Method for adjusting a laser beam, device for providing an adjusted laser beam and optical arrangement

Abstract

The invention relates to a method for adjusting a laser beam (1), comprising: measuring a beam profile (15, 16) of the laser beam (1) after passing through a beam shaping device (4) by means of at least one location-resolving detector (6a) of a detector device (6); determining a beam quality characteristic (D) of the laser beam (1) on the basis of the measured beam profile (15, 16); and adjusting an adjustable optical device (5) in order to change at least one characteristic of the laser beam (1) before it enters the beam shaping device (4); wherein, in order to adjust the laser beam (1), the adjustable optical device (5) is adjusted in particular a plurality of times in accordance with the determined beam quality characteristic (D), preferably until the beam quality characteristic (D) reaches a predefined value (D S ). The invention also relates to a device (3a) for providing an adjusted laser beam (1) and to an extreme ultraviolet radiation generating device.

Description

Technical Field

[0001] The invention relates to a method for conditioning a laser beam, a device for providing a conditioned laser beam, and an optical arrangement, in particular an extreme ultraviolet (EUV) radiation generating device. Background Art

[0002] When a laser beam passes through a beam-shaping device—i.e., a device that changes at least one optical property of the laser beam—the beam quality of the laser beam is typically also affected (usually deteriorated). When adjusting the laser beam through such a beam-shaping device, numerous optical edge conditions and interference factors in the beam-shaping device can result in the laser beam being adjusted to the design parameters only providing inadequate beam quality for the laser beam emerging from the beam-shaping device. Therefore, such beam-shaping devices are typically optimized empirically to achieve good beam quality, i.e., only using qualitative criteria for adjustment, as no quantitative criteria are available. This makes adjustment only possible by experts and difficult to reproduce.

[0003] US 2013 / 0146569 A1 discloses a method for adjusting a laser beam that acts on a workpiece during a machining process. A laser machining system receives a laser beam to which a beam quality characteristic is assigned and adjusts the laser beam in such a way that the beam quality characteristic changes based on the properties of the workpiece, the properties of the machining process, or a combination of the two. The beam quality characteristic can, for example, be the diffraction index (M 2 value) or beam parameter product (Strahlparameterprodukt). Summary of the Invention

[0004] Invention Task

[0005] The present invention is based on the object of providing a method for regulating a laser beam, a device and an optical arrangement for providing a regulated laser beam, which method, device and optical arrangement enable regulation based on objective beam quality characteristics.

[0006] Subject matter of the invention

[0007] One aspect of the present invention relates to a method for adjusting a laser beam, the method comprising: measuring the beam profile of the laser beam after passing through a beam shaping device by means of at least one preferably point-resolving detector of a detection device; determining a beam quality characteristic of the laser beam based on the measured beam profile; and adjusting an adjustable optical device to change at least one characteristic of the laser beam before it enters the beam shaping device; wherein, in order to adjust the laser beam, the adjustable optical device is adjusted in particular multiple times according to the determined beam quality characteristic, preferably until the beam quality characteristic reaches a predetermined value.

[0008] According to the present invention, the beam quality of a laser beam passing through a beam shaping device is optimized by measuring the beam profile of the laser beam after passing through the beam shaping device and determining an objective or uniquely defined beam quality characteristic of the laser beam based on the measured beam profile. An adjustable optical element is arranged upstream of the beam shaping device, through which the laser beam passes, and is adjusted according to the determined beam quality characteristic or the corresponding determined value of the beam quality characteristic to optimize the beam quality. This makes it possible to dispense with subjective criteria for adjustment without requiring a relatively precise understanding of interfering effects occurring in the beam shaping device.

[0009] In principle, it is known to determine the beam position and / or propagation angle of a laser beam using detectors, such as quadrant detectors or cameras, and to act on a control device, such as an adjustable optical element, to stabilize the beam position and propagation angle. However, this control is not based on optimizing the beam quality, but rather solely on direct feedback signals, such as offsets at the detector, which minimize the deviation between actual and desired values. However, when passing through a beam shaping device, there is usually no direct relationship between the actual and desired values, or this relationship is very complex. Instead, using beam quality characteristics as objective criteria allows for control without requiring precise knowledge of the interfering effects that occur in the beam shaping device. Therefore, beam shaping devices can be referred to as "black boxes" for control purposes.

[0010] Adjustable optical components are typically adjusted automatically using an optimization algorithm that is implemented in suitable software and / or hardware and provides control signals for actuating the adjustable optical components or actuators provided for this purpose. Optimization is typically performed until a predetermined value for the beam quality characteristic is reached, thereby achieving an adjustment target or termination criterion.

[0011] Alternatively, the adjustable optical system for adjustment can be manually adjusted by an operator. In this case, it is advantageous to provide the operator with instructions on how to change the adjustable optical system depending on the respectively determined value of the beam quality characteristic. If the adjustable optical system offers more than one adjustment option for optimizing the beam quality characteristic, manual adjustment or optimization is difficult to implement in practice, as this necessarily results in relatively complex instructions.

