to excessive power of only 4), and beam 80 leaving the first FPI 10 can be sent to the (particularly identical) second FPI 72. The desired overall effect is then produced, but the load on the R layers of the two FPIs 10, 76 is reduced by a coefficient of 6.25. This basic principle can be extended and optimized, for example, by using more than two interferometers to further reduce the radiative load, or by using multiple interferometers with optimized, different R values to maximize decoupling efficiency while minimizing the radiative load.
[0097] In summary, the present invention relates to an optical device 20, 20a, 20b for filtering laser radiation, when viewed in overview of all the accompanying drawings. Devices 20, 20a, 20b have a wavelength selection 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 relative to the laser beam arriving at the Fabry-Perot interferometer 10 at an angle greater than 0° and less than 6°. The Fabry-Perot interferometer 10 is configured not only to couple out unwanted radiation components 30 from the primary laser beam 24 arriving at the Fabry-Perot interferometer 10, but also to couple out unwanted radiation components 52 from the secondary laser beam 46 reflected by the laser beam target 40. Preferably, devices 20, 20a, 20b include at least one additional Fabry-Perot interferometer 76. Further preferably, devices 20, 20a, and 20b include a telescope device 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 device 20, 20a, or 20b. Preferably, the laser device 56 includes an amplifier system 58 arranged between the two devices 20a and 20b. Particularly preferably, the laser device 56 is configured to generate EUV radiation. For this purpose, the laser beam target 40 may be in the form of a droplet.
[0098] List of reference numerals
[0099] 10 First Fabry-Perot Interferometer
[0100] Plates of the Fabry-Perot interferometers 12a and 12b
[0101] 14. Laser beam focusing
[0102] 16. The reflected components of laser beam focusing
[0103] 18 Wavelength Selective Elements
[0104] 20, 20a, 20b Devices for beam filtering and radiation decoupling
[0105] 22 Laser beam source
[0106] 24 Primary laser beams
[0107] 26. Polarization direction
[0108] 28 Wavelength Selective Elements
[0109] 30 Radiation components filtered from the primary laser beam
[0110] 32 First Absorber
[0111] 34. Filtered radiation components of the primary laser beam
[0112] 36 Polarizer
[0113] 38 λ / 4 phase shifter
[0114] 40 Laser beam targets
[0115] 42 Circularly Polarized Radiation
[0116] 44 Focusing element
[0117] 46 secondary laser beams
[0118] 48. Partially linearly polarized radiation
[0119] 50. Attenuated radiation component returned.
[0120] 52. Returning (secondary) radiation components eliminated by FPI
[0121] 54 Second Absorber
[0122] 56 Laser Equipment
[0123] 58 Amplifier System
[0124] 60 Radiation components propagating from the amplifier system along the direction of the laser beam source
[0125] 62 Radiation components returning from the laser beam target
[0126] 64. Magnified, circularly polarized radiation
[0127] 66 λ / 4 phase shifter
[0128] 68 beam focusing
[0129] 70 Telescope device
[0130] 72 Widened beam
[0131] 74 Beams originating from FPI
[0132] 76 Second FPI
[0133] 78 The beam that reaches the first FPI10
[0134] 80° away from the first FPI10 beam
Claims
1. A laser device (56) comprising a laser beam source (22) for emitting a primary laser beam (24), a laser beam target (40), and means (20, 20a, 20b) for guiding the primary laser beam (24), wherein, The beam path of the device (20, 20a, 20b) extends between the laser beam source (22) and the laser beam target (40), wherein the device (20, 20a, 20b) is configured to guide a primary laser beam (24) on the beam path and to guide a reflected secondary laser beam (46), wherein the device (20, 20a, 20b) has components through which the beam path is guided: a. Wavelength selection element (28); b. A first Fabry-Perot interferometer (10), wherein the first Fabry-Perot interferometer is arranged in the beam path at an angle greater than 0° and less than 6° relative to the optical axis of the Fabry-Perot interferometer (10); c. Polarizer (36); d. λ / 4 phase shifter (38), The laser beam target (40) is configured to emit EUV radiation when irradiated by the primary laser beam (24). The devices (20, 20a, 20b) have the following characteristics: A first absorber (32) is used to absorb the component (30) of the primary laser beam (24) reflected by the first Fabry-Perot interferometer (10); and / or A second absorber (54) is used to absorb the component (52) of the secondary laser beam (46) reflected by the first Fabry-Perot interferometer (10). In the primary laser beam (24), the wavelength selection 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).
2. The laser device according to claim 1, wherein, The first absorber (32) and / or the second absorber (54) are respectively configured as radiation detectors.
3. The laser device according to claim 1 or 2, wherein, The first Fabry-Perot interferometer (10) has two plates (12a, 12b) made of transparent material, the opposing sides of the two plates having a reflectivity R between 0.3 and 0.
9.
4. The laser device according to claim 1 or 2, wherein, The wavelength selection element (28) is constructed in the form of a diffraction grating.
5. The laser device according to claim 1 or 2, wherein the device has a telescope device (70) for widening the primary laser beam (24) in front of the first Fabry-Perot interferometer (10).
6. The laser device according to claim 1 or 2, wherein the device has a second Fabry-Perot interferometer (72) arranged behind the first Fabry-Perot interferometer (10).
7. The laser device according to claim 1 or 2, wherein the laser device has an amplifier system (58) arranged in the beam path between the laser beam target (40) and the laser beam source (22), wherein, The device (20a) is arranged between the amplifier system (58) and the laser beam source (22), wherein the laser device (56) has a further device (20b) arranged between the amplifier system (58) and the laser beam target (40). The additional device (20b) is configured to guide a primary laser beam (24) on the beam path and to guide a reflected secondary laser beam (46), wherein the additional device (20b) has components through which the beam path is guided: a. Wavelength selection element (28); b. A first Fabry-Perot interferometer (10), wherein the first Fabry-Perot interferometer is arranged in the beam path at an angle greater than 0° and less than 6° relative to the optical axis of the Fabry-Perot interferometer (10); c. Polarizer (36); d. λ / 4 phase shifter (38), The laser device (56) has an additional λ / 4 phase shifter (66) arranged between the amplifier system (58) and the additional device (20b).
8. The laser device according to claim 7, wherein, The laser beam target (40) is constructed in the form of a droplet.