[0012] In one variant, the beam quality characteristic is maximized or minimized by adjusting an adjustable optical element. As mentioned above, the adjustable optical element is typically not adjusted randomly, but rather is adjusted in a targeted manner based on an optimization algorithm. Adjustment is typically performed in multiple adjustment steps, in which the adjustable optical element is acted upon in order to gradually change (at least) one characteristic of the laser beam. The decision regarding the degree / direction of adjustment and—in the case of multiple adjustment options—the selection of the appropriate adjustment option for the next adjustment step can be made using an optimization algorithm, for example, using a gradient method. Adjustment ends when the adjustment target is reached, i.e., when the beam quality characteristic reaches a predetermined value for maximization or minimization, or when a termination criterion, such as a predetermined number of adjustment steps, is reached.

[0013] In one advantageous variant, the deviation of the beam profile from a predetermined beam profile, in particular a Gaussian beam profile, is determined as a beam quality characteristic. For this purpose, at least one image or intensity distribution of the laser beam is typically recorded using a detector, such as a spatially resolved camera, in at least one plane, typically perpendicular to the propagation direction of the laser beam. In the simplest case, the beam profile is a single image or intensity distribution of the laser beam in a plane perpendicular to the propagation direction. Alternatively, the beam profile can be a beam caustic in the propagation direction of the laser beam. To determine the beam profile, multiple images are typically recorded in multiple planes offset in the propagation direction of the laser beam.

[0014] In one advantageous embodiment, the deviation of the intensity distribution of the laser beam on the detector from a predetermined intensity distribution, in particular a Gaussian intensity distribution, is determined as a beam quality characteristic. In this case, the beam profile is measured by recording a single image or a single two-dimensional intensity distribution on the detector. The beam quality characteristic is determined by comparing the intensity distribution with a predetermined two-dimensional, for example Gaussian, intensity distribution. Typically, the predetermined intensity distribution is not an image or a (two-dimensional) intensity distribution, but rather described using (geometric) parameters. In addition to Gaussian intensity distributions that are rotationally symmetric about the beam center of gravity or beam center point of the laser beam, other non-rotationally symmetric intensity distributions can also be used for comparison. Advantageously, the beam profile of the laser beam deviates from a rotationally symmetric intensity distribution, for example because the laser beam has an elliptical beam profile.

[0015] (Two-dimensional) Gaussian intensity distribution I G (x,y) can be described, for example, by the following formula:

[0016] I G (x,y)=I0 EXP([(x-x0) 2 +(y–y0) 2 ] / (2 w0 2 )) (1)

[0017] Among them, I0, x0, y0, and w0 form the parameters of the Gaussian intensity distribution.

[0018] The detected intensity distribution I(x,y) and the Gaussian intensity distribution I G The deviation D of (x,y) can be expressed, for example, by the following error integral:

[0019] D=∫∫ x，y (I(x,y)-I G (x,y)) 2 (2)

[0020] The deviation in the form of the error function D is determined numerically and minimized during the adjustment.

[0021] In one embodiment, at least one parameter of the predetermined intensity distribution is varied when determining the deviation. In principle, when determining the deviation D, one or more of the parameters I0, x0, y0, and w0 of the Gaussian intensity distribution defined above can be varied (independently), or the parameters can be fixedly predetermined. The decision as to whether certain parameters are varied or fixedly predetermined depends on the desired properties of the laser beam. For example, the center position x0, y0 and the maximum intensity I0 of the Gaussian intensity distribution can be varied or freely selected, while the range of the beam diameter w0 is fixedly predetermined.

[0022] It is understood that, depending on the respective application, other fitting functions or error functions (1) and (2) are also possible for determining the value of the deviation D. For example, in the case of a laser beam with a non-rotationally symmetrical, for example elliptical, intensity distribution, different weightings can be applied depending on the distance from the beam center of gravity to determine the deviation D in the direction of the two axes of the ellipse. Furthermore, the detected intensity distribution can be corrected, for example, by subtracting a calibrated background image, or individual regions of the intensity distribution can be weighted differently in the error function depending on their influence on the actual application.

[0023] In another variant, the beam parameter product or the diffraction index is determined as the beam quality characteristic. For this purpose, it is typically necessary to determine the beam profile in the form of the beam caustics of the laser beam, at least approximately. For this purpose, the intensity distribution of the laser beam is usually measured in a plurality of planes spaced apart from one another, for which purpose a detector can be moved along the propagation direction of the laser beam. Alternatively or additionally, a plurality of images or intensity distributions can be recorded on a plurality of detectors that are arranged at a fixed distance from one another. Based on the beam caustics, the diffraction index (M) of the laser beam can be determined. 2 value), that is, the divergence angle of the laser beam Divergence angle of an ideal Gaussian beam with the same diameter at the beam waist Ratio Beam parameter product (BPP) = M 2 λ / π can also be determined in this way and with a known laser wavelength λ. Ideally, an ISO-compliant caustic measurement of the laser beam can be performed for this purpose.

[0024] As an alternative to the aforementioned beam quality characteristics, other (objective) beam quality characteristics can also be used for the adjustment. The aforementioned beam quality characteristics can also be modified appropriately depending on the respective application; for example, if the fringe fields are not important for the further propagation of the laser beam, then these fringe fields of the laser beam can be disregarded.

[0025] In another embodiment, when adjusting the adjustable optical element, the beam position of the laser beam, the orientation of the laser beam, the caustics of the laser beam, and / or the aberrations of the laser beam are adjusted. The beam position of the laser beam can be understood as the position of the laser beam in a plane perpendicular to the propagation direction when it enters the beam shaping device. The orientation of the laser beam can be understood as the angle of incidence of the laser beam when it enters the beam shaping device. The caustics of the laser beam can be understood as the aperture angle of the laser beam. (divergence angle or convergence angle) and the diameter of the laser beam. The aberrations that are varied by means of adjustable optics can be, for example, astigmatism, which can be influenced in the adjustable optics, for example, by means of cylindrical lenses or the like.

[0026] In one embodiment, when adjusting the laser beam, at least one optical element of an adjustable optical system, in particular at least one reflector and / or at least one lens, is adjusted. The adjustable optical system can include one or more adjustable (plane) reflectors. In this case, the adjustment is usually achieved by tilting the reflector or mirrors around a corresponding tilting axis or rotation axis, so that the beam position and / or orientation of the laser beam can be adjusted. The adjustable optical system can also include one or more adjustable lenses, such as spherical lenses or cylindrical lenses. In this case, the adjustment is usually achieved by moving the corresponding lens along the beam axis or propagation direction of the laser beam. In this way, in particular, the caustics of the laser beam can be adjusted. In the case of cylindrical lenses, the astigmatism of the laser beam can also be adjusted if necessary.

[0027] In another embodiment, the beam-shaping device is selected from the group consisting of an optical amplifier, an optical isolator, an optical modulator, an optical delay path, and an optical polarizer. The beam-shaping device can, for example, include one or more apertures, non-uniform optical gain or absorption, non-planar reflective optical elements, etc., which produce non-deterministic or only highly calculable interference effects when the laser beam passes through them. The beam-shaping device can, for example, be an optical amplifier, an optical isolator, such as a Faraday rotator, an optical (intensity) modulator, such as an electro-optical modulator or an acousto-optic modulator, an optical delay path, such as a multi-pass delay path, an optical polarizer, such as a beam splitter plate mounted at the Brewster angle, etc.

[0028] In another embodiment, the beam shaping device, in particular in the form of an optical modulator, and the adjustable optical system are arranged in the laser resonator. The laser resonator also has a laser-active medium and usually two resonator mirrors, between which the resonator path is formed. One or both resonator mirrors can be part of the adjustable optical system and, for example, can be moved or flipped in order to perform adjustments. The laser resonator can in particular be a seed laser for generating a pulsed seed laser beam, which is amplified in one or more optical amplifiers. The optical modulator within the laser resonator is usually used to generate the pulses.

[0029] The beam shaping device can of course also be arranged outside the laser resonator in the beam path. For example, it can be an optical amplifier or amplifier stage to which the laser beam of a (seed) laser source, a preceding amplifier stage, etc. is fed.

[0030] In one variant, the (entire) laser beam is directed onto the (at least one) detector (only) during the adjustment time, or a power component of the laser beam is permanently directed onto the (at least one) detector.

[0031] In the first case, a switchable optical element, for example in the form of a flip mirror, is typically used. This optical element can be introduced into the beam path of the laser beam and removed therefrom in order to direct the laser beam onto a detector or to feed it to an application for control. In this case, after passing through a beam shaping device, the beam path of the laser beam can be switched between a measuring beam path for control and a useful beam path for using the (controlled) laser beam. In the useful beam path, for example, a (further) optical amplifier stage or, in the case of an EUV radiation generating device, a target material, in particular in the form of tin droplets, onto which the laser beam is focused, can be arranged.

[0032] In the second case, a (typically small) power component of the laser beam is permanently coupled out of the (useful) beam path of the laser beam. For this purpose, a decoupling device, for example in the form of a beam splitter device, can be used. This is particularly useful if the laser beam is to be deflected onto a detector within the laser resonator. Permanent decoupling allows the laser beam to be adjusted during the ongoing operation of an optical arrangement in which a beam shaping device is arranged. However, in this case, it is usually necessary to only slightly adjust the adjustable optical components during the respective adjustment steps to avoid undesirable effects on the respective application during the adjustment.

[0033] Another aspect of the present invention relates to a device for providing a regulated laser beam, the device comprising: a beam shaping device; a detector device having at least one preferably point-resolved detector for measuring the beam profile of the laser beam after it passes through a beam influencing device; an analysis device for determining a beam quality characteristic of the laser beam based on the measured beam profile; and an adjustable optical device for changing at least one characteristic of the laser beam before it enters the beam shaping device, wherein the analysis device is configured to adjust the laser beam, in particular multiple times, according to the determined beam quality characteristic, preferably until the beam quality characteristic reaches a predetermined value.

[0034] As mentioned above, manual adjustment of the laser beam passing through a beam-shaping device, which may non-deterministically change the optical properties of the laser beam, is typically only possible by experts and is difficult to reproducible. The aforementioned apparatus for providing a laser beam enables automatic adjustment of the laser beam passing through the beam-shaping device. Therefore, the beam-shaping device is equipped with an automatic adjustment unit that performs adjustments autonomously without operator intervention in the adjustment process.

[0035] In one embodiment, the beam shaping device is selected from the group consisting of an optical amplifier, an optical isolator, an optical modulator and an optical delay path, and an optical polarizer. As mentioned above, non-deterministic interference effects can occur in these devices, and precise knowledge of these non-deterministic interference effects is not necessary for the adjustment described here.

[0036] In another embodiment, the adjustable optical system comprises at least one adjustable optical element, in particular at least one adjustable lens and / or at least one adjustable mirror, for varying at least one characteristic of the laser beam. At least one actuator or motor drive is typically used for (automated) adjustment of the at least one optical element. An analysis device implemented in suitable software and / or hardware (e.g., in the form of a computer program, a programmable component, such as an FPGA, etc.) generates control signals for controlling the actuator or motor drive to achieve adjustment of the at least one optical element. The actuator can, for example, be configured to move and / or rotate the optical element. For this purpose, a corresponding holder for the optical element can be configured to be movable and / or rotatable by means of the actuator. The actuator can also act on the optical element to vary its geometry, for example, to change the radius of curvature of a lens or mirror. The adjustable optical system can, in particular, comprise two optical elements that form a beam telescope, i.e., the two optical elements are arranged relative to each other substantially at a distance of their focal lengths, wherein one or both of the optical elements are movable along the beam axis of the laser beam.

[0037] In another embodiment, the beam shaping device, in particular in the form of an optical modulator, and the adjustable optical system are both arranged in a laser resonator, which additionally includes a laser-active medium. In this case, the device for providing the regulated laser beam typically constitutes a laser source, in particular a seed laser source, for generating the regulated (seed) laser beam. In this case, the beam shaping device can be, for example, an electro-optical modulator or an acousto-optic modulator, which can be used, for example, to generate ultrashort pulses. For pulse generation, the laser resonator can be operated, for example, with a (classical) Q-switch or using cavity dumping.

[0038] In another embodiment, the device, or more precisely the detector arrangement, includes a switchable optical element for directing the laser beam onto the detector (only) during the adjustment period and / or a beam splitter arrangement for (permanently) directing a power component of the laser beam onto the detector. As described above, in the first case, the beam path of the laser beam is switched between a useful beam path and a measurement beam path by means of the switchable optical element. In the second case, the beam splitter arrangement couples the typically small power component of the laser beam out of the useful beam path and feeds it to the detector.

[0039] The evaluation device can be configured to maximize or minimize the beam quality characteristic when adjusting the adjustable optical element. As described above in conjunction with the method, an optimization algorithm can be implemented in the evaluation device for this purpose, which controls the adjustment of the adjustable optical element as a function of the beam quality characteristic.

[0040] In another embodiment, the evaluation device is designed to determine the deviation of the beam profile from a predetermined beam profile, in particular a Gaussian beam profile, as a beam quality characteristic. As described above in conjunction with the method, this allows a single, unambiguous criterion for the beam quality of the laser beam to be provided.

[0041] In one embodiment, the evaluation device is configured to determine a deviation of the intensity distribution of the laser beam at the detector from a predetermined intensity distribution, in particular a Gaussian intensity distribution, as a beam quality characteristic. The evaluation device can be configured in particular to vary at least one parameter of the predetermined intensity distribution when determining the deviation. The numerically determined deviation of the intensity distribution from the Gaussian intensity distribution constitutes a particularly simple criterion for the beam quality of the laser beam. In this case, to determine the beam quality characteristic, it is sufficient to record a single image of the intensity distribution of the laser beam using a detector, for example, in the form of a camera.

[0042] The evaluation device can be designed to determine a beam parameter product or a diffraction index of the laser beam as a beam quality characteristic. In this case, a beam profile in the form of caustics of the laser beam is measured along the propagation direction or beam direction of the laser beam. In this case, a detector (e.g., a camera) is typically designed to be movable in the propagation direction of the laser beam in order to determine the intensity distribution of the laser beam in multiple mutually offset planes. The device can also include two or more detectors spaced apart in the beam direction of the laser beam in order to determine the intensity distribution in multiple mutually offset planes. The determination of the beam parameter product and / or the diffraction index can be carried out, in particular, in the form of an ISO-compliant caustic measurement.

[0043] Another aspect of the present invention relates to an optical arrangement, in particular an extreme ultraviolet radiation generating device, comprising at least two devices configured as described above and preferably arranged one after the other in the beam path of a laser beam. When the laser beam passes through two or more devices one after the other (sequentially or in a chain), each of the devices can be adjusted individually. In a chain of devices, or beam-shaping devices, such as optical amplifiers or amplifier stages, it is often necessary to readjust the entire chain, i.e., all subsequent amplifiers, if, for example, the amplifier at the beginning of the chain fails or needs to be re-adjusted, which results in a significant loss of time. In the optical arrangement described herein, this can be remedied by equipping at least the most critical beam-shaping device with the aforementioned automatic adjustment function. For this purpose, any existing optical components are configured to be adjustable upstream of the respective beam-shaping device, and a detection device having at least one detector is arranged downstream of the respective beam-shaping device. To determine the beam quality characteristics, an evaluation device connected to the detector can be integrated into the optical arrangement, for example, in a machine controller present there, which also controls the adjustable optical components. Ideally, this eliminates the need for any mechanical manipulation of these devices. This adjustment can be performed, in particular, as a regular, automatic adjustment with fixed, predetermined adjustment cycles, or alternatively, when the performance falls below or exceeds certain performance criteria (e.g., the optical transmission / gain of the optical arrangement, optical beam quality, etc.).

[0044] If integration into the machine control is not possible, the evaluation device can optionally be installed or run on a mobile computer (laptop) that is connected to the corresponding detectors and, if necessary, the adjustable optical system for signal processing. In this case, even inexperienced operators can perform adjustments independently. By determining objective beam quality characteristics and assisting the operator with adjusting the adjustable optical system when adjustments are performed manually, critical adjustment steps can be significantly accelerated.

[0045] The optical arrangement can, in particular, be an EUV radiation generating device for generating EUV radiation. Such an EUV radiation generating device typically has a vacuum chamber in which a target material, for example, in the form of tin droplets, can be introduced into a target region to generate the EUV radiation. The EUV radiation generating device has a beam guidance device for guiding (at least) one laser beam into the target region in order to transition the target material into a plasma state and thereby generate the EUV radiation. The EUV radiation generating device typically has a seed laser source for generating a pulsed seed laser beam, which is amplified in a plurality of optical amplifiers (amplifier stages) before being focused in the target region by means of a focusing device. Such EUV radiation generating devices typically include a plurality of beam shaping devices, for example, in the form of a seed laser source or an optical modulator arranged therein, in the form of one or more optical amplifiers (coaxial amplifiers or power amplifiers), and, if necessary, in the form of optical isolators, for example, in the form of Faraday rotators. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Further advantages of the present invention are apparent from the description and the accompanying drawings. Similarly, the features mentioned above and those yet to be listed can be used individually or in any combination. The embodiments shown and described are not to be understood as an exhaustive enumeration, but rather as a number of exemplary features for describing the present invention.

[0047] The accompanying drawings show:

[0048] Figure 1 A block diagram of a device for providing a regulated laser beam is shown, which device has a beam shaping device in the form of an optical amplifier, adjustable optics, and a spatially resolving detector;

[0049] Figure 2 Shows something like Figure 1 block diagram of a laser beam forming device having a beam shaping device in the form of an electro-optical modulator arranged in a laser resonator;

[0050] Figure 3a A schematic diagram showing a two-dimensional intensity distribution of a laser beam measured by means of a spatially resolved detector;

[0051] Figure 3b Show Figure 3a One-dimensional cross section of the intensity distribution and Gaussian intensity distribution;

[0052] Figure 4 Show that there is a basis Figure 1 Equipment and according to Figure 2 Illustration of an extreme ultraviolet radiation generating device. DETAILED DESCRIPTION

[0053] In the following description of the figures, the same reference numerals are used for identical or functionally identical components.

[0054] Figure 1 A block diagram is shown with five blocks or optical modules, which are successively passed through by a laser beam 1. The first block forms a beam source 2 for providing a laser beam 1 with defined optical properties or a predetermined beam quality. Beam source 2 can, for example, be a laser source or another optical module that provides a laser beam 1 with a predetermined beam quality. The second, third, and fourth blocks together form a device 3a for providing a conditioned laser beam 1. Device 3a includes a beam shaping device 4 in the form of an optical amplifier. Optical amplifier 4, due to uneven gain, can produce interference effects that affect, or worsen, the beam quality of laser beam 1. To condition laser beam 1 so that it emerges from device 3a with the desired beam quality, an adjustable optical component 5 is arranged upstream of optical amplifier 4, and a detector device 6 is arranged downstream of optical amplifier 4, which is connected to an evaluation device 7 for signal processing. The conditioned laser beam 1 emerging from the device 3 a is fed to a fifth block, which in the example shown is a further beam shaping device 8 in the form of a further optical amplifier.

[0055] Figure 2 A block diagram is shown, which is the same as the Figure 1 The difference to the block diagram shown in is essentially that the first block relates to a laser-active medium 2, which is arranged in a laser resonator 9. The laser resonator 9 is delimited by two end mirrors 10a, b, of which the second end mirror 10b is partially transparent, so that it forms a decoupling mirror for coupling the laser beam 1 out of the laser resonator 9. Alternatively, the decoupling of the laser beam 1 from the laser resonator 9 can also take place at a (polarization) beam splitter. This is usually the case when a beam shaping device in the form of an optical modulator 4, for example an electro-optical modulator or an acousto-optical modulator, is arranged in the laser resonator 9 for generating ultrashort laser pulses, as in Figure 2 From Figure 2 , it can also be seen that the adjustable optical system 5 of the device 3 is also arranged within the laser resonator 9, while the detection device 6 is arranged outside the laser resonator 9. However, the latter is not absolutely necessary, that is, the detection device 6 can also be arranged within the laser resonator 9.

[0056] exist Figure 2In the example shown in FIG, the device 3 for providing a conditioned laser beam 1 comprises a laser resonator 9, the first four blocks of the block diagram—that is, also the laser-active medium 2—and an evaluation device 7. Thus, the device 3 forms a (seed) laser source for providing a pulsed (seed) laser beam 1. It should be understood that the arrangement of the laser-active medium 2, the adjustable optical element 5, and the beam-shaping device 4 in the laser resonator 9 is essentially arbitrary. In particular, the laser-active medium 2 can be a solid or gaseous laser-active medium.

[0057] The adjustable optical system 5 has at least one adjustable optical element and can be designed in different ways. Figure 1 In the block diagram presented in , the adjustable optical system 5 comprises two transmissive optical elements in the form of lenses 11a, b, which form a beam telescope and are arranged relative to each other at a (nominally) spacing of their focal lengths f1, f2. Figure 1 The example shown in FIG involves spherical lenses 11a, b, of which the second lens 11b can be moved along the beam path or along the propagation direction of the laser beam 1. A motor drive is used for moving the second lens 11b, which is Figure 1 Indicated by a double arrow in the figure. Additionally or alternatively, the first lens 11a in the beam path can also be designed to be adjustable, in particular movable, and / or both lenses 11a, b can be movable along the propagation direction of the laser beam 1. By moving the second lens 11b, the divergence of the laser beam 1 can be influenced, i.e., a (slightly) converging or diverging laser beam 1 can be generated from the collimated laser beam 1. When using aspherical or cylindrical lenses, other characteristics of the laser beam 1 can be influenced. For example, optical aberrations such as astigmatism can be selectively generated or corrected using a movable cylindrical lens.

[0058] exist Figure 2 In the illustration, the adjustable optical device 5 has two deflection mirrors 12a, b, which are both configured to be adjustable and are adjusted by means of Figure 2 The respective motor rotary drives indicated by the double arrows in FIG. 1 can be rotated about respective rotation axes perpendicular to the drawing plane. If only the second deflection mirror 12 b is rotated, the angle of incidence δ of the laser beam 1 upon entering the beam shaping device 4 can be adjusted. This also changes the beam position X of the laser beam 1 upon entering the beam shaping device 4. P ,Y P If the two deflection mirrors 12a, b are rotated appropriately, a parallel displacement of the laser beam 1 can be produced in order to adjust the beam position X upon entry into the beam shaping device 4 independently of the angle of incidence δ of the laser beam 1. P ,Y P .

[0059] exist Figure 1 and Figure 2 The detector device 6 shown in FIG has a spatially resolving detector 6a in the form of a camera, which is connected to an evaluation device 7 for signal processing. Figure 1 The detector device 6 shown in FIG. 1 has a switchable optical element in the form of a tiltable deflection mirror 13, which can be switched from Figure 1 The position shown in FIG. 1 (in which the deflection mirror is arranged outside the beam path of the laser beam) is tilted by means of an electric motor drive to Figure 1 In the position indicated by the dashed line (in which the deflecting mirror 13 is arranged in the beam path of the laser beam 1 and directs the laser beam 1 onto the detector 6 a during adjustment).

[0060] exist Figure 2 The detector device 6 shown in Figure 1 The difference of the detector device 6 shown in FIG. 1 is that, instead of the switchable device 13, a beam splitter device 14 is provided for (permanently) directing the laser beam 1 onto the detector 6a. The beam splitter device 14 is designed as a polarization beam splitter in the example shown and enables a predetermined power component P of the laser beam 1 to be directed to the detector 6a. D (eg <1%) is directed to the detector 6a, while a significantly larger power component P N The effective power is available when the (regulated) laser beam 1 emerges from the device 3 .

[0061] Figure 3a Shown in Figure 1 The beam profile 15 of the laser beam 1 is shown on the detector surface of the detector 6a, ie in a plane perpendicular to the propagation direction z of the laser beam 1. Figure 3a In the illustration of , the beam profile 15 is represented by a line of constant intensity of the intensity distribution I(x,y) of the laser beam 1 , which is measured in a spatially resolved manner by a detector 6 a in the form of a camera. Figure 3b Shows the direction extending in the X direction. Figure 3a The one-dimensional cross section of the intensity distribution I(x,y) at the center of the detector surface (i.e., when y0=0) is shown as a dot-dashed line in FIG. Figure 3b As can be seen from the figure, the intensity distribution I(x,y0) extends irregularly and deviates from the (radially symmetric) Gaussian intensity distribution I G (x,y), in Figure 3b The Gaussian intensity distribution in the X direction is also shown in FIG. G A one-dimensional cross section at (x,y0).

[0062] Two-dimensional Gaussian intensity distribution I G (x,y) can be described by the following formula:

[0063] I G (x,y)=I0 EXP([(x–x0) 2 +(y–y0) 2 ] / (2 w0 2 )) (1)

[0064] Among them, I0, x0, y0, w0 form a Gaussian intensity distribution I G (x,y) parameters.

[0065] In a first approximation, the parameters x0 and y0 can be identified by means of the center point of the detector surface (x0=y0=0). The parameters I0 and w0 can also be predetermined, for example, by irradiating the detector 6a for calibration with a laser beam 1 having a Gaussian beam profile.

[0066] The detected two-dimensional intensity distribution I(x,y) and Gaussian intensity distribution I G The deviation D of (x, y) can be represented, for example, by the following error integral or functional:

[0067] D=∫∫ x，y (I(x,y)-I G (x,y)) 2 (2)

[0068] Among them, I G (x,y) is parameterized as in equation (1).

[0069] In order to be based on Figure 3a By adjusting the laser beam 1 according to the beam profile 15 on the detector 6 a shown in FIG, the following method sequence can be performed:

[0070] First, the laser beam 1 passes through the beam shaping device 4 and the beam profile 15 is measured in the form of the intensity distribution I(x,y) by means of the detector 6a. In the next step, the beam quality characteristic in the form of a deviation is determined based on the intensity distribution I(x,y) and based on equations (1) and (2). It is checked whether this deviation D has reached a predetermined value D corresponding to the control target. S .

[0071] If this is not the case, an adjustment step is performed, in which the adjustable optical element 5 is adjusted, for example by Figure 1 The optimization algorithm determines how the adjustable optical element 5 is to be adjusted based on the determined deviation D, more precisely based on the respectively determined values ​​of the deviation D and, if necessary, based on the deviation D determined in the corresponding preceding adjustment step.

[0072] In the next step, the deviation D is determined again and thus the influence of the adjustment of the adjustable optical element 5 on the beam quality is determined again. If the adjustment target is reached, ie the deviation D is less than the predefined value D S , the adjustment is ended. If this is not the case, the optimization algorithm makes a new decision based on the value of the deviation D (and based on the deviation determined in the previous adjustment step): how to adjust the adjustable optical element 5. If the deviation D increases during the last adjustment of the adjustable optical element 5, the opposite adjustment can be carried out, for example, Figure 1 The second lens 11b of the adjustable optical element 5 is moved to the left in order to reduce the deviation D. The direction and magnitude of the movement of the second lens 11b are selected by an optimization algorithm, which iteratively minimizes the deviation D and sets the deviation D when the adjustment target is reached, that is, when the deviation D reaches a predetermined value D. S Terminate adjustment.

[0073] The steps of the above method, in particular the optimization algorithm, can be executed in the analysis device 7. The analysis device 7 can be implemented, for example, in the form of suitable software and / or hardware (for example, a control computer). When the deviation D is determined according to equation (1), the Gaussian intensity distribution I G At least one of the (x,y) parameters I0, x0, y0, w0 is varied in order to minimize the deviation D from the corresponding measured intensity distribution I(x,y), but this is not absolutely necessary.

[0074] Combine Figure 3a The beam quality characteristic described in Item 1b in the form of a deviation D is a particularly simple criterion for the beam quality of the laser beam 1 , which can be determined without great effort.

[0075] Alternatively, as a beam quality characteristic, the deviation of the beam profile 16 in the form of beam caustics in the propagation direction z of the laser beam 1 from a predetermined beam profile, in particular from a Gaussian beam profile, can be determined, as Figure 2 In this case, the beam caustics of the laser beam 1 are measured by measuring the intensity distribution I(x,y) or the beam diameter of the laser beam 1 in a plurality of planes offset from one another in the propagation direction z. Figure 2 The detector 6a shown in FIG can be moved by means of a motor drive along a suitable guide, as indicated by the double arrow. Based on the beam caustics 16 of the laser beam 1, the M of the laser beam can be determined in a known manner. 2 Value, that is, the divergence angle of the laser beam Divergence angle of an ideal Gaussian beam with the same diameter at the beam waist The ratio of Beam parameter product (BPP) = M 2 λ / π can also be determined in this way (and with a known wavelength λ of the laser beam 1). Ideally, an ISO-compliant caustic measurement of the laser beam 1 can be performed. The laser beam 1 can be adjusted or optimized in a manner similar to that described above for the deviation D, i.e., by reducing or minimizing the beam parameter product BPP or the diffraction index M. 2 , until they reach a predetermined value BPP S Or M S 2 .

[0076] Figure 4 An optical arrangement in the form of an EUV radiation generating device 20 is shown, which has a seed laser source 3 for providing or generating a (regulated) laser beam 1, as in Figure 2 The laser beam 1 in the example shown is a CO2 laser beam which is irradiated by means of a gaseous laser-active medium 2 (see Figure 2 ) is generated. In addition to the seed laser source 3, the extreme ultraviolet radiation generating device 20 also includes an amplifier arrangement 22 with three optical amplifiers or amplifier stages 3a, 3b, 3c, a beam guiding device 24 (not shown in detail), and a focusing device 25. The focusing device 25 is used to focus the laser beam 1 generated by the seed laser source 3 and amplified by the amplifier arrangement 22 into a target area in a vacuum chamber 28, into which target area a target material 26 is introduced. When irradiated with the laser beam 1, the target material 26 transitions into a plasma state and emits extreme ultraviolet radiation therein, which is focused by means of a collecting mirror 27. The seed laser source 3 and the amplifier arrangement 22 together form a driver laser arrangement 29 of the extreme ultraviolet radiation generating device 20.

[0077] exist Figure 4 In the extreme ultraviolet radiation generating device 20 shown in FIG, the first amplifier stage 3a is as shown in FIG. Figure 1 The device 3a shown in FIG is designed in such a way that the first amplifier stage 3a automatically adjusts the laser beam 1. This is advantageous because after trimming the first amplifier stage 3a, only the first amplifier stage 3a needs to be adjusted in the above-described manner, without having to adjust the subsequent amplifier stages 3b, 3c. It is understood that the second and third amplifier stages 3b, 3c can also be adjusted in the same manner as in FIG. Figure 1 Constructed in the manner shown in Figure 1 The optical amplifier described in Figure 2Different beam shaping devices 4 of the electro-optical modulator described in can be adapted in the above-described manner, such as optical isolators, optical (multi-pass) delay paths, optical polarizers, etc. which can also be provided in the EUV radiation generating device 20 or other optical arrangements.

Claims

1. A method for adjusting a laser beam (1), comprising: Measuring a beam profile (15, 16) of the laser beam (1) after passing through a beam shaping device (4) by means of at least one detector (6a) of a detector device (6); Based on the measured beam profile (15, 16), a deviation of the beam profile (15, 16) from a predetermined beam profile is determined as a beam quality characteristic of the laser beam (1), wherein a deviation of an intensity distribution I(x, y) of the laser beam (1) on a detector (6a) from the predetermined intensity distribution is determined as the beam quality characteristic; and adjusting an adjustable optical element (5) to change at least one characteristic of the laser beam (1) before entering the beam shaping device (4); wherein, in order to shape the laser beam (1), the adjustable optical element (5) is adjusted according to the determined beam quality characteristic, wherein the beam shaping device (4) comprises an optical amplifier, an optical isolator, an optical delay path and / or an optical polarizer, Obtain the intensity distribution I(x,y) of the laser beam (1) on the detector (6a) and the Gaussian intensity distribution I G The deviation of (x,y) is used as a beam quality characteristic: D=∫∫ x，y (I(x,y)-I G (x,y)) 2 。 2. The method according to claim 1, wherein When adjusting the laser beam (1), the beam quality characteristic is maximized or minimized.

3. The method according to claim 1, wherein The Gaussian intensity distribution is described by the following formula: I G (x,y)=I0EXP([(x-x0) 2 +(y–y0) 2 ] / (2w0 2 )), Where x0, y0 are the center positions of the Gaussian intensity distribution, I0 is the maximum intensity of the Gaussian intensity distribution, and w0 is the beam diameter.

4. The method according to claim 3, wherein: When obtaining the deviation, the Gaussian intensity distribution I G At least one parameter I0, x0, y0, w0 of (x, y) changes.

5. The method according to any one of claims 1 to 4, wherein When adjusting the adjustable optical device (5), the beam position of the laser beam (1), the orientation of the laser beam (1), the caustics of the laser beam (1) and / or the aberrations of the laser beam (1) are adjusted, wherein the beam position of the laser beam is the position of the laser beam in a plane perpendicular to the propagation direction when the laser beam enters the beam shaping device, wherein the orientation of the laser beam is the angle of incidence of the laser beam when it enters the beam shaping device.

6. The method according to any one of claims 1 to 4, wherein When adjusting the laser beam (1), at least one optical element of the adjustable optical system (5) is adjusted.

7. The method according to any one of claims 1 to 4, wherein The beam shaping device and the adjustable optical element (5) are both arranged in a laser resonator (9).

8. The method according to any one of claims 1 to 4, wherein During the adjustment, the laser beam (1) is directed onto a detector (6a), or a power component P of the laser beam (1) is directed onto a detector (6a). D The sensor (6a) is permanently guided to the sensor.

9. A device (3, 3a) for providing a modulated laser beam (1), comprising: Beam shaping device (4); a detector device (6) having at least one detector (6a) for measuring a beam profile (15, 16) of the laser beam (1) after passing through the beam shaping device (4); An analysis device (7) for determining, based on a measured beam profile (15, 16), a deviation of the beam profile (15, 16) from a predetermined beam profile as a beam quality characteristic of the laser beam (1), wherein the analysis device (7) is configured to determine a deviation of an intensity distribution (I(x,y)) of the laser beam (1) on a detector (6a) from a predetermined intensity distribution as the beam quality characteristic; and An adjustable optical element (5) for changing at least one property of the laser beam (1) before entering the beam shaping device (4), wherein the evaluation device (7) is designed to adjust the adjustable optical element (5) in accordance with the determined beam quality property in order to adjust the laser beam (1), wherein the beam shaping device (4) comprises an optical amplifier, an optical isolator, an optical polarizer and / or an optical delay path, Obtain the intensity distribution I(x,y) of the laser beam (1) on the detector (6a) and the Gaussian intensity distribution I G The deviation of (x,y) is used as a beam quality characteristic: D=∫∫ x，y (I(x,y)-I G (x,y)) 2 。 10. The apparatus according to claim 9, wherein The adjustable optical system (5) has at least one adjustable optical element for varying at least one property of the laser beam (1).

11. The apparatus according to claim 9 or 10, wherein: The beam shaping device and the adjustable optical element (5) are both arranged in a laser resonator (9).

12. The device according to claim 9 or 10, further comprising: a switchable optical element (13) for directing the laser beam (1) onto the detector (6a) during adjustment, and / or A beam splitter device (14) for dividing the power component P of the laser beam (1) D Guided to the detector (6a).

13. The apparatus according to claim 9, wherein The Gaussian intensity distribution is described by the following formula: I G (x,y)=I0EXP([(x-x0) 2 +(y–y0) 2 ] / (2w0 2 )), Where x0, y0 are the center positions of the Gaussian intensity distribution, I0 is the maximum intensity of the Gaussian intensity distribution, and w0 is the beam diameter.

14. The apparatus according to claim 13, wherein The evaluation device (7) is designed to, when determining the deviation, use the Gaussian intensity distribution I G At least one parameter I0, x0, y0, w0 of (x, y) changes.

15. An optical arrangement comprising: At least two devices (3, 3a) according to any one of claims 9 to 14.

